Meticulous Study On Flexural Behavior Of Fiber-reinforced Lightweight Aggregate Concrete Beams Reinforced With FRP Bars

Fiber-reinforced polymer(FRP)bars are of particular interest in construction structures due to their excellent resistance to corrosion as well as their light weight and high tensile-strength properties.Lightweight aggregate concrete(LWC)is a green building material that meets the sustainable development needs.The combined use of LWC and FRP in large-space prestressed structure system could contribute to enlarging the span as well as improving the durability in complex environment such as high corrosivity and altitude cold.Research on this new style of structural member is still in its infancy.Revealing the cooperative working performance between FRP and LWC in views of material mechanical property and component performance,studying the mechanism of stiffness degradation and the law of crack development of FRP-reinforced LWC beams,and establishing calculation method with definitude physical meaning could promote the application of this type of component.In this paper,an experimental study of fifteen LWC beams reinforced with FRP bars and nine beams prestressed with unbonded carbon fiber-reinforced polymer(CFRP)tendons was conducted to investigate their flexural behavior and serviceability performance.The influences of the addition of steel fibers and the application of prestressing force were analyzed.A finite element model for the purpose of detailed analysis was developed.Calculation methods based on serviceability performance were established for both prestressed and nonprestressed elements.The major contributions of the work presented in this thesis are listed as follows:1.Toughening mechanism of fibers and bond performance between FRP bars and LWC.Transmission characteristics of cohesive force between the fibers and the cement paste as well as the anti-cracking mechanism were studied in different stages of fracture development from aspects of microcosmic and macroscopic.Consequently,toughening and strengthening mechanism of the fibers were revealed.The whole process of bond-slip between the FRP bars and the LWC were experimentally investigated.On this basis,modified bond-slip models were developed.The results showed that in the process of pulling out,the steel fibers restrained the development of cracks by bonding with the cement paste.The proposed bond-slip models provided reasonable theoretical results.2.Flexural behavior of LWC beams reinforced with FRP bars and unbonded prestressed beams reinforced with FRP bars.Nine LWC beams reinforced with glass fiber-reinforced polymer(GFRP)bars,six with CFRP bars and one with steel rebars as well as nine beams prestressed with unbonded CFRP bars were tested under four-point bending.The experimental results in terms of ultimate flexural strength,deflection,crack width,and mode of failure were presented and the influences of the parameters on the serviceability were analyzed.The test results showed that the failure surfaces in the compression zone crossed the interior of the lightweight aggregates and were relatively smooth,which was obviously different with those of normal weight concrete specimens.Both increasing the FRP reinforcement ratio and the application of the prestressing force reduced the deflection and decreased the crack width of the beams.Adding steel fibers improved the rigidity and restrained the crack propagation of the specimens at low load levels.Furthermore,given the same reinforcement ratio,specimens with larger clear span length tended to yield smaller maximum crack widths at low load levels.3.Detailed finite element model of LWC beams reinforced with FRP bars.An elastoplastic model along with progressive damage was developed and employed to simulate the rupture of GFRP.Embedding the concrete damaged plastic(CDP)model,three-dimensional nonlinear finite-element(FE)models using an explicit algorithm were established to finely simulate the behavior of beams reinforced with FRP bars cast using LWC.This study lays a foundation for expanding the database of the structural performance of such components.The results revealed that the theoretical flexural capacities and the deflections at service load obtained based on the established FE models matched well with the experimental values.This confirmed that the compression constitutive model of LWC reasonably described the stress distribution and the crushing failure characteristics.The inclusion of the CDP model of LWC was capable of reflecting the tension stiffening caused due to the bond between the longitudinal bar and the concrete.The influence of the steel fiber content on the beam deformation after cracking was quantified rationally.4.Analysis model of performance in ultimate limit state.Using the micromechanical model of residual stress of steel fiber-reinforced concrete,the stress distribution in the weak plane of the normal section in ultimate limit state was specified.On the basis of a reasonable simplification of the stress distribution,the flexural capacity models of FRP-reinforced beams failed due to balanced failure and concrete crushing were improved.In the case of the beams pretressed with unbonded FRP tendons,the strain of prestressed and auxiliary tendons in ultimate limit state were described quantitatively,and thus the discrimination criterion of the failure mode was proposed.Combining the compression constitutive model of the LWC and the strip analysis method,the ultimate compressive strain of the LWC in normal section was calibrated based on the experimental and FE results.Furthermore,stress-block factors for the concrete were modified.5.Calculation models of deformation and crack width at service load.Taking into account that the distribution of FRP strain is a key parameter that governs the beam stiffness and cracking behavior,the tension stiffening strain approach in axial tension was introduced.On the basis of the definition of concrete compressive toughness index,the equation of nonuniformity coefficient of strain in FRP bars was modified incorporating the effects of the aggregate type,steel fiber content and FRP reinforcement ratio.For unbonded prestressed specimens,decomposing analytical procedure into a bonded beam analysis and an unbonded member analysis,an algorithm considering the difference of strain increase mechanism between the two types of the tendons was employed to deflection and crack width calculation.The estimations closely matched the measured values.In addition,combining the bond-slip models for “low slip stage” and the residual stress model of steel fiber-reinforced concrete,an applicable crack width model based on bond behavior was proposed.Based on the developed discrimination criterion of the failure mode and the established deflection and crack width models,considering the safety of the failure and the economy of the reinforcing,the calculation methods of the FRP-reinforced and pretressed steel fiber-reinforced LWC beams were proposed in which the requirements of serviceability were set as control conditions,and the failure mode as well as the flexural capacity were set as checking conditions.In this study,meticulous analysis on flexural behavior of fiber-reinforced lightweight aggregate concrete beams reinforced with FRP bars were performed.Deflection and crack width models for service stage were established.The failure mode criterion and flexural strength calculation method were improved.A calculation theory based on service performance was proposed which provided technical support for the design and engineering application of this type of component.