An Overview of Evaluating the Compressive Strength of Concrete
Concrete plays a pivotal role in construction and infrastructure industries. In any building structure, in conjunction with the steel reinforcement design, it is essential to select the correct grade and mix design of concrete. To ensure reliability, the concrete blend must be proportioned or engineered as per the requirement of the structural engineers. Moreover, reinforced concrete has many important engineering properties such as tensile strength, density, elastic modulus, coefficient of thermal expansion, and creep coefficients. By evaluating the compressive strength of concrete, we can find out useful information on many of these properties.
The compressive strength of any material may be referred to as its resistance to failure when the material is subjected to compressive forces. As far as concrete is concerned, its compressive strength is determined by many different factors including cement strength, the composition of the concrete content, water-cement ratio, and quality control during the process of concrete manufacturing. Evaluation of concrete’s compressive strength helps determine the highest level of compressive strength that a given concrete material can sustain without fracture when it is subjected to a gradually applied load.
Evaluating Concrete Compressive Strength
When measured by a compression test, the term compressive strength indicates the strength of hardened concrete. The test is carried out by a compression testing machine and involves crushing cylindrical concrete. In this test, concrete’s ability to withstand a load before its failure is evaluated. It helps contractors with critical information concerning concrete’s strength characteristics and its potential increase over time.
In most instances, concrete requires 28 days to achieve its full strength. This is why on-site concrete testing must be performed during a pour. The compressive strength of concrete tends to increase over time because of hydration. While making concrete, a paste is formed by cement and water, which coats each sand and particle in the mix. This cement paste then hardens through hydration and becomes stronger. Major compounds present in cement create chemical bonds with the molecules of water during hydration. Therefore, in the presence of water, the strength of concrete increases over some time.
AS 1012.9 is the Australian Standard for the measurement of compressive strength of concrete. A 100mm diameter and 200mm high concrete cylinder is used in this test. The concrete sample is compacted on-site into the mould. Generally, two samples are taken, and cured to Australian Standards. The cylinders are placed in an environment-controlled bath after they are demoulded.
Following this, the pair of cured concrete cylinders are weighed and measured before individually placing them into a Compressive Strength test rig. Force is applied until the concrete cylinders experience failure and the findings are recorded. Then the compressive strength is calculated to the nearest 0.5 MPa and an average is also calculated for the pair. Finally, the results are compared to the concrete strength grade, and a pass or fail decision is made based on the specification requirements.
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