Concrete is a heterogeneous material that consists of cement, aggregates, and water as basic constituents. Several cementitious materials and additives are added with different volumetric ratios to improve the strength and durability requirements of concrete. Consequently, performance of concrete when exposed to elevated temperature is greatly affected by the concrete type. Moreover, post-fire properties of concrete are influenced by the constituents of each concrete type. Heating rate, days of curing, type of curing, cooling method, and constituents of the mix are some of the factors that impact the post-fire behavior of concrete structures. In this paper, an extensive review was conducted and focused on the effect of concrete constituents on the overall behavior of concrete when exposed to elevated temperature. It was evident that utilizing fibers can improve the tensile capacity of concrete after exposure to higher temperatures. However, there is an increased risk of spalling due to the induced internal stresses. In addition, supplementary cementitious materials such as metakaolin and silica fume enhanced concrete strength, the latter proving to be the most effective. In terms of the heating process, it was clear that several constituents, such as silica fume or fly ash, that decrease absorption affect overall workability, increase the compressive strength of concrete, and can yield an increase in the strength of concrete at 200 °C. Most of the concrete types show a moderate and steady decrease in the strength up until 400 °C. However, the decrease is more rapid until the concrete reaches 800 °C or 1000 °C at which it spalls or cannot take any applied load. This review highlighted the need for more research and codes’ provisions to account for different types of concrete constituents and advanced construction materials technology.

The mechanical properties of concrete based mainly on flexural and compressive bearing capacity. Generally, researchers have an interest in the evaluation of compression property through the importance of the flexural performance of the material in the constructions, namely the significance of each mechanical property based upon the position of the structural element. The present experimentally work is directed toward improving the flexural strengths performance of ordinary hardened cement paste (HCP) at ambient and after elevated temperatures exposure. The used parameters were different pozzolanic materials with different replacements ratios to cement mass and different levels of temperature. Results proved the significant contribution of pozzolanic material to enhance the flexural properties of HCP after being exposed to elevated temperatures. The low content of CaO, the high grinding fineness, and the physical morphology of the used pozzolanic materials, made their adoption effective to HCP after exposure to elevated temperatures. Using 3%, 12%, and 15% of silica fume (SF), metakaolin (MK), and fly ash (FA), respectively, showed the highest heat endurance among the other replacements. However, the optimum replacement of MK has shown a better heat endurance than the optimum replacements of SF and FA. On the other hand, the spalling has occurred at high replacements of SF. Finally, the results are supported by means of thermo-gravimetric, SEM, and computed tomography investigations.

This paper presents a wide experimental study, in which it evaluates the performance of high-strength paste exposed to elevated temperatures up to 900 °C. Several factors have been investigated at the age of 90 days, i.e. metakaolin (MK) dosages, water to binder ratio (w/b) as well as elevated temperatures. Results proved that MK improves the relative residual compressive strength and relative residual bending strength showing a gain up to 52% and of 71% at 500 °C, respectively, compared to the pure cement paste in case of 0.3 w/b. The maximum use of MK is not more than 12%, and the optimum dosages were 9, 12, and 12% of MK replacements for 0.3, 0.35, and 0.4 w/b, respectively. The optimum dosage could change with changing w/b ratio and this up to the density of the microstructure which is controlled by the amount of w/b ratio and the packing effect of MK amount. In addition to the mechanical properties, the adoption of MK decreases the cracking of the specimens at elevated temperatures. SEM investigations show the positive physical morphology contribution of MK, specific surface area as well as its chemical composition for decreasing the Ca(OH)₂effect. Different phases that formed during temperatures elevation are illustrated by TG analysis. Results showed the reason behind using MK on the cracking enhancement and mechanical properties improvement after high temperatures exposure. Meanwhile, the obtained optimum MK dosages at ambient temperature are not similar to that obtained at elevated temperatures.