National Concrete Pavement Technology Center

Project description

Concrete’s water to cementitious (w/cm) ratio is the most important indicator of the overall strength and durability of the structure. A method to determine the w/cm ratio on-site is important and needed for quality assurance and quality control. There are currently limited standardized techniques for the measurement of w/cm on-site and those techniques can be very time consuming. A developed in-situ microwave moisture meter exists that relies on the difference in the dielectric constant of concrete from that of water to estimate the w/cm ratio. In this report, the accuracy and precision of the meter is studied. Several paste, mortar, and concrete mixes with known w/cm ratios as well as aggregates with known moisture contents were measured with the Cementometerᵀᴹ device. An AASHTO T318-02 microwave test was also performed in comparison to the Cementometerᵀᴹ on known w/cm values. Statistical methods included a t-test determining the accuracy of the device.For precision, linear regression analysis, absolute difference, and sum of squared error calculations were made between the Cementometerᵀᴹ predicted w/cm and actual w/cm contents of the mixtures. Overall, the Cementometerᵀᴹ device was found to precisely measure moisture levels in aggregates, but in concrete none of the modes were precise (all R² values were less than 0.065) nor accurate (p-value less than 0.044). On the other hand, the in-lab AASHTO microwave method produced statistically precise (R² of 0.62 or more) and accurate (p-value of 0.91) readings, and thus the in-lab AASHTO T318-02 method is recommended over the in-situ Cementometerᵀᴹ for w/cm ratio determination.