Showing papers by "Jun-Jie Zeng published in 2023"

Behavior and model evaluation of large-rupture-strain (LRS) FRP-confined concrete-encased high-strength steel columns under axial compression

Abstract: Fiber reinforced polymers (FRP)-confined concrete-encased steel columns (FCSCs) have become increasingly popular. However, few studies have been conducted on the FCSCs with a large-rupture-strain (LRS) FRP tube. To this end, the axial compressive behavior of FCSCs with a polyethylene terephthalate (PET) FRP tube (PFCSCs) is investigated in this study. A total of eighteen circular stub columns (i.e., two unconfined specimens, four PET FRP-confined concrete columns (PFCCs), and twelve PFCSCs) were tested under axial compression, in which the PET FRP tube thickness, the shape and nominal yield strength of the encased steel sections were the focal points. The results indicate that the steel strength can be fully utilized because local instability of the encased steel sections can be well suppressed by the surrounding concrete. The specimens with a circular steel tube had better performance than those with a cruciform steel due to the additional confinement provided by the circular steel tube. Moreover, a model assessment suggested that the models of Lin et al. and Zeng et al. could provide relatively satisfactory predictions for the ultimate stress and strain of confined concrete in PFCSCs.

Punching Shear Behavior of FRP Grid-Reinforced Ultra-High Performance Concrete Slabs

Abstract: Fiber-reinforced polymer (FRP) grid-reinforced ultra-high performance concrete (UHPC) slabs are new structural solutions that take advantage of the mechanical properties of FRP and UHPC. However, the punching shear behavior of this new slab type has not been characterized. Therefore, punching shear tests were conducted to eight carbon fiber-reinforced polymer (CFRP) bars or grid-reinforced UHPC square slabs (600 mm side width × 40 mm thick). Several influential factors [e.g., use of CFRP bars or grids as flexural reinforcements, type of strengthening short fiber, steel fiber (SF) content, and presence of shear reinforcements] were investigated. The test results showed that FRP grids and short fibers helped to distribute the applied loads and dissipate the input energy; therefore, more cracks were observed, and higher punching shear capacities were achieved. Furthermore, increasing the reinforcement ratio in the FRP grids led to a more significant postcrack ductility response, which increased the punching shear capacity by 17%. In addition, SF addition could enhance the initial cracking load of the slab (Vcr), and polyethylene (PE) fiber addition could intensify the postcrack ductility response, both enhanced the punching shear capacity. The installation of shear reinforcements (eight pieces of 80 mm long CFRP grid strips) appeared to be more cost-effective than increasing SF content. Finally, compared with the current design provisions for conventional reinforced concrete slabs, a more accurate (average error of 8%) punching shear model was proposed for FRP grid-reinforced UHPC slabs with or without SF additions. However, the robustness of the proposed model will be assessed with more test data in the future.

Tri-axial compressive behavior of steel fiber-strengthened expansive concrete

Abstract: Concrete is widely used with external confining devises such as fiber-reinforced polymer (FRP) tubes and steel tubes. Expansive concrete and steel fiber-reinforced expansive concrete has been proposed to prevent concrete shrinkage, and even to provide some pre-stresses in the confining devices so as to enhance confinement efficiency. However, the confinement mechanisms on expansive concrete and steel fiber-reinforced expansive concrete have not been fully understood yet. In this study, tri-axial compression tests were conducted to 52 concrete cubes with different expansive agent contents and steel fiber additions, in which 12 concrete cubes were under true tri-axial compression with different lateral confining pressures. It is found that concrete cubes under true tri-axial compression typically exhibited oblique shear failure, and their strength enhancements were limited by the magnitudes of minor principal stress. Furthermore, the effects of expansive agent content and steel fiber volume fraction on the axial stress-strain relationship were investigated. Results showed that the CaO-based expansive agent addition should be at least more than 4% to make a difference for the axial stress-strain curves. Concrete cubes with 8% expansive agent addition exhibited a steeper initial ascending slope, but the strength enhancements diminished with increasing confining pressures. Steel fiber addition could significantly enhance the peak axial stress (by 10–20%), and the enhancement effect became more prominent with increasing confining pressures. Therefore, the combined use of expansive agent and steel fiber has synergistic and complementary effects.

Axial compressive behavior and design-oriented model for large-rupture-strain (LRS) FRP-confined concrete in rectangular columns

Abstract: Polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) composites, which is a kind of low-carbon production and have a large rupture strain, may have prosperous application outcomes. PET FRP wrap has been explored as confining reinforcement for concrete, and the majority of investigations are limited to PET FRP-confined concrete in circular and square columns, whereas PET FRP-confined rectangular columns have barely been touched. To fill this research gap and gain a fundamental understanding of the confinement mechanism, comprehensive compressive tests on 36 columns were conducted in this study, in which the confinement thickness, cross-section aspect ratio, and specimen size were the focal points. All specimens exhibited a three-segment behavior (i.e., strain hardening-softening-hardening response). In addition, it is demonstrated that specimen size had an insignificant effect on the axial stress-strain response of small and medium columns with a height less than 400 mm, provided that their confinement levels were the same (or similar). Furthermore, the minimum effective confinement stiffness for PET FRP-confined rectangular columns to achieve sufficient confinement was 0.004, which was extracted from all available test results. A design-oriented model with improved performance was proposed and the simulated full axial stress-strain showed close agreements with the test results.