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Circular concrete columns confined with a fiber reinforced polymer (FRP) jacket fail because of the rupture of the FRP jacket due to hoop tension at an average hoop strain considerably lower than the FRP tensile strain at failure obtained from tensile tests of flat coupons. This well-established phenomenon, referred to as premature rupture, is governed by the interaction between a heterogeneous material (i.e., concrete) and a brittle material (i.e., FRP) and has been difficult to explain. The present study adopts a meso-scale model, the so-called Lattice Discrete Particle Model (LDPM), for the simulation of concrete, in conjunction with the Spectral Stiffness Microplane Model (SSMM) for simulating the fracturing behavior of the FRP jacket. The numerical predictions, experimentally validated, demonstrate clearly that due to the heterogeneity of concrete, the circumferential strain is highly non-uniform around the circumference of the column right from the beginning of the loading process rather than uniform as conventionally assumed or expected. This strain non-uniformity is the main reason for the premature rupture of the FRP jacket. In addition, stress concentrations at the finishing end of the FRP jacket are also shown to have a significant effect on the premature rupture of the FRP jacket.
Ultra-high-performance concrete is typically defined as an advanced cementitious material that has a compressive strength of over 150 MPa and superior durability. This article presents the development of a new type of ultra-high-performance concrete, namely, ultra-high-performance seawater sea-sand concrete. The development of ultra-high-performance seawater sea-sand concrete addresses the challenges associated with the shortage of freshwater, river-sand and coarse aggregate in producing concrete for a marine construction project. When used together with corrosion-resistant fibre-reinforced polymer composites, the durability of the resulting structures (i.e. hybrid fibre-reinforced polymer–ultra-high-performance seawater sea-sand concrete structures) in a harsh environment can be expected to be outstanding. The ultra-high strength of ultra-high-performance seawater sea-sand concrete and the unique characteristics of fibre-reinforced polymer composites also offer tremendous opportunities for optimization towards new forms of high-performance structures. An experimental study is presented in this article to demonstrate the concept and feasibility of ultra-high-performance seawater sea-sand concrete: ultra-high-performance seawater sea-sand concrete samples with a 28-day cube compressive strength of over 180 MPa were successfully produced; the samples were made of seawater and sea-sand, but without steel fibres, and were cured at room temperature. The experimental programme also examined the effects of a number of relevant variables, including the types of sand, mixing water and curing water, among other parameters. The mini-slump spread, compressive strength and stress–strain curve of the specimens were measured to clarify the effects of experimental variables. The test results show that the use of seawater and sea-sand leads to a slight decrease in workability, density and modulus of elasticity; it is also likely to slightly increase the early strength but to slightly decrease the strengths at 7 days and above. Compared with freshwater curing, the seawater curing method results in a slight decrease in elastic modulus and compressive strength.
Extensive research has been conducted on the behavior of fiber reinforced polymer (FRP)-confined concrete in both circular and rectangular concrete columns. In the former columns, the stress-strain behavior of FRP-confined concrete is now well understood and can be closely predicted, but the same cannot be said about rectangular columns. This paper presents a new attempt at understanding and modeling the confinement mechanism in square columns as a special case of rectangular columns, leading to a new stress-strain model. The salient features of the new model include a more rigorous definition of the effective confinement area and a corner hoop strain-axial strain relationship based on advanced finite element results as well as a more reliable definition of the ultimate condition. The proposed model is analogous in approach to analysis-oriented stress-strain models for FRP-confined concrete in circular columns and represents a more advanced and robust method for modeling the stress-strain behavior of FRP-confined concrete in square columns than the existing empirically-based stress-strain models. The approach is also easily extendable to FRP-confined concrete in rectangular columns. The proposed model is shown to be accurate and perform better than the existing stress-strain models of the same type in predicting existing test results.
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Cylinders, cones, spheres and tori are some of the common basic shell elements. Steel shell structures such as silos, tanks, pressure vessels, offshore platforms, chimneys and tubular towers generally consist of two or more of these basic shell elements. Axisymmetric intersections featuring meridional slope mismatches between the connected elements are common features in steel shell structures. High bending and circumferential membrane stresses are developed in these intersections, and their buckling and collapse strengths are a key design consideration. This paper presents a summary of recent research on the stress, stability and strength of axisymmetric steel shell intersections. Particular attention is paid to intersections formed from cylindrical and conical segments as these are more common and have been more extensively researched. A simple approximate method for extrapolating the knowledge gained on these intersections to those containing curved shell segments is also suggested.
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Extensive research has been conducted on the replacement of steel rebars with fibre-reinforced polymer rebars to eliminate the steel corrosion problem in conventional steel bar–reinforced concrete ...
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This paper presents an accurate bond strength model for carbon-fibre-reinforced polymer (CFRP) strips near-surface mounted (NSM) to concrete where debonding failure happens in the concrete layer adjacent to the interface between FRP and concrete. Both bonded joints with a sufficient bond length and those with an insufficient bond length are covered by the proposed model. The bond strength model was developed on the basis of an existing analytical solution as well as a recently proposed bond-slip model for such bonded joints. Numerical comparisons between the proposed bond strength model and 51 test specimens collected from the existing experimental studies as well as the only existing bond strength model for such joints demonstrate the accuracy of the proposed model and its superiority over the existing bond strength model, especially for joints with insufficient bond lengths.
