CC 203
CONCRETE TECHNOLOGY
TOPIC 2
CONCRETE WORKS AT THE
CONSTRUCTION SITE
Lecturer name:
SAPLIZA BIN MAHMUD
SUB TOPIC 2
Factor that cause of occurrence material
segregation and bleeding.
Fresh concrete.
Concept of hard concrete
Preparation of concrete at site.
Concept of transporting
concrete
Method of compaction concrete
Concept of casting and placing concrete.
Method of curing concrete.
Standard laboratory testing on concrete
mixture
SUB TOPIC 2.1
Factor that cause of occurrence material segregation and
bleeding.
- Describe the segregation and bleeding of
concrete.
- Show the segregation and bleeding of
concrete mix.
- Effect of segregation and bleeding on
quality of concrete.
Segregation Separation of the constituent materials of concrete.
Source - over limit of compacting
Can divide by 3 types cause :
a) Coarse aggregate separating out or settling down from the concrete mix.
b) Paste separating away from coarse aggregate.
c) Water separating out from the material being of lowest specific gravity.
Coarse
aggregate
falls to
bottom
Segregation on site
Bleeding
Bleeding in concrete is sometimes referred as water gain
It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete.
Source – over limit of compacting
Bleeding on site
Effect of segregation and bleeding on
quality of concrete
a) Strength of concrete
b) Surface cracking.
c) Bonding of material in concrete mix
SUB TOPIC 2.2
Fresh concrete
- Relate the workability of wet concrete with
water cement ratio.
- Testing on workability of concrete
a) Slump test.
b) Vebe test.
Relate the workability of wet concrete with
water cement ratio.
One of material in concrete mixture.
If volume of water are over limit the strength and density of concrete is reduced.
If volume of water are low limit the concrete will be difficult to do and the hydration process is not perfect.
Slump Test
Purpose :
To determine the workability of concrete at site. ( BS 1881: Part 102:1983 )
Apparatus :
Cone and base plate.
Circular tamping rod 12mm.
Ruler steel and concrete mixing equipment.
Concrete.
Procedure:
Prepare materials for the concrete mix according to the mix ratio as directed.
Clean the surface of the cone, then lay on a flat
base plate.
Include the concrete mix into the cone of 3 layers and each layer compacted by 25 times using a tamping rod stuffing until full.
Flatten the top and clean the concrete surface.
Slowly and carefully remove the cone by lifting it vertically and place the cone next the concrete mix.
Place the rod horizontal at cone and concrete slump.
Measure and record the distance slumps the concrete.
Repeat the testing with add water cement ratio.
No. of testing 1 2
Mix Ratio 1 : 2 : 4 1 : 2 : 4
Mass of materials ( kg ) 28 kg 28 kg
Volume of water ( liter ) 2.3 L 2.8 L
Height of cone ( mm ) 300 mm 300 mm
Height of slumps ( mm ) 275 mm 245 mm
Distance of slumps ( mm ) 25 mm 55 mm
* Range BS( Limit slump 0 – 150 mm )
Data result:
b) Vebe test
Objective : to determine the workability of fresh concrete
Apparatus
Concrete
Vebe consistometer mechine
PROCEDURE i) A conventional slump test is performed, placing the slump cone inside the cylindrical part of the consistometer. ii) The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot. iii) The electrical vibrator is switched on and a stop-watch is started, simultaneously. iv) Vibration is continued till the conical shape of the concrete disappears and the concrete assumes a cylindrical shape. v) When the concrete fully assumes a cylindrical shape, the stop-watch is switched off immediately. The time is noted.
SUB TOPIC 2.3
Concept of hard concrete.
- Criteria of hard concrete
a) Compression strength
b) Tensile / bending strength
c) Durability ( Ketahanlasakan )
d) Permeability ( Resapan )
e) Resistant to chemical reaction
f) Changing of shape
g) Shrinkage
h) Creeping ( rayapan )
i) Fire resistant
a) Compression strength
Most important property of hard concrete.
Generally considerad for the design.
For ordinary construction compressive strength in a range of 20-40 MPa.
Low range (cast in-situ), High range (precast)
Testing concrete prepared in standard cube 100 or 150mm for 7,14,21 and 28 days.
b) Tensile / bending strength.
Should be high to resist cracking from shrinkage and temperature changes.
Measured using one or more for
i) Direct tension test
ii) Split cylinder test
iii) Flexural test.
c) Durability ( Ketahanlasakan )
Concrete should be design to serve its purpose
Factors that influence durability can be divided into:
the environment and usage factors (outside control)
the composition and production factors
d) Permeability ( Resapan )
The low permeability of concrete is essential to increase the resistance properties of cooled action, chemical action and protects the place of steel corrosion occurs.
To obtain a low permeability, the concrete should be fully compacted and the preservation of good done.
Permeability can also be reduced by using low water content and increase the cement content.
e) Resistant to chemical reaction
Portland cement concrete is often attacked by acids such as organic acids and other food processing, especially when the work performed.
Chemical elements are commonly attacked concrete sulphate salt present in the soil and in sea water.
Resistance properties of concrete sulphate attack depends on the type of cement used, the cement content and water cement ratio.
f) Changing of shape
Elastic deformation that occurs at that time and depending on the value of applied stress.
Drying shrinkage that occurs in a long time and does not depend on the stresses in the concrete.
Creep occurred in the long run, but depending on the value of the stresses on the concrete.
As other materials, the concrete will also experience expansion and contraction due to temperature changes.
g) Shrinkage
Shrinkage of concrete depends on the amount of drying that occurs.
Thus it is affected by humidity and ambient air temperature, air flow rate on the surface and the rate of surface area and volume of concrete.
In normal conditions, contraction was influenced by the amount of available water in the concrete during the mixing and the cement content.
For a small concrete, usually assumed that half of the drying occurs in one of the first month and the balance within six months thereafter.
h) Creeping ( rayapan )
Creep of concrete depends on the value of the stresses in it.
For stresses up to 1 / 3 of the cube strength is considered to be directly proportional to the creep stress.
Creep is affected by temperature and moisture environment, the cement content in concrete, water cement ratio, cement type and nature of aggregates.
The mass of concrete also has some influence on the creep, but much less than its effect on contraction.
The parameters used to evaluate the creep is the ratio of applied stress on concrete cubes at one point of time.
i) Fire resistant
The combustion temperature is below 400[degrees] F (200[degrees] C). At this temperature, only the free water in the concrete will be lost, and the strength loss is minimal. Mineralogical changes in hydrated cement occur at higher temperatures. The most important factor is consider is the selection of aggregate. The differential thermal movement between the cement paste and the aggregate is what can cause damage. Quartzite aggregate is the most prone to fire damage by cracking through the quartzite aggregate and bond failure between the cement paste and the aggregate. Limestone aggregate exhibits better fire resistance when exposed to low temperature fire. Lightweight aggregate also performs well. For this situation, a 6 1/2-sack lightweight concrete should provide all the heat resistance needed.
At higher temperatures (above about 800[degrees]F), high-heat-resistant concrete is generally needed. Light-weight aggregates have proven to be more heat resistant and so that is the first step--use lightweight. When temperatures get extremely high, above 1000[degrees]F, calcium aluminates cement produces a fire-resistant concrete.
SUB TOPIC 2.4
Preparation of concrete at site.
- The batching method of concrete.
a) Volume method.
b) Weigh method.
- The mixing method of concrete.
a) Manually.
b) Machinery.
The batching method of concrete.
a) Volume method.
using the measuring box and the conventional method, which has long been used to work at construction site.
This method can not be used for work that requires accuracy because of the volume of concrete depends on its density.
