Research On Seismic Behavior Of Bridge Pier Model With Polypropylene-Engineered Cementitious Composite In Plastic Hinge Regions

Polypropylene fiber reinforced engineered cementitious composites(PP-ECC)have the key features of strain hardening and multi-crack steady cracking under tensile load.PP-ECC is attractive for bridge engineering to solve the drawbacks of ordinary concrete such as low energy dissipation and easy cracking.It is an ideal substitute for high energy dissipation parts.It not only improves the seismic capacity of the piers,but also meets the national green environmental protection and sustainable development strategy.Therefore,it has broad prospects for development and application.In this dissertation,the basic mechanical properties of PP-ECC material and mechanical performance of PP-ECC piers under lowcycle repeated loads are investigated through experimental and theoretical methods.The main research objectives,methods and results are summarized as follows:1.Comprehensive literature review on the past studies on the seismic behavior of fiber reinforced cement-based composites and reinforced concrete piers are collected and elaborated.The mechanical properties of ECC material are introduced,and the influencing factors of the seismic behavior of reinforced concrete bridge piers are summarized.Based on the basic principle of orthogonal test,16 PP-ECC mix proportions are designed by considering four influencing factors,i.e.,water-binder ratio,fly ash-cementitious materials ratio,fiber content and air entraining agent content.Through reasonable selections of loading fixture,loading mode and data acquisition system,uniaxial tensile tests are carried out on 16 mix proportions,and the optimal design mix proportion which satisfies the ECC behavior is determined.Four-point bending tests and scanning electron microscopy tests are carried out on the PP-ECC materials to study the mechanical properties under bending load and the microstructure of PP-ECC after failure.(Chapter 1?2)2.Six PP-ECC pier specimens and two RC pier specimens were designed and manufactured.The objectives of the test,loading fixtures,loading mode,test contents and test procedures are introduced.The failure process of 8 pier specimens under low-cycle repeated loads is described and analyzed,the failure modes of PP-ECC and RC piers at different stress stages are summarized,and the first cracking loads and displacement test values are compared with the theoretical calculation ones.(Chapter 3)3.Based on low-cycle repeated load test results on the 8 pier specimens,the effects of design parameters such as axial compression ratio,volume hoop ratio and PP-ECC height on the seismic behavior of bridge pier specimens are discussed.The hysteretic curves,skeleton curves,displacement ductility,bearing capacity degradation,stiffness degradation,momentcurvature of pier bottom section,reinforcement strain,hysteretic dissipation energy and other seismic behavior indexes of PP-ECC and RC pier specimens are compared and analyzed.The seismic behavior of bridge piers partially using PP-ECC is evaluated comprehensively.The experimental results showed that the higher the axial compression ratio is,the more severe the strength degradation and stiffness degradation of the piers presented after the peak load;the horizontal load and equivalent viscous damping coefficient increased with the increase of axial compression ratio while the ductility coefficient,the ultimate displacement angle and the curvature of the plastic hinge area decreased;the higher the stirrup content in the plastic hinge area at the bottom of the piers is.The greater the initial stiffness is,the better ductility is,and the greater the curvature of the plastic hinge area is,but the horizontal bearing capacity decreased to some extent.The higher the height of the PP-ECC material in the plastic hinge area is,the larger the displacement ductility coefficient is,the slower the bearing capacity and the rate of stiffness degradation is,and the greater the curvature of the plastic hinge area is,which indicated better seismic behavior.(Chapter 4)4.Based on the hysteretic curves and skeleton curves under low-cycle repeated loads,as well as the hysteretic behavior of PP-ECC piers,the parameters and calculation formulas of each feature point in the skeleton curve model are determined by theoretical derivation and statistical regression method.In addition,the calculation formulas and laws of loading stiffness and unloading stiffness are proposed.The measured skeleton curves are compared with the calculated ones,which verified the correctness of the skeleton curves.Combining the skeleton curves with the stiffness degradation laws in accordance to certain hysteretic rules,the suitable restoring force model for PP-ECC piers is obtained.(Chapter 5)5.Existing calculation formulas for the equivalent plastic hinge length of reinforced concrete bridge piers are summarized.Main factors that affects the length of equivalent plastic hinge are discussed.Then,the test values of PP-ECC bridge equivalent plastic hinge length is standardized to analyze and evaluate the applicability of the proposed calculation formula for the equivalent plastic hinge length.Finally,the formula for calculating the equivalent plastic hinge length of PP-ECC piers is proposed through the multiple linear regression analysis of the experimental data of PP-ECC piers.The formula for calculating the equivalent plastic hinge length is verified by other test results.The values obtained by the proposed formula for calculating the equivalent plastic hinge length of the piers are well consistent with the experimental ones in the average sense,but are slightly better than the values obtained from other calculation formulas.This indicates that the proposed formula can be used to calculate the equivalent plastic hinge length of piers using ordinary concrete or PP-ECC.(Chapter 6)...