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Concrete In Australia : December 2014
Concrete in Australia Vol 40 No 4 57 FEATURE: DURABILITY Resistivity test criteria for durability design and quality control of concrete in chloride exposures Prof C. Andrade, Center for Research in Security and Durability of Structures and Materials National Research Council of Spain Modern trends are to specify performance rather than the concrete characteristics. This performance approach demands definition of a durability controlling parameter, such as the chloride diffusion coefficient, with its corresponding test and model to predict the time to steel corrosion. This paper describes the use of concrete electrical resistivity to be used as a durability performance parameter and also to provide complementary information on the setting period, mechanical strength and degree of curing. Also explained is how to design the concrete mix to obtain a target resistivity. 1.0 INTRODUCTION Design for concrete durability is increasingly required as present prescriptive codes and standards have proven inadequate to provide enough durability in aggressive environments. Several proposals exist based on modelling the mechanisms of attack (Tuutti, 1982; Sagüés, 2003; Martin- Pérez et al, 2001) or on the so called “performance” concepts or the use of “durability indicators” (Baroguel-Bouny, 2002). Nevertheless, their effective incorporation into the standards seem to be slow and a worldwide controversy exists on which is the best approach, due to the lack of sufficient tradition and experience with these new proposals. The service life of reinforcement is usually modelled by assuming two periods: the time to initiation of corrosion, ti and its propagation, tp. Thus, tl = t i + tp (Tuutti, 1982). The calculation of the duration of tl is usually undertaken by considering that the aggressive agents penetrate through concrete cover by diffusion and, therefore, Fick’s law is used to calculate a diffusion coefficient able to predict the concentration of the aggressive agent at a certain depth, at several periods of time. Present models of carbonation and chloride penetration are usually based on assuming the diffusion as the transport mechanism. However, these models are composed of parameters, such as the surface concentration and the diffusion coefficient, whose variation with time, has not been fully quantified, and not having at present enough experience and calibration of these variations, the use of these models to predict the time to reinforcement corrosion is very risky. The practice has shown that different predictions may differ by decades , which is not an acceptable difference. On the other hand, the resistivity can be monitored with time as it is a non destructive test method, and therefore, its evolution with time may be fully assessed. In this paper, a proposal is presented based on the use of the electrical resistivity (Andrade, 1993, 2004, 2010; Polder et al, 2000). The main advantage is that it is a non-destructive measurement and then it can be repeated with a relatively low cost. It can be measured in specimens and in the built structures. 2.0 DEFINITION AND INFLUENCING PARAMETERS IN CONCRETE RESISTIVITY Concrete resistivity is a volumetric property that indicates the ability to transport electrical charges through the material. It is quantified through Ohm’s law: R=V/I=ρI/A (1) where R is the electrical resistance, which can be measured by applying a voltage and measuring the current, I, circulating. The ratio between voltage and current is equal to the resistivity multiplied by a “geometric factor”, I/A, where I = the distance between electrodes and A is the cross section area. The most common laboratory method of measurement is shown in Figure 1b. Two electrodes are placed on two parallel faces of a concrete disc and a voltage is applied. The more common field method is that known as the “four points Wenner method” shown as Figure 1a) (Morris et al, 1996; ASTM G57, 1995; Wenner, 1915; PrUNE 83-988 Part 1 and Part 2). The electrical resistivity of water saturated concrete is an indirect measurement of the concrete pore connectivity. The higher the porosity, the lower the resistivity, due the higher volumetric fraction of pores. On the other hand, while resistivity is related to porosity and connectivity, in non- water saturated concrete, it is also an indication of its degree of saturation (McCarter & Gavin, 1989). This relation can be expressed through Archie’s law (Archie, 1942), where ρ0 = the resistivity of the pore solution (Andrade, 2012), f v is the volumetric fraction of water and τ is the tortuosity factor, τ: ρ=ρ0.W -τ (2) Regarding the influence of the chemical composition of pore solution, r0, its impact in the total resistivity is small providing the concrete remains alkaline. At high pH values the pore solution resistivity varies from 30-100 Ω.cm, which is comparatively very small taking into account that the concrete resistivity after several days of hardening is in the range of several hundred Ω.cm. However, when concrete carbonates, then the pore solution is much more diluted and the electrical resistivity of the pore solution may significantly increase and start to be influencing. In chloride contaminated concrete, the 57-64 - Andrade.indd 57 57-64 - Andrade.indd 57 21/10/14 2:22 PM 21/10/14 2:22 PM