Concrete is a fundamental building material used in various structures and projects, from residential buildings to bridges and roads. As such, it is essential to ensure that concrete is of high quality and meets specific standards to guarantee the safety and longevity of the structure. One way to assess the quality of concrete is to measure its compressive strength, which is the maximum amount of load it can withstand before failing. Concrete testing cubes are commonly used to determine the compressive strength of concrete, and understanding their use and testing procedure is crucial for ensuring the reliability and safety of concrete structures.
What is the compressive strength of concrete?
Compressive strength is a critical property of concrete that determines its ability to withstand pressure on its surface without cracking or deflecting. This property is often referred to as the characteristic strength of 150-mm cubes of concrete after 28 days, which is the standard testing procedure for measuring the compressive strength of concrete. In simple terms, this means that samples of concrete are compressed using a machine to determine the maximum load they can withstand before breaking.
Significance of compressive strength of concrete
When designing and constructing structures like buildings, bridges, and highways that are meant to sustain enormous loads or pressure, the compressive strength of concrete is a vital factor to take into account. In most cases, it is expressed as a measurement in terms of units of force per unit area, such as megapascals or pounds per square inch.
Understanding the compressive strength of concrete is crucial for ensuring the safety and longevity of structures. It allows engineers and builders to accurately predict the maximum amount of pressure that concrete can withstand and design structures accordingly. This, in turn, helps to prevent costly and dangerous failures that can result from inadequate design or construction.
Which factors affect the compressive strength of concrete?
The compressive strength of concrete is influenced by several factors, and it is essential to understand them to achieve the desired strength and quality of concrete.
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Coarse aggregate
Coarse aggregate is a crucial component of concrete, and it has a significant impact on its compressive strength. Initially, it was believed that increasing the size of coarse aggregates would result in higher compressive strength. However, research has shown that while larger aggregates initially increase strength, they also decrease it exponentially over time. The primary reason for this decrease in strength is the reduced surface area available for bonding between the cement matrix and the aggregates. Therefore, it is essential to select the optimal size of coarse aggregates to achieve the desired compressive strength of the concrete.
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Air entrainment
Air entrainment is the process of adding microscopic air bubbles to concrete intentionally. This technique was initially developed to prevent damage from freezing and thawing in cold climates, but it has other advantages as well. It increases the workability of concrete at a lower water/cement ratio, which can result in lighter concrete with higher compressive strength. Air entrainment also improves the durability and resistance of concrete to chemical attacks, erosion, and abrasion.
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The ratio of water to cement
The water-to-cement ratio is a crucial factor that can significantly impact the compressive strength of concrete. Cement is the primary binding material in concrete, and it requires water for hydration. However, excess water can lead to significant reductions in the compressive strength of concrete. This is because the water in the concrete matrix dries, leaving interstitial gaps between the aggregate and cement grains. These spaces can transform into cracks during compressive strength testing, leading to a decrease in strength. Therefore, it is crucial to maintain the optimal water/cement ratio to ensure that the concrete has the desired compressive strength and meets the required quality standards.
Which test is used to determine the compressive strength of concrete?
A concrete testing cube is a small cube-shaped sample of concrete that is used to determine the compressive strength of concrete. The concrete testing cube is a small sample of concrete that is representative of the concrete used in a construction project. By subjecting the concrete cube to a compression test, the maximum compressive load that the cube can withstand before it fails can be determined. This load is then used to calculate the compressive strength of the concrete.
Concrete testing is an essential tool in the quality control and quality assurance processes for construction projects. It allows for the testing of concrete strength in a controlled environment, which can help prevent problems that could occur in the actual structure during its service life.
How to measure compressive strength with a concrete testing cube?
To measure the compressive strength of concrete using a concrete testing cube, the following steps can be taken:
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Apparatus for cube test
The apparatus required for the cube test includes a compression testing machine. This machine applies a load gradually on the concrete cube until it fails. The machine must be calibrated regularly to ensure accurate results.
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Concrete cube preparation specimen
The concrete used for the test specimen should have the same proportion and materials as the concrete used in the field. For cube testing, 15 cm size of 6 cubes size mix M15 or above is taken.
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Mixing of concrete for cube test
The concrete for the cube test can be mixed either by hand or by a laboratory batch mixer. In the non-absorbent and waterproof platform, cement and fine aggregates are mixed until the mixture is thoroughly blended and is a uniform colour. The coarse aggregate is then added and mixed until it is uniformly distributed through the batch. Water is added to achieve the desired consistency, and the concrete is mixed until it appears to be homogeneous.
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Procedure to make cubes for test
The mould used for cube testing should be cleaned and oiled. Approximately 5 cm thick layers of concrete are filled in the mould. Each layer is compacted with not less than 35 strokes per layer with a tamping rod. The top surface is levelled and smoothed with a trowel. The cubes are then stored in a moist environment for 24 hours.
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Curing of cubes
After 24 hours, the cubes are removed from the moulds and marked, and then submerged in clean fresh water. The temperature of the water must be maintained at 27 ± 2 degrees Celsius. The cubes are kept in the water for the specified curing time.
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Concrete cube test procedure
After the specified curing time, the cubes are removed from the water, and excess water is wiped off the surface. The dimensions of the cubes are then measured to the nearest 0.2 mm. The bearing surface of the testing machine is cleaned, and the cube specimen is placed on the machine in such a way that the load is applied to the opposite sides of the cube cast. The specimen should be aligned centrally on the base plate of the machine, and the movable portion should be rotated gently with hands so that it touches the top surface of the specimen.
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Loading procedure
The load is applied gradually and without any shock at the rate of 140 kg per cm2 per minute continuously until the specimen fails. The maximum load is recorded, and any unusual features in the type of failure are noted.
By following these steps, the compressive strength of the concrete can be accurately measured with a concrete testing cube.
FAQs
What is a concrete testing cube?
A concrete testing cube is a standardised specimen made of concrete used for measuring its compressive strength.
Why is compressive strength important in concrete testing?
Compressive strength is important in concrete testing as it indicates the ability of concrete to withstand loads or stresses without breaking or cracking.
How are concrete testing cubes made?
Concrete testing cubes are made by mixing concrete, pouring the mixture into cube-shaped moulds, compacting the mixture, and curing it for a specific period of time.
What is the procedure for testing compressive strength using a concrete testing cube?
To test the compressive strength of a concrete testing cube, it is placed in a compression testing machine and gradually loaded until it fails. The load at failure is divided by the cross-sectional area of the cube to calculate the compressive strength.
What factors can affect the compressive strength of concrete?
The compressive strength of concrete can be affected by several factors, including the water-cement ratio, type of cement, type and amount of aggregates used, curing conditions, and the age of the concrete.
