Sometimes it becomes necessary to know the compressive strength of the concrete at an early stage. Especially when the schedule is tight and you need to progress faster. Accelerated Compressive Strength Test (ACT) is the way to find out the early stage compressive strength of concrete.
Normally compressive strength of concrete is measured at 7 days and 28 days. But sometimes like in Prefabrication projects high early stage strength allows to remove shuttering in 24 hours of placing the concrete. This enables to reduce the cycle time and results in savings on cost.
Most commonly used technique to get higher early stage strength is accelerated curing. This is done by steam curing at atmospheric pressure, warm water curing and autoclaving.
When it is becomes necessary to know the compressive strength of concrete at a early stage. Accelerated Curing Test is used. In this method the cube is cured with accelerated means of curing and the strength is checked at 28 hours.
IS : 9013-1978 details the method of making , curing the test specimens and determining compressive strength of accelerated cured concrete.
Accelerated Compressive Strength Test (ACT)
The test specimen (cube) is stored in humid air of at least 90% humidity for 23hrs.
The cubes are covered by a steel plate to avoid distortion during the use.
The cubes are kept in the curing tank and remains totally submerged for 3.5 hours. The temperature of the water in tank has to be 100°C when the cubes are placed. The temperature of water should not drop more than 30°C and should within 15minutes return to boiling.
After 3.5 hours of curing with hot water the cubes are than cooled by immersing them in cooling tank at 27 ±2°C for 2 hours.
After the cooling period the cubes are tested for accelerated compressive strength (Ra) test.
Ra = Accelerated Compressive Strength of Concrete in N/mm²
Compressive strength at 28 days can be found by following formula:
Predicted 28 days compressive strength (R28) = 8.09 + 1.64 Ra
Hope the article was clear. Read more about concrete grades here.
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