Experimental investigation of the bond strength between new to old concrete using different adhesive layers Khuram Rashid a,b , Madiha Ahmad b,c , Tamon Ueda d , Jun Deng a,⇑ , Kiran Aslam b , Imran Nazir b , Muhammad Azam Sarwar b a School of Civil and Transportation Engineering, Guangdong University of Technology, Guangdong, China b Department of Architectural Engineering and Design, University of Engineering and Technology, Main GT Road, 54890 Lahore, Pakistan c Department of Architecture, University of Lahore, Pakistan d College of Civil and Transportation Engineering, Shenzhen University, China highlights  Different bonding systems between old to fresh concrete were experimentally investigated.  SBR-latex was the best adhesive for composite structures under tension-shear stress.  CFRP showed promising behavior as an adhesive for repaired systems under tensile or shear stress. article info Article history: Received 7 November 2019 Received in revised form 1 March 2020 Accepted 19 March 2020 Keywords: Interfacial bond strength Adhesive layers Tension and shear stresses Failure mode abstract This work was designed to evaluate the performance of adhesive layers between substrate concrete and freshly overlaid concrete, under the action of tensile and shear stresses. Four types of adhesive layers that are frequently used in real applications (i.e., cement paste, epoxy bonding agent, styrene butadiene rub- ber (SBR)-latex and carbon fiber reinforced polymer (CFRP)) were selected. Composite specimens were tested to evaluate their performance under tension, shear and tension-shear stress by split, bi-surface and slant shear strength tests. SBR-latex showed a promising result for repaired systems under the com- bined action of tensile and shear stresses due to its superior adhesive property via the formation of a polymer film. As expected, CFRP showed the highest strength when such composite systems were under the action of tensile or shear stresses and had a strong potential for use for jointing precast members. Cement paste, which is a widely used adhesive repair material, showed poor resistance in the tension test due to its brittle nature. In addition, the roughness of the fractured surfaces was quantified and the failure modes were explained at the meso-level. The outcomes of this work may guide designers in the selection of appropriate types of adhesive layers for a composite structure that may be under tensile, shear or tensile-shear stress. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Cementitious materials that have better characteristics than substrate concrete may be beneficial choices for repair. However, the interface between old and new concrete is still considered as the weakest zone [1,2] and must be strengthened and evaluated in depth. During service life of repaired structure, a composite specimen may be exposed to tension stress, shear stress or a combination of both types of stresses due to any mechanical or environmental loading [2]. Normal and shear stresses are generated at the inter- face of concrete slab poured with cementitious repair material due to the self-equilibrated state. These stresses are caused by restrained drying shrinkage. Similarly, normal and tensile stresses are generated at the interface between the soffit of the repairing concrete (RC) beam and repair material. Several methods have been developed to assess the bond strength of composite speci- mens and to determine the interfacial strengths of tension stress, shear stress or their combined action [3–5]. The split tensile strength test is reported in ASTM C 496-04 standard for bulk spec- imens [6]. Both the cylinder and prism have been numerically shown to produce an essentially uniform tensile stress across most https://doi.org/10.1016/j.conbuildmat.2020.118798 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Civil and Transportation Engineering, Guangdong University of Technology, Guangdong, China. E-mail address: jdeng@gdut.edu.cn (J. Deng). Construction and Building Materials 249 (2020) 118798 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat