Determination of qualitative parameters in pavements using ground penetrating radar (GPR) method

Voids of asphalt and water content of its base are among important properties of pavements that play significant roles in the bearing capacity of the pavements. A conventional method for quality control of pavements is to take cores. However, this method is time-consuming and expensive. In addition, it provides discontinuous data from the pavements. In this regard, one of the new challenges for engineers who construct roads is to determine qualitative parameters of pavements using non-destructive methods. Ground penetrating radar (GPR) is a non-destructive geophysical method that has been considered for this purpose. This method, opposite to the above-mentioned conventional method, is practically fast and relatively inexpensive. It also provides continuous data from pavements. In this method, a transmitter antenna sends an electromagnetic pulse to the ground reaching subsurface structures or anomalies. These pulses are reflected by electrical inhomogeneities or anomalies, and then, the reflections are received by a receiver antenna. One of the effective and recent applications of this method is the qualitative control of pavements although the primary aim of using this method in pavements is to determine the thicknesses of the layers below the pavements including asphalt, base and sub-base layers.
    In this research work, 12 asphalt samples containing same materials but of different densities were prepared in laboratory. Void contents of the asphalt samples were determined using D2726 ASTM standard instructions. Similarly, 9 base samples containing same materials but of different moisture contents were also prepared in laboratory then, applying the surface reflection method, dielectric constant values of asphalt and base samples were calculated using 800 MHz and 250 MHz GPR shielded antennas, respectively finally, it was attempted to establish a mathematical relationship between dielectric constant values and void contents of the asphalt samples. In a similar manner, a mathematical relationship was established between dielectric constant values and moisture contents of the base samples our results demonstrated that the dielectric constant values of the asphalt samples increased exponentially with increasing the density values of the asphalt samples via decreasing the void contents of the asphalt samples and vice versa. Furthermore, the amounts of dielectric constant of the base samples increased with increasing the moisture contents of the samples linearly. This showed that as the moisture contents of a base sample increased, the attenuation of the electromagnetic waves, generated by the GPR transmitter antenna, intensified in the base sample and the depth of penetration of the electromagnetic waves was remarkably reduced.
    It should be noted that the quantitative results, presented in this paper, contain reasonable errors, mainly because the surfaces of the samples are relatively small in comparison to the effective surface of the electromagnetic waves emitted from the GPR antenna. To obtain more accurate dielectric constant values from the tests, it is necessary to use GPR antennas with higher central frequencies than those used in this research work, and to use samples with larger surfaces than those used in the carried out experiments however, the results of this laboratory research work can be generalized to the real field work. Moreover, the results of this research work indicated that how engineering properties of pavements were well controlled and obtained by the use of non-destructive geophysical GPR method without needing to take cores from the pavements.