°Cd Effect of surface preparation of substrate on bond tensile strength of thin bonded cement-based overlays S. Asad Ali Gillani a,b , Ahmed Toumi a *, Anaclet Turatsinze a a Université de Toulouse, UPS, INSA, LMDC (Laboratoire Matériaux et Durabilité des Constructions), 135, avenue de Rangueil, F-31077 Toulouse, France b Civil Engineering Department, University of Engineering and Technology, Lahore 54890, Pakistan Received 15 March 2019; received in revised form 24 November 2019; accepted 26 November 2019 Abstract The paper focuses on the debonding between a cement-based overlay and a concrete substrate under a mechanical loading. Two categories of substrate surface preparation were considered: Sawing the Substrate and Sand Blasting of the casting surface of the substrate. Untreated substrate surface used in its original state was taken as the Reference Surface. The overlay material investigated was a cement-based mortar. Direct tensile tests perpendicular to the overlay-substrate interface were carried out at the age of 28 days and the results (tensile strength, residual post peak behavior) are presented and discussed. Attention was paid to the Sand Blasted surface which is a more realistic model of practical field conditions. The debonding propagation under monotonic and fatigue loading was monitored for sand blasted surfaces using the Digital Image Correlation technique. Results indicate that both surface preparation technique has its own efficiency level and gives different roughness. Among the techniques used, greater bond strength improvement was observed with the sawn substrate surface than with the sandblasted one. Moreover, under cyclic loading, major debonding along the interface occurs from the first cycle to the 20000th cycle and is negligible thereafter. Keywords: Substrate surface preparation; Cement-based overlays; Bond tensile strength; Fatigue; Interface debonding; Digital image correlation 1. Introduction Concrete has been utilized all over the world as an important construction material in dams, offshore structures, pavements, buildings etc. During its service life, many changes occur in its structure due to different physical, chemical and mechanical effects. Because of these changes, concrete structure always requires regular repair and rehabilitation in order to maintain a proper serviceability level. The normal trend all over the world is to repair the old, damaged structure rather than rebuild it. There are various techniques for repairing a damaged structure, some of which involve overlaying existing concrete. The major problem often observed after a few years in service is a cracking of the overlay, followed by the debonding along the interface. The two fundamental causes of such distress have been well described in previous research [1-5]. These are related to shrinkage cracking and/or mechanical load induced cracking of the overlay with resulting peeling and bending moment. A common practice is then to increase the surface roughness of the existing concrete substrate * Corresponding author E-mail addresses: asadgillani@uet.edu.pk (S. A. A. Gillani); toumi@insa-toulouse.fr (A. Toumi); anaclet@insa-toulouse.fr (A. Turatsinze). Peer review under responsibility of Chinese Society of Pavement Engineering. before applying new concrete. In the light of previous research, this surface roughness of the substrate is considered among the important parameters for a good bond between substrate and overlay [6-10]. Several methods are used to increase surface roughness and each method has its own relative efficiency level. Preparation techniques such as sand-blasting, shot-blasting, wire- brushing, chipping and water jetting are commonly used to remove the top weak layer. In some cases, a pneumatic hammer is also used to remove the top, weak layer of concrete substrate but the general opinion is that this method results in interface weakness because the high impact energy induces micro-cracking in the substrate [8- 12]. Garbacz et al. [13] determined the characteristics of concrete surface roughness and its relation with adhesion in a repair system by using four different methods of substrate surface preparation. They found that maximum roughness was obtained by shot blasting and minimum roughness by grinding. More aggressive techniques such as shot-blasting and milling produced more surface cracking. Bett et al. [14] repaired and strengthened reinforced concrete columns by jacketing and they also mention that, before the jacketing was carried out, all models were roughened by light sandblasting. The influence of the surface roughness appears to be more significant when cement mortars or polymer modified cement mortars are used, since when epoxy resins are used, failures do not occur frequently at the interface [15]. ISSN: 1997-1400 DOI: https://doi.org/10.1007/s42947-019- 0101-5 Chinese Society of Pavement Engineering. Production and hosting by Springer Nature International Journal of Pavement Research and Technology Journal homepage: www.springer.com/42947 Chinese Society of Pavement Engineering