About the Expediency of Replacing the Penetration Index of Bitumen With the Coefficient of Its Temperature Sensitivity

Bitumen is a typical thermoplastic binder, which can be fluid or brittle depending on the temperature. The criterion for its flowability is the softening point (TS.P.) according to the ring and ball experiment, and the brittleness (Tbr) which is the corresponding to the low temperature determined by the Fraas method. The plasticity interval for these parameters varies widely depending on the penetration grade and type of bitumen. To assess this change, the penetration index has been used for almost 100 years, which is normalised by the standards of different countries depending on climatic conditions and road categories. The disadvantages of this conditional indicator are: limiting the temperature zone of its definition to the interval of change in penetration from 25 °C to the softening point and the impossibility of assessing the shear resistance of bitu-men within the range: from temperature of the softening to the temperature of 25 °C, and even more so, from softening to brittleness temperature. Replacing this criterion with a physical indicator of the temperature sensitivity of bitumen makes it possible to determine the shear resistance of any bitumen under any temperature conditions and to predict their effect on the shear resistance and crack resistance of asphalt concrete. The proposed indicator is based on regularities, namely: the penetration of bitumen which is a characteristic of its shear resistance. This resistance is the same for all road bitumen: at the softening point it is 0.015 MPa, and at the brittle point it is 31 MPa. The temperature dependence of the shear resistance and penetration is identical. It makes possible to determine the shear resistance of bitumen in the temperature range from Tbr to TS.P. The temperature dependences of penetration for different bitumen have different slopes and intersect at the average temperature corresponding to a stress of 0.21 MPa or a penetration of 31 × 0.1 mm. Setting the temperatures corresponding to the penetration of 1.25 × 0.1 mm, 31 × 0.1 mm and 800 × 0.1 mm makes it possible to determine the shear stress of bitumen at any temperature within the penetration index from -2.0 to +2.0. The validity of the established regularities is evidenced by the result of processing the literature data concerning bitumen of the ‘sol’ and ‘sol-gel’ types.