Volume 5, Issue 7 (July 2008)
Catching the Elusive Water-Cement Ratio Using Petrographic Methods—and Their Evaluation
Petrographers use a variety of techniques for estimating the w/c and w/cm of hardened concrete. The estimates are routinely accepted by the concrete industry. Generally, four general methods are popular today: (1) water-droplet absorption; (2) scratch hardness; (3) combination of 12 or so microscopical and physical observations of the paste; and (4) methods where thin sections of concrete are impregnated with blue- or fluorescent-dyed epoxy. Sometimes combinations are used. There are few specific details in the literature for assessing the precision of the w/c using these methods except for the fluorescence technique, where there has been controversy about its claimed accuracy. The degree of interpretation of data from each method is based upon the comfort of petrographers in extending their expertise to provide that estimate. The deftness, skill, and experience of the petrographers will usually dictate their comfort zone. However, sometimes confounding that estimate is the existing concrete condition. The acceptability of the petrographic estimates depends on: (1) competency of the petrographer; (2) validity of the technique(s) used; (3) ability of the petrographer to qualify the techniques used; (4) the purpose(s) to which the data will be used; and (5) needs of those who either want to accept the estimates or debunk the estimates—for whatever reason.