Strength and Permeability Studies of Self-Compacting Concrete at Elevated Temperatures

Abstract

One of the major problems being faced by all the nations across the world is the disposal of waste materials and industrial by products due to its ever increasing quantities. There are several types of waste materials and industrial by products. Some of these can be put to use. The utilization of such materials in self-compacting concrete (SCC) makes it cost effective and helps in reducing disposal problem also. Fly ash is one of such industrial by products generated from combustion of coal in the thermal power plants. Another such by product is foundry sand obtained from ferrous and nonferrous metal casting industry and successfully used as a land filling material. But due to rapid increase in disposal cost, the use of fly ash and foundry sand for land filling is becoming a problem. Hence in the present research, awareness for using fly ash and foundry sand together as replacement of cement and fine aggregate are explored. Pozzolanic concretes are used extensively throughout the world where oil, gas, nuclear and power industries are among the major users. Due to their superior structural performances, environmental friendliness, and energy conserving implications, the applications of such concretes are increasing day by day. These types of concretes are exposed to elevated temperatures for considerable periods of time in the above industries, apart from the usual risk of fire. Generally concrete is believed to be an excellent fire proofing material, but there is extensive damage at high temperatures. At high temperatures, chemical transformation of gel weakens the matrix bonding, which brings about a loss of strength of fly ash concrete. Fly ash is used as a mineral addition in concrete to improve its strength and durability characteristics. Fly ash can also be used either as an admixture or as a partial replacement of cement or fine aggregates or total replacement of fine aggregates and as supplementary addition to achieve different properties of concrete. In the present research, the experimental investigation was carried out to evaluate the strength properties (compressive strength, splitting tensile strength, and modulus of elasticity, mass loss, and porosity) and permeability (rapid chloride permeability) studies of SCC mixes at elevated temperatures up to 300ºC. Cement was replaced with three percentages (0%, 30%, 40%, and 50%) of fly ash and fine aggregate was replaced with 10% of foundry sand by weight. A total of four SCC mixes (SCC1, SCC2, SCC3, and SCC4) were developed. The control mix (SCC1) was developed without fly ash and foundry sand. Mix SCC2 was with 30% fly ash and 10% foundry sand, mix SCC3 was developed with 40% fly ash and 10% foundry sand, and the mix SCC4 was with 50% fly ash and 10% foundry sand. The specimens of each SCC mixture were heated up to different temperatures (100°C, 200°C, and 300°C). In order to ensure a uniform temperature throughout the specimens, the temperature was held constant at the maximum value for one hour before cooling. Tests were performed for compressive strength, splitting tensile strength, modulus of elasticity, mass loss, porosity, and rapid chloride permeability, after curing periods of 28, 91, and 365 days. Test results showed that the compressive strength, splitting tensile strength, modulus of elasticity, mass loss, porosity, and rapid chloride permeability of SCC mixes made with 30%, 40%, and 50% of fly ash as cement replacement was lower than the control mix at all ages and that the strength of all mixes continued to increase with age. Test results also indicated that there is little improvement in compressive strength within the temperature range of 200°C-300°C as compared to 27°C-200°C. But the rate of splitting tensile strength and modulus of elasticity loss was higher than that of the compressive strength loss at elevated temperatures and with the increase in percentage of fly ash. In this research X-ray diffraction (XRD) and Scanning Electron Microscopic (SEM) studies were also made to explain the observed residual compressive strength increase between 200°C-300°C. Statistical analyses of the results were carried out at 28, 91, and 365 days of age. The results of correlation analysis depict that there is good correlation between actual properties and predicted properties