Effects of deviations from thermo-rheologically simple behavior of asphalt mixes in creep on developing of master curves of their stiffness modulus

The paper presents the deviations from the thermo-rheologically simple behavior of asphalt mixes and their effects on the developing of master curves of stiffness modulus. Eleven different mixes were tested in three point bending test under constant load at temperatures -20 degrees C, -10 degrees C and 0 degrees C, and in some cases at + 10 degrees C. The test method was developed at the Gdansk University of Technology, Poland. The applied stress level was approximately equal to 30% of the flexural strength. It was found that the stiffness moduli of tested asphalt mixes at a certain low temperature and at a long time of loading greater than 600 s or 1200 s tended to the constant values unrelated to the time of loading. It resulted from the diminishing of viscoelastic flow of the material at long loading time at low temperature. Therefore, the stiffness modulus at long loading time was dependent upon temperature but was almost not related to the loading time. In most cases at temperatures -20 degrees C and at -10 degrees C a clear deviation from the time-temperature superposition principle was observed. These deviations were dependent on the type of bitumen used, type of asphalt mixture and the value of applied stresses. The deviations from thermoreologically simple behavior affected the formation of the master curve of stiffness modulus and might cause increase of thermal stresses in asphalt layers of pavements and disappearance of stress relaxation.