On-site concrete construction quality can be accomplished in three distinct phases:
- Quality Control Before Concreting
- Quality control during concreting
- quality control after manufacture
first stage (quality control before concreting)
This phase of quality control consists of two phases.
- Checking specification requirements in respect of excavations, forms, reinforcement and embedded fixtures etc.
- Control tests on concrete materials (ie on cement, aggregate and water)
The quality of cement is ascertained by conducting compressive strength test on cement cubes. However for effective control of cement:
- Testing should be done once initially for each source and then once every two months
- should be protected from moisture
- Should be re-tested after 3 months of storage, if long-term storage in unavoidable
- Large lumps found in cement bags should be discarded.
The concrete aggregate must conform to the specified values in accordance with the standard specification.
The quality of concrete is affected by various physical and mechanical properties of the aggregate, i.e. size, grading, durability, specific gravity and water absorption etc. These properties of aggregate must be tested before it is used for the production of concrete.
The amount of harmful substances and organic impurities should also be tested.
Sand deposition is also an important property in many ways. It gives incorrect results when batching volumes. This increases the water cement ratio which in turn reduces the strength.
For an effective control set:
- Needs to be tested once initially for approval of source
- The latter should be tested once or twice a day for moisture content and allowed for moisture content in the aggregate.
The water quality should be checked for the requirements specified in the relevant standard. Chemical analysis will be done for approval of the source. In case of suspended impurities, it is necessary to freeze the water for some time to allow them to settle. In case of doubt, the concrete cubes made from this water are tested. The average compressive strength of at least three cubes or cylinders or specified size prepared with water proposed for use for 28 days shall not be less than 90% of the average strength of three identical concrete cubes prepared with distilled water.
Phase 2 (Quality Control during Concreting)
Careful supervision is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing during concrete manufacturing. The following precautions should be taken during the concreting operation.
- The concrete mix must be designed in a laboratory with the materials used on site.
- Concrete should be batched according to weight as far as possible. If load batching is not possible, volume batching may be permitted through proper supervision in the presence of the Engineer-in-Charge.
- The mixer must be charged to full capacity during mixing. The ingredients should be fed in the proper order. The speed of the mixer should be between 15 to 20 revolutions per minute. The mixing time should in no case be less than 2 minutes. Separation 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, the slump test or compaction factor test should be done to check the workability of concrete. About three tests should be done for every 25 square meters3 Of concrete.
- Care should be taken that there is no separation during the transportation of concrete.
- Concrete should not be dropped from a height of more than 1 meter. If the drop height is more than 1 meter a chute should be used.
- Concrete should be kept in its final position as far as possible to avoid re-handling.
- Vibrators should be used to compact the concrete. The insertion distance of the internal vibrator should not exceed 0.6 m. It should be pulled out slowly so that no holes are left in the concrete. The frequency of the vibrator should not be less than 7000 cycles/min.
- The curing should be done for a specified period of time so that the concrete can develop the required strength. Once the concrete has hardened, it should be covered with a hessian.
- The form should correspond to the final form of the work structure. This should be checked before starting the concreting. The inside of the forms should be cleaned and oiled. The forms should be deleted after the specified period.
- Concrete should be protected from hot and cold weather at an early age. Concreting 4.5. should not be used at temperatures lower than0C and 40. at over0 Water and aggregates should be cooled in very hot weather. Retarders of approved quality may be used.
- Water and aggregates should be heated in very cold weather. Accelerators of approved quality may also be used.
step 3 (Quality control after manufacture)
Once the concrete has been laid and compacted, compression tests are performed on the cubes made of this concrete. For ordinary concrete, cubes are made from concrete formed at the work site.
Hardened concrete should be checked for correctness in dimensions, shapes and sizes as per the design specification. The general surface appearance of the concrete should also be checked. Dimensions are ascertained by various measurements. The reinforcement should have adequate solid cover and if the reinforcement is visible in the part of the structure, the part should be rejected or necessary action taken accordingly.
The strength of concrete is generally determined from cube or cylinder samples tested in 28 days. If the received power is less than the typical minimum, one or more of the following steps may be taken.
- Load testing and measurement of deflection and/or stress (the quality of the structure can then be ascertained by calculating the strength of the concrete)
- Cutting cores from structures and testing them for strength
- Non-destructive tests such as Schmidt Rebound Hammer or Ultrasonic Pulse Velocity Test. These tests only give a very rough idea and are mainly used to ascertain the uniformity of construction.
- Chemical analysis of hardened concrete.