Surface Resistivity Testing of Concrete
A proposed new ASTM test method will be used to quantify the resistivity of concreting materials that are widely used for construction around the world. WK37880, Test Method for Measuring the Surface Resistivity of Hardened Concrete Using the Wenner Four-Electrode Method, is being developed by Subcommittee C09.66 on Concrete's Resistance to Fluid Penetration, part of ASTM International Committee C09 on Concrete and Concrete Aggregates.
Resistivity is a material property that quantifies the degree to which an object prevents the passage of an electrical current. While the solid material in concrete has a relatively high resistivity, the pores are partially to fully saturated with a concentrated alkaline solution that has a relatively low resistivity. Thus, electrical current flows primarily through the pore solution, giving an indirect measure of the quality of the microstructure.
The Wenner four-electrode test in WK37880 consists of four equally spaced, co-linear electrodes that are placed in contact with a concrete cylinder specimen. An alternating current is applied to the outermost electrodes and the voltage between the middle two electrodes is used to determine the resistance. The sample resistivity is calculated from the resistance, the distance between the electrodes and the dimensions of the cylinder.
The resistivity is the intrinsic material property that is independent of the specimen size, so it is the useful quantity for characterizing a concrete mixture. The original Wenner equation for the resistivity was devloped for a type of slab geometry. Therefore, a geometry correction factor is need to account for the cylindrical geometry.
"The proposed test method addresses this correction explicitly, and provides a means of estimating this correction factor for a wide variety of commonly encountered electrode spacing distances and concrete cylinder dimensions," says Kenneth A. Snyder, a member of C09.66 and leader of the inorganic materials group of the Materials and Structural Systems Division, Engineering Laboratory, National Institute of Standards and Technology.
"The concrete resistivity is affected by changes in the microstructure, the pore solution composition and the temperature of the specimen," says Snyder. "Therefore, the resistivity could be used as an economical quality control tool to monitor production, construction and exposure processes that might influence the ultimate durability of the concrete."
Once approved, the proposed standard will be used by engineers, testing laboratories and researchers who work with concrete. Specifiers could use WK37880 for material qualification, while concrete manufacturers would find it helpful for materials development and optimization. The proposed standard could also facilitate materials development and optimization for concrete manufacturers, and assurance of materials compliance for owners.
CONTACT Technical Information: Kenneth A. Snyder, National Institute of Standards and Technology • Gaithersburg, Md. • Phone: 301-975-4260 | ASTM Staff: Scott Orthey • Phone: 610-832-9730 | Upcoming Meeting: June 9-12 • June Committee Week • Indianapolis, Ind.