Alternative in-situ Water-Cement Meter Using a Parallel-Plate Capacitor Concept

Project Details









Amanda Bordelon




Admixtures; Capacitors; Measuring instruments; Moisture content; Prototypes; Water cement ratio

Project description

One of the best predictors of strength and performance for concrete is the water-to-cement (w/c) mass ratio. As useful as this number is, it is difficult to control the water content in the field because often time contractors will add unknown amounts of water to improve on-site workability of the concrete. If excess water was added on site, the strength and durability may be compromised. A challenge in the industry has been to have a device that can be used on-site before the concrete has hardened, which can accurately and precisely predict whether the water content has been increased. Currently Quality Assurance/Quality Control personnel are required to verify the water-to-cement ratio by performing one of two methods: either 1) take a sample of the fresh concrete to a laboratory site to perform the American Association of State Highway and Transportation Officials (AASHTO) T 318 standard for w/c ratio using a microwave, or 2) wait until concrete has hardened and test core samples in compressive strength to back-calculate an estimated w/c ratio. Both methods require samples to be taken off-site or waiting until after the concrete is hardened. The use of an in-situ w/c meter allows for an immediate on-site determination of the concrete mixture's w/c ratio. The company named NDT James has created a device called a Cementometer䋢, for which they have advertised as being "accurate" (James Instruments 2010) for estimating such on-site in-situ water-to-cement contents. However, statistical data using the device (shown in Figure 1), as tested by the Michigan Department of Transportation (Michigan DOT) (Peterson and Sutter 2011) or preliminary testing at the University of Utah has indicated otherwise, that the device is not accurate nor precise. The theory behind how the Cementometer䋢 device appears technically sound, as it relies on a microwave frequency to create electromagnetic permittivity, of which a dielectric constant can be measured. The dielectric constant for water is significantly higher (around 80) than solids like cement and aggregates (around 2 through 11), and higher than air (around 1) (Nave 2012). As such, the net relative permittivity should correlate well with amount of free water in the concrete. Since the existing meter is proprietary, there is little to no information available about its fundamental settings (frequency, current, or voltage) or internal calculations. Furthermore, the design of the device uses two probes, for which there could be lost electromagnetic rays into the surrounding concrete. A parallel-plate capacitor design (Liao et al. 2004) would be more practical to have an averaged relative permittivity reading across the concrete, rather than just based on the material between the two probes. The main objectives of this research are to develop and then evaluate the precision of a prototype parallel-plate capacitor style device to measure the fresh concrete mixture's water-to-cement content. This prototype device will be compared against existing standard methods for evaluating water-to-cement contents