This research describes the development of an entirely new method for determining chloride quantity in bridge decks using nondestructive GPR technology in combination with a limited number of cores for calibration. Chloride infiltration into concrete is the major cause of corrosion induced delamination in steel reinforced bridge decks and repairing delaminated concrete is a major cost factor in bridge deck rehabilitation. Knowledge of the quantity and location of chlorides in bridge deck concrete is an important factor in decisions relating to the type and extent of repairs. Traditionally, the measurement of chlorides involves core sampling and laboratory testing of concrete samples. While this gives an accurate measure of chloride at the location of the core, it cannot readily determine the distribution of chlorides throughout the deck or the maximum or minimum chloride amounts unless a great number of cores are taken. The GPR method developed here has the potential to provide that information and provide bridge owners with detailed information on the quantity and location of chlorides in their bridge decks, thereby improving the effectiveness of repairs, reducing the cost of maintenance and repairs, and potentially increasing the lifespan of the bridge deck.
GPR signal attenuation in bridge deck concrete occurs as a result of the conductive nature of the concrete when water and chlorides are introduced. The current research approach focuses on the use of non-contacting GPR and the measurement of signal attenuation to determine the quantity of chloride in the concrete, and specifically to demonstrate the possibility of utilizing radar technology along with limited coring (two or three core samples) and laboratory chloride measurements to produce an accurate and quantitative, spatial mapping of chlorides in bridge decks. The results of this research show that this is possible, based on in-situ field testing which was confirmed by analytical modeling and laboratory experimentation.
A three-pronged approach was taken consisting of (1) the development of an analytical model, which described through mathematical derivation the GPR losses or attenuation in chloride contaminated concrete, (2) laboratory experimentation with sand/gravel test specimens mixed with varying concentrations of chloride and moisture, and (3) field testing on an asphalt overlaid, concrete bridge deck using laboratory chloride measurements to calibrate GPR signal attenuation measurements. In each of these approaches the attenuation-chloride relationship for bridge deck concrete was derived and quantified. The analytical modeling and laboratory experiments established and confirmed the fundamental theory behind this technique. The field tests on a bridge deck demonstrated the feasibility of the technique in-situ and showed that GPR can predict with good accuracy, and spatially map bridge deck chlorides based on measurement of signal attenuation.
The final report is available.
