The interlayer bond strength between binder and wearing course and several possible treatments of enhancing the contact surface roughness and the interlocking are investigated. For this purpose, conventional methods, such as shear tests, but also laser image acquisition of the binder upper surface have been used. The mechanical outcomes of a shear test device and the binder surface roughness parameters, have been compared looking for a relation between the shear performance and the surface characteristics. The comparison between the roughness average and the root mean square of the profile heights with the maximum shear stress shows the achievement of the same strength level for treatments with similar roughness parameters, as proved by the statistical analysis. Furthermore, the comparison between the roughness parameter kurtosis and the maximum height of the profile with the slope of the response curve before the peak and residual shear stress, demonstrates a better locking for more high peaks.

This paper shows the interlayer fatigue performance evaluated by the Sapienza shear testing machine. The tests can be performed by placing double layer specimens in several inclinations, in order to reproduce a variety of expected ratios between the normal and the shear stress. The machine manages loads with any kind of waveform and is particularly able to simulate the stress trends expected in field in points located just outside the wheel path. In this experimentation, the machine applies a triangular waveform load computed by a linear elastic multilayer program at the depth of the first pavement interface. A fatigue law is estimated considering the number of repetitions that causes the interface failure. A linear regression in the log-log graph suitably fits the experimental results for the analyzed conditions of temperature and interface type.

This research aims to investigate the influence of the normal load on the shear behavior of double-layer asphalt specimens. The LCB shear test device proposed by Miro has been chosen as model for the design and the development of two new shear tests in the laboratories of the Sapienza University of Rome; under both shear tests, it possible to apply a normal load so as to reproduce the composed state of stress carried out by the vehicular loading. Several compression levels are investigated, paying attention especially to the load application modes and to the effect of the normal stress on the interlocking properties. The study parameters chosen for the analysis are the maximum shear stress and the slope of the final branch of the response curve or the residual shear stress, in relation to the failure behavior due to the devices. The results of tests performed on the first machine, when the normal load is applied, show a direct proportionality with the normal load and the slope of the response curves after the peak remain constant because it is related to the friction features. With the second machine, which was adjusted to evaluate the shear behaviour for high interface displacements, the peak shear stress and the residual one were also evaluated, showing the increasing in relation to the compression applied during the test. The two machines provide different but comparable results.