Principles of Ground Penetrating Radar (GPR) for Bridge Deck Condition Evaluation
Ground penetrating radar operates by transmitting short pulses of electromagnetic energy into the pavement using an antenna attached to a survey vehicle. These pulses are reflected back to the antenna with an arrival time and amplitude that is related to the location and nature of dielectric discontinuities in the material (air/asphalt or asphalt/concrete, reinforcing steel, etc). The reflected energy is captured and may be displayed on an oscilloscope to form a series of pulses that are referred to as the radar signal. The signal contains a record of the properties and thicknesses of the layers within the deck, as shown schematically.
Bridge deck deterioration can be inferred from changes in the dielectric properties and attenuation of the GPR signal in concrete (see Maser and Roddis, 1990; SHRP C-101, ASTM D 6087-08). GPR was originally developed for overlaid decks, since access to the structural concrete surface via other traditional methods is limited. The concepts have been shown to work equally well on non-overlaid decks. The variation of the dielectric constant of the deck concrete is used as one measure of deterioration (Maser, 1990). The dielectric constant is a measure of density, chloride and moisture content, and large variations in the dielectric constant can indicate concrete scaling. Where there is an overlay, these variations can also indicate advanced stages of overlay debonding.
The attenuation (loss of signal strength) of the radar signal, as measured from the top rebar reflection and/or the bottom of the deck, is used as a measure of concrete delamination. This is because contaminated and delaminated concrete will cause the GPR signal to dissipate and lose strength as it travels through the deck and reflects back from the rebar and the bottom.
The GPR equipment used for this inspection project is a dual 1-GHz or 2-GHz horn antenna system manufactured by GSSI, Inc. of Nashua, NH. The survey vehicle is equipped with an electronic distance-measuring instrument (DMI) mounted to the rear wheel of the survey vehicle, providing continuous distance data as the GPR data is collected. The data collection and recording is controlled by either the SIR-20 or SIR-30 GPR system operated from within the survey vehicle. Using this system, the GPR data is collected with a series of longitudinal passes, each at fixed offset from the curb, at a collection rate of at least 4 scans per linear foot. The overall objective of the survey layout is to generate passes at 3-foot transverse spacing from curb to curb. The GPR survey is carried at speeds up to 50 mph; no lane closures are required while data is collected.
The bridge deck analysis is carried out with Infrasense's proprietary software winDECAR® using the following steps:
(1) Identification of the beginning and the end of the deck in each radar file, and check of the radar distance measurement against the known length and other features within the deck;
(2) Identification of features (AC overlay, top rebar, bottom of deck) that appear as dielectric discontinuities in the GPR data (see example data, Figure B3);
(3) Setup of the analysis for all of the passes for a given deck, computation of concrete dielectric constant, rebar depth, (AC thickness, if applicable), concrete attenuation, and approach slab bottom amplitude;
(4) Mapping the results and calculating quantities.
The analyzed GPR data is presented in the form of contour plots. The potential areas of delaminated and scaled concrete are identified by a threshold, which is calculated from the data. The combined influence of surface dielectric constant and attenuation is used as the measure of deterioration. For the approach slabs, areas identified as having a relatively high amplitude at the approach slab bottom/ supporting material interface are identified as indications of either an air gap or high moisture levels.
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