Rebars and defects detection by a GPR survey at a L'Aquila school damaged by the earthquake of April 2009

After the earthquake of L'Aquila (Abruzzo Region, Italy) occurred on the 6th April 2009, Ground Penetrating Radar investigations of public infrastructures were performed in order to provide an early damage assessment about structural elements such as beams and pillars. In particular, here we present the results of a 1500 MHz GPR survey on a cracked beam of a public building of L'Aquila to verify the goodness of a restoration work by means of epoxy resin injections. First a classical processing routine has been performed in order to focus the rebars hyperbolas and possible reflections coming by the injections; a data volume was built and several depth-slices are here presented. The survey allowed to check the rebars geometry and the reliability of the intervention based on the epoxy resin injections. Limitations of the classical data processing approach are a great complication in imaging the scene also due to the risk of introducing subjectiveness elements in the data by the operator and to the poor resolution achieved for the deeper rebar layer. The possibility of repeating the measurements on the opposite face of the beam in order to detect the second rebar layer was here adopted as a solution of the problem; however this way of operating becomes unacceptable during an earthquake post-crisis phase. The possibility of overcoming these drawbacks was offered by the microwave-tomography (MT) technique. The MT technique were first tested by processing one of the collected profiles acquired on the joist and the result confirmed the ability of the technique to achieve good focusing also of the deeper layer of rebars.

I. Introduction

The GPR is a well-established geophysical technique for cavity and pipe detection [1], stratigraphy studies, archaeology and polluted monitoring areas [2], [3], [4]. Moreover, the GPR is particularly suitable when used in the frequency range between 1000 MHz and 2000 MHz, as an effective tool for non destructive testing in order to check rebars and defects in the reinforced concrete [5], [6]. Main difficulties related to the GPR technique are connected to the data processing and interpretation.