Study On Stress-strain Relationship Of Concrete Specimens Confined With "Thin" Stirrups Under Axial Compression

The "thin" steel bars refer to straightened stress hardening bars, which cannot conform to the weight and ductility specifications. Compared with normal steel bars,the cross-sectional area of "thin body" steel bars is reduced after being cold drawn and straightened, leading to increased yield strength and decreased ductility. As the "thin" steel bars are produced in the straightening process of the steel coils, they are often used as beam and column stirrups due to the small diameter, having a certain market share. Currently there are limited studies on stress-strain curve of concrete confined with "thin" stirrups and the reinforcement of buildings with "thin" stirrups.Axial compression and finite element analysis are conducted to quantify the concrete specimens confined with 16 rectangular prism specimens and "thin" stirrups with a17.9% average weight-loss rate respectively. Both rectangular stirrups(volume stirrup ratio between 0.69%-1.38%) and rectangular diamond composite stirrups(volume stirrup ratio between 1.18%-2.36%) are adopted in this study. The experimental and theoretical results show that:(1) The stress-strain curve of concrete confined with "thin" stirrups and normal stirrups are similar. There is no obvious reduction observed in the descent stage of the stress-strain curve of concrete confined with "thin" stirrups.(2) There is little difference between the concrete stress-stirrup strain curve of concrete specimens confined with "thin" stirrups and normal stirrups. Elastic relations can be observed in the ascent stage of the curve, while the descent stage of the concrete stress-stirrup strain curve are smooth after exceeding the peak value of the stress. For some of the specimens, the strain of test points next to the stirrup hook in both sides decreases when the strain of the stirrup is more than 1%. Meanwhile disconnecting phenomenon of the specimens with stirrups is observed. The strain of stirrups in the other two sides continues to increase until exceeds the range of stirrups strain(2%). Subsequent result cannot be measured and no stirrup pull off phenomenon is observed throughout the test process.(3) After disconnecting with the concrete, the stirrups connected with the hook in both sides are unloaded, and bond slip happens between the stirrups in the other two sides and the concrete, which turn out to be one of the reasons leading to integrity of the stirrups. Another reason is that, since the minimum stirrup spacing of this test is 74 mm, a lot of concrete is found stripping out from stirrups, forming a gap between the stirrups and concrete. Concrete deformation due to subsequent load cannot cause squeeze effect on the stirrups. If the spiral stirrups with a high density are used,concrete cannot stripped out, which may lead to snap of the "thin" stirrups.(4) The finite element software ABAQUS is used to numerically simulate and analyze the specimens. The concrete damaged plasticity nephogram achieved from simulation coincides well with the final destructive pattern. In addition, the stress distribution of vertical section is consistent with theoretical functional mechanism.(5) The stress-strain curve of concrete confined with "thin" stirrups is consistent with finite element simulation results at low reinforcement ratio. With high reinforcement ratio, the descent stage of the finite element analysis results is much smoother.(6) A big difference in the descent stage of concrete stress-stirrup strain curves is observed between the Finite element simulation and experiment results. The descent stage of the experiment results is much smoother than finite element analysis.This is because that the bond slip between stirrups and concrete is not take into consideration in the finite element analysis. In addition, significant cracking and fall-off rather than slight elastoplastic deformation is happened during the descent stage of the concrete stress, which cannot be totally presented by current plastic damage model. Moreover, fracture mechanics are also involved in this problem.(7) Mean value and variance of the test results cannot be achieved due to the quantity limitation of the specimens. Given the factors such as inhomogeneity of the concrete, eccentric loading, and the measuring error, statistical test results cannot be obtained. The finite element software simulation of concrete stress-strain curve of specimens confined with "thin" and normal stirrups shows that the descent stage of their stress-strain curves are similar when rectangular stirrups are used. However, thedescent-stage ductility of the "thin" stirrup specimen is slightly better than normal stirrup specimen when rectangular-diamond composite stirrups are utilized.