Cohesion of bitumen: its opportunities and prospects

The consistency of bitumen as a leading indicator is usually evaluated by the depth of a needle penetration. This characteristic shows the extent (measure) of bitumen resistance to penetration of a standardised needle under specified load and time. Each penetration value corresponds to specific shear rate time function with its own average shear rate. Therefore, the comparison of consistency by penetration is not sufficiently informative. To predict the strength of asphalt concrete in the field, it is necessary to determine the strength of the binder at a given temperature and strain rate. At temperatures above 0°C, cohesion (maximal shear resistance) of a bituminous film at a plane-parallel shear can be used as a measure of strength. In this paper, existing types of cohesion-meters are considered; a cohesion-meter adopted for the research is described in which a shear of a bituminous film with a thickness of 200 µm is carried out at a shear rate of 1 s−1 and at a temperature of 25°C. The dependencies of cohesion on the temperature of bitumen of different types of structure and of the sol-gel type bitumen at different shear rates are presented. The dependence of cohesion on the penetration of bitumen obtained by oxidation and vacuum distillation is considered, the effect of polymer and warm-mix additive on cohesion is shown. The relationship between shear strength and loss modulus values of bitumen at the equivalence of shear rate and angular frequency is established, as well as the relationship between cohesion and shear resistance that is determined in a rotational viscometer. The suggestion was made to possibly use the shear resistance determined at a temperature of 60°C and a fixed shear rate (1 s−1) as an indicator of cohesion for predicting the rut resistance and shear strength of asphalt concrete.