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Concepts, applications, and measurement techniques
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The so-called rapid chloride permeability (RCP) test is a widely accepted approach for assessing the durability of concrete. Originally proposed by Whiting1 and since standardized as ASTM C12022 and AASHTO T 227,3 the test method allows the measurement of the total electrical charge passing through a concrete specimen subjected to a standard voltage. While it does provide an indication of the concrete's ability to resist chloride ion penetration, the RCP method is neither rapid nor capable of direct measurement of chloride permeability. Rather, the test was developed by correlating a measured charge to the total chloride penetrating to a depth of 1-5/8 in. (41 mm) of reference slabs subjected to a 90-day ponding test (AASHTO T 2594). The chloride permeability categories are thus qualitative.
As an alternative, electrical resistivity measurement can be used for the performance-based evaluation of concrete. Resistivity test procedures, including sample preparation, are much easier and faster than that of the RCP test. Also, the resistivity value can be directly related to the chloride diffusion coefficient of concrete using the Nernst-Einstein equation.5
Several techniques have been developed and studied for measuring the electrical resistivity of concrete, including the bulk electrical resistivity and surface electrical resistivity. In this article, different approaches in the measurement of concrete electrical resistivity are discussed. The correlations between the resistivity measurements and certain durability characteristics of concrete are reviewed.
Electrical Resistivity
Electrical resistivity measurement techniques are becoming popular among researchers and scholars for the quality control and durability assessment of concrete (for example, refer to References 6 and 7). The adoption of these techniques into standards and guidelines has been rather slow, with only surface electrical resistivity adopted as a test method by the American Association of State Highway and Transportation Officials (AASHTO TP 958).While ASTM Committee C09 is also developing a standard procedure for evaluating the surface electrical resistivity of concrete, the only resistivity test method that has been standardized to date is ASTM C1760,9 and this is used for measuring the bulk electrical resistivity. Thus, a gap exists between the state-of-the art knowledge and the current industry practice.
The concepts
Durability of concrete depends largely on the properties of its microstructure, such as pore size...