Jean-Paul Balayssac

Evaluation of concrete water content and other durability indicators by electromagnetic measurements

In order to use non destructive techniques (NDT) for the survey of reinforced concrete structures, it is important to show their ability to measure the cover concrete characteristics related to durability, in particular the concrete water and chloride contents. For this purpose, tests with two electromagnetic methods (GPR and capacitive probes) and impact echo method were carried out on 81 slabs of 9 different concrete mixes. Concrete porosity was ranging between 12.5 and 18%. Measurements were carried out at five different water contents. A real structure was also tested in situ. The NDT results are compared to concrete performance indicators such as porosity and water content, as well as chloride profiles. The comparisons show the complementarity of the methods to perform a pertinent diagnosis of concrete structures.

Numerical Simulations and Laboratory Tests to Explore the Potential of Ground-Penetrating Radar (GPR) in Detecting Unfilled Joints in Brick Masonry Structures

The objective of this work is to define the sensitivity of the ground-penetrating radar (GPR) signal to detect deep unfilled joint defects in the inspection of brick masonry structures and, in particular, to look for deep unfilled joint defects. This definition will help the manager to quantify the volume of mortar to be reinjected in case of reinforcing work. As a first approach, a numerical modeling of a GPR antenna with a central frequency of 1.5 GHz is used to define the sensitivity of radar waves to detect unfilled joint defects. The simulations are carried out in a separated bistatic configuration. For each transmitter position, several signal acquisitions are implemented using a regularly spaced crescent pattern for the receivers. A specific algorithm for the processing of the simulated signals has been developed that uses the inverse methods applied in the time domain and specifically a method of phase focusing to locate the defects. The processing analyzes the travel times of the reflected signals by making the assumption that each point of the modeled space is a scattering point. The calculation of the travel time, which helps to identify the signal corresponding to each point of space, is made by using an estimated speed of the direct wave between transmitter and receivers in the material, which is then regarded as representative of the whole of the simulated environment. A parametric study allowed limits to be set in terms of size, orientation and depth of the defect. Early results are promising and show that unfilled joints can be detected in the depth of the masonry structure with good accuracy. The last stage of this work is to test the validity of these algorithms on a full scale model with different kinds of unfilled joint defects before using them on real structures.

Ruptures of prestressing cables

The structures involved are prestressed concrete structures. The aim is to detect either failures of prestressing cables (or strands) or as a preventive measure to detect corrosion or damages before failure.

Non destructive evaluation of concrete contamination by chloride: Comparison of methods

ABSTRACT This paper aims to analyse the sensitivity of some NDT methods (radar, resistivity and capacitive technique) to chloride contamination. In laboratory, four different concretes with different water to cement ratio and two kinds of aggregates were conditioned with different saturation degrees of two solutions (30 g/l or 120 g/l of NaCl). NDT measurements show that the attenuation of radar waves is sensitive to the presence of chlorides but depending on the saturation degree of the solution while velocity is not affected. Resistivity is also very sensitive. The real permittivity measured at low frequency with capacitive technique is modified by the presence of chlorides. All the techniques were carried out on a real site. From the analysis of laboratory results, an attempt of evaluation of both moisture content and level of chloride contamination is proposed and compared to values obtained from cores extracted on a very near area.