Environmental effects on subsurface defect detection in concrete without solar load

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] This study investigated the application of infrared (IR) thermography for the detection of subsurface damage in concrete. The deterioration of concrete as a result of corrosion of embedded reinforcing steel is a widespread problem. Cracking in the concrete that develops at a subsurface level manifests as delaminations that lead to spalling. These delaminations develop due to the expansion of the reinforcing steel during corrosion processes, and as such typically occur at the level of the reinforcing steel mat. As a result, this deterioration is not observable during typical visual inspections, and nondestructive evaluation (NDE) techniques such as thermography are needed to detect this deterioration in its early stages, when mitigation and repair strategies can be effective. Thermography relies on ambient environmental conditions to induce thermal gradients in the concrete that are necessary to make subsurface features observable in a thermal image. The objective of this study was to characterize the environmental conditions that enable the detection of subsurface defects in concrete bridges, and evaluate the effects of environmental parameters such as diurnal temperature variations, humidity and wind speed. The study focused on concrete surfaces not exposed to radiant heating from the sun to develop data to support guidelines for the application of IR imaging for detecting subsurface defects in concrete. A large concrete test block was constructed with embedded Styrofoam targets at various depths to simulate the effects of a subsurface delamination in the concrete. The surface temperature variations of the block were monitored continuously over a period of three months with a thermographic camera, and recorded together with onsite weather station data.