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Automation and Robotics Based Technologies for Road Construction, Maintenance and Operations
“Building and construction is one of the major industries around the world. Construction industry is labor-intensive and is conducted in dangerous situations; therefore the importance of construction robotics has grown rapidly. Applications and activities of robotics and automation in this industry started in the early 90s aiming to optimize equipment operations, improve safety, enhance perception of workspace and furthermore, ensure quality environment for building occupants”
Construction productivity on large projects, including road construction, has been constant or declining since the 1970s. This has been coupled with a dramatic increase in construction labor cost and shortage in funding for new road construction and maintenance. At the same time,
highway construction costs have been increasing, even after correcting for general inflation (Refer Table 1). One viable solution is partial or full automation of a number of work tasks. Automation is particularly germane due to the relative simplicity, repetitiveness, and large volume of work involved with roadways. Since Today’s construction projects are characterizing by short design and build period, increased demands of quality and low cost. These problems can be approached by a flexible automation using robots based on computer assisted planning, engineering and construction management. Especially in high labor cost countries, automated and robotized construction technologies can compensate increasing demand on construction projects. Automated and robotized construction process lead to a continuous working time through the year. Introduction of
Sonjoy Deb, B.Tech, ‘Civil’Associate Editor
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robotic technology would result in better working and health conditions, and advanced mechatronics know how and skills. The reduction of construction and repair rehabilitation time would improve cost benefit analysis of construction project or critical maintenance activities likes in roads due to faster availability and return on investment.
In addition to any strictly financial benefits, an expected advantage of automated road construction equipment is improvement in work safety and health. In some instances, laborers will be completely removed from the work loop and thus prevented from being run over by the working machine or other vehicles. In other cases, the health hazards associated with the worker’s proximity to carcinogenic materials may be reduced. Refer Figure 1 for a project sensor-based compactor.
Work Breakdown Structure of Road Construction
Following are the works that exists in a typical road construction or repair/maintenance projects-
1. Cut and fill operations: These initial works involve mass transport of earth material within and outside the immediate road construction location to provide the desired sections and profiles of the terrain prior to the commencement of construction. Heavy excavation and off-the-road hauling equipment are typically used for this purpose (Nunnaly 1980).
2. Grading: This task involves the sieving and breakdown of small rock and soil pieces to the desired maximum size, as well as the creation of exact profiles and sections of road at each station. Specialized grading machinery is typically utilized.
3. Base preparation and placement: This work consists of the placement of gravel base on the graded soil. Typical work tasks include gravel dumping, screeding, and compaction. Heavy trucks, screeders, and drums are typically used for this purpose.
4. Surface material placement: This set of construction tasks involves the placement of hot bituminous material, concrete mix, or other surface type, as well as vibration and screeding. Specialized surface-placement equipment is used for this purpose.
5. Curbing and guardrail placement: This work involves the forming and placement of temporary or permanent curbs and guardrails. The tasks include fabrication of curb and guardrail sections as well as their transport and placement.
6. Road maintenance: Maintenance work involves a variety of continuously performed tasks, including snow removal, road painting, grass mowing, brush cleaning, sign placement, pothole and crack filling, and others.
As with other construction activities, labor requirements in road construction are closely associated with the equipment tasks outlined here. They include the operation of excavators and hauling trucks during cut and fill, operation of graders, manual support of road-base placement, curb /guardrail installation, and maintenance tasks.
Year (1) Gross national product defla-tor Index 1972 x 100 (2)
Standard highway cost index 1972 = 100 (3)
Standard highway cost index con-struction dollars 1972 = 100 (4)
Percent change cost by decade (5)
1940 29 26 90
1950 54 48 89 -1
1960 68.7 58 84 -6
1970 91.5 91 99 +18
1980 178.6 255 143 +44
Table 1: Highway Construction Cost Indexes 1940-1980 (Commerce 1986)
Figure 1: Project sensor-based compactor
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Automation In Construction - General
The project success from the project management’s view point is achieved when the project is completed with the lowest possible cost, the highest quality, no accidents, etc. In other words, success means bringing each of the project performance indicators (PPI)- such as cost, schedule, quality, safety, labor productivity, materials consumption or waste, etc. to an optimum value.
Applying automation and robotics in construction is addressed from the perspective of raising building projects performance to serve the client and the environment. Robotics and automation systems in construction industry can achieve the following advantages:
- Higher safety for both workers and the public through developing and deploying machines for dangerous jobs.
- Uniform quality with higher accuracy than that provided by skilled worker.
- Improving work environment as conventional manual work is reduced to a minimum, so the workers are relieved from uncomfortable work positions
- Eliminating complains about noise and dust concerning works such as removal, cleaning or preparation of surfaces
- Increasing productivity and work efficiency with reduced costs.
Automation in Road Construction and Maintenance - Road paving robots show high level of automation through providing the followings:
- automated reception of asphalt- automatic control of asphalt conveyance- automatic control of asphalt spreading- automatic steering control with mechanical sensor and
automatic control of paving speed- automatically controlled start/stop of all paving functions
In addition, tasks can be performed automatically based on an artificial vision and a laser range sensor.
Similarly, Longitudinal Crack Sealing Machines can fill and seal cracks running along the road, for example between lanes and the shoulder. The process is remote-controlled by the driver, and the machine can fill cracks at up to five miles per hour. In comparison a manual sealing operation would take a large crew all day to complete two miles. Robots are also helping to remove roadside litter and debris, another hazardous, labor-intensive operation.
Automation In Road Construction – Specific
Three major categories of road construction and maintenance equipment exist: mechanized equipment, numerically
controlled (NC) hard automation equipment, and semi-autonomous/autonomous (flexible, soft automation) equipment. While mechanized equipment has been used on road construction sites for many years, NC equipment constitutes the state of the art utilized in practice, and autonomous equipment is still in the research and development stage. The major utility of mechanized road construction equipment is its ability to apply large forces over an extended period of time in various work tasks, such as excavation, trenching, and hauling. This capability significantly contributes to task productivity and efficiency in large-volume works. Almost exclusively, this is due to hydraulic force actuation and transmission hardware. This equipment is currently well suited for rough handling in outdoor construction environments due to the lack of, or only minimal, inclusion of naturally fragile electronic devices. Equipment operation requires human support for each executable work task.
Numerically controlled (NC) equipment has the capability of executing repetitive, large-volume tasks with little or no operator assistance. However, the work environment is restricted to the conditions in which only one task or a sequence of identical tasks is required. Also, prior to the execution of work, the removal of any obstacles in the path of the working machine is mandatory. Thus, operator assistance is required when an unexpected obstacle or other operational difficulty is encountered. In some cases, guide wires or light-emitting diodes (LEDs) may be used as established reference points for mobile machines. Autonomous (robotic) road construction equipment presents the highest level of technical sophistication compared with mechanized and NC equipment. Depending on its level of autonomy, the equipment is capable of partially or fully independent execution of one or a variety of tasks. The operational autonomy of equipment is achieved by the use of sensory data obtained from the environment. The use of sensor data requires subsequent processing and use in the actuation of relevant machine actions. Thus, robotic machines may be capable of acting intelligently in reaction to unforeseen work-site conditions within a limited range of possibilities. If the site conditions become too complex to be recognized and acted upon by the machine, an operator’s assistance may also be requested. Also, automatic shutoff of the equipment operation should occur when an unacceptable type of hazard is encountered. This type of equipment can be reprogrammed to suit differing sets of job-site requirements and different types of compatible construction tasks. Refer Figure 2 for dragline project excavator, which is fully automated and adds to the construction efficiency. Refer Figire 3 for the automated excavator of the University of Sidney.
Table 2 lists some examples of numerically controlled and
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The following areas of technology constitute the basis for development of automated road construction and maintenance machines (Hendrickson 1989).
- Manipulators - Stationary, articulated manipulator arms are essential components of industrial robotics. The role of a manipulator arm is to move an effector tool into a proper location and orientation relative to a work object.
- End Effectors - A variety of end effectors can be employed on robot arms. Typical end effector tools and devices on automated road construction and maintenance equipment include discharge nozzles, sprayers, scrapers, and sensors.
- Motion Systems - Mobility and locomotion are essential features for road construction and maintenance equipment. A variety of mobile platforms can support stationary manipulator arms for performance of required tasks.
- Electronic Controls - Controllers are hardware units designated to control and coordinate the position and motion of manipulator arms and effectors. A controller is always equipped with manipulator control software enabling an operator to record a sequence of manipulator motions and subsequently to play back these motions a desired number of times.
- Sensors - Sensors convert environmental conditions into electrical signals. An environmental condition might be a mechanical, optical, electrical, acoustic, magnetic, or other physical effect.
Hard Automation (NC) Equipment
The equipment examples described in this section are designed for the execution of repetitive construction and maintenance tasks typically performed on roadways. This equipment requires a substantial amount of site preparation before the intended work tasks can be executed. No sensors are employed on the equipment for site data acquisition. Thus, all equipment control functions requiring judgment based on the external environment data are performed by an operator. The motivation for development of these machines came primarily from the expected economic payoff in high-volume highway works.
Some Practical Developments
A. Societe Nicolas of France has developed a multipurpose traveling vehicle (MPV) used for a variety of maintenance tasks (Point 1988). The main tooling on the vehicle is intended for mowing grass around roadway curbs. It can cut a width of 2.5 m in two passes. It is claimed
Equipment Example
Type of task (1)Numerically-Con-
trolled (NC) (2)Autonomous (3)
Cut and fill Grading
Base preparation and placementSurface material
placementRoad maintenance
--
-Miller formless
systemsMiller formless systems Socite
Nicolas, Secmar
Carnegie MellonSpectra-Physics,
Agtek--
U.S. Air Force
Table 2: Examples of Automated Equipment for Road Construction and Main-tenance Tasks
autonomous equipment for the types of road construction and maintenance tasks used commonly.
Relevant Core Technologies of Automation in Highway Works
Figure 2: Dragline project Excavator for Road construction
Figure 3: The Automated excavator of the University of Sydney
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that the MPV can save up to 50% on mowing costs compared with traditional mowing equipment.
B. Miller Formless Systems Co. has developed four automatic slipform machines, M1000, M7500, M8100, and M9000, for sidewalk curb and gutter construction. All machines are able to pour concrete closer to obstacles than with alternative forming techniques. They can be assembled to order for the construction of bridge parapet walls, monolithic sidewalk, curb, and gutter, barrier walls, and other continuously formed elements commonly used in road construction.
C. Secmar Co. of France developed a prototype of the integrated surface patcher (ISP) (Point 1988). ISP is used primarily for hot resurfacing repairs, including surface cutting, blowing, and tack coating with emulsion, as well as for repairs requiring continuous treated or nontreated granular materials. The unit is suitable for deep repairs using aggregate/bitumen mix, cement-bound granular materials, and untreated well-graded aggregate, as well as for sealing wearing courses with granulates.
Automated Equipment of Future
Developments in this automated road construction and maintenance equipment will lead to the future expansion of advanced technology in high-volume road works. Several new types of machines will be developed for a variety of tasks.
- In cut and fill works, further progress is expected in the autonomy of task performance. Excavators, backhoes, and off-the-road dump trucks will navigate autonomously around construction sites with the use of signals emitted from reference locations and received by location sensors mounted on the equipment. The excavation will be performed with little or no monitoring by an operator thanks to the use of surface modeling and object-detection algorithms executed in real-time by on-board controllers.
- In grading works, the dissemination of laser-controlled blade operation will be augmented by autonomous grader navigation around job sites.
- In base preparation and placement works, automation of equipment assignments will also play an important role in productivity improvement. The efficient movement of gravel trucks, compacting drums, vibrators, screeders, and other equipment over large work areas will be enhanced with automated work scheduling techniques. The equipment will be able to determine its work area, proceed to the job location, and execute an optimum sequence of operations based on dispositions provided by on-board controllers.
- In surface material placement works, equipment autonomy will improve the introduction of autonomous navigation and the use of material property sensors during placement. Such quantities as thickness of asphalt layers, consistency of mix, and layer profiles will be monitored and corrected automatically with the use of sensor-equipped robotic controllers.
- In curbing and guardrail placement works, proliferation of numerically controlled equipment will continue. Standards for dimensions, quality, weight, and placement procedures will be developed for the use of NC equipment. In road maintenance tasks, a variety of new devices integrating autonomous equipment mobility with smart sensors, including artificial vision, and dextrous manipulator end effectors will be employed.
New capabilities of the existing machines will be created from the advancement of fundamental research in robots technology. Improved sensor designs, more efficient robot controllers, and innovative end effectors will all contribute to redefinition of current equipment work procedures. Entirely new types of equipment that integrate several tasks from across the presented taxonomy may also be developed. This will be possible if the development cost of one machine can be spread over several applications unrelated at present. Thus, a systematic approach to the development of functional modules of robotic machines may prove advantageous.
Conclusion
The performance of robotic technology is increasing rapidly and we can support its advancement by designing, engineering, managing the construction processes and products in a robot oriented way. On the engineer level we need robotic and mechatronic construction engineers, managers and architects education. The workers need mechatronic and robotic training and qualifications. The realization of automation and integration of advanced technologies in the construction field can be supported, if the guidelines for automation oriented construction systems are followed and took into the thinking process. Together with a slightly modified design, the effective prefabrication and automated assembly on-site are processes, which can be linked together through a sophisticated computer integration and interface management. A systematic approach to the development of automated road construction and maintenance equipment, based on a thorough ergonomic and economic analysis of relevant work tasks, will result in determining the most feasible alternatives for equipment operational modes. It is anticipated that numerically controlled (NC) equipment will prove sufficient and successful for a majority of routine, high-volume tasks. Autonomous equipment is desirable for tasks traditionally requiring
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continuous monitoring of machine work by an operator who customarily can take only a limited number of actions when required to correct task execution.
Reference
1. Cho, Y., Haas, C.T., Liapi, K. and Sreenivasan, S.V. (2002). A framework for rapid local area modeling for construction automation, Automation in Construction, 11, 6, 629-641.
2. Concrete construction robot, Series of construction robots which have been awarded prizes from the Architectural Institute of Japan, www.takenaka.co.jp/.../53_crobo/53_crobo.htm
3. Berlin, R. (1994). Development of the multi-purpose mobile robot for concrete surface processing, Proceedings of the 11th International Symposium on Automation and Robotics in Construction (ISARC), Brighton, UK, 133-140.
4. Kalay, Y.E. and Skibniewski, M.J. Automation in Construction journal, www.iaarc.org/_old/frame/publish/autcon.htm
5. Kim Y.S. and Haas, C.T. (2000). A model for automation of infrastructure maintenance using representational forms, Automation in Construction 10, 1, 57-68.
6. Fell, A. (2001), Robots at work help make highways safer,
University of California newspaper, Davis, dateline ucdavis, http://www.dateline.ucdavis.edu/092801/DL_caltrans .html
7. Hendrickson, C , and Au, T. (1989). Project management for construction: Fundamental concepts for owners, engineers, architects and builders. Prentice Hall, Englewood Cliffs, N.J.
8. Herbsman, Z., and Ellis, R. (1988). “Potential application of robotics in highway construction.” Proc, 5th Int. Symp. on Robotics in Constr., Japan Industrial Robot Association, Tokyo, Japan, June, 299-308.
9. Kobayashi, T., et al. (1988). “Study on a robotic system for pavement cutting work.” Proc, 5th Int. Symp. on Robotics in Constr., Japan Industrial Robot Association, Tokyo, Japan, June, 289-298.
10. Nunnaly, S. (1980). Construction methods and management. Prentice Hall, Inc., Englewood Cliffs, N.J.
11. Paulson, B. (1985). “Automation and robotics for construction.” J. Constr. Engrg. andMgmt., ASCE, 111(3), 190-207.
12. Point, G. (1988). “Two major innovations in current maintenance: The multi-purpose vehicle and the integrated surface patcher.” 67th TRB Annual Meeting, Transp. Res. Board, 29.
13. Automation and Robotics for Road Construction and Maintenance By Miroslaw Skibniewski1 and Chris Hendrickson, 2 Members, ASCE.
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64 The Masterbuilder - April 2013 • www.masterbuilder.co.in
Low RPM Low Wind Velocity Aero-generator
Nowadays, electric power supply of state electricity board is very uncertain and the voltage is also unreliable. There is frequent load shading, interruptions, voltage
fluctuation and shutdowns usually without any notice to consumers. This results into stoppage of working and mental
agony. Also, the cost per unit is increasing day by day. In future the electrical energy is going to be more and more scarce and more and more costly. Especially, those, who own a small office with computers, have to suffer a lot due to such uncertain supply of power. In houses and small offices/ establishments,
Avinash D. Shirode
There are various methods of generation of electrical power. Thermal, hydraulic, atomic are conventional and solar; wind, bio-mass, fuel cells etc. are non-conventional methods. Unfortunately, these sources are thought of on large scale generation (on MW scale) which are not within the reach of an individual. Also, the present day cost of solar cells & wind power generators for small requirements is quite exorbitant. The wind speed is very fluctuating, inconsistent, unreliable and varies from time to time through out the year from place to place. There are ‘wind zones’ and ‘wind maps’ earmarked for India. It is speculated that at most of the places in our country windmills will not be viable due to low wind speed. Hence, a generator, which will produce the energy even at very low speed, is required. This paper discusses the generation of electrical power (1 to 5 kW range), at a reasonably low cost, by the alternator, which is driven by wind energy.
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normally, generator run by petrol /diesel /kerosene is used to supply electricity when there is no grid power. The use of such appliance is not always satisfactory as every time a person has to go to generator to start it and shut it down. Also, even for 15 watt bulb, the generator has to be kept running with full consumption of fuel. It makes lot of noise and creates smoke and fumes. Thus, there is a great nuisance of noise & air pollution.
To overcome the said problem, inverter and UPS system with large capacity batteries to give sufficient backup (time & wattage) is provided. This arrangement works well for some years even without changing batteries. But, even such an arrangement fails when there is great scarcity of power. If, for example, there is no power for two full days in a week and almost 4-5 hours on other days, even a very efficient battery back up system fails as the batteries cannot get sufficiently charged due to such frequent and long duration power cuts.
It is well known that solar energy with the help of photovoltaic cells can be used. But, harnessing the solar energy is very costly. Wind energy has been harnessed in the windmill farms. These farms require large investments and at present are available mostly in MW range.
At present there is no contrivance to overcome these shortcomings of the present situation. Therefore, there is a dire need for a set of equipment, which will generate just sufficient electrical energy for domestic needs of 1 to 5 kW.
The wind energy, though not consistent, is quite cheaper to solar energy. This does not create any pollution, environment friendly, totally free of running cost, maintenance free and satisfying.
Windmill
The Windmill consists of:
1) Propeller Blades 2) Tail Vane 3) Alternator4) Supporting Tower 5) Storage Batteries6) Inverter
1) Propeller Blades
The blades of suitable size and shape are made up of either fiber glass/ dried Dewdar wood/ FRP/ Plastic material/ aluminium sheets etc., in the aero foil shape. The shape of blades is the most important factor. The blades are to be dynamically balanced. It should sweep maximum wind to produce sufficient torque to rotate the blades even at low wind speeds.
2)Tail Vane
The direction of wind is not same or uniform all the time. The tail vane is used to bring the blades in front of the wind so
that the blades get maximum available wind force to keep rotating most of the time. The design of tail vane and blades depend on each other. Additionally, the tail vane is also used to balance the weight of blades.
3)Alternator or Electrical Generator
The alternator is the main component of the wind mill. It produces electricity.
The idea of making an alternator was actually thought from the spinning toy, ‘Bhingaree’ i.e. large diameter with thin width, which the children play in villages.
The alternator consists of a stator and a rotor. The stator and a rotor comprise of two circular shaped, concentric rings. The inner ring is rotated by a shaft, which is connected to propeller blades. Bearings are used for smooth rotation of the shaft. The outer ring is stationary and does not rotate with the shaft. Permanent magnets of equal size, shape and magnetic strength and copper wire coils of equal size, shape, turns and electrical characteristics are pasted on inner & outer rings to generate electricity. The rotating ring is dynamically balanced. This whole arrangement of the stator and rotor of the alternator is covered by aluminium /G. I. /m. s. sheet on both sides to protect the alternator from weather. The three phase A. C. current output is obtained which is converted to D. C. current through bridge rectifier and stored in batteries.
