Defect detection of concrete structures using both infrared thermography and elastic waves Chia-Chi Cheng, Tao-Ming Cheng, Chih-Hung Chiang ⁎ Chaoyang University of Technology, Wufong, Taichung, Taiwan ABSTRACT ARTICLE INFO Article history: Accepted 14 May 2008 Keywords: Infrared thermography Concrete Elastic waves Impact echo There are many nondestructive testing techniques that can be applied to assess the condition of existing concrete structures with little expenditure of human labor. The objective of the current research is to pilot- study how one can combine active infrared thermography (IRT) with elastic wave techniques. In such a manner, the fast scanning of a large structure is followed by elastic wave probing at a given small area. Concrete slabs with embedded defects are heated by means of lamps prior to acquisition of thermal images. The thermal images recorded show clear indication of hidden defects of various depths and areas. At present, the results of IRT provide only limited quantitative information regarding the depth of hidden defects in concrete. Depth of defects is readily deduced based on spectral analyses of received elastic wave signals. The fusion of information gathered from IRT and elastic waves provides effective and efficient means for inspection of the building exterior. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Tiles are popular decorating materials for both interior and exterior of concrete structures. The bonding quality of tiles to the concrete substrate often deteriorates due to poor workmanship, high humidity, and variations in ambient temperature. Deficient performance of tiled-wall systems could seriously affect the functions and safety of concrete structures. Very limited information is currently available regarding the durability and performance of tiled-wall systems. For external tiles on non-load bearing walls, the performance-related factors include gravity load, wind load, and structural vibration. In addition, environmental parameters such as solar irradiance, high temperature and moisture, and heavy rainfall are also important in durability problems [1]. Infrared thermography (IRT) is an effective tool for fast and qualitative inspection of a structure's state. Many tests have been reported for defect detection using IRT in both laboratories and outdoors. For example, it is used successfully to locate subsurface flaws in FRP laminates bonded to concrete [2]. Typically the results are rated by the contrast and resolution of the thermal images acquired in a certain distance from the surface of the structure. The objectives of this study are twofold. First we try to apply active IRT inspection to concrete specimens. Mock-up testings of a tile-wall system are explored to further examine the applicability of IRT inspection of a concrete structure. To enhance the characterization of a subsurface defect, elastic wave methods including coin tapping and impact echo are also applied. Results will be analyzed based on a combined data set obtained from IRT and elastic wave techniques. 2. Background of infrared thermography When an object is heated, its temperature is increased due to energy absorption. Heat exchange between the object and the surrounding is a dynamic process involving conduction, convection, and radiation. Heat radiation may be viewed as the propagation of electromagnetic waves consisting of a range of wavelengths. Infrared radiation is placed between visible and microwaves in the electro- magnetic spectrum, covering the wavelengths from 0.75 to 14 μm. From the NDT perspective, infrared thermography is based on the principle that heat transfer in any material is affected by the change in material thermal properties, especially those caused by a subsurface defect. Infrared radiation from the surface of an object is detected and registered using an infrared camera. Temperature differences on the surface, which may be related to the subsurface defects, can thus be localized based on the thermal images. Much work has been reported for the theory and instrumentation of IRT and related NDT applications [2–5]. While technology advancement makes it easy to obtain the thermal images, the contrast and resolution still are in need of enhancement for practical NDT of concrete structures. Drawing from the abundant experiences in IRT of other applications [3,4,6], image contrast and defect characterization can be improved for IRT inspections of concrete structures. For example, the self-referencing technique is very promising in defect detection even with non- Automation in Construction 18 (2008) 87–92 ⁎ Corresponding author. No.168 Gifong E. Rd. Wufong, Taichung, 413, Taiwan. Fax: +886 4 23742325. E-mail address: chiangc@mail.cyut.edu.tw (C.-H. Chiang). 0926-5805/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.autcon.2008.05.004 Contents lists available at ScienceDirect Automation in Construction journal homepage: www.elsevier.com/locate/autcon