**1. Introduction**

High-performance concrete (HPC) is a concrete mixture, which possesses high durability and high strength when compared to conventional concrete [1]. This concrete contains one or more of the cementitious materials such as fly ash, silica fume, or ground granulated blast-furnace slag (GGBFS) and usually a super plasticizer. High strength of concrete is achieved by reducing porosity, inhomogeneity, and microcracks in the hydrated cement paste and the transition zone. Consequently, there is a reduction of the thickness of the interfacial transition zone in high-strength concrete. The densification of the interfacial transition zone allows for efficient load transfer between the cement mortar and the coarse aggregate, contributing to the strength of the concrete. For very high-strength concrete where the matrix is extremely dense, a weak aggregate may become the weak link in concrete strength [2].

The importance in the behavior 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 [3]. During a fire, the temperature may reach up to

1100°C in buildings and even up to 1350°C in tunnels, leading to severe damage in a concrete structure. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the material is progressive, but because this process is slow, significant 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 concerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids) [4].

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 [5].

Many research have demonstrated that changes in the strength of concrete as a function of temperature 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 temperature.
