Development of a Surface Resistivity Specification for Durable Concrete
Surface resistivity testing is a state-of-the-art method of testing concrete’s ability to resist chloride ion penetration. This electrical test is performed by passing an electrical current through a saturated concrete specimen, and has recently gained steady support as a superior testing method to currently specified durability tests such as the rapid chloride permeability test (RCPT) which is time consuming and exhibits significant variation. Over the past two decades research and implementation of surface resistivity has provided insight into the benefits of this test, namely cost and time savings, for durability testing of concrete. The goals of this research were to evaluate the factors affecting surface resistivity measurements of concrete produced with North Carolina materials, as well as to identify target values indicative of durable concrete performance for North Carolina Department of Transportation (NCDOT) performance-based specifications. Twenty-four different concrete mixtures were produced using materials typical of that specified by the NCDOT for use in concrete bridge and pavement construction. The mixtures included variations to the w/cm, cementitious material content, fly ash replacement percentage, and substitution of a portland limestone cement. Mixtures were also proportioned in a way that would represent a range of designs typical of concretes used in both structural (e.g., bridge) and pavement construction. These mixtures were used to evaluate fresh and hardened mechanical and durability properties, and to support the development of a surface resistivity specification and performance targets. Compared to mixtures with higher water to cementitious material (w/cm) ratios, test results for mixtures with lower w/cm showed the generally established trend that lower w/cm ratios provide benefits to both mechanical and durability performance. Mixtures including fly ash also exhibited superior durability performance, although exhibit delayed development of mechanical strength at early ages as expected. In order to allow these fly ash mixtures to develop the proven improved durability performances, revision of the NCDOT 28 day 4,500 psi compressive strength specification should be considered. NCDOT’s decision to increase allowable fly ash replacement rates from 20% to 30% should have little to no impact to mechanical properties at later ages. NCDOT’s decision to allow use of portland limestone cement (PLC) in mixtures was supported, as the influence of PLC on mechanical and durability properties was not significant. Mixtures with improved durability performance could be promoted by NCDOT through specification provisions promoting lower w/cm mixtures, which could readily be achieved through use of WRAs, fly ash, and optimized aggregate gradations.Results of this study were used to develop a surface resistivity specification for use by NCDOT. Performance specifications allow concrete manufacturers to innovate and leverage their experience, adjusting mixture inputs with sustainability, economy, and constructability in mind, and producing concrete more finely tuned to perform under specific conditions. The implementation of a surface resistivity specification could prove to be beneficial in the areas of early-age durability indication, as well as time and cost savings for use in future applications for North Carolina’s roadway and bridge infrastructure.