The Experiment Study Of Shear Behavior Of Concrete Box Girder

Shear strength in reinforced concrete has been a concern in academia for quite along time. Unfortunately, the shear failure mechanism of reinforced concretemember have been imperfect, moreover, empirical and semi-empirical formulamethod has been used to calculate the shearing resistance in different building codesat home and abroad. Meanwhile, different theoretical models, experimental data anddesigning habits lead in inconsistent results of the shearing capacity formula in eachcode. Box girders, with the characteristic of closed section, have excellent spatialmechanical behavior. Therefore, they are widely used in bridge structures. However,among the previous researches about the shear strength of box girders, there is noconcrete code for the calculation of shear strength. Generally, the box girders havebeen simplified as I-beam. In addition, three-dimension prestress system should beusually applied in large-span box. In representative design codes at home and abroadfor on concrete structures, the vertical prestress on the shear capacity of concretebox girder with three-way prestressed has not been considered.How to reduce weight and enhance durability becomes one of the importanttasks which will greatly affect the development of bridge. The mechanical anddurability properties of high performance concrete make it an ideal candidate foruse in developing new solutions to pressing concerns about ordinaryreinforced concrete. So far, most of the studies and experiments of materialproperties have been carried out on high performance concrete, few works havebeen successfully conducted on shear performance.This paper is ground on the item of National Natural Science Fund “Structuralbehavior of super-long span cable-stayed based on high performance materials"(51078134) and Specialized Research Fund for the Doctoral Program of HigherEducation "Study on mechanical properties amd design of Reactive PowderConcrete box girder"(20120161110021). The shear performance of concrete boxgirder, reinforced thin-wall box girders with vertical prestressed tendon and highperformance concrete box girder were studied.The main research contents are asfollows:（1）Four reinforced concrete beams are made at the scale of1:10and1:5tomainly study the different shear resistance behaviors between box girder andequivalent T-shape girder and compare size effect among them. The result shows that the shear resistance of box girder is8.5%lower than T-shape girder when the ratio ofreinforcement is equal, therefore, it is not completely safe to measure shearresistance of box girder with equivalent T-shape beam formula. When the size isdoubled, the ratio of experimental value and theoretical value of box girders isreduced by3.9%and3.0%for T-shape girders, according to the comparison.Concerning that the actual size structure box girder is larger, the enough safetymargins of employing existing method of shear resistance calculation in technicalcodes need to be further studied. The shear bearing capacity of tested beams hasbeen analyzed with Double Section method(Response-2000)，a simplified analysismethod, Signal Section method and finite element method (VecTor2software), andthe average ratio of calculating data to testing data are0.92,0.88,0.81,0.88,respectively. And the explanation on the basis of the principle of fixed pressure fieldanalysis method can better predict the shear of the box beam. It indicates that Theshear bearing capacity of box girder can be well calculated with modifiedcompression field theory.（2）Shear behavior of two reinforced thin-wall box girders with HRB400verticalprestressed tendon and four reinforced girders with high strength vertical prestressedtendon(1860MPa) have been studied. Whether the vertical tendons are prestressed ornot and the ducts are grouted or not have been comparatively studied. Thedevelopment law of damage form of experimental beam girders, load deflectionrelationship, the average strain of web, the vertical prestressed reinforcement stress,cracking load and the shear bearing capacity are anal yzed. Through the comparisonof test girders, it is found that the vertical prestressing force can improve thediagonal cracking load and shear capacity, however, the grouting quality for verticalprestressed ducts is of great importance to shear behavior of the box girder. Whetherthe vertical prestressed tension is tensioned or not has little impact on shearreinforcement strain increment. The stretch-draw vertical prestressed tendon canbring about effective performance of high strength shear reinforcem ent and theincrease of inclined crack angle. The vertical prestressed improves shear bearingcapacity through raising the level of the strength of shear reinforcement.According to the characteristic of vertical prestress tendon，calculation methods ofshear behavior of reinforced concrete girders are placed based on ModifiedCompression Field Theory. At the same time, nonlinear finite element program of isprogrammed. The calculation results from this method are in good agreement withthe experimental results. （3）Shear behavior of ten prestressed Reactive Powder Concrete（RPC）boxgirders were investigated. The main parameters in tests are the shear-span to depthratio and the stirrups ratio in bending shear zone. The experimental study is toinvestigate crack patterns and width, load-deflection relationship, diagonal crackingbehaviors, ultimate shear capacity, the average web strain and principal straindirection as well as stirrup strain. The test results show that under the condition ofthe laboratory general maintenance, the cube compressive strength, the cubesplitting strength and elastic modulus of the RPC with2%volume content of steelfiber is about110MPa,9MPa and42000MPa, respectively. Shear capacity increasedwith stirrup ratio, which have little effect on the cracking load and the critical angleof inclined cracks; shear capacity and the critical angle of inclined cracks decreaseswith the growth of shear span ratio. Experimental study show that by using the steelfiber in RPC beams, the cracks demostrate ‘small and dense’ feature。The initialinclined cracks always appeared on one of webs of the box girder. On the contrary,The width, spacing and angle of inclined cracks on both webs of box girder behavevery similarly. The characteristic of box girdr with thin webs effects should factorinto cracking loading. The shear strength of test results form this study and otherliterature have a good agreement with the calculated value, The mean ratio of thecalculated value and experimental value is0.901, and the variance is0.103, Thecomparison between the calculating results and the numerical calculating resultsshows that the proposed formula is reasonable.