The ratio of each substance is calculated based on the volume of concrete.
volume of cement is calculated based on the volume of one bag / weight of cement and sand as well as for coarse aggregate measured by volume of the same bag
Density of cement ( BS = 1442kg/m2 ) to determine the measure of box :
1bag cement = 50kg/ 1442kg/m3
= 0.035 m3
The measuring of box is
( 0.33m x 0.33m x 0.33m = 0.035m3 )
b) Weigh method.
using the principles of the scales.
suitable for control purposes in the laboratory for testing purposes.
Example:
Mix ratio (1:1.9:3.8 ) and W/C = 0.5
1 bag cement x 50kg = 50kg
1.9 x 50kg fine agg. = 95kg
3.8 x 50kg coarse agg. = 190kg
Volume of water = 25L
The mixing method of concrete.
a) Manually.
Mixing by hand.
Can’t to used or efection for the high workability.
Mixing for are small quantity mixture.
Manual method on site
b) Machinery.
i )Types T – drum are lean
This machine are used drum for mixing the materials of concrete.
When mixing are finish the concrete will be throut.
The shape of machine is cone and spin in one direction.
The angle of casting is 400 to 600 .
ii )Types NT – drum are not lean ( Cylinder )
This machine shape like cylinder and same are machine Types T.
The drum spin in vertical part.
The concrete mixing well throut at bottom part of machine.
SUB TOPIC 2.5
Transporting method of concrete.
- Definition of ready mix concrete.
- Transporting method of concrete on site.
a) Wheelbarrow.
b) Chute.
c) Pump.
d) Crane.
Ready mix concrete
Ready-mix concrete is a type of concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a work site, by truck mixers.
Transporting method of concrete on site.
a) Wheelbarrow
Used for concrete transporting in small quantity.
Used to transport the concrete materials such as cement and aggragate.
b) Chute.
Tools to transporting concrete from height to the lower place.
Made from PVC pipe , wood and other materials.
Chute limited for 3m and diameter is 300mm.
c) Pump.
fast method and economical if careful planning.
using pipe ɸ 6” to pumped from the mixing place.
used for large quantities concrete.
can carry as long 30m – 50m vertically and 300m – 500m horizontal direction.
have two type is plunger pump and press pump.
used in difficult to utilise lorry and trasport machine.
d) Crane.
Have two types:
i) mobile crane.
ii) tower crane.
i) Mobile crane
Used for project large and in the construction of high structures less than 30m.
The concrete include in large container have a trap door at bottom.
ii) Tower crane
Used for building higher than 30m
Use skrip depend on load of size
Size of skrip is 1394kg @12.57m3
If the concrete placing is high, a large of skrip should be used to make it more economical.
SUB TOPIC 2.6
Casting and placing of concrete.
- Method of casting and placing for
concrete.
- Procedures of casting and placing on site.
Method of casting and placing
Casting
Space to casting the concrete in formwork at each structure member much be less than 1.2m for avoid the segregation cause.
Must be casting the concrete layer by layer.
Placing
Must be placing early after mixing with water.
Maksimum times is 11/2 hour.
Procedures of casting and placing on site.
Concrete mix must be near at site.
Must be placing in a uniform layer by layer for avoid separation materials.
For structure concrete wall and column must be placing in layer thickness 450mm.
Concrete must be quickly as possible to prevent from hardening.
Make sure every layer must be compact before the next layer.
Avoid for cold joint.
Avoid bar reinforcement move during the casting of concrete work.
51
Structural Assessment & Repair
Cold Joints
Cold joints are places of discontinuity within a member where concrete may not tightly bond to itself (old and new layer)
To achieve proper bond and water-tightness, the surface of hardened concrete must be free of dirt, debris, and laitance
New Placement on
Hardened (or Initial
Set) Concrete.
Cold
Joint
Laitance
Hardened
Concrete
SUB TOPIC 2.7
Compacting method of concrete.
- Method of compacting for concrete.
a) Steel rod.
b) Vibrator machine. ( internal , external
and platform )
- The advantages and disadvantages of
compacting concrete using vibrator and
steel rod.
Method of compacting for concrete.
a) Steel rod
Using for laboratory testing.
