Electrical Testing of Cement-Based Materials: Role of Testing Techniques, Sample Conditioning, and Accelerated Curing

摘要

These projects examined the potential for using electrical testing on concrete as a potential surrogate for obtaining information on ion and fluid transport. Electrical measurements are particularly attractive for use in quality control as they are easy to perform, are performed rapidly, and can be directly related to fluid transport. This work describes how electrical resistance measurements should be corrected for geometry to obtain a geometry independent resistivity or conductivity. Further, this work reviews and discusses several factors that influence the resistivity that is measured including porosity, pore connectivity, the role of temperature on degree of hydration (activation energy), temperature effects on conduction, ionic leaching, and the role of temperature and leaching on porosity. The goal of this work is to provide an overview of the main factors that should be considered when conducting electrical property testing, when using electrical properties for quality control/quality assurance, or if using these materials for acceptance. As a part of this project a series of testing devices were purchased and provided to the INDOT district testing labs for the purpose of providing training and providing information on the variability associated with testing. A round robin study has shown that a coefficient of variation of 4.36 % was obtained leading to a within laboratory precision of 12.8 % and a multi-laboratory precision of 36.0 %. This work has shown that temperature can dramatically influence the resistivity. The work has shown that in general for a standard 100 mm x 200 mm test cylinder, the ratio of surface resistivity to uniaxial resistivity, specifically the factor discussed in Section 5.1, is 1.9 for a homogenous material; however if the material is heterogeneous (due to drying or leaching) this value changes. This project has also illustrated the importance of ionic leaching. It was shown that accelerated curing requires temperature correction as well as control of ionic leaching. A direct correlation is discussed between electrical resistivity and the rapid chloride permeability test. At the current time, resistivity tests show promise in a quality control program, as a method of monitoring the variation associated with production. If used in this way, a trial batch could be used to develop target values, similar to that which is currently done with the maturity method. It is anticipated that an agency could eventually have the potential to use resistivity for mixture qualification; however, additional work is needed to correlate the resistivity with performance or service life models.