As a fundamental construction material in modern engineering, concrete is extensively utilized in structural buildings across various industries due to its exceptional workability, mechanical properties, durability, constructability, and safety. In recent years, the pursuit of increasingly innovative structural designs and the escalating demands and complexities of construction technologies, coupled with the rapid advancements in materials science, have led to the emergence of Ultra-high Performance Concrete (UHPC), whose preparation and production techniques have undergone significant enhancements. Consequently, catering to the diverse requirements of construction materials in different service environments, multiple novel UHPC materials have been developed and gradually introduced into pivotal construction projects, encompassing high-rise or super high-rise buildings, long-span bridges, underground structures, nuclear power projects, and massive dams. Nevertheless, fire remains one of the primary risks confronting modern engineering safety, posing a severe threat to the structural stability and service life of UHPC components. Under elevated temperatures, UHPC experiences a sudden imbalance between internal and external temperatures, triggering a series of physicochemical degradations such as dehydration, decomposition, embrittlement, and thermal deformation incompatibility of hydration products. Notably, these detrimental effects are more pronounced in UHPC with higher strength grades. Therefore, conducting systematic research on the elevated performance of UHPC and proposing feasible enhancement measures are of paramount importance for enhancing the fire resistance of structural engineering.The elevated stability of UHPC is paramount to ensuring the long-term safe service of building structures in fire environments. Therefore, there is an urgent practical need to develop novel construction materials with exceptional elevated performance through a systematic overview of mechanical properties and degradation mechanisms of UHPC at elevated temperatures. In this paper, the macro-mechanical properties of UHPC at elevated temperatures, encompassing compressive strength, tensile strength, flexural strength, bond strength and elastic modulus, were comprehensively reviewed. The thermal decomposition of hydration products, pore structure and pore water, C-S-H gel, and the interfacial transition zone characteristics of UHPC at elevated temperatures were further summarized. Additionally, the degradation mechanisms of concrete properties at elevated temperatures and their applicability in UHPC were discussed, and the critical issues pertaining to UHPC’s elevated performance in existing research were elucidated. Lastly, some insights into future research directions and prospects were proposed. Based on the conclusions summarized in this paper, valuable references can be provided for the design of UHPC materials with enhanced elevated resistance, as well as for the inspection, evaluation, and repair of structures after fire events.
Summary and prospects
As one of the most prevalent safety risks faced by modern building structures, fire can lead to elevated spalling failure of UHPC, resulting in rapid loss of load-bearing capacity, posing significant challenges to the serviceability, lifespan, structural stability, and safety of structural engineering. Conducting relevant research holds practical application value in pushing the limits of UHPC materials and structures in terms of elevated resistance and fire prevention capabilities. The main conclusions of this paper are as follows: 1) At elevated temperatures, the strength of UHPC is primarily influenced by factors such as its strength grade, temperature, cooling method, raw material composition, moisture content, reinforcement or fiber type, and specimen dimensions. As the temperature increases, the compressive strength, flexural strength, tensile strength, and bond strength of UHPC decrease, with the loss of strength exhibiting distinct trends within different temperature ranges. 2) At elevated temperatures, UHPC undergoes phenomena such as evaporation and diffusion of pore water, deterioration of pore structure, decomposition of C-S-H gel, and failure of the interfacial transition zone, which initiate the formation of microcracks and accelerate their propagation. 3) Regarding the spalling behavior of concrete at elevated temperatures, scholars have proposed theories including the vapor pressure theory, thermal stress theory, and thermal cracking theory. While these theories can, to some extent, explain the degradation mechanisms of ordinary concrete in extreme temperature environments, they remain controversial, particularly when applied to UHPC.Despite the extensive research conducted by scholars worldwide on UHPC in the context of building fires and elevated environments, as well as the significant achievements made in experimental and theoretical analyses of fire resistance for concrete materials or structures, the current literature reveals a lack of effective technical measures to enhance the elevated resistance of UHPC, necessitating further in-depth investigation. This field is plagued by numerous critical issues that urgently require research and resolution: 1) The elevated environment inevitably exerts negative effects on the performance of UHPC, and the enhancements achieved through existing research on improving the thermal resistance remain insufficient. It is thus imperative to develop novel intelligent elevated-resistant concrete that can efficiently prevent, resist, and insulate heat, while also integrating rapid fire warning response capabilities, through advancements in concrete material composition design, mix proportion optimization, and structural directional control. 2) At elevated temperatures, UHPC undergoes simultaneous processes of secondary hydration enhancement from unhydrated cement particles and elevated degradation of hydrated product phases. The key to deeply analyzing and comprehensively understanding the degradation mechanisms of UHPC at elevated temperatures lies in achieving in-situ microscopic characterization of hydrated product phases and accurately quantifying the contribution ratios and development patterns of these two processes to the strength of UHPC. 3) The spalling behavior of UHPC at elevated temperatures is complex and multidisciplinary. The current theories on elevated failure can only partially explain the elevated degradation of UHPC, making it difficult to predict the randomness and uncertainty of UHPC's elevated peeling and establish a correlation mechanism. It is crucial to comprehensively explore the elevated failure mechanisms of UHPC by integrating multiple disciplines such as material mechanics, thermodynamics, fracture mechanics, and blast dynamics.