Authored by Amit Kenny*
Abstract
Reinforcement corrosion is a common cause for reinforced concrete structure deterioration. One of the main causes for reinforcement corrosion is chloride ingress to the concrete. Published work hints a relationship between the susceptibility to chloride induced corrosion, referred as the chloride threshold, which is the chloride concentration at corrosion initiation, and the interface between the concrete and the rebar, known as interfacial transition zone (ITZ). This paper presents an investigation of the relationship between the chloride threshold for chloride-induced corrosion and the properties of the ITZ around embedded rebar was investigated using 16 different concrete mixes. ITZ properties were extracted using automated image analysis. The chloride threshold was found to decrease with the distance between the rebar surface and the concrete solids. In the case of horizontal rebar, the chloride threshold decreases with ITZ thickness as well. These results agree with the theory of concentration polarization in localized corrosion and may explain the variety of chloride threshold values reported in the literature.
Keywords: Concrete; Chloride threshold; Rebar; Corrosion; Interfacial transition zone
Abbreviations: BSE: Back Scattered Electron Microscopy; CCCL: Chloride Corrosion Concentration Limit or Chloride Threshold; EIS: Electrochemical Impedance Spectroscopy; HRWR: High Range Water Reducer; ITZ: Interfacial Transition Zone; RC: Reinforced Concrete
Introduction
Corrosion damage is typically the biggest item on the maintenance bill of reinforced concrete (RC) structures. In the USA, direct costs due to corrosion of RC infrastructures are estimated at 0.25% of the GNP, which corresponds to $16.6 billion a year [1]. Chloride-induced corrosion is the main cause for corrosion damages in bridges in the USA [2]. When the chloride content near the rebar surface exceeds a certain limit, referred to as the chloride corrosion concentration limit (CCCL, also denoted as “threshold” in the followings), depassivation of the steel occurs and the steel is susceptible to corrosion; indeed, the corrosion hazard to the embedded reinforcement increases drastically above this level [3]. Most of the attention in research and practice is directed towards reducing the rate of chloride ingress into the concrete by reducing concrete permeability and increasing the transport length (increased concrete cover) [4-7]. A change in the chloride threshold can have a much larger effect on the lifetime of RC than do the transport properties of the concrete or the cover Crete thickness. Thus, for example, according to LIFE-365 model [8], a 20% increase in CCCL increases the time to initiation of corrosion by 28%, whereas a 20% reduction in the diffusion coefficient increases the time to initiation of corrosion by only 10%. The range of chloride threshold values presented in the literature is wide and includes [C-]/[OH-] ratio ranging from 0.12 to 3.0; 0.03% to 4% free chloride from cement mass; and 0.04% to 2.42% total chloride from cement mass [9-18]. It is difficult to compare the various results since it is not always possible to convert from one representation method to another due to missing data in the reported works. The considerable spread of chloride threshold values encountered in the literature may be the result of the high number of variables that influence the chloride threshold such as cement chemical composition, temperature, steel composition, to name but a few [16], [19]. Investigation of steel bars embedded in simulated pore solution containing various concentrations of chlorides offers a more reliable representation of the chemical processes that take place between the chloride ions and steel surface. This synthetic situation does not, however, represent the true situation that occurs in concrete that contains voids, aggregates, and chlorides of different availability (i.e. free and bound). Inconsistent investigation of the variables and published data make it difficult to extract reliable information about the influencing variables. Thus, determination of the most appropriate method to represent the chloride threshold value is, in itself, controversial. Angst and Vennesland [17] reviewed the pros and cons of several representation methods, namely free chlorides and total chlorides, and chloride concentration relative to cement, total binder, concrete and [OH-]. All methods are presented in the literature, but no conclusion is reached as to the preferred method.
