Structural Behavior of Self-Compacting Concrete Incorporating Cement Kiln Dust Subjected to High Temperature

Abstract

Laboratory investigations have been performed to identify the effect of incorporating cement kiln dust (CKD) on fresh, mechanical, hardened and microstructural properties of Self-Compacting Concrete (SCC). The characteristic of CKD and its effect as a partial replacement (10%, 20%, and 30% by weight) of Portland Cement (PC) in SCC were studied in this research program. This study also explored the influence of elevated temperatures (300°C, 500°C, and 700°C) in the electric furnace with a heating treatment time of 2 hours on the mechanical, hardened and microstructural properties of the studied concrete. This study highlights the differences in structural behavior between beams tested at ambient laboratory temperature and beams subjected to combined thermo- mechanical loading. For sustainable development, cement usage requires to be reduced. CKD is potential to be incorporated into the SCC mixs. Four SCC mixes are used in this study. One without CKD (reference mix). The other three (replaced cement mixes) use different replacement ratios. The fresh properties were investigated using slump flow, T500, V-funnel, L-box, segregation resistance and fresh density tests. Tests were conducted to evaluate the mechanical and hardened properties, include compressive- and indirect tensile strengths, density, elastic modulus, ultrasonic pulse velocity, pull-out test, impact resistance, absorption and mass loss. To analyze the SCC microstructure, tests of FESEM-EDS, XRD, TGA and MIP were performed in the present work. Twelve reinforced SCC beams reinforced by 12mm, 10mm and 8mm steel rebar in diameters were prepard for structural tests. The beams with dimensions 100×150×1200 mm were used to evaluate the structural behavior of 20%CKD-inclusion beams. The investigations have shown that incorporating CKD in SCC results slightly reduced its fresh density, filling and passing abilities. However, enhancing its resistance to segregation. The mechanical properties of SCC were slightly declined up to 20% CKD-inclusion. Compressive strength, indirect tensile strength, elastic modulus and impact resistance decreased by 12%, 14%, 5% and 4%, respectively, II compared to SCC without CKD. Interestingly, pull-out tests indicate an excellent bond strength of SCC even with 30% CKD replacement ratio in the ambient tested group. It was found that elevated temperatures had a greater effect on the bond strength and impact resistance than indirect tensile and compressive strength. The results showed that the mechanical and hardened properties of the reference SCC mix were significantly higher than SCC with CKD at all tested temperatures.The microstructure analyses reveal valuable chemical and physical observations of the SCC matrix. After heating beyond 500°C, the chemical and physical changes in the SCC samples with considerable development in mass loss, voids and pores rather than micro-thermal cracks are also measured and monitored by microstructure tests. It is found that incorporating CKD waste in SCC significantly improves its structural behavior in terms of stiffness. In contrast, a minor reduction in toughness, ductility, and fire resistance is observed. In addition, there was a slight decline in load capacity and ultimate deflection of CKD beams at ambient testing. The beams tested under thermo-mechanical loading showed a greater deterioration in flexural strength. Structural testing showed that CKD beams were able to sustain the applied load (70% of their ultimate capacity) without failure for about 1 hour of fire exposure, even after the temperature at the rebar exceeded 570 °C. This study confirms that 20% CKD can be potentially used in producing self-compacting concrete with almost the same strength as plain SCC; it can also be structurally involved.