Thursday, July 8, 2021

Concrete Work- Standard Operating Procedures

CONCRETE WORK



Introduction:

This standard operating procedure enumerates the scope of Concrete Works to be executed in the Tower area as specified in the approved drawings and at places when instructed by the Tower-in-charge.

Responsibility:

The respective Tower-in-charge is responsible to execute the work as per work procedure.

Safety & protection:

Ensure the workers use safety appliances like safety belts at height, helmets, safety goggles, safety shoes, etc.

Ensure the working area is kept clean for easy access.

Proper illumination to be provided during the work if required.

Educate the workmen through pep-talk meetings. 
 

Preparatory Works:

Ensure calibration of weigh batcher, water dispenser is done every six months.

The amount of the water added shall be adjusted to compensate for any variations in the moisture content. To allow for variations in the mass of aggregates due to variation in the moisture content, suitable adjustments in the masses of aggregates shall be made. It should be done by QC Department.

Ensure all equipment related to the concrete are available and is in working condition.

Production of Concrete:


Production of concrete should be done as per approved mix design and by weight batching only. In any case, do not allow volume batching.

1st batch of days production should be made in front of Tower-in-charge and should be checked for the required slump. If any variation in produce concrete is observed should be informed to the QC department and after correction batching should proceed.

It is a must to monitor the consistency of the mix in each batch of concrete. The first three batches should be done in front of the Tower-in-charge and then further should be monitored by the concrete supervisor.

The concrete shall be mixed in a mechanical mixer. The mixing shall be continued until there is a uniform distribution of the materials & the mass is uniform in color & consistency.

If there is segregation after unloading from the mixer, the concrete should be remixed.

All Concrete will be conveyed from the mixer and/or Batching Plant to the place of final deposit in buckets, transit mixers, containers, without segregation.

Concrete will be transported with all precautions to prevent loss of water by evaporation in hot weather and heat loss in cold weather conditions. No concrete will be transported and placed in the open, while it rains. 

All equipment used for transporting and placing concrete will be thoroughly cleaned after each period of placement. 

The mix after discharging from the mixers shall be transported by wheelbarrows, buckets, pumps, etc. without causing segregation and loss of cement slurry and without altering its desired properties, with regard to water-cement ratio, slump, cohesion, and homogeneity.


Placing: 


Before placing the Concrete all reinforcements steels, embedded parts, water stoppers shall be correctly placed, aligned, and securely held in positions and formwork fixed. Concrete will be deposited in successive layers in its final position, as nearly as practicable, to avoid segregation due to re-handling. 

The concrete shall be placed & compacted before the initial setting of concrete commences & each layer of concrete shall be compacted fully before the succeeding layer is placed 

For locations where direct placement is not possible and in particular through narrow forms, suitable drop chutes will be provided to confine the concrete in movement. 

Long troughs, chutes, and pipes will be kept free from coatings of hardened concrete. Open troughs and chutes shall be of metal or metal lined. 

Concrete shall be placed carefully to avoid displacement of reinforcements, embedded parts. Concrete shall not be permitted to fall freely from a height of more than 1.5 m or to strike the forms at an angle.


Placement of Concrete by Pumping:

The receiving hopper of the pump shall have a hydraulically/electrically operated remixing blade to keep the concrete mixed continuously and maintain consistency and uniformity.

The pipelines will be made of seamless and high-quality precision steel pipes. At the face of the placing boom, a heavy-duty flexible hose may be attached to facilitate Concrete Pouring. Strong Couplings will be used to clamp both the flexible and rigid pipes in proper alignment. 

Accessories like curved sections of rigid pipes, swivel joints, rotary distributors, pin and gate valves to prevent backflow, air vent for downhill pumping, etc. shall be provided as per the requirements.

At the start of the pumping operation, cement slurry or pumping aid shall be allowed to pass through the pipeline to lubricate the inner surface of the pipes. 

During the placing of concrete first, place the concrete in the column area and then in the beam. Do not allow rollover concrete from beam to column.


Compaction: 

Each layer of Fresh Concrete shall be thoroughly compacted by vibrators to the minimum practicable consolidated volume.

The concrete to be vibrated shall be placed in position in level layers of suitable thickness not greater than the effective length of the vibrating needle. It is advisable to have concrete layer thickness not more than 600mm(in vertical members) for effective vibration.

A vibrator needle shall be inserted vertically inside the Concrete and shall be allowed to penetrate through by its own weight. Vibration shall not be stopped when the needle is immersed in Concrete. 