Several authors emphasized surface voids on the steel as the main durability-related problem of reinforced concrete. Observations of corroding RC structures show that corrosion initiates at these voids and that good quality of the steel-concrete interface can delay initiation of corrosion [20-24]. Glass and Reddy [25] found that the chloride threshold rises sharply when the percent of voids drops below 2% of the interface surface. In their work, they intentionally created voids by applying insufficient vibration to low slump concrete. These voids were, however, macroscopic and do not offer a good simulation of the true situation in well-consolidated concrete. In addition to the void size itself, various solid precipitants are present on the steel surface. For example, Glass et al. [26] concluded that hydration products found on steel surface are similar to those in the bulk cement paste, whereas Horne et al. [27] concluded that these products are actually all calcium hydroxide, but at different concentrations that vary also between vertical or horizontal oriented bars. The influence of voids on the corrosion behavior of embedded steel can be understood through the localized corrosion mechanism. Galvele [28] proposed a model for the concentration polarization of pH between a metal surface within a pit and an aqueous buffer solution over the pit opening. For a specific corrosion rate, the pH at the metal-solution interface decreases as the distance between the metal and the buffer increases, and the metal is at greater risk for a higher corrosion rate. Since concrete is a strong buffer at high pH values, we can infer that the risk for corrosion increases with the distance between the steel and the concrete. Thus, for every single point on the steel surface, the distance to the closest concrete component determines its susceptibility to localized corrosion. The point at which this distance is maximal will be the most susceptible to corrosion and will determine the resistance to corrosion. Parameters of minimal distance between the steel and the concrete are, therefore, considered to be an important characteristic of the interfacial transition zone (ITZ) with respect to RC durability. The above short review demonstrates the difficulties involved in isolating the various parameters that affect the initiation of steel bar corrosion in reinforced concrete. This paper presents a statistical analysis of the relationships between the chloride threshold and ITZ properties, mix composition, and fresh mix properties. An unbiased statistical method was used to eliminate biased interpretation of the results. Sixteen different concrete mix compositions were prepared in which steel was embedded in two directions to produce a range of ITZ structures. Over 1,300 images were analyzed using an automated image analysis developed for this study [29]. The results were used to identify possible relationships between chloride threshold limit and ITZ parameters that are affected by mix composition and fresh mix properties as published in [30]. The model proposed by [28] was applied in a modified way, where the steel-concrete distance at the steel concrete interface represents the distance from a buffer in Galvele model and was used in the current work to explain the corrosion results.
Experimental
Mix preparation
Sixteen different concrete mixes were produced. Variations in mix properties were intended to create variety of rebar-concrete interface, while keeping constant chemistry. The mix variables were: (1) w/c ratios between 0.40 and 0.65; (2) water / powder ratios ranging from 0.91 to 1.36 at constant w/c ratios of either 0.45 or 0.52 where the powders include all the particles smaller than 0.15 mm, which makes part of the aggregates. The amount of powders was adjusted by adding fine lime powder. The mixes were designed to yield different ITZ properties. The composition of the concrete mixes is given in Table 1. The concrete was mixed according to the following procedure: coarse aggregates were premixed with 70% of total water for 1 min. and allowed to absorb water for an additional 5 min in rest. Fine aggregates, cement, powder, the rest of the water, and admixture were then added and mixed for an additional 3 min. Other mix properties are described in detail elsewhere [30]. Mix notation: The letter W followed by two digits represents the water to cement ratio; the letter C (in some mixes) followed by two digits represents addition of fine lime powder expressed as percent of cement weight. The letters H and V represent rebar orientation relative to cast direction-horizontal or vertical, respectively. Thus, for example, W45C16-H denotes a sample with a w/c ratio of 0.45, added lime powder at a ratio of 16% of cement weight, and horizontal rebars (relative to cast direction). Mix W40 was cast twice. The second cast was notated as W40B2.
To read more about this article...Open access Journal of Current Trends in Civil & Structural Engineering
Please follow the URL to access more information about this article
To know more about our Journals....Iris Publishers
No comments:
Post a Comment