Colette Sirieix

Electrical resistivity measurement applied to cracking assessment on reinforced concrete structures in civil engineering

Abstract Non-destructive evaluation appears more and more important in the civil engineering economic stakes. In this context electrical resistivity measurements get sensitivity to parameters allowing to assess concrete structures conditions. This article analyses the ability of the resistivity measurement to study cracks in concrete. Its ability to detect and to locate cracks and spalling is shown with on site measurements on a damaged slab. Then specific studies on such disorders allow to distinguish the influence of their characteristics. The sensitivity of the method to cracks depth, according their moisture conditions, is assessed by computation. Experimental works on a reinforced concrete beam, ideally cracked, confirmed these results. Some assumptions based on measurements realised on a size-one structural component allow to assess the general influence of crack opening and bridging degree between crack lips. Qualitative results show the similar effects on measurement of various cracking parameters. Prospective works presented in the paper lead us to say that electrical resistivity method applied to civil engineering structures is a relevant tool for the assessment of structural damage.

Electrical resistivity monitoring with buried electrodes and cables: noise estimation with repeatability tests

Electrodes and cables are sometimes embedded or buried permanently into the ground in order to minimize electrode location errors during long electrical resistivity monitoring. This procedure is efficient and useful. In order to evaluate the feasibility and suitability of this technique for a long monitoring of water content in a rock mass, electrical resistivity data have been repeatedly acquired from a buried array. Forty-eight steel electrodes composed the array, connected to two multicore cables, which have been buried in a trench since January 2006. A fast resistivity-meter was used to carry out dipole-dipole electrical measurements, over a period of one year, starting in December 2006. Data acquisitions of repeated resistivity measurements have been realized in order to distinguish between variations in electrical resistivity due to noise and to changes in water content. Around once every month, successive dipole-dipole array measurements on the same day (between three and eleven arrays with around 10 minutes for one array) have been performed. Using all the pseudosections recorded the same day, the coefficients of variation have been calculated for each data point. Results show that the variability of the data is six times greater when the data from the first array of the day-series are taken into account. The data collected from this first array are therefore significantly noisier than the following measurements and must be removed for correct interpretation. We propose two main explanations for this effect: 1) polarization of the system array/clay at the time of the first acquisition and 2) damage encountered into the buried cables. Despite this damage, we have shown that the electrical data can be consistent and correctly exploited if the first acquisition of the day is not taken into account. In conclusion, on the basis of the results presented here, we recommend that further studies be made using buried equipment to systematically carry out several acquisitions before starting the long monitoring. We also recommend to remove data from the first acquisition whenever they are found to be significantly noisier than data of the following acquisitions.

Detection of landfill cover damage using geophysical methods

On closed landfills, impermeable covers are capping the waste in order to minimize water infiltration and accumulation of leachate inside the waste. In France, the cover composition depends notably on the kind of stored waste. In cases of hazardous waste, the cover must be composed of a drainage layer under the top soil and a geomembrane or a Geosynthetic Clay Liner (GCL) associated with an underlying 1.0 m thick low permeability material to ensure its tightness. However, this protection cover is sometimes damaged leading to an escape of landfill gazes and an unusual increase of leachate within the waste after rainy events. As leachate treatment is very expensive, it appears necessary to locate the weakness zones of the cover and assess their sizes in order to limit maintenance cost on landfills. In order to detect damages in the cover, the 2 following geophysical methods have been carried out on a French landfill: cartography with an Automatic Resistivity Profiling (ARP©), the Electrical Resistivity Tomography (ERT) and the Self Potential method (SP). The joint use of these methods has given us the opportunity to determine the contribution of each of them for covers characterization. The ARP survey has put in evidence the lateral heterogeneity of cover materials on the whole landfill. The ERT has confirmed the variability of the cover composition but also provided information about the cover thickness and the damaging of the GCL. SP measurements have revealed a negative anomaly at the top of the landfill, possibly linked with a thin and damaged cover and therefore a greater proximity of waste. Finally, manual augers holes have enabled to associate electrical resistivity properties with different materials used in the cover and the damaged GCL. This study on a hazardous waste landfill shows that geophysical methods associated with manual auger holes have allowed to improve the knowledge of the cover. Thus, the damaged areas detected thanks to measurements performed on site may be useful for the landfill manager who can optimize the drilling survey necessary to check the nature of defects and then choose a suited remediation of the cover.

