Methods of Test for Determination of Strength Properties of Natural Building Stones, Part III: Tensile Strength
1974 Edition

Test specimens must be cylindrical cores with a minimum diameter of 50 mm. The height should be twice the diameter, ensuring a height of at least 100 mm. During testing, the specimen is placed between two steel plates measuring 25 mm in width, 10 mm in thickness, and equal in length to the specimen's height. Load is applied continuously at a rate of 200 N/s until failure occurs. Tests are performed on specimens dried at 70 ± 5 °C for 48 hours, with the average tensile strength reported from five specimens if their results vary within ±15%.

Specimens are conditioned either by vacuum saturation or oven drying. For saturation, specimens are immersed in water at 20–30 °C under a vacuum of 50 to 100 mm Hg for 4–5 hours, then weighed hourly until two consecutive weights differ by no more than 0.1%. For drying, specimens are oven-dried at 70 ± 5 °C for 48 hours, cooled in a desiccator to room temperature (20–30 °C), and weighed hourly until mass variation between two measurements is within 0.1%. Achieving constant mass ensures reliable and reproducible testing results.

The testing apparatus must have adequate capacity to apply a uniform load at 200 N/s without shock. It should be equipped with two steel bearing plates at least 10 mm thick with hardened surfaces; one plate features a spherically seated ball bearing aligned centrally to allow slight rotation and tilt, and the other is a plain rigid block. The bearing faces must have a width greater than 25 mm, a length at least equal to the specimen length, and a flatness deviation not exceeding 0.0125 mm. Load application continues until specimen failure, with the maximum load recorded to ±1% accuracy.

Split tensile strength (S) is calculated using the formula S = (2W) / (π d L), where W is the maximum load at failure in newtons, d is the specimen diameter in millimeters, and L is the specimen length in millimeters. The specimen is loaded until failure, and the maximum load recorded. This calculation is performed for each of five specimens, with the average value reported as the tensile strength, expressed in N/mm² (MPa).

A minimum of five specimens must be tested under each condition. The average tensile strength is calculated from these results. Individual specimen results should not differ by more than ±15% from the average. If this variation is exceeded, the test must be repeated with a new set of specimens. When the quality of rock is noticeably variable, an increased number of samples should be selected to represent the range of properties accurately.