The alternator can be designed in two ways either for large outputs or for small outputs within the said kW range.
If the alternator is designed for large outputs, the magnets of large size are used. In such an eventuality, the magnets are pasted at equal distance from each other to the inner side of the outer stationery ring and coils are pasted at equal distance from each other to the outer side of the inner concentric rotating ring. This design requires slip ring & brushes and makes alternator bulky. This also requires maintenance of setting of brushes.
If the alternator is designed for small outputs, the magnets of small size are used. In such a case, the magnets are pasted to inner rotating ring and coils to outer stationery ring. Such an arrangement eliminates the need of slip ring & brushes and also helps the continued rotation by way of ‘flywheel effect’ obtained due to weight of the magnets. The disadvantage of this system is that comparatively high initial torque is required to start the rotation of inner ring due to its higher weight.
However, it is to be understood that such a distinction of arrangement with respect to placement of magnets and coils is purely arbitrary and both the said arrangements can be used with equal effectiveness in all the cases.
The assembly consists of blades on one side, the alternator
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consumption current, the excess is stored in batteries and utilised when the reverse occurs. The number of batteries and their current capacity depends on generating capacity, wind condition and the consumption. Normally, two batteries of 12 volts are used in series to produce 24 volts of system voltage. But, even 8 batteries can be connected to produce 96 volts. More the system voltage more is the current output at the same RPM. However, the initial RPM (torque) to produce system voltage increases proportionately. Where there is sufficient wind speed almost throughout the day and all seasons, higher system voltage is recommended. More efficient, maintenance free, long life batteries, though initially little more costly, can be used for long life.
6) Inverter
An inverter of a suitable required capacity is used to convert the DC power from the batteries to usable 230 volts AC power.
7) Lighting System
Since the electrical power generated by wind energy is very precious and hence has to be utilized very meticulously. The CFL tubes consume very little power and its illumination is equally high compared to conventional lighting system available in the market. These energy efficient/ energy saving bulbs & tubes are initially costly but by way of saving power they turn out more economical. The power appliances like washing machine, air conditioner, refrigerator, electrical motors etc. require 3-4 times initial starting current and brings over load on batteries and hence not easily used. The ‘soft start’ technology can be effectively applied to start and use these appliances.
Normally, in most of the affordable houses, the battery back up system (inverter/ UPS) and wiring is already done where the batteries are charged by grid power by investing money. The cost of windmill, even if taken only as an alternative charging media, is quite reasonable. However, with effective working of windmill system, one may not require grid power at all and can say GOODBYE to State Electricity Board. The costing can be substantially reduced if it is commercially manufactured on large scale and can be brought within a reach of common man if government gives some subsidy on such products. This windmill can be erected at any normally windy place. This can be used at remote areas where grid power has not reached. It is best suited for residential colonies, apartments, schools, hospitals etc. The investment can be recovered within 3-4 years as the running cost is almost zero. This gives pollution free, noise free, maintenance free constant power at constant voltage which safeguards electrical appliances too.
8) Patent
Patent has been granted for 20 years from 18th June 2002 by Govt. of India for Alternator.
in middle and tail vane on the other side. The whole assembly is erected at a reasonable height from ground level on supporting tower. The whole assembly rotates in 360 degrees as per the direction of wind guiding the tail vane. The supporting tower could be placed on top of overhead water tank or top of staircase tower of the building. Depending upon the generator capacity, it will be quite heavy and cumbersome to erect the same at such height.
Alternately, the alternator, which is quite heavy in weight (mainly due to the weight of magnets), is not mounted on top of tower along with the blades and tail vane assembly but is placed at the bottom of the tower. This facilitates easy observation and maintenance of the alternator and reduces the weight on tower and thus, relaxing the requirement of balancing of blades & tail vane system. The rotations from propeller blades to alternator shaft are transmitted at right angles with the assembly of bevel gears through flexible/ solid shaft. However, there are some losses in transmission of rotations from blades through gearbox and long flexible/ solid shaft to alternator. A pulley and belt system can also be used. The losses are negligible and far outweighed considering the advantages of placing the alternator at bottom of tower. This is also advantageous from the safety of the alternator.
The basic design concept of alternator is to generate electricity at very low wind speed, i.e. low R.P.M. of the shaft. This can be achieved by using more number of small magnets than using less number of bigger magnets. Also, the number of turns, diameter, quality and current carrying capacity of wire plays important role in design of alternator.
It is also seen that using more number of magnets, the diameter of alternator becomes very big and requires big lathe machines for turning/ machining operation and costly also. Hence series of small diameter alternators rotating on a common shaft, individually producing small voltage, to make up required system voltage can be used.
4) Supporting Tower
The wind speed normally increases at greater heights from ground level. The supporting tower is designed considering its height, loads coming on it and the wind speeds encountered at the location and is suitably fabricated out of steel material with a foundation strong enough to support the tower. The steps are welded to structural members of the tower for easy climbing up/down and working. Also, a removable pulley arrangement is made to hoist the blades and tail vane assembly in place.
5)Batteries
Since the availability of wind and the wind speed is very much fluctuating, the electricity generated needs to be stored in batteries. When the generating current is more than
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Future Work
Different types of alternators, using different materials to make it light weight, and maintenance free, for inner & outer rings, like aluminium casting, can be used. Instead of blades to rotate shaft, the cylindrical impeller with pelton wheel type louvers can be used. For large diameter alternators, the spokes of inner rotating ring can be made in blade shape which will support the ring as well as work as blades, thus making it compact, light weight and cheap. Even the blades can be made of aluminium casting which is light and rust proof. Small alternators with self rotating inner spoke blades can be erected on street poles to light 100-200 watt bulbs with a small dry battery. It can be used on car top carrier or in front/ sides keeping set of batteries in dickey to use for car A.C. and other gadgets. Energy can be created by rotating a fly wheel with the alternator ring by battery power input just to start the rotation and keeping the flywheel rotating by inertia effect.
Conclusion
The components of the windmill to produce about 2500 Watts of power have been designed and got fabricated locally and erected on the terrace of the house and is working smoothly and efficiently since August 2002. Experiments were carried out for different alternators (magnets on stationary ring / magnets on rotating ring), different size of magnets, flexible and solid shafts, wooden and fiber blades, alternator on top of tower attached to blade & tail vane assembly and detached from assembly (at bottom of tower) etc. Light weight and proper profile blades, perfectly dynamically balanced rotating ring of alternator, smooth working high speed bearings and balancing tail vane will make the wind mill work very efficiently to produce electricity at low wind speeds. The cost will depend on the capacity of alternator, number of batteries and height of supporting tower but is definitely within the reach of needy persons.
Author’s Bio
Born in 1948, Mr. Avinash Shirode graduated as a Civil Engineer in 1970 and completed his post-graduation in Structural Engineering from the Indian Institute of Science, Bangalore in 1972. While he is an aerospace engineer by employment in the Vikram Sarabhai Space Center and has been involved in several industrial World Bank projects as an architectural engineer, Mr. Avinash Shirode’s interests and engineering skills run across different engineering disciplines including software, electrical and mechanical engineering, agricultural engineering and even pharmaceutical engineering. He has been awarded several patents by the Govt. of India and is a winner of several prestigious national and international awards and felicitations. An active member of several technical, educational and social institutions as office bearer, Mr.Shirode has also published several technical papers & reports in national and international journals. He is also a poet, writer, thinker, traveller, visionary, orator, spiritual meditator and donor.
Nasik-based engineer, Avinash Shirode, has developed a low RPM (revolutions per minute) and low wind velocity aero-generator windmill (alternator) for domestic use. Shirode, who is an aerospace engineer by employment at the Vikram Sarabhai Space Centre, has been involved in several industrial World Bank projects as an architectural engineer. A multi-faceted
Rooftop wind Turbines for Homes May Soon be a Reality
personality, Shirode is keen in making the product commercially available, and is ready to tie-up with potential manufacturers who can manufacture the same by acquiring the license on suitable terms.
The invention, for which a patent has been granted for 20 years, could be the answer to frequent power cuts. While generators run by petrol/diesel/kerosene and inverters are used, their performance is not always satisfactory, apart from the issues of noise and air pollution. The wind mill has been developed taking into considerations all these factors.
One of the key highlights of the aero-generator is the fact that it does not need strong wind currents for generating electricity. Typically the wind speed available in most Indian urban areas does not exceed 4—5 m/s. This factor has been a deterrent to people using wind energy in large numbers, with the present use not being economically or technically viable to cater to the needs of domestic household requirements. This had led to the need for developing a windmill that could generate sufficient electrical energy for domestic use of 1 to 5 kW and that could be adapted to a very low wind speed throughout the year.
The new invention of Shirode starts generation of electricity (charging current) at as low as 0.6 meter per second wind speed and as low as 4 RPM for the 12 V system. This innovative product harnesses considerably what other wind mills loose in the wind speed range of 0.6 m/s to 0.33 m/s wind velocity range. The windmill according to its inventor is within the reach of the common man and could be the answer to domestic energy requirements. Easy to erect, the windmill can also be used in remote areas where grid power has not yet reached.
With the running cost claimed to be almost zero, investments could be recovered in quick time. The windmill is ideally suited for residential areas, schools, hospitals, etc.
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Formwork, Insulation, Wall thickness and Fly Ash: Do They Affect Concrete Maturity?
Concrete is the most widely used construction material and its formability is one very important property. Several different types of formwork are available in
the market. One way of classifying them is based on whether they are stripped or not: (1) conventional formwork and (2) stay-in-place (SIP) formwork made using different materials such as steel, PVC (poly vinyl chloride), FRP (fiber-reinforced polymers), EPS (expanded polystyrene), etc. Typically, wood formwork is used to form concrete. However, the amount of wood that can be harvested has been reduced, increasing the cost and reducing the availability of wood. In addition, it is environmentally advantageous to decrease the amount of wood needed in construction.
New concrete forming technologies designed to reduce wood consumption include reusable and SIP formwork. After being used, reusable metal and wood forms must be removed, cleaned, transported and then stored. These systems limit design versatility since they generally come in large, flat panels. Unlike traditional formwork that are stripped after concrete has gained enough maturity, the SIP forms remain an integral part of the structure; some even
provide structural strength and ductility (Kuder, Gupta et al. 2009), some provide higher R value and some just provide a finished surface. Some forming systems such as Insulated Concrete Forms (ICF) increase the insulative properties and R value of the concrete walls and some SIP systems also integrate insulation in the forming system. However, the effect of such highly insulated walls on concrete hydration at early-ages is not fully understood. One such category of SIP forms are the plastic forming systems that are also more versatile than wood and metal because various shapes can be easily manufactured given its flexibility.
Since the SIP forms are not stripped, hence never exposing the surface of concrete, it is very important to ascertain that concrete in the forms has met or exceeded the project specifications. One such type of forming system is a SIP system that utilizes PVC panels and connectors as formwork (Octaform Systems Inc, 2009). This forming system can be used with and without insulation and its effect on the maturity of concrete is not fully understood.
On the material side, fly ash is a commonly used Supplementary
Rishi Gupta1 and Katie Kuder21Faculty & Program Coordinator, Department of Civil Engineering, British Columbia Institute of Technology 2Assistant Professor, Dept. of Civil and Environmental Engineering,Seattle University
Use of different forming material, insulation, and stripping time can significantly affect the maturity and hence the strength gain of concrete within such forming systems. This information can be vital in determining the stripping time of scaffolding and formwork. In this project, maturity and compression tests were performed on specimens (simulating scaled-down walls) formed using a PVC stay-in-place (SIP) forming system with and without insulation. These findings were then compared to data obtained from walls formed by wood formwork, which is the material typically used in the field. The various parameters studied in this project were wall thickness, type of forming material, insulation, and addition of fly ash. Results indicate that with an increase in wall thickness, the peak temperature and the temperature development index (TDI) increase proportionally. TDI is defined as the area under the temperature versus time curve measured from the dormant temperature to the peak temperature. The data show that the proposed TDI is a good indicator of the extent of the hydration reaction, and with further research the relationship between temperature development and strength gain of concrete could be clearly identified. Both wood forming when compared to the SIP system, and insulated systems when compared to un-insulated systems, increase the peak temperature and TDI. Use of fly ash in concrete results in a lower temperature peak and TDI and a delay in reaching peak temperature. However, use of concrete containing fly ash in insulated SIP systems has a higher TDI than a conventional concrete mix formed in wood forms, indicating better concrete maturity at the same age.
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Cementing Material (SCM) which enhances the fresh properties of concrete including increased workability (Mindess, Young et al. 2003; Malhotra 2006; Mehta 2009). High volume fly ash contents are now replacing cement because (1) this results in lower consumption of cement, hence reducing the energy required to produce cement and also reducing the associated green house gas emissions, (2) production of many self consolidating mixes require high contents of fly ash, and (3) this results in cost-savings and is a more sustainable process since an industrial by-product (fly ash) is now being utilized which otherwise would end-up in a landfill. However, addition of fly ash can decrease the rate of the hydration reaction, negatively impacting the construction process as the stripping of forms may be delayed. The effect of using fly ash in concrete on the maturity of concrete was studied in this project. During the hydration reaction, heat is generated and released to the surroundings; the rate of the reaction is proportional to the heat generated. The dissipation of this heat of hydration to the environment will depend on the type of forming material used, thickness of the concrete mass, and use of insulation (Khan, Cook et al. 1998; Wang, Zhi et al. 2006). The effect of using insulation, wood or a PVC SIP system on the maturity of concrete was studied in this project. The maturity of concrete was evaluated by calculating a Temperature Development Index (TDI), which is described later.
The TDI is a close function of the hydration process and hence it is important to note the different stages of the hydration process. The first stage is the rapid heat evolution, which occurs very quickly, the concrete then moves into the dormant stage where the concrete is workable. The dormant stage ends with the initial set and moves into the acceleration stage as the reaction begins to accelerate. The concrete remains workable until the final set where the greatest temperature is achieved. During the deceleration stage, the reaction slows down and temperature is reduced, bringing the concrete to a steady state. A practical and effective way to evaluate this hydration process is to monitor the temperature released by the hydration reaction over time. The temperature data also serves as an indicator of the rate of reaction, as temperature increase is proportional to the heat generated.
Materials and Forming Systems
Concrete was prepared in a rotary drum mixer using Type I cement (manufacturer- Lafarge), river sand, coarse aggregate with maximum size 10 mm, Class F fly ash (Plant- Centralia), and admixtures including superplasticizer (product- Glenium 3000 NS) and air entrainer (product- MB VR Standard). For constructing the wood forms, lumber meeting the following specifications was used: 23/32 inch DF-DF plywood, 48/24 span rated. The forms were oiled using a release agent (WD-40) before pouring concrete.
A concrete mix design typical of what is used in field construction with the PVC SIP system was used. The control concrete mix had a water-cement ratio of 0.49 with 350 kg/m3 of cement, 1160 kg/m3 coarse aggregate, 700 kg/m3 of sand, dosage of 600 ml/m3 of superplasticizer and 200 ml/m3 air entrainer. Another mix was prepared by replacing 40% of the cement with fly ash by weight.
PVC SIP Forming System
The SIP forming system used in this study is briefly described below. This forming system is composed of PVC panels, connectors, and braces that form cells. The panels are typically 150 mm wide and come in variable heights. The connectors, which are available in various widths, are placed perpendicular to the panels and have openings that are provided for placement of rebar and to allow the concrete to flow through the wall. Figure 1 (a) shows one such cell of the forming system braced with standard connectors, T-connectors, and the 45° braces to the panels. Insulation is also available (Figure 1 (b)) for an increased thermal mass, leading to higher energy efficiency.
The panels that make up the interior and exterior of the formwork and can be curved to conform to the shape needed for the specific application, such as an aquaculture tank shown in Figure 2 (a). Once the vertical formwork has been assembled and raised, wood bracing is used, as shown in Figure 2 (b). This bracing is similar to the bracing required when the wood formwork is used and is removed once concrete within the forms has gained sufficient strength.
Figure 1. Components of SIP formwork cell (a) Top vies of cell containing all components, (b) Schematic of cell with insulation (Octaform 2009)
Specimen Preparation
To compare the influence of PVC SIP formwork on the heat of hydration (maturity/temperature release), the results from PVC SIP system were compared to wood formwork. The SIP test wall configuration, shown in Figures 3 and 4, was designed to have three rectangular cells. This configuration was chosen so that the two extra cells on either side of the middle cell would eliminate the temperature effects of closeness to the end of the wall (boundary effects). The sides of the end cells were filled with wood pieces to prevent concrete from flowing
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out of the wall. The final interior dimensions of the wall were: 460 mm (18”) length, 300 mm height, and variable width (ranging from 100 to 300 mm). For the wood formed walls, plywood was assembled to match the interior dimensions of SIP system’s dimensions.
standards (C192) and poured into small wall-shaped formwork 300 mm in height supported by a bracing system (previously described). A tamping rod was used to ensure the concrete was well compacted within the formwork.
Thermocouples (Type K) were embedded into the central cell at five locations, which are shown in Figure 5. One thermocouple was placed in the center of the test specimen to evaluate temperatures in the middle. Four thermocouples were placed around the central thermocouple to provide a more accurate depiction of temperatures throughout the test specimen, particularly locations closer to the formwork. It was hypothesized that, if thermocouple location was critical, the thermocouples located closer to the faces of the wall would be more affected by the ambient temperature than those in the middle, and the centrally-located thermocouple would reach the highest peak temperature as it was surrounded by the largest thermal mass during curing. In addition, a sixth thermocouple measured the ambient temperature in the lab. The thermocouples were attached to a data acquisition system and the temperature was recorded for the duration of the test.
Figure 1. Components of SIP formwork cell (a) Top vies of cell containing all components, (b) Schematic of cell with insulation (Octaform 2009)
(a) (b)
To evaluate the effect of the PVC formwork on the hydration of concrete, concrete was cast inside the PVC formwork and was compared with concrete cast inside traditional wood formwork. It was initially hypothesized that the PVC SIP formwork would contain heat and moisture during the hydration process, therefore increasing the rate of the reaction and the ultimate strength. Three variables were introduced to simulate the varying field conditions to which concrete is typically exposed: concrete composition, wall thickness and insulation. The matrix of variables tested is shown in Table 1 below.
Figure 3. Schematic (plan view) showing dimensions of the formed specimens
Test set-up
During casting, the concrete was mixed according to ASTM
Figure 4. Bracing of specimen constructed using PVC SIP
Concrete type Formwork type Specimen width (mm)
100 200 300
Normal (NC)SIP
Wood
Fly Ash (FA)SIP
Wood
Normal (NC)SIP with 50 mm insulation - -
Wood with 50 mm insulation - -
Fly Ash (FA)SIP with 50 mm insulation - -
Wood with 50 mm insulation - -
Table 1: Test matrix for the variables investigated
Figure 5. Thermocouple locations in the middle SIP formwork cell
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Temperature measurements
Before the concrete was mixed and poured into the formwork, the data acquisition system was started to obtain the initial ambient temperature. The thermocouple wires were then placed in the concrete as described earlier. Temperature readings were collected at a rate of 3 readings per minute, each reading being an average of 100 scans. Once the test had run for an amount of time determined by previously conducted preliminary tests indicating complete hydration, the data acquisition system was stopped and the data was saved for analysis.
Compression testing
Cast cylinders
To determine the compressive strength, cylinders (100 x 200 mm) were cast according to ASTM C 31. Cylinders were de-molded after 24 hours and moist cured for 56 days. Testing was done using a Riehle hydraulic testing machine with a 300 kip load cell. Specimens were loaded by displacement-control at a rate of 0.085 mm/min. The data acquisition system was set up to measure the applied load at a rate of 25 readings per second, each reading being an average of 1000 scans. Four cylinders were tested for each mix type (NC and FA). Neoprene caps were used in lieu of capping or grinding of cylinders.
Cores
To study the effect of the PVC SIP system and insulation on the concrete compressive strength, drilled cores were extracted after monitoring the temperature for 36 hours. This was also done to determine if there was any correlation between temperature and strength development. Cores were taken from various 200 mm walls and subjected to compressive testing equipment as described above. The various configurations from which were extracted are shown in Table 2.
