Experimental Research On Axial Compression Behavior And Creep Behavior Of Ultra-high Strength Concrete Columns Under High Axial Compression

With the development of high-rise building structure, high strength concrete is more and more widely used because it has the characteristics of compact structure, high strength and good durability. Concrete is a kind of brittle material and the higher the concrete strength the brittleness of concrete is more obvious. The ductility reduction will limit the development of the ultra-high strength concrete structure. In order to improve the brittle characteristics and the ductility of high strength concrete, we can add into high strength and high elastic modulus PVA fiber material and confine concrete with transverse reinforcement. With high strength stirrup confined super high strength concrete, which can not only play the advantage of high strength and can ensure the ultra-high strength concrete has enough ductility. Ultra-high strength concrete is generally used for high-rise buildings with low story columns, which will bear larger axial pressure and has large axial strain, so it is very important to consider the change of long term compression characteristics caused by long-term high axial force in the design. However, there is relatively few research on high strength and ultra-high strength concrete in China. In this paper, axial pressure characteristics of high strength stirrups confine ultra-high strength concrete with PVA fiber and compressive creep characteristics of ultra- high strength concrete columns under high axial force were studied. The detail works are summarized as follows:(1) The experimental phase of this investigation consists of 12 confined columns and three unconfined columns subjected to concentric loading conditions to investigate the effects of concrete strength, stirrup spacing and stirrup form on the stress-strain curves, bearing capacity and ductility of confined concrete.(2) Based on axial compression test results, the relationships between the effective constraint coefficient and confining reinforcement characteristic value with the strength and the ductility were analyzed. A simplified stress-strain model for confined concrete was developed after summarizing and analyzing the former typical model.(3) Six large size ultra-high strength concrete columns confined by high strength stirrups under long-term high axial force were studied to study the shrinkage and creep properties of high strength concrete. Based on the 209 R ACI(1992) creep model, the modified creep prediction model was proposed by introducing the influence coefficient of the strength correction factor and the reinforcement effect coefficient.