Measurements of the Thickness of In-place Concrete with Microwave Reflection

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Previous microwave reflection measurements made on simple, unreinforced concrete blocks have shown that the transit time of a microwave through concrete is linearly related to its thickness. In this study measurements were conducted on concrete slabs that were built to simulate pavements and on an actual continuously reinforced concrete pavement to determine whether this type of rapid nondestructive measurement could be used in lieu of coring in the inspection of newly built concrete pavements for compliance with slab thickness specifications. It was found that reflections from the bottom of a concrete pavement slab may often be too weak and difficult to identify in a radar profile; therefore, the transit time of a microwave through reinforced concrete slab would often be difficult to measure with reasonable accuracy. The results obtained with radar measurements made on simulated pavement slabs showed that radar was only able to indicate whether a slab is too thin (i.e., less than 8 in); but those measurements made on an actual pavement yielded reasonably good agreement between the thickness determined by coring and those determined by reflection measurements in conjunction with a calibration procedure. Based on these mixed results, it would be difficult to predict how reliable radar would be for precise quantitative measurement of slab thickness of a particular pavement until an actual radar scan of the pavement has been conducted. If the reflection from the bottom of the slab appeared to be identifiable, then the calibration procedure could be used to determine the slab thickness for the entire pavement. Otherwise, radar can be used at least as a screening tool to spot areas in the pavement that appear to be to thin. These areas would then be cored to verify the radar readings.

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