Results and discussion
Data averaging
The temperature data were recorded from all six thermocouples over time. After analyzing the data, it was noted that the temperature readings from the five embedded thermocouples did not vary significantly with the location of the thermocouples; therefore the average curves were deemed suitable for analysis. This finding is illustrated in Figure 7. Note that the bottom and the top thermocouples reach the second lowest and the highest temperatures, respectively, even though these were located approximately the same distance from the center of the wall.
Formwork type Concrete type Insulation
SIPNormal Present
Fly Ash Absent
WoodNormal Present
Fly Ash Absent
Table 2: Various specimen types used for extracting core samples
Each wall was cored with a concrete coring machine as shown in Figure 6, with a 100 mm diameter drill. Three cores were taken from each wall: one from the middle cell, and one from each of the two side cells. Each 100 mm diameter core was then cut down to a height of 200 mm and tested for compressive strength.
Figure 6. Coring 100 mm diameter sections for compression testing from SIP formed specimens
Figure 7. Typical plot: temperature versus time as a function of thermocouple location for a 200 mm thick wall formed with wood
Temperature Development Index (TDI)
From the averaged data, the peak temperature (Tp) was determined along with the time at which the peak (tp) occurred. Figure 8 presents a typical plot of average temperature versus time, indicating the Tp and tp. Throughout the testing program,
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the ambient temperature in the lab fluctuated, resulting in varying initial temperatures from specimen to specimen. This situation would also be typical of a construction site where the ambient conditions will be different from day to day. Figure 8 presents the average temperature versus time for two tests performed during primary testing. The ambient temperature recorded during each test is not presented in the plot for clarity. The variation in ambient temperature was less than ±1°C in each test and hence the effect was considered negligible. Although it was expected that the 100 mm FA specimen would achieve a lower peak temperature than the 200 mm FA specimen because it contained a smaller volume of concrete, the results show just the opposite. As research has shown, ambient temperatures affect the rate of the hydration process (Wang, Zhi et al. 2006); warmer temperatures speed up the hydration process and contribute to higher peak temperatures, while colder temperatures slow down the hydration process and contribute to lower peak temperatures. Because of the effect of ambient temperatures on the hydration process, and that ambient temperatures were not controlled during testing, there was no linear relationship between hydration and ambient temperature. Therefore this experimental project cannot directly account for this effect.
the 100 mm specimen with fly ash (Table 3). Refer to Figure 8 for identification of critical points for the temperature analysis and an illustration of the areas calculated. After analysis it was seen that the 100 mm FA specimen (shown in Figure 8) achieved a smaller A1 calculation in comparison to the larger, 200 mm FA specimen. The area calculation was conducted for each test and the areas were compared. These values were used as an indication of temperature development during hydration. The A1 value representing temperature rise and maturity immediately after the dormant stage was found to be more relevant than that of A2 and is used extensively throughout this report. The authors have called this value the “Temperature Development Index” (TDI or simply A1). The results from these comparisons are discussed later.
Figure 8. Average temperature versus time curve for 100 and 200 mm thick walls
To minimize the effect of the ambient temperature during the analysis, it was proposed that the area under the hydration curve be calculated and analyzed. This area was split into two smaller areas: A1 being the region bound by the initial minimum temperature (indicating the dormant period) and the peak temperature and A2 being the region bound by the peak temperature and the final temperature at 30 hours. Preliminary tests (not reported here for maintaining brevity) had indicated that the internal temperature in specimens more or less dropped to ambient temperature after 30 hours (Lowrie, Sommer et al. 2007). In certain specimens the peak temperature was very similar to the ambient temperature and sometimes lower than that recorded at 30 hours. In such cases, the value of A2 would be negative as in the case of
Figure 9. Average temperature versus time for 100, 200 and 300 mm thick walls formed with SIP formwork with normal concrete (NC) and 40% fly ash replacement (FA)
Wall TypePeakTemp
(°C-hr)
Time atPeak (hrs)
A1 (TDI) (°C-hr)
A2 (°C-hr)
NormalConcrete
100 mm 200 mm 300 mm
24.08 27.79 29.69
11.9 13.3 15.0
13.91 25.93 40.75
22.36 30.38 60.80
Fly Ash100 mm 200 mm 300 mm
22.07 25.81 27.39
13.0 15.5 16.4
5.72 24.04 30.08
-0.19 28.95 36.79
Table 3: Temperature at peak and A1 (Temperature Development Index, TDI), A2 for the SIP system
Wall TypePeakTemp
(°C-hr)
Time atPeak (hrs)
A1 (TDI) (°C-hr)
A2 (°C-hr)
NormalConcrete
100 mm 200 mm 300 mm
25.31 29.12 29.85
15.013.515.7
24.12 38.71 53.71
34.07 60.93 76.99
Fly Ash100 mm 200 mm 300 mm
24.09 24.02 26.11
12.1 16.7 18.1
8.44 22.65 32.47
3.99 29.67 37.85
Table 4: Temperature at peak and A1(Temperature Development Index, TDI), A2 for the Wood formwork
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Forming Systems
A plot of temperature versus time for the SIP system for varying composition and wall thickness is summarized in Figure 9 and Table 3. Overall temperature test results for concrete formed using the wood forms are summarized in Figure 10 and Table 4.
Effect of fly ash
For the walls formed using the SIP system, with the addition of fly ash, the peak temperature and TDI decreased for all wall thicknesses, however, the time to peak when comparing the same wall thickness increased indicating that the extent of the reaction is reduced with the addition of fly ash. Similar trends were observed for the specimens formed using wood.
Effect of wall thickness
Figure 11 presents the average TDI for the two mixes tested when wall thickness is varied for the SIP system. In comparing the wall thickness among the SIP specimens (Figure 9), the data show an increase in temperature, time to peak temperature, and TDI as the thickness is increased for both mixes tested. The increase in wall thickness from 100 to 300 mm increased the TDI by more than 192% for NC SIP specimens and 210% for FA SIP specimens. The data shows that larger walls reach a higher peak temperature at a later time and achieve a greater TDI than smaller walls. In general, an increase in wall thickness is correlated with an increase in peak temperature, time to peak temperature, and total temperature development in the hydration reaction. Similar results were observed for wood formed specimens and the results are presented in Figure 11. For wood formed specimens the increase in wall thickness from 100 mm to 300 mm increased the TDI by 120% for NC specimens and 280% for FA specimens. This result implies that the TDI and hence the strength gain in thin walls is significantly lower
when compared to thicker walls. Hence, there is a need to be closely monitor and consider the strength gain of such wall before stripping the forms especially when high volumes of fly ash are used.
Figure 10. Average thermocouple temperature versus time for 100, 200 and 300 mm thick walls formed with wood with normal concrete (NC) and 40% fly ash replacement (FA)
Figure 11. TDI (A1) as a function of wall thickness for walls formed with PVC SIP and wood formwork for NC and FA
Effect of insulation
Figure 12 presents temperature versus time for walls formed with SIP formwork with and without insulation and with and without fly ash. Incorporating a 50 mm thick insulation with the SIP system resulted in greater TDIs and higher peak temperatures (Figure 12). Use of insulation increased the peak temperature and TDI by 10% and 52% respectively for the normal concrete mix, and increased 13% and 83%, respectively with the use of fly ash. Figure 13 presents average temperature versus time for wood formed specimens with and without insulation. Similar to the results of the SIP specimens, the data indicate that when insulation is used, the peak temperature is greater in comparison to walls without insulation. The NC specimen with insulation also achieved a greater temperature development in comparison to the NC specimen without insulation; however, this trend was not visible in the FA specimens. When insulation was used with the normal concrete mix, peak temperature and TDI increased by 15% and 19%, respectively. When insulation was used with the fly ash mix, the peak temperature increased by 9%, while the TDI decreased by 12%. The reasons for this decrease are not clear and warrant further investigation.
Wood formed vs. SIP system: Comparison
Figure 11 presented earlier is a plot of the average temperature development for the SIP system and wood formed walls of various wall thicknesses. For both the wood and SIP system formed walls, there was an increase in temperature development with an increase in wall thickness due to the increased thermal mass of the additional concrete from the larger walls. The results show that the wood formed specimens achieve greater temperature development in comparison to the SIP system, particularly when the normal concrete mix is
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used. In general, the
addition of fly ash appears to slow the hydration process, lowering the total amount of temperature developed, and cause the specimen to reach a lower peak temperature at a later time for both the wood and SIP walls. In the case of the 200 mm SIP fly ash specimen, a slightly higher temperature development was achieved in comparison to the wood formed specimen. When varying the composition, there is a greater difference in the TDI for the wood walls, an average of a 49% difference, than for the SIP walls, with an average of a 31% difference. This may indicate that the SIP system may contain more moisture and develop more cumulative temperature relative to wood formwork during the hydration process when fly ash is used.
In general, the inclusion of insulation for both wood and SIP systems increases the peak temperature during hydration, and contributes to greater temperature development. In general the peak temperatures for specimens containing insulation occur later in comparison to specimens without insulation. For the specimens tested, the inclusion of insulation with the SIP system appears to have a more significant effect on temperature development and peak temperatures achieved in comparison to the wood system. This finding may be a result of the wood system itself providing insulation and the additional insulation having little effect. It is interesting to note that the SIP system used with the fly ash mix and insulation achieved a greater temperature development than the wood forming system used with the normal concrete mix and no insulation. These results are an indication that the SIP system used in combination with fly ash and insulation more positively contributes to the hydration process in comparison to standard wood forms used with the normal concrete mix.
The compression test results for cored specimens along with the analyzed temperature data is presented in Table 5. In comparing the wood and SIP formed walls, the wood formed walls generally achieved a higher temperature development than the SIP formed walls for all size walls. To understand this correlation, the R-Value of each formwork material was
determined. The R-Value for the PVC SIP is reported as 0.60 (Octaform Systems Inc., 2009) while that for 20 mm (¾”) plywood is reported as 0.90 (TECO, 2010). The lower R-Value of the SIP indicates that it is less resistant to thermal change than wood, and hence may explain why it achieved lower temperature development overall in non-insulated systems.
Compression Testing
Cast cylinders: When 40% cement was replaced with fly ash,
Sample CompressiveStrength (f’c) (MPa)
Standard Deviation (MPa) TDI / A1 (°Chr) Avg. Peak Temp
(Tp) (oC)Avg.Time at
Peak (tp) (hrs)
Normal ConcreteSIP 17.86 4.83 42.24 29.45 14.34
Wood 20.13 7.38 43.50 31.00 11.84
Fly AshSIP 12.07 2.69 24.31 25.42 17.87
Wood 12.41 1.72 34.53 26.57 17.99
NC InsulatedSIP 15.05 0.92 64.01 32.54 16.00
Wood 12.51 7.52 44.52 28.80 17.33
FA InsulatedSIP 20.40 0.35 51.68 35.50 12.80
Wood 5.85 0.24 30.25 28.89 14.87
Table 5: Compressive strength data from cored samples (36 hours after casting) and the corresponding temperature data
Figure 12. Average thermocouple temperature versus time for walls formed with SIP formwork with NC and FA with and without insulation (Insul)
Figure 13. Average thermocouple temperature versus time for walls formed with wood formwork with NC and FA with and without insulation (Insul)
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the compressive strength decreased from an average of 32 + 3 MPa to 29 + 6 MPa at 56 days.
Cores: Cores were taken from four 200 mm walls and four 250 mm walls with 50 mm insulation (and 200 mm concrete): two each from SIP NC, SIP FA, Wood NC, and Wood FA. These cores were tested and averaged for each wall. The compression testing data is summarized in Table 5. It should be noted that the compressive strengths reported in Table 5 are at an age of 36 hours and hence significantly lower than that measured for cast specimens tested after 56 days. One of the other methods of comparison for these walls was the TDI (A1 in Table 5), which has been explained already.
Treating the compressive strength for FA insulated wood specimen as an anomaly, a reasonable correlation between f’c measured for cored samples and TDI was observed. This correlation existed only when the same concrete type and formwork configurations were considered. The general trend shows that when the TDI increases there is an increase in compressive strength. However, when comparing the insulated walls to the non-insulated walls, similar compressive strengths were measured for dissimilar TDIs. This may be attributed to limited number of cores and the high standard deviation observed in the compressive test results; as high as 60% for the NC insulated wood specimen). Establishing a straightforward correlation between f’c and TDI was difficult also because TDI corresponded to thermal activity up to peak (time to peak ranged between 11 and 18 hrs), whereas all coring occurred at 36 hours, hence making a direct comparison more difficult. Further research is necessary to clearly establish this correlation.
Conclusions
1. The proposed TDI was an effective method of analyzing the temperature data. TDI could be effectively used to minimize the effect of different ambient conditions and to capture the hydration that occurs immediately after the dormant hydration stage.
2. The wood forming system contributes to higher peak temperatures, which occur later when compared to the PVC SIP forming system. The extent of hydration process does appear to be greater in the wood system for a normal concrete mix. This corroborates well with the R value for both forming systems.
3. The SIP system used in combination with a high volume fly ash mix and insulation achieved greater temperature development in comparison to the non-insulated wood forming system used with normal concrete. These results indicate that the insulated SIP system used with fly ash more positively contributes to the hydration process in comparison to the noninsulated wood formed system used with the normal concrete mix.
4. Special attention is required to ensure strength gain before stripping forms especially for thin wall cast using concrete containing high volume fly ash.
Further Research
In addition to wood and one type of SIP formwork, further research should be done to compare the effect of other forming systems used in the industry on maturity of concrete. In particular, larger wall sizes should be tested to better simulate the conditions experienced in the field. Stripping time variability should be incorporated into this testing. Validity of TDI should be examined by conducting tests at extreme ambient conditions to simulate colder winter climates and warmer summer climates. Further research is suggested to clearly establish the relationship between concrete compressive strength and TDI by having a larger sample size of cored specimens.
Acknowledgements
The authors would like to thank Octaform Systems Inc. for sponsoring this project and for providing the materials and technical expertise for this project. The authors would also like to acknowledge the contributions of the Seattle University senior design team that was comprised of Kristian Lowrie, David Sommer, and Nikki Wheeler.
References
- Khan, A. A., W. D. Cook, et al. (1998). “Thermal Properties and Transient Thermal Analysis of Structural Members During Hydration.” ACI Materials Journal 95(3), 293-303.
- Kuder, K. G., R. Gupta, et al. (2009). “Effect of PVC Stay-in-Place Formwork on Mechanical Performance of Concrete.” Journal of Materials in Civil Engineering 21(7), 309-315.
- Lowrie, K., D. Sommer, et al. (2007). Effect of PVC Stay-In-Place Formwork on the Hydration of Concrete, Seattle University: 40.
- Malhotra, M. (2006). “Reducing CO2 Emissions: The Role of Fly Ash and Other Supplementary Cementitious Materials.” Concrete International, 42-45.
- Mehta, P. K. (2009). “Global Concrete Industry Sustainability: Tools for Moving Forward to Cut Carbon Emissions.” Concrete International, 45-48.
- Mindess, S., F. J. Young, et al. (2003). Concrete. Upper Saddle River, Prentice Hall.
- Octaform Systems Inc., Technical Guide (accessed October, 2009), <http://www.octaform.com/index.php? page=technical-guides>
- TECO, Panel R Values (accessed June 2010), <http://www.tecotested.com/prod-info>
- Wang, K., G. Zhi, et al. (2006). Developing a Simple and Rapid Test for Monitoring the Heat Evolution of Concrete Mixtures for Both
Laboratory and Field Applications. N. C. P. T. Center.
Publishers Note: This paper was presented at the Proceedings of the One Day Seminar on Modern Formwork Systems for Building Construction Held in IIT Madras, Chennai. The Masterbuilder was the official Media Partner for the above event.
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Certain Safety Considerations for Formwork
Formwork, the temporary mould and support for fresh concrete until the concrete is strong enough to support its own weight and other construction loads, itself requires
a support called ‘falsework’. In many codes, formwork and falsework together are called ‘formwork structure’ or just ‘formwork’ - which last will be the terminology used in this paper.
The problem with formwork is that it is ‘temporary’. In many under-developed and even some developing countries, the word ‘temporary’ is automatically associated with lack of need for planning, design and care, and with neglect of appearance, strength, and safety. As the owner pays only for the finished permanent structure and not the temporary structure, least cost (including cheapest labour and materials, and in the worst case scenario, low compensation for accident and fatality claims) are often the easiest way to cut costs on this ‘non-essential’ item.
In advanced countries however, it is recognised that most accidents and in fact most fatalities and property damage occur during the brief construction stage and not during the long usage phase of a structure. The business case for safety in these countries also has amply demonstrated the wisdom of preventing or mitigating the effects of accidents as against paying for large compensation and work disruption costs due to accidents. This is exactly why hazards present in formwork must be identified, and the risks arising from them must be assessed and controlled.
In this paper, not being sufficiently familiar with Indian practices in regard to formwork safety - except as a lay observer during his visits to India - author will focus on his experience with Singapore practices, in the hope that Indian professionals may make their own comparisons and draw their own lessons for local application.
Basic Safety Requirements
The basic safety requirement is set in the Singapore Workplace Safety and Health Act of 2006 as the responsibility of every
employer, as far as is reasonably practicable, to protect every employee from injury and ill-health at the workplace.
This aim of providing a ‘safe place to work’ is achieved by adopting guidelines provided by the Ministry of Manpower and Workplace Safety and Health Council, including the following:
- Risk assessment and control, before work starts [Ref.1].- Safe Work Procedure for every activity at the workplace
which may involve risk.- Permit to Work for all hazardous activities such as work at
height.- Construction Reg. 2007, Sec. 22(2) reads: “In a worksite,
every open side or opening into or through which a person is liable to fall more than 2m, shall be covered or guarded by effective guard-rails, barriers or other equally effective means to prevent fall.”
- Construction Reg. 2007, Sec. 63(2) reads: “Any formwork structure that (a) exceeds 9m in height; (b) consists of any formwork which is supported by shores constructed in 2 or more tiers; or (c) consists of any formwork where the thickness of the slab or beam to be cast in the formwork exceeds 300mm, shall be designed by a P.E.”
Figure 1 depicts formwork for a condominium block in Singapore.
Hazards in Formwork
‘Hazards’ are potential dangers. Hazardous activities in formwork design, erection, use and dismantling are as follows:
- Incorrect or incomplete formwork design- Erecting frames and bracing- Erecting bearers and joists- Placing deck and beam formwork- Moving around on formwork during rebar placement,
concreting, and curing- Dismantling formwork
N. KrishnamurthySafety and Structures Consultant, Singapore
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In erection, use and dismantling phases, most activities involve following common hazards:
- Climbing up to or down from formwork, usually by ladders- Working at height with unprotected edges on platforms- Tripping and falling at level- Falling through gaps and holes in formwork- Falling from incomplete or badly designed formwork- Hit by formwork components- Carrying heavy loads- Struggling with awkward shapes- Fitting damaged connections and components- Handling sharp objects and corrosive materials- Working in harsh (sunny, cold, wet, windy, dusty, noisy
etc.) environments- Uneven, sloping and cramped work surfaces- Overloading of formwork
In addition to these, dangers may also arise from inadequate supervision, material flaws etc. To cover all these in a paper would be an onerous task. The author will therefore focus only on the following factors in this paper:
1. Some design considerations,
2. Working safely at height, and,3. Manual handling of heavy loads.
Some Design Considerations
Factor of safety
India has its own design norms, and they are likely to be world class. Problems may arise during implementation, and in the safety culture that may be prevalent in various enterprises.
Author has seen some excellent formwork in big projects in cities. (Fig. 2.)
But more commonly, especially with formwork for residential and office building floors, a common sight that greets one is a forest of supposedly vertical and straight but actually twisted, bent, de-barked tree branches leaning at all angles some as much as 20 degrees to the vertical, supporting the beam and slab formwork. (Fig. 3.)
Other Asian countries also use natural timber for falsework. In the Far East, bamboo is common, with the advantage that bamboo is straight and nearly uniform in size along its length, In India we use all kinds of timber which are twisted, bent, and non-uniform along their length.
Having just finished an assignment on the formwork code committee in Singapore, author is very conversant with the need for strict and conservative design for formwork and other temporary structures, as already mentioned in the Introduction.
Fig. 1. Author with site engineers in front of extensive formwork for a condominium construction in Singapore.
Fig. 2. A recent picture taken by author in India.
In the past, load factors of 1.5 were commonly used for falsework design. Often wear and tear in use, and poor field conditions of connections and erection encroached into this factor, and in certain cases resulted in accidents involving injuries including fatalities and property damage.
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Current Singapore Formwork Code [Ref. 2] stipulates a minimum “Load safety factor” of 2.0 to be applied to all designs by whatever method, and for all testing, so that the designed or tested capacity is at least twice the maximum requirement under the worst combination of loadings.
What is the corresponding design requirement for Indian construction with such timbers?
Inclined shores
There is another aspect of such ad-hoc arrangement of shores that raises the question: If formwork has to be approved to satisfy design criteria, how are sloping shores handled?
An inclined member AB at an angle to the vertical subjected to a vertical compressive force V will develop a horizontal component H, which would be 18% and 36% of V for angles of 10° and 20°. This horizontal component will tend to increase the angle . (Fig. 4.)
Then, how come we have not had all inclined members slide and fall down? That is because the horizontal components have been successfully resisted, as at top they may be nailed
to some boards, and at bottom the friction and random projections will usually prevent sliding.
If in a particular case everything is fine when erected, but when wet the friction coefficient vanishes, and/or when the load increases the slide resistance is inadequate, disaster may strike.
Smart people may think that they can cancel out the slope effects by arranging adjacent members AB and A’B’ sloping in opposite directions. But there will still be the same horizontal separating force H at the top and bottom, and if the resistance to opening up at top and bottom is not enough, woe be unto the formwork! (Fig. 5.) Of course, someone who knows what is happening can easily take care of this problem by two simple ties at or near AA’ and BB’ - but this is not much in evidence.
Sloping shores may be the fast and cost-effective way to use available poles without cutting them down to required size. It may also be true that they have worked well for decades, and the permanent structures that emerge from these temporary structures of whatever shape, have been finished beautifully.
The point author is making here is that any structural resistance to failure is not by design, but by chance. Contractors have just been lucky, and professionals have not even considered, let alone provided for the horizontal component. That they survive is because of modifications by trial and error. Potential for failure continues to exist.
Author shows special concern about this sloping shore, because in a court case in which he was involved, he demonstrated that it was exactly such an undesigned inclined strut - although it happened to be a straight steel rod - that might have contributed to the formwork failure.
In this day and age, when India is contributing globally to the cyber era and space effort, engineers should be a little more scientific,
Fig. 3. A common sight?
Fig. 4. Forces on an inclined prop. Fig.5. Props sloping opposite ways.
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contractors a little more professional, and the owners who pay for all this a little more considerate of essential expenses in what they do, at least in the interests of ultimate structural safety, if not for the sake of appearance.
Working Safely at Height
Working at height has been the most hazardous activity all over the world from time immemorial, and continues to attract the maximum number of accidents and the maximum number of fatalities. There are many ways in which safety may be ensured while working at height [ Ref. 3], as follows:
A. Guardrail and toeboard (Fig.6A)
B. Work restraint, attachment to lifeline (Fig. 6B)
C. Retractable lifeline (Fig. 6C)
D. Auxiliary scaffolding (Fig. 6D)
E. Safety net below (Fig. 6E)
F. Safety harness (Fig. 6F)
In providing risk control against falling from height, collective control for all workers (A, D, or E) is better than individual control (B, C, or F); fall prevention (A, B, C, or D) is better than ‘fall arrest’ (meaning termination of a fall before hitting the base) to reduce the effects of fall impact after one has fallen (E or F).
In terms of hierarchy of safety then, A or D is the best, and F is the worst. The full-body harness (E) also comes with a number of other auxiliary requirements for effective deployment, including proper fit, sufficient fall distance, strong anchorage, and prompt rescue. [Ref. 4]
All these requirements are mandatory according to the Singapore Code of Practice for Working Safely at Height. [Ref. 5]
Manual Handling of Heavy Loads
In formwork - in common with most construction and factory activities - regularly carrying loads larger than about 25kg is an insidious risk, not sudden and dramatic like falling from height, but slowly causing musculoskeletal disorder (MSD) and escalating to permanent damage of the spine over a period of about an year.
Musculoskeletal disorders (MSD) are among the most common worker complaints in the West. In Asia and other under-developed countries however, it is not reported as much or taken as seriously, possibly because natives of these countries are more pain tolerant than citizens of the more developed countries, or because management will not do anything about it, or both. It may also be that both management and workforce do not realise that what starts as a little persistent discomfort can escalate into a permanent painful problem. In any case,
Fig. 6. Safeguards for working at height.
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most do not recognise it as a problem, and even workers who experience it resign themselves to it as their lot in life, enduring lifelong discomfort if not suffering as a consequence.
So workers regularly carry heavy loads over long distances or keep doing repetitive physical activity; supervisors and bosses let them, expect it from them, and even order them to do so. The reason is simple: Labourers (by very name) have always been doing it. If they don’t, who will? They are paid for it, aren’t they? We are not forcing them against their will!
This topic comes under ‘Ergonomics’ the science of work posture. Author’s recent paper [Ref.6] covers many aspects of construction ergonomics.
Why is this important? What do we do about it? The answers are not simple. It becomes a matter of safety culture in a society, the concern of the more powerful groups of people for the weaker and less fortunate sections of society. Author hopes that once he explains his stand, professionals will rethink about how we are using or abusing our fellow human beings.
Many do not know that each kilogram of weight we bend and pick up and carry in front of our body develops a force of about 12kg on our low back muscle and bone. (Fig. 7.)
So a 50kg cement bag will put a load of 600kg on the back of a worker. An average Asian’s back is designed by nature to carry a maximum force of about half that (after allowing for the force imposed by our own torso weight), which means that nobody should be carrying more than 25kg on a regular basis.
Australia, where the average person would be larger in size and stronger than Asians, legislated a few years ago that no worker should carry more than 20kg routinely. UK had done likewise a few years earlier when their workers complained about 40kg hollow concrete blocks.
Singapore recommends a limit of 25kg for worker loads.
Author is not sure about any limitations mandated in India, but purely on humanitarian grounds he appeals to employers not to burden their workers with more than 25kg in their normal work.
If any activity requires lifting and movement of larger loads, mechanical aids like trolleys may be provided for moving the heavier weights around; two or more workers may be deployed to lift them on to trolleys, or carry them for short distances. Even the simple expedient of rotating the task between different workers would reduce exposure to risk to more tolerable levels. Proper procedure to lift heavy loads by squatting and getting up with the load is also easily learnt.
Needless to say, this analysis and recommendations for this particular hazard, apply to white collar non-construction workers too, such as office and lab assistants.
Conclusion
Author has highlighted a few of the hazards in formwork design, erection, use and dismantling with which he has personal experience in Singapore. Not all the hazards may be perceived as equally critical in India. But in a nation committed to democracy and concern for all citizens, the risks described and the solutions proffered by the author may serve to trigger improvement of overall safety culture.
References
1. Krishnamurthy, N., Introduction to Risk Management, (Self-Published), Singapore, May 2007, 86p, ISBN: 978 -981-05-7924-1.
2. SS580:2012 (ICS 91.080.99), Code of Practice for Formwork (Formerly CP23), SPRING, Singapore, Nov. 2012, 40p.
3. Figures 5A to 5E sourced from “Falls from height during the floor slab formwork of buildings: Current situation in Spain”, by Jose M. Adam, Francisco J. Pallarés, and Pedro A. Calderón, Copyright 2009 National Safety Council and Elsevier Ltd.
4. Krishnamurthy, N., “Full Body Harness - Blessing or Bane?”, The Singapore Engineer, Magazine of the Institution of Engineers, ‘Health and Safety Engineering’ issue, August 2012, p. 18-22.
5. WSH Council, Code of Practice for Working Safely at Height, Workplace Safety and health Council, Singapore, October 2009, 50p.
6. Krishnamurthy, N., “Ergonomics at the Construction Sites”, The Singapore Engineer, Magazine of the Institution of Engineers, ‘Health and Safety Engineering’ issue, February 2013, p. 20-27.Fig. 7. Forces on vertebrae
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An Insight into Formwork Pressures Using Self-Consolidating Concrete
Utilization of Self-Consolidating Concrete (SCC) in cast-in-place concrete has been relatively slow to progress in developing countries partly due to the lack of
understanding of pressures exerted on formwork systems. It is generally understood that the pressure is greater than ordinary (nonflowing) concrete, but quantification and prediction of the pressure has been difficult due to the many types of SCC available and the various construction methods used during placement. Without adequate prediction methods, the only options left to the engineer, according to ACI 347, are to either design for full hydrostatic pressure or to monitor
pressure during placement and adjust the rate of placement accordingly. The assumption of full hydrostatic pressure would generally create formwork that is too costly to justify the use of SCC. Since formwork pressure is a life-safety related issue, with significant construction collapse as a consequence, the only viable option remaining is to monitor the form pressure during placement until prediction models can be developed and validated. The monitoring performed in this study is intended to be used for validation of prediction models and to promote understanding of the factors that influence lateral formwork pressure during construction with SCC.
Sonjoy Deb, B.Tech, ‘Civil’Associate Editor
“Self consolidating concrete (SCC), also known as self compacting concrete, is a highly flowable, non-segregating concrete that can spread into place, fill the formwork and encapsulate the reinforcement without any mechanical consolidation, but lack of knowledge of formwork pressure is a concern and reason for not using SCC in large scale in developing countries”.
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Factors Affecting Formwork Pressure
(A) Materials
Formwork pressure exists as long as the concrete is in a plastic state, and its rate of decay is related to the rate of the stiffening of the concrete mixture. It follows that the lower the yield stress and plastic viscosity of the concrete are (i.e. high flowability), the greater the initial lateral pressure. Conversely, a faster rate of stiffening brings about a faster rate of decay of the lateral pressure. As is evident, SCC can be formulated in various ways and consequently does not have any specific composition. Researchers have found that any of following parameters aggregate content and size, w/cm, cement type and content, silica fume, fly ash, slag, ground limestone filler, superplasticizer type and content, and VMA type and content can affect the lateral pressure characteristics. To be more specific, they have found that increasing aggregate content and size gives rise to a lower initial lateral pressure. In contrast, rich concrete mixtures develop greater pressure than normal and lean mixtures. This is attributed to internal friction of coarse aggregate carrying some of the hydrostatic load. However, if the content of the fines (the paste component of the concrete) is increased, the ability of the coarse aggregate to carry loads decreases, and the lateral pressure is increased. Increasing the w/cm or/and superplasticizer content increases the lateral pressure and vice versa. However, different superplasticizers have been found to affect differently both the initial lateral pressure and its rate of drop after casting. Researchers also observed that addition of supplementary cementitious materials (SCM), such as fly ash, silica fume, or granulated blast-furnace slag, affect the lateral pressure and more specifically its rate of decay. This is attributed to the thixotropic nature of the concrete that changes with the inclusion of these materials. However, the literature mainly provides qualitative information on the effects of the various ingredients listed above and occasionally, disagreements on the effects of some ingredients are encountered.
(B) Placement Conditions
The placement rate is a critical parameter for formwork pressure of SCC. The higher the rate of placing, the higher the lateral pressure is, and to reiterate, this pressure may be as high as full hydrostatic pressure. Conversely, if placement rates are reduced, the concrete mixture, especially that possessing thixotropic properties, undergoes structural build-up, and the pressure is reduced. Similarly, the placement method has a significant effect on formwork pressure. Concrete pumped into the fromwork from the bottom of the form exhibits higher pressures than that placed from above. These effects are to a great extent related to the shearing forces to which the plastic concrete is subjected to during placement, being greater once the concrete is pumped into the forms from the bottom up. Since the mechanical properties of concrete are temperature
dependant, the higher the initial temperature of the concrete and/or the ambient temperature, the lower the lateral pressure is. Subsequently, a higher rate of pressure decay is recorded. This is attributed to a faster rate of structure build-up and hydration takes place at higher temperatures. The set time of concrete has similar effects. After placement, mixtures with longer setting times display longer lateral pressure cancellation time.
(C) Formwork Characteristics
There is little data pertaining to the effects of formwork dimensions and formwork pressure. Few researcher have showed that smaller cross-sections exhibit lower maximum pressure. It was explained that this relationship was due to an arching effect that limits lateral pressure. The presence of reinforcement theoretically helps to decrease the formwork pressure because it can hold part of the concrete load, although this may be a negligible effect in SCC. Research also has shown that the type of formwork used has an effect on formwork pressure. Specifically, rigid and smooth formwork materials result in higher lateral pressure and lower rate of pressure drop after placement. The roughness of the forms also plays a role due to the dynamic friction that develops upon concrete placement. It was shown that the application of demolding agents, such as oil, to the formwork can decrease friction and lead to an increase in lateral pressure.
Characterization of the placement process of Self Consolidating Concrete
Fresh concrete is an age-stiffening, thixotropic material. Without agitation, concrete begins to gain/regain its shear strength. Concrete in a mixer or transit truck is agitated continuously, which destroys any tendency to build up a thixotropic structure, and remixed at high speed upon reaching the construction site. After concrete is discharged into the bucket it is at rest. Concrete is discharged from the bucket and flows into the form. For pumped construction, the concrete is agitated until it is in the forms. However after the concrete has reached its final position in the form it is not in a state of flow/failure. Because formwork pressures are influenced by the behavior
Figure 1: Geokon 4820 earth pressure cell
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of concrete at rest in the forms, measurements at near zero shear rates (namely static yield strength or stiffness before flow), after periods of rest are relevant to formwork pressure. The static yield strength reflects the stress needed to initiate flow in an at-rest material while the dynamic yield stress reflects the stress needed to maintain flow after the at-rest structure has been destroyed. Static yield strength can be measured directly in a rheometer in a strength growth test, during which a very low shear rate is applied to the concrete and the build-up in stress before flow is monitored. The initial state of the concrete sample for the stress growth test must be representative of the concrete in the form.
Measurement of Field Formwork Pressures
Lateral pressures have been measured with small diameter, commercially available, pressure transducers, various earth pressure cells, tie force measurements or strain in form elements. Rate of pour, concrete consistency uniformity are problematic in field investigations. Concrete head is difficult to measure and maximum head is limited by the project. Many projects have not measured the changes in concrete behaviour over the pour durations, typically 4-5 hours or less.
Some Field Measurement Studies
(Source Concrete International Journal January 2012, article by by N.J. Gardner, Lloyd Keller, Robert Quattrociocchi, and George Charitou)Field The field investigation of form pressure in Charleston, SC, from June 2005 to February 2006 is discussed here for general information of the reader on the article insight.
Mixtures were designed for a required initial slump flow of 600 to 700 mm (24 to 28 in.). In addition, the laboratory flow properties were determined using rheometers: an IBB rheometer for the Charleston and London mixtures. As the investigation progressed, more on-site material characterization was done by measuring the on-site slump flow and stiffening characteristics of the concrete. The test results emphasized the sensitivity of the SCC stiffening behavior to variations in water content, temperature, and admixture types and dosages. The lateral formwork pressures were measured using 125 mm (5 in.) diameter, Geokon 4820, vibrating wire pressure cells ( Refer Figure 1). Pressure measurements were recorded using a scanning data logger. In-form concrete elevation data were taken by personnel using tape measures and stopwatches.
Citadel, Charleston, SC
Prior to construction, a baseline mixture, a mixture with a reduced water-cementitious material ratio (w/cm), a mixture with reduced paste, and a mixture with increased coarse aggregate were chosen to investigate the effects of proportions on formwork pressure. For all mixtures, the maximum aggregate size was 20 mm (3/4 in.). As the project progressed, modified
mixtures were added to the program and other mixtures were abandoned without being used in the field. The project was a university residence hall with 150 and 400 mm (6 and 16 in.) thick shear walls. Placement heights were 3.5 m (11.5 ft) (Refer Figure 2).
A single residence unit between door blockouts, shown in Figure 2, required about 5 m3 (6 yd3) of concrete. For such a small quantity of concrete, placement by pump could be completed in as few as 10 minutes, a rate of placement of 18 m/h (60 ft/h). Initially, two sets of four load cells in vertical rows were used to monitor form pressure. The maximum concrete head above the lowest gauge was 3.1 m (10 ft). Early results showed that the upper cells experienced only hydrostatic pressure, so the top cells were not installed for later placements. After inspection of the results, the placement sequence was modified to reduce the rate of placement without excessively slowing down construction. Concrete placement was alternated between adjacent residence units so that the first lift was half of the form height. This lift was
Figure 2: Crane view of the Citadel, Charleston, SC, site
Figure 3: Lateral pressure versus time from start of data logger for Labatt’s Brewery, London, ON, with concrete temperature of 17°C (63°F). Hypothetical hydrostatic pressures are provided in the legend
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allowed to rest for about 20 minutes while concrete was placed in the forms for the adjacent unit. Eventually, two different mixtures were placed on the same day, using four sets of three load cells. Two instrumented forms were used for each mixture. The results for the February 2, 2006, placement are shown in Figure 3, along with the hypothetical hydrostatic pressures. The negative gauge readings are due to the load cell being only partially submerged during form filling. Most of the measured pressures were close to hydrostatic, regardless of the mixture proportions. Discontinuous placing (placing the concrete in lifts with a rest period between lifts) reduced the maximum pressures.
Form Pressure Models
Numerous models and code regulations aiming at predicting form pressure are reported in the literature. However, in most cases the models do not pertain to SCC. The models available include the following input parameters: pore water pressure, rate of casting, vibration, setting time, consistency, form permeability and surface texture, form dimensions, coarse aggregate, temperature, and concrete unit weight. Of the few models pertaining to SCC, thixotropy has been identified as a main factor that affects the lateral pressure and its rate of decay. Accordingly, both the Sherbrooke and Northwestern research groups have shown that thixotropy can be monitored and quantified by measuring the area confined in the hysteresis loop (of shear rate vs. shear stress plot) obtained by studying the rheological characteristics of the mixture or by evaluating the drop in shear stress between initial and equilibrium states determined at different shear rates. The resulting area in the hysteresis loop or in the drop in shear stress vs. shear rate is used to quantify the energy required for structural breakdown during mixing and the ensuing structure build-up once the mixture is at rest. Indeed, Khayat and his coworkers successfully implemented this concept and made a connection between the rate of pressure decay and the degree of thixotropy.
Conclusion
The lateral pressures developed by Self Consolidating Concrete are dominated by the performance of the admixtures. Form pressures are determined by the rate of concrete placement relative to the rate of development of concrete stiffness /strength. The rate of concrete placement must not be increased nor admixtures changed or substituted without consideration of their effects on formwork pressure. Reducing the rate of concrete placement by scheduled placement i.e. bucket or programmed interruptions of pumping, allows the concrete to gain shear strength, reducing the maximum form pressures. Mix design and qualification should be done prior to start of construction. Testing for production, mixture selection/qualification and formwork selection must be done in concert and concrete control parameters established to ensure
compliance. Changes in the water content of the aggregates can significantly affect the stability of the mixture and strict control for moisture compensation needs to be instituted at the ready-mix plant. Rigorous on-site quality control is required to ensure mix compliance and consistency. When concrete arrives on site, if the initial slump flow is too low it can be brought into compliance using High Range Water Reducer (HRWR). However this may change the stiffening behavior of the concrete which would change the maximum formwork pressures. Whether or not HRWR has been added on site, the stiffening behavior of the concrete should be measured on one of the first batches of concrete delivered. During the field testing described in this paper, various approaches for characterizing concrete rheology were tried. The flow parameters are sensitive to the conditioning of the concrete, agitated or not agitated, prior to measurement. The standard rheometer testing protocol at relatively high shear rates was found not appropriate for quality control during construction.
Reference
- www.concreteinternational.com.
- The National Ready-mix Concrete Research Foundation And The Strategic Development Council, American Concrete Institute
- Alexandridis,A. and Gardner, N.J. 1981, Mechanical behavior of fresh concrete. Cement and Concrete Research, Vol.ll, pp. 323-339.
- Assaad J.J., Harb J. And Khayat K.H. 2009. Use of Triaxial Compression Test on Mortars to Evaluate Formwork Pressure of Self-Consolidating Concrete, ACI Materials Journal V.106, No.5, September-October, pp.439-448.
- Gardner N . J. 1985 Pressure of concrete on formwork - a Review. ACI Journal September/October 1985, No. 5, Vol. 82, pp.744-753.
- Gardner N.J., Keller L., Charitou G. and Quattrociocchi, (2011) Field Investigation of Wall Formwork Pressures using Self Consolidating Concrete, Concrete International.
- Gregori A., Ferron R.P., Sun Z. and Shah S.P. 2008, Experimental Simulation of Self-Consolidating Concrete Formwork Pressure, ACI Materials Journal V.105, No.1, January-February, pp.97-104.
- McCarthy R. and and Siwerbrand J. (2011) Comparison of Three Methods to Measure Formwork Pressure when using SCC, Concrete International, V.33, No.6, June, pp.27-32.
- Proske T. and Gauber C.-A. 2008, Formwork Pressure using Self-Compacting Concrete – Experiment and Model. Internet –Publication 208, Technische Universitat, Darmstadt, Germany.
- Schjodt R. 1955, Calculation of Pressure of Concrete on Forms. Proceedings ASCE V.81 680-1 to 680-15.
- Field Investigation of Formwork Pressures Using Self-Consolidating Concrete, by N.J. Gardner, Lloyd Keller, Robert Quattrociocchi, and George Charitou, Concrete international January 2012.
Formwork
luminum formwork systems come with a host of advantages and it is not surprising to find A
them making an entry into the Indian market. Hi-Lite Systems, which is the first company in the world to design aluminum shoring systems, is now offering its products in India. The wide range is ideally suited for infrastructure, as well as real estate projects.
The aluminum shoring frames on offer from the company are known for their reliability and quality. The life span of aluminum shoring frames is calcu-lated to be more than 20 years. What is more with 70% scrap value after 20 years, they make for excellent invest-ment for contractors. Just to put things into perspective the corresponding scrap value for steel shoring frames is estimated to be zero. Aluminum shor-ing frames also score high on the cost front, since they cost almost the same as steel frame, while being more dura-ble and offering a higher scrap value.
Another major advantage with Hi-Lite systems is that they can be erected about 10 times faster than steel frame.
A Time & Motion study has clearly indi-cated this advantage of aluminum shor-ing frames over steel frames. The dura-bility of aluminum frames has also been proven. A drop test, which was displayed at IIT, conclusively proved the fact. In a free fall test of aluminum frame from 20' height in all directions and that too repeatedly on Concrete Floor, the frames showed no defects. In order to ensure higher strength of aluminum, the company uses pure bil-lets and not scrap metal. Tests done at IIT show that the Aluminum beams are
50% more stronger than in cases where scrap metal has been used, while also ensuring longer life and enhanced safety.
With incidents of formwork failure often reported from different parts of india, safety is another aspect that influences purchase decisions of con-tractors. For one there are no loose parts when it comes to Hi-Lite systems. They come integrated with couplers and pins. They are very safe for workers to climb, erect tower, with non-slippery rungs. The safety factor has meant that these aluminum frames are recom-mended by civic authorities, against steel frames in order to avoid acci-dents. All these aspects make them ideally suited for both residential, as well as commercial projects.
The company has lined up ambi-tious expansion plans for the Indian market. The aluminum formwork sys-tems are now being manufactured in India. The company also offers value added services including help with financing. Full time site support ser-vices is another key value added ser-vice offered by Hi-Lite Systems. With its emphasis on stringent quality stan-dards and excellent technical support services, aluminum shoring frames from Hi-Lite Systems are making rapid inroads into the Indian market.
Only -Aluminum Shoring Frame–23.2MT/Tower
Mantra “Speed–Speed & Speed”
of Construction
110 ·The Masterbuilder - April 2013 www.masterbuilder.co.in
For further details:
Hi-Lite Systems India Pvt. Ltd.VIRAG H BHACHECH(VP – SALES & ENGG)5B1, Fifth Floor, J.P. Tower - 7/2, Nungambakkam High Road,Chennai - 600034 Mob: 09409541938H.O.: Toronto Canada,Tel No.: 001-647-880-4032E-mail: [email protected] Project in Gujarat (RES & COMM)
Gaumont Disneyland – PARIS
Project in NF – CANADA (Ideal for Slabs located at higher heights – panel of size 5mt x 5mt x 45’ high weighs only 1500 Kgs – Includes Weight of Aluminum Frames & Aluminum Beams -can be flown from one floor to another very easily –as shown)
Project in FRANCE
Winnipeg Airport CANADA
An Info Marketing Feature
he Indian formwork market is witnessing the entry of several new innovative products and T
technologies. A good example is the “aluminum telescopic flying frames” from Hi-Lite Systems, a globally reputed industry leader in the field. The aluminum telescopic flying formwork system has come as a boon to contractors who are almost always under pressure to complete projects within time, since it helps in speeding up the construction process. In fact, Hi-Lite Systems has been able to achieve truly remarkable cycle time of as less as 3 days with the system.
The aluminum telescopic flying form is ideally suited for multiple sto-ries, as well as infrastructure projects. The light-weight and easy to handle system can be assembled into modu-lar component system, which ensures quicker turnaround times. The tele-scopic design ensures that the modu-lar component system adjusts for dif-fering floor heights and for other com-plex geometries. A good example is available in the form of the Casino Structures in Atlantic City, New Jersey, where Hi-Lite systems, which have
been able to achieve 3 days cycle time by using aluminum telescopic flying form. The system can be used on typi-cal floors and non-typical floor too, where floor heights are different. In this case just by pulling the pin out, the tele-scopic struts can be extended, as per the floor height, before the pin is adjusted again.
In one of the company's current pro-jects in Toronto for Collavino Group (builders of World Trade Center), too it has been able to achieve 3 days cycle time. In India also the company has been able to replicate the success sto-ries of the west and speed up the con-struction process. The company has been able to achieve a cycle time of 5 days for a project of L & T. The com-pany has supplied its products to 60
construction sites of L & T India, which is a testimony to its quality and efficacy. The stripping of the system is also not labor, time or cost intensive. In fact, the stripping of the system can be done by a minimum number of construction workers.
The light weight of the formwork sys-tem means reduced crane time. In fact, in the case of lighter sized frames, there may not be a requirement for cranes at all, something that is not pos-sible with either timber or steel forms.
The aluminum telescopic flying form system ensures that there is sig-nificant reduction in labor costs, as well as crane time, while also improving the construction quality. With its ability to increase productivity, it is not surprising that the aluminum telescopic flying formwork system from Hi-Lite Systems is finding an increasing number of takers in India.
Only Mantra Speed Speed & Speed
3 Days Cycle Time
of Construction–
For further details:
Hi-Lite Systems India Pvt. Ltd.VIRAG H BHACHECH(VP – SALES & ENGG)5B1, Fifth Floor, J.P. Tower - 7/2, Nungambakkam High Road, Chennai - 600034, Mob: 09409541938H.O.: Toronto Canada,Tel No.: 001-647-880-4032E-mail: [email protected]
Harrah’S Casino – 46 Storey
Revel Tower – 50 Storey
Borgata Tower 1 & 2-50 & 48 Storey-Atlantic City-NJ-USA
40 Towers in Singapore
Project in Toronto – 3 -Days Cycle Time
Project in Philadelphia & HI-RISE in ABU -DHABI
An Info Marketing Feature
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128 The Masterbuilder - April 2013 • www.masterbuilder.co.in
Pre-Cast Concrete Elements in Construction - Emerging Scenario in India
Pre-cast construction is gaining significance in Indian scenario in general and Urban areas in particular. Pre-cast construction can be broadly classified in to three
categories.
- Project Specific Pre-Cast- General Pre-Cast for Sector Specific- Precast for Non Structural Elements
While the first and last categories are very much prevalent in India for quite some time. The First Category is gaining more popularity with the rapid urban infrastructure growth in India. The Non Structure Category is widely present but still to attract Major organized players. General Pre-Cast which is Sector Specific Such as Buildings, Power Distribution, Water Supply etc., is available scattered in India. Again this Category requires involvement of major players.
Though advantages of Pre-Cast are accepted by every stake Holder, the application is limited because the industry didn’t attain the scale it should be in. The Note presents case study pertaining to Building Precast category, by discussing various elements that can make the total MSB completely pre-cast. Also provides case study or example plants set up in Hyderabad city of India.
General
Cast In Place (CIP) VS. Pre – Cast
The current practice in India is the CAST IN PLACE (CIP) Reinforced Concrete Structures. The other construction method is using structural steel. In general most commercial and residential buildings are of CIP in nature. Steel structures are normally restricted to Industrial Building and sparsely Commercial Buildings. The ratio between Cast In Place (CIP) and steel structures is approximately 70 : 30. The third construction is using Precast Concrete which is primarily limited to Bridges and Railway Sleepers and other non structural elements.
Problems faced in CIP Structures
a) Safety issues: The biggest problem faced during construction
is failure of scaffolding. More structurally stable scaffoldings are cost prohibitive for ordinary commercial structures. (Pic. Of Collapse of Buildings at Khammam, Andhra Pradesh as published in News Papers is placed below.)
b) Concrete Placement using conventional systems in uncontrollable weather conditions.
c) Reinforcement Placement at higher altitudes.d) Quality of finishes highly dependent on working conditions
and form of support structures.e) The current structural system used is a rigid frame with
infill masonry walls. This system is very inefficient in the resistance of lateral loadings like earth quakes etc. Lot of Redundancy in structures.
f) Even most advanced techniques like tunnel forms with 9hrs cycle time take 8 months for 19 storied building.
Why Pre-Cast Construction?
Precast concrete offers solution to all of the problems and setbacks faced by Cast in Place Concrete (CIP).
The following are the main advantages:
a) Rapid Erection: Rapid Speed of Erection & Fast Construction resulting in earlier occupancy and reduced financing costs. Eg. 19 Storied Building can be completed in 3 months time.
b) Quality Assurance: Materials, Workmanship and Finishes. No transit loss in Quality. Greater specialization in trades leading to higher quality in workmanship.
c) Longer spans are possible with Precasting and Prestressing.d) Ease of Construction due to total Pre – Engineering.e) Aesthetics: Both Structural and Architectural finishes are
possible. Architectural finishes may be achieved acid itching, sand blasting or brick veneer or any other finish as specified by the Customer/Architect.
f) No form working is required at the job site. Limited space for Construction and minimum number of workers at site
g) The floor height can be reduced by eliminating the beams and shallow depths of slabs.
h) No prolonged usage of heavy machinery at site.i) No intensive labour at site for construction. The erection
P. Surya PrakashChief Consultant Satya Vani Projects and Consultants, Hyderabad.
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crew is limited 5 to 6 personnel.j) Better coordination with other specialized trades during
the planning and fabrication stage.k) Construction activities are limited to Precasting Erection,
Mechanical & Electrical services and the total coordination is done at the design and fabrication stage.
l) Precast Systems generally use walls for structural stability. The structural wall systems are more efficient and economical than the conventional CIP framed structures.
Pre –Cast Elements:
The following Precast Elements may be commonly used as illustrated in the photographs placed below:
- Columns- Lite wall panels- Structural wall panels- Single Tees, Double Tees & Quad Tees- Inverted Tees- Hollow cores- Spandrels- Rectangular beams- Stairs- Balconies, Architectural elements etc.
Case Study
As Per the request of the Manufactures the Case study is not published but will be presented in the work shop.
Case A: General Precast factory for Columns, Beams, Solid Slabs and Hollow core.
Case B: Specific Plant for Residential shear wall Construction.
Pre-Cast Construction
Modern Construction of the Buildings and various other Structures are done with Reinforced Cement Concrete by freshly mixed material, which can be molded into any shape. The relative quantities of Cement, Aggregates and Water mixed together control the properties in the wet state as well as in the hardened state, which is poured on the Steel Reinforced according to the requirements.
The Reinforced Concrete combines concrete and steel bars by simply putting them together and letting them act together as they may wish.
Pre-stressed Concrete, on the other hand, combines high strength concrete with high strength steel in an active manner. This is achieved by tensioning the steel and holding it against the Concrete, thus putting the Concrete into compression. This active combination results in a much better behaviour of the two materials. Steel is ductile and now is made to act
in high tension by Prestressing. Concrete is a brittle material with its tensile capacity now improved by being compressed, while its compressive capacity is not really harmed. Thus Pre-stressed Concrete is an ideal combination of two modern high strength materials.
Pre-cast Concrete is a material used to clad the exterior building envelope where each building design can be a custom creation, reflecting desired aesthetic expressions through colours, textures and physical sizing of pre-cast components. One must think of material, initially fluid in nature, with the ability to assume any design form from the mould into which concrete is poured. The subsequent curing, finishing and site installation ultimately provides a wall assembly which could be lean and sleak or strong and massive or perhaps very omate and sculptured emulating detailed stonework found in architecture in previous centuries. Pre-cast can be considered as a plastic material in its uncured stage, with infinite shapes, sizes and panel configurations.
Existing Pre–Cast Technology
Current Precast techniques are limited to specific customized projects and are usually Precast at the job site. Quality of field Precast elements cannot be assured because of batching, placement of concrete, placement of reinforcing, method of compaction, method of forming and finally quality of prestressing. Railway sleepers are also precast.
The application of Precast Concrete in Buildings require special connection details not commonly used in India. This expertise and skill is a common practice in advanced countries in Europe and USA.
This expertise can be acquired through collaborative effort with firms abroad.
Tecnology
It has been envisaged to import the existing Technology and expertise from USA with modifications to suit Indian conditions. Almost all commercial and residential CIP buildings currently built in India can easily be converted into precast, prefabricated concrete structures.
Process Involved in Production of Pre – Cast And Pre – Stressed Concrete Elements:
The following Production Process is recommended to be followed for obtaining the optimum levels. Proper planning of Plant layout and Equipment sizing is very important.
Equipment & Site Preparation
The basic equipment and site preparation required to support Pre-cast & Pre-Stressed Concrete elements production is listed below.
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- Serviced land of a minimum of 20 acres.- Building or production shed with cover is considered.- Levelled compacted storage yard.- Concrete batching plant capable of producing “zero”
slump concrete.- A minimum of 1.5 Cum output capacity is recommended.- Overhead cranes or Gantry cranes of 7 to 10 Tons capacity
of two units are recommended over the production area.- Construction of concrete stressing abutments and casting
pallet platform.- Mechanic shop and concrete testing facility.- Curing system with low pressure, high recovery, hot water
boiler with circulating pump and pipes, steel boiler with feed pipes or electric heating.
- Temperature control capability is recommended.- Yard cranes or Gantry cranes and Fork lift Trucks.- Front end loader for Raw Material handling.- Flat bed trailers or buggies to transport finished product
from production area.
Production Sequence
A typical Hollow core production should follow the recommended setup procedures and production sequences as listed below:
a. Cleaning and release agent application to pallet surface with Ultra-Span Service vehicle.
b. Pulling pre-stressing cables with Service vehicle and stressing cables with hydraulic jack.
c. Place extruder on pallet and prepare unit for extrusion.d. Start-up batching plant, and prepare mix for deliver.e. Deliver the mix to extruder and start production.f. After first line is cast, mark product and cover with curing
blankets.g. When curing is completed, test product for concrete
strength. If concrete strength is at the required level start distressing operators.
h. Distress the cables and start cutting operations on each pallet.
i. Lift off product from pallets and transport to storage area.j. Repeat cycle on a continuous base.
Equipment and Construction Requirements
The model hollow core production plant and other casting yard is based on specifications as follows:
a. Land requirement .b. Factory Building of the size c. Concrete bases for casting palluts and stressing abutments.d. Levelled and compacted storage yard area.e. A Production capacity of f hollowcore slabs per year
based on
f. A hollowcore slab production of the size of 20 Cms x 120 Cms.
g. A batching plant capable of producing “Zero” slump concrete.
h. Concrete curing equipment.i. Boiler and circulating pipes or supply pipes.j. Clean water and utility services.k. Electrical supply of 3 phase 440 Volts.l. Concrete Delivery System or Equipment.m. Cutting, Stripping, Cleaning and Storage Handling
Equipment.n. Overhead cranes or Gantry cranes over production area.o. Yard Material Handling Equipment.p. Clean and graded raw material supply and storage facility.q. Concrete and Material Testing Laboratory Facility.
Plan for Transfer of Technology
The transfer of Technology for production of Pre-Cast & Pre-Stressed Concrete Elements, visualised being:-
a) Transfer of Documents
- Design Drawings - Process Sheets- Tooling Documents - Technical Standards
b) Deputation of Engineering Specialists
c) Training of Operational Staff in the New Technology Process
Plan for Construction of Project Infrastructure
It is proposed to complete the infrastructure facilities for setting up of the Plant for the manufacture of Pre-Cast & Pre-Stressed Concrete Elements within a period of 12 months as detailed below: -
Plant Machinery & Equipment
It is Proposed to complete the process of selection of the Plant Machinery & Equipment within 3 months from the date of go ahead of establishing the Project. Thereafter the process of Ordering, Procurement, Erection and Installation of the Plant Machinery & Equipment will be completed within a period of 9 months.
Civil Works
The Construction of the Factory Buildings and the Auxillary Buildings including Concrete Bases and Storage Yard shall be planned to be completed within a period of 12 months from the date of go ahead of the Project.
Plan for Manufacture
It is envisaged that the Pilot Project would commence within 12 months from the date of go ahead of the Project and the
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Trial runs will commence thereafter. It is expected that the Process of Production will stabilize within 24 months from the date of go ahead of the Project by which time all the facilities required would be installed and the training of the Personnel for the Production would be completed.
Market Potential
Ready Mixed Concrete (RMC) is being used by about 80% of the Potential Users. This conversion process has taken over 15 years. The Potential Users have stated that survey products increase the convenience at site. The total sales+production of RMC in Hyderabad are expected to be 1,500,000 cubic mts./ annum with a growth of 15% year to year. This constitutes about 8% of cement usage out of total sales of 1.5 million tons/annum in Hyderabad.
The price and product flexibility for different buildings of the survey products is the crux of the problem. The larger contractors, consultants and builders like L&T, MECON, TCE, TPL and NCC may start suggesting or forcing their sub-contractors to use and using these products quickly. Using it for lower value mass housing projects where standard products are required will also improve market potential.
There is regular demand for pre-stressed concrete poles by railways and electricity board and concrete sleepers from railways. Since it is a regular market and there is severe competition for this range of products. These are not part of the survey products.
- There may be different kinds of machinery, which could not be supplied by one single agency. In such a case, all the agents shall be engaged.
- Design, Supply, erection, supply of shop drawings, their approvals shall be in the scope of Agencies.
- However, the Architect shall approve the shop drawings.
- The ancillary mechanical items required for erection of the machinery shall be procured by the concerned Agency only.
- Necessary Base Plates, Bed Foundations etc. required for erection of machinery and machinery parts shall be executed by respective agencies only in collaboration with the Civil or Other Contractors / Agencies.
- Synchronization of two or more machines or machine parts of different companies shall be made in presence of respective agencies, architect, client and engineer-in-charge and they shall be finished to the satisfaction of all the concerned.
Technology
Technology can be improved in operations of all the Processing Plants and Tools
- To avoid under utilization of efficiency of machinery - To save energy, fuel and lubricants wherever possible - To have optimum utilization of men, material and machinery - To have risk free operations - To minimize rejections - To have improved quality of out comes.
Batching plant
- Optimum utilization of materials to increase the production, crushing strengths etc.
- More production in minimum time with technically accepted out come.
- Minimum operations to have the increased out put - Conveyance of ingredients and final mix with easy and
minimum operations to required spot in minimum time either through conveyer belt, buckets or hopper system.
Forming
Forming requires Transfer Of Technology for improving methodology, equipment and optimization.
- Minimum shuttering to use for required casting- Reusing the same shuttering for more number of times- Same shuttering patterns to use for different patterns of
castings- Minimum operations of shuttering for different patterns of
castings
Fabrication
- Optimum tonnage of steel to obtain required strength to castings
- Minimizing number of rows of steel bars to get required stressing
- Cutting of bars without wastage
Pre-Stressing
Pre-Stressing requires Transfer Of Technology for improving methodology, equipment and optimization. This is proposed to be had from R.A.I.
- Conservation of energy in obtaining required stressing
Casting
Casting requires Transfer Of Technology for improving methodology, equipment and optimization. This is proposed to be had from R.A.I.
- Conveying the mix to casting spots.- Minimizing the wastage of mix during casting- To minimize the rejections to maximum extent
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- To minimize the stripping operations to save labor and time.- New methods of Compaction- New techniques of Surface and needle vibrations
Curing
Curing requires Transfer of Technology for improving methodology, equipment and optimization.
Effective and accelerated curing with minimum time period of not more than 9 (nine) hours
- Minimum quantity of steam, degree and intensity of super-saturation of steam (if steam is used)
- Optimum time requirement for quick and accepted setting with acceptable results of strengths
- To minimize the types of chemicals used, if chemicals are used
Stripping
- New methods to strip the castings without causing damage- To minimize the labor and time - To strip more number of castings at a time
Handling
Handling requires Transfer of Technology for improving methodology, equipment and optimization.
- Easy handling and conveying the material and castings to required spot with minimum labor, time and energy.
- Risk free handling during loading and unloading
- Usage of hardware like Wire ropes, slings, hooks, D-shackles, clamps, different types of knots to tie the blocks etc.
Stacking
- To stack more quantity of material and castings in minimum place without forming a heap of abnormal size.
- Stacking to have easy loading and unloading- Stacking to have easy movement of vehicles- Stacking materials in order of their movement and priority
of use.
Loading & Shipping
- The equipment can be of indigenous.- Loading into trucks with maximum castings to the full
capacity of the truck- To cause trouble free and effective operations - To minimize the loading and shipping time- To have shipping formalities in minimum time inorder not
to consume much time of trucks.
Erection, Jointing and Finishing
Erection, Jointing and Finishing require Transfer Of Technology for improving methodology, and optimization
The equipment can be indigenous.
- Minimum labor and time to erect as per the drawings and requirements
- Risk and trouble free operations- Causing no damage to the castings- Firm and Effective jointing to with stand and absorb all
types of loads and eddy effects - Smooth finish and good look
Safety certification
Safety Certification requires Transfer Of Technology for improving methodology. Process of inspection
- Conduction of tests- Safety measures - Certification methods
Engineering
- Engineering can be improved through Softwares.- Different kinds of packages of various makes can be studied.- New systems and packages can be improved though the
existing resources.- The information can be had through Foreign and Inland
Companies.- Tie-ups can be had with Foreign Companies for supply of
new technology and engineering.- Techniques can be improved in Engineering.- Preparation and study of Drawings like Construction
Drawings, Structural Drawings, Architectural Drawings, Shop Drawings and As-built Drawings.
- Drawings of all kinds like Civil, Electrical, Mechanical, Plumbing etc.
- Detailing the drawings- Arriving quantities from drawings with ease and
systematically.- Optimum number of engineering personals to be engaged
to commensurate the workload.- Planing the process of works, procurement of materials
and monitoring the achievements by conducting review meetings and emphasizing the need of achieving targets fixed.
- Estimating, Rate Analysis, Tendering, Documentation shall also be improved through Transfer of Technology.
Management
- Managerial skills shall be updated and improved for
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effective running of the Plant.- Demand, Supply and penetration of new product into
the market shall be studied with respect to other similar category of products of other companies.
- Necessary promotional activities and strategies shall be implemented to promote the product in the market.
- Customers who have accustomed to use conventional products shall be tuned to new product.
- Administration shall learn the techniques of extracting more out turn with minimum core staff.
- Shifting, transferring, removing and recruiting new staff frequently may effect the production.
- Finance wing shall update their knowledge in Finance Management.
- Methods to minimize or avoid transfer of funds from one head to other shall be improved.
- Technical staff shall also be trained in principles of finance management to avoid wrong bookings / under wrong heads.
- Sales tax, income tax, excise duty etc.are to be maintained duly updating the knowledge for not to be answerable to respective Govt. Departments.
Training
- Training is an effective mode of transfer of technology.- It can be of practical and theoretical.- Training can be had either at the Factories of concerned
companies or at the plant site.- Training shall be given to the staff and workers in various
fields in rotation system. - It shall be given for every year to update the techniques.- Training in erecting, commissioning and running the
machinery, their maintenance and attending the repairs that occurs generally.
- Smooth and effective running of machinery and plant shall be the main motto of the training.
- Training to all categories of staff and workers shall be given in their respective fields.
- Staff and workers deal with handling, erection, jointing, loading etc. laborious jobs shall be trained not to face unpleasant situations.
- Engineering Personal, Production Managers, Production workers, Marketing, Finance and Administration staff and Managers shall also be sent for trainings in their respective fields.
- Technical staff and workers shall be trained in reading the gauges, measures and meters for measuring various parameters and variables
- They shall be trained in using various instruments used for different purposes and at different places.
Publishers Note: This paper was presented at the Proceedings of the One Day National Workshop on Precast Concrete Buildings in India Practices, Possibilities & Prospects Held in ICSR Auditorium, IIT Madras, Chennai. The Masterbuilder was the official Media Partner for the above event.
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U.S. Experience with Seismic Design and Construction of Precast Concrete Structures
U.S. Codes and Standards
State and Local Codes, Model Codes
The building code development and adoption process in the United States is quite complex. State and local building codes, which are the legal codes that must be followed for design and construction, are typically based on a model code. There have been three model codes in the recent past, the best known of which was the Uniform Building Code (UBC). These have now largely been replaced by the International Building Code (IBC). While much of the country has adopted the IBC, isolated big cities such as Chicago continue to use the older regional codes, now called the “legacy” codes.
Standards
The model code organizations do not have resources to develop code provisions on every aspect of design and construction covered by the building code. Thus, it is common for the model codes to adopt standards. The ASCE 7 Minimum Design Loads for Buildings and OtherStructures and the ACI 318 Building Code Requirements for Structural Concreteare two important standards that are adopted by all model codes for design loads on structures and for concrete design and
construction provisions, respectively. The latter document is a standard and not a code, even though the word Code appears in its title. The various standards published by the ASTM International are also widely adopted by all the model codes as well as by many other standards such as ACI 318.
Seismic Design Criteria
Seismic Zones
Until relatively recently, seismic design criteria in building codes depended solely upon the seismic zone in which a structure was located. Zones were regions in which seismic ground motion on rock, corresponding to a certain probability of occurrence, was within certain ranges. Under the UBC, which had significant worldwide influence, the U.S. was divided into Seismic Zones 0 through 4, with 0 indicating the weakest earthquake ground motion. The level of seismic detailing (including the amount of reinforcement) for concrete structures was then indexed to the Seismic Zone. Also indexed to the seismic zone were height limits on structural systems, minimum requirements concerning the analytical procedure that must form the basis of seismic design, and other restrictions/limitations/ requirements.
S. K. Ghosh Associates Inc.Palatine, IL and Aliso Viejo, CA
This paper discusses the seismic force-resisting structural systems that are recognized by U.S. codes and standards and that are in common use. Innovative structural systems that are newly recognized by U.S. codes and standards and their application are also discussed. The precast building market in the United States is dominated by parking structures and office buildings. This is very different from the situation in Europe. And this provides context to much of the discussion in this paper. The structural systems discussed are mainly geared towards the parking structure and the office building. The other piece of information needed for context is that some degree of seismic design of structures is required in most of the United States. Thus seismic force-resisting systems are the focus of much of the discussion that follows.
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Seismic Design Categories
The most recent development in the mechanism for triggering seismic design requirements and restrictions is the establishment of Seismic Design Categories (SDCs). It was recognized that building performance during a seismic event depends not only on the severity of sub-surface rock motion, but also on the type of soil upon which a structure is founded, and that building design requirements should depend not only on the building configuration, but also on its intended occupancy. As a result, the SDC is a function of location, building occupancy, and soil type.
Table 1 shows correspondence (not equivalency) between Seismic Zones and SDCs. Seismic design criteria that were applicable in the UBC to structures located in Seismic Zone 3 or 4 are now typically applicable in the IBC to structures assigned to SDC D, E, or F.
frames or shear walls or combinations thereof. The different categories of frames and shear walls of cast-in-place and precast concrete that are recognized by ACI 318-11, ASCE 7-10, and the 2012 IBC are shown in Table 3.
Note that there is no intermediate precast concrete moment frame and that there is no intermediate cast-in-place concrete shear wall. Note also that ASCE 7-10 makes a distinction between ordinary reinforced concrete and ordinary precast concrete shear walls.
The elements shown in Table 3 make up the various seismic force-resisting systems of cast-in-place and precast concrete, as described below.
Seismic Force-Resisting Systems
The basic structural systems that may be used to resist earthquake forces are listed in ASCE 7-05 Table 12.2-1. A general description of each of the seismic force-resisting systems is given below.
For concrete structural members within a building assigned to SDC D, E, or F that are not proportioned to resist forces induced by earthquake motions, the deformation compatibility requirements of ACI 318-11 Section 21.13 must be satisfied (ASCE 7-10 Section 12.12.4). In short, every structural component not included in the seismic force-resisting system in the direction under consideration must be designed to be adequate for vertical load-carrying capacity and the induced bending moments and shear forces resulting from the design story drift .
Moment-Resisting Frame Systems
This is a structural system with an essentially complete space frame providing support for gravity loads. Lateral forces are resisted primarily by flexural action of the frame members. The
1997 UBC Seismic Zone 0, 1 2A, 2B 3, 4
1997 UBC Seismic Zone A,B C D, E, FTable 1: Correspondence between UBC Seismic Zones and IBC Seismic Design Categories
Table 2 shows the sections and subsections of Chapter 21 of ACI 318-11 that are applicable to a structure, depending on its SDC. The detailing required as a minimum for SDC A and B (Columns 2 and 3 of Table 2) is termed ordinary detailing. The detailing required as a minimum for SDC C (Column 4 of Table 2) is termed intermediate detailing. The detailing required as a minimum for SDC D, E, or F is (Column 5 of Table 2) is called special detailing. It should be noted that while intermediate or special detailing is fully allowed for a structure assigned to SDC B, neither ordinary nor intermediate detailing is allowed for a structure assigned to SDC D, E, or F; special detailing is the minimum requirement.
Seismic force-resisting systems of concrete consist of
Component resisting earthquake effect, unless otherwise notedSeismic Design Category
A (None) B (21.1.1.4) C (21.1.1.5) D,E,F (21.1.1.6)
Analysis and design requirements
None
21.1.2 21.1.2 21.1.2, 21.1.3
Materials None None 21.1.4-21.1.7
Frame members 21.2 21.3 21.5, 6, 7, 8
Structural walls and coupling beams None None 21.9
Precast structural walls None 21.4 21.4+, 21.10
Structural diaphragms and trusses None None 21.11
Foundations None None 21.12
Frame members not proportioned to resist forces induced by earthquake motions None None 21.13
Anchors None 21.1.8 21.1.8
*In addition to requirements of Chapter 1 through 19, except as modified by Chapter 21. Section 22.10 also applies in SDC D, E, and F.+As permitted by the legally adopted general building code of which ACI 318-08 forms a part.
Table 2: ACI 318-11 Detailing Requirements for Different Seismic Design Categories*
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entire space frame or selected portions of the space frame may be designated as the seismic force-resisting system.
forces is provided by shear walls. No interaction between the shear walls and the frames is considered in the lateral force analysis; all of the lateral forces are allocated to the walls.
As in the case of dual systems, the concept of the building frame system loses its appeal for structures assigned to SDC B, since there is little to be gained from assigning the entire lateral resistance to the shear walls in the absence of any special detailing requirements for the frames. As noted above, a shear wall-frame interactive system, where there is no 100% shear wall requirement, is more practical and economical in such cases.
Undefined Structural Systems
Undefined structural systems are any systems not listed in ASCE 7-10 Table 12.2-1. The seismic design coefficients are to be substantiated based on approved cyclic test data and analysis.
Special Moment Frames, Intermediate and Special Shear Walls of Precast Concrete
Explicit provisions for precast concrete structural elements with intermediate or special detailing were first introduced in ACI 318-02. Until then, precast concrete structures could be built in areas of moderate or high seismicity only under an enabling provision of ACI 318, which became part of all the model codes adopting ACI 318. The provision allows precast concrete construction in a highly seismic area “if it is demonstrated by experimental evidence and analysis that the proposed system will have a strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure….” The enforcement of this vague, qualitative requirement was, for obvious reasons, non-uniform. The need for specific enforceable design requirements for precast structures in regions of moderate and high seismicity existed for a long time. The provisions introduced in ACI 318-02 have evolved some through the 2005, the 2008, and the 2011 editions of ACI 318.
ACI 318 presents two alternatives for the design of precast lateral-force-resisting systems. One choice is emulation of monolithic reinforced concrete construction. The other alternative is the use of the unique properties of precast concrete elements interconnected predominantly by dry connections (jointed precast). A “wet” connection uses any of the splicing methods of ACI 318 to connect precast or precast and cast-in-place members, and uses cast-in-place concrete or grout to fill the splicing closure. A “dry” connection is a connection between precast or precast and cast-in-place members that does not qualify as a wet connection.
Figure 1 illustrates the scope of the design provisions for precast concrete structures assigned to intermediate or high seismic design categories (C, D, E, or F).
Moment Frames Shear Walls
Ordinary RC moment frames(cast-in-place or precast)
Ordinary RC shear walls(cast-in-place or precast)
Intermediate RC moment frames(cast-in-place only)
Intermediate precast shear walls(precast only)
Special RC moment frames(cast-in-place or precast)
Special RC shear walls(cast-in-place or precast)
Table 3: Different Categories of Moment Frames and Shear Walls Recognized by U.S. Codes and standards
Bearing Wall Systems
This is a structural system without an essentially complete space frame that provides support for the gravity loads. Bearing walls provide support for all or most of the gravity loads. Resistance to lateral forces is provided by the same bearing walls acting as shear walls.
Dual Systems
A dual system is a structural system with the following essential features:
1. Resistance to lateral forces is provided by moment-resisting frames capable of resisting at least 25 percent of the design base shear and by shear walls.
2. The two subsystems (moment-resisting frames and shear walls) are designed to resist the design base shear in proportion to their relative rigidities.
The 2012 IBC and ASCE 7-10 recognize dual systems in which the moment-resisting frame consists of special moment frames and dual systems in which the moment-resisting frame consists of intermediate moment frames.
The concept of the dual system loses its validity in buildings assigned to SDC B, since it is questionable whether the moment frames, which are required to have only ordinary detailing, can act as a back-up to the ordinary shear walls (the inelastic deformability of both systems are comparable). In areas of low seismicity, utilizing a shear wall-frame interactive system is more logical. In this system, the shear walls and frames resist the lateral forces in proportion to their rigidities, considering interaction between the two subsystems at all levels. There are additional requirements imposed by ASCE 7-05 Section 12.2.5.10. It is important to note that a shear wall-frame interactive system is not allowed in structures assigned to an SDC higher than B.
Building Frame Systems
This a structural system with an essentially complete space frame that supports the gravity loads. Resistance to lateral
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Special Moment Frames (ACI 318-11 Section 21.8)
Emulative Design (Sections 21.8.2, 21.8.3) - Precast frame systems composed of concrete elements with ductile connections (Section 21.8.2) are expected to experience flexural yielding in connection regions. Precast concrete frame systems composed of elements joined using strong connections (Section 21.8.3) are intended to experience flexural yielding outside the connections. Strong connections include the length of the coupler hardware. Capacity design techniques are used to ensure that the strong connection remains elastic following the formation of plastic hinges in the connected members. Additional requirements are provided to avoid hinging and strength deterioration of column-to-column connections.
constructed using precast concrete must satisfy all ACI 318 requirements for cast-in-place special structural walls (Section 21.9), and must also satisfy the connection requirements imposed on Intermediate Precast Structural Walls (Section 21.4).
Non-Emulative Design (Section 21.10.3) – Special structural walls constructed using precast concrete and unbonded post-tensioning tendons and satisfying the requirements of Section 21.10.2 are permitted provided they satisfy the requirements of ACI ITG-5.1.
The non-emulative or jointed systems are of particular interest in U.S. practice. The Precast Seismic Structural Systems (PRESSS) research program played an important role in the development of non-emulative systems. The program had two primary objectives: to develop comprehensive and rational design recommendations needed for a broader acceptance of precast concrete construction; and to develop new materials, concepts, and technologies for precast concrete construction in regions of different seismic hazards. As the key element to the final phase of the PRESSS research program, a 6/10-scale five-story precast concrete building was constructed and tested under simulated seismic loading at the University of California, San Diego (UCSD). The structure combined five different seismic force-resisting systems in regions of varying seismic hazard. Of these, the hybrid frame (utilizing post-tensioning strands as well as mild reinforcement in the beams and through the beam-column joints)and the precast post-tensioned shear wall system are highly suitable as components of the seismic force-resisting systems in structures assigned to SDC D, E, or F. Much effort has gone into their codification in recent times. The hybrid system in particular has seen important applications.
Applications of Presss Systems
Several commercial structurescompleted in the field have demonstrated the viability of the jointed precast concrete special moment frame system. The most prominent building using this system is the 39-story Paramount apartment building in San Francisco.
A recent application of the hybrid moment frame in a parking structure is shown in Figure 3.
An application of both the hybrid frame and the unbonded post-tensioned shear wall was found in Santiago, Chile by a PCI team sent to investigate damage from the February 2010 earthquake. The precast manufacturer Preansa constructed a five-story structure at their convention/exhibition site that usedunbonded post-tensioned walls and frames following the research of the PCI PRESSS program. The structure is braced in the short direction by post-tensioned shear walls placed at the ends of the building. The post-tensioning strands are
Figure 1: Seismic design requirements for precast/prestressed concrete structures
Non-Emulative Design (Section 21.8.4) - Special moment frames constructed using precast concrete and not satisfying the requirements of Sections 21.8.2 and 21.8.3 are permitted, provided they satisfy the requirements of ACI 374.1-05, Acceptance Criteriafor Moment Frames Based on Structural Testing. The design procedure used for the structure must not deviate from that used to design the test specimens, and acceptance values must not exceed values that were demonstrated by tests to be acceptable.
Intermediate Precast Structural Walls (ACI 318 Section 21.4)
Connections between precast wall panels or between wall panels and the foundation are required to resist forces induced by earthquake motions and to provide for yielding (of steel elements or reinforcement only) in the vicinity of the connections. Elements of connections that are designed not to yield must develop at least 1.5 times the specified yield strength of the reinforcement.
Special Structural Walls (ACI 318 Section 21.10)
Emulative Design (Section 21.10.2) - Special structural walls
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located near the center of the walls. In the other direction, there are three bays framed with unbonded post-tensioned moment-resisting frames. A view of the structure is shown in Figure 4. Although the erection of the structure was complete, the building was unfinished at the time of the earthquake. The first floor was in operation as a kitchen for the convention center. The upper floors remained to be completed. The structure experienced no damage from the earthquake.
Figure 2: 39-story precast prestressed concrete Paramount Tower in San Francisco
Figure 3: Parking Structure under Construction at University of California, Davis Medical Center (Courtesy: Ray Bligh, Watry Design Group, Redwood City, CA)
Figure 4: Building Framed with Unbonded Post-tensioned Walls and Frames
Concluding
The application of precast structural systems in parking structures, office buildings, and other construction is increasingly successful throughout the United States including its regions of moderate and high seismicity and is being increasingly recognized in U.S. codes and standards.
This includes innovative structural systems that do not emulate cast-in-place reinforced concrete construction.
This Paper was Presented at FIB - Days 2012 International Conference held at Chennai.
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Zero Energy System for Precast Concrete: An Overview
As the name implies, the premise of zero energy system is to eliminate the need to input mechanical or thermal energy into the process of consolidating and curing
precast concrete elements. The Zero Energy System involves the use of self-compacting concrete to eliminate all external energy required for placing and consolidating concrete and no external energy for heat curing of concrete. The heat generated by hydration of cement is used for accelerating the strength development in concrete. Heat loss is minimized by insulating the forms and covering the concrete after placing with insulating mats. The concept of a Zero Energy System (ZES) was first introduced to the precast industry in Italy in September 2001. This is achieved by the use of innovative chemical admixtures and a proper mixture design. The ZES has a number of distinct advantages overthe conventional precast manufacturing process, which include economic, quality and safety considerations.
