Infrared Thermography and Modeling to the Concrete Deck with Internal Defects as a Non-Destructive Testing

This work is aimed to investigate the temperature characteristics at the surface of the concrete decks, to detect the existence of internal defects buried in the solid in such a way that enables the quantitative determination of temperature distributions from the concrete. For this purpose, it is supposed that the signature for temperature characteristics is performed experimentally by means of Infrared (IR) thermography. A quantitative analysis of computational model developed by FVM implement that temperature distributions on the surface of the concrete with the internal defects have a significant feature by the dimension of defect, thermal conductivities of a variety of materials filled in the defect. According to the distance reaching the defects from the surface, the thermal imaging affected by temperature differences are compared, The simulations are also evaluated with the thermal pattern acquired by a qualitative IR thermography in a local district of the concrete deck Nomenclature i, j subscript, tensor, g gravitation, m 2 /sec T temperature, °C u velocity, m/sec x coordinate, m symbol β therrmal expansion factor, 1/°C ρ density, m/kg ∆ difference, Introduction Thermal imaging technology using the infrared thermography is one of prospective non-contact and non-destructive technologies. Measuring temperatures of each point on the structure expanded to the broad object area, it provides an image of temperature distribution on an object. Over the year, the importance of infrared imaging technology in NDT for mechanical and electrical facilities has been increasing popularity mainly because it makes remote inspection possible by sensing the thermal radiation emitted from the warm object. With development of infrared imaging systems, this test features safety based on non-contact, high-speed measuring, and easy record preservation. IR thermal imaging as the method of NDT is used for inspecting whether outer finish materials of buildings are exfoliated. Since asphalt covers the deck, it is not possible to see with naked eyes until exfoliation goes further. Also, it is not easy to detect and predict in a variety of deteriorations to be + Corresponding author . Tel. : +82-41-330-1283; Fax:+82-41-330-1269 E-mail address: kwt@kongju.ac. kr Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 938-943 doi:10.4028/www.scientific. net/KEM.270-273.938 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,17:38:03) Title of Publication (to be inserted by the publisher) processed inside the concrete bridge deck slab from asphalt exfoliation. Acoustic NDT is one of typical NDT method applied to detect this kind of internal defects. This conventional test requires tester's subjective determination since sound should be made by hammering exfoliated area or dragging chain on the deck, and it takes much time. [1] These weaknesses are supplemented by infrared thermal imaging technology. Especially, thermal imaging system can be a useful tool for predicting and detecting inner defects from thermal image presented by differences of temperature distributions occurring due to defects existing in the structure through non-destructive test: detection of the defects of fuselage, heat control for buildings, detection of humidified areas and inspection of waterproof treatment. Fig. 1 shows the diagram of IR imaging system. In the figure, the thermal imaging system converts the infrared light emitted from thermally heated objects into electrical signal via infrared sensor and infrared camera (called scanner), and transforms it into an image. [2] Fig. 1. Principle of infrared thermography Fig. 2. Experimental apparatus in this study Experimental Apparatus and Numerical Method The Thermovision® 900 System produced by AGEMA Infrared Systems AB was used in the experiment Fig. 2 shows the experimental apparatus. It offers improved accuracy and resolution with greater image definition. Experiments was conducted with several concrete ducts for investigating thermal characteristics from infrared thermal imaging and then models to analyze the internal defects set below the surface of concrete structure by using numerical computation. In this experimental testing method, D4788-88 in ASTM was applied and, as the following reference, D4580 of ASTM was also applied. [1,3] Modeling to the IR Thermal Imaging. Measuring defects of concrete structures including bridge deck is very important in calculating endurance period of structure, and in terms of structure usability. However, measuring inner defects of existing structures is very difficult. To solve this problem, non-destructive test of concrete is advocated as an alternative technology in Fig. 3. [4] In this section, a model formula capable of undertaking thermal analysis in consideration of the thermal characteristics and air conditions of concrete is established. An experiment results are compared to verify the numerical computations. Experimental analysis. Two experimental slabs as concrete mold are made. They are filled in with artificial exfoliation.In Fig. 3, a slab contains steel reinforcing, and the other does not. Thermocouple(K-type) per slab is installed to measure the temperature of each area of the slab. Table 1 shows the type of defect materials, sizes and positions. In Fig. 4, experimental slabs with placed concrete was cured for more than one month in its natural state and locations of defects were inspected by using infrared thermal imaging system mentioned above. A steel for reinforcing steel Key Engineering Materials Vols. 270-273 939 Title of Publication (to be inserted by the publisher) concrete is SS400 D10. Coin marking center of defect location was placed on the concrete slabs, and infrared thermal images were captured for analysis through specific program of AGEMA900. [4,5,6] Fig. 6 shows a typical thermal imaging of he concrete slab. Three points on the first row and two points below are centers of defects A, B, C, D and E in the figure. Four coins were put on the four edges of point C in order to get thermal images. Top of Fig. 6 is marked hot since it is exposed to solar radiation. Defects A and C is shown to be hotter, making existence of exfoliation known. However, it was impossible to discern B with smallsized defect and three defects located deep, i.e., D, E and F. It can be used for evaluating deterioration of concrete on the asphalt-covered bridge deck. Numerical Analysis. In the case of numerical computation modeling of thermal images, temperature distribution found inside concrete must be analyzed by distinguishing it from defects