The Prediction Model Of Concrete Shrinkage And The Shrinkage Effects Of Widened Bridge Structure

In the projects to widen or renovate the existing bridges, the difference ofshrinkage between the new and old concrete will have a significant effect on themechanical properties of the composite structure. It is of vital importance to providea precise prediction of shrinkage strain for analysis of the concrete shrinkage and itseffects on the structure. However, there are existing great differences by usingdifferent specifications model to do the calculation. The applicability of theseexisting models still need further study. Besides, the box section is commonly usedin the modern long-span bridge. The problem is that the inner surface of box girderdo not expose to the environment, which lead to the slow speed of drying. Thus, itsshrinkage development is definitely different from the solid sections. There is littlefocus study both at home and abroad.Therefore, the research focused on the shrinkage mode of box-shapedcomponent of structure, as well as the shrinkage effects of widening box girderbridge, with the National Natural Science Fund project "Research on the basic theoryof the existing concrete bridge widening design". The article involves mainly thefollowing investigations and corresponding results:(1) With integration of research at home and abroad, the kind of shrinkage andthe influencing factors were discussed from the mechanism. Furthermore,comparative study on those representative concrete shrinkage prediction modelswere made, including the CEB-FIP series model, ACI209series model, B-P seriesmodel, GL2000model, etc.. Their theoretical basis, factors and prediction accuracywere analyzed emphatically.(2) Through180days observation of shrinkage on6sets of concrete specimens,the development of shrinkage strain with different thickness and section law wereachieved. The results showed that: the shrinkage developed rapidly in the early stage,but became slow in the late stage; the shrinkage values calculated by the predictionmodel were less than the measured ones; the value decreased with the increment ofthe theoretical thickness of test piece; in the first180days, the box-shapedcross-sectional test piece shrinkage value was greater than the solid cross-sectionalinterview pieces.In addition, according to the principle of least squares, the concept of "boxsection impact factor" was proposed on the based of fitting the experimental data. Then, the contraction formula on China’s Road and bridge code was amended, andthe revised calculating results matched well with the experimental values.(3) On the background of a continuous prestressed concrete box girder bridgeon Chang-Zhang Expressway extension project, the pole finite element model wasestablished to perform theoretical calculations of effects caused by concreteshrinkage when the bridge was widened. The results indicated that: the shrinkagedifference between the new and old beam have great effect on superstructure,including horizontal bearing reaction, lateral displacement, lateral bending moment,and the longitudinal axis force of main beam; No matter whether the joints betweenthe new and old main beam was rigid or hinge, there was little difference in theshrinkage effect. When the lateral restraint of the bearings became stronger, thelateral displacement and lateral bending moment became smaller, and thelongitudinal axis force became greater, and the horizontal bearing reaction decreasedafter the first increases. The longer time the new beam store before connect with theold beam, the smaller the shrinkage effect, but the reduce speed decreased with thegrowth of the storage time. The effect of shrinkage varied according to differentprediction model of shrinkage strain, and the prediction model had a significantimpact on the determination of the storage time of new beam.