Friction and cohesion coefficients of composite concrete-to-concrete bond M.E. Mohamad a,⇑ , I.S. Ibrahim a , R. Abdullah a , A.B. Abd. Rahman a , A.B.H. Kueh b , J. Usman a a Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia b Construction Research Centre, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia article info Article history: Received 11 March 2014 Received in revised form 26 August 2014 Accepted 22 October 2014 Available online 29 October 2014 Keywords: Interface shear strength Surface roughness Roughness parameter Friction and cohesion coefficients abstract Experimental study is conducted to quantitatively assess the effects of different surface textures on the friction and cohesion coefficients of concrete-to-concrete bond under different normal stresses. The top surface of concrete base specimens are treated with five different surface textures; surface ‘‘left as-cast”, deep groove, indented, and wire-brushing in longitudinal and transverse directions. The roughness pro- file of the treated concrete base is measured using a portable stylus roughness instrument. In addition, the ‘‘push-off” test method is conducted to determine the relationship between the roughness profile and the interface shear strength. Results show that the mean peak height, R pm has the most significant influence on the pre-crack interface shear strength where the correlation coefficients, R 2 ranged from 0.9009 to 0.9209. Analytical equations are then proposed to predict the friction and cohesion coefficients by integrating R pm into the proposed equations. The comparison shows a good concordance with the experimental results within an acceptable range. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Precast concrete slab is normally tied with cast-in-place con- crete topping by composite action at the interface between the concrete layers. The composite action between the concrete layers plays a significant role in achieving monolithic behavior of the slab system. Such action, development and provision by the precast concrete slab and the concrete topping are achieved via mutual and adequate interface shear and bond strength. The horizontal shear transfer crossing between two members of concrete must be maintained through concrete cohesion, friction and dowel action from the projecting shear reinforcement [1–3]. The type of surface roughness at the interface influences the cohesion, friction and the bond strength between concrete layers [4–6]. To characterize the horizontal shear strength at the interface between concrete layers cast at different times, design codes such as and ACI 318 [1], Eurocode 2 [2], and CEB-FIB Model Code 2010 [3] recommended certain design values in which based on the sur- face texture and the surface with projecting shear reinforcement links. In this study, the comparison is made on the surface texture. ACI 318 [1] mentioned the interface shear strength is only catego- rized by two types of surface texture; rough and very rough at full amplitude of 6.4 mm. The compressive strength concrete is not specified in ACI 318 [1] as a function of interface shear strength which is only based on qualitative assessment. On the other hand, the CEB-FIB Model Code 2010 [3] and Eurocode 2 [2] mention the compressive strength concrete of concrete layers and categorized the degree of roughness from very smooth to very rough surfaces. Eurocode 2 [2] states that the friction and cohesion coefficients at interface of concrete layers are influenced by the degree of the roughness. Also, the recommended roughness height for rough sur- face should be at least 3 mm and for indented or very rough surface at least 5 mm. The friction coefficient ranged from 0.50 to 0.90, while the cohesion coefficient ranged from 0.025 to 0.50 which are postulated for surface profile from very smooth to very rough. However, the selection of these values may be subjective as it is difficult to distinguish the characteristic of surface roughness pro- file between very smooth and smooth as well as between rough and very rough. CEB-FIB Model Code 2010 [3] quantifies the surface roughness by using average roughness, R a which is determined as the mean value of texture heights along a certain length, l m . The surface texture concrete is measured and categorized from very smooth to very rough by applying the roughening method. Very smooth surface is cast against steel formwork, thus R a is not measurable. http://dx.doi.org/10.1016/j.cemconcomp.2014.10.003 0958-9465/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: ezy11887@gmail.com (M.E. Mohamad), iznisyahrizal@utm.my (I.S. Ibrahim), redzuan@utm.my (R. Abdullah), baharfka@utm.my (A.B. Abd. Rahman), kbhahmad@utm.my (A.B.H. Kueh), jamilonline05@gmail.com (J. Usman). Cement & Concrete Composites 56 (2015) 1–14 Contents lists available at ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp