Strengthening of RC slabs with reinforced concrete overlay on the tensile face Hugo Fernandes a,⇑ , Válter Lúcio b , António Ramos b a FCT – Universidade NOVA de Lisboa, Portugal b CERIS - ICIST, FCT – Universidade NOVA de Lisboa, Portugal article info Article history: Received 21 October 2015 Revised 19 August 2016 Accepted 10 October 2016 Keywords: Slab strengthening Concrete layer Reinforced concrete Steel connectors Interfacial bond abstract Strengthening of concrete structures with a new concrete layer has been commonly used for columns, beams and slabs. This technique is economic and efficient for structural strengthening since it uses the same base materials, steel and concrete. It is usually applied on the compressed face of the concrete ele- ment due to concrete’s recognized behaviour under compression, posing several challenges to control cracking and resistance when applied on the tensile face. For assessing the performance of the strengthening method, twelve slab specimens were designed and tested monotonically. The main parameters to assess in this work were the debonding behaviour and load, and the relationship between the latter and the relative displacements at the interface of the two concrete layers. The performance of the strengthened structures strongly relies on the interaction of the two concrete layers, with this being the main subject of the research about overlaid concrete. The load transfer capacity of the interface depends on the interface shear strength, which in turn is highly depend- able on substrate roughness, cleanliness and curing conditions of the newly added layer. Interface perfor- mance may be improved by using steel connectors crossing the interface, properly anchored on both layers. The importance for these elements grows as the existing concrete is more deteriorated, since adhesion strength will decrease with lower quality concrete. This paper presents the experimental research for the application of bonded concrete overlays on the tensile face of reinforced concrete slabs, mainly aimed at office buildings and parking facilities, where spatial clearances or inaccessibility to the lower side of the slabs are recurrent. A ductile behaviour upon debonding was achieved for the specimens with reinforcement crossing the interface, and a debonding load up to three times that of the reference specimens without reinforcement crossing the interface. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Strengthening of concrete structures by adding a new concrete layer is well known when applied to the compressed face of con- crete elements. Examples of that are beams and columns strength- ened with concrete jacketing. Depending on the intended reason for strengthening/retrofitting the existing structure should be relieved of existing loads until strengthening is applied. This is more important with damaged or deteriorated structures since loading will further aggravate the existing structural condition. Bonded concrete overlay becomes a relevant strengthening and repairing technique since the base material has to be replaced with new concrete. When applied on the tensile face of concrete elements, bonding between two concrete layers is affected by the normal stresses that appear due to the difference in stiffness of the concrete layers. This technique relies on the quality of bond between the two concrete layers, therefore varying with surface preparation, and if steel con- nectors are installed crossing the interface. If no connectors are used, adhesion is the only component of the resisting mechanism acting on the interface, and brittle failure shall occur. This relies strongly on roughness, which allows for interlocking of the two layers and consequently bonding stresses to develop along the interface. With steel connectors crossing the interface between the two concrete layers, three components of the resisting mecha- nism illustrated in Fig. 1 shall develop [1]: 1. Adhesion, due to chemical bond between the two layers, and mechanical interlocking, if macroscopic surface roughness is present, shall be considered for slips up to 0.5 mm [2]. http://dx.doi.org/10.1016/j.engstruct.2016.10.011 0141-0296/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: HDPFernandes@fct.unl.pt (H. Fernandes). Engineering Structures 132 (2017) 540–550 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct