Study On Mechanical Behavior And Structural Reliability Of Widened Concrete Bridges

With the comprehensive application of concrete bridge widening technique in the reconstruction project of existing highways,it is of increasingly urgent to establish the specific design theory,methodology as well as guidelines associated with widened concrete bridges,and precise analysis methodology of mechanical behavior and structural reliability regarding these widened concrete bridges are the prerequisites.However,existing studies on mechanical behavior of widened concrete bridges,especially those on the long-term shrinkage and creep properties,are almost all finite element analysis on one specific structure for each study,making the universality of the research results and conclusions very weak.In the meantime,investigations on the structural reliability of widened concrete bridges are very limited,which is far behind with the practical needs from engineering community.Accordingly,equipped with experimental researches and theoretical analysis,this thesis focuses on the shrinkage and creep properties as well as structural reliability of widened concrete bridges.The main contents are arranged as follows:1.Experimental study on free shrinkage properties of concrete and modification of shrinkage prediction model?1?A 500-day observation of free shrinkage deformation associated with 18concrete specimens was conducted,accordingly,the influence of member size and cross-sectional shape on free shrinkage of concrete were quantitatively investigated.The results show that:member size simultaneously affects the ultimate value and development rate of shrinkage,and the larger the specimen size,the smaller the ultimate value and the slower of development rate of shrinkage.For those hollow box specimens sealed at both ends,both the shrinkage deformation at each stage and the shrinkage development rate are lower than those of the corresponding solid rectangle specimens with the same theoretical thickness,indicating a larger theoretical thickness of those hollow box specimens due to a sealed box cell.The shrinkage of the hollow box specimens can be predicted by the same model for the solid rectangle section,while the inner cell circumference of the hollow box should be reduced to 0.45 of its original value when determining the theoretical thickness.?2?Based on the experimental results,the shrinkage prediction model suggested by the current Chinese design code for highway bridges?JTG D62—2004?was modified,and the applicability and feasibility of the modified model were verified by the test results in presented research and relevant literatures.2.Experimental study and theoretical analysis on restraint shrinkage behavior of new-old-concrete composite components?1?The restraint shrinkage strains of 18 new-old concrete composite specimens with different geometric sizes and interfacial states?including chiseled and chiseled accompanied by post-installed shear rebar?were continuously measured for 300 days.The results show that:measured shrinkage strains of new concrete near the interface?targets 2-2'?and those near the top surface?targets 1-1'?are significantly smaller and slightly larger than free shrinkage measured from the reference specimen,respectively,which has the same geometric size as the new concrete in the composite specimen.The measured restraint shrinkage strains of all targets decreases with the increase of specimen size.For specimens with same geometric size,specimen with chiseled interface accompanied by post-installed shear rebar shows larger and smaller measured strain at targets 1-1'and targets 2-2'than that of specimen with only chiseled interface,respectively.?2?An analytical method for restraint shrinkage effects of new-old-concrete components was proposed by incorporating the inverse method in plane stress theory and the age-adjusted effective modulus method?AAEM?for concrete creep effect calculation.Compared with the existing analytical methods based on elementary beam theory,the proposed plane-theory-based method shows both higher accuracy and applicability.Besides,the influence of longitudinal interfacial slip between the new and existing structures can be included in the proposed method by setting reasonable value of the interfacial shear stiffness.The generic stiffness values of the two investigated representative interfaces in this study,i.e.,interface pre-treated by chiseling and chiseling accompanied by post-installed shear rebar,were evaluated based on the measured strain results of the corresponding composite specimens,and the results are 4×10¹⁰ N/m³ and 9×10¹⁰ N/m³,respectively.?3?Using the proposed plane-theory-based analytical method,the restraint shrinkage stresses of the tested new-old-concrete composite specimens as well as a practical widened concrete slab bridge were evaluated.The results show that:under the effects of shrinkage and creep difference between new and existing concrete,the new and