Impact of curing solution on concrete surface resistivity and formation factor

Formation factor (F) is the ratio of a concrete’s bulk resistivity to its pore solution resistivity and is inversely related to porosity and pore connectivity. Limitations of the unitless parameter F are of interest as measurements of concrete resistivity are increasingly common in research and could become part of performance-based concrete specifications. However, to avoid direct measurements of pore solution conductivity necessary to determine F, curing in a synthetic pore solution (i.e., “Bucket Test”) has been proposed. But, studies are lacking to support the implementation of this approach. To that end, this investigation compares surface resistivity (SR) and F results for seven concrete mixes based on two distinct curing solution regimes: (1) AASHTO TP119 Bucket Test (BT) curing and (2) conventional limewater curing. Formation factors for all mixes were calculated over the first 56-days of hydration to understand how the use of that metric, along with measurements of SR, influence interpretations of durability. NIST’s pore solution conductivity calculator was used to show that the BT’s synthetic pore solution is not an accurate proxy for mix designs including Class F or Class C fly ashes. Time-series SR measurements show that the choice of curing solution regime affects results, and therefore, how mixes can be compared with each other. Further, the percent difference between the calculated formation factors for the two curing regimes was at least 130% for all mixes at 56-days, showing that F as a metric can vary widely for the same mix design depending on the composition of the curing solution. The universal approach offered in AASHTO TP119’s default conditioning option attempts to simplify a concrete's F calculations by assuming the concrete's pore solution resistivity is the same as the synthetic curing solution's resistivity. However, creating and maintaining a synthetic pore solution increases the curing's experimental complexity, but does not enable any more accurate F calculations that would make the added experimental complexity worth it compared to the current industry norm of limewater curing. For this reason, the current industry standard curing condition – limewater curing – is recommended for formation factor calculations, including AASHTO TP119. •Calculated concrete formation factor (F) for a range of concrete compositions are compared.•Standard limewater curing is compared with a mock pore solution, as given in a provisional standard.•