On-Site Measurement of the Water-Cementitious Ratio and Heat of Hydration of Delivered Concrete

In this study, AASHTO T318-15 was adopted to estimate the water content of fresh concrete mixes and then revised to have better precision. The additional step required sieving out the coarse aggregate after drying the sample in a microwave oven, and it was then used in the calculation of the absorbed water and cementitious material content. Several laboratory batches, as well as on-site water-cementitious (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 that the accuracy of the revised method was increased to an average percentage error of 2.16% from the actual w/cm ratio, while the method based on AASHTO T318 was 6.2%. For cases with high chemical admixtures 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. A simple on-site measurement for the heat of hydration of fresh concrete was conducted using an insulated 1-meter cube. The cube was designed to be well insulated with 8-cm of insulation material on each side. It acted as a semi-adiabatic calorimetry since the heat loss was small. The temperature-time history at the center was used to calculate the adiabatic temperature rise (ATR) of the concrete. An average heat loss characteristic parameter was obtained for each delivered batch using the measured concrete temperature-time history after five days of curing. A finite element model (FEM) was developed to analyze the cube at different ambient environmental conditions. A table with the required insulation for various ambient conditions with a range between -10 to 30 °C ambient temperatures was also provided so that the setup can accurately estimate the ATR using only the temperature measurement at the center of the cube. The w/cm ratio and the heat of hydration test methods were evaluated in three on-site castings in the state of West Virginia. Two of the on-site castings contained Grade 100 ground granulated blast furnace slag. The third on-site casting used Class F fly ash concrete. Several compressive strength cylinders were made and cured at a temperature of 23 °C. The materials used in each on-site casting were also collected to perform compressive strength, adiabatic, and isothermal heat of hydration tests in laboratory conditions. The compressive strength measured on-site was lower than the laboratory batches which indicates a higher w/cm ratio was received on-site. The adiabatic temperature rises calculated from the cube’s center temperature compared well with the results from both the adiabatic and isothermal calorimetry. Results show that the on-site temperature measurement using the 1-m cube can be considered as a simple and accurate approach to measure the heat of hydration of a delivered concrete batch

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 mate- rial (w/cm) ratio tests, were performed on concrete mixes containing ordinary Portland cement, ground-granulated blast fur- nace 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 calcula- tions 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