International Concrete Abstracts Portal

Concrete-filled fiber reinforced polymer (FRP) tubes provide an alternative to conventional reinforced and prestressed concrete columns. The tube helps turn normal strength concrete core into a high performance concrete, i.e., one with high strength, ductility, and durability. The over-reinforcement in concrete-filled tubes helps avoid split rupture of the FRP reinforcement in tension, which is catastrophic in nature. A total of 8 concrete-filled FRP tubes were tested as beam-columns under a combination of axial and flexural loads. The tubes were 9 feet long, and had an outside diameter of 13 11/16 in. with a wall thickness of about 0.55 in.. The study showed that over-reinforced concrete-filled FRP tubes generally behave well as beam columns. They deflect to a lesser extent than the corresponding under-reinforced sections. They are also more efficient than under-reinforced sections, because a smaller portion of their sectional capacity is consumed by secondary moments and P-D effects. Their failure, while in compression, is considered to be gradual and progressive.

While ductile behavior of a reinforced concrete member can be given by flexural longitudinal steel bars, high strength continuous fibers can contribute as shear and confining reinforcement, which can provide excellent durability as well. Composite R/C members encased by fiber-mesh-mortar tubes seem to be therefore one of the most effective systems. In this research, the following aspects of the system were investigated by experimental studies: (a) Tensile behavior of fiber-mesh-mortar plate—Tensile properties of the fiber-mesh-mortar plates were investigated. The test results showed tension stiffening effect, which reduced crack spacing. (b) Compressive capacities of concrete columns with fiber-mesh-mortar tubes—Uniaxial compressive tests of concrete columns cast in fiber-mesh-mortar permanent forms were conducted. Improvements in strength and ductility were obtained. The confining effect was governed by fiber type, mesh directions and number of mesh layers. (c) Shear and flexural behavior of composite R/C beams encased by fiber-mesh-mortar tube—Composite R/C beams fully encased by fiber-mesh-mortar tubes were tested. The fiber-mesh-mortar tube reduces surface crack spacing on the lateral sides of the beams and improved shear/flexural capacities.

A total of 15 concrete columns were cast and tested to investigate the influence of replacing longitudinal and/or tie steel bars by an equal volume of amount of glass fiber reinforced polymers (GFRP) bars on the behavior of concrete columns. The columns were subjected to concentric monotonic axial loading. The concrete block for all columns was 450 ¥ 250 ¥ 1200 mm. The results indicated that replacing the longitudinal steel bars by GFRP bars reduced the axial capacity of the column by 13%. The results also showed that regardless of the type of the longitudinal bars, replacing the steel ties by GFRP ties reduced the axial capacity of the column by 10%. However, the study revealed that replacing the steel ties by GFRP had, up to about 80% of the ultimate load, no influence on the load-axial shortening curve. Furthermore, the results indicated that the currently used ACI formula to estimate the axial capacity of the column overestimated the actual capacity of the column reinforced longitudinally and or transversely by GFRP bars.

A mathematical failure analysis model was developed to predict the behavior and failure of FRP dowels subjected to tensile and shear forces. The model consists of two sub-models. To model dowel action, the beam on elastic foundation (BEF) model was adopted and modified by introducing two indices, namely a displacement level index to accommodate the concrete subgrade stiffness, and a tension index to accommodate the cable effect. The Tsai-Hill failure criterion, in which a failure factor was introduced, was used as the failure criterion for the dowels subjected to tensile and shear forces. The failure analysis model was used to predict the ultimate dowel shear force and corresponding displacement. Analytical predictions were compared with the test results of the CFRP dowel specimens and a good agreement was observed.

As part of a research project on “Fire resistance of concrete elements strengthened with externally bonded FRP reinforcement”, a synthesis report is presented on the influence of elevated temperatures on the thermal and mechanical properties of FRP materials and resins, and on the influence of elevated temperatures on the behaviour of the interface between concrete and FRP. Also a survey of the results of fire tests on FRP strengthened or reinforced concrete members is given.