Experimental and numerical studies on compressive behavior of winding FRP grid spiral stirrups confined circular concrete columns

Currently, fiber reinforced polymer (FRP) is commonly used as main reinforcement as well as stirrups. Especially in a column, conventional pultruded FRP rod spiral reinforcement is limited by the bending radius and is prone to premature bond failure between FRP spiral and concrete, resulting in inefficient concrete confinement. An automated winding of a novel grid spiral stirrup was proposed to overcome these drawbacks. In this paper, axial compression tests were carried out on FRP grid spiral stirrup confined concrete columns (FGSCC). The effects of key parameters (stirrup width-to-thickness ratio, stirrup type, and stirrup spacing) on the compressive behavior of FGSCC were mainly investigated. The ultimate axial load and ultimate axial strain for the specimen with 50 mm stirrup spacing was almost two times that of the specimen with 150 mm stirrup spacing. In addition, a numerical model was established using ABAQUS. The model adopted a modified concrete damage plastic model (CDPM), and the dilation angle to circumferential pressure relation equation applicable to the FGSCC model was proposed. The existing confined concrete stress–strain model was modified based on experimental results. The results of the parametric analysis showed that the increase in concrete strength substantially increased the ultimate axial load of the FGSCC, while the ultimate axial strain was reduced. The ultimate axial load of FGSCC was increased by 7.3 % when the concrete strength was increased from 31 MPa to 51 MPa. The cross-section area of the grid spiral stirrup had a great effect on the compressive properties of FGSCC. When the cross-sectional area of the FRP grid spiral stirrup was increased from 27 mm² to 75 mm², the ultimate axial load increased by 75.3 % and the ultimate axial strain increased by 26.1 %.