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Thermography uses temperature sensors to detect voids in or below concrete, areas of delamination and areas of poor compaction. Air in voids, and in regions of delamination and poor compaction, conducts heat at a much lower rate than good quality concrete and hence the presence of these anomalies affects heat flow through concrete. These differences in heat flow cause small localised differences in surface temperature. The technique uses still or video cameras that respond to radiation in the infra-red range.
When carrying out thermographic surveys, the sun is often used as the heat source. (Alternatively strong artificial lighting can be used.) In this case, areas of voiding or delamination will tend to have a warmer surface as the heat is conducted away more slowly. Surveys can also be carried out at night using the stored heat. In this case, the heat flow is from the concrete or underlying ground to the environment. The surface of voided or delaminated areas is now cooler.
Thermography has the advantage that it is essentially non-destructive and that large areas can be surveyed in a short period. Its main uses are on large area structures such as highway carriageways, airfield pavements and car-park decks.
Acknowledgement: The Concrete Society
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