A recently proposed technique for the recycling of concrete is to directly reuse large pieces of coarsely crushed demolition concrete (referred to as recycled concrete lumps or RCLs) with fresh concrete in new construction. This novel recycling process is simpler and more cost-effective than the traditional way of recycling concrete as aggregates. The resulting concrete, referred to as compound concrete in this paper, features greater heterogeneity, especially when the fresh and the recycled concretes have rather different strengths. The interfaces between RCLs and fresh concrete, as internal weaknesses, may also compromise the performance of the compound concrete. One possibility to improve the properties of this compound concrete is to supply it with a substantial amount of confinement as may be available in concrete-filled FRP tubes, which are attractive as columns in corrosive environments. This paper therefore presents and interprets the results of an experimental study on the compressive behavior of FRP confining tubes filled with compound concrete. The influences of various parameters, such as the mix ratio and the strength ratio between the old and the fresh concretes as well as the FRP tube thickness, are investigated. The test results indicate that with a significant level of confinement, the ultimate axial stress and the ultimate axial strain of the compound concrete containing RCLs are comparable with those of the fresh concrete, regardless of the strength of the old concrete. This conclusion is important as it means that the weaknesses of compound concrete containing RCLs can be largely eliminated through lateral confinement. The applicability of an accurate stress-strain model previously developed for FRP-confined normal concrete to FRP-confined compound concrete with RCLs is also examined.

A large amount of research has been conducted on recycled aggregate concrete (RAC) due to its social, environmental, and economic significance. However, the in situ application of RAC has so far been mainly limited to nonstructural purposes, as the performance of RAC, in both the short and long term, is inferior to its normal concrete counterpart. Existing research has shown that the performance of concrete in compression members can be significantly enhanced through external confinement using steel tubes and fiber-reinforced polymer (FRP) tubes/wraps. Some recent research has examined the behavior of steel tubes filled with RAC, but the research on the behavior of RAC confined with FRP has been rather limited. Research is therefore needed to better understand the stress-strain behavior of and develop a reliable stress-strain model for FRP-confined RAC to facilitate the design of members with FRP-confined RAC. This paper presents the results of the first systematic experimental study on the axial compressive behavior of FRP-confined RAC in which 18 FRP-confined RAC cylinders were tested. The effects of the replacement ratio of coarse aggregate (ratio between the mass of recycled coarse aggregate to the total mass of coarse aggregate) and degree of FRP confinement are investigated. Both the axial and lateral strain responses of the specimens are examined in detail. The test results show that specimens with a replacement ratio of 20% behave similarly to that of normal concrete, but specimens with a replacement ratio of 100% exhibit a lower strength and a different stress-strain response. The applicability of two existing stress-strain models for FRP-confined normal concrete to FRP-confined RAC is also examined.

Hybrid fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular members (DSTMs) are a new form of hybrid members that consist of an outer tube made of FRP and an inner tube made of steel, with the space between them filled with concrete. The existing studies on hybrid DSTMs have been mainly focused on their use as compression members, with only a very limited number of studies on their use as flexural members [i.e., hybrid double-skin tubular beams (DSTBs)]. This paper presents the first ever experimental study on large-scale hybrid DSTBs with headed shear studs; the effect of an integrated deck is also examined. The main parameter examined in the experimental program was the section configuration. The test results show that both the DSTBs and the DSTB-deck unit possessed a very ductile response, and that the headed shear studs effectively reduced or eliminated slips between the steel tube and the concrete. This paper also presents a theoretical model based on conventional section analysis. The predictions from the theoretical model are in reasonably close agreement with the test results.

Carbon fiber reinforced plastic (CFRP) cables are superior to steel cables in cable-stayed bridges due to their light weight and excellent durability. It is a kind of flexible structural component with strongly weak shear stiffness. Hence, understanding the dynamic properties of CFRP cables poses a significant issue in bridge engineering. In this paper, the parametric vibrations of CFRP cables is performed and compared with steel cables by using a non-linear finite element method. The CFRP and steel cables of a cable-stayed bridge with a span of 1400 m, designed by the same criteria, are studied as typical cases. The material damping, aerodynamic damping and viscous dampers are simulated in the analysis, and the variations of the material damping and aerodynamic damping due to amplitude variations are taken into consideration. The dynamic responses of cables due to stochastic and harmonic support excitations are investigated. Numerical results show that the vibration amplitudes of steel cables are greater than that of CFRP cables. The parametric vibration of CFRP cable is more sensitive to damping than that of steel cable. The effects of material damping and viscous dampers on parametric vibration of both steel and CFRP cables are very significant, while that of aerodynamic damping is inferior to the former two dampers.

This article presents test results of a recent study on the axial compressive behaviour of fibre-reinforced polymer–confined compound concrete–filled thin steel tubes. The usage of compound concrete, which is a mixture of fresh concrete and large pieces of recycled concrete lumps, can recycle waste concrete in a simple but effective way. Totally, three series of tests were conducted, with the parameters including the relative strength between fresh concrete and recycled concrete lumps, the volumetric percentage (i.e. mix ratio) of recycled concrete lumps, the diameter-to-thickness ratio of the steel tubes, and the thickness of the fibre-reinforced polymer jackets being investigated. The stress–strain curves of the steel tube and compound concrete core were derived and the effects of different parameters were then examined and discussed. An existing stress–strain curve model of fibre-reinforced polymer–confined normal concrete-filled steel tubes was also found performing well in predicting the behaviour of fibre-reinforced polymer–confined compound concrete-filled steel tubes.