POST FIRE BEHAVIOUR OF GRANULATED BLAST FURNACE SLAG AGGREGATE CONCRETE FILLED STEEL TUBE

Abstract

The substantial increase in steel production within India has brought about a noteworthy rise in the output of granulated blast furnace slag (GBS). One potential solution to address the environmental issues linked to GBS production is to employ it as a substitute for natural fine aggregate in concrete, particularly within the concrete filled steel tube (CFST). However, comprehensive literatures exist concerning the behaviour of CFST columns incorporating GBS aggregate at ambient temperature conditions. But, literature regarding the post fire behaviour of CFST with GBS aggregate is hardly available. Hence, the present study aims to investigate the post fire behaviour of GBS-incorporated infill concrete CFST. In the present work, 30 CFST and 30 concrete filled double skin steel tubes (CFDST) were prepared using GBS as fine aggregate in infill concrete. i.e., no natural fine aggregate was used in concrete preparation. Three different shapes of the specimens, i.e. circular, square and rectangular, were used to check their effect on the post fire performance. Moreover, the length of the specimen was also varied. The specimens were heated using a laboratory muffle furnace for 2 hours. The heating temperatures varied as 200o C, 400o C, 600o C and 800o C. After the heating period, the power supply to the furnace was cut off, and the specimens were allowed to cool inside the furnace. The axial compression load was applied to these cooled specimens using a compression testing machine. A displacement control loading method was used for the experimentation with a constant loading rate of 0.5 mm per minute. The maximum displacement applied to each specimen was 30 mm. The load and displacement data were collected using a data collection computer system, and the deformation shapes of the specimens were captured. Moreover, a three-dimensional finite element model (3D-FEM) was developed to simulate the behaviour of GBS-incorporated unheated and heated CFST specimens. The commercially available software ABAQUS-2021 was used for developing the model. Several code provisions were also used to predict the ultimate load-carrying capacity of the tested specimens. Different standard code provisions were used to calculate the ultimate load-carrying capacity of the above specimens and compared with their experimental values. An insignificant reduction in the load-carrying capacity was observed for the specimens heated at 200o C, 400o C and 600o C. This was due to free-Magnesium Oxide and free-Calcium Oxide, which causes initial expansion in concrete while hydrated with water. This initial expansion compensated the shrinkage in concrete upto some extent. However, the reduction in the load carrying capacity was significant for the specimens heated at 800o C due to the loss in material properties of concrete and steel. The circular CFST and CFDST specimens showed a gradual i post-peak behaviour compared to the square and rectangular specimens, which showed a steeper post-peak behaviour. It was observed that temperature had a negligible effect on the initial stiffness of the specimens. The residual strength index value was higher for the circular specimens due to their better confinement than the square and rectangular specimens. Local bulging deformation shape was observed for circular short specimens. For square and rectangular short specimens, local buckling deformations shapes were observed. For intermediate specimens, global buckling with some local buckling at the ends was observed. Moreover, Euler’s buckling shape was also observed for the intermediate CFST and CFDST specimens heated at 800o C. The proposed 3D-FEM model predicted the load-displacement behaviour and deformation shapes of the tested specimens with acceptable accuracy. The ultimate load-carrying capacity predicted by the code provisions did not show any specific conservative result for the above tested specimens.