Revised Method for Rapid Determination of On-Site Water-Cement Ratio using Microwave Oven

The water-cement ratio (w/c) of delivered concrete is one of the most important parameters of the material's quality. In this study, the AASHTO T318-15 guideline was adopted to estimate the water content of fresh concrete mixes and was revised for better precision. The additional step required sieving out the coarse aggregate after drying the sample in a microwave oven, and using it in the calculation of the absorbed water and cementitious material content. The cementitious content was assumed to be proportional to the mix design ratios. Several laboratory batches, as well as on-site water-cementitious material (w/cm) ratio tests, were performed on concrete mixes containing ordinary Portland cement, ground-granulated blast furnace slag, and Class F fly ash. The results of the experiments indicated the accuracy of the revised method was increased to have an average percentage error of about 2.16% from the actual w/cm ratio whereas the method based on AASHTO calculations was 6.2%. For cases with high chemical admixture dosages, washing vinegar was used to wash out the particles around the dried sieved coarse aggregate to calculate the w/cm ratio with a more precise mass for each sample. The correlation between the measured and calculated compressive strength using the measured amount of w/cm ratios provides evidence for the method's accuracy. Therefore, the revised method can be used as an accurate and practical process of measuring the on-site w/cm ratios of fresh concrete mixes.