Title: Laboratory Evaluation of Nuclear Gage for Measurement of Water and Cement Content of Fresh Concrete

Author(s): David Whiting and Mohamad Nagi

Publication: Materials Journal

Volume: 96

Issue: 1

Appears on pages(s): 101-108

Keywords: air entrainment; calibrating; cement contents; concrete; tests; water content

DOI: 10.14359/435

Date: 1/1/1999

Abstract:
A laboratory test program was undertaken to determine the effects of variation in concrete constituents and proportions of response of a nuclear water/cement content gage. A total of 14 separate test series were evaluated in the program. The evaluations included study of the effects of the following variables on the nuclear gage’s determination of cement and water contents: 1) air content; 2) hold time; 3) Classes C and F fly ash; 4) ground, granulated blast furnace slag (GGBFS); 5) maximum size of coarse aggregate; 6) limestone coarse and fine aggregates; 7) basalt coarse aggregate; and 8) temperature. The testing showed that the cement content probe is sensitive to all materials containing calcium atoms; therefore, the gage must be calibrated with exactly the same materials as will be used on the job in question. While the water probe is, in theory, only sensitive to hydrogen atoms, there are some effects from other materials; therefore, calibrations using job materials are also required. With proper calibration, the cement gage is capable of determining cement content of fresh concrete to within approximately 10 to 20 lb/yd3 (6 to 10 kg/m3). The water gage is capable of determining water content to within approximately 2 to 4 lb/yd3 (1 to 2 kg/m3). From the predicted standard errors in water and cement contents, an error in calculated water-to-cement ratio (w/c) of as much as 0.03 may be expected. As entrained air decreases concrete density, which appears to have an effect on the gage response, it is necessary to control air content within very close tolerances (± 1 percent) in both laboratory calibrations and field work with the gage.