Methods of physical tests for hydraulic cement, Part 8: Determination of transverse and compressive strength of plastic mortar using a prism
1988 Edition

The standard mortar mix ratio specified is Cement : Standard Sand : Water = 1 : 3 : 0.5 by mass. This translates to 450 grams of cement, 1350 grams of standard sand (divided evenly into fine, medium, and coarse fractions), and 225 grams of potable or distilled water. The mixing process involves adding water first, followed by cement, mixing at low speed for 30 seconds, gradually adding sand fractions over the next 30 seconds, then mixing at a medium speed for 30 seconds. After a rest period with scraping and covering, the mortar is mixed again to ensure uniformity, preparing it for specimen casting.

Specimens should be demoulded between 20 to 24 hours after moulding, except for 24-hour tests where demoulding occurs 15 to 20 minutes before testing. If specimens lack sufficient strength at 24 hours, demoulding may be postponed by an additional 24 hours with proper documentation. After demoulding, specimens are immersed in water maintained at 27 ± 2°C, kept upright and separated to allow water contact on all faces. Water used for curing must be replaced every 14 days. Prior to testing, specimens are removed from curing water less than 15 minutes beforehand, transported in water-filled containers to prevent drying, and wiped clean to remove surface deposits.

The transverse (flexural) strength test requires a setup with three rollers of 10 mm diameter: two bottom rollers spaced either 100 mm or 106.7 mm apart and one top roller positioned centrally. The bottom rollers are designed to freely rock to ensure even load distribution. The testing machine should be electrically operated, capable of applying loads under 10 kN with load accuracy within ±1% in the upper 80% of the range, and able to apply load at a rate of 50 ± 10 N/s. For compressive strength tests, a standard compression testing machine compliant with relevant standards is used, equipped to apply and measure compressive forces accurately.

During compressive strength testing, the load should be increased at a rate of 1 N/mm² per second until approximately half of the anticipated crushing load is reached. Beyond this point, higher loading rates may be employed; however, the total test duration must be no less than 10 seconds. For flexural strength tests, the load applied via the rollers should increase progressively at a rate of 50 ± 10 N/s to ensure consistent and controlled loading conditions.

Flexural strength is calculated using the load at failure (P) applied to the prism and depends on the support span (L) of either 100 mm or 106.7 mm. The formulas used are: For L = 100 mm, R = 0.00234 × P; for L = 106.7 mm, R = 0.00254 × P, where R is the flexural strength in N/mm². Compressive strength is determined by dividing the maximum load applied at failure (P) by the cross-sectional area (A) of the specimen, calculated as fc = P/A, with results expressed in N/mm².