Mechanical properties of concrete at high temperature—A review

Fire response of concrete structural members is dependent on the thermal, mechanical, and deformation properties of concrete. These properties vary significantly with temperature and also depend on the composition and characteristics of concrete batch mix as well as heating rate and other environmental conditions. In this chapter, the key characteristics of concrete are outlined. The various properties that influence fire resistance performance, together with the role of these properties on fire resistance, are discussed. The variation of thermal, mechanical, deformation, and spalling properties with temperature for different types of concrete are presented.

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Properties of Concrete at Elevated Temperatures

The paper presents the impact of high temperature on cement concrete. The presented data have been selected both from the author's most recent research and the published literature in order to provide a brief outline of the subject. The effect of a high temperature on concrete covers changes taking place in cement paste, aggregates, as well as the interaction of these two constituents, that result in changes of mechanical and physical characteristics of concrete. This paper presents the effects of a high temperature on selected physical properties of concrete, including colour change, thermal strain, thermal strains under load, and transient thermal strains. In addition, changes to mechanical properties are discussed: stress-strain relationship, compressive strength, and modulus of elasticity. Moreover, the phenomenon of explosive spalling and the main factors that affect its extent are analysed in light of the most recent research. Interest in the behaviour of concrete at a high temperature mainly results from the many cases of fires taking place in buildings, high-rise buildings, tunnels, and drilling platform structures. During a fire, the temperature may reach up to 1100 ◦ C in buildings and even up to 1350 ◦ C in tunnels, lead- ing to severe damage in a concrete structure (1). However, in some special cases, even much lower temperature, may cause explosive destruction of concrete, thus endangering the bear- ing capacity of the concrete element. Nevertheless, concrete is considered a construction material that satisfactorily preserves its properties at high temperature. Owing to concrete's fairly low coefficient of thermal conductivity, the movement of heat through concrete is slow, and thus reinforced steel, which is sensitive to high temperature, is protected for a relatively long period of time. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the ma- terial is progressive, but because this process is slow, signifi- cant temperature gradients are produced between the concrete member's surface and core inducing additional damage to the element. Fundamental issues related to the impact of high temperature on concrete involve identification of the complex changes that take place in concrete while heated. This con- cerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids). The analysis is complicated due to the fact that cement concrete is a composite consisting of two substantially different constituents: cement paste and aggregates. The effects of the various changes taking place in heated concrete are the alterations of its physical, thermal, and mechanical properties. Research has demonstrated (1, 2), that changes in the strength of concrete as a function of tem- perature are related to, inter alia, concrete composition the type of aggregate used, the water/cement ratio, the presence of pozzolana additives, etc. Important factors are also the rate of heating and the time of concrete exposure to high tempera- ture. The increase in temperature results in water evaporation, C-S-H gel dehydration, calcium hydroxide and calcium alu- minates decomposition, etc. Along with the increase in tem- perature, changes in the aggregate take place. Due to those changes, concrete strength and modulus of elasticity gradu- ally decreases, and when the temperature exceeds ca. 300 ◦ C, the decline in strength becomes more rapid. When the 500 ◦ C threshold is passed, the compressive strength of concrete usu- ally drops by 50% to 60%, and the concrete is considered fully damaged. The Eurocode method of calculating the load- bearing capacity of reinforced concrete members subjected to a fire is based on this assumption. In the 500 ◦ C isotherm method, sections of concrete surface where the temperature had exceeded 500 ◦ C are omitted from the calculations (3).

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Behaviour of cement concrete at high temperature

Properties of fresh and hardened concrete

The term Ultra-High Performance Concrete (UHPC) refers to a relatively new class of advanced cementitious composite materials whose mechanical and durability properties far surpass those of conventional concrete. This class of concrete has been demonstrated to facilitate solutions that address specific problems in the United States highway bridge infrastructure. Initial material development research on UHPC began more than two decades ago. First structural deployments began in the late 1990s. First field deployments in the U.S. highway transportation infrastructure began in 2006. For this study, UHPC-class materials are defined as cementitious-based composite materials with discontinuous fiber reinforcement that exhibit compressive strength above 21.7 ksi (150 MPa), pre- and post-cracking tensile strength above 0.72 ksi (5 MPa), and enhanced durability via a discontinuous pore structure. This report documents the state of the art with regard to the research, development, and deployment of UHPC components within the U.S. highway transportation infrastructure. More than 600 technical articles and reports covering research and applications using UHPC have been published in English in the last 20 years, with many more published in other languages. The report includes information about materials and production, mechanical properties, structural design and structural testing, durability and durability testing, and actual and potential applications. The report concludes with recommendations for the future direction for UHPC applications in the United States.

https://rosap.ntl.bts.gov/view/dot/26387/dot_26387_DS1.pdf

Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community

Synopsis This paper deals with the bond strength of steel and concrete at high temperatures because, although bond strength is of major importance in the determination of the behaviour of concrete structures in fire, there is no detailed information available on this problem. For this reason a test apparatus was developed which enabled pull-out experiments to be carried out on cylindrical concrete specimens with short embedment lengths. During the tests the slip at the non-loaded end of the concrete specimen and the pull-out force were reassured. In the temperature range 20 to 800°C the pull-out system enabled two different types of experiment to be carried out: force–slip measurements at thermal steady state and at non-steady state conditions. In these experiments normal-weight concrete made with Portland cement and siliceous aggregate and three different types of reinforcing steel (ribbed steel bars, plain round bars and deformed prestressing bars) were used. Differences between the results for the diff...

Bond strength at high temperatures

Effect of temperature on steel and concrete for PCRV's

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Ulrich Schneider

Papers

Bond strength at high temperatures

Effect of temperature on steel and concrete for PCRV's

Recommendation of RILEM TC 200-HTC: Mechanical concrete properties at high temperatures-modelling and applications : Part I. General presentation

Compressive strength for service and accident conditions

Behaviour of mortar and concrete at extremely low temperatures