Dynamic shear tests for the evaluation of the effect of the normal load on the interface fatigue resistance Cristina Tozzo ⇑ , Nicola Fiore 1 , Antonio D’Andrea 1 Sapienza University of Rome, Via Eudossiana 18, 00100 Rome, Italy highlights  Investigation of the interface shear fatigue behavior between two asphalt layers.  A regression law incorporating all the possible stress combinations is proposed.  Isodamage curves quantify the influence of normal pressure on fatigue resistance. article info Article history: Received 10 November 2013 Received in revised form 6 March 2014 Accepted 7 March 2014 Available online 28 March 2014 Keywords: Fatigue analysis Dynamic shear tests Strength and testing materials Hot mix asphalt Bond strength abstract This paper focuses on investigating the fatigue performance of the interface between asphalt layers. The aim is to highlight the relationship between the state of stress and the interface fatigue failure. Several combinations of normal pressure and shear load amplitude have been tested to derive a comprehensive law. A new model is proposed to join the contributions of normal stress and shear stress to the interface fatigue behavior. The model is a regression surface in a 3D space where the stresses are the independent variables while the dependent one is the number of repetitions corresponding to characteristic moments of the test. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Road pavement is a layered structure composed of different materials. Its life depends not only on the strength and stiffness of its individual layers but also on the bond between them. Several factors affect the interface behavior, and several issues affect the pavement when the interlayer bonding is poor. The most common distress that can be caused from a low bond- ing level between layers is slippage cracking, especially at intersec- tions and sharp curves. Premature fatigue, top down cracking, potholes, distortions and surface layer delamination can be other associated distresses. Due to the structural integrity decay of the pavement associated with these distresses, the analysis of the interface bond condition is very important. 2. Background Shear testing is the most popular method to experimentally evaluate the strength behavior at the contact between pavement layers. Many devices have been developed to work in stress control or in strain control modality and to apply various combinations of shear and normal load. Pure direct shear tests are most widely used due to their simple arrangement. The Leutner test [1] included in the SN 671961 since 2000, and its evolution, the Layer-Parallel Direct Shear Test (LPDS) [2] are direct shear tests. The same configuration is also used in the FDOT device [3] and in the testing machine used by Mrawira and Damude [4]. All of them are without normal loading and with sev- eral shear deformation rates. Alternatively, direct shear tests with normal loading have been used to highlight the strong relationship between the normal load and the interface shear strength. The response curve for this test is characterized by a peak and a residual friction trend [5], which is also used for interface response modeling. The first device capital- izing on this latter approach was developed by Uzan et al. [6]. Other devices of the same type include the ASTRA apparatus from http://dx.doi.org/10.1016/j.conbuildmat.2014.03.010 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +39 0644585124. E-mail addresses: cristina.tozzo@uniroma1.it (C. Tozzo), nicola.fiore@ uniroma1.it (N. Fiore), antonio.dandrea@uniroma1.it (A. D’Andrea). 1 Tel.: +39 0644585116. Construction and Building Materials 61 (2014) 200–205 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat