Jinlong Pan

Effect of Concrete Composition on FRP/Concrete Bond Capacity

External bonding of fiber-reinforced plastics (FRP) to concrete members has been established as an efficient and effective method for structural strengthening and retrofitting. Direct shear test is often employed to study the crack-induced debonding failure in reinforced concrete members flexurally strengthened with FRP composites. In many existing models, the bond capacity (which defines ultimate load capacity of the specimen in the direct shear test) is considered to be strongly dependent on the compressive or tensile strength of the concrete. However, since debonding behavior is affected by interfacial friction due to aggregate interlocking within the debonded zone, the concrete composition should also play an important role in determining the bond capacity. In this study, the direct shear test is performed with 10 different compositions of concrete. The test results indicate that the bond capacity has little correlation with either the concrete compressive or splitting tensile strength. On the other h...

Mechanical Behavior of Fiber-Reinforced Engineered Cementitious Composites in Uniaxial Compression

Polyvinyl alcohol (PVA) fiber reinforced engineered cementitious composite (ECC) is a class of high performance cementitious composites with pseudo strain-hardening behavior and excellent crack control when subjected to uniaxial tension. However, the compressive behavior of ECC has not been well characterized in the literature. In this paper, uniaxial compression tests were carried out on ECC with five different mix proportions and compressive strength ranging from 35 MPa to 60 MPa. Complete stress-strain curves were obtained. Based on the test results, the compressive parameters, such as the elastic modulus, engineering strain at the peak stress, the Poissons ratio and the toughness index, were studied. A new constitutive model was proposed to express the pre- and post-peak mechanical behavior of ECC under uniaxial compression. The proposed model showed a good agreement with the experimental curves. The model proposed should be a valuable reference for the nonlinear analysis of ECC material in the part of structures under uniaxial compression.

Flexural Behaviors of ECC and Concrete/ECC Composite Beams Reinforced with Basalt Fiber-Reinforced Polymer

The use of fiber-reinforced polymer (FRP) reinforcement in structural engineering has attracted great attention due to high tensile strength, good fatigue performance, and inherent corrosion resistance. Engineered cementitious composite (ECC) is a class of high-performance cementitious composites with pseudo-strain-hardening behavior and excellent crack control. Substitution of concrete with ECC can avoid the cracking and durability problems associated with brittleness of concrete. In this paper, six FRP-reinforced ECC, or ECC/concrete composite beams with various longitudinal and transverse reinforcement ratios and ECC thicknesses, are tested in bending. According to the test results, FRP-reinforced ECC beams show much better flexural properties in terms of load-carrying capacity, shear resistance, ductility, and damage tolerance compared with FRP-reinforced concrete beams. For the FRP-reinforced ECC beam without stirrups, final failure occurs in shear. However, the ultimate load capacity and deformation are comparable to the FRP-reinforced concrete beams with properly designed stirrups, and the failure process is ductile due to the strain hardening behavior of ECC materials. For ECC/concrete composite beams, strategic application of ECC can lead to considerable increase of energy dissipation capacity. When a layer of ECC is placed in the tension zone, the crack width along the beam can be well controlled. High residual strength and stiffness of the composite beam can hence be obtained. In addition to experimental work, a theoretical model is proposed to predict the moment-curvature responses of FRP-reinforced ECC beams. Model results are found to be in good agreement with test data. Theoretical analysis is then conducted to illustrate the effect of reinforcement ratio, compressive strength, and thickness of ECC on the ultimate moment, curvature and ductility of beams.

FRP Debonding from Concrete Beams under Various Load Uniformities

In recent years, the bonding of fiber reinforced polymer (FRP) has been accepted as an effective and efficient method for strengthening and retrofitting concrete structures. A lot of research efforts have focused on the flexural strengthening of concrete beams with FRP plates. However, in most investigations, experimental testing was limited to simply supported beams under three-point or four-point bending, which may not represent the real conditions of the strengthened members. In this investigation, to study the failure of the FRP strengthened beams under practical situations, a special waffle tree system is designed to test the beam specimens under quasi-distributed loading conditions. According to the test results, for beams bonded with thin FRP plate, enhancing load uniformity can significantly increase the moment capacity without changing the failure mode. However, for beams with thick FRP plate, there will be a change of failure mode from crack-induced debonding to concrete cover separation and a corresponding reduction in the moment capacity. The transition of the failure mode indicates that the load uniformity plays an important role in determining the failure behavior of the strengthened beams. For both crack induced debonding and plate end failure (concrete cover separation), several existing analytical models have been employed to calculate the ultimate debonding moment and compared with the test results. The results show that most existing models are not able to properly predict the failure behavior of the strengthened concrete beams under different levels of load uniformity.

