PSI - Issue 64

Hernán Xargay et al. / Procedia Structural Integrity 64 (2024) 1790–1797 Hernán Xargay / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The development of Structural Health Monitoring (SHM) strategies for critical infrastructures is of great interest, particularly in nuclear plants where containment structures are required to maintain integrity and remain sealed to ensure radiation safety and durability. In addition, these structures may be exposed at moderate and even to high temperatures in service or in accidental scenarios (Naus (2010)), highlighting the necessity for early detection of any emerging cracks to assess their severity, origins, and possible corrective interventions. Given that conventional in service visual inspections are typically annual, a continuous real-time monitoring system capable of early detection would be of utmost importance. In this regard, Acoustic Emission (AE) stands out as a promising approach due to its unique advantage for the volumetric monitoring of structural elements. From the point of view of physics, AE is defined as the release of energy as transient elastic waves emanating from material stress points, which can result from micro-structural events like crack propagation and friction as fracture progress. These events generate ultrasonic waves that can be detected on the element surface by piezoelectric sensors, transforming them into electrical signals for further analysis. A set of suitable parameters can be calculated in order to characterize the digitalized AE waveforms. Significant research, including studies by Ohno et al. (2014) and Saliba et al. (2015), has explored AE monitoring in mortar and concrete specimens under three-point bending tests. However, there are still many aspects to delve of this technique. One of them is to characterize how the thermal effect affects the recorded signals and their implications for the calibration of an intelligent SHM system equipped with alert criteria. Exposed to elevated temperatures, cement-based materials undergo irreversible changes that impact the hardened cement paste through both chemical and physical transformations, including moisture loss. The aggregate phase and the interfacial transition zone are also affected, leading to microstructural changes, decomposition, and thermal incompatibility among component bonds (Ma et al. (2015)). Primarily, temperatures up to 200 °C result in the evaporation of both free and adsorbed water, affecting pore structure without significantly altering mechanical properties. As temperatures approach to 500 °C, there is a notable reduction in water content and dehydration of the calcium silicate hydrates occurs. By 450 °C, calcium hydroxide in the cement paste decomposes into calcium oxide and water. Until these temperatures, aggregate materials remain relatively unchanged, with the exception of siliceous aggregates. At 573 °C, a critical transformation of α - quartz to β -quartz in aggregates take place, leading to harmful expansions. Escalating the temperature further to 600 °C initiates the chemical breakdown of hydrated calcium silicates, which is the main strengthening component of cementitious matrix. From 600 °C to 800 °C, the dissociation of calcium carbonate happens, significantly diminishing the material's strength capacity (Arioz (2007)). The most relevant physical and mechanical properties of cement-based composites (i.e., cohesion, friction, stiffness, strength, cracked configurations and durability parameters) can suffer severe consequences when the material is submitted to long-term high temperature exposures (Culfik and Ozturan (2002)). Many researches as Ripani et al. (2020) have addressed that the mechanical and physical properties of mortars, such as cohesion, stiffness, and durability, are adversely affected by long-term exposure to high temperatures. Concrete behaves in a more ductile manner with reduction of both Young's modulus and Poisson's ratio as temperature rises. In addition, a decrease in fracture energy and a significant reduction in tensile and compressive strength are observed at temperatures above 400°C (Nielsen and Bicanic, (2003)). Xargay et al. (2018) analyzed the benefits of adding of micro- and macro-fibers as a mitigation strategy of thermal damage and its impact on AE behavior post-exposure to high temperatures. This work aims to investigate the influence of thermal-induced damage on the AE responses of normal-strength cementitious mortar during fracture process. The effects of a wide range of temperature levels (100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C), applied through a thermal cycle in an electrical furnace, are analyzed and compared in three-point bending tests. The measurements are carried out in residual state, i.e., ambient temperature allowing to monitor both load and AE activity. This approach addresses a gap in the existing literature by exploring both AE monitoring technique and thermal effects on the fracture behavior of Ordinary Portland Cement (OPC) mortars, an area not extensively covered in previous research despite the substantial focus on their mechanical characteristics and AE analysis.