Thermal spalling in NSC: experimental studies and findings

Concrete is a fundamental material in modern construction, but it faces significant durability problems under extreme thermal conditions, which can lead to structural failures. This study addresses the phenomenon of thermal spalling in Normal Strength Concrete (NSC), focusing on how high temperatures affect its structure through experimental investigations. It identifies the key factors influencing spalling, including heating regimes, concrete mix design, and the role of thermo-chemical reactions. To gain insights into this process, a series of controlled experiments was carried out on three types of NSC mixtures, with the samples heated to temperatures of 600°C, 900°C, and 1200°C. Additionally, Scanning Electron Microscopy (SEM) was utilized to study the changes in microstructure in NSC under high temperature conditions. The experiments demonstrated explosive spalling at 610 °C, and deterioration in the concrete at temperatures surpassing 700°C, where there was notable release of chemically bonded water, resulting in structural weakening. The results indicate that at 900 °C the concrete showed signs of porosity, micro-cracks, and the breakdown of hydration components, like calcium hydroxide and calcium silicate hydrate (CSH), and the aggregates spalled. Moreover, at 1050 °C the aggregates near the heated surface were melted. These microstructural alterations were more pronounced at 1200°C, leading to a substantial loss in material integrity. The findings suggested that the extent of spalling and reduction in strength were influenced by variables like heating speed, temperature variations, and release of chemically bonded water within the concrete matrix. The SEM findings underscore the critical impact of high-temperature exposure on the microstructure of NSC, contributing to a deeper understanding of thermo-chemical spalling mechanisms and their implications for structural durability. Despite the progress made, this study requires further research on large-scale structural elements and prestressed or restrained concrete specimens to develop comprehensive predictive models.