High performance concrete (HPC) has opened manifold opportunities for the construction of concrete structures. According to Forster [1
], “High performance concrete is a concrete made with appropriate materials combined according to a selected mix design and properly mixed, transported, placed, consolidated and cured so that the resulting concrete gives excellent performance in the structure in which it will be placed, in the environment to which it will be exposed and with the loads to which it will be subjected for its design life”. Hence, HPC should be workable, pumpable, and easily compactable within the confines of any formwork or reinforcement. After a proper curing regime, it must achieve the desired level of strength to carry the expected loads during its design life. Also, it must withstand different detrimental factors when exposed to the environment.
HPC can be made with the same basic materials used for ordinary concrete; the major differences are the mix proportions and water-to-binder (W/B) ratio. The W/B ratio of HPC is much lower than that of ordinary concrete. While ordinary concrete has a W/B ratio between 0.50 and 0.75, it ranges from 0.20 to 0.35 for HPC [2
]. This reduction in W/B ratio is not only obtained by increasing the amount of cement, but also by simultaneously decreasing the amount of mixing water in the presence of superplasticizer (SP). In addition, the use of a suitable supplementary cementitious material (SCM) such as silica fume or fly ash improves the strength and durability of HPC. In particular, the use of silica fume is essential for obtaining very high strength HPC [3
]. Most SCMs are obtained as industrial by-products. For example, silica fume is obtained as a by-product of silicon and ferrosilicon alloy production whereas fly ash is obtained as one of the residues generated by coal combustion in power plants. The use of SCMs in concrete provides both economic and environmental benefits as they decrease the demand for cement and eliminate the disposal problems caused by industrial wastes. The successful use of SCMs obtained as industrial by-products would reduce the environmental load to the landfill sites as well as the greenhouse gas emissions from cement factories.
HPC has been used to construct many novel concrete structures in the world. In 1988, 120 MPa HPC was used in Seattle, USA to construct the columns for a 52-storey building at 2 Union Square [4
]. In the same year, 100 MPa HPC went into the columns of one of the world’s tallest concrete building [5
]. In 1984, an experimental column was poured in the La Laurentienne building, Montreal with 90 MPa HPC [6
]. Toronto’s Scotia Plaza, completed in 1987, had HPC with an average compressive strength of 93 MPa [7
]. The Hibernia Gravity Base Structure, constructed in Newfoundland, Canada, incorporated HPC for strength and durability [5
]. In Europe, HPC was stated to be developed for the construction of large offshore platforms and bridges, but very rarely for the construction of high-rise buildings [2
]. HPC was used to build huge offshore structures in Norway [8
]. Later HPC was used to construct highway pavement in Norway due to its superior resistance to abrasion [9
]. In France, many researches were conducted in the Laboratorie Central de Recherche [11
] to master the characteristics and behaviour of HPC. Moreover, the Institute of Reinforced and Prestressed Concrete (IBAP) at the Ecole Polytechnique Federale in Lausanne undertook several research activities concerning the serviceability of concrete structures. All of those efforts produced advances in the scientific and technological fields of HPC [12
]. In Asia, Japan made a significant contribution to the development of HPC. The researchers from the concrete laboratory of the University of Tokyo in Japan developed super-workable HPC [13
]. The Southeast Asian countries also used high strength HPC. In the Kuala Lumpur City Centre (KLCC) project, Malaysia, 80 MPa high strength HPC was used. In the Bioyette Tower of Bangkok, Thailand, 80 MPa high strength HPC was also used to reduce the cost and ease the construction [14
One of the most essential practices that must be given high priority for stringent quality control in the production of HPC is the curing of concrete at a proper temperature. Many researchers have investigated the effects of curing technique, temperature, and environmental and climatic impacts on the mechanical and durability characteristics of various concretes including HPC [15
]. Ait-Aider et al.
] reported based on their findings that under hot climates, increasing the water amount in the concrete mix to a certain limit can contribute positively by maintaining the required workability of the mix and compensating the water amount that was lost by evaporation. This additional quantity of water did not show any marked detrimental effect on the strength of the hardened concrete [15
]. Meanwhile, Al-Gahtani et al.
] found that, for blended cement concretes produced with Type I cement (normal portland cement), silica fume and fly ash, the concrete specimens cured by wet burlap covering provided more promising results in terms of strength development, in comparison with the specimens cured with water-based or acrylic-based curing compounds. However, the specimens cured with acrylic-based curing compounds indicated greater efficiency in reducing the plastic and drying shrinkages compared with the concrete specimens cured with wet burlap or a water-based curing compound. Ibrahim et al.
] researched the contribution of different curing techniques to the strength and durability characteristics of normal portland cement and silica fume concretes. They found that the strength and durability properties of the concrete specimens cured with selected curing compounds were superior compared with the specimens cured using wet burlap. The performance of different concretes was analysed based on their compressive strength, water absorption, and chloride permeability. While there was no significant distinction in terms of strength observed, the specimens cured with a bitumen-based curing compound exhibited the best durability characteristics with respect to water absorption and chloride permeability. Nassif and Petrou [19
] found that the curing of concrete at very low, near-freezing temperatures may result in 20% and 25% loss in the 28-day stiffness and strength of the concrete, respectively, and the replacement of 20% portland cement with fly ash cannot compensate for the detrimental effects of cold weather concreting. This was established after experimental tests on the concrete slab specimens under various curing regimes between 20 °C and −5 °C. Furthermore, Nie et al.
] researched the feasibility of using local mineral admixtures as supplementary cementitious material in concrete subjected to severe sulphate environmental conditions in northwest China. The chemical, mechanical, and durability characteristics of concrete were analysed in their study. They reported that the incorporation of mineral admixtures such as Class F fly ash and slag improves the resistance of concrete to the harsh environment [20
The temperature in concrete and surroundings are expected to influence the performance of different concretes including HPC [15
]. The increased temperature of both fresh concrete mix and the surrounding air during the execution of concrete work influences the properties of hardened concrete. The increased temperature of curing affects the compressive strength, elasticity, and durability properties of HPC. This is because the increase in temperature affects the hydration of cement in concrete [15
]. The hydration of cement decreases significantly when the ambient relative humidity during curing of concrete is below 80% [24
]. Therefore, the early drying of concrete at higher temperature may stop the cement hydration before the capillaries are blocked by hydration products. The covercrete is more sensitive to drying since it is more prone to losing water. The formation of a network of capillaries in covercrete may provide easy passage for the intrusion of aggressive species that cause deterioration of the concrete structures [25
]. Early drying can also lead to more shrinkage cracking and this would aggravate the deterioration process of concrete. Hence, a favorable temperature must be maintained while curing the concrete, especially during the early period for preventing premature drying and interrupted cement hydration. Moreover, a disconnected pore structure or a compact microstructure with reduced porosity is required to enhance the durability performance of concrete. This can be achieved by curing the concrete at a proper temperature.
The present study investigated the effects of three different medium temperatures of 20, 35 and 50 °C as well as two industrial by-products, silica fume and fly ash, on the key hardened properties such as compressive strength and dynamic modulus of elasticity of HPC. The hardened concrete specimens were tested for the aforementioned hardened properties after curing in water at 20 °C for selected periods (3, 7 and 14 days) and then exposing them to a medium temperature of 35 or 50 °C until the day of testing. A number of concrete specimens water cured at 20 °C were also tested to observe the effect of silica fume and fly ash on the initial surface absorption (ISA) and moisture movement into concrete.