Pedram Zohrevand

Behavior of Ultrahigh-Performance Concrete Confined by Fiber-Reinforced Polymers

Over a decade of studies have demonstrated the benefits of ultra high performance concrete (UHPC) in terms of damage tolerance, energy absorption, crack distribution, and deformation capacity. However, little information is available on the confinement behavior of UHPC, especially when confined with fiber-reinforced polymers (FRP). Sixteen UHPC-filled FRP tubes with different fiber type and tube thickness were tested under monotonic uniaxial compression. All specimens failed by rupture of the tube at or near the midheight. Similar to conventional concrete, test results showed significant enhancements in the ultimate strength and strain of UHPC—up to 98% and 195%, respectively, compared with its unconfined counterpart. The experimental results were compared with a number of available confinement models. Although one of the models provided a reasonable fit for the stress-strain response in most cases, all models generally underestimated the effectiveness of FRP confinement at higher confinement ratios. The study demonstrated the need for confinement models that could accurately predict the behavior of FRP-confined UHPC in terms of the stress-strain relationship and the respective ultimate strengths and strains.

Cyclic Behavior of Hybrid Columns Made of Ultra High Performance Concrete and Fiber Reinforced Polymers

The unique features of ultra-high-performance concrete (UHPC) in damage tolerance, energy absorption, and deformability were combined with the superior performance of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) to develop a novel hybrid system of FRP tube and UHPC, and the cyclic behavior of this system evaluated. Four specimens were tested. Two were steel-reinforced: one with conventional concrete (RC), and the other (RUHPC) with UHPC within twice the plastic hinge length and conventional concrete for the remainder of the column length. The other two had FRP tubes: one filled with conventional concrete (CFFT), and the other (UHPCFFT) filled with UHPC within twice the plastic hinge length and conventional concrete for the remainder of the column length. Each column was tested as a cantilever under a constant axial load and reverse cyclic lateral loads applied incrementally in displacement control. Each of the tubed specimens without any internal reinforcement achieved the same flexural strength and ductility as its steel-reinforced counterpart. Specimen UHPCFFT showed significantly higher flexural strength and initial stiffness, lower residual drift, and relatively similar energy dissipation as compared with Specimen RC. The proposed hybrid system can be optimized for strength and ductility as a viable alternative to the conventional RC column.

Stress-Strain Model of Ultrahigh Performance Concrete Confined by Fiber-Reinforced Polymers

The application of ultrahigh performance concrete (UHPC) as an alternative to conventional concrete has grown rapidly in recent years. However, to date, little is known about the confinement behavior of UHPC, knowledge that is necessary to develop design guidelines for UHPC columns. In a previous study, the authors investigated the stress-strain behavior of a series of UHPC-filled fiber-reinforced polymer (FRP) tubes with different fiber types and thicknesses under uniaxial compression. The FRP confinement was shown to significantly enhance both the ultimate strength and strain of UHPC. It was also shown that the existing confinement models are incapable of predicting the behavior of FRP-confined UHPC. Therefore, in this study, two commonly used FRP confinement models are recalibrated based on test results of FRP-confined UHPC. A model was further modified based on the stress-strain model of unconfined UHPC to better capture the linear response of UHPC before the activation of FRP confinement. A comparison of the three models showed that a recalibrated model provides the most accurate prediction of the stress-strain behavior of FRP-confined UHPC in terms of the stress-strain curve and ultimate strength and strain.

Seismic Response of Ultra-High Performance Concrete-Filled FRP Tube Columns

The seismic response of a novel hybrid column made of a fiber-reinforced polymer (FRP) tube filled with ultra-high performance concrete (UHPC) was studied. A methodology was proposed to estimate the maximum ground acceleration capacity of five UHPC-filled FRP tubes (UHPCFFT) and one reference concrete (RC) column based on the results of their pseudo-static tests. The proposed analytical methodology can be applied to any pseudo-static test. The analysis showed 20% higher maximum ground acceleration capacity for the steel-free UHPCFFT column with a thin FRP tube, as compared to its RC counterpart. The results were further verified using a nonlinear dynamic simulation.

Cyclic Behavior of FRP Concrete Bridge Pier Frames

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) were initially developed without any internal steel reinforcement as viable alternatives to conventional RC columns in nonseismic regions. Studies have shown that extending CFFT application to seismic regions requires a moderate amount of internal steel reinforcement. To date, cyclic performance of CFFTs as part of a structural frame has not been assessed. This paper describes testing of four one-sixth-scale two-column bents: a control RC, a glass FRP-concrete frame (GFF), a carbon FRP-concrete frame (CFF), and a hybrid glass/carbon FRP-concrete frame (HFF). Each frame was tested under reverse cyclic lateral loading with a constant axial load. Specimen HFF with hybrid FRP tubes demonstrated the highest moment capacity and initial stiffness, with an increase of 200 and 70%, respectively, over the control specimen. Specimen GFF showed no sign of cracking up to a drift ratio of 15% with considerable residual strength, whereas specimen CFF had the least ductility. Glass FRP tubes extended the plastic hinge length of the pier columns to twice that of the control RC frame.

