Effect of palm oil fuel ash on self-compacting concrete short columns subjected to fire

Abstract

There have been quite a significant research and development activities in producing self-compacting concrete (SCC) through the use of palm oil fuel ash (POFA) since 2011. POFA was used as a partial replacement to Portland cement since it improves strength and durability properties of SCC. However, the study on the application of selfcompacting POFA concrete to structural members that exposed to elevated temperatures as in the case of fire is quite limited. This research, therefore, focuses on the effect of POFA on self-compacting concrete short columns subjected to fire. Assessment of the microstructure, physical and chemical characteristics of the binders were carried out for characterization. Mechanical properties and microstructure of SCC produced with POFA at elevated temperatures were also carried out. For this purpose, two mixes of SCC containing 0% and 15% of POFA were prepared. The variables considered in the study include the percentage of POFA which is 0 and 15%, the concrete cover of 25 and 35 mm and the longitudinal steel reinforcement ratios of 2% and 3%. The columns produced were made of normal strength and had 600 mm height and 150 mm square cross – section. Twenty four columns were cast altogether. Eight columns out of these columns were unheated and serve as a control while the remaining sixteen columns were heated in an automatic electric furnace to 750 °C and this temperature was maintained for 2 and 4 hours exposure time. After cooling down, all the columns were tested under axial compression load up to failure. Furthermore, POFA self-compacting concrete standard specimens were fabricated and tested for mechanical properties in 27 – 1000 °C temperature range at 28 days. Various techniques which include the use of scanning electronic microscope and Xray diffraction were used to study the microstructure of the hardened SCC in 27 – 1000 °C temperature range at 28 days. Results from characterization confirmed that POFA was a good pozzolanic material and satisfied the specified physical and chemical properties requirements. Results from elevated temperature mechanical property tests revealed that there was an increment in residual compressive and gradual loss in the residual flexural, splitting tensile strengths and modulus of elasticity for SCC produced with and without POFA at 400 °C temperature. The loss in residual modulus of elasticity, splitting tensile, compressive and flexural strengths fluctuated sharply at 400 – 600 °C, 600 – 800 °C and 800 – 1000 °C temperatures. However, residual mechanical properties of SCC produced with POFA reduced at faster rate than SCC produced without POFA at 800 - 1000 °C temperatures. Results from elevated temperature microstructures showed that the transformation of calcium silicate hydrate (C-S-H) gel into distinctive phases and formation of micro-cracks, voids and pores were noticed on the hardened SCC at temperature above 600 °C. Results from ultimate axial capacity tests showed that all the unheated columns produced with POFA were improved on strength at 28 days. The residual axial capacity tests also showed that all the columns exhibited similar non-linear reduction trends with time of exposure. Heating the columns at 750 °C caused a severe reduction in the strength of all the columns. However, irrespective of concrete covers and percentage of steel reinforcements used, the loss in residual strength was high in the columns produced with POFA than the columns produced without POFA for 2 and 4 hours exposure time. The residual strength loss is in the range of 51% - 63% and 47% - 59% for columns produced with and without POFA. The performance of SCC containing POFA does not significantly improve the concrete performance in resisting fire. However, further research needs to be carried out on POFA so as to improve this short coming.