As per BIS (IS 516) the compressive strength of a concrete mix is determined by curing concrete cubes (150mm) for 28 days, temperature maintained at 27+/- 2^{0}C.This test determines the grade of concrete.

But under special requirements (time constraints) to determine grade of concrete it may not be feasible to wait as long as 28 days, BIS has recommended accelerated curing test (on accelerated-cured concrete test specimens) to determine grade of concrete in about 28 hrs, governed by IS: 9013.

Although, Indicative strength can be achieved by testing the cubes after 3 days (50% of 28 days strength) and 7 days (60% of 28 days strength) from casting, a more standard method is commonly called Accelerated Curing Test.

Accelerated Curing is a method used to get high early compressive strength in concrete structure. Accelerated Curing is mostly used in pre-cast industry where high early strength reduces costs, by decreasing the formwork removal time, also repair works are accelerated curing, in order to make its functional at early age.

Raising the temperature of curing water speeds up the cement hydration process, thereby both curing of concrete and achieving high early compressive strength.

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The following two methods can be used for Accelerated Curing Test determining Compressive Strength of Concrete.

- Warm-Water Method
- Boiling Water Method

Boiling Water Method is commonly used for achieving accelerated strength.

**Accelerated curing tank used in laboratory for concrete testing**

## accelarated Curing by Boiling Water Method

- Test specimens are to be casted and left undisturbed for 23 hours +/- 15 mins in moist air of atleast 90% humidity and at a temperature of 27 +/- 2 deg C
- The test specimens are lowered into the curing tank and totally immersed in water for a period 3.5 hours +/- 5 mins. The water in the curing tank should be maintained at a temperature of 100 deg C.
- After curing for 3.5 hours in the curing tank, the test specimens are to be removed from the boiling water and cooled by immersing them in a cooling tank with water at a temperature of 27 +/- 2 deg C for 2 hours
- The cubes are to be demoulded once cooled

## Compressive Strength Testing & analysis of results

- The test specimens once removed from the curing tanks are to be tested for their compressive strength in accordance to IS: 516
**.** - Based on the correlation obtained between normally cured concrete and accelerated cured concrete, the following formula are derived for 28 days compressive strength of concrete mix:

*R _{28} (Strength at 28 days) = 8.09 + 1.64 Ra*

**Where Ra is the average compressive strength of concrete test specimens used in the Accelerated curing test**

The 28 days compressive strength of the concrete mix can be calculated by using the following formula.

**Correlation between Accelerated strength and 28 days compressive strength**