Performance-based RCPT specification for geopolymer concrete: Modified voltage protocol and chloride resistance classification

The reliability of the Rapid Chloride Permeability Test (RCPT), ASTM C1202, for geopolymer concrete (GPC) is often debated, particularly due to the use of 60 V and the lack of a standardised rating scale tailored to GPC. Although prior studies have noted problems with RCPT in GPC and proposed partial remedies, a fully validated framework tailored to GPC is still lacking. Standardised testing conditions and clear links between RCPT results and chloride transport behaviour remain undeveloped. To this end, this study examines the effect of applied voltage on RCPT results and recommends a modified methodology to establish a suitable rating scale for GPC. Sixty GPC mixtures, characterised by varying mix designs and strengths, were assessed, and their key properties – compressive strength, conductivity, formation factor, and charge passed – were analysed in relation to chloride diffusion coefficients. The formation factor was incorporated to normalise pore solution conductivity, thereby enabling assessment of microstructural transport behaviour. Findings confirm that voltages above 10 V often cause premature test termination; therefore, 10 V is identified as the recommended upper operating voltage for reliable RCPT testing in GPC under the investigated conditions. Following these results, performance criteria for the modified RCPT and formation factor are proposed, providing a reliable basis for assessing chloride permeability in GPC. Additionally, prescriptive requirements are outlined to guide the selection of suitable GPC mixtures for specific applications. This study delivers a validated framework for reliable RCPT use in GPC, defining voltage, rating scale, and formation factor limits to enable standardisation and adoption in design codes.