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 waveform. The waveform contains a record of the properties and thicknesses of the layers within the pavement.
The sequence of scans is frequently coded in color or gray scale to produce the “b” scan representation, which provides the equivalent of a cross sectional view of the pavement, with the individual pavement layers showing up as horizontal bands. The white and black bands indicate stronger reflections and occur when the dielectric contrast is greater. The grey regions indicate weaker reflections and occur when there is little dielectric contrast. Layer thickness is calculated from the arrival time of the reflection from the top and bottom of each layer, and the dielectric constant of the layer.
Computation of the dielectric constant of the surface layer can be made by measuring the ratio of the radar reflection from the pavement surface to the radar amplitude incident on the pave¬ment. The incident amplitude on the pavement is determined by measuring the reflec¬tion from a metal plate on the pave-ment surface, since the metal plate reflects 100% of the incident energy. A similar calculation can be made for the dielectric constant of the base material. Changes in base moisture content have a strong effect on the base dielectric constant, and thus the base dielectric constant can be used as an indicator of high moisture content.
Figure 1 – Measurement Setup
Figure 2 – Pavement Setup
The calculations described above are automated in Infrasense’s winDecar©data analysis software program for computing pavement layer thickness and changes in pavement layer properties. The analytical techniques described above serve as the basis for data analysis carried out during Infrasense’s pavement projects.