Experimental Studies Of Precast Bridge Piers With UHPC Connection

With the acceleration of urbanization process in China,traffic congestion is becoming increasingly serious.To solve this issue,a large number of urban viaducts and intercity rail transit bridges have been planned and built in China.In this context,the traditional on-site casting construction methods has not fully met the requirements of modern bridge construction,while prefabricated assembly construction technology has become an effective way and an important means to achieve green and low-carbon bridge construction in the new era due to the advantages of reduced construction period,less traffic disturbance,high construction quality,low noise and minimal environmental pollution.While the prefabricated assembly construction technology is flourishing,various novel materials are also developing rapidly,among which Ultra-High Performance Concrete(UHPC)has attracted a lot of attention from world’s scholars due to its superior material properties.UHPC has the excellent characteristics of high strength,good ductility and strong anchorage performance,which provides a new potential solution for the pier-footing(or cap beam)connection of precast bridge piers.In this study,theoretical and experimental studies on the reasonable structure and seismic performance of the UHPC connected precast bridge piers are conducted based on two different connection methods of UHPC connected precast bridge piers.The main contents are as follows:(1)Based on the extensive literature review,the current state of research of prefabricated assembly technology and the application of UHPC in the field of precast bridge engineering are presented and summarized.(2)In view of the two new UHPC connection types of precast bridge piers investigated in this study,namely the constant-section-dimension-connection at pier bottom and the enlarged-head-connection at pier bottom,the structural characteristics and construction technology of the two connection types are elaborated in detail.The seismic performance of the UHPC precast bridge piers with the two different connection types and their corresponding cast-in-place reinforced concrete bridge piers are studied by using Open Sees finite element analysis software.The analysis results show that under the model parameters selected in this section,the hysteretic energy dissipation characteristics of the precast bridge piers connected with UHPC are comparable to that of cast-in-place bridge piers,and the damage mode of the piers under cyclic loading is the bending damage of the core concrete crushed in the standard section above the UHPC connection at the bottom of the pier,while the UHPC connection section at the bottom of the pier has no damage indicating that both UHPC connections are reliable.(3)Eight UHPC precast bridge pier models using the constant-section-dimensionconnection at pier bottom(refer to as type Ⅰ connection),are designed and built,in which the design parameters of the pier height,the length of the UHPC connecting segment(i.e.,the anchorage length between UHPC and reinforcing bars),and the stirrup ratio of the UHPC connection segment at the bottom of the pier are considered.To compare the seismic performance between the UHPC connected precast piers and that of cast-in-place reinforced concrete piers,two cast-in-place normal reinforced concrete pier models are designed and built as well.The effects of the above-mentioned design parameters on the damage process,failure mode and hysteretic energy of precast bridge piers with the UHPC connection are studied by quasi-static tests.The test results show that the failure mode of the high pier is generally characterized by the bending failure of the normal concrete crushing of the standard portion above the UHPC connection section at the pier bottom.While the short pier is subjected to the combination of bend and shear deformation,and the shear force has a reduction effect on the anchorage between UHPC and reinforcing bars,resulting in the failure mode of the bond-slip failure between UHPC and reinforcing bars in the pier.In addition,for the precast pier with lap reinforcement of 32 mm,when the anchorage length between UHPC and reinforcing bars is less than 10d(d is the diameter of the reinforcing bars)or the stirrup ratio of UHPC connecting portion is less than the minimum stirrup ratio required by the specifications(i.e.,the unilateral stirrup ratio of potential plastic hinge area in the rectangular sectional pier column shall not be less than 0.4%),it may lead to the bondslip failure between UHPC and reinforcing bars.Finally,based on the experimental results,the finite element models are refined and their validity are discussed.(4)Two 1:2.5 large-scale bridge pier models with the same cross-section dimensions and reinforcing are designed and tested by quasi-static cyclic loading.One is a UHPC-connected precast pier with enlarged-head-connection at pier bottom(refer to as type Ⅱ connection),and the other is a cast-in-place normal reinforced concrete pier.The test results show that the failure modes of the precast pier with type Ⅱconnection and the cast-in-place reinforced concrete pier are similar,both of which are the bending failures of the normal core concrete in the standard portion of the pier body.The seismic performance between the two specimens is unsignificant,and the UHPC connected precast pier with type Ⅱ connection is emulative of cast-in-place pier accordingly.(5)Based on the results of the finite element analysis and quasi-static tests,the design method and process of UHPC-connection precast piers incorporating the capacity-protected concept are proposed.The validity and feasibility of the design method are validated by a design example of a continuous girder bridge.