Investigation of Mix-Variation Effect on Defect-Detection Ability Using Infrared Thermography as a Nondestructive Evaluation Technique

Abstract

Infrared thermography (IRT) is a promising nondestructive evaluation technique that has been widely used for bridge deck inspection. It is a quick and fairly simple method for providing preliminary information about defected areas in bridge decks. However, detection capabilities and sensitivity to environmental conditions and material properties impose certain challenges for the application of IRT. In this paper, the effect of various concrete mixtures on passive IRT was investigated. Four concrete mixtures (i.e., conventional, high-strength, self-consolidated, and lightweight concrete) were considered in the evaluation. Various defects with relatively small sizes placed at different depths were simulated and planted in cast-in-place slabs. The casting and testing took place in the United Arab Emirates (UAE), which is considered a hot-weather region. The defected slabs were imaged, and the existence of defects was judged visually. A minimum of 0.8°C was used for successful detection to make a more confident judgment for the existence of a defect. In addition, the images were compared with those of nondefected slabs to enhance the judgment of defect detection. Thermal conductivity of each mix was estimated using the Maxwell–Eucken two-phase composite-material model, and rapid chloride penetration was tested and used as an indication of concrete density. The slabs were imaged during the cooling cycle. Results showed that mix variation had a significant effect on IRT. High-strength concrete provided the highest detection possible among the mixes. Results also showed that the higher the density and thermal-conductivity coefficient, the better the defect detection using IRT under ideal imaging conditions. For the UAE, ambient conditions are favorable for passive IRT because the results indicated minimal sensitivity to wind, relative humidity, and temperature, as opposed to other locations in the world where passive IRT would not be applicable. Nevertheless, because of its limitations, IRT would better serve as a health-monitoring technique using baseline comparisons rather than as an independent inspection tool for bridges.