Compressive strength test

• Cement, fine aggregate and coarse aggregate (up to 38) to be used for making concrete are weighed in the ratio to be used in the field and are of hand mixing or by machine mixing            
• Now water is added and the entir batch mixed until the concrete appears to be homogeneous and has the desired consistency test specimens recommend are 150×150×150 mm cubes or cylinders of 150 diameter and 150 mm height 
• Mixed concrete is filled into the  mould in layers of 50 mm and each layer is tamped with 16 mm in diameter and 600 mm long 35 times or with a vibrator  
• Test specimens are stored at a temperature 27+-3 °c at 90% humidity for 24+_1/2 hour form the time of addition of water to the dry ingredients 

• the After this time specimens are removed  from the moulds and placed in water and kept there until taken  out just before the test     

                • usually specimens are tested for 7 day  or 28 day  strength but IS code only suggest 28                         days strength only
                • specimen is placed between the plates of the compression testing machine with the care
                 that the axis of specimen is aligned with the cantre of thrust of the spherically seated plate
               • compression testing machine should be able to apply gradual load of 14 N/ mm 2 / minute                    until the specimen is crushed.
              • Average of the three values is taken as the compressive strength of concrete , provided the                 individual variation is not more than +- 15 per cent of the average

              NOTE- iS IS cade also recommends use of cubes of size 100×100×100 mm provided the                                   aggregate size does not exceed 19 mm