The bridge deck condition is defined by the signals returned to the receiver; the speed at which they return; and other properties of the signal, including amplitude, attenuation, signal polarity, and wavelength. The signal responses are different for various interfaces due to the contrast in the electrical properties of adjacent layers. The electrical properties of primary interest are the electrical conductivity (inverse of resistivity) and relative dielectric permittivity (dielectric constant). These two properties govern the ability of the EM energy to penetrate that particular medium and the speed at which EM waves propagate through the medium, respectively. The contrast in the dielectric constant between adjacent materials dictates the strength of the reflected energy to the surface.
EM waves cannot penetrate into metals (e.g., rebar or metallic expansion joints). Even dense wire screens and thin foils are impermeable to EM waves. The penetration of the GPR’s EM waves is fair to good in most construction materials (concrete, hot mix asphalt, or engineered pavement soils). GPR signal’s penetration in concrete that is moist and high in free chloride ions (e.g., fresh concrete) or other conductive materials can be adversely affected. For example, the GPR signal will be highly attenuated in a reinforced concrete deck that has undergone severe corrosion induced deterioration, and limit the depth of assessment. Dielectric constant for typical construction materials, including concrete, are shown in the following table 1.
Table 1. Relative Dielectric Permittivity for Different Materials.
Material |
Dielectric Constant* |
Air | 1 |
Clay | 25-40 |
Concrete | 8-10 |
Crashed base | 6-8 |
Gravel | 4-7 |
Hot mix asphalt |
4-8 |
Ice
|
4
|
Insulation board
|
2-2.5
|
Sand |
4-6 |
Silt |
16-30 |
Silty Sand |
7-10 |
Water (fresh) |
81 |
*Moisture can significantly increase the dielectric values
(e.g., for gravel it may go from 5 to above 20)