Prediction of Reinforcement Corrosion in Concrete and Its Effects on Concrete Cracking and Strength Reduction

ACI Materials Journal, Jan/Feb 2008 by Li, Chun-Qing, Yang, Yang, Melchers, Robert E

Based on extensive research on reinforcing steel corrosion in concrete in the past decades, it is now possible to estimate the effect of the progression of reinforcement corrosion in concrete infrastructure on its structural performance. There are still areas of considerable uncertainty in the models and in the data available, however. This paper uses a recently developed model for reinforcement corrosion in concrete to improve the estimation process and to indicate the practical implications. In particular, stochastic models are used to estimate the time likely to elapse for each phase of the whole corrosion process: initiation, corrosion-induced concrete cracking, and structural strength reduction. It was found that, for practical flexural structures subject to chloride attacks, corrosion initiation may start quite early in their service life. It was also found that, once the structure is considered to be unserviceable due to corrosion-induced cracking, there is considerable remaining service life before the structure can be considered to have become unsafe. The procedure proposed in the paper has the potential to serve as a rational tool for practitioners, operators, and asset managers to make decisions about the optimal timing of repairs, strengthening, and/or rehabilitation of corrosion-affected concrete infrastructure. Timely intervention has the potential to prolong the service life of infrastructure.

Keywords: cracking; serviceability; steel corrosion; strength.

(ProQuest: ... denotes formulae omitted.)

INTRODUCTION

The corrosion of reinforcing steel in concrete is recognized as a significant problem for concrete infrastructure subjected to chloride environments (Bentur et al. 1997). Corrosion induced structural deterioration is a gradual process with a commencement time not always obvious from external examination. Once reinforcement corrosion becomes active, however, it almost invariably causes concrete cracking; excessive deflection, and, eventually, the loss of structural ultimate strength, with potentially catastrophic consequences. There has been extensive research on steel corrosion in concrete in the past decades (ACI Committee 365 2000; Andrade et al. 1993; Castel et al. 2000; Hong and Hooton 1999; Melchers and Li 2006; Pantazopoulou and Papoulia 2001; Roberts et al. 2000; Weyers et al. 1994); and it is now possible to provide a reasonable estimate of the whole process of reinforcement corrosion in concrete infrastructure. It is important to note that this also allows its effects on structural performance to be estimated and enables the service life of corrosion-affected concrete infrastructure to be predicted using various theoretical frameworks developed in the past few years (Frangopol et al. 1997).

Despite these significant advances, there are still areas of considerable uncertainty in the various models and in the data available. In an effort to provide some improvement, Melchers and Li (2006) recently developed a phenomenological model for the corrosion of reinforcing steel bars in concrete as a function of time (Fig. 1). The model has a number of features in common with earlier models but differs from them in important ways (Tuutti 1982; Weyers et al. 1994; Bentur et al. 1997; Francois and Arliguie 1999). In principle, the model applies to the steel bar at a generic cross section of a reinforced concrete member. The model divides the corrosion process into two stages with six detailed phases. The two stages of corrosion initiation and propagation are similar to those of Tuutti (1982), but the detailed phases comprising them are derived from the mechanics of corrosion (Melchers 2003). As shown in Fig. 1, Phase D1 is the diffusion of chlorides into the concrete and the commencement of leaching of hydroxyl ions out of the concrete. When there are cracks present in the member (for example, flexural members), the local time to initiation t^sub ic^ is governed by the time of occurrence of the local crack(s) (Li 2002). When no cracks occur in the member, t^sub ic^ tends to be the initiation time t^sub i^ , which is governed by the rate of diffusion and therefore the permeability of the concrete (Melchers and Li 2006). At t^sub i^ , the chlorides will have reached the steel but the balance between the concentrations of the Cl- and (OH)- ions and the pH may not be such that active corrosion will actually commence (which is denoted as t^sub ac^).

During Phase C0, the rate of corrosion tends to increase because the pH will typically reduce due to the leaching of hydroxyl ions out of the concrete. In Phase C1, the propagation of corrosion is governed by the rate of oxygen and water supplies and the conditions at the steel corroding surface. Because of microcracking (for example, caused by stress) and the resulting loss of influence from Cl- and (OH)- ions and the greater permeability, the environment external to the concrete will increasingly control the corrosion rate. As corrosion progresses, there will be an increasing build-up of corrosion products and associated increased radial stresses, causing longitudinal cracking and, eventually, concrete spalling. Moreover, the increasing build-up of corrosion products on the corroding surfaces will contribute to an increasing resistance to oxygen diffusion (that is, the rate of oxygen supply to the corroding surfaces). Phase C2 denotes the period when this controls the rate of corrosion. Eventually, the rate of oxygen diffusion to the corroding bars through the rust layer will become so low that anaerobic corrosion activity will set in (Melchers 2003). This is shown as Phase C3 in Fig. 1.