Diameter steel rod 12mm.
b) Vibrator machine. (internal , external
and platform)
Internal vibrator
Widely used by electric or diesel power.
Compaction of this type has different sizes of vibrating pins 10mm to 25mm.
Must be done carefully so that the reinforcement in the mold is not move or damaged.
Internal vibrator
External vibrator
is done outside the mold side of the concrete until the mixture became compressed.
Usually it is used in workshops and performed by the power to make beam, piling, box culvert.
External vibrator
Platform vibrator
is a table supported by a number of springs that run by electricity which allows the face of the vibrating table.
usually used in workshops to make the precast concrete.
Concrete parts shall be made in formwork and placed on the table to vibrator for compact.
Platform vibrator
SUB TOPIC 2.8
Method of curing concrete.
- Purpose of curing.
- Method of curing concrete.
a) Covered with wet materials( sacks, plastic, wood dust ) .
b) Flooding or spraying with water.
c ) Sweeping with chemicals.
d ) Using a vapor steam.
61
Methods of Curing
Water Spraying/Ponding
Must be carried out thoroughly. A continuous fog spraying or sprinkling of water is required
Ponding is accomplished by building earth or sand dikes around the perimeter of the concrete slab to retain a pond of water within the enclosed area
The water needs to be at a temperature close to that of the concrete surface
Maintain presence of water
62
Wet Covering
63
Maintain presence of water Wet Covering
Surfaces can be effectively cured by using hessian or other absorbent materials, providing they can be kept continually wet and are uniformly positioned
Waterproof Paper
Can also be used on unobstructed horizontal surface
Polythene Sheeting
Can be used similar to waterproof paper
White sheeting will reflect the sun’s rays and will be useful in warm weather
Polythene sheeting can cause discoloration or mottling of the hardened concrete
64
Plastic Sheets
65
Curing of Concrete
Gunny Sack
66
67
68
Prevent loss of mixing water
Spray-on-membranes Liquid membrane-forming materials containing
waxes, resins, etc. are applied by spraying, brushing or rolling on horizontal or vertical surfaces to retain moisture or retard evaporation
The material should be sprayed evenly as soon as the free water has evaporated
Curing compounds may prevent a bond between the hardened concrete and any screed or wet concrete applied later
69
Accelerate Hydration Use of Heat
Curing time can be reduced in a number of different ways, including space heating, electrical heating and steam curing
Space heating
area ready for concrete can be temporary enclosed by the use of screens, tarpaulins or plastic sheeting.
This enables oil or electric space heaters or fires to provide and maintain in this limited area a temperature which will keep the concrete at least above 5oC
70
Accelerate Hydration Electrical curing
Replacing braziers as a form of accelerated curing, primarily in precast factories, to assist in the reduction in the number of moulds
Low voltage electricity is used and by embedding wire heater elements between plywood laminations, thermal efficiency is high and the method clean and easy to use
Steam curing
Favourable method of curing for precast and concrete masonry block
Structural members are covered with tarpaulins and pipes fed underneath conveying moist hot steam
71
Curing in Hot Weather
In hot weather, exposed concrete must be kept cool, both during mixing and throughout the curing period
Problem in hot weather concreting due to the faster rate of hydration and increased rate of evaporation (concrete temperature, relative humidity, wind velocity, and air temperature)
When evaporation is not controlled, concrete are prone to thermal cracking
Lowering the temperature of coarse agg. decrease the concrete temperature
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Cont.
The common methods for cooling concrete in hot weather application are:
Cooling the mixing water with chipped ice
Shading the aggregate storage and water tank
Painting the truck mixers white
Avoidance of overmixing
Keeping the humidity high and air temperature low with fog sprays around the area where concrete is placed
Use retarder in the mix
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Curing Period
Curing period cannot be prescribed in a simple way but,
If the temperature is above 10oC, ACI 308-92 lays down a minimum of
3 days : RHPC (type III)
7 days : OPC (type I)
14 days : LHPC (type IV)
BS 8110:Part 1:1985 lays down the normal curing periods for different cements and exposure conditions