**Objective**

The tests are required to determine the strength of concrete and therefore its suitability for the job.** **

**Reference**** Standards**

IS : 516-1959 – Methods of tests for strength of concrete.

**Equipment & Apparatus**

- Compression testing machine (2000 KN)
- Curing tank/Accelerated curing tank
- Balance (0-10 Kg)

**Procedure**

- Representative samples of concrete shall be taken and used for casting cubes 15 cm x 15 cm x 15 cm or cylindrical specimens of 15 cm dia x 30 cm long.
- The concrete shall be filled into the moulds in layers approximately 5 cm deep. It would be distributed evenly and compacted either by vibration or by hand tamping. After the top layer has been compacted, the surface of concrete shall be finished level with the top of the mould using a trowel; and covered with a glass plate to prevent evaporation.
- The specimen shall be stored at site for 24+ ½ h under damp matting or sack. After that, the samples shall be stored in clean water at 27+2
^{0}C; until the time of test. The ends of all cylindrical specimens that are not plane within 0.05 mm shall be capped. - Just prior to testing, the cylindrical specimen shall be capped with sulphur mixture comprising 3 parts sulphur to 1 part of inert filler such as fire clay.
- Specimen shall be tested immediately on removal from water and while they are still in wet condition.
- The bearing surface of the testing specimen shall be wiped clean and any loose material removed from the surface. In the case of cubes, the specimen shall be placed in the machine in such a manner that the load cube as cast, that is, not to the top and bottom.
- Align the axis of the specimen with the steel platen, do not use any packing.
- The load shall be applied slowly without shock and increased continuously at a rate of approximately 140 kg/sq.cm/min until the resistance of the specimen to the increased load breaks down and no greater load can be sustained. The maximum load applied to the specimen shall then be recorded and any unusual features noted at the time of failure brought out in the report.

**Calculation**

Compressive strength is calculate using the following formula** **

**Compressive strength (kg/cm ^{2}) = W_{f }/ A_{p}**

Where

W_{f} = Maximum applied load just before load, (kg)

A_{p} = Plan area of cube mould, (mm^{2})** **

**Safety & Precautions**

- Use hand gloves, safety shoes & apron at the time of test.
- After test switch off the machine.
- Keep all the exposed metal parts greased.
- Keep the guide rods firmly fixed to the base & top plate.
- Equipment should be cleaned thoroughly before testing & after testing.

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Dear Sir,

I have a doubt about the calculation, We have compression cylinders Length 4.538inch and Dia 4inch, sample is in cylinder shape, I got the value during the test is 411.8 KN, Can you please help me how to calculate the compression test, test is carrying as per ASTM C39.

Expecting your reply at the earliest.

Regards

Haseeb

First convert your load value from kN to lbf

i.e 411.8 kN = 92576.32 lbf

Calculate cross-sectional area of concrete cylinder

A = (3.141*4*4)/4 = 12.564 in2

Compressive strength of concrete = Load/Area = 92576.32/12.564 lbf/in2 = 7368.38 lbf/in2

As per ASTM C39, if L/D is less than or equal to 1.75, then the compressive strength calculated as above must be multiplied by a correction factor.

In our case L/D = 4.538/4 = 1.134, which is less than 1.75.

So multiply the compressive strength as calculated above with a correction factor of 0.90.

So final compressive strength is = 7368.38 x 0.90 = 6631.542 lbf/in2 or 45.72 MPa.

after removing the cylinder on the water tank, how many hours should we let it dry before the compression strength test? ’cause if it undergo strength testing right after the removal on the water tank its interior are a bit wet and brittle and its strength really gets low.

Wat if u want to calculate or establish the standard deviation for the grade of concrete