existing part of the composite specimen bears eccentric tension and eccentric compression,respectively,and the composite structure behaves integral in-plane bending.The restraint on free shrinkage of new concrete by old concrete induces relative large interfacial shear stress,as well as maximum tensile and compressive stress in the ends of interface in new and existing concrete,respectively.?4?Compared with the new-old-concrete composite specimens,as the width-to-span ratio of the analyzed widened slab bridge is significant larger,i.e.,2.75,the nonlinearity associated with the longitudinal distribution of interfacial shear stress caused by concrete shrinkage and creep is much obvious,and shear stress concentration appears in the ends of the beam.On the other hand,the longitudinal distributions of longitudinal normal stress along the interface are much uniform.The longitudinal normal stress in the mid-span section of new and existing beam are both nonlinearly distributed along the transverse direction,indicating the inapplicability of the Bernoulli's hypothesis of plane remaining plane for this case.3.Analytical analysis of shrinkage and creep effects of widened concrete bridges considering the effect of splice joint?1?Considering the local interfacial deformations of new and existing beams caused by interfacial stress,a beam-theory-based analytical model as well as explicit solutions for shrinkage and creep effects of widened concrete bridges were suggested.As the proposed solutions associated with the shrinkage and creep stresses are very simple-in-form and proved to have high accuracy,they are supposed to have relative high applicability.?2?Parametric analysis on a widened new-old slab structures with a span of 5m was conducted using the proposed method,the results show that:the elastic modulus of splice joint shows slight influence on the shrinkage and creep effects of widened concrete structures.With the increase of the width of splice joint,the interfacial stresses become more uniformly distributed along the interface,and the maximum values of all interfacial stress components decrease significantly.Therefore,in practical bridge widening project,it can be a suitable means to reduce the harmful shrinkage and creep effects of widened structures by increasing the width of splice joint.4.Uncertainty analysis of shrinkage and creep effects of widened concrete bridges?1?By combining the age-adjusted effective modulus method?AAEM?-based FE analysis as well as Latin hypercube sampling?LHS?-based Monte Carlo simulation,a general algorithm for uncertainty analysis of shrinkage and creep effects in widened multi-girder bridge was proposed.Case study on a widened 25m-span simply-supported concrete bridge with T-shape girders indicates that:normal distribution is an acceptable approximation for the probability distribution of the maximum axial forces in mid-span that induced by concrete shrinkage and creep of the new bridge.the largest expected values of the axial forces among all girders are those associated with the two girders that adjacent to the splice joint.The mean value as well as standard deviation of axial force increase with time,indicating that the uncertainty of shrinkage and creep effects increase with time before converging at a relative high level.?2?Sensitivity analysis shows that:among various factors,the modeling uncertainty factors of concrete shrinkage and creep are the two most influential variables for the uncertainty of shrinkage and creep effects in widened concrete bridges.5.Time-variant reliability analysis of widened concrete bridges?1?Considering the effects of concrete shrinkage and creep,structural deterioration and live-load redistribution on resistance and load effects,a general probabilistic framework and the corresponding computational algorithms for time-variant reliability of widened concrete bridges were proposed.?2?Case study shows that:the variation of flexural moment capacity of all girders increases significantly when considering the shrinkage and creep effects.Concrete shrinkage and creep have more significant effects on girder reliability than that of live-load redistribution and reinforcement corrosion,especially for the girders adjacent to splice joint,which are subjected to maximum shrinkage-and-creep-induced axial force.The reliability indices of the existing and new girders adjacent to splice joint reaches the threshold value?4.2?at about 15 and 50 years after widening,respectively,though these two girders are both subjected to minimum live load moment in among all girders after widening.?3?In addition to the shrinkage and creep effects,capacity correlations among different girders also have a considerable impact on the system reliability of widened bridges.When capacity correlations and the effects of concrete shrinkage and creep were simultaneously considered,the reliability index of the bridge system reaches the threshold value in about 70 years after widening.