Chloride-induced corrosion on reinforcing steel: from the fundamentals to the monitoring techniques M.F. Montemor * , A.M.P. Sim~ oes, M.G.S. Ferreira Grupo de Corros~ ao e Efeitos Ambientais, DEQ, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract One of the most important causes for reinforcing steel corrosion is the presence of chloride ions. They cause localised break- down of the passive film that initially forms on steel as a result of the alkaline nature of the pore solution in concrete. The harmful chloride ions can be originated from the use of contaminated mix constituents or from the surrounding environment. The determination of a critical level, above which serious problems can occur, has been one of the main goals of investiga- tion. Unfortunately, it is difficult to establish such a value since the chloride level is influenced by several factors. Thus, after concrete contamination, it is of fundamental importance to follow the activity of chlorides and the state of the reinforcing rebars. In this respect, the use of electrochemical techniques such as polarisation resistance, electrochemical impedance, galvanostatic pulse and potential measurements have shown to be powerful tools. Nevertheless, the interpretation of the results becomes sometimes a difficult task. A large number of authors have dedicated several studies to the interpretation of such measurements and a highly dispersed number of interpretations can be found in literature. The aim of this paper is to present an overview on the state-of-the-art of the most important aspects of the corrosion process initiated by chlorides, its development and monitoring techniques. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Concrete; Reinforcement corrosion; Chlorides; Corrosion monitoring 1. Introduction The use of cement, the most important ingredient of concrete, is known since the construction of pyramids in old Egypt, where it was used as a binding agent. Now- adays concrete is one of the most widely produced ma- terials on the earth, with consumption above dozens of billions of tons. The concrete industry involves millions of euros being the basis of the modern society devel- opment. Contrary, to the common belief, concrete is not free of severe degradation problems. Apart from structural design failures, the most important cause of concrete degradation is the corrosion of the reinforc- ing steel. This problem has reached alarming propor- tions in the past three decades, leading to very high repair costs, sometimes above the initial construction cost, or in extreme situations, to the final collapse of the structure. The most important causes of reinforcement corro- sion are (i) localised depassivation of the reinforcing steel due to the ingress of chloride ions and (ii) complete depassivation of the reinforcement due to acidification of the interstitial solution in consequence of reactions of the cement matrix with carbon dioxide present in the atmosphere. Theharmfulchlorideionsmaybepresentinconcrete, as result of the use of contaminated ingredients on the manufacture of the mix, or as result of an external con- tamination prior to construction. This situation arises from exposure of the structures to water and marine atmospheres or to the use of de-icing salts (NaCl, CaCl 2 and MgCl 2 ) a necessary practice in cold climates. After initiation of the corrosion process, the accu- mulation of corrosion products (iron oxides and hy- droxides), occupying a volume several times larger than that of the original iron [1] leads to internal stresses that result in cracking and spalling of the concrete cover. At this stage the intrusion of aggressive agents, oxygen and * Corresponding author. Tel.: +351-21-8417234. E-mail address: pacfatima@alfa.ist.utl.pt (M.F. Montemor). 0958-9465/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0958-9465(02)00089-6 Cement & Concrete Composites 25 (2003) 491–502 www.elsevier.com/locate/cemconcomp