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READY MIX CONCRETE:
AN OVERVIEW
BY
BHUPENDRA SINGH RAGHAVA
M TECH - CONSTRUCTION MANAGEMENT
SEM- I
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READY MIX CONCRETE RMC is a specialized material in which the cement
aggregates and other ingredients are weigh-batched at aplant in a central mixer or truck mixer, before delivery to
the construction site in a condition ready for placing bythe builder.
Thus, `fresh' concrete is manufactured in a plant awayfrom the construction site and transported within the
requisite journey time. The RMC supplier provides two services, firstly one of
processing the materials for making fresh concrete andsecondly, of transporting a product within a short time.
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HISTORY OF RMC First introduce by Architect Jurgen Heinrich Magens, he
get his patented of RMC in Germany in 1903.
The first concrete mixed off site and delivered to aconstruction site was effectively done in Baltimore,
United States in 1913 just before the First World War.
The first concept of transit mixer was also born in 1926 in
the United States. In 1939, the first RMC plant was installed in United
Kingdom and in 1933 first specifications on RMC was
published in United Kingdom.
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Figure: Transit mixer in late 1940s and Concrete pump in late 1940s
(Ambuja Cement Literature Vol. 90)
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In India RMC was first initially was used in 1950 duringthe construction sites of Dams like like Bhakra Nangal,Koyna.
RMC was first fully established in Pune in the year 1991by private companies which was close later.
RMC was successfully established sometime after 1994 inIndia.
As per the available record upto 2003, there are around 76RMC plant in 17 cities with a total capacity of around3875 CuM/hr, producing 3.8 million CuM of concrete peryear.
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ADVANTAGES OF RMC SUPERIOR QUALITY
REDUCED THE WASTAGES OF MATERIAL
ELIMINATE THE PROBLEM OF STORAGE OF
MATERIAL REDUCE SUPERVISION & LABOUR COST
SAFE & HYGENIC CONDITION
FLUCTUATION OF RAW MATERIAL COST & NON-AVABILITY OF MATERIAL ELIMINATE
REDUCES THE PROBLEM OF INSPECTION, CHECKING& TESTING
ROADS & FOOTHPATH ARE NOT BLOCKED
IMPROVE THE ENVIRONMENT
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SCOPE OF RMC IN INDIA The World Bank's India Cement industry Restructuring Project"
under which a technical study report on the development of market
for bulk cement in India was made in 1996, proved to be positive
development towards modernization of cement distribution system inIndia, including setting up Ready mix concrete Plants.
The objective of this technical study was to formulate an action plan
for the development of market for bulk cement in large cement
centres in India and for gradual shift. from the traditional mode of
transportation in bags to bulk transportation through setting up ofready mixed concrete plants in different parts of the country. The
recommendation of the action plan provided a useful guidance
towards expanding bulk cement market thus paving a way for
installation or ready mixed concrete plants in India
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According to Cement Manufacturers Association, RMC is being
increasingly recommended for all major public construction work
such as highways, flyovers.
In India it can grow to consume 40-45 percent of cement by 2015
through setting up of RMC plants in various consumption centers.
According to European Ready Mixed Concrete Organization
(ERMCO) .The marketing of RMC should no more be in terms of
strength grades only, but a combination of strength durability
classification as per the Concrete Codes which improves thesellability of RMC in terms of the requirements of the projects.
Appropriate environmental, safety and health regulations for the
working force need to be kept in mind in the management and
operation of RMC.
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Figure : RMC Market Potential (CE&CR, June 2001, pg.no.30)
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TYPES OF BATCHING
PLANTS1) Centrally Mixed Concrete or Wet batch mixer
In Centrally mixed concrete, the mixing is done at a central plant& the mixed concrete is then transported usually in the truckagitator truck which revolves slowly so as to prevent segregationand undue stiffening of the mix.
2) Truck mixed concerte or Dry batch concrete plant
In the truck mixed concrete, the materials is batched at a centralplant but are mixed in a mixer truck either in transit to the site or
immediately prior to the concrete is being discharged. Transit-mixing permits a longer haul and is less vulnerable in case ofdelay, but the capacity of a truck used as a mixer is only 63 percent, or even less, of the drum while for central-mixed concrete is80 percent.
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BATCHING AND MIXING
PLANTSa) Storage of Materials
i) Cement
Cement is generally stored in silos. The loading of cement is done with the help of pneumatic
blower systems
Figure: Pneumatic blower system on a
cement bulker unloading cement
into silos.(Ambuja Cement Literature Vol. 12)
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ii) Water
Water is generally stored in tanks located close to the plant. Itis accurately measured by a water gauge and microprocessorcontrolled system.
The modern plants have new electronic MFM 85 moisturerecorders .
Figure:
Electromechanical weighing system used
on modern batching plant for cement and aggregates (Amubuja
Cement Literature Vol.no.12)
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iii) Aggregates
Vertical Production Plant
In this the aggregates are stored above the batching and
mixing elements, in one or more silos.These plants are not suitable for relocation at short
intervals of time.
As the aggregates are stored in silos it is relatively easy to
protect the aggregates from very low temperature inwinter period.
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Figure: Schematic view of the Vertical Batching Mixing
Plant with vertical aggregate and cement storage silos.
( Ambuja Cement Literature Vol.12)
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Horizontal Production Plant
They can be again broadly classified into four types
i) Star pattern aggregate storage
ii) Storage in tall silo
iii) Storage in pocket silo
iv) Inline aggregate storage silos
The star bin storage of aggregates is most popular in India
mainly because of climate conditions.
A storage capacity of upto 1500 CuM is possible
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Figure: Horizontal Batching Mixing Plant
with vertical aggregates & cement silos
(Amubuja Cement Literature Vol.no.12)
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Figure: Rear view of the batching mixing plant with star pattern having four
aggregate storage bins
(Amubuja Cement Literature Vol.no.12)
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b) MIXING ARRANGEMENTS
There are various types of concrete mixers used on the
concrete production plant. The two basic types are free fall
mixers and power mixers
Free fall mixer consists of a rotating drum with bladefixed on the drum's interior. As the drum rotates, the
material inside is lifted and dropped.
Most of the imported plants have power mixer. The power
mixer sets in motion the materials positively. Thematerials get thoroughly mixed by rotating arms.
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Figure: Cross sectional view of the single shaft
compulsory power mixer.
(Amubuja Cement Literature Vol.no.12)
Figure: Twin shaft compulsory power mixer
(Amubuja Cement Literature Vol.no.12)
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c) CONTROL SYSTEMS
Figure: a) Fully automatic control panel of a modern
mixing plant.
b) Microprocess Control System on the modern
mixing plant.
(Amubuja Cement Literature Vol.no.12)
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d) TRANSPORTATION EQUIPMENT: - TRANSIT MIXER
Normal capacity 4 to 12 CuM
Hydrulic Drive of Mixer Separate engine or driven by truck engine
Water tank capacity 192 to 2000 litres
Mixer trucks Twin axles for 4 CuM capacity
Three axles for 6 to 7 CuM capacity, three/four
axles for 8 to 10 CuM capacity, Semi trailer
truck for 10 to 12 CuM.
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SAMPLING OF CONCRETE As per IS 4926-2003 After the truck-mixer has re-mixed
its delivery on site allow at least the first one-third of a m3
of concrete to be discharged prior to taking any samples.
Take at least 4 incremental samples from the remainder of
the load avoiding sampling the last cubic metre of
concrete.
As per BS 1881: Part 101 & Part 102, describe the
recommended sampling methods for ready mixed concrete
in British code. Using a standard scoop, this can collect
about 5kg of normal weight concrete.
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SAMPLING OF CONCRETE BS1881: Part 102(The Alternative Method) An initial discharge of at least 0.3 m3 is made before
a sample of six scoopfuls is collected from the moving
stream; The sample is then re-mixed on a non-absorbent
surface and split into two equal parts. Each part is then
tested or slump, with the average of the two tests recorded
as the test result.The Alternative method of collecting
sample
(CE&CR, June 2001, pg. no. 35)
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TESTING OF CONCRETE COMPRESSIVE TEST
As per IS 456 -2000, a result is the average of the
compressive test specimens made for the same sample of
concrete. Individual variation should not be more than 15
Percent of the average.
SLUMP TEST
According to ASTM C94-94, the time during which thecement and moist aggregate are allowed to remain in
contact is limited to 90 minutes; BS 5328: part 3: 1990
allows 2 hours.
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The 90 minutes limit can be relaxed by the purchaser.With the use of retarding admixtures, the time limit can be
extended to 3 or even 4 hours.
The United States Bureau of Reclamation provides for an
extension of 3 to 6 hours in the time of contact between
cement and wet aggregate in transport prior to mixing.
In IS 4926-2003, The general requirement is that concrete
shall be discharged from the truck-mixer within 2 h of thetime of loading. However, a longer period may be
permitted if retarding admixtures are used or in cool
humid weather or when chilled concrete is produced.
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NEED TO BE SPECIFIED FOR
RMC Characteristic strength or grade (N/mm2) Target workability or slump in mm required at site
Maximum water to cement ratio
Minimum cement content
Maximum aggregate size
Type of cement
Mineral admixture and its proportion (Kg/m3) Maximum aggregate size
Rate of gain of strength (for formwork removal or prestressing etc.)
Maximum temperature of concrete at the time of placing (in extreme climatic conditions or in caseof massive concrete pours)
Type of surface finish desired.
Method of placing
Rate of supply desired to match the placing and compaction speed planned at site. Quantity of concrete required.
Lift and lead of concrete transportation and placement at site.
Frequency of concrete testing
Details of materials and their required tests.
Permeability tests required (if any)
Placing of concrete in formwork to be under scope of RMC supplier (if required)
Mode of measurement.
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INFORMATION TO BE
SUPPLIED BY THE PRODUCER Name or number of the ready-mixed concrete depot.
Serial number of the ticket.
Date.
Truck number.
Name of the Purchaser.
Name and location of site.
Grade or mix description of the concrete.
Specified target workability.
Minimum cement content (if specified).
Type of cement and grade (if specified).
Maximum free water-cement ratio (if specified).
Nominal maximum size of aggregate. Generic type or name of any chemical and
Quantity of concrete in m.
Time of loading.
Signature of the plant operator.
A statement warning the purchaser of the precautions needed to be taken when working with cement
and wet concrete.mineral admixtures included.
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CHECKS BY CONSUMER
BEFORE ORDERING THE RMC Reliability of the plant and transit mixers
Calibrations of all measuring devices and their accuracy.
Mode of operation of plant should preferably be fully
automatic and not manual. Quality of materials proposed to be used.
Adequacy of quantity of materials proposed to be used.
Compliance of concrete specifications based on the mix
parameters specified. Adequacy of testing facilities
Time likely to be taken by transit mixers from plant to site andback.
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CHECKS NEEDED AT SITE
PRIOR TO RECEIPT OF RMC Reinforcement layout
Adequacy of formwork
Openings and chutes provided Adequacy of manpower and equipment
Proper approach for transit mixers
Proper platform to receive concrete.
If pumping is proposed, the location of the pumpshould be approachable from both sides
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CHECKS NEEDED AT SITE
DURING CONCRETING Proper co-ordination between the RMC supply and placing and
compacting gangs.
Proper signaling or communication at site is necessary.
Workability of concrete within accepted limits. Adequacy of cohesiveness of concrete for pumpability.
Water addition or chemical admixtures are not added duringtransportation .
Temperature of concrete at the time of receipt at site (ifspecified).
Continuous and steady supply at site and speedy unloading
Retempering should not be allowed.
UNNECESSARY RESTRICTIONS ON
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UNNECESSARY RESTRICTIONS ON
SUPPLIERS OF RMC BY
PURCHASER Insistence on use of cement and admixtures of specific brands
Inappropriately low water to cement ratio
Restriction on use of water reducing admixtures
Insisting on Indian Standard method of concrete mix design
Frequency of testing
Fixed slump insisted upon
Ambiguous specifications
Concrete Field strength should not be less than Target strength
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THE CONSTRAINTS FACED BY
RMC PRODUCERS AT PRESENT High Cost Delayed payments and long credit by consumers
RMC plants in cities are not permitted to be installed inresidential zones.
High cost of the plant and equipment results in high capital
costs. Bad quality of roads and traffic congestion and intermittent
signals often delay the deliveries in metros.
Availability of trained and skilled manpower for operations andmaintenance of plant and equipment.
Delays in placing, compacting and finishing at consumer's endcauses delays in unloading of transit mixer and stiffening of theconcrete mix.
Quality of formwork and its adequacy
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PROMOTING RMC IN INDIA Ban any storage of materials at any construction site.
No public space shall be occupied by building materials during
construction.
Government should come out and insist on RMC and quality.
Transit mixers movements should be allowed through out 24
hrs.
Government should given some taxes & import duties benefit.
The government and local bodies can encourages the use of
ready mixed concrete
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CONCLUSION DURABLE CONSIDERATION
OVERALL ECONOMY
REALISATION OF THE ADVANTAGES BY
ENGINEERS AND CONSTRUCTION INDUSTRY
WIDELY ADOPTED THROUGH OUT THE WORLD
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REFRENCES Kaushal Kishore (June 2001), RMC: Some Technical Aspects,
Civil Engineering & Construction Review.
Sanjay Bahadur(June 2001), RMC- A Non-Negotiable Approach toDurable Urban Infrastructure, Civil Engineering & Construction
Review. Kshemendra Nath (June 2001), RMC- The Need for Partnership,
Civil Engineering & Construction Review.
Danish Rashid (June 2001), RMC: The Indian Answer to WorldClass Construction, Civil Engineering & Construction Review.
IS 4926-2003, Standard on Ready mixed concreteCode ofPractice, BIS, New Delhi.
RMC on the move (Oct. 2003), Ambuja Technical Literature, Vol.No. 90
Mechanisations of concreting, Part I- Batching, Mixing &Transporting (Dec. 1996), Ambuja Technical Literature, Vol. No. 12
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THANK YOU
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