The vibrator shall penetrate fully into the layer being placed and also penetrate about 100 to 200 mm in the already consolidated lower layer for good bond and homogeneity between the two layers and to prevent the formation of cold joints. The vibrators shall be withdrawn slowly out of the concrete so as to allow the concrete to flow back into the hole behind the vibrator.

The spacing of insertion shall be at intervals of about 450 mm to 700 mm depending upon the mix. The specified spacing between the inserting positions shall be maintained uniformly throughout the surface of the concrete so that the concrete is uniformly vibrated.

The spacing shall provide some overlapping of the area vibrated at each insertion. It is better to insert the vibrating head at a number of places than to leave it at one place for a long time. 

The range of action & degree of consolidation can be recognized from the rising air bubbles & the formation of a smooth textured surface around the vibrating head. 

The vibrator needle shall be kept clear from forms. The vibrator shall be prevented from coming into contact with the reinforcement bars, embedded parts. Over vibration shall be avoided.


Compaction of concrete at junctions with hardened concrete:

The hardened concrete surface shall be prepared by hacking or roughening & removing laitance, greasy matter & lose particles.

The cleaned surface shall be wetted. Cement paste of creamy consistency or Cement  SBR (Nitto Bond) mix shall then be applied to the wet surface of old concrete & fresh concrete vibrated against it. 

Curing of Concrete:

Exposed surfaces of concrete shall be kept in a damp or wet condition by ponding (with vatta of 50mm thick) or by covering with a layer of sacking, canvas, or similar materials. 

The curing shall be started after the initial setting time of concrete & the same shall be continued for a minimum of 10 days. 
The curing water shall be the same as that is used for construction purposes. 

Sampling & strength of concrete:-


Minimum 12 cubes (4 samples) for concrete irrespective of its quantity should be made.

Samples should be distributed over whole concrete batches and should be taken randomly.

3 cubes should be made from each sample by filling the cube in 3 layers and each layer should be compacted with a tamping rod 35 times.

Cubes after finishing should be embossed with binding wire or nail or any suitable mean and should show;

Grade of concrete.
Date of concrete.
Location of Concrete.

Cube Number should be started with 1 and first 3 cubes of number 1,2,3 should be of the same batch and next 4,5,6, number cube should be of the same batch taken at another interval and so on.

Before casting cubes, ensure cube molds are in perfect alignment and oiled when it is dismantled for cleaning.

Note: In all cases, the 28 days compressive strength specified shall alone be a criterion for acceptance or rejection of the concrete.

Frequency of sampling when concrete quantity is more: 

The minimum frequency of sampling of each grade (For RMC):

Quantity of concrete in Cum            Number of SPECIMENS/Grade


1 Cum  to 5Cum                                  6 Nos

6 Cum to 15 Cum                                3 Nos

16 Cum to 30 Cum                             3 Nos

31 Cum to 50 Cum                            6 Nos

51 and above                                    18+ three additional specimens for each additional 50 m3  or part thereof.


Test results of sample: 

The result is for the average strength of three specimens.
The individual variation should not be more than + 15 percent of the average. If more, the test results of the sample are invalid.


Stripping of formwork:

The following striking period shall be adopted when the ambient temperature does not fall below 150C:
1. Vertical formwork to columns, walls, 
2. beams  18  24 hours.
3. Soffit formwork to slabs  7 days.
4. Soffit formwork to beams  14 days.

Records:

Records of concrete poured indicating place, quantity and other particulars will be maintained.

INSPECTION RECORDS:

The following records pertaining to Concrete Works will be prepared and maintained:

 1. Concrete Pour Card  
 2. Post Concrete Inspection 
 3. Test Results of Fine Aggregate  
 4. Test Results of Coarse Aggregate  
5. Sieve Analysis of Fine Aggregate    
6. Sieve Analysis of Coarse Aggregate  
7. Combined grading for All In Aggregate with 20 mm MSA  Batch Weight Field Concrete Data   
 8. Concrete Request/program   
 9. Concrete Dispatch details   
10. Calibration of Weighing Scale  

Concrete surface:

1. The face of concrete for which formwork is not provided other than slabs shall be floated to a smooth finish. The floating shall be done so as not to bring an excess of mortar to the surface of the concrete. 

2. Slab top should be broomed finish.

3. After casting of column and walls, do insert 20mm metal halfway into it to form a construction joint.

4. All Concrete surfaces which receive plaster should be hacked properly with hacking tools.




Team
CBEC India

2 comments:

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