1001 Concrete Repair, Rehabilitation and Retroftting IV – Dehn et al. (Eds) © 2016 Taylor & Francis Group, London, ISBN 978-1-138-02843-2 Bond of reinforcing bars in cracked concrete P. Desnerck, J.M. Lees & C. Morley Engineering Department, University of Cambridge, Cambridge, UK ABSTRACT: Cracks are inherent in reinforced concrete structures. The origin of cracks can be mani- fold. At early stages they can be caused by plastic settlement, shrinkage, drying shrinkage, etc. Typically, the most severe forms of cracking in reinforced concrete structures are those caused by the corrosion of the reinforcing bars. Corrosion products are expansive in nature and tend to generate tensile stresses in the concrete surrounding the corroding bars. As soon as these stresses exceed the tensile capacity of the concrete, cracks around and along the reinforcing bars are formed. In assessing existing structures engineers often notice severe cracking due to corrosion or longitudinal cracks due to plastic shrinkage e.g. in the anchorage zones. The section loss of these bars can easily be taken into consideration when performing load bearing capacity checks, but questions arise with respect to the remaining bond capacity of the rebars in the cracked concrete. This study aims to quantify the influence of different parameters on the bond strength of reinforcing bars in cracked concrete. Rather than performing accelerated corrosion tests, it focuses on the effect of cracking itself (in absence of corrosion products) so the results can be used for non-corrosion related cases as well. Parameters under investigation include the confinement, concrete cover, crack direction and crack extension. Results show that single cracks as narrow as 0.03 mm can have a significant influence on the obtained bond strengths. The last two phenomena have a direct influence on the bond of the reinforcing bar. On one hand, a weak interface layer of brittle corrosion products surrounding the reinforcing bar increases the relative displacement of the bar with respect to the concrete at certain load levels. On the other hand corrosion products are expansive in nature and tend to gener- ate tensile stresses in the surrounding concrete. At low levels this expansion may be advantageous but once these stresses exceed the tensile capacity of the concrete, cracks around and along the reinforcing bars develop (Task Group Bond Models 2000). The reduction in confinement due to these cracks then leads to a progressive reduction in the bond strength. Hence, understanding of the link between corrosion rates, the induced cracking (and crack widths) and the reduction in bond capacity is essential. To investigate crack formation due to corro- sion, researchers (Rodriguez et al. 1994; Andrade et al. 1993; Al-Sulaimani et al. 1990; Clark & Sai- fullah 1994; Almusallam et al. 1996; Clark & Sai- fullah 1993; Alonso et al. 1998) have undertaken accelerated corrosion tests on reinforced concrete specimens where impressed currents of 0.003 to 10 mA/cm 2 were applied. Table 1 summarizes results found in literature on the corrosion levels to cause cracking of the concrete cover. The corrosion levels are expressed as a percentage of bar cross-sectional area loss (section loss expressed as a uniform metal 1 INTRODUCTION Concrete is an inhomogeneous material with a relatively low tensile strength. Therefore it is often used in combination with steel reinforcement so that the steel can resist tensile stresses after crack- ing. The load bearing capacity of reinforced con- crete structures depend highly on the interaction between reinforcing bars and the surrounding con- crete (Task Group Bond Models 2000). Over time the bond can degrade due to deterioration of the reinforcement. Cracks are inherent in reinforced concrete struc- tures and are caused by a number of different types of actions (BRE Centre for Concrete Construction 2000). One of the most severe forms of cracking in reinforced concrete is the result of the corrosion of the reinforcing bars. Due to carbonation and chlo- ride ingress a favourable environment is created for corrosion to initiate and corrosion products to be formed (BRE Centre for Concrete Construc- tion 2000). Fib bulletin No. 10 (Task Group Bond Models 2000) categorises the effects of corrosion into 3 potential consequences: •฀ loss of reinforcing bar section •฀ creation of a weak interfacial layer at the rein- forcing bar/concrete interface •฀ volumetric expansion of the reinforcing bar