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Behavior of Steel Fiber-Reinforced High-Strength Concrete Columns under Uniaxial Compression

This paper presents tests that were performed on square large-scale steel-fiber-reinforced high-strength concrete (HSC) columns under concentric compression loading. The experimental program was mainly designed to examine the effect of the volumetric steel-fiber ratio on the behavior of reinforced HSC large-scale elements subjected to axial compression loading. The test program was also designed to examine the combined confinement effect of steel fibers and transverse steel reinforcement. Thus, the test variables studied herein are the steel-fiber volumetric ratio and the volumetric ratio, yield strength, and spacing of the transverse steel ties. The results show that adding discrete fibers to HSC mixtures in reinforced concrete columns not only prevents the premature spalling of the concrete cover but also increases the strength and ductility of the axially loaded reinforced member. This behavior was predicted by the proposed fiber-reinforced concrete stress-strain model, which takes into account most of the parameters that influence confinement effectiveness: the concrete strength; the spacing, yield strength, volumetric ratio, and configuration of the transverse reinforcement; the distribution of the longitudinal reinforcement; and the diameter, length, shape, volumetric ratio, and frictional bond strength of the fibers. Predictions were found to be in good agreement with experimental results.

Seismic Performance of Circular High-Strength Concrete Columns

Little experimental research is available for high-strength columns (HSC) under combined cyclic flexure and axial loads. This paper seeks to fill this gap by presenting results from tests of six large-scale spirally reinforced HSC circular columns under reverse cyclic flexure and constant axial loads. The tests were conducted to examine the post-elastic behavior and the ductility level reached by HSC circular columns designed according to the 2004 Canadian Standards Association A23.3 requirements for transverse steel reinforcement. The columns were subjected to constant axial loads and a cyclic horizontal load-inducing reversed bending moment. Findings show that columns designed according to the confinement reinforcement requirements of the Canadian standard can achieve adequate ductility if the transverse reinforcement chosen accounts for the axial load level and the transverse-steel yield strength. Concrete columns that have different transverse-steel yield strength and different axial-load levels will have adequate sectional ductility.

Seismic behavior of synthetic fiber-reinforced circular columns

Ductile behavior and high energy-dissipation ability are two essential properties for a reinforced concrete column part of a structure in a moderate to high seismic region. Concrete design codes ensure ductile behavior of columns by setting a requirement for a minimum amount of transverse steel reinforcement. Studies have shown, however, that use of fiber-reinforced concrete (FRC) can enhance the post-peak behavior and hence, the ductility and energy dissipation ability of concrete columns subjected to axial force and bending moment. Therefore, the inclusion of macro fibers in the concrete mixture, combined with a reduced amount of lateral reinforcement, can be an alternative to the conventional lateral reinforcement required by the codes. Moreover, the higher resistance to crack growth and the excellent durability of FRC over nonfibrous concrete can result in a higher cost-effective value. In this regard, tests on large-scale circular synthetic fiber-reinforced concrete (SNFRC) columns subjected to a combined constant axial load and reversed cyclic flexure simulating earthquake loading were carried out. The aim of this test program was to examine the influence of adding synthetic fibers to the concrete mixture on the behavior of normal-strength concrete (NSC) columns. The results show that, in terms of ductility and energy dissipation, SNFRC columns outperformed NSC columns. The results also show that the larger the amount of lateral reinforcement the smaller the influence of the fibers on the columns behavior. Based on the test results, the amount of confinement steel required by concrete design codes could be reduced when SNFRC is used.

Behavior of Circular Reinforced-Concrete Columns Confined with Carbon Fiber–Reinforced Polymers under Cyclic Flexure and Constant Axial Load

AbstractDeficient concrete-bridge columns can be effectively upgraded by bonding on the exterior surface fiber-reinforced polymer composite sheets with the fibers oriented in the columns’ circumferential direction, thus providing additional confinement. The efficiency of this rehabilitation method has been proven by the results of a number of cyclic loading tests performed on column specimens. The number of these tests, however, is much less than similar ones performed on reinforced-concrete columns without fiber-reinforced polymer confinement. This paper presents cyclic flexural test on reinforced-concrete columns confined with conventional circular hoops and carbon fiber–reinforced polymer and subjected to different axial-load levels. In addition to enriching the available database, the tests in this research program were programmed such that failure would occur in the composite sheets to validate an innovative stress-strain model that considers the passive confinement provided by both transverse-steel ...

Model of Circular Reinforced Concrete Members Subjected to Axial Load

Existing models that describe the behavior of reinforced concrete columns that are confined by transverse reinforcement have an empirical or semi-empirical basis. Many of these models refer to the uniaxial stress-strain relationship of the confined concrete, assuming that the transverse steel has yielded. This paper proposes a theoretical model of circular RC members confined by transverse reinforcement. The analysis is based on formulation of an equivalent, fully confined, concrete column. The equivalency is based on a preliminary elastic analysis of a cylinder confined by a sequence of equally spaced lateral elastic-perfectly plastic stiffening rings. Results of worked examples show that within a Reduced Cylinder Radius (RCR) there is a zone of uniformly distributed stresses in which the tangential stress is equal to the radial stress and the shear stresses are equal to zero. This finding leads to application of the concept of RCR that was observed in elastic solutions without leaving out the confinement influence near the cylinder surface. Thus, the discrete steel ties can be replaced by an equivalent tube and the equivalent confined cylinder can then be solved in the plastic range of the concrete material with the relatively simple boundary conditions at the concrete-tube interface. Using the Imran and Pantazopoulou (2001) concrete plasticity model, a computer program was written to solve this problem. Initial results of circular columns that were analyzed with this model show a very good agreement with published test results.