| Post-tensioned bonded prestressed concrete is widely used in engineering.The current manufacturing technique cannot guarantee the completely density of grouting.Besides,there is no reliable method for detecting grouting holes.There are many engineering accidents in China and other countries caused by grouting holes.To solve the grouting problems,the initial retarded-bonded prestressed concrete structures are invented based on bonded and unbonded prestressed concrete structures in 1980 s.The retarded-bonded prestressed tendons are not only suitable for the prefabrication process,but also have a flexible structural form of composite integral type.The retarded-bonded prestressed tendons not only have the advantages of unbonded prestressed tendons(convenient construction and flexible arrangement),but also have the advantages of the prestressed tendons(high efficiency in using material properties and anti-corrosion property).In this dissertation,the retarded-bonded partially prestressed concrete beam was taken as the research object.Its performance under static and fatigue loads was studied experimentally and theoretically.The main research works of this paper are as follows:(1)The research starts with the bond behavior experiment between the retardedbonded tendon and concrete.The bond performances of the retarded-bonded tendon under unidirectional and bidirectional tension are investigated by the pull-out test and the tension test respectively.The experimental study investigated the influence of the nominal diameter,concrete strength,cover thickness,bond length and external restraint on the bond performance.The experiment result showed that the bond behavior between retarded-bonded tendon and concrete characterized by plastic flow.The bond stress with 1.0 mm slip was recommended as the marked bond strength.Based on the results of the pull-out test,the average bond-slip constitutive curves and the bond-slip constitutive curves with 95% strength guarantee were fitted according to five classical bond-slip constitutive law,which provided the support for the numerical simulations.(2)The experimental study on the static performance of retarded-bonded partially prestressed concrete beams was carried out.Compared with bonded and unbonded prestressed specimens,the anti-crack performance,stiffness degradation,ultimate bearing capacity,stress distribution and crack development of retarded-bonded partially prestressed concrete beams with different prestress ratios and reinforcement forms were investigated.The experiment result showed that the stiffness,bearing capacity,crack control ability of retarded-bonded prestressed specimens were slightly better than those of bonded prestressed specimens.Taking all bending cracks and the bending cracks which extended to the edge of the compression zone as samples,the ratios of the maximum crack width with 95% guarantee rate and average crack width were 1.646 and 1.582 respectively.The calculation methods in the current code are still suitable to estimate the performance of the retarded-bonded prestressed flexural member under the ultimate limit state and serviceability limit state.(3)The fatigue experimental research of the retarded-bonded partially prestressed concrete beams under constant and variable amplitude fatigue loads was carried out.The stiffness of beam,concrete and steel strain amplitude,and crack development were analyzed.The evolution of the overall,local and main fracture cracks under fatigue load was investigated.Focusing on the characteristics of the three-stage evolution of the fatigue performance,the similarities and differences of the degradation of the beams’ performance under constant and variable amplitude fatigue loads were summarized.The experiment results show that although the fatigue failures of all the test beams were caused by the fracture of tensile ribbed bars,the fracture of the strands’ wires were observed through cutting the fracture sections.In the stable crack development stage,compared with the crack length under the static load,the total length of the cracks increased by 140%～220%.In addition to the characteristic of larger crack width,the main fracture crack also characterized by lateral extension and bifurcation at the top of the crack.The probabilities of bifurcation happen to the bending-shear cracks and the pure bending cracks were 9.41% and 3.03%,respectively.In the stable crack development stage,the ratio of the average height to the average length of cracks was between 0.6 and 0.9.The larger the fatigue load amplitude was,the smaller the ratio was.According to the stress amplitude of the tensile ribbed bars,the parameters of four commonly used linear and nonlinear damage accumulation criteria and the corresponding S-N curves were fitted.Among them,the Manson criterion and the Corten-Dolan criterion were desirable for better life prediction results.Based on the Manson criterion,the calculation formulas of residual deformation and stiffness degradation were deduced and fitted.(4)On the basis of the traditional Cohesive Zone Model(CZM),through the definition of the mixed(I+II)mode fracture criterion to cohesive element and the global contact behavior,a numerical concrete tension and compression simulation method at meso scale was established.The application scope of the CZM was extended from tension simulation to compression simulation at meso scale.The unification of the tension and compression simulation methods was realized.Through the simulation analysis of the whole process of concrete under uniaxial tension and compression,and through the study of multi-factors(fracture energy,mixed mode fracture criterion,frictional behavior,interface strength,aggregate shape,aggregate type),the following conclusions were drawn.When the ratio of element size to model side length size was1/150,precise simulation results can be obtained with economical calculation cost.When the residual compressive strength of concrete was roughly equal to 1/2～1/3 times the maximum compressive strength,the main shear bands were formed,and the concrete was under a relatively stable state.The increase of the edges of aggregates could reduce the internal force of concrete under compression,which led to the intensify of strength’s degeneration.The tensile and compressive strength of the models with regular pentagon aggregates were close to those of the models with arbitrary polygonal aggregates.(5)The simplified and refined meso-scale numerical simulation methods were established for the simulation of bond behavior retarded-bonded tendons and ribbed bars.In the simplified numerical simulation method,the bond-slip relationship was equivalently represented by the connection elements.In the refined numerical simulation,based on the modified CZM,the refined retarded-bonded tendon model was constructed according to the actual structure and material properties of the tendon.A quantitative analysis method was proposed for the development of primary and secondary cracks in the process of bond behavior.The study showed that the bond-slip law included in GB50010 and proposed by Wu et al.were suitable for describing bond behavior under bidirectional tension state.The reasons for limit of the current bond behavior of retarded-bonded tendon were the low elastic modulus and low strength of the force-transmitting layer(sheath and retarded-bonded material),of which the sheath which made of high-density polyethylene was the main limitation factor.(6)A numerical simulation method for macro-meso concrete members was proposed.In this new method,the bonding relationship between reinforcement and concrete,the cracking of concrete,and the bond behavior translation of reinforcement can be taken into consideration.The simulation and experimental results could achieve good agreement on the crack development and the overall mechanical response.Through the variable parameter study on bond strength of reinforcement and residual effective tensile area of ribbed bars,the following conclusions were drawn.On the premise of ensuring the anchorage,the change of the bond strength between 0.1 and100 times the measured value will not have a significant impact on the bearing capacity of the specimen.In the stable stage of crack development,the crack width was roughly inversely proportional to the square root of the bond stiffness.When the area defect rate of the ribbed bars exceeded 10%,the main fracture crack would show obvious bifurcation development characteristics.(7)Theoretical research on crack development behavior of flexural members with retarded-bonded reinforcement was carried out.A calculation method of crack width based on bond stiffness was proposed.Under the classical model,the average crack spacing and the distribution of crack spacing were calculated by statistical simulation method.The classic model was modified according to the numerical simulation results.The concrete strain,bonding stress,and the nonuniformity coefficient of reinforcement strain between cracks were analyzed in the parameter study on the nominal diameters of reinforcements,concrete strengths,and cover thicknesses.On the basis of the law of horizontal development,a method for calculating the crack width of center-to-center tensile specimens considering the bond stiffness was established and proposed.The calculation method reveals and reflects the nonlinear and almost-linear development law of crack width in the initial and stabilization stage,which is caused by the transformation of bond stiffness.Based on the data of the static test beam,the established calculation method was extended to the calculation of crack width of flexural members.The proposed method,and the crack width calculation method included in the standards of China,American and fib were used to calculate the crack width of static test beams in this research and the beams in the literatures.The reliability of the proposed method was confirmed through the comparison of the calculation results. |
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