The brittle fracture of tars and bitumens

As part of a study of the cracking and disintegration of bituminous road‐surfacing materials, measurements have been made of the tensile strength of tars and bitumens, of tars to which filler has been added, and of tars after exposure to the action of weather. Measurements have also been made of the Fraass brittle temperatures of the materials. Under conditions of test in which there is no flow, tars, pitches and bitumens have the same brittle tensile strength of about 6 kg./cm.2; this value, which was obtained for materials tested in moulds that imposed lateral restraint, increases to about 30 kg./cm.2 when the specimen has no support at the position of fracture. Weathering of the materials has little effect on the brittle tensile strength, but the measured tensile strength increases considerably when the cross‐sectional area of the test specimen is reduced. A mathematical analysis of the Fraass brittle test has shown that it is an equi‐viscous temperature test measuring the temperature at which the material has a viscosity of approximately 4 × 109 poises. Although the addition of filler to tar increases the brittle tensile strength, no significant change is produced in the Fraass brittle temperature because there is a corresponding increase in viscosity.