Zhimin Wu

Experimental Study on the Bond Behavior of Reinforcing Bars Embedded in Concrete Subjected to Lateral Pressure

Owing to its importance in the design and assessment of steel-reinforced concrete structures, the bond behavior of reinforcing bars embedded in concrete has been widely investigated. In this paper, 91 specimens were cast and tested to study the effect of lateral pressure on the bond behavior of plain and deformed round bars. On the basis of the experimental results, it is found that, for plain round bars, the bond capacity increases significantly with an increase in lateral pressure, whereas the slip corresponding to the peak bond stress reduces dramatically once the lateral pressure is applied. Through using the deformed reinforcing bars with noncentrosymmetric surface configuration, it is found that the bond behavior is closely related to the direction and magnitude of the lateral pressure. When the lateral pressure is applied parallel to the transverse rib, the bond capacity increases with an increase in lateral pressure. When the lateral pressure is perpendicular to the transverse rib, the bond capaci...

Bond Behavior of Plain Round Bars in Concrete under Complex Lateral Pressures (with Appendix)

The bond behavior of reinforcing bars is highly dependent on confinement conditions and is of great importance in the design and analysis of concrete structures. This paper presents an experimental investigation into the bond behavior of plain round bars subjected to uniaxial and biaxial lateral pressures with different pressure ratios. The bond parameters are analyzed with respect to the average lateral pressure. The results show that the residual and ultimate bond strengths increase with the increase of the average lateral pressure, but their ratio stays invariable. The slip at the ultimate bond stress first reduces remarkably, and then increases gradually with an increase in lateral pressure. Compared with lateral pressure, the bar size and the strength of concrete have little influence on the bond strength ratio. Finally, a constitutive model of bond stress-slip with the lateral pressure effect is presented and shown to correlate well with the experimental data for different pressure ratios and levels.

Numerical Method for Mixed-Mode I–II Crack Propagation in Concrete

AbstractA crack-propagation criterion is proposed in this paper for mixed-mode I–II fracture in concrete. In this criterion, crack propagation is initiated when the difference between the stress-intensity factor at the crack tip caused by the external force and that by the cohesive stress satisfies the crack-initiation equation. On the basis of this criterion, a numerical method is developed to simulate mixed-mode I–II crack propagation in concrete. To verify the criterion, three sets of experimental data are selected: one is obtained from the self-conducted test and the other two are collected from the literature. The numerical results show that the calculated load versus crack-mouth opening and sliding displacement curves and crack trajectories are in good agreement with experimental results. Therefore, once the mode I initial cracking toughness, elastic modulus, fracture energy, possion’s ratio, and tensile strength of concrete are available, the whole mixed-mode I–II fracture process in concrete can b...

Crack Extension Resistance Curve of Concrete Considering Variation of FPZ Length

Owing to its importance in evaluating the fracture behavior of concrete, the crack extension resistance curve of concrete has been widely studied, both experimentally and theoretically. In this paper, a numerical approach is developed for the crack extension resistance curve of concrete by considering the variation of the fracture process zone (FPZ) length during the whole fracture process. In this approach, the FPZ length is determined by using the linear asymptotic superposition assumption. Dividing the whole fracture process into three different stages of the cohesive stress distribution within the FPZ, the crack extension resistance curve is formulated by superposition of the intrinsic fracture toughness of concrete and the fracture toughness caused by the cohesive stress within the FPZ. The developed numerical approach is applied to the tested and simulated standard three-point bending notched concrete beams. The effect of the variation of the FPZ length on the crack extension resistance curve is evaluated on the basis of the numerical results. The crack extension resistance first increases with an increase in ratio of the effective crack length to the beam depth and then reaches a plateau value when the FPZ is fully developed. When the effective crack length is normalized to the beam depth, the crack extension resistance is basically independent of the beam depth within the beam size range studied.

Bond Behavior of Plain Round Bars Embedded in Concrete Subjected to Biaxial Lateral Tensile-Compressive Stresses

Owing to its importance to the assessment and design of reinforced concrete structures, the bond behavior of steel bars in concrete has been extensively studied. This paper presents an experimental investigation into the bond behavior of plain round bars embedded in concrete subjected to biaxial orthogonal lateral tensile-compressive stresses. A total of 174 pull-out specimens with different strengths of concrete, bar diameters, and combinations of lateral stresses were tested. The experimental results showed that, for a given lateral compressive stress, the bond strength decreases, but the slip at the peak bond stress increases as the lateral tensile stress increases. For a given lateral tensile stress, the bond strength increases, but the slip at the peak bond stress decreases with an increase in lateral compressive stress. Based on the experimental results, an empirical bond stress-slip relationship is proposed. It is shown that, for different strengths of concrete, bar diameters, and combinations of lateral stresses, the empirical bond stress-slip relationship is in good agreement with the experimental results.

Analytical Solution for Fracture Analysis of CFRP Sheet–Strengthened Cracked Concrete Beams

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.

Fracture Behavior of Site-Casting Dam Concrete

An experimental investigation of the fracture properties of dam concrete obtained directly from a dam construction site is presented. The specimens were all cast at the construction site of a super-high arch dam. The complete load-crack mouth opening displacement (P-CMOD) curves and fracture parameters for specimens with dimensions of 800 × 800 × 450 mm (31.5 × 31.5 × 17.7 in.) at 28, 90, and 180 days and 1000 × 1000 × 450 mm (39.4 × 39.4 × 17.7 in.) and 1200 × 1200 × 450 mm (47.2 × 47.2 × 17.7 in.) at 180 days were obtained. The fracture parameters of the site-casting dam concrete can represent the real fracture properties of the super-high arch dam and can be directly applied to the super-high arch dam. The parameters obtained in this paper are significantly different from those of other studies.

Fracture mechanisms of rock-concrete interface: Experimental and numerical

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China under the grant of NSFC 51478083, 51421064, and 51109026, and the fundamental research funds for the Central Universities under the grant of DUT14LK06.

Experimental Study on Bond Behavior of Deformed Bars Embedded in Concrete Subjected to Biaxial Lateral Tensile Compressive Stresses

AbstractThe bond behavior of deformed reinforcing bars, which is important to the assessment and design of reinforced concrete structures, is greatly influenced by the stress state in concrete. Because the effect of biaxial lateral tensile-compressive stresses on the bond behavior of deformed bars is seldom studied, an experimental investigation is presented in this paper. A total of 252 pull-out specimens with different strengths of concrete, bar diameters, and combinations of lateral tensile-compressive stresses were included in the test. The results show that the failure mode of specimens with deformed bars is influenced by the strength of concrete, ratio of the cover depth to bar diameter c/D, and the coupling effect of lateral stresses ζ(=pt/ft+pc/fcu). The bond strength and the slip at the peak bond stress decrease by increasing ζ, whereas the deteriorating ratio depends on the strength of concrete and c/D for specimens failed by pull-out and splitting, respectively. Regardless of ζ, the ratio of th...

Effect of Loading Rate on Bond Behavior of Deformed Reinforcing Bars in Concrete under Biaxial Lateral Pressures

AbstractThe bond of reinforcing steel bars is closely related to the stress state in the surrounding concrete. However, most of the current studies are limited to static monotonic loading and only few studies on the dynamic bond behavior of deformed bars under lateral pressures are reported in the literature. The purpose of this paper is to present an experimental investigation on the effect of the loading rate on the bond behavior of deformed bars subjected to biaxial lateral pressures. The test results indicate that the bond parameters are influenced by lateral pressures, loading rate, bar diameter, and concrete strength. Finally, an empirical bond stress-slip relationship for deformed steel bars is proposed and validated against experimental results.