PSI - Issue 64

2

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

M. Pedram et al. / Procedia Structural Integrity 64 (2024) 621–628

622

Keywords: Concrete bridge; Subsurface defect; Infrared thermography (IRT); Structural health monitoring (SHM); Finite element analysis (FEA), LUSAS.

1. Introduction

The global population of ageing concrete bridges is on the rise, posing significant challenges in terms of maintenance requirements. This surge not only places a substantial burden on budgets but also undermines efforts to achieve Net-zero Carbon goals by 2050 (Ichi and Dorafshan, 2022, Pedram et al., 2022c, Pedram et al., 2022b). Hence, cutting-edge research is essential to tackle the practical obstacles hindering the deployment of sensors for non-destructive, contactless, rapid, wide-area, and remote condition monitoring of concrete components in bridges. Among the non-destructive testing techniques (NDT), IRT has shown potential for fast, contactless, non-contact, and wide-area condition monitoring of bridges (Kashif Ur Rehman et al., 2016, Watase et al., 2015, Doshvarpassand et al., 2019, Pedram et al., 2021). Conducting experimental studies in the laboratory, manufacturing concrete slabs and components with defects of varying shapes and depths, and testing them under different conditions, is laborious, expensive, and time-intensive. In this context, numerical solutions serve as indispensable complementary computational tools. FEA has emerged as an efficient numerical method and has been utilised to explore the impact of delamination size and shape at different depths (Hiasa et al., 2017). The results obtained from FEA serve as a foundation for making decisions about the requirements for IRT, considering factors such as the camera's specification and the available thermal excitation mechanism. Previous studies showed that the formation of a lateral heat flow in the concrete cover over the defect was the condition for the decisive detection of a true subsurface defect with a thermal camera (Pedram et al., 2022c). In addition, ASTM D4788-03 standard (ASTM-D4788-03) recommends a 0.5° safe detectable (minimum detectable) thermal contrast. Other criteria recommend thermal contrast of at least 10-20 times the camera ’ s noise equivalent temperature difference (NETD)(Hiasa et al., 2017), 1°C or 2°C (Washer et al., 2010) for the plausibility of the detection of subsurface defects. The comparative experiments of three thermal excitation mechanisms showed that the thermal excitation that directly correlates with the amount of heat input or output to the slabs significantly affects the magnitude of thermal contrast on the surface (Pedram et al., 2022a). This paper proposes a novel approach to improve the practical implementation of IRT for monitoring concrete bridges. For this purpose, parametric FE models of concrete slabs cast for an experimental programme (Pedram et al., 2022a) were developed and analysed in the LUSAS software package. The results of the analysis were validated against those of the experimental programme. The input parameters for the FEA were the initial temperature of concrete, air temperature (final temperature), and convection and radiation coefficients as well as the thermal parameters of concrete and the Styrofoam used to simulate subsurface void. This paper presents the variation of thermal contrast on the surface with the depth of subsurface defects using a numerical method. It compares the trends with those of the experimental programme. As a proposal to remediate the challenges and uncertainties in the practical implementation of IRT, FEA estimates the amount of energy required to achieve safe detectable thermal contrast at various depths based on standard and empirical criteria for IRT inspection of concrete bridges (ASTM-D4788-03, Washer et al., 2010, Hiasa et al., 2017). 2. Material and methodology 2.1. Experimental temperature measurements The IR temperature records collected during convection tests of slabs with simulated subsurface defects as described in (Pedram et al., 2022a) are adopted as the experimental data for this paper. In those tests, a range of initial temperatures was selected and adjusted by placing the slab test specimens overnight in an environmental chamber. Initial temperatures of -20°C, 0°C, 40°C, and 60°C were specifically chosen to enable examination of the impacts of both positive and negative temperature ranges (warming up and cooling down). Upon reaching the desired target initial temperature, each sample was taken out from the chamber, encased in an insulation box, and