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About the author: Gary Wojcik performed this study as a NIST/National Research Council post-doctoral research associate at NIST where his work focused on the effects of environmental conditions on early-age properties and durability of concrete. He worked with the New York State Department of Transportation studying the interaction of atmospheric conditions and curing concrete bridge temperatures and moisture as a student in the Department of Earth and Atmospheric Sciences at the University at Albany, State University of New York. He is currently a meteorologist at Northrop Grumman Corporation in Chantilly, VA. The National Institute of Standards and Technology is an agency of the Technology Administration, U.S. Department of Commerce.
X-ray absorption (or more properly, x-ray attenuation) techniques have been applied to study the moisture movement in and moisture content of materials like cement paste, mortar, and wood. An increase in the number of x-ray counts with time at a location in a specimen may indicate a decrease in moisture content. The uncertainty of measurements from an x-ray absorption system, which must be known to properly interpret the data, is often assumed to be the square root of the number of counts, as in a Poisson process. No detailed studies have heretofore been conducted to determine the uncertainty of x-ray absorption measurements or the effect of averaging data on the uncertainty. In this study, the Poisson estimate was found to adequately approximate normalized root mean square errors (a measure of uncertainty) of counts for point measurements and profile measurements of water specimens. The Poisson estimate, however, was not reliable in approximating the magnitude of the uncertainty when averaging data from paste and mortar specimens. Changes in uncertainty from differing averaging procedures were well-approximated by a Poisson process. The normalized root mean square errors decreased when the x-ray source intensity, integration time, collimator size, and number of scanning repetitions increased. Uncertainties in mean paste and mortar count profiles were kept below 2 % by averaging vertical profiles at horizontal spacings of 1 mm or larger with counts per point above 4000. Maximum normalized root mean square errors did not exceed 10 % in any of the tests conducted.
Key words: cement paste; moisture; mortar; non-destructive testing; Poisson process; x-ray absorption.