Method of Tests for Strength of Concrete
1959 Edition

The standard defines specific specimen sizes to ensure consistent testing. Cubes typically measure 15 x 15 x 15 cm, with an alternative size of 10 x 10 x 10 cm permitted when the maximum aggregate size is 2 cm or less. Cylindrical specimens are generally 15 cm in diameter and 30 cm in length, maintaining a length-to-diameter ratio of 2. Cylinders with diameters not less than 7.5 cm are allowed if the diameter is at least three times the maximum aggregate size. For flexural strength tests, prisms of 15 x 15 x 70 cm are standard, with 10 x 10 x 50 cm allowed if aggregate size does not exceed 19 mm. These dimensions ensure representative and reliable test results.

Specimens should initially be kept in a moist environment with at least 90% relative humidity at approximately 27 ± 2°C for 24 ± 1 hours after mixing. Following this, specimens are removed from molds, marked for identification, and submerged in clean water or saturated lime solutions maintained at 27 ± 2°C. The curing water should be refreshed weekly. Specimens must remain moist and should never be allowed to dry out before testing. When transported from the field, specimens should be wrapped in damp materials to maintain moisture and stored in water at 24 to 30°C for at least 48 hours before testing to ensure proper hydration and accurate strength measurement.

The compressive strength test requires a calibrated testing machine capable of applying load at a controlled rate of approximately 140 kg/cm² per minute, with an accuracy within ±2% of the maximum load. Two hardened steel bearing platens are used: one featuring a spherical seating to allow rotation and tilt, and the other a rigid flat platen. Both platens must be at least the size of the specimen and maintain flatness within 0.01 mm when new, not exceeding 0.02 mm during use. Extensometers may be employed for strain measurement during loading cycles to enhance accuracy.

The modulus of elasticity can be determined by two principal methods. The electrodynamic method is a non-destructive test measuring the natural frequency of longitudinal vibrations in concrete prisms, allowing specimens to be reused for subsequent tests. Alternatively, the static method involves compressive loading of specimens equipped with extensometers to measure strain during the last two load cycles. Stress-strain curves are plotted, and slopes are calculated for each extensometer. If the difference between the two slopes is within 15%, the average is accepted as the modulus; otherwise, the test is repeated or discarded. This ensures reliable quantification of concrete’s elastic properties.

Concrete specimens should be compacted immediately after mixing to prevent premature setting. The mould is filled in approximately 5 cm layers, with the concrete evenly distributed around the mould’s perimeter. For manual compaction, a standardized tamping bar is used to apply uniform strokes: at least 35 for 15 cm cubes, 25 for 10 cm cubes, and 30 for cylinders per layer, ensuring penetration into underlying layers. The mould sides are gently tapped to eliminate voids left by tamping. Alternatively, vibratory compaction may be employed following relevant procedures. Once compacted, surfaces are leveled with a trowel and covered with a plate to minimize evaporation, ensuring fully consolidated specimens free from segregation or excess laitance.