Xinyan Guo

Fatigue Life Prediction of RC Beams Strengthened with Prestressed CFRP under Cyclic Bending Loads

The application of prestressed carbon reinforced polymer (prestressed CFRP) in reinforced concrete (RC) members can improve the mechanical properties of strengthened structures and strengthening efficiency. This paper proposed a semi-empirical prediction formula of fatigue lives of the RC beams strengthened with prestressed CFRP under bending loads. The formula is established based on the fatigue life prediction method of RC beams and fatigue experimental data of non-prestressed CFRP reinforced beams done before. Fatigue effect coefficient of the formula was confirmed by the fatigue experiments of the RC beams strengthened with prestressed carbon fiber laminate (prestressed CFL) under cyclic bending loads. Fatigue lives of the strengthened beams predicted using the formula agreed well with the experimental data.

Fatigue Crack Propagation Behavior of RC Beams Strengthened with CFRP under High Temperature and High Humidity Environment

Numerical and experimental methods were applied to investigate fatigue crack propagation behavior of reinforced concrete (RC) beams strengthened with a new type carbon fiber reinforced polymer (CFRP) named as carbon fiber laminate (CFL) subjected to hot-wet environment. -integral of a central crack in the strengthened beam under three-point bending load was calculated by ABAQUS. In finite element model, simulation of CFL-concrete interface was based on the bilinear cohesive zone model under hot-wet environment and indoor atmosphere. And, then, fatigue crack propagation tests were carried out under high temperature and high humidity (50°C, 95% R · H) environment pretreatment and indoor atmosphere (23°C, 78% R · H) to obtain curves and crack propagation rate, , of the strengthened beams. Paris-Erdogan formula was developed based on the numerical analysis and environmental fatigue tests.

Prestress Loss of CFL in a Prestressing Process for Strengthening RC Beams

A prestressing system was designed to strengthen reinforced concrete (RC) beams with prestressed carbon fiber laminate (CFL). During different prestressing processes, prestress loss was measured using strain gauges attached on the surface of CFL along the length direction. The prestress loss was 50–68% of the whole prestress loss, which is typically associated with CFL slipping between the grip anchors. Approximately 20–27% of the prestress loss was caused by the elastic shortening of the RC beam. An analytical model using linear-elastic theory was constructed to calculate the prestress loss caused by CFL slipping between the anchors and the elastic shortening of the strengthened beams. The compared results showed that the analytical model of prestress loss can describe the experimental data well. Methods of reducing the prestress loss were also suggested. Compared to other experiments, the prestressing system proposed by this research group was effective because the maximum percentage of prestress loss was 14.9% and the average prestress loss was 12.5%.

Experimental Study on Bond Behavior of FRP-Concrete Interface in Hygrothermal Environment

As the technique of fiber-reinforced polymer (FRP) composite material strengthened reinforced concrete structures is widely used in the field of civil engineering, durability of the strengthened structures has attracted more attention in recent years. Hygrothermal environment has an adverse effect on the bond behavior of the interface between FRP and concrete. This paper focuses on the bond durability of carbon fiber laminate- (CFL-) concrete interface in hygrothermal condition which simulates the climate characteristic in South China. Twenty 100 mm × 100 mm × 720 mm specimens were divided into 6 groups based on different temperature and humidity. After pretreatment in hygrothermal environment, the specimens were tested using double shear method. Strain gauges bonded along the CFL surface and linear variation displacement transducers (LVDTs) were used to measure longitudinal strains and slip of the interface. Failure mode, ultimate capacity, load-deflection relationship, and relative slip were analyzed. The bond behavior of FRP-concrete interface under hygrothermal environment was studied. Results show that the ultimate bearing capacity of the interface reduced after exposure to hygrothermal environments. The decreasing ranges were up to 27.9% after exposure at high temperature and humidity (60°C, 95% RH). The maximum strains () of the specimens pretreated decreased obviously which indicated decay of the bond behavior after exposure to the hygrothermal environment.

Experimental study on fatigue crack propagation rate of RC beam strengthened with carbon fiber laminate

The experimental research on fatigue crack propagation rate of reinforced concrete (RC) beams strengthened with carbon fiber laminate (CFL) is carried out by MTS system in this paper. The experimental results show that, the main crack propagation on strengthened beam can be summarized into three phases: 1) fast propagation phase; 2) steady propagation and rest phase; 3) unsteady propagation phase. The phase 2-i.e. steady propagation and rest stage makes up about 95% of fatigue life of the strengthened beam. The propagation rate of the main crack, da/dN, in phase 2 can be described by Paris formula, and the constant C and m can be confirmed by the fatigue crack propagation experiments of the RC beams strengthened with CFL under three-point bending loads.

