The compressive strength of hardened cement is the most important of all the properties. Take 555 gms of standard sand (Ennore sand), 185 gms of cement (i.e. ratio of cement to sand is 1:3) in a non-porous enamel tray and mix them with a trowel for one minute, then add water of quantity P/4+3.5 percent of combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than 3 minutes nor more than 4 minutes. Immediately after mixing, the mortar is filled into a cube mould of size 7.06 cm. The area of the face of the cube will be equal to 50 sq. cm. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating table.
Keep the compacted cube in the mould at a temperature of 27° +2° C and at least 90 percent relative humidity for 24 hours.
Where the facility of standard temperature and humidity room is not available, the cube may be kept under wet gunny bag to simulate 90 percent relative humidity. After 24 hours the cubes are removed from the mould and immersed in clean fresh water until taken out for testing
Three cubes are tested for compressive strength at the periods mentioned below. The periods being reckoned from the completion of vibration. The compressive strength shall be the average of the strengths of the three cubes for cache period respectively.
Ordinary Portland Cement – 3 and 7 days
Rapid Hardening Portland cement 1 and 3 days Low heat Portland Cement 3, 7 and 28 days.
