Optimal Design And Mechanical Properties Of Bridge Continuous Decks Based On Engineered Cementitious Composites (ECC)

Bridge engineering plays an important role in highway construction.Due to the increasing pressure of transportation,several kinds of diseases occur in traditional bridge expansion joints,which affect the normal operation and social benefits of highway bridges.Therefore,the research and application of jointless bridges have gradually emerged.Among them,the simply supported girders with continuous decks have been widely used in small and medium-span bridges because of the advantages of their definite stress and reasonable economy.However,traditional bridge continuous decks are designed based on normal concrete with brittle characteristics.As a result,exfoliation and crack will be observed on the surface of the structure,leading to the intrusion of harmful substances and corrosion of steel bars,and seriously reduces the safety of the bridge.Engineered Cementitious Composites(ECC)with high ductility,fine cracks and superior durability has been introduced in bridge continuous decks to improve the structural performance and overcome the durability problems.This paper developed suitable ECC material and investigated its basic behavior,bond property between ECC and existing concrete and the structural behavior of ECC bridge continuous decks through experiments and finite element simulations.Relevant optimization suggestions were proposed based on the results.The main contents of the research are as follows:(1)Mix design of ECC was carried out.Through 12 groups of tests,the influence of fly ash content,fiber content,sand-binder ratio and water-binder ratio on mechanical properties of materials was clarified and the benchmark mix ratio was determined.In order to reduce the shrinkage deformation of ECC,the influence of the addition of expansion agent on the mechanical properties and free shrinkage of ECC was further analyzed.By adjusting the dosage of the expansion agent,the optimized mixture was obtained as follows: with fly ash content of 60% by weight,the sand-binder ratio of 0.3,the water-binder ratio of 0.26,expansion agent content of 3%,PVA fiber content of 2%.The average cube compressive strength,tensile strength and ultimate tensile strain at 28 d were 40.2MPa,6.06 MPa and 3.19%,respectively while the drying shrinkage deformation was 525με at 90 d.(2)The basic properties of ECC materials with the optimized mixture were investigated.The uniaxial compression performance,four-point bending performance,resistance to chloride ion penetration and frost resistance were tested through experiments.The results show that ECC has good toughness under uniaxial compression and exhibits strain hardening characteristics under bending load.Also,ECC has better resistance to chloride-ion penetration than ordinary concrete,and the freezing durability index can reach 62.1%.(3)The bonding performance of the interface between ECC and existing concrete was studied.According to the interfacial splitting tensile test and interfacial shear test,the bonding behavior of the ECC and the existing concrete is better than that of the new and old concrete.The bonding tensile strength can be enhanced through the improvement of interfacial roughness and the utilization of bonding agents.Besides,bonding shear behavior can be improved by the bonding agent and interfacial steel bars.Increasing the number of steel bars is beneficial to increase the load value of the platform section in the load-slip curve and control the interface crack width.(4)The mechanical behavior of bridge continuous decks based on ECC was investigated with the nonlinear finite element method.The real bridge model was first established based on ABAQUS and the deformation characteristic and stress distribution of the continuous decks were analyzed under the circumstance of lane load,change of temperature,vertical temperature gradient and the coupling of lane load and lowering the temperature.The results indicate that the plastic zone only appears when applying lane load,lowering the overall temperature and the coupling of the above circumstance and the maximum tensile strain is less than the limit value of ECC.Secondly,the local model of ECC bridge continuous decks was established to explore the interfacial connection between the bridge continuous deck and the main girder as well as its working performance under fatigue load.The results show that it is necessary to apply the interfacial steel bar to ensure a reliable connection.The maximum tensile strength and ductility of the continuous deck haven’t reached the limit state,which reveals its favorable working condition.Finally,the parametric analysis were conducted in order to investigate the influence of the main design parameters on the mechanical behavior of the bridge continuous decks.(5)Given the optimal design of the bridge continuous decks based on ECC,the following suggestions were proposed in this paper.Firstly,an appropriate amount of expansion agent is recommended to be used to reduce the shrinkage of ECC while relevant curing methods are needed to be adopted to prevent water loss.Secondly,suitable treatment of the interface is needed to be conducted before casting and the interfacial steel bars are suggested to be arranged.Finally,the length of the unbonded area and the reinforcement ratio could be 3.75%-5% of the span and 1%-1.2%,respectively.The thickness of the continuous decks could be the same as or slightly higher than the pavement.