Underlying Concept of ZES
The Zero Energy System takes into consideration all the aspects of the manufacturing process of precast elements. Apart from eliminating vibrations, reducing energy consumption and labour, the durability of the concrete itself is also enhanced. Energy in the sense of the Zero Energy System comprises more than the direct costs of electricity and oil needed for the operation of a precast plant. It comprises all of the energy resources that must be mobilized in precast production; not only fossil fuels and electricity must be considered as forms of energy, but also aspectsof direct labour, material consumptions and productivity.
Major Constituents of ZES
There are two major constituents of ZES, (a) High Early Strength High Range Water Reducer, (b) Self-Consolidating Concrete.
Sonjoy Deb, B.Tech, ‘Civil’Associate Editor
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- High Early Strength High Range Water Reducer (HES-HRWR)A key component to the ZES is a HES HRWR. This admixture is based on anext-generation polycarboxylate ether (PCE), which provides excellent dispersion via asteric mechanism and improved hydration kinetics. This generation of PCE is believed to cover less of the surface of the cement grain, thereby actually providing accelerated hydration as compared to a non-admixture treated reference. This also contributes to the early age compressive strength development than other PCE’s. Even further improvements to the early strength development and workability retention properties of the HES HRWR have recently been made.
- Self-Consolidating Concrete (SCC) Mixture Design with a Viscosity Modifying Admixture (VMA) Another important component of the ZES is the use of a SCC mixture design. These mixtures are highly fluid and typically have slump flow values of greater than 550 mm.Therefore, these mixtures must be designed to exhibit adequate dynamic and staticstability. This includes suspension of the coarse aggregate and control of bleeding.The HES HRWR will typically impart improved stability over earlier generation PCE’s,but is not always adequate for some applications. Early generation concrete mixtures relied upon a higher fines content to achieve the required stability. This approach ofteninvoked concerns on the long-term creep and shrinkage characteristics of such concretes. It also made it impossible for precast manufacturers who were interested in high coarse aggregate contents for architectural finishes to use SCC. An improved approach isto use a more conventional mixture design and to incorporate a VMA. Use of the appropriate type and amount of VMA for the application will adjust the rheology of the concrete to provide the desired stability.
Commonly Available Commercial Products
Glenium ACE by BASF is high early strength high range water reducer and is most commonly used for ZES system. The two key elements of the Zero Energy System are Rheodynamic concrete and Glenium ACE, a hyperperforming superplasticizer for Rheodynamic technology. Rheodynamic concrete, an optimized self-compactingconcrete, provides a concrete mix with exceptional placing characteristics, accelerated cement hydration forearly strength development and high-quality concrete.
About Glenium Ace: An essential component of the Zero Energy System is Glenium ACE, a superplasticizerof the latest generation of polycarboxylateether (PCE) polymers, especially developed for precast applications.Glenium ACE molecules are rapidly adsorbed on the surface of the cement grains and act through electrostatic and steric repulsion to powerfully disperse the individual particles of cement. The molecular
structure of polycarboxylate etherpolymers is essential for the early developmentof strength. With conventional PCE superplasticizers,the molecules cover the entire surface of the cement grain and build a barrier against contact with water. Therefore, the hydration process takes place slowly.The unique, proprietary molecular structure ofGlenium ACE exposes increased surface of thecement grains to react with water. As a result ofthis effect, it is possible to obtain earlier development of the heat of hydration, faster development of the hydration products and, as a consequence, higher strengths at very early age. This advantage can even be utilized at low temperatures. Refer Figure 1for Glenium Ace molecular structures.
Figure 1: Glenium Ace molecule (Source: BASF Construction Chemicals)
Benefits of the Zero Energy System
Increased Productivity
Productivity in the precast industry dependsdirectly on the speed at which concrete cures, regardless of which manufacturing processis used. The unique principle of High Early Strength High Range Water Reducer acting on the cement molecules, significantly increases hydration kinetics without disturbing hydrate morphology. The natural exothermic heat produced in the first few hours is capable of speeding up the crystallization processes, developing faster material strength. The substantial improvement in performance incomparison with traditional superplasticizerstherefore optimizes the efficiency of the mix and reduces the production cycle, potentially doubling output Increased.
Early Strength Development
The strength development of ZES PCE and that of Traditional PCE is shown in Figure 2 below. It clearly shows in case of ZES PCE (in this case it is Glenium ACE 30 of BASF Chemicals)there is a significant strength gain between 6 to 12 hours than that of Traditional PCE which achieves the same in 20 hours. This again adding to the productivity increase.
Minimized Heat Curing
The energy required for heat curing is oneof the key parameters when calculating thecost of precast concrete elements. It is therefore an important economic factor. One
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of the objectives of the Zero Energy System is to optimize the amount of energy required in the production cycle to achieve specification requirements.The action of the unique Glenium ACE polymer combined with the control of manufacturing and placing parameters allows optimum use of natural energy for hydration, making it happen sooner! External energy supplies can therefore be reduced or eliminated, removing the need for heat curing.This feature of the Zero Energy System not only has economic benefits; it alsoimpacts on the durability of concrete by limiting any microcracks that may result from heat curing (thermal shock, temperature gradient, desiccation, etc.).
concrete requires trained and skilled labor. With the use of SCC as part of the ZES, the concrete easily flows into the formwork and is generally self-leveling. As a result, only minimal labor is required toget the concrete into its intended location. When compared to conventional concrete, areduction is labor costs is generally possible.
Improve concrete quality and intricacy of shapes
The use of the ZES can improve both the engineering properties and aesthetic properties of the finished element. It has been reported steam curing can compromise the later age compressive strength development. The engineering properties, such as bond to reinforcement, top-bar effect, creep, and diffusivity are all comparable or better than the properties of similar conventional mixtures. Drying shrinkage is generally better in SCC mixtures, especially when a VMA is used. The surface appearance of the finished elements is generally improved when compared to conventional concrete. There is little or no need to surface repairs, grinding or reworking as a result of the excellent self consolidation of these mixtures. This is true also for thin, complex elements as shown in Figure 4.
Figure 2: Compressive Strength Comparison (Source: BASF Construction Chemicals)
Figure 3: Curing Cycle (Source: BASF Construction Chemicals)
Elimination of Vibration
The energy required to place concrete is a further key factor in calculating the costs of precast elements. However, vibration is also a recognized nuisance factor: noise for workers and near by residents plus the physical stress for people involved in placing concrete. One of the objectives of the Zero Energy System is its ability to eliminate the energy required to place precast concrete.The flow and water reduction action of High Early Strength High Range Water Reducer, combined with aviscosity-modifying agent (VMA), enables the robust and direct formulation of self-compacting concretes, which can be placed without vibrations when combined with a compatible manufacturing process.The Zero Energy System therefore reduces the related costs whilst improving working conditions in the precast concrete plant. Refer Figure 4 for flowing concrete mix.
Reduce labor requirements
The placement, consolidation and finishing of conventional
Figure 4: No Vibration Required (Source: BASF Construction Chemicals)
Figure 5: A thin, intricate precast element that was formed using the ZES. Note the fine details andgood surface appearance.
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Application Areas
The flexibility of the Zero Energy System is suitable for a diverserange of markets: components for buildings, civil engineering,drains, artstone, agriculture, roads and highways, railways, telecommunications, etc.The Zero Energy System can be used in the casting of girders, prestressed floor support slabs, floor slabs, panels, segments, pipes, blocks, vaults, cross-beams, façade units, paving and copings.The Zero Energy System is an innovative and upcoming concept for the precast industry.
Conclusion
The use of the Zero Energy System (ZES) has a number of economic benefits for the precast manufacturer. Depending on the specific interests of the producer, these can include possible reductions in energy and material costs, reductions in equipment repairs and maintenance, reductions in worker health claims or increased productivity. Additionally, the use of the ZES results in a finished precast element with improved aesthetics and engineering properties from that made with conventional concrete. Also the flexibility of the Zero Energy System ensures the optimum utilization of energy required to the precasting of concrete elements. For some manufacturing processes, depending on the cycletime, the ambient temperatures and the composition of themix, the full benefits of the Zero Energy System can be compounded by combining the effectiveness of ZES PCE with the technology of self-compacting concrete.Concrete can be placed without vibration, achieving the required performance without the need of heat curing.
Reference
1. Corradi, M., Khurana, R., Magarotto, R., and Torresan, I., “Zero Energy System: AnInnovative Approach for Rationalized Precast Concrete Production,” Proceedings of the17th Intl. Congress of Precast Industry, Istanbul, Turkey, May 1-4, 2002, 8 pp.
2. Parker, D., “A Mixed Blessing: A Dramatic Improvement in Precast ConcreteProduction Efficiency …,” New Civil Engineer (UK), November 15, 2001, pp. 6-8.3. Daczko, J. A., Kurtz, M. A., Bury, M. A., and Attiogbe, E. K., “Zero Energy System forPrecast Concrete Production,” Concrete International, Vol. 25, No. 4, April 2003, pp.103-107.
4. Khurana, R., Magaratto, R., and Torresan, I., “New Generation of PolycarboxylateSuperplasticizier to Eliminate Steam Curing of Concrete,” Proceedings – InternationalCongress on ‘Challenges in Concrete Construction’, Dundee, Scotland, September 5-11,2002, pp. 213-224.
5. Bury, M. A. and Christensen, B. J., “The Role of Innovative Chemical Admixtures inProducing Self-Consolidating Concrete,” Proceedings of the First North AmericanConference on the Design and Use of Self-Consolidating Concrete, Center forAdvanced Cement-Based Materials, Northwestern University, Evanston, IL, November12-13, 2002, pp. 141.
6. Christensen, B. J. and Bury, J. R., “Evaluation of a New Generation Synthetic HRWRin Precast SCC Mixtures,” to be published at Second International Conference on theDesign and Use of SCC, Chicago, Ill, October 2005.
7. Daczko, J. A. and Kurtz, M. A., “Development of High-Volume Coarse AggregateSelf-Compacting Concrete,” Proceedings of the 2nd International Conference onSelf-Compacting Concrete, Tokyo, Japan, October 23-25, 2001.
8. Zero Energy System for Precast Concrete, Dr. Bruce J. ChristensenDegussa Construction Chemicals Asia Pacific.
9. BASF, The Chemical Company.
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Precast Manufacturing Facility
The traditional method for construction of buildings has been cast-in-place construction. This paper describes a typical precast and prestressed concrete plant facility
and its operations which can provide products such as double tees, hollow core slabs, flat slabs, beams, spandrels, columns, wall panels and stair flights for structures which have modular construction. Residential buildings, office buildings, multistorey parking structures, industrial warehouses, data centers, stadiums, justice facilities, and bridges which are designed and detailed for precast construction can be constructed economically and in shorter amount of time using precast and prestressed concrete manufacturing facility.
Plant Manufacturing Facility
Precast and prestressed plant manufacturing facility typically has following departments:
I. Quality ControlII. Mold ShopIII. Steel ShopIV. Structural Plant/ Casting AreaV. Product StorageVI. Applied Finishes AreaVII. Architectural Plant
I. Quality Control
Quality Control tests enable precast plant to assure uniformity and accuracy in the manufacturing of precast pieces, as well as structural reliability in the finished products. Plant should adhere to strict industry standards governing both the quality of products and the specifics of manufacturing and erection process.
Pre-Pour Inspection: The total setup process is inspected before concrete is ordered. The size and shape of the forms are measured, including haunches, block-outs, recesses, and special forming conditions. The reinforcing and hardware are checked for size, shape, finish, and location. Quality Control calculates strand elongations and stresses the prestress
strand in the product. After the product is inspected and approved by Production and Quality Control, the concrete is ordered.
Quality Control is involved in a project from job planning through building erection. Prior to the placing of concrete into the forms, Quality Control inspectors perform specific tests to ensure that the concrete used in the fabrication of Plant’s precast components is of the highest quality, and of the proper mix and strength to meet individual component specifications.
Material Testing: Aggregate gradation is done from random portions of stock pile. Cement is sampled weekly and tested for normal consistency, early stiffening, fineness and strength.
Concrete Testing: Concrete Tests for Slump/Flow, air content, temperature, initial set, compressive are performed by quality control.
Post Pour Inspection: The precast concrete piece is visually inspected for cracks, chips or honeycomb. If any of these visual flaws exist, the cause is determined and repairs are performed. The piece is also inspected for air voids, blemishes, and color. Once the piece has been approved for storage, it receives a tag, stamp, or paint mark signifying that it has received approval from Quality Control.
Self Consolidating Concrete: Self Consolidating Concrete
Amit Kumar P. Patel, P.E.Project Engineer with Blue Ridge Design, Inc. Winchester, VA, USA
QC Performing slump Test
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(SCC) is a recent innovation in precast/prestressed concrete technology. Also, known as self-compacting concrete, SCC is a high performance concrete with the ability to flow easily into restricted spaces within formwork without segregating, and without vibration. Introduced to the precast industry in 1989, SCC is ideal for structural and architectural concrete pieces requiring smooth surfaces, and offers the benefits of faster placement, improved consolidation, improved uniformity, reduced labor costs, elimination of vibrators, and improved finish.
II. Mold Shop
The primary function of Plant’s Mold shop is to construct a form of the piece to be cast. Many of the structural forms are comprised of steel, and the Mold Shop is responsible for building the blockouts, bulkheads, or other accessories which make a piece unique. Architectural molds are constructed on large, flat casting beds – and require that a more complete package be supplied.
The forms are typically built within tolerances of plus 0” to minus 1/8”. Due to the weight of the concrete when it is poured, it is common for the molds to expand slightly after the concrete has been poured. By keeping the tolerances within a strict range, the amount of expansion is restricted. Too much expansion could cause the finished precast concrete piece to be larger than original designed, resulting in potential erection problems at the piece joints.
Architectural forms receive a fiberglass finish; however, structural molds receive a special resin designed for the precast industry. A wide range of steel shapes are also employed in building the forms. Cardboard tubes or PVC pipe are occasionally used for small holes in the piece.
Sure cure and compressive strength Testing
Mold Shop
Applying resin finish to structural mold pieces
Form Finishes: The type of finish specified is dependent upon the role the completed piece has in the finished structure. The following is a breakdown of the form finishes commonly used by Plant.
Commercial Grade (CG): This finish should be specified only when the product will not be visible in the completed structure, or when the function of the building does not require a finished surface.
Standard Grade (SG): The finish may be used where products are exposed to view but the function of the building does not require a special finish. The surface will be suitable for an applied stucco type finish, but will not be suitable for painting (e.g., stems of inverted tee beams and double tees in parking structure).
Finish Grade (FC): This finish is to be used on bottom-in-form, visually exposed, structural members (e.g., bottom and face of ledges on inverted-tee-beams and spandrels; bottom of double-tee flange).
Finish Grade B (FB): This surface is suitable for painting (especially with at textured or “sand” paint); however, some surface blemishes will be visible.
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Finish Grade A (FA): This finish is suitable for painting with any type of paint.
Architectural Finish (AF): This finish is suitable as a frame around a Veneer Finish.
The finished fiberglass form is used by the plant for approximately 20 castings. At the end of its pour series, the form is inspected by the mold shop, and much of the framework returned to the mold shop to be refurbished and utilized on another form.
Another important function of the Mold Shop is to interpret drawings and create a form plan. The form plan is distributed to other departments to maintain consistency in visualization of the form. Constructing the form plan typically involves visualizing the three-dimensional piece from a flat drawing, but also envisioning the negative of the piece. Laps, drafts, and stripping joints are added to the form so that the finished concrete piece will easily strip out of the mold, and so the formwork will not become locked into the piece.
III. Material Handlers and Steel Shop
Material Operations: The steel Shop is responsible for construction of the steel cages and mesh mats used to reinforce precast pieces. Production of the steel reinforcement cages or mesh mats begins with scheduling and material handling. Advanced order materials specified by Engineering are compared with the information contained on the shop cards, to make certain all of the steel required for a piece has been noted. Schedules and pour cards are then generated for each bed in the plant. This system eliminates the need for the Steel Shop to “hunt and gather” required materials, which in turn contributes to a smoother and more efficient production process.
Steel Shop: Like Material Operations, the Steel Shop acts as a
support group for production. A production schedule is given to the Steel Shop, and from this schedule the Steel Shop crew runs a bed schedule to determine what steel materials are required for each pour. The steel is first sheared by the cut and bend crew on the automated shear line, and then bent, before being set to the steel tiers where the mats will be tied. Once the mats are tied, the material handlers deliver the finished pieces to the appropriate beds in the plant.
Steel Shop Material Conveyance cart
Galvanized corbel plate is delivered to production
The Steel Shop operates on a 3-day lead time. On the first day, the steel needed for a pour is cut and bent. On the second day, the steel is tied into a cage and prepared for production. On the third day, the cage is delivered to the bed to be cast into the concrete. The steel shop produces 15-25 mesh mats per day, depending upon the complexity of the mats and production’s needs. All the plates and inserts that are used for connections have to be galvanized when they are exposed. When they are not exposed they can be primed.
Plant’s steel shop is critical to the speed, efficiency, and productivity of precast manufacturing process. Having an onsite dedicated Steel Shop enables the plant to cast the beds every day
Cages ready for delivery to production
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IV. The Structural Plant
Structural elements differ from architectural components in that they comprise most of the frame and floor system of a building and are usually gray concrete.
Double Tee production: In a mid-size plant, five lines of double tee forms, each measuring 450’ long are used. Three lines will be used for casting daily. Pour rate is typically six to eight pieces depending on the length of each piece. The average length of double tees is 60’ and the standard depths vary from 24” to 34”. Standard widths are 8’, 9’, 10’, 11’-4”, 12’. There are some plants which go upto 13’- 4” and 15’ wide. Flange thickness can be 2”, 4”, and 4 5/8”. 4 5/8” thickness is used for 2-hour fire ratings. Strand pattern in Double tees can be depressed or non-depressed as per the design requirement. For a mid-size plant, approximately 275 cubic yards of double tees for three lines of 450’ can be poured.
Pour Activities:
a. Delivery of concrete: Concrete is transported to plant from central RediMix plant by trucks.
b. QC of Concrete: Quality control inspectors perform the required tests.
c. Placement of Concreted. Vibration of Concrete: Vibrating screed ensures thorough
consolidation of the concrete.e. Finish Concrete: After vibration and concrete bleed water
has come to the surface, the required finish is applied.
Cages tied with stirrups are delivered to production
Hydraulic ram for prestressing strands
Steel strands in the pretensioning process
Casting Double Tee Finished Double Tees in yard
Placing concrete into Double Tee stem Vibration screed for consolidation of concrete
Below is the list of Wet Finishes applied to precast pieces:
Magnesium Float (MF): Roof double tees, flat slabs, floor double tees where sub-floor is to be applied, flat slabs or double tee where the surface will be covered.
Smooth Magnesium Float (SMF): Hotel room floors, and roof double tees.
Wet Trowel (WT): Columns if exposed but not architectural. Interior surfaces where a finish slightly better than SMF is desired. Finish is not suitable for exposed surfaces.
Hard Trowel (HT): Exposed surface of Columns; and surface that has to be painted.
Light Broom (LB): Interior surfaces of exposed wall panel and spandrel beams.
Standard Untopped Finish (SUF): Top finish for untopped parking deck double-tees and inverted-tee beams that match the double tees.
f. Curing Process: Two critical items that affect the initial cure cycle are moisture and heat retention. To retain moisture, covers that hold moisture are used. Additionally, moisture
Applying Light broom finish to Double Tee Double Tee with a light broom finish
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may be added in a mist form to replenish surface moisture. As an alternative, curing compounds can be used to retain moisture.
Concrete produces heat of hydration during the initial strength development phase. The retention of this heat can be used disadvantageously to provide heat for accelerated curing. Accelerated curing is the use of added heat to increase the rate of hydration reactions. The accelerated curing cycle is generally 12-16 hours. Once the accelerated curing cycle is complete, the product is removed from the form. In this way, forms are re-used on a daily basis. Plants have used hot oil radiant heat, propane gas heaters, natural gas heaters, and radiant air heaters for the curing process. Plain tarps and insulated tarps are used on beds. Enclosing forms with insulation can enhance the retention of heat for accelerated curing process.
