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Geophysical testing as a non destructive testing method for the evaluation of masonry |
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The geo-electric measuring technique, originally used in geology, is well known for nondestructive testing of massive structures. The technique is frequently used to evaluate injections of grouts and polymers for the consolidation of ancient masonry. The technique is quite simple. A current is sent through the structure and between two measuring electrodes at the surface the induced potential field is determined. The apparent resistivity is defined as the potential difference divided by the current, times a proportionality factor depending on the electrode's configuration. The measured potential field is influenced by voids, cracks and the overall situation of the masonry that enables to evaluate the need of consolidation and after injection, the evaluation of the execution of the injection work. The wanted information however, is partly hidden by many elements. To name a few of them: limited dimensions of the structure, moisture content, presence of steel objects in the wall and the use of different building materials. A method to eliminate the disturbance of the boundary effect and the holding position of the measuring electrodes has been developed. A relative difference map is obtained from which the relevant information is much easier to read. Now research is focussing on eliminating the other disturbing effects, i.e., the moisture content. Another goal of the research is to make a real time evaluation of polymer or grout injection possible. The penetrating grout will completely change the electrical behavior of the wall and hence produce big sudden differences in the resistivity of the material. The development of a monitoring system based upon the geophysical measurement technique could mean a useful tool for on site injection works and make it possible to adapt the process parameters. Doing an injection will no longer be a "blind" work. |
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