Evolutionary And Micro Mechanisms Of Multi-Scale Fiber Reinforced Cementitious Composites After High Temperature Exposure

Cementitious composites show multi-scale physical characteristics,multi-stage cracking development.failure process.and multi-temperature stage performance degradation at high-temperature of fire disaster.Based on these theoretical and engineering backgrounds,micron sized calcium carbonate whisker(CW)was introduced with steel and polyvinyl alcohol(PVA)fiber together,to build multi-scale fiber-reinforced cementitious composite(MSFRC).This project was supportted by NSFC:Performance and mechanism of a new type hybrid fiber reinforced cementitious composite based on multi-level structure(51478082),and NSFC:Rheological properties and mechanism of a new hybrid fiber reinforced cementitious composite(5167811 1).In this thesis,mechanical properties of MSFRC after high temperature exposure were inverstigated;microscopic composition,mechanism.and the corresponding calculation models of MSFRC were developed.The main research activities and outcomes of this thesis are summarized as following:(1)Physical and chemical mechanisms of CW are clarified.The results provided insight answers to the basic problems that restricted CW from being widely used in cementitious composites.The presence of aragonite and calcite CW has significant impact on hydration heat,rheological properties,strengths,hydration products,and interface elements.The roles and mechanisms of aragonite CW and calcite CW