V. Product Storage Area
Product Dry Finish: The precast product is removed from the casting bed with an overhead crane, and placed on flat or stretch trailers depending upon the type of product. Tractors or yard mules lift the trailer and move it to the dry finish area or applied finish area. At the dry finish area, fins are removed, chips and spalls are repaired, edges are ground, strand is cut and patched, rust is removed, and surface bug holes are filled (according to tolerances), etc. Once the product is completed, it is moved to the storage area were the travel lift unloads and places it in storage until the piece is shipped for erection. The storage area’s sub-grade should be stabilized to avoid soft spots where one end of the product may settle. Settling can create twisting or tensile stresses which can result in cracking and damage to the product.
Product Handling and Shipping: From the time a precast unit is stripped out of the molds to the moment it is unloaded at the jobsite, a number of handling steps are involved. For each of these steps, it is imperative that systematic and well thought out procedures are implemented. In general, the number of times a unit is lifted and handled should be kept to a minimum. There are five basic handling steps that a precast unit undergoes between casting and erection: stripping plant-to-yard hauling, yard roll-up (when applicable), yard storage, and shipping. In all cases, it is crucial to ensure that the proper equipment including rigging, chains and related devices is used.To facilitate this process, engineering prepares “handling tags” that describe through diagrams the methods and procedures for handling the precast units. These handling tags are affixed to each precast unit in a way that prevents them from being easily removed. The handling tags serve as a general guide only, by showing dunnage locations, pick points, or whether a panel is to be stored flat or on edge. However, the handling instructions will not indicate what types of racks, trailers, chains, or rigging must be used.
The following sections describe the general procedures that should be followed during each of the handling phases.
a. Stripping: After ensuring that that minimum concrete strength is achieved prior to stripping, piece is either stripped flat or rolled out of the form. The handling instructions indicate which of these two methods are to be used. Strip the piece using the lifting devices at the location shown on the piece details.
Lifting Loops in Beams Lifting loops with rebar cage
b. Plant to Yard Hauling: Piece is hauled on trailer lifts from the plant to the yard either flat or in an inclined or upright position. Load pieces according to the handling instructions and loading diagrams with dunnage or rocker frames located at the correct location. Secure all loads to the trailer using the appropriate quantity of chains or straps.
Travel Lift used for moving pieces Erection burkes for rolling the panel
c. Yard Roll-Up: Roll piece onto their edge as per the handling tags using loops or lifting devices. Precast piece should be rolled on edge as quickly as possible, to help minimize the potential for warping, bowing or cracking.
d. Yard Storage: Store the piece at the dunnage locations as noted on the handling tags. If piece is stored on a yard rack, ensure the capacity of the rack is not exceeded. Pieces that are treated with an architectural finish should not be stacked in the flat position. For other pieces stored
Ramp Walls with corbels are stacked Spandrel beams stored in a yard
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flat, do not stack more than three-four pieces high. When stacking, always make sure dunnage points are aligned from top to bottom.
e. Shipping: Pieces are loaded on trailers according to the handling instructions and load diagrams with dunnage located at the correct position. Secure all loads to the trailer using the appropriate quantity of chains. If it is necessary to ship a load with only one precast unit on a set of frames, offset toward the left side of the trailer as viewed from behind. The amount of offset will depend upon the size, weight, and configuration of the panel and will be shown on the load diagram. Single piece loads will always be loaded on the curbside of the trailer.
removed from the surface using abrasive material, partially exposing a portion of the coarse aggregate, per approved sample.
Precast stair in storage yard L-Beam is loaded on the trailer for shipping
Product Locator System: Plants utilize a locator system to track the storage location of each piece of product in storage yard. The storage yard is divided into aisles, and each aisle is numbered. Yard personnel log the information into the computer each day and it is used by loading crews to locate pieces for shipment. This information is also included on the “Control of Erection Sequence.” It is utilized by project managers to help them locate product for architect, owners, and owner representatives to view.
VI. Applied Finishes Area
Three types of applied finishes are provided: Sandblast, Acid wash and Retarded
a. Sandblasting: Sandblasting of surfaces offers three degrees of exposure: light sandblasting, medium sandblasting, and heavy sandblasting. This process is suitable for exposure of either large or small aggregates.
Light Sand Blast (LSB): Entire surface skin of cement and sand is removed using abrasive material, but the coarse aggregate is not exposed.
Medium Sand Blast (MSB): More of the sand and cement is
Column is loaded on the trailer for shipping
Aisle of Plant Storage
Sandblasting an architectural wall panel
Heavy Sand Blast (HSB): More of the sand and cement is removed from the surface using abrasive material, so that the coarse aggregate are the major surface feature.
b.Acid Wash: Acid etching dissolves the surface cement paste to reveal the sand with only a small percentage of coarse aggregate being visible.
Acid Etch (A): Acid is brushed or sprayed onto the panel.
c. Retarded: Retardation involves the application of a specialized chemical to the concrete surface (normally the mold surface). Retarders are extremely sensitive to changes in the rate of hydration, so their effectiveness will vary dependent upon different temperatures, humidity, or water content of the face mix.
Retarder Wash (RW): Aggregate is exposed by retarder and water washing to achieve desired results. The amount and type of retarder used depends on depth of matrix, size of aggregate, room temperature, mix temperature, and how long the panel will be left in the form.
Retarder Blast (RB): Aggregate is exposed by retarder and sandblasting to achieve desired results.
Thin Brick
Brick snaps is brick-inlay system that enables precast plants to quickly produce brick-embedded concrete walls. Brick liner system offer precast plants unlimited options for producing creative brick facades by accommodating any brick size, shape, pattern, and point devised. Advantages of brickembedded concrete over conventional masonry are:
Structural and aesthetic value; Simplified engineering; No
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flashing, lintels, or weep activities; No efflorescence; Reduced construction time; Recommended in seismic zones; No sand, mortars, or mixers on site.
stone (granite, limestone, or marble), brick ceramic, or quarry tile. The use of natural cut stone usually requires the use of a bond breaker, per approved sample.
Form Liner (FL): A liner is placed within the form to achieve a desired effect on the face of a panel. A release agent is usually applied so the liner will not stick to the panel. Special attention is given to the liner joints to prevent reading these lines in the panel face.
VII. Architectural Plant
Architectural precast concrete can be cast in almost any Thin brick (approximately ½” thick) is packaged face down into individual plastic holders that snap together on the pour surface
Concrete is poured on the back of the brick assembly and finished in the normal manner
The brick embedded concrete panel is removed from the form and snap hold-ers are removed from the face. The panel is washed with hot high-pressure water
The result is a hard concrete panel with cast-in-brick that is beautiful, economical and manufactured at faster pace.
Wall with architectural finish stored inclined
Parking structure with brick façade on wallpanels and spandrels beams
Brick Liner System
A brick patterned elastomeric form liner is made to specifications. Thin bricks are placed into the liner pockets and concrete is poured over the back of the bricks. This integrally casts the brick tiles into the concrete.
Other Finishes
Veneer Facing (VF): Includes materials such as natural cut
Architectural form being built
Architectural form ready to be cast
Casting of Architectural wall panels
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color, form, or texture. This is achieved by varying aggregate and matrix color, size of aggregates, finishing processes and depth of exposure. Combining color with texture accentuates the natural beauty of aggregates. Color can be achieved through the natural color of the aggregates, color of the cement, or through the addition of pigments. Aggregates range in color from white to red, black and green. Natural gravel provides a wide range of warm earth colors and various shades of gray. Cement may be gray, white, buff, or a mixture of colors. Pigments are also available in a wide rage of colors (red, ivory, cream or buff, yellow, brown, gray, black, green, olive, turquoise, blue, and white). White cement is used with pigments to maintain uniformity of color throughout the panels. The amount of pigment utilized directly influences the shade and intensity of color. Several colors can be poured in the same panel, and several pigments can be used to achieve a desired color.
Typically 1½” and 1¾” architectural face mix with a structural gray backup mix is used. The use of separate face and backup mixes dpedends upon economics, the type of finish required, and the configuration of the product. Product can be structural in nature as well as architectural. An example of this would be a column, It would require one uniform concrete mix throughout. In addition to color reveals, recesses reliefs, and liners can all be used to achieve architectural effects.
A full size sample is produced, including the proper matrix and finish in accordance with planned production techniques. Producing a sample helps to avoid problems with color and materials prior to production. Batch plant should maintain consistency of the architectural mixes.
Architectural Sample Area
The sample area provides owners, architects, and others a place to see a variety of colors, finishes, and textures from previous projects. Previous work of a similar nature can serve as a useful visual standard and highlight potential problems.
Specific project samples can be made for each job sold. The architect approves the 12”x12” sample for color and finish. A larger mock-up piece can then be fabricated using regular production techniques. The architect approves the mock-up for color and finish, enabling production to begin. This reinforces the the architect that we can duplicate the 12”x12” sample color and finish for the project.
Conclusion
- Precast concrete plants can produce precast components in short amount of time even before the site is ready. The components can be stored in a yard and transported as per the construction sequence.
- The quality control department in precast concrete plants can ensure that the concrete products will exhibit high standards of quality and uniformity.
- Architectural precast components with reveals, copings, brick facade and wall panels that serve primarily as the exterior facade of a structure can be produced with range of choices for colors, designs, and textures.
- Because precast concrete products are manufactured in a controlled environment project time is not lost waiting for ideal weather conditions.
- Precast Plants can provide organized construction industry where engineers, technicians and laborers can be stationed at one place and products can be delivered around 200 mile radius from where the plant is located.
- Small scale manufacturers who can provide the necessary inserts, plates, etc. can support these plants.
- To increase the infrastructure in a developing country like India in the next decade, there should be at least 3-5 plants in each state which can help build the required infrastructure. However, there is a need to provide adequate training to students, engineers, architects, detailers, and construction crews to promote precast and prestressed concrete construction. Erection crew should be aware of erection sequence; welding and bolting process to keep the structure stable when it is in the erection phase.
12”x12” sample for Architects review and approval
Full-size sample can be fabricated
Publishers Note: This paper was presented at the Proceedings of the One Day National Workshop on Precast Concrete Buildings in India Practices, Possibilities & Prospects Held in ICSR Auditorium, IIT Madras, Chennai. The Masterbuilder was the official Media Partner for the above event.
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Formwork for Precast - An Overview
Precast concrete buildings are structures made up of numerous small individual elements of concrete cast at an off-site location. These precast elements such as
beams, columns, slabs and walls are transported to the site for assemblage and erection. Wind and earthquake loads are resisted by coupling of beams to columns for moment frame resistance, and coupling of wall elements together for composite shear wall resistance. Thus generally in precast concrete buildings the individual element on its own plays no role in gravity and lateral resistance. It is the assembly of all these elements by proper connections which gives the building its stability against vertical and lateral resistance.
Precast concrete usually is either ordinary reinforced concrete or prestressed reinforced concrete. Prestressing gives advantages of reduced cross-sections and steel requirements (reduced weights). However prestressing needs additional equipments, abutments etc.
Precast Concrete is either a factory-cast (off-site) precast or site-cast (on-site) depending on the volume of work and logistics. Factory cast is typically more popular. Factory cast precast gives more control to the producer and the designer with better options for prestressing, architectural finishes and grade of concrete. A better quality can be obtained as workers and supervisors are well trained and experienced. Work does not hamper due to bad weather.
Site-cast precast is adopted when the project volume is so large that setting up a plant at site is economical. It is also adopted when the transport of precast products becomes very expensive or difficult due to large distances and adverse
road conditions. Setting up of long line prestressed beds is difficult on site and may not be economical, hence most of the site-cast precast is non-prestressed.
Wet concrete is poured in forms (moulds) and stripped out when it attains certain minimum strength. It is stored in a storage area and later transported to the site for erection. Forms are basically either stationary steel plate forms or Tilting tables or battery moulds or moving Carrousel systems with production pallets. Selection of a system depends upon the volume of production of a particular element and flexibility desired in production. There are various patented systems for forms systems available in India.
Prestressed systems are usually long line systems wherein large number of elements are produced in a single bed. Typical elements produced in such a system are Hollowcore
Mangesh Kumar HardasDirector, Precision Precast Solutions Pvt. Ltd.
The principles for formwork for precast concrete remain mostly the same as that for conventional in situ construction. However there are a few nuances arising due to the fact that concrete is cast away from the location where the element is supposed to be for its service life. The forms used for precast are of better quality in dimensions and straightness as no one expects to do any plastering (and thus hide the inconsistencies in formwork) at site. Beautiful shapes and architectural finishes can be achieved which otherwise are very difficult or even impossible to achieve in a conventional in situ construction. Formwork for precast can be used multiple times and at the same time the quality of concrete achieved is much better. This paper touches at the requirements of formwork for precast and overviews the systems generally used as in building construction.
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planks, Double Tee floor elements, Spandrels and Inverted Tee girders. A prestressed bed needs stressing abutments at the ends and a long form is in between. Generally the forms for prestressing elements are either self stressing forms which take the hydrostatic forces of concrete and compressive forces from prestressing, or non-self-stressing or free forms which take only hydrostatic forces leaving the compressive forces coming from prestressing to the abutments. End abutments for stressing is a good solution but sometimes one needs setup for small quantity of elements where self stressing beds can be used. Sometimes post tensioning is also done within the factory for small number of elements.
The forms must be designed properly so that they do not deform during any of the operations of production - pouring concrete, vibrating, stressing, distressing and stripping the element out of form. The end product must comply with the specified tolerances as specified in the BIS codes.
Material for Precast forms
The forms for precast concrete are also called as Moulds (US: Molds). These moulds can be made up of Wood, Steel, Aluminium, Fiberglass, Plastic, Concrete or even EPS (Expanded Polystyrene) as long as it retains it shape against the hydrostatic pressure of concrete, provides product tolerances, and is able to withstand the vibrations, the impacts of placing the rebars and the forces of stripping. Generally good quality fiberglass and wood forms can be reused about 50 times. Steel forms have a very large reuse capacity. For complex shapes of elements as used in architectural precast, forms made with wood fiberglass or concrete are used. EPS forms have limited reuse and mostly used in Architectural precast where the shapes are complex. EPS is also used as sacrificial formwork.
Comparison with Conventional Formwork
Unlike cast in situ formwork, precast formwork can be vibrated in place using vibrating tables thereby giving excellent quality.
Cast in situ formwork needs extensive shoring/ propping which precast formwork does not.
In case of precast formwork it is very important to maintain shapes and dimensional accuracies (tolerances) or else the product may not fit at its place at the time of erection.
Precast formwork presents unlimited possibilities of architectural
finishes such as brick, stone, ribbed finish sand blasted or acid itched exposed aggregates.
Steam curing and heating of the bed is possible in precast formwork which increases the rate of strength gain of concrete.
Design Considerations
Maximum reuse of formwork is the key to economy. The Architect must keep the number of different shapes to a minimum and design shapes which can be stripped easily, preferably cast in single pour. Even so, it should achieve the desired edges, surfaces and textures.
Typically forms should be made for standard cross sections of columns, beams etc. The Architect should try to use these standard sizes as much as possible so that new forms are not required to be made.
The form side(s) of the precast are usually on exterior of the building. When a panel is cast horizontal, the bottom side may be exposed aggregate, rubber form lined (to give desired texture) or just plain surface. The upper surface of the concrete in the mould which is not as smooth is on the interior of the building.
In case of forms with fixed sides, the vertical faces should have draft (slope) of about 1:5 to 1:12 depending upon the width of the section - this would make it easy to remove.
The interior edges of the form should be radiused or chamfered at least 10mm to avoid edge damage during stripping. This can be done using chamfer strips made up of wood or steel.
Figure 1 - A long moving Prestressed bed showing blockouts
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In long line - prestressed method of casting during detensioning of strands concrete shortens, and so the inside forms need to be removed before detensioning. The design should be such that these inside forms can be removed without disturbing the strands.
The form surface against which concrete is cast should be smooth. These are cleaned by wirebrush, scrapping, scrubbing and even chipping. The form sheet should be thick and strong enough to maintain its smooth surface. The plywood used is raisin coated.
If steel bed is chosen, which normally is the case, magnetic systems can be used to fix side forms. Side forms are needed for not only defining the boundaries of the panel but also for door and window openings.
Formwork construction techniques vary, but generally heavier construction gives more dimensional stability and helps reduce transmission of vibration and results in longer life. Fabrication tolerances are typically half the product tolerances. The surface roughness of the steel used is about 0.15 micron. The steel forms have thickness of plates of about 5mm to 8mm and have gussets at every 200mm to 500mm depending upon the forces. Sometimes the Steel plates are made of Chrome Molybdenum Steel.
Sometimes accelerated curing is achieved by heating. To do so, elobrate piping is done under the form bed and hot water or steam is passed through it. To reduce heat loss, insulation should be installed under the beds. The pipe for heating is above the insulation.
Long forms usually have slopes and drainage should be provided.
Formwork for precast wall panels
Wall panels are cast individually or on a long bed when prestresssed. Generally the bottom platform is a steel plate of at least 5mm thick mounted on a concrete. The side forms are usually are fixed rail/channel or wooden. The blockouts are also wooden.
In a long line method, there is a long form of about 50 to 100m with side fixed rail on one side which makes the common side for all the panels. The second rail is usually movable and is kept such that it is on the largest width in the pack. Others in between are wooden. Sometimes the bed is capable of vibrating.
Figure 2 - Tilting Table
Figure 3 - Battery Mould
Tilting tables are used to cast wall panels. These tables are equipped with heating and vibrating bed as well. Tilting tables are hydraulically operated and are horizontal at the time of casting. At the time of striping, tilting tables tilt to almost vertical – thus need lifting inserts only on the edges. They also reduce the steel required or can be stripped quickly.
Battery moulds are designed for the vertical fabrication wall panels. Each layer can have a variable area and reinforcement. They consist of bulkheads between which 5 to 10 panels can be simultaneously formed. Vibrators facilitate the effective compacting of concrete. Battery moulds offer to produce architectural wall panels with both inside and outside surfaces as smooth.
Another system is based on production pallets (a steel table) which pass through various workstations manually over a set off protruding wheels before concrete products are complete. Various transport systems (such as central shifter, side shifters, and rollers) transport the pallets from workstation to workstation. Each workstation has a role – preparing, concreting, curing and stripping. This system offers the flexibility of horizontal casting and economizes on tilting table.
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Strong magnetic systems are available which help in fixing the side forms. The force is more that 500kgs and th
A fully automated system called carrousel system is also available. It is computer controlled and gives a very rate of production. Lattice Girder Slabs can be made with such a system.
Double wall formwork is essentially the same but it additionally needs a system to rotate one half of the already cast and set slab all around and keep it on the one which is recently concreted.
Formwork for Columns and beams
Usually precasters keep standard width and height forms. Column forms are usually non prestressed and can be made up of steel or wood. These can also be made in specially
Figure 4 - Formwork for IT beams
Figure 5 - Steel Formwork for Round Column
fabricated battery moulds. Rectangular beams can be cast in similar way but special forms are needed for Inverted Tee beams. The sides of these forms can be detached. Long line prestressed forms have arrangements for prestressing steel. They need permanent abutments and hence are fixed in place.
Formwork for Hollowcore slabs
Formwork for Hollowcore beds need steel plates firmly mounted on a foundation and abutments at both ends to take prestressing force. Manufacture of hollowcore is a propriety system and a hollowcore machine manufacturer normally provides the beds as well. No side forms are required as hollowcore production needs a very dry mix concrete and remains their on its own. Some machine manufacturers recommend concrete beds to cast the hollowcores on.
Conclusions
Formwork or precast is needed more in the plant and less at the construction site. The principles of structural design of formwork remain the same. Tolerances required for the finished product and the forces coming on it govern the design of formwork. Precast concrete products do not need any finishing (such as plastering) on site. By using coloured aggregates and formliners beautiful patters can be achieved. Companies can fabricate their own formwork or choose from the various systems available in the market based on the production needs.
Figure 6 – Steel Formwork for Rectangular Column
Publishers Note: This paper was presented at the Proceedings of the One Day Seminar on Modern Formwork Systems for Building Construction Held in IIT Madras, Chennai. The Masterbuilder was the official Media Partner for the above event.
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Email: [email protected], [email protected], Website: www.styleearth.net, www.styleearthprecast.com
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