Behavior of FRP-confined self-compacting concrete

Abstract

With the advantages of excellent corrosion resistance and high strength-to-weight ratios, FRP composites are being increasingly used in both structural retrofit/strengthening and new construction. In the new construction area, hybrid FRP-concrete members based on filament-wound FRP confining tubes with fibers oriented close to the hoop direction have great potential in practical applications. Such members include concrete-filled FRP tubes with longitudinal internal steel reinforcement and FRP-steel-concrete double-skin tubular columns (DSTCs). This thesis is concerned with the behavior of self-compacting concrete (SCC) used to fill FRP confining tubes to form hybrid members of various forms. In such hybrid members, the use of SCC is highly attractive as it simplifies the task of construction and facilitates the proper flow of concrete into small spaces (e.g. the annular space in a DSTC). While the substitutability of SCC for normal concrete (NC) is widely accepted based on numerous studies, little knowledge exists on the differences between SCC and NC when they are under substantial confinement from an FRP tube. As extensive work has been published on FRP-confined NC, this thesis presents a systematic study on the behavior of FRP-confined SCC as found in concrete-filled FRP confining tubes, and compares this behavior with that of FRP-confined NC. Following a literature review that identifies the existing knowledge gaps, this thesis presents an experimental study on the behavior of SCC confined by an FRP wrap under axial compression. The FRP wrap had fibers oriented only in the hoop direction and was formed on the concrete column after the concrete had hardened. As a result, these tests did not involve a number of complicating effects associated with concrete-filled FRP tubes, including the complexity in determining the hoop elastic modulus, the effect of the axial stiffness of the FRP tube and the shrinkage of the SCC. The test results are carefully presented and compared with an accurate stress-strain model for FRP-confined NC (i.e. Jiang and Teng{174}s model). The comparisons indicate that the model can provide reasonably close predictions for the axial stress-axial strain behavior of moderately-confined and heavily-confined normal strength SCC and heavily-confined high strength SCC but fails to make close predictions for the axial stress-axial strain behavior of weakly-confined SCC and moderately-confined high strength SCC. In addition, the model does not provide accurate predictions for the lateral expansion of FRP-confined SCC. The thesis next presents a study on the behavior of SCC in glass FRP (GFRP) tubes by testing a series of SCC-filled GFRP tubes under axial compression. An expansive admixture was included in the SCC mix for some of the specimens to compensate for the shrinkage of SCC. The test results of SCC-filled GFRP tubes indicate that a suitable amount of expansive admixture should be used in such columns to create a better confinement condition. The test results are again compared with predictions from Jiang and Teng's model developed for FRP-confined NC. These comparisons again show that the lateral expansion behavior of SCC confined by GFRP tubes cannot be properly predicted by the model, which however can predict the axial stress-axial strain behavior of SCC confined by GFRP tubes reasonably closely, particularly when the shrinkage of SCC is compensated for through the use of an expansive admixture.