GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.2 107 and for the water content estimation by means of Time Domain Reflectometry-TDR techniques (Topp et al. , 1980). These approaches, while being generally very precise and accurate, provide just punctual information and are also very time consuming and quite expensive. GPR can provide a continuous survey of a road segment: this is a very important factor since both the moisture content and thickness may have strong lateral variations. Moreover, the GPR is a fast and quite cheap technique. Applications of GPR on road surfaces can be divided into four main categories: 1- surveys for planning of new roads; 2- investigations for the renovation of existing roads; 3- quality control or security investigations during road projects; 4- targeted surveys to the management of the road surface (Saarenketo, 2009). In recent years, various applications have been tested for different purposes, ranging from thickness mapping to quality controls, with the development of techniques to achieve an accuracy of 2.5 mm (Maser et al. , 2003) and absolute errors of 2.9% by comparing direct data and radar profiles (Al Qadi et al. , 2003). Moreover, examples of detection of discontinuities, defects and studies of infiltration are reported (Saarenketo and Scullion, 2000, Grote et al. , 2005), as well as measurements aimed to the moisture content and the density estimation and to the quantitative characterization of the materials forming the pavements (Benedetto and Pensa, 2007; Benedetto, 2010; Plati and Loizos, 2013; Shangguan et al. , 2014). In surveys performed on infrastructures with high traffic, the major advantages of GPR surveys are the continuous profiling, as well as the speed and accuracy. Such technique continues to be the only one that can provide meaningful subsurface information at an acquisition speed close to the highway velocity limits. The development of GPR application in infrastructures open to the traffic started in the seventies of last century with systems mounted or dragged by a van; from 1980s in Finland the method become the most popular and used as a routine surveys tool in various road applications (Saarenketo, 1992). Then, it soon developed especially in north Europe and nord America and actually it is a worldwide used technique for different purposes of the roads monitoring. The GPR hardware is generally mounted on a survey van and made up of a control unit, cables, a positioning System (GPS); air or ground coupled antennas may be mounted on the van or dragged. The instrument is triggered by both constant time intervals or electro-mechanic odometers. The system can also record simultaneously data using different frequencies. This aspect is very important because the higher frequencies allow to reach higher resolution levels but within limited depths, while antennas transmitting at lower frequencies penetrate deeper even if the degree of detail is reduced. We describe the method for GPR data acquisition by using the R.Ex. system showing applications for: 1) the analysis of the thickness and the shallow stratigraphy in an airport transit area; 2) thickness evaluation in a newly constructed aircraft apron; 3) thickness lateral changes on recently re-paved roads. Method. Esplora srl and the geophysical group of the Department of Mathematics and Geosciences of the University of Trieste developed a prototype of a GPR equipment support for roads investigation, that has been named R.Ex.-Road Explorer (Fig. 1). It is composed by a tow entirely made in fiberglass in order to avoid electromagnetic (EM) noises caused by metal supports; it includes special anti-shock boxes to support two ground coupled GPR antennas with different frequencies. The wheels of the tow contain an encoder for the trace positioning, connected to the central unit. The latter is positioned inside the van that drag the R.Ex. system; depending on the conditions of wear and alteration of the surface, the van can reach speeds up to 50-60 km/h, without altering the GPR records. A Malå ProEx GPR is used to transmit simultaneously two EM waves with different frequencies by connecting two antennas to the central unit. All the antennas are shielded in order to avoid or limit spurious signals. One of them is in the frequency range 800-500 MHz,