Methods of physical tests for hydraulic cement, Part 6: Determination of compressive strength of hydraulic cement (other than masonry cement)
1988 Edition

Per IS 4031 Part 6 Clause 6.1.2, the recommended mix for mortar cubes consists of 200 grams of cement, 600 grams of standard sand, and water amounting to (P plus 3.0)% of the combined weight of cement and sand, where P is the water percentage for standard consistency from IS 4031 Part 4. The water and ambient temperatures during mixing should be maintained at 27 ± 2°C, using potable or distilled water. The dry cement and sand are mixed for one minute, followed by adding water and mixing for 3 to 4 minutes until a uniform color is achieved. This ensures consistent and reproducible mortar strength for testing.

According to IS 4031 Part 6, the curing environment must be carefully maintained to ensure reliable test outcomes. Initially, freshly filled moulds should be kept in a moist closet or moist room for 24 hours following vibration compaction. After demoulding, specimens are to be submerged immediately in clean, fresh water maintained at 27 ± 2°C. The curing water should be refreshed every seven days to maintain quality. Additionally, the moulding room temperature is controlled at 27 ± 2°C with a relative humidity of 65 ± 5%. Specimens must remain moist and should not dry out prior to testing to ensure proper hydration and strength development.

IS 4031 Part 6 mandates the use of standard cube moulds, generally 70.6 mm in dimension, which are firmly clamped on a vibration table during preparation. A vibration machine complying with IS 10080-1982 is utilized to compact the mortar in the mould to eliminate voids. A hopper is affixed to the mould during filling to facilitate uniform placement and remains until vibration is completed. The compression testing machine should have a self-aligning platen and apply load steadily at a rate of 35 N/mm² per minute. Environmental conditions including temperature at 27 ± 2°C and relative humidity at 65 ± 5% are maintained throughout the process to ensure consistent results.

The vibration compaction procedure, as per IS 4031 Part 6, involves placing the assembled cube mould on the vibration table and securing it with clamps. After mixing, the mortar is poured partially into the mould, then prodded 20 times within approximately 8 seconds using the specified rod to expel entrapped air. The remainder of the mortar is then filled via the hopper and again prodded similarly. The mould is vibrated for 2 minutes at a frequency of 12,000 ± 400 vibrations per minute. This method ensures thorough compaction, minimizing air voids and achieving uniform density in the test specimens.

IS 4031 Part 6 specifies that three mortar cubes are tested at each curing age. The cubes are loaded on their sides without packing, applying load at a uniform rate of 35 N/mm² per minute. The average compressive strength of the three specimens represents the test outcome. Results are rounded off following IS 2:1960 guidelines to the appropriate decimal places. If the average strength meets or surpasses the specified strength for the cement grade, the batch is accepted. If it falls below, the batch is rejected. This process ensures reliable and consistent evaluation of hydraulic cement strength.