ASTM C 120 - Modulus of Rupture

This standard describes the test methods for the determination of the breaking load, modulus of rupture and modulus of elasticity of slate. Since the modulus of elasticity is not important for roofing slates only the other two parameters will be shortly introduced.

For the determination of the modulus of rupture at least 10 sawn specimens with sizes 5 x 4 in. (125 x 101.6 mm) are needed.The specimens has to be dried in an oven until the mass doesn't change.

The testing machine has to be accurate to 1 % within the range from 100 to 2,000 lbf (0.500 to 10 kN). A load up to 1,000 lbf/min (5.000 N m^-1) is applied to the specimen until failure and the breaking load to the nearest 5 lbf (20 N) has to be recorded.

The report has to comprise the span length, the width of the specimen, average thickness of the specimen along the center line and at the edges as well as the breaking load of each specimen.

Furthermore, the average of the breaking loads and the breaking load across the length of the slate shingle has to be stated. All determinations, weight data and conditioning time for each specimen has to be reported.

The classification of the roofing slates is as follows: