Experimental study on high strength concrete encased steel composite short columns

Abstract

This paper investigates on the axial compressive behavior of high strength Concrete Encased Steel (CES) composite short columns. Six column specimens made of high strength concrete were tested under monotonic pure compression. The structural performance was investigated including failure pattern, load-carrying capacity, load-deformation response, and post peak ductility. The major parameters affecting the ultimate strength of such composite columns are studied including concrete strength, spacing of transverse reinforcement bars, and the inclusion of steel fibers in the high strength concrete. Based on the experimental work, it is found that the failure mode of CES columns with high concrete strength C120 is characterized by concrete cover spalling, which triggers a sudden drop of compression capacity once the peak resistance is reached. With the decrease of transverse reinforcement spacing, the ultimate load capacity of high strength CES columns can be enhanced due to better concrete confinement but the failure is still brittle with abrupt unloading from the peak load. The test results also indicated that the provision of 0.5% volume of steel fiber is able to prevent the concrete cover spalling, and hence the load-carrying capacity can be enhanced. Finally, the test results together with additional test data collected from the literature are used for comparison with codes’ prediction using the EN 1994-1-1 and JGJ 138-2016. The code methods give excellent prediction of CES columns with normal strength steel and concrete grade up to C90, but they over-estimate the maximum resistance of CES columns with concrete strength higher than 90 MPa. To extend the current design methods for CES members with higher strength concrete, two analytical methods were proposed which shows good correlation with test results, thus enabling the design of CES members with higher strength of concrete.