Fatigue test of RC beams strengthened with prestressed CFLs

Applying prestress to fiber reinforced polymer (FRP) can be used more efficiently since a greater portion energy of its tensile capacity is engaged. Based on carbon fiber laminate (CFL), fatigue tests are made to find out the fatigue behavior of reinforced concrete (RC) beams strengthened with prestressed CFL. The interfacial debonding is a main failure mode for RC beams strengthened with prestressed CFLs under the cyclic loading. Furthermore, it has been found that the stress value of CFLs decide whether the additional prestressing has a negative or positive effect on the fatigue behavior of the strengthened beam, and the excessive prestressing would reduce the fatigue life of the strengthened beam.

Fatigue Performance of RC Beams Strengthened with CFRP under Overloads with a Ladder Spectrum

Vehicle overload is detrimental to bridges and traffic safety. This paper presents a study on the fatigue performance of typical reinforced concrete (RC) beams of highway bridges under vehicle overload. A definition method of vehicle overload and a construction method of overload ladder spectrum were first proposed based on traffic data acquisition, statistical analysis and structural calculation of the highway bridges in Guangzhou. A fatigue experimental method was also proposed with the three-ladder vehicle overload spectrum, and the fatigue tests of 15 RC beams strengthened with carbon fiber reinforced polymer (CFRP) under three loading levels were then carried out. The fatigue performance and the failure mechanism of the strengthened beams were presented and discussed, and two fatigue life prediction methods were proposed with the established modified Palmgren-Miner rule and the loading level equivalent method respectively. The results showed that the fatigue performance of the strengthened RC beams was severely degraded under overload ladder spectrum compared with that under constant amplitude cyclic load, and the life prediction methods were proved effective.

Fatigue Performance of SFPSC under Hot-Wet Environments and Cyclic Bending Loads

A new structural material named “steel fiber polymer structural concrete (SFPSC)” with features of both high strength and high toughness was developed by this research group and applied to the bridge superstructures in the hot-wet environments. In order to investigate the fatigue performance and durability of SFPSC under hot-wet environments, the environment and fatigue load uncoupling method and the coupling action of environment and fatigue load were used or developed. Three-point bending fatigue experiments with uncoupling action of environments and cyclic loads were carried out for SFPSC specimens which were pretreated under hot-wet environments, and the experiments with the coupling action of environments and cyclic loads for SFPSC specimens were carried out under hot-wet environments. Then, the effects of hot-wet environments and the experimental methods on the fatigue mechanism of SFPSC material were discussed, and the environmental fatigue equations of SFPSC material under coupling and uncoupling action of hot-wet environments and cyclic bending loads were established. The research results show that the fatigue limits of SFPSC under the coupling action of the environments and cyclic loads were lower about 15%. The proposed fatigue equations could be used to estimate the fatigue lives and fatigue limits of SFPSC material.

Experimental Study on Bond-Slip Constitutive Relation between CFL and Concrete

Externally bonded reinforcing technique with fiber reinforced polymer (FRP) has been widely used in civil engineering. The performance of the interface between FRP and concrete is one of the key factors affecting the behavior of the strengthened structures. This paper presents a detailed study on the bond-slip mechanism between carbon fiber laminate (CFL) and concrete based on double-shear tests. 8 specimens with different bonded length and width of CFL were tested under static loading. Strain gauges along the CFL face and displacement sensor were used to measure longitudinal strains and slip of the interface. The bond-slip constitutive relation of the interface between CFL and concrete was analyzed with the testing results. Compared with four different bond-slip models, a shear stress-slip model was proposed based on the experimental data.

Effects on the Fatigue Lives of RC Beams Strengthened with CFL at Elevated Temperature under Cyclic Bending Loads

The fatigue behavior of RC beams strengthened with carbon fiber laminate (CFL) was experi- mental studied and theoretical analyzed under cyclic bending loads in several different temperatures (5 , ℃ 20 , 50 and 80 ), which the temperatures approached the weather in subtropical zone such a ℃℃ ℃ s southern China. The effect of the temperature on fatigue lives of the RC beams strengthened with CFL was discussed. Furthermore, a calculation equation of the fatigue lives of the strengthened beams was proposed under the coupling action from the temperature and the load conditions. Using the equation, the fatigue lives and fatigue limits of the RC beams strengthened with CFL were estimated accurately from the coupling action from the temperatures and the bending loads. The preliminary validity check was completed with the verify tests. Fatigue behavior and durability of RC beams strengthened with fiber reinforced polym er (FRP) was a cutting edge subject in civil engineering field (1-9). Temperature variation affected the integration fatigue lives of RC beams strengthened with FRP in practical service environment and the research in this area was seldom mentioned in China and abroad. This paper discussed the fatigue behavior of RC beams strengthened with carbon fiber laminate (CFL) mainly with experimental method under cy- clic bending loads in several different temperatures, which the temperatures approached the weather in subtropical zone such as southern China.