Abstract Concrete-to-concrete interface are widely used in composite construction in practical engineering, such as between precast concrete elements and cast-in-situ concrete, or between new and old concrete elements. In modern high-speed railway, ballastless track made of concrete composite structure are utilized due to its better performance such as lower maintenance requirements and higher track stability. The structure usually consists of prefabricated elements, cast-in-situ concrete elements as well as filing layers. Ballastless track works well when these composite elements are integrated. The Bond strength between different concrete layers is generally assumed to be high enough to avoid cracking under usual monotonically increasing short-term loading. However, cracking or even delamination at the interface occurs due to repeated actions caused by different sources. This phenomenon causes durability problems and higher maintenance costs. In severe cases, cracks at interface could also lead to safety risks. In order to study the debonding mechanism that occurs at the interface between concrete layers, interface resistances regarding shear and tensile directions has been studied. Experimental investigation has been conducted to study the damage mechanism of interface. Concrete and cement-asphalt mortar have been used to obtain the stress-slip correlation of the interface. Keywords: interface experiments, bonding behaviour, ballastless track, interface damage model 1. Introduction 1.1. Background In practice, concrete interfaces exist in many cases, such as prefabricated elements sitting on cast-in-situ concrete or new concrete pouring on old concrete for strengthening and rehabilitation. Nowadays, high- speed railway lines (HSRL) are developing quickly around the world. In order to offer a higher track stability and passenger comfort, the slab track systems instead of traditional ballasted track are widely used in the new built HSRL. Although there are different variants of slab tracks, prefabricated elements connected with cast-in-situ concrete or cementitious mortar are quite common structures. However, existing experience showed that cracks occurs often at interface, which cause additional maintenance. In some cases, the damage of interface could lead to very severe condition and arouse safety risks, such as debonding of interface leading to uplifting of the continuous slab track (CST) under extreme temperature loads. What’s more, observations have been found that two interfaces in composite structures with a filling layer between the top and bottom concrete layers exhibit different properties. The top interface exhibits much lower interface strength than the bottom interface (Su, M. 2017). Even though there are a lot of researches on interface resistance, especially the shear-friction theory for ultimate concrete shear strength, the interface damage evolution and testing methods for determination of interface fracture mechanism still requires much research. As a result, preliminary testing for concrete interface damage are presented in this article. EXPERIMENTAL STUDIES ON THE INTERFACE BETWEEN CONCRETE AND CEMENT-ASPHALT MORTAR Sisi Zhang 1 , Boso Schmidt 1 and Steffen Marx 1 1 Institute of Concrete Construction, Leibniz University Hannover, Hannover, Germany Corresponding author email: zhang@ifma.uni-hannover.de