BENEFITS OF MEASURING HALF-CELL POTENTIALS AND REBAR CORROSION RATES IN CONDITION SURVEYS OF CONCRETE BRIDGE DECKS. FINAL REPORT

The benefit of using half-cell potential measurements in condition surveys of concrete bridge decks was examined using data collected from several decks by visual inspection, sounding with chain drags, and measurement of half-cell potentials. As suspected, half-cell potentials on a deck were observed to fluctuate from survey to survey, mostly in response to seasonal fluctuations of temperature and moisture in the concrete. Consequently, in contrast to the interpretation guidelines recommended in ASTM C 876, the numerical value of each measured half-cell potential by itself is a poor indicator of the probability of corrosion occurring on a rebar, and even less of an indicator of the condition of the surrounding concrete. Instead, the potential observed at each location should be considered in relative terms, i.e., in relation to potentials observed at the surrounding concrete. Therefore, when all the potentials observed were plotted on an iso-potential contour map, the locations of active rebar corrosion and delaminated concrete were found to be highly associated with high negative potential gradients. In addition, due to the localized nature of rebar corrosion, the grid spacing of 4.0 ft recommended in ASTM C 876 for use in surveys of bridge decks (and the 5.0-ft spacing used by many state transportation agencies) was found to be too large for locating existing active corrosion and the associated damage to concrete. It was determined that a grid spacing of no more than 2.0 ft should be used. When half-cell potential surveys are performed in a sufficiently small grid spacing and the collected half-cell potentials are plotted on contour maps of isopotential lines, the locations of all existing active rebar corrosion and damaged concrete are indicated with a high degree of accuracy by relatively high potential gradients. When combined with the other inspection techniques, such a survey would, therefore, be extremely useful for estimation of repair quantity. The benefit derived from being able to measure the corrosion rates of rebars in an existing concrete bridge deck, as part of condition surveys, was also examined. The 3LP device appeared to be a convenient tool for measuring corrosion rates. It yielded rebar corrosion rates that correlated reasonably well with metal losses observed in rebars specimens extracted from the decks. Further, it appeared that metal losses of 3 to 6 percent by weight of the rebars may be the threshold metal loss necessary to initiate delamination in reinforced concrete decks. Rebar corrosion rates also vary with location in a deck and with temporal fluctuating conditions in the concrete. Therefore, until appropriate methods have been developed for determining the representative rebar corrosion rate for a deck (that accounts for these variations) and relating it to future concrete damage or remaining service life, the benefit of measuring rebar corrosion rates in condition surveys of concrete bridge decks will not be fully realized.