Effects of Solar Loading on Infrared Imaging of Subsurface Features in Concrete

Abstract

This paper addresses the effect of solar loading on thermal imaging for the detection of subsurface deterioration in concrete bridge components. The deterioration of the concrete resulting from corrosion of embedded mild reinforcing and prestressing steel results in delamination and spalling that can affect the strength and serviceability of a concrete structure. The ability to detect this deterioration during its early stages, when mitigation efforts can be employed, can provide a useful tool for maintenance and inspection personnel. Infrared thermography is a tool that has the potential to detect these subsurface defects to compliment inspection efforts and provide an additional means of condition assessment for a bridge. However, the technique relies on thermal gradients developing in the concrete such that a temperature contrast exists between damaged concrete and sound concrete. The environmental conditions at the bridge, such as direct solar loading, ambient temperature variation, and wind affect the thermal gradient in the concrete, and hence, the ability to image subsurface features. The effects of direct solar loading on the detection of subsurface targets in a concrete test block have been studied. Quantitative measurements of the thermal contrast that appear in thermal images of the test block are reported and analyzed. The effect of the depth of the embedded target is discussed, as well as the timing of inspection (relative to sunrise) that resulted in maximum contrast in thermal images.