What Is Quality Control Of Concrete?
Concrete is one of the most important and widely used materials in the construction industry. Concrete consists of raw materials including cement, sand, aggregates, admixtures, and water. All these raw materials are mixed up in a certain proportion to form concrete, according to the requirements.
Concrete is widely used for the construction of elements like columns, beams, walls, lintels, slabs, copings, and foundations so on. The prepared concrete, as a material, should maintain a certain level of quality standards to get accepted for its utilization for the construction of various elements of concrete.
The quality of concrete should be checked, maintained, and also should be corrected if the level of quality standards does not match the required level of quality. If the quality of concrete is inferior, it may have an adverse effect on the construction.
The quality of concrete is checked in three diﬀerent stages of construction. They are as follows:
- Quality control of concrete before concreting,
- Quality control of concrete during concreting, and
- Quality control of concrete after concreting (or after construction of an element).
Let us study all the three stages of quality control of concrete in detail.
Stage 1: Quality Control Of Concrete Before Concreting
Before the concrete is even prepared, the quality of raw materials shall be checked, and only then they are mixed in a required proportion to form fresh concrete. The quality of raw materials is directly related to the quality of concrete.
The quality control of concrete in this stage is done in two diﬀerent steps. They are as follows:-
Step -1: Test for quality of materials used in concrete
Step – 2: various tests on fresh concrete for its quality check.
Step – 1: Quality Tests Of Concrete Materials
Concrete is made up of cement, sand, aggregates, and water, respectively. So it is necessary to have a check for the quality of all of these materials.
The quality of cement is usually diagnosed by carrying out tests for compressive strength on cement cubes. There are a few points which are important and should be taken care of to control the quality of cement before the commencement of concreting. They are:-
Cement should be tested before it’s added to the concrete mix.
The cement should be subsequently checked once every two months.
Cement should be kept far away from moisture.
Cement should be retested after every three months if it is stored for a longer period.
If large lumps of cement are found in the cement bags, it should not be accepted.
Aggregates (Fine and Coarse)
Aggregates have diﬀerent properties such as shape, size, grades, water absorption, durability, specific gravity, etc. All of these properties should be checked for the sample of aggregates which are to be used while preparing the concrete. Also, impurities are to be removed before mixing them into the concrete mix.
Some points to remember while checking the coarse aggregates.
The coarse aggregates having a rough surface are preferable over the smooth aggregates for better bonding.
The aggregates should be hard and strong enough to provide proper strength to toes concrete.
If the aggregates are submerged underwater for 24 hours, the water content should not exceed 10%.
The quantity of flaky and elongated aggregates shall be equal to or lesser than 35 percent of the total quantity of aggregates in the concrete.
20 mm and 10 mm aggregates shall be taken in the ratio of 70:30 or 60:40 to allow better compaction and to achieve a higher density of concrete.
Some points to remember while checking the quality of sand or fine aggregates.
Sand used for making concrete shall retain on 150-micron sieve and should pass 4.75 mm sieve.
Sand should have a lesser amount of clay in it, as the presence of clay makes the sand cohesive. The amount of clay should be less than 8%.
Sand containing more than 5 % moisture is not preferable for concrete.
Water is an important element of concrete. The quantity and quality of water decide the quality of concrete. The quality of water should be checked for:
- Chemically harmful substances.
- Impurities in water.
To ensure the quality of water, at least three cubes of concrete should be cast using that particular sample of water, and the compressive strength of concrete after 28 days should be measured. If the compressive strength is less than 90 %, it is advised to reject the sample of water.
Stage 2: Quality Control Of Concrete During Concreting
For quality control of concrete during concreting, the following points are kept in mind:
The design mix of concrete should not diﬀer from the mix design given in the specification.
The ingredients of concrete in a concrete mixture should be properly mixed.
Approximately 15 to 20 revolutions of mixture machine are recommended to give them a proper mix.
It should be taken care that segregation should not take place.
Slump test shall be conducted after every 25 m3 of concreting done, to ensure the perfect workability of concrete.
The dropping height of concrete should not exceed 1m, to avoid segregation.
Once the concrete is poured, proper vibration shall be done so that the voids in the concrete are removed.
After the hardening of concrete, proper curing for 7 days should be done.
Stage 3: Quality Control Of Concrete After Concreting
After the concrete is hardened, the quality of concrete is tested in the following ways:
Chemical tests and analysis of concrete are done.
Core is obtained from the concrete element and they are tested for the strength of concrete.
Non-destructive test like rebound hammer test is performed on the concrete structure.
The concrete structure is tested for its dimensions and shapes.
Also, a check for clear cover is done. If, in some parts, the reinforcements are visible, that particular part of the structure is rejected.
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