The response of ground penetrating radar (GPR) to changes in temperature and moisture condition of pavement materials

Abstract

The use of geophysical techniques to assess geotechnical and pavement structures can provide much useful information to the engineer. The development of ground penetrating radar (GPR) in recent years has led to its increasing use for pavement and geotechnical investigations, and the technique involves recording the amplitude and travel time of electromagnetic GPR signals reflected from features within the ground or structure of interest. Depths can be determined, and features of interest such as different layers, excess moisture, voids and changes in materials can be identified. The interpretation of GPR data depends largely on the ‘dielectric constant’ of the material(s), which governs the passage of GPR signals through a material and the amount of signal energy reflected from features within a structure. This paper reports an investigation of pavement material samples, conducted under controlled conditions, using GPR. The effect of changes in material moisture and temperature on the dielectric constant, and hence the passage of GPR signals, was investigated. Core samples of bituminous material obtained from highway pavement sites were used to conduct a series of laboratory tests, in which the temperature of the material was controlled in the range from -5 to +45 degrees C, and the dielectric constant and GPR signal velocity were determined. Also, the materials dielectric constant and signal velocity were determined under dry and soaked moisture conditions. The test programme allowed an assessment of the effect of changes in materials temperature and moisture condition to the response of data obtained during GPR investigations. The results of the testing showed that both moisture and temperature can have a significant effect on the data obtained from GPR surveys of pavement structures. Each material has a ‘dielectric constant’, which is a measure of its relative (to a vacuum) dielectric permittivity, and it is a critical parameter in the practical application of GPR data. The dielectric constant affects the velocity at which GPR signals will travel through the material, affects the ‘reflection coefficient’ (governing how much energy is reflected when there is a change in material), and also affects the resolution of data that is obtained. Therefore, understanding what factors affect the dielectric constant, and the degree to which they affect it, can assist in conducting GPR surveys and interpreting and understanding the data. The measurement of the dielectric properties of asphalt materials can also be useful to assess density and provide information on compaction quality control. The presence of relatively low dielectricity air (in air voids) within materials will affect the overall dielectric properties of an asphalt mix, so it is possible to assess the compaction (i.e. density) of pavement material by measuring dielectric properties of the asphalt (as described by Saarenketo, 1997). In GPR investigations, the dielectric constant can be determined by calculating the GPR signal velocity within the material. The velocity is related to the dielectric constant by the relationship shown in Equation 1, below: