Stiffness Modulus of Bitumen: Resilient vs. Dynamic Modulus

Q4.A). What is stiffness modulus of bitumen?

Ans. The stiffness modulus of bituminous mixtures is fundamental to the analysis of the stress–strain response of pavement under traffic loading. It can be measured using various methods such as the resilient modulus test, indirect tensile test, and uni-axial direct tensile test. Thus, stiffness is usually characterized by the ratio of the stress (force per unit area) to the corresponding displacement (also known as deflection). In mechanics, this ratio is called modulus. The larger the modulus, the stiffer the pavement.

Q4.B) Write a short note on dynamic modulus of bituminous mixtures?

Ans. The dynamic modulus represents the stiffness of the asphalt material when tested in a compressive-type, repeated load test. The dynamic modulus is one of the key parameters used to evaluate both rutting and fatigue cracking distresses in the MEPDG. The bitumen complex modulus was obtained by performing a frequency sweep test using a Dynamic Shear Rheometer (DSR) and modeled based on the modified Huet-Sayegh formulation.

Q4.C). What is the difference between resilient modulus and dynamic modulus?

Ans. It is often measured at three different temperatures (approximately 40, 70, and 100°F) although the test at the middle temperature (70°F) is most often used in analysis. The dynamic modulus represents the stress-strain response under continuous cyclic loading. With the current trend toward developing mechanistic flexible pavement design and the need for more reliable deafen procedures, accurate characterization at hot-mix asphalt (HMA) properties is needed. Resilient and dynamic modulus tests were performed at five temperatures on two typical mixes used in the Commonwealth of Virginia to compare the test result. The dynamic modulus was measured at six frequencies at each of the testing temperatures, and the resilient modulus test was performed at one loading time.

The study found that the size of the specimen statistically affected the measured resilient modulus value. Resilient modulus values obtained in the 100-mm-diameter specimens were higher than those obtained in the 150-mm-diameter specimens at all testing temperatures. No statistical differences were observed in the resilient modulus of the two mixes. However, statistical differences were found in the dynamic modulus of the two mixes. A strong relation between the dynamic modulus test performed at 5 Hz and the resilient modulus was found. Three different pavement structures were analyzed to estimate the variation of their bottom-up fatigue life when different moduli were used for the HMA layer. It was found that the measured dynamic moduli resulted in the highest fatigue life estimates for the three considered pavements. It is concluded that the dynamic modulus test provides a better characterization of HMA than the resilient modulus test because it provides full characterization of the mix over temperature and loading frequencies.