Research On The Fire Resistance And Spalling Prevention Of Prestressed Ultra-high Performance Concrete Beams

Prestressed ultra-high performance concrete structure could take full advantages of prestressed tendons and ultra-high performance concrete（UHPC）,and has a certain development potential.Compared with normal strength concrete（NSC）and high-strength concrete（HSC）,UHPC achieves extreme compressive and tensile strengths,durability and flowability due to the refined microstructure and well-designed chemistry.However,the very dense microstructure makes UHPC susceptible to explosive spalling at elevated temperatures due to the vapor pressure build-up in matrix.Spalling could result in the attenuation of sectional dimension,the increase of sectional temperature,and the exposure of steel bars.Stress relaxation and high temperature creep of prestressed tendons also occur,resulting in substantial degradation of mechanical properties and reduction of load-bearing capacity of members,so it could be difficult to meet the fire resistance standard.Therefore,it is of great significance to carry out experimental research on the fire-induced spalling behavior of UHPC and the fire resistance performance of prestressed UHPC beams.Aiming to solve the above problems,this thesis has carried out the following research work:（1）Two hundred and twenty-eight UHPC cubes with variations in parameters such as curing regime,fiber type,and water-binder ratio were tested without external loading in the ISO 834 fire.The spalling behavior was obtained by fire-resistance cameras and the influences of curing regime,fiber type,and water-binder ratio to the spalling of UHPC were also analyzed.The spalling test results showed that the curing regimes directly affected the vapor pressure by controlling the moisture content of UHPC,and significantly influenced the spalling behavior.On the basis of vapor pressure theory,H150 two-stage hot air curing was proposed to inhibit the spalling of UHPC in fire.The hydration and pozzolanic reaction in the matrix could be further activated with the application of H150 two-stage hot air curing.Meanwhile,almost all the free water could be consumed and evaporated after the curing,which dramatically improved the spalling resistance of UHPC in fire.The addition of steel fiber and polypropylene fiber can postpone the critical time critical temperature of spalling,and the fluctuation of water-binder ratio in the range of 0.18 to 0.22 had a limited effect on the spalling.（2）Based on the free-external loading spalling test,twelve full-scale UHPC beams with various curing regimes and load ratios were tested under thermal-mechanical coupling conditions.The influences of curing regime and load ratio on the spalling behavior of UHPC beams were analyzed.The cross-sectional temperature distribution,deflection-time curve,and spalling evolution process were obtained.The results showed that H150 can prevent the spalling of UHPC beams with high-stress levels and high heating rates.The UHPC beams cured under standard curing spalled severely,the loss of cross-sectional area was more than15%,the cross-sectional temperature was significantly increased after spalling,and the fire resistance was reduced by more than 20 min.（3）Based on the prevention of spalling,fire resistance tests on eight prestressed UHPC simply supported beams with external loading were carried out to explore the effects of the cover thickness of prestressed tendons,load ratio,and bond condition type,under ISO 834 standard fire.The results showed that the cover thickness and load ratio significantly influenced the fire resistance of the UHPC beams,the fire resistance increased with the increasing of cover thickness and decreasing of load ratio.Bonded prestress UHPC beams showed better fire resistance performance than unbonded prestress UHPC beams,and beams fabricated as under-reinforced beams at ambient temperature may fail as lightly reinforced beams under fire exposure.（4）Based on ABAQUS software,the refined model was established by using the sequential thermo-mechanical coupling method.The accuracy of the model was verified by comparing the model with the experimental results,and the range of parameters was further expanded.The results showed that the cover thickness,load ratio,bond type,and sectional dimension have vital influence on the fire resistance of prestress UHPC beams.According to these parametric studies in this thesis,Eurocode 2: BSEN 1992-1-2 might overestimate the fire resistance of the beams with small sectional dimensions by up to 15.4%.Therefore,the fire resistance suggestions for PRPCBs are presented in this thesis.