Recent specifications for concrete bulk resistivity tests (ASTM C 1876 and AASHTO TP 119) recommend a submerged standard (simulated) pore solution (SPS, solution conductivity = 78.74 mS/cm) for curing concrete specimens. The rationale for the bucket test is that immersing concrete specimens in a soak solution similar to their pore solution would eliminate the need to determine pore solution resistivity for calculating the formation factor (FF) of concrete mixtures. However, the thermodynamic modeling predictions of 91-day pore solution concentration (PSC) for eight high performance concrete (HPC) mixtures evaluated in the current study showed SPS to be a close representation only for reference ordinary Portland cement (OPC) and binary silica fume mixtures. In contrast, the average PSC of Class F and Class C fly ash mixtures was approximately 12% to 20% lower and 20% to 40% higher, respectively, than the SPS. Accordingly, an innovative matching pore solution (MPS) curing approach was developed in which mixtures are grouped based on the influence of supplementary cementitious materials (SCMs), that is, their type and replacement levels of the long-term PSC of concrete mixtures and, thereby, cured in a simulated solution matching the average PSC of a particular group. Based on experimental work in the current study, HPC mixtures under MPS demonstrated a lower coefficient of variation (COV) and more comparable (<10% difference) bulk resistivity (BR) and surface resistivity (SR) measurements compared with SPS. Moreover, the MPS improved the reliability in FF determination and FF-based transport property prediction for HPC mixtures, as verified by lower mean absolute error and improved R2 between FF-predicted diffusion coefficients and experimental measurements.
Performance-based approaches to evaluate fly ash effectiveness in suppressing alkali silica reaction (ASR) require testing for a range of replacement levels to determine optimum fly ash dosage. Current approaches of ASR evaluation, that is, determining optimum fly ash dosage for ASR mitigation, are impractical as a mix design tool for new concrete construction. Therefore, in the present work a screening tool is developed to predict optimum fly ash dosage to suppress ASR in concrete mixes. The screening tool uses water-soluble (readily available) alkali from fly ash and cement to determine pore solution alkalinity (PSA) of concrete mixes for different fly ash replacement levels. The optimum fly ash dosage in concrete to mitigate ASR is estimated based on the PSA and aggregate threshold alkalinity (THA) relationship, that is, PSA should be ≤THA to make the mix ASR mitigated. Results from the screening tool demonstrate an 82% reliability in predicting fly ash dosage necessary to keep expansions below 0.04% based on AASHTO TP 142 tests for ASR expansion. In addition, a multiple non-linear regression model was developed to predict water-soluble alkali from fly ashes in place of testing using 50 experimental data points from laboratory measurements and literature studies. Overall, the screening tool presents a rapid and reliable approach to determine the optimum fly ash content required for ASR mitigation based on pore solution composition of mixes and aggregate reactivity.
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