Cristina Tozzo

Interlayer Shear Testing under Combined State of Stress

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.

Prediction of Fatigue Failure at Asphalt Concrete Layer Interface from Monotonic Testing

This study investigated the characterization of interface shear behavior in asphalt concrete through the estimation of the stress ratio (SR). This parameter, originally identified as the ratio between predicted interface stress from a finite element model (FEM) and interface shear strength at the corresponding normal stress, was assumed to be dynamic. As part of the experimental plan, monotonic tests on double-layered asphalt specimens were performed. Dynamic evaluations of the number of repetitions to failure under several stress conditions, equal to or higher than stresses computed from an FEM of the pavement structure, were also performed. The failure curves of the two testing modalities show similar patterns on the Mohr plane. The Hoek–Brown shear strength failure criterion and the three-dimensional surface that best fits the dynamic outcomes were considered. In this scenario, the SR referred to the proportion between the applied shear stress conditions in the dynamic modality and the maximum stress from monotonic tests. For the same predicted failure repetitions, SR assumed a constant value. Correlating monotonic and dynamic results could be an important approach both in furthering knowledge of interface shear strength and in predicting information about failure under repetitive loading applications based on simple monotonic tests.

Recycling Dredged Sludge in Asphalt Pavement

An experimental investigation of an innovative recycling method to reuse dredged sludge in asphalt pavements is presented. According to this method, semifinished products or agglomerates, made of dried sludge and bitumen, are produced. These products may be used to partially substitute fine sand and bitumen in the asphalt mix. In this paper, the practical aspects of the manufacture, storage, and transportation of these agglomerates are simulated in order to demonstrate the feasibility of the method. The technical properties of asphalt mixes prepared with these new products are compared to those of a reference asphalt mix, in terms of Marshall characteristics, stiffness, complex modulus, permanent deformation, and water sensibility. Environmental and economic issues are also analyzed. A simplified cost factor analysis shows that this method is economically advantageous compared to the disposal of contaminated sludge.

Investigation of Dilatancy Effects on Asphalt Interface Shear Strength

Dilatancy is described as the shear-induced volume change in granular materials. An inclined shear test was used in this study to measure dilatancy on double layer asphalt specimens, where, due to the configuration of the device, dilatancy cannot be inhibited. Under monotonic loading conditions, three angles of 30°–45°–60° between the specimen axle and the horizontal were considered, so as to set different ratios between normal and shear force. A guillotine shear test was used to compare allowed and inhibited dilatancy at different levels of applied normal stress. The dilatancy angle ψ and the dilatancy speed were introduced as useful parameters to characterize the phenomenon. The results proved that, by inhibiting dilatancy, the interface shear strength increases of a constant amount regardless of the applied normal pressure. Moreover, the comparison of the test outcomes between the two devices highlighted the non-contemporaneity between the dilatancy speed peak and the shear stress peak in the inclined shear test. It can be attributed to the fact that the volumetric expansion of the specimen is countered by the normal stress, which increases proportionally to the shear stress during the test.