National Concrete Pavement Technology Center

Project description

The focus of this project is the investigation of a new method for determining chloride quantity in bridge decks using nondestructive ground penetrating radar (GPR) technology in combination with a limited number of cores for calibration. Traditional measurement techniques involve 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. 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 radar to determine the quantity of chloride in the concrete, and specifically to demonstrate the possibility of utilizing GPR 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 and is further confirmed by analytical modeling and laboratory experimentation. It should be noted, however, that these results are based on limited experimental data and further testing of the accuracy and reliability of the method is recommended. A three-pronged approach was taken consisting of (1) the development of an analytical model, which describes through mathematical derivation the GPR losses or attenuation in chloride contaminated concrete, (2) laboratory experimentation with sand and gravel test specimens 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 attenuation. In each of these approaches the attenuation-chloride relationship for bridge deck concrete has been derived and quantified. Together, they define the fundamental theory for this technique and the methodology for putting it into practice.