Research On The Cause Crack Of Small Prefabricated Box Beam In High-altitude Severe Cold Regins

With the development of the Silk Road Economic Belt,the government accelerated the northwest area transportation infrastructure construction,but northwest area has a harsh climate: cold,dry,big temperature difference between day and night and so on.The survey found that a large number of small prefabricated box beam more cracks appear in the construction process.Thus,this thesis relying on a highway project,through field research,bridge visual inspection,comparative load experiment,conservation experiment and finite element analysis,research on the cause crack of small prefabricated box beam in high-altitude severe cold regins.At the same time,the thesis makes concrete compressive strength experiment,the annular restraint cracking experiment and proposes crack remediation measures.Here are the research contents of this paper:Through on-site investigation and crack detection of small prefabricated box beam,analysising the law of cracks,the paper gets the following results.Cracks position distributing is irregular,no obvious features.Crack width between 0.1mm and 0.15 mm are 81.5%.Crack width than 0.2mm have 7.37%.Crack length between 1m and 5m are 80% and between 5m and 8m are 16%.Select three group defective bridge span and comparison bridge span conduct load test,the paper gets the following results.Prefabricated small box girders static force performance satisfies the design requirements.During the test,the existing cracks are not expansion and new cracks are not appeared.This shows that the cracks are non-structural stress cracks.According to fracture characteristics and the natural environment,it is determined shrinkage cracks and temperature cracks.The shrinkage cracks are generated due to lack of conservation and the temperature Cracks are produced due to the large temperature difference between day and night.Through the concrete compressive strength test and Constraints ring cracking test under the different conservasion conditions,the paper gets the following results.The case with the same concrete mixing ratio and the raw materials,the strength and shrinkage performance of concrete are greatly influenced by the curing conditions.Standard curing is the best,steam curing is followed,the film coated sand conservation and sand sprinkling water conservation maintenance effect is not very different,natural conservation is the worst.When there are cracks in the concrete,the width and length of cracks expand rapidly in a short time and after reaching a certain value are basically stable.Through the finite element model of precast small box girder are respectively applied temperature gradient load,which are come from standard and the high-altitude severe cold regins,the paper gets the following results.At the different temperature gradient model,small prefabricated box girder's temperature stress and deflection are different.Therefore when calculating the temperature stress,according to the natural environment,we must select the appropriate temperature gradient mode.Under the temperature gradient model in the high-altitude severe cold regins,prefabricated small box girders temperature stress and deflection increases gradually,wherein the temperature change value and the temperature stress the basic exhibit a proportional relationship.When T₁ changes 1?,the upper edge temperature stress increases about 0.14 MPa,and the tensile stress of the lower edge is increased by about 0.07 MPa.When T1 is equal to 30?,the maximum tensile stress of the lower edge is 2.80 MPa,which has exceeded the maximum allowable tensile stress of C50 concrete.Using crack dispensing closure reinforcement method and CFRP bonding method on the prefabricated small box girders by reinforcement,monitoring results show that after reinforcement of prefabricated small box girders can meet the design requirements and reinforcing effect is satisfactory.