Concrete quality is crucial in construction due to its prominence in any project. It directly impacts factors like durability, load-bearing strength, and resistance to environmental elements such as wind, snow, and water. Therefore, ensuring high-quality concrete is vital for the construction of buildings and structures. Throughout the concreting process, quality checks are conducted at various stages to maintain desired standards.
Stages of Concrete Quality Checking
- Preconcrete quality checks
- Quality checks while concreting
- Post concrete quality checks
1.Preconcrete Quality Checks
This quality check stage includes two steps. The first step involves verifying specification requirements for excavation, forms, reinforcement, and embedded fixtures. The second step involves conducting control tests on concrete ingredients such as cement, aggregate, and water.
Cement
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.
Read More on quality checks on cement: Tests on Cement at Construction Site To Check Quality of Cement
Aggregate
Concrete aggregates must adhere to specified values outlined in standard specifications. The quality of concrete is influenced by various physical and mechanical properties of aggregates, such as shape, grading, durability, specific gravity, and water absorption. It is crucial to conduct tests on these aggregate properties prior to using them in concrete production. Additionally, the presence of deleterious materials and organic impurities should be assessed. The bulking of sand is also a significant property that can impact the concrete in multiple ways. It can lead to incorrect results when volume batching is performed and increase the water-cement ratio, consequently reducing the strength. To effectively control aggregates:
- Initial testing for source approval is necessary.
- Subsequent daily testing, once or twice, should be conducted to determine moisture content, and adjustments should be made considering the moisture content of aggregates.
Water
Water quality must meet the standards specified for its designated purpose. Chemical analysis is necessary to approve the water source. When dealing with suspended impurities, allowing water to settle is crucial. Doubtful water samples are tested by creating concrete cubes, and the average 28-day compressive strength of these cubes should be at least 90% of the average strength of three similar cubes prepared with distilled water.
2.Checks while Concreting
Careful supervision is crucial for all concrete manufacturing operations, including batching, mixing, transporting, laying, compacting, and curing. Several precautions need to be taken during these operations.
Firstly, the concrete mix should be designed in the laboratory using the materials intended for on-site use. Whenever possible, concrete should be batched by weight. If weight batching is not feasible, volume batching may be permitted under proper supervision with the presence of the engineer in charge.
During mixing, the mixer should be fully charged, and the materials should be added in the correct sequence. The mixer’s speed should range from 15 to 20 revolutions per minute, and the mixing time should not be less than 2 minutes. It is essential to avoid segregation when unloading the concrete from the mixer.
Workability of concrete, a vital property in its fresh state, should be checked using tests such as the slump test or compaction factor test. Approximately three tests should be conducted for every 25 m3 of concrete.
Transportation of concrete should be carried out with care to prevent segregation. Concrete should not be dropped from a height exceeding 1 meter, and chutes should be used if the drop height is greater.
To minimize rehandling, concrete should be placed at its final position whenever possible. Vibrators should be utilized for compacting the concrete, ensuring that the insertion spacing of internal vibrators does not exceed 0.6 meters. The vibrators should be withdrawn slowly to avoid leaving voids in the concrete, and their frequency should be at least 7000 cycles per minute.
Proper curing is necessary to ensure that concrete achieves the required strength. As soon as the concrete hardens, it should be covered with hessian. The curing period should be specified and adhered to.
Formwork should match the final structure and should be inspected before starting concreting. The inside of the forms should be cleaned and oiled, and the forms should be removed after the specified period.
Concrete should be protected from extreme hot and cold weather during its early stages. Concreting should not be performed at temperatures below 4.5°C or above 40°C. In very hot weather, water and aggregates can be cooled, and approved retarders may be used. In very cold weather, water and aggregates should be heated, and approved accelerators can be employed.
3.Post Concreting Checks
Compression tests are performed on concrete cubes after laying and compacting. For ordinary concrete, cubes are made on-site. The dimensions, shape, and sizes of hardened concrete are checked against design specifications. The general surface appearance is also assessed. Reinforcement should have sufficient concrete cover, and any visible reinforcement in a structure must meet the specifications or be rectified. Concrete strength is typically determined by testing cube or cylinder samples after 28 days. If the strength falls below the specified minimum, several steps can be taken:
- Load testing and measuring deflection or strain: By calculating the concrete strength backwards, the quality of the structure can be determined.
- Cutting cores from the structure for strength testing.
- Non-destructive tests such as the Schmidt rebound hammer or ultrasonic pulse velocity test. These tests provide a rough indication and mainly assess construction uniformity.
- Chemical analysis of hardened concrete.
Important Quality Tests Of Concrete
Various tests are conducted to evaluate the quality of concrete. Among these tests, the slump test is carried out both before the concrete leaves the batching plant and upon its arrival at the construction site. The slump test measures the consistency and workability of the concrete.
Another significant test is the compressive strength test, which determines the concrete’s ability to withstand applied loads. This test is crucial in assessing the structural integrity and durability of the concrete.
The water permeability test is conducted to evaluate the concrete’s resistance to water penetration. It helps determine the concrete’s ability to prevent water ingress, which is vital for maintaining its long-term performance and preventing deterioration.
The rapid chloride ion penetration test is performed to assess the concrete’s resistance to chloride ion penetration. Chloride ions can cause corrosion of reinforcement, so this test is important in evaluating the concrete’s durability in chloride-rich environments.
Water absorption test is conducted to measure the amount of water absorbed by the concrete. It provides valuable information about the porosity and permeability of the concrete, which affects its durability and resistance to external elements.
Lastly, the initial surface absorption test is carried out to determine the rate at which water is absorbed by the concrete’s surface. This test helps assess the concrete’s surface quality and its ability to resist the ingress of water and other substances.
These tests play a crucial role in evaluating the quality and performance of concrete, ensuring its suitability for various construction applications.