USEFUL GUIDE FOR CONCRETING

METHODOLOGY 

1. INGRADIENTS FOR CONCRETE

     a) 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

b) AGGREGATE

• 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

c) 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

d) 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              

2. MIXING OF CONCRETE

     a) 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:

• 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

3. 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. 

4. 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.

5. TESTING OF CONCRETE

      a) 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.

     b) 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.

TOOLS REQUIRED FOR CONCRETING

The following tools must be available at site to ensure correct work.

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 

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