INFLUENCE OF GPR ANTENNA POLARITY ON DETECTABILITY OF VOIDS IN PLASTIC TENDON DUCTS IN P-T CONCRETE BEAMS
Dr MR Clark, TRL Limited, Crowthorne, UK
Dr A Giannopoulos, University of Edinburgh, UK
Professor MC Forde, University of Edinburgh, UK
Several test beams with known defects were designed in order to validate the results of earlier numerical simulations of GPR. Beams with both metal and plastic ducts were constructed. Note that the metallic ducts represent p-t beams prior to the 1992 Highways Agency moratorium on the use of p-t bridge beams (DTP Press Notice 260, 1992), whilst the plastic ducts represent post-moratorium p-t beams. A uniform cross-section of 400mm x 450mm and a length of 2000mm for all the beams was employed. For the results presented here one of these beams incorporating a plastic duct having 63mm external and 50mm internal diameters was used. A steel tendon with a diameter of 20mm was included at the centre of the duct.
In this concrete beam - to be illustrated in the paper - the duct is completely grouted apart from an 800mm long section that is completely voided. The main aim of the experiment was to detect the voided section of the duct when scanning along the side of the beam with GPR. For this investigation the cover depth of the duct was 145mm.
Both experimental and numerical modelling results suggest that the optimum orientation of the radar's antennas is perpendicular to the long axis of the ducts containing the post-tensioning tendons. Full details of the experimental results are given in the paper.