Studies of using infrared flash thermography (FT) for detection of surface cracks, subsurface defects and water-paths in building concrete structures

Abstract

Aging of building structures poses a serious safety concern in Hong Kong and detection of surface cracks and subsurface defects plays a vital role in building pathology. Present available NDT techniques are inadequate for assessing building integrity, in particular for concrete structures. Therefore, more effective NDT techniques are urgently needed. This thesis presents a novel NDT technique for building inspection using pulsed thermography with short duration heat pulse, namely flash thermography (FT). The technique is capable of detecting: (1) surface cracks, (2) subsurface defects, such as voids and debonds and (3) subsurface water path detection in concrete structures. The degree of deterioration of concrete structures is normally assessed by monitoring the formation of surface cracks and subsurface defects. Corrosion of re-bars which can be accelerated by water seepage within the concrete structure is also a major concern. Flash thermography allows full-field and non-contact qualitative observation and quantitative measurement of thermal radiation from the object surface. During inspection, the structure surface is excited with a heat-pulse of short duration. Surface crack is detected based on the difference in heat emission of cracks and intact region. This method is superior to the traditional inspection practice through visual inspection. Subsurface defects are detected by recording a sequence of thermograms and processing of the thermograms allows examination of the surface rate of temperature change. The presence of subsurface defects, either air or water will exhibit anomalies in the temperature change distribution, and hence debond and water path can be detected. Detectability of defects is enhanced by applying further processing technique, such as time-derivatives method. Through the consideration of time parameter and reference calculation, depth of subsurface defects and water path can also be estimated by depth evaluation method-logarithmic second derivative peak (LSDP) method. Unlike traditional moisture content measurement which requires contact and point-by-point measurement, the present method is a full-field and non-contact technique, which exhibits key advantages of both high inspection rate and high confidence level.

Date of Award	2 Oct 2008
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yiu Tommy LO (Supervisor) & Michael HUNG (Supervisor)