Multiscale Study On Mechanism And Thermo-mechanical Properties Of Fire-resistant Ultra-high Performance Concrete

With the development of underground structures towards “deep sea” and “deep ground”,the scale of tunnel engineering is increasing,and the surrounding environment is becoming more and more complex,which also puts forward higher requirements for the safety and durability of underground structures in extreme environments.Due to the semi-closed environment of underground structures,the fire in tunnels would lead to fast heating rate and high peak temperature,and accordingly the concrete structures are vulnerable to high-temperature damage.Therefore,the research on novel fire resistance methods of concrete structures has important theoretical significance and practical value for improving the fire safety of infrastructure.In this thesis,a new type of fire-resistant concrete is proposed by incorporating thermally-triggered polymer(TTP)composites in concrete,and the active solution of concrete structure(i.e.,synergistic effect of fire-resistance and structural behavior)can be realized through the foaming reactions of TTP composites.In what follows,a series of high-temperature tests,mechanical analysis and micro characterization are carried out to investigate the effect of TTP fibers on thermal and mechanical properties of ultra-high performance concrete(UHPC)at elevated temperatures.Furthermore,based on the multi-scale structures of cement-based materials,multi-level homogenization calculation models are established to predicted the thermal and elastic properties of fire-resistant UHPC,and the optimization design of UHPC can be achieved through parameter sensitivity analysis.The main research contents and results are as follows:(1)Mechanism and optimal design of fire-resistant concrete.Based on melting extrusion and cooling molding technology,fiber-and particle-type TTP composites are fabricated by using polypropylene as matrix resin and intumescent flame retardants as inclusions,and inorganic modifiers are introduced to improve the thermal stability and micromechanical properties of the composite.As revealed by one-side heating test of fire-resistant concrete slabs,the melting of polypropylene matrix in TTP composites at around 160 ℃ helps release the intumescent flame retardants,and the foaming,overflow and char formation of flame-retardant products between 200 and 600 ℃ can effectively improve the fire resistance of concrete.In addition,fiber-type TTP composites are proved more effective in promoting the connectivity of pores in concrete matrix,while granular TTP composites are easier to achieve higher content in concrete and accordingly have better performance in cooling the concrete.(2)Heat transfer performance of fire-resistant UHPC and multi-scale modeling of the thermal properties at elevated temperatures.The temperature field distribution and thermal conductivity of fire-resistant UHPC slabs is analyzed under standard heating condition,suggesting that TTP fibers are capable of slowing down the heating rate and temperature gradient,reducing the maximum temperature and thermal conductivity of UHPC,and prolonging the time duration to reach the fire resistance limit.Based on the equivalent medium theory,a multi-scale calculation model is established to predicted the thermal conductivity of fire-resistant UHPC considering the effect of interfacial thermal resistance.Following the step-by-step homogenization process,the proposed model can elevate the of thermal properties from equivalent inclusion level to fiber reinforced concrete level,and the reliability of the predicted results are verified at different levels.A the UHPC matrix level,the effective thermal conductivity is affected by the volume fraction and type of aggregate,and the effect of interfacial thermal resistance.At the fire-resistant UHPC level,the effective thermal conductivity is mainly affected by the content of TTP fibers.(3)Residual mechanical and microstructural properties of fire-resistant UHPC.When exposed to high temperature,the plain UHPC experiences explosive spalling at around 600 ℃,while no spalling occurs in fire-resistant UHPC at up to 800 ℃.Due to the decomposition of hydration products and evaporation of free water,the microstructural properties of UHPC degrade with temperature,which finally leads to the strength loss at elevated temperatures.The degradation degree of mechanical properties follows: flexural properties > compressive properties > splitting tensile properties > interfacial bonding properties.In contrast,TTP fiber can promote the residual mechanical properties of UHPC,as the flexural toughness of UHPC beams can be increased by more than 76% at 300 ℃ when the 2.0% volume fraction of TTP fibers are incorporated.The mechanism for mitigating the high-temperature damage of fireresistant UHPC can be explained by: TTP fibers effectively enhance the pore connectivity of UHPC,especially the capillary medium pores,which help prevent the explosive spalling;the foaming reaction of TTP fibers and the overflowing and char formation of flame-retardant products help reduce the thermal conductivity and heating rate of UHPC,and accordingly slow down the degradation speed of micro-and macro mechanical properties of UHPC.(4)Multi-scale calculation model of elastic properties of fire-resistant UHPC at elevated temperatures.Based on the modified Mori-Tanaka homogenization method,a multi-scale model is proposed to predict the elastic modulus of fire-resistant UHPC as a function of temperature.By incorporating the interface spring model and the particle size distribution function,the effects of interfacial bonding effect and size effect are taken into account.The predicted results are validated by experimental results at C-SH scale,cement paste scale,mortar scale and fire-resistant UHPC scale.For UHPC matrix with high-modulus inclusions,both the elastic properties of aggregate and interfacial bonding performance affect the effective elastic properties of the composites,while the interfacial bonding has no significant effect on the effective elastic modulus of when incorporating low-modulus inclusions into fire-resistant UHPC.In addition,the influence of interfacial bonding on the elastic properties is gradually weakens with the increasing size of the inclusions,and maximum effective elastic modulus can be obtained through reasonable design of the inclusion size distribution.