The investigation of the validity of a single k-value for fly ash in the range of W/C ratio critical for the standards

•The cementing efficiency factor decreases significantly with decreasing W/C ratio.•The prescriptive k-value led to degradation in strength class at low W/C ratio.•The equivalent W/C ratios differed significantly after testing actual performances.•The effective k-values were relatively low in strength and high in durability.•A performance-based approach by measuring k-values at different W/C can be safer. The prescriptive k-value of the standards should cover a range of W/C ratio in fly ash concretes. This range corresponds to the requirements for the environmental exposure classes. The results of the present study showed a significant decrease of the k-value at low W/C ratio for compressive strength. The k-values measured at 0.60, 0.50 and 0.40 – W/C ratios enabled safer results. The durability performances of the FA concretes were generally better. The equivalent W/C ratios (and effective k-values) were considerably different from the nominal ones. The results indicated that the performance-based k-values could be more reliable than the prescriptive k-value.