Effectiveness of Externally Applied CFRP Stirrups for Rehabilitation of Slab-Column Connections

AbstractThe application of externally bonded carbon fiber-reinforced polymer (CFRP) stirrups is by now proven as an effective technique to strengthen slab-column connections for punching shear. However, there has not been adequate study into its effectiveness for repair and rehabilitation of existing slab-column connections with different levels of prior damage. The punching performance of predamaged slab-column connections rehabilitated with CFRP stirrups was investigated for the first time in this study. Two series of 2/3 scale slab-column connections with two different concrete compressive strengths (low and normal) were rehabilitated using CFRP stirrups at different damage levels. The specimens were all tested under a concentric punching load. Test results showed that externally applied CFRP stirrups could restore the punching shear capacity of fully damaged slabs to levels above their undamaged conditions. The lower concrete strength proved to have no detrimental effect on the effectiveness of the CF...

Effect of column parameters on cyclic behavior of ultra-high-performance concrete-filled fiber-reinforced polymer tubes

A novel hybrid column made of fiber-reinforced polymer (FRP) and ultra-high-performance concrete (UHPC) was developed in a previous study by the authors. The steel-free UHPC-filled FRP tube (UHPCFFT) system proved promising as an alternative to conventional reinforced concrete (RC) columns. This study investigates the effect of column cross section, type of FRP tube, and amount of longitudinal steel reinforcement on the cyclic behavior of UHPCFFT columns. Accordingly, six column specimens, including one control RC and five UHPCFFTs with different FRP tubes, steel reinforcement ratios, and diameters, were made and studied under pseudo-static tests. Clear and strong correlations were established between the initial stiffness and strength of UHPCFFT systems and the stiffness index and reinforcement index, respectively. All UHPCFFT columns exhibited significantly lower residual displacement and slightly lower ductility, as compared to Specimen RC.

Assessment of Cyclic Behavior of Hybrid FRP Concrete Columns

Previous experimental studies have shown superior performance of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) under static or pseudostatic loading. This study has focused on the effects of fiber type and architecture and the combined shear and flexure on the cyclic behavior of CFFT columns. One control RC and five CFFTs with different fiber types and architecture and shear span-to-depth ratios were tested under a constant axial load and reverse cyclic lateral loads. One of the tubes was off-the-shelf filament-wound product, whereas the other four were made using hand layup in the laboratory. The flexural strength and initial stiffness of the CFFT columns were shown to be dominated by the longitudinal tensile strength and stiffness of the FRP tube, respectively. On the other hand, the modulus of elasticity of the FRP tube in both the longitudinal and hoop directions was shown to be a dominant factor in the ductility of CFFT columns. The CFFT column with the combination of carbon fibers in the longitudinal direction and glass fibers in the hoop direction showed the highest energy dissipation, and all nonslender and slender CFFT columns showed a similar mode of flexural failure. An analytical study was carried out to comprehensively investigate the effects of fiber architecture and shear span-to-depth ratio on the cyclic behavior of CFFT columns.

Punching Shear Enhancement of Flat Slabs with Partial Use of Ultrahigh-Performance Concrete

AbstractPrevious studies have clearly shown that flat slabs made of ultrahigh-performance concrete (UHPC) have considerably higher punching shear strength than their counterparts made of conventional concrete. However, it may not be economically feasible to construct an entire slab out of UHPC. As such, the objective of the research reported in this paper was to determine the optimal use of UHPC within the critical punching shear area, while the remainder of the slab is made of normal concrete. Ten flat slab specimens with two different steel reinforcement ratios and three different areas (or depths) of UHPC were tested under a concentric load to study the effect of UHPC on punching shear capacity of flat slabs. Test results showed that full-depth application of UHPC within the area enclosed by a perimeter located at a distance equal to the slab thickness from the column face could enhance punching shear capacity of the slab by 70%. This configuration was deemed to be the optimal application of UHPC in fl...

Behavior of Ultrahigh-Performance Concrete Confined by Steel

AbstractUltrahigh-performance concrete (UHPC) offers a superior alternative to normal-strength concrete (NSC) due to its significantly higher compressive and tensile strengths, improved ductility, and enhanced durability. This paper presents an experimental study of the stress-strain behavior of UHPC confined by conventional transverse steel reinforcement. Test results are compared with two confinement models, as well as a large amount of available data on steel-confined NSC and high-strength concrete (HSC), and limited data available for UHPC confined by fiber-reinforced polymers (FRP). The study shows a potential threshold of confinement ratio beyond which the confinement effectiveness of UHPC exceeds that of HSC and nears that of NSC. This may have implications on the minimum level of confinement reinforcement for UHPC. For the most part, steel reinforcement is more effective than FRP as the confining device for UHPC, except for the very high end of confinement ratios, where FRP may become more effecti...