Jamal Rhazi

Design and Calibration of a Large Open-Ended Coaxial Probe for the Measurement of the Dielectric Properties of Concrete

The subject of this paper is the design and calibration of an open-ended coaxial probe for the nondestructive measurement of the dielectric properties of concrete. Measurements are made between 100-900 MHz, frequencies which are often used in geophysics and civil engineering for ground penetrating radar inspection. The probe is calibrated using measurements on saline solutions in conjunction with three different mathematical techniques for comparative study. Measurements of mortar and concrete specimens having different water/cement ratios were made in order to observe the standard deviations due to their heterogeneous nature. Similar to the case of relatively homogeneous rock specimens (limestone and granite), the standard deviation for heterogeneous concrete samples do not exceed 5%. In addition, the effect of the concretes porosity on its dielectric properties was clearly observed: measured permittivity between 4-4.5 at 900 MHz for porous concrete, and between 6.5-7.5 at 900 MHz for dense concrete.

Application of Jonscher model for the characterization of the dielectric permittivity of concrete

The study of electromagnetic waves propagating in concrete is a complex problem. Understanding the phenomenon of interaction between the wave and the matter is related to the knowledge of the variation process of concretes electromagnetic properties in terms of its physical characteristics. In particular, dielectric permittivity of concrete is affected by moisture content and change in the frequency of the electromagnetic field applied. In this study, we apply the three-parameter Jonscher model (n, χr, e∞) to show the dispersive aspect of the concrete. The validation of this model is carried out through tests on mortar and concrete at the laboratory, on the one hand, and by comparison of the results with data obtained previously by other researchers, on the other hand. The Jonscher model matches very well the experimental measurements of the concrete. At different moisture levels, heterogeneities and porosities, the results obtained are very good. This shows that this model is very effective and very suitable to represent the dielectric properties of concrete.

Modelling dielectric-constant values of concrete: an aid to shielding effectiveness prediction and ground-penetrating radar wave technique interpretation

A number of efficient and diverse mathematical methods have been used to model electromagnetic wave propagation. Each of these methods possesses a set of key elements which eases its understanding. However, the modelling of the propagation in concrete becomes impossible without modelling its electrical properties. In addition to experimental measurements; material theoretical and empirical models can be useful to investigate the behaviour of concretes electrical properties with respect to frequency, moisture content (MC) or other factors. These models can be used in different fields of civil engineering such as (1) electromagnetic compatibility which predicts the shielding effectiveness (SE) of a concrete structure against external electromagnetic waves and (2) in non-destructive testing to predict the radar wave reflected on a concrete slab. This paper presents a comparison between the Jonscher model and the Debye models which is suitable to represent the dielectric properties of concrete, although dielectric and conduction losses are taken into consideration in these models. The Jonscher model gives values of permittivity, SE and radar wave reflected in a very good agreement with those given by experimental measurements and this for different MCs. Compared with other models, the Jonscher model is very effective and is the most appropriate to represent the electric properties of concrete.

Assessment of concrete slab quality and layering by guided and surface wave testing

This paper presents an experimental study on the investigation of concrete properties by guided and surface wave nondestructive testing. Applications were made on two large slabs simulating homogeneity and layering in concrete, respectively. An efficient nonintrusive method was used to evaluate the concrete quality by solving the modal propagation problem of Lamb guided waves and Rayleigh surface waves. Lamb waves were used to determine the Poissons ratio and the Youngs modulus of the concrete slabs. Rayleigh waves were identified using Lamb wave fundamental-modes; thereafter, the inverse problem of Rayleigh waves was solved to evaluate the variation of shear wave velocity with depth and thus characterize the layered slab. The obtained results demonstrate the high potential of this tool that can easily be used for in-place assessment 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.

USE OF JONSCHER MODEL FOR ESTIMATING THE THICKNESS OF A CONCRETE SLAB BY TECHNICAL GPR

The thickness measurement of concrete is one of the most important commercial applications of ground-penetrating radar (GPR) technique. This paper describes a procedure for estimating the thickness of concrete slab for difierent moisture contents (MCs) in frequency domain, as in Impulse-Response (IR) Method, over the radar frequency band (100MHz{2GHz). The method is based on predicting the re∞ected frequency spectrum through a concrete slab using Jonscher model. The procedure is explained and examples of results are presented.

Impact resonance method for damage detection in RC beams strengthened with composites

There are numerous successful applications of fibre-reinforced composites for strengthening the civil engineering infrastructure. Most of these repairs are being continuously or intermittently monitored for assessing their effectiveness and safety. The impact resonance method (IRM), a non-destructive technique, utilized in civil engineering exclusively for determining the dynamic concrete properties, could be a valuable and viable damage detection tool for structural elements. The IRM gives useful information about the dynamic characteristics of rectangular and circular concrete members such as beams and columns. In this experimental program, a 1.2-m-long reinforced concrete beam strengthened with a carbon fibre-reinforced polymer (CFRP) plate has been employed. The CFRP-strengthened beam has been loaded in fatigue for two million cycles at 3 Hz. The load amplitude was from 15 to 35% of the anticipated yielding load of the beam. Throughout fatigue testing the cycling was stopped for IRM measurements to be taken. The obtained data provided information about changes in modal properties such as natural frequencies of vibration. These results have shown the successful use of the IRM for detecting fatigue damage in concrete members strengthened with composites.