Quality of concrete construction on site can be accomplished in three distinct stages as follow
Quality control before concreting
Quality control during concreting
Quality control after construction
Stage-1 (Quality Control Before Concreting)
This stage of quality control consists of two steps.
Checking of specification requirements regarding excavation, forms, reinforcement and embedded fixtures etc.
Control test on concrete ingredients (i.e. on cement, aggregate & water)
Quality of cement is ascertained by making compressive strength tests on cement cubes. However for effective control cement:
Should be tested initially once for each source and subsequently once for every two months
Should be protected from moisture
Should be retested after 3 months of storage, if long storage in unavoidable
Should be rejected if large lump are found in cement bags.
Concrete aggregates should confirm to specified values as per standard specification.
The quality of concrete is affected by different physical and mechanical properties of aggregate, i.e. shape, grading, durability, specific gravity and water absorption etc. these properties of aggregated should be tested before using it for concrete production.
The quantity of deleterious materials and organic impurities should also be tested.
Bulking of sand is also an important property in several ways. It gives wrong results when volume batching is done. It increases water cement ratio which in turn reduces strength.
For effective control aggregates:
Are required to be tested once initially for approval of source
Should subsequently be tested once or twice daily for moisture content and allowance should be made for moisture content of aggregates.
The quality of water should be checked for the requirements as specified in respective standard. Chemical analysis shall be conducted for approval of source. In case of suspended impurities, it is necessary to store water for some time to allow them to settle down. In case of doubt concrete cubes made with this water are tested. Average 28 days compressive strength of at least three cubes or cylinders or specified size, prepared with water proposed to be used shall not be less than 90% of the average strength of three similar concrete cubes prepared with distilled water.
Stage-2 (Quality Control During Concreting)
Careful supervision during concrete manufacture is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing. Following precautions should be taken during concreting operation.
The concrete mix should be designed in the laboratory with the materials to be used on site.
As far as possible concrete should be batched by weight. If weight batching is not possible, then volume batching may be permitted through proper supervision in the presence of engineer in charge.
During mixing the mixer should be charged to its full capacity. The materials should be fed in proper sequence. The speed of the mixer should be range from 15 to 20 revolutions per minute. The mixing time should not be less than 2 minutes in any case. Segregation should be avoided while unloading the concrete from the mixer.
Workability of concrete is an important property of concrete while concrete is in its fresh state. Therefore slump test or compaction factor test should be performed to check workability of concrete. About three tests should be carried out for every 25 m3 of concrete.
Care should be taken so that no segregation takes place during transportation of concrete.
Concrete should not be dropped from a height of more than 1 m. if the drop height exceeds 1 m chutes should be used.
To avoid rehandling of concrete it should be placed at its final position as far as possible.
Vibrators should be used for compacting concrete. The insertion spacing of internal vibrators should not be more than 0.6 m. It should be drawn out slowly so that no holes remain in the concrete. The frequency of vibrators should not be less than 7000 cycles/minutes.
Curing should be done for a specified period so that concrete develops requisite strength. Concrete should be covered with hessian as soon as it becomes hard.
The form work should correspond to final form of the structure. It should be checked before concreting is started. The inside of the forms should be cleaned and oiled. The forms should be removed after the specified period.
Concrete should be protected from hot and cold weather at early ages. Concreting should not be done at temperature below 4.50C and above 400 In very hot weather water and aggregates should be cooled. Retarders of approved quality can be used.
In very cold weather water and aggregates should be heated. Accelerators of approved quality can also be used.
Stage-3 (Quality Control After Construction)
Once the concrete is laid and compacted, compression tests are made on the cubes made out of this concrete. For ordinary concrete, cubes are made out of the concrete made at work site.
The hardened concrete has to be checked for trueness in dimensions, shape and sizes as per design specification. General surface appearance of concrete should also be checked. Dimensions are ascertained by different measurements. Reinforcement should have adequate concrete cover and if the reinforcement is visible in part of a structure, the part should be rejected or necessary actions should be taken accordingly.
Concrete strength is normally to be ascertained from cube or cylinder samples tested at 28 days. In case the strength obtained is less than the specific minimum, one or more of following steps may be taken.
Load test and measurement of deflection and / or strain (the quality of the structure can then be ascertained by calculating back the concrete strength)
Cutting cores from the structures and testing them for strength
Non destructive tests like Schmidt rebound hammer or ultrasonic pulse velocity test. These tests give only a very rough idea and are primarily used to ascertain the uniformity of construction.