Chloride induced corrosion is a main cause for damages on concrete structures and thus
responsible for a large share of the cost for the rehabilitation of concrete structures. On bridge decks covered with asphalt the concrete surface can not be inspected visually or with common potential techniques. As a result the planning of the repair of bridge decks is usually based on a small number of probes and the experience of the engineers involved. Therefore, a non-destructive method for the localisation of zones with increased chloride contents on bridge decks is desirable.
Ground-Penetrating-Radar (GPR) is a non-destructive inspection method that can be applied on large objects such as bridge decks. Today, it is mainly used for structural problems such the concrete cover of rebar or the localisation of tendons. There, asphalt pavement does not cause significant problems. Several authors have suggested GPR as a method for the localization of damages and zones with increased chloride contents and a number of pilot studies has been carried out. However, no final conclusion has been reached on the applicability of this approach.
To examine the suitability of GPR as an inspection tool for chlorides.
Laboratory experiments and testing on bridges.
A field test was carried out on two bridges on Swiss motorway A12, Pont sur la Denève and Pont sur la Veveyse de Fégire to examine the suitability of the method.
In a laboratory experiment it was shown that both chlorides and moisture in concrete have a measurable influence on the damping of the radar signal and thus on radar amplitudes. This effect seems to be large enough to be applied for the inspection of real bridges. The computation of the quotient of the reflection amplitudes at the concrete surface and the bottom of a concrete specimen resulted in values that were twice as high for specimens containing chloride and moisture than for dry ones without added chlorides. Thus, this approach should also be stable under field conditions where additional effects will be present. An analysis of the velocity of the radar signal also showed the influence of chlorides and moisture.
However, this effect was much smaller than the impact on the damping of the signal. As the analysis of the signal velocity is a good deal more complex than that of the damping, this approach has to be considered not very promising at present.
Although the value of chloride inspections using Ground Penetrating Radar could not be defined conclusively during the field tests, the results of this research project are indicating that radar is applicable to this problem. A detailed verification of this statement with the help of a field test is recommended.
The cost for chloride inspections of bridge decks depends on numerous factors and was therefore exemplified for the Pont sur la Denève. Radar data were recorded and analysed along parallel lines over the whole length of 122m and the width of 24m. The time required for the preparation and execution of the data acquisition, the analysis of the data and the preparation of the report was about 90 man-hours. In addition, there is some work that has to be carried out by the awarding authority such as the closure of single lanes during radar data acquisition and the sampling and analysis of the probes. If the proposed method should become routine, there will certainly be space for an optimization and thus for cost reduction. When compared to the cost for the rehabilitation of bridge decks, the expenses for radar surveys are certainly economically justifiable.