Geophysical methods applied to characterize landfill covers with geocomposite

We attempt to characterize with geophysical methods the state of landfill covers to detect damages that can induce preferential water pathways and unusual increase of leachate within the waste mass. The geophysical methods used were the Electrical Resistivity Tomography (ERT), cartography with an Automatic Resistivity Profiling (ARP©), and the Self Potential method (SP). We worked on experimental parcels reproducing common defaults on landfill covers (clay material and geocomposite) and on a larger scale on a french landfill cover. The joint use of these methods gives us the opportunity to test their ability to detect defects. Results on the parcels have shown a good detection of the larger cracks (0.10 m) on the compacted clay cover but a less easy detection of defaults on the geocomposite. Results on the landfill have shown conductive zones correlated with important SP variations that could indicate a preferential infiltration zone in the cover.

Uncertainty analysis and probabilistic segmentation of electrical resistivity images: the 2D inverse problem

In this paper, we present the uncertainty analysis of the 2D electrical tomography inverse problem using model reduction and performing the sampling via an explorative member of the Particle Swarm Optimization (PSO) family, called the Regressive-Regressive PSO (RR-PSO). The procedure begins with a local inversion to find a good resistivity model located in the nonlinear equivalence region of the set of plausible solutions. The dimension of this geophysical model is then reduced using spectral decomposition, and the uncertainty space is explored via PSO. Using this approach, we show that it is possible to sample the uncertainty space of the electrical tomography inverse problem. We illustrate this methodology with the application to a synthetic and a real dataset coming from a karstic geological set-up. By computing the uncertainty of the inverse solution, it is possible to perform the segmentation of the resistivity images issued from inversion. This segmentation is based on the set of equivalent models that have been sampled, and makes it possible to answer geophysical questions in a probabilistic way, performing risk analysis. This article is protected by copyright. All rights reserved

Quality of concrete condition assessment using several non-destructive techniques

Non-destructive techniques (NDT) are often seen as a practical and efficient way to assess the material and structural condition of existing reinforced concrete structures. However, assessment cannot be reduced to measurement and interpretation, and asset managers and structural engineers often need a quantitative assessment. A combination of several techniques can offer precious help. This article intends to show what kind of improvement can be expected from such a combination. Examples are taken from a series of on-site case studies and laboratory experiments. The focus is on the assessment of water content and concrete porosity as these material properties are key factors regarding their present condition as well as their durability in an aggressive marine environment. Material variability, uncertainties on measurements and model error condition the accuracy of the parameter estimation. Their relative contribution to the quality of material assessment is analysed.

Characterisation of geometrical properties of concrete using passive infrared thermography

ABSTRACT This paper talks about thermal response of concrete during natural warming up. An experimental study is carried out. Experimental warming up curves are compared with numerical modelling based of the Fourier equation. In the last part we study the influence of density, specific heat and thermal conductivity on diffusivity coefficient. Study proves that infrared passive thermography performed to monitor the natural warming up of concrete is sensitive to variable thickness of concrete.

On the Geoelectrical Characterization of an Old Landfill Cover

Two geoelectrical methods were used to characterize the state of an old French landfill cover. The objectives were to locate different materials used for the covering such as clay material or geomembrane and to identify heterogeneities that could be linked to possible defects in the cover due to fractures or cracks. These damages can induce preferential water pathways and unusual increase of leachate within the waste mass. The geoelectrical methods used were the electrical resistivity cartography with an Automatic Resistivity Profiling (ARP©) and the Self Potential method (SP). Results have put in evidence three distinct zones with different geoelectrical signatures that are correlated with three different phases of landfill covering. ARP also seems to have detected the presence of geomembrane installed all around the old landfill to improve its stability and geodrains over two closed alveoli. Local differences in apparent resistivity and self-potential signals have also been identified and could result in default in the clay cover due to thickness variations but also in different lithology, compaction and water content.