Potential pitfalls in assessing chloride-induced corrosion of steel in concrete
A. Poursaee
a
, C.M. Hansson
b,
⁎
a
School of Civil Engineering, Purdue University, West Lafayette, IN, USA
b
Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada
abstractarticle info
Article history:
Received 27 November 2007
Accepted 29 January 2009
Keywords:
Corrosion
Electrochemical properties
Pore solution
Acceleration
Potentiostatic polarisation
Electrochemical assessment of the condition of embedded reinforcing bar (rebar) in concrete is being carried
out increasingly routinely, both in the laboratory and in the field. However, because of the perceived need to
produce results very rapidly, the results may not, in fact, be representative of the actual behaviour of the
rebar. This paper describes some of the pitfalls the authors have encountered in their own work and have,
therefore, analysed and quantified, together with others which have appeared in the literature.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The most common cause of deterioration of reinforced concrete
structures is corrosion of the reinforcement caused by localised
breakdown of the passive film on the steel by chloride ions. Because of
the impact of this deterioration on both safety and the economy, the
ability to accurately and reliably measure the corrosion condition of
the reinforcing bars (rebars) in the concrete is essential. In this paper,
the authors describe some of the difficulties and problems in such
measurements that they have encountered in their work and in the
literature.
In practice, chloride ingress into reinforced concrete takes many
years. This means that the concrete has time to mature and the steel
has time to equilibrate in the alkaline environment, before the steel
encounters chlorides and corrosion is initiated. However, for labora-
tory experiments, it is impractical to wait many years to obtain results
and most of the problems described below arise because of the desire
to accelerate the onset of corrosion. Furthermore, for those assessing
the condition of structures in the field, it appears that one main
objective is to make as many measurements as possible in as short a
time as possible, in order to reduce costs. This push for speed is, again,
the cause of some of the problems described below. Other problem
areas are the use choice of appropriate electrochemical techniques
and specimen details. One problem, which cannot readily be over-
come, is that almost all laboratory tests are conducted on young,
immature concrete and at relatively constant ambient conditions. This
is not the same environment as that encountered by steel in real
structures at the time that chlorides penetrate the concrete cover. This
problem is beyond the scope of this paper but should be borne in mind
when extrapolating research results to the field.
2. Accelerated laboratory testing
2.1. Admixed chlorides
The most common method for accelerating the corrosion of steel in
concrete is to contaminate the concrete with chloride at the time of
mixing. This is a completely acceptable method, for example, to
determine maximum allowable limits of chlorides in concrete
components, beyond which rebar corrosion might be a problem.
However, for other applications, it may not be appropriate. Firstly, it
takes time for steel to become passive in uncontaminated concrete, as
illustrated in Fig.1 [1]. If chlorides are mixed into the concrete, the steel
does not have time to passivate before encountering chlorides and so
the concept of localised passive film breakdown as the first step in
corrosion initiation must be modified. It is probable that the mill scale
present on most carbon steel reinforcing bars also results in localised
corrosion but the mill scale is not as protective as a passive film.
Secondly, there are extraneous effects of chlorides on the concrete
itself. These can include: (i) acceleration of cement hydration which
increases the porosity of the concrete [2]; (ii) an increase in the
conductivity of the pore solution; (iii) changes in the pH of the pore
solution and (iv) changes in the amount of chemically bound chlorides
[3]. Moreover, the chloride cation influences the direction and
magnitude of some of these effects: the pH is increased by NaCl and
KCl but decreased by CaCl
2
and, probably, by MgCl
2
[3]; the amount of
chloride binding when the chloride is CaCl
2
is significantly higher than
when it is present as NaCl and KCl [4]. Despite the greater degree of
binding, the rebar corrosion rate induced by CaCl
2
is greater than that
Cement and Concrete Research 39 (2009) 391–400
⁎ Corresponding author.
E-mail address: chansson@uwaterloo.ca (C.M. Hansson).
0008-8846/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cemconres.2009.01.015
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