Corrosion behavior of reinforcing steel embedded in chloride contaminated concretes with and without metakaolin

Abstract

One of the most remarkable drawbacks of the reinforced concrete structural elements is the corrosion of reinforcement. Especially, in marine structures or in such environments where chloride contamination risk exist this issue gains much importance. In this study, chloride contamination of the concrete was implemented by adding 0%, 1.5%, 3%, and 5% NaCl by weight of the total binder content, into concrete. To assess the influence of high reactivity metakaolin (MK) incorporation on the corrosion resistance of the chloride contaminated concrete, 5% and 15% MK replacement levels by the total weight of the binder were assigned in concrete production. The corrosion behaviors of reinforcing bars embedded in concretes were monitored through accelerated corrosion test, corrosion current density and the corresponding corrosion rate by linear polarization technique (LPR). Moreover, the electrical resistivity of concrete was measured. Analysis of the test results indicated that MK was proved to be effective in inhibiting the active corrosion of concrete even at severe chloride contamination levels.

Introduction

Reinforced concrete (R/C) is the most commonly used composite material in structural practices due to ease in applications and lower cost of construction. Besides, reinforced concrete structures offer good service under certain environmental conditions. The worldwide demand for high performance concrete with improved corrosion resistance has increased and it is expected that it will be widely used in construction industry during next decades. The corrosion resistance of concrete has an important effect on the durability and hence its performance. Therefore, it can be said that concrete performance depends mainly on the environmental conditions and the quality of the concrete.

The presence of chloride ions R/C plays a major role in reinforcement corrosion and hence for the durability and service life of R/C structures [1]. The existence of chlorides within reinforced concrete accelerates the initiation of reinforcement corrosion and results in severe deterioration of concrete structures. Once the chloride content at the reinforcement reaches a threshold value and enough oxygen and moisture are present, the reinforcement corrosion will be initiated [2]. When corrosion is initiated, active corrosion results in a volumetric expansion of the rust around the reinforcing bars against the surrounding concrete [3]. It is known that, in well designed and high quality concrete, the risk of corrosion is expected to be minimal since it provides chemical and physical conservation to the embedded steel reinforcement bars. The corrosion of rebar in concrete is generally considered as an electrochemical process [4], [5], [6], [7], [8]. Therefore, the use of electrochemical techniques for the appraisal of corrosion behavior of R/C in this regard, becomes a prominent field of durability study.

Metakaolin (MK) obtained through proper calcinations of kaolin and having pozzolanic properties has been used as an additive for cement [9]. The studies, regarding the improvement of the mechanical, shrinkage, and some durability properties of the concrete by MK have been carried out by the researchers [10], [11], [12], [13], [14], [15], [16]. Nevertheless, there is still a gap in the literature regarding the corrosion resistance of the concretes modified with MK. Batis et al. [17] studied the effect of metakaolin on the corrosion resistance of cement mortar. They used a poor Greek kaolin with low kaolinite content. The Greek kaolin was thermally treated and ground to the appropriate fineness. Moreover, a commercial metakaolin of high purity was also used. Several mixture proportions were used to produce mortar specimens, where metakaolin replaced with either sand or cement. For evaluation of the corrosion resistance of the metakaolin modified concretes, the following criteria considered: corrosion potential, mass loss, electrochemical measurements of the corrosion rate by the Linear Polarization method, and carbonation depth. They reported that the use of metakaolin, either as a sand replacement up to 20% w/w, or as a cement replacement up to 10% w/w, improved the corrosion behavior of mortar specimens.

In this study, the effectiveness of MK replacement by weight of the total binder content on the corrosion behavior and electrical resistivity of chloride contaminated concretes were investigated experimentally. For this purpose, two replacement levels of MK were assigned to produce mineral admixed concretes. For comparison, a reference plain concrete group was produced, as well. To evaluate the degree of the deterioration of the chloride contamination, four sodium chloride concentrations (0%, 1.5%, 3%, and 5%) were considered. Corrosion behavior of reinforcing bars embedded in concretes was monitored through accelerated corrosion test and linear polarization resistance (LPR) test. Moreover, being an important indicator of reinforcing steel corrosion, the electrical resistivity of concrete was also measured at the end of the specified curing periods.