Effect of Concrete Composition on Interfacial Parameters Governing FRP Debonding from the Concrete Substrate

For concrete beams strengthened with FRP plates, FRP debonding may occur from the bottom of an intermediate flexural or shear/flexural crack in the span. As debonding failure occurs within the concrete, interfacial friction resulted from aggregate interlocking between opposing surfaces of the debonded zone plays an important role in the debonding behavior. Interfacial debonding can be analyzed with a three-parameter model. To investigate the effect of concrete composition on the debonding behavior, the direct shear test was conducted with ten different compositions of concrete. Interfacial parameters corresponding to each concrete composition were then derived according to the analytical model. It is found that the concrete compressive or splitting tensile strength shows little correlation with interfacial parameters. However, the debonding initiation strength and residual shear strength correlates well with the surface tensile strength, and the maximum sliding between FRP and concrete is mainly governed by the aggregate content. Test results indicate that the composition of concrete should be considered explicitly in the investigation of FRP debonding from a concrete substrate.

Mechanical Behavior of ECC and ECC/RC Composite Columns under Reversed Cyclic Loading

AbstractEngineered cementitious composite (ECC) is an advanced composite material with strain-hardening and multiple-cracking behaviors. The substitution of conventional concrete with ECC can signi...

Elastoplastic time history analysis of reinforced engineered cementitious composite or engineered cementitious composite–concrete composite frame under earthquake action:

Engineered cementitious composite is a class of high-performance cementitious composites with pseudo-strain hardening behavior and excellent crack control capacity. Substitution of concrete with engineered cementitious composite can greatly reduce the cracking and durability problems associated with low tensile strength and brittleness of concrete and can significantly increase structural seismic resistance. In this article, a pair of beam–column joints with various matrix types has been tested under reversed cyclic loading to study the effect of substitution of concrete with engineered cementitious composite in the joint zone on the seismic behaviors of composite members. After that, a simplified constitutive model of engineered cementitious composite under cyclic loading is proposed, and the structural performance of steel reinforced engineered cementitious composite members is simulated by fiber beam elements. The accuracy of the model is verified with test data. Finally, three frame structures with different matrixes subjected to earthquake actions were numerically modeled to verify the contribution of ductile engineered cementitious composite material to structural seismic resistance. The seismic responses or failure mechanisms, deformation patterns, and energy dissipation capacities for each frame structure are analyzed and compared. The simulation results indicate that the application of engineered cementitious composite can reduce the maximum story drift ratio, and the distributions of the dissipated energy are more uniform along the building height when engineered cementitious composite is strategically used in ground columns and beam–column joints of the frame structure. The seismic performance of the reinforced engineered cementitious composite-concrete composite frame is found to be even better than the frame with all concrete replaced by engineered cementitious composite.

Analytical and numerical modelling of FRP debonding from concrete substrate under pure shearing

External bonding of fiber reinforced polymer (FRP) composites on the concrete structures has been proved to be an effective and efficient way to strengthen concrete structures. For a FRP strengthened concrete beam, it is usually observed that the failure occurs in the concrete and a thin layer of concrete is attached on the surface of the debonded FRP plate. To study the debond behavior between concrete and FRP composites, an analytical model based on the three-parameter model is developed to study the debonding behavior for the FRP-to-concrete joint under pure shearing. Then, nonlinear FEM analysis is conducted to verify the proposed analytical model. The FEM results shows good agreement with the results from the model. Finally, with the analytical model, sensitivity analyses are performed to study the effect of the interfacial parameters or the geometric parameters on the debonding behavior.

A Novel Constitutive Model for Concrete under Uniaxial Compression

In this paper, a novel constitutive model of concrete has been proposed by introducing a new parameter, namely, cracking Poisson’s ratio (νcr), to account for the effect of localization due to cracking. By fitting the curve between the dimensionless strain (ε/εpr) and cracking Poisson’s ratio (νcr), νcr can be expressed as an 3rd order polynomial function of dimensionless longitudinal strain (ε/εpr). The constitutive model for the softening regime can then be proposed with the parameters of dimensionless strain and cracking Poisson’s ratio. Finally, Validity of the proposed model is verified by the test results of cylinder specimens of C30.

The Effect of Fiber Orientation on the Mechanical Properties of SHCC

Strain-Hardening Cementitious Composites (SHCC) are materials featuring strain-hardening behavior accompanied by formation of multiple cracks. The distribution of fiber orientation in SHCC members is affected by member thickness due to the limited freedom of rotation for fibers near the surfaces. This paper first demonstrates how to acquire fiber orientation distributions for various member thicknesses from geometrical consideration. The distribution of fiber orientation is found to be between ideal 2D distribution and 3D distribution, so the tensile performance should be in between as well. Constitutive law for a single crack is computed based on obtained distributions. Stress-strain curves for tensile members are also simulated. This study reveals the theoretical effect of member thickness on SHCC behavior. Compared to laboratory data obtained from small-size specimens, a thickness-dependent reduction factor for mechanical properties (mainly tensile strength and ductility) should be considered in the design of real structural members.