Methods for determining stresses at key temperatures

Bitumen, having a pronounced thermoplasticity, reacts like a Newtonian or anomalous liquid, an elastic-viscous-plastic and even a solid body with temperature decreasing. On the W. Heukelom diagram, three states are distinguished by penetration values: ≤1.25 dmm is solid; from 1.25 to 800 dmm is elastic-viscous (viscoelastic) and > 800 dmm is fluid. The division into states is carried out according to the temperatures at the above-mentioned penetrations. For a long time, penetration has been considered as an empirical characteristic, which is difficult to perceive as a mechanical characteristic, which is necessary to predict the behavior of bitumen and asphalt concrete under various temperature conditions. To remedy this shortcoming, many attempts were made to convert penetration into a mechanical quantity, in the beginning into stiffness modulus (Van der Poel), later into viscosity (many researchers). None of these attempts led to the normalization of the areas of temperature-rheological behavior of bitumen by viscosity values or stiffness moduli due to the technological purpose of viscosity and the impossibility of determining the stiffness modulus by penetration. The closest to solving the problem in the early 60 s were J. Carré and D. Laurent. They solved the problem of transforming penetration into viscosity by developing a method for obtaining stress and shear rate when a needle is immersing. This work is based on the fact that the Penetration Index is a characteristic of bitumen resistance to shear when the needle is immersing. At the same time, each penetration has its own speed and stress. The purpose of the work is to substantiate and determine the shear stress corresponding to penetration: 800 dmm; 31 dmm; 1.25 dmm and at 25 °C. The obtained results indicate that τ800, τ31 and τ1.25 are close to 1320 Pa, 2.1·105 Pa and 33·106 Pa respectively. The functional dependences obtained from them are semilogarithmic. They are identical to the W. Heukelom diagram DRB with the difference that the penetration is replaced by shear stress over the entire temperature range from softening point (T800) to breaking point temperature (T1.25).