Critical review and data modelling of interlayer shear resistance in asphalt pavements.

In current times, the world has seen a great increment in the use of all types of vehicles for distinct reasons in order to fulfill our society’s demand of transportation. Such increase has led to the higher solicitations to the roads’ infrastructures, which, consequently require a better design to provide the proper performance to withstand the ever-increasing traffic demand. One particular design element that has been overlooked are the interlayers of asphaltic pavements. The term ‘interlayer’ refers to the exact part where two of the main layers of a pavement are in contact, and they can be treated with a tack coat/emulsion to improve the pavements’ response under loading. For such reason, this study focused on performing a critical review of the interlayers shear response by analyzing results obtained from Leutner Shear tests. Such review focused firstly on analyzing the constitutive models that have been proposed in the past to describe the mechanical behavior under shear loading. Within such review, the effect of only shear loading was described as well as the contribution given by the presence of normal loads to the peak shear resistance. Then, the Leutner shear test was described, along with a critical review and description on the expected results to be obtained from such test. As this test measures the shear displacement and the shear loading acting on the interlayer of double-layered specimens, it is to plot a stress – displacement curve, from which, the Interlayer stiffness modulus can be obtained from the first segment of the plot (before the peak shear stress is achieved), as the slope from the curve at a given point. Additionally, by using a combination of the test results, various mathematical models were tested in order to find the best fit to the experimental results As the main scope of this work was to provide with an adequate criterion for selecting the Interlayer stiffness modulus (K) to be used while designing or modelling in different asphalt pavements design software, such as BISAR, KENPAVE or SHELL, also a critical review on how the interlayer characteristics are taken into account in such software was performed. After defining the previous point, the way to calibrate the Interlayer stiffness modulus is provided. It is focused on the fact that this value, depends on the current stress/strain states of the interlayer, and therefore changes as the stress/ strain change. The first method is based on the modelling of the Leutner test data, for which an approximation of the stress – strain curve was done by proposing a mathematical model. Then, the Interlayer stiffness modulus was defined as the value of the derivative of such equation at a given point. However, the analyzed software only allows the input of a single value, rather than a function. For such reason, an iterative method was proposed, which consisted in using an initial value of K, equal to Kmax, to evaluate the results (from the software) in terms of the shear stress and then, recalculating the K value under the current shear stress, and so on, until the convergence is reached.