CONCRETING
6.1. KNOWLEDGE OF ACTIVITY
A. INGREDIENTS
CEMENT
Use appropriate grade of cement as recommended by the consultant and confirming to
IS 456:2000.
Manufacturers test certificate should accompany each consignment and cement should
be consumed within 1 month of their manufacturing date.
Cement should be stored properly in stacks not more than 10 bags, clear from the walls
by 1 metre and protected at all time from moisture. However, do not wrap the bag with
polythene sheets, sweating may occur.
Identify each lot/ consignment of cement and follow a first in first out (FIFO) method of
issue of bags.
Each consignment of cement should be tested in-house for consistency, setting time, and
strength.
AGGREGATE FOR CONCRETE
For RCC works coarse aggregates confirming to IS 383 and IS 2386 having maximum
size of 20mm and down.
It should be obtained from crushed granite, trap, and basalt quarry. It should be
chemically inert, rounded or angular in shape and free from dust, foreign matter and not
thin porous, laminated or flaky.
Fine aggregate shall be gritty sand with FM range between 2.6 to 3.2, silt content should
not be more than 5% and there should be no traces of salt. Aggregate with specific
gravity less than 2.6 should not be used.
If dirty, wash sand before use. Sand should be clear of organic material
WATER
Water should be clean, fresh and free from oil, acid, alkali and organic matter
Potable water is considered good for concreting and curing.
IS 456:2000 gives permissible limits of solids.
ADMIXTURES
Use of admixture/ additive should be done only after obtaining technical clearances from
technical consultant
Dosage, point of application, the desired results should be clearly understood.
Accelerators, retarders, plasticizers, integral-waterproofing compound are the additives
commonly used.
Expiry date of the chemical should be checked before use.
A) MIXING OF CONCRETE
IN-SITU MIXING
Ensure adequate stock of all ingredients of concrete i.e., cement, aggregate, water and
any admixture if used for the day’s work
Use calibrated farma (calibration done once in a month) for measurement or in case of
weigh batching, use a weigh batcher machine.
Charge the rotating drum with 25% of required water to rotate 3-4 times.
Generally 20 to 25 litres is recommended for 1 bag of cement. The water
cement ratio is kept 0.4 to 0.5. The moisture content of aggregates has to be checked at
frequent intervals
Load the hopper with ½ the coarse aggregate followed by ½ the fine aggregate. Above
this the entire quantity of cement should be spread. Then half the quantity of fine
aggregate followed by balanced coarse aggregate should be loaded. This type of
sandwiching is done to avoid spilling of cement while discharging into the drum.
Immediately after loading the drum balance quantity of water is added.
The drum should be rotated for at least 1 ½ to 2 minutes. The rotation of the drum is
about 15 to 20 revolutions per minute.
If plasticizer is used, one litre of water should be held back. Add requisite quantity of
plasticizer to the water and mix it thoroughly. This mixture is poured into the drum after
the drum has rotated for about one minute. The drum should be rotated for at least 1
minute after adding the plasticizer mixture to ensure proper dispersion.
Minimum cement content per Cum of concrete is as given below:
Grade Characteristic Characteristic Minimum
strength after 7 strength after 28 days cement content
days (N/mm2) (N/mm2) per Cum
M10 7 10 220 kgs
M 15 10 15 280 kgs
M 20 13.5 20 320 kgs
M 25 17 25 360 kgs
M 30 20 30 390 kgs
No grade below M 20 shall be used for RCC works.
Concrete should be placed in position and compacted within half an hour of mixing of the
same
Always keep a supervisor on the mixer to monitor the works.
Use large bandlies, wheelbarrows, hoist machine to expedite transporting process. Wet
the surface of the carrier to avoid moisture loss due to adsorption
Be careful to avoid segregation and wastage during transportation. In case of
segregation, remix the concrete before placing.
Concrete by vision should be homogeneous in colour, well graded, fluffy and it should
form a shape of ball when rounded in the hands.
B) BATCHING PLANT MIX (RMC)
Preference for ready-mix concrete from reputed companies possessing automated
batching plant, of at least 30 cum per hour capacity supported by adequate number of
transit mixers.
The logistics should be worked out so as to receive at least 1 load of 6 cum every 10 to
15 minutes.
Check the empty transit mixer for preset concrete in the drum. Preset quantity affects
the quantum of concrete received.
Concrete should be received with a delivery challan and data cycle sheet showing grade,
quantity, water cement ratio, slump, cement content, time of manufacturing, dosage of
additive added, weight of various ingredients of concrete.
Go through the cycle data before accepting the concrete
Do not accept concrete after 3 hours of mixing
Do not recharge the concrete by adding additive, water, cement without prior permission
for the project in-charge
Slump recommended for RMC mix is 100mm to 125mm
Place the concrete pump on horizontal, hard even surface and keep the approach area
free of materials to facilitate easy maneuverability of transit mixers. The approach should
not slope towards the concrete pump.
The horizontal section of pipe should be at least 7 metres before the vertical bend is put.
Reduce the length of pipe to the minimum to reduce frictional losses and excessive strain
on the pump.
The pipeline should be independently supported on vertical segment and not on the
shuttering. The vibrations due to pumping should not affect or displace shuttering works.
Pump rich slurry (one bag) before pumping concrete to lubricate the inner surface of the
pipes.
In case of intermittent supply keep the current transit mixer unloading in progress until
the next mixer arrives at site. This avoids choking of pipeline due to non-pumping for
long duration. Avoid reverse stroke to clear a choke. The fluid gets flushed out and the
problem gets compounded.
Do not bend the flexible end pipe by more than 1350 while concreting.
In constricted areas do not unload directly from the pipe. Use bandly and mumty to shift
and place concrete in such areas.
Be excessively careful while passing the ball while clearing the concrete in the pipeline
after the day’s work is over. Due to negligence somebody could get hurt.
Employ professional hands for connecting pipeline. They will do a neater and faster job
without any safety problems.
Leave the pipeline, under the pump and adjacent areas clean of concrete dumping after
the work is completed.
B. PLACING AND COMPACTION
Do not use chutes longer than 4.5m or inclined at more than 45 to horizontal to pour
concrete.
Place cement concrete to the required depth and vibrate it until entrapped air inside
concrete is released.
Pour concrete first into beams and then over the slab portion.
BOND NEW CONCRETE TO PREVIOUSLY POURED CONCRETE
If the previous pour was less than 4 hours prior, the laitance film and porous layer shall
be removed from surface of previous pour.
If the previous pour was more than 4 hours prior but less than 30 days old observe the
above process and expose aggregate with wire mesh and wash with clean water.
If the joint is more than a month old use bonding agents as per specification of the
manufacturer.
If the beam is more than 500mm deep, concrete it in layers.
Columns, beam, slab level of concrete top should be pre-marked before start of
concreting works.
Check the slab level during concreting with line dori method and also the leveling
instrument.
Use appropriate needles for vibration depending on the aggregate size, reinforcement
spacing and element to concrete. For big footings use 60mm needle, for slabs use 40mm
needle, and for starters and precast lintels use 25mm needle. Vibrators should operate
for a speed not less than 10,000 rpm. Ensure that just adequate vibration is only done.
Over vibration is very bad.
Use wooden runner to ram so as to form slurry on top of the concrete surface.
Clean the shuttering works of all spilled concrete immediately after concreting.
Leave vertical construction joints after consultation with the structural engineer.
Match the vibrators, masons to the rate of concreting.
All surfaces of shuttering shall be moistened before placing of concrete to reduce water
loss due to absorption.
Slab should be sprayed with water after covering with hessain cloth 3 hours after
concreting, i.e. initial setting is over and ensure that hessain cloth will remain wet till
ponding of water.
Pond water to a depth of 25mm using bunds the next day and keep water standing for 7
days at a stretch.
In case ponding is not possible, cover it with hessain cloth and keep it moist throughout.
DO NOT IGNORE CURING OF SLAB TO FACILITATE MARKING OF COLUMNS FOR THE
UPPER SLAB.
The beam bottoms should be cleaned using high-pressure cleaners.
C. COLUMNS, WALLS AND STARTER
Remove the shuttering the next day and wrap with moist hessain cloth after spraying the
column with water to reduce the heat of hydration
The columns and walls should be cured for a minimum of 7 days.
Starters are to be cured for 3 days.
D. TESTING OF CONCRETE
SLUMP TEST
Conduct slump cone test, one during the start, one at midday and one towards the close
of days work.
Again any load under suspect can be checked for slump on case-to-case basis.
CUBE TEST
The sample is made at point of discharge of mixture.
Take 3 samples after 7 days and 3 samples after 28 days strength.
Every 100 cum or 10 batches of concrete (whichever is smaller) should be represented
by a sample of test cubes.
For concrete manufactured at site, there must be representative samples for the day’s
work and for different elements like footing, slab, column and wall.
Test results should be discussed with the project in-charge even if the results are
satisfactory
In case of cube failure, the element to be identified and non-destructive test to be
conducted. Cube results of the RMC supplier for the same batch may be checked.
6.1.1. TOOLS TO BE USED BY TRADESMEN
Required tools must be available at site to ensure correct work. Basic tools of the concreting
gang are:
1. Trowels
2. Shovels
3. Line dori
4. Gum boots
5. Vibrator
6. Dumpy levels
7. Spirit levels 1-3m
8. Measuring tape
9. Masons trowels
10. Floating trowels
11. Screed finishing darby
12. Steel trash tracks
13. Hammer
14. Leveling trowels
15. Wheelbarrow
16. Long handle Aluminium leveler
17. Masons brushes and buckets
18. High pressure water cleaners
6.2. INSPECTION METHODOLOGY FOR QUALITY ASSURANCE
1. Personnel
Ensure all personnel are wearing gumboots and rubber gloves
Ensure personnel at wearing safety belts during concreting at heights or on periphery of
structures
2. Before Concreting
Ensure base plate have been cleaned and sealed with sealing tape so as there is no
leakage of slurry
All reinforcements are in place especially chairs
All cover blocks damaged during reinforcement layout are replaced
All sunken areas (sunken beams, toilets and cutout areas) have been properly cleaned of
dust and oily substances using high-pressure water cleaners
3. During Concreting
Ensure proper grade of concrete as recommended by the consultant is employed
Ensure proper expansion joints as per working drawings and protected
Check slab and beam for parallelity using spirit levels before and after concreting
Check the top surface of fresh concrete for eveness of top surface
Ensure sufficient compaction using correct needle size of vibrator
4. After Concreting
Curing carried as per guidelines
Columns are covered with a damp hessain/ jute cloth for 7 days
Slabs – water level is maintained during ponding
If honeycombing seen after de-shuttering, immediate touch up without delay
Ensure the cube test results are available at the end of the term indicated
6.2.1. TOOLS TO BE USED FOR QUALITY INSPECTION
1. Dumpy levels to check finished slab level
2. Spirit levels 1-3m
3. Measuring tape
4. Plumb bob
5. All related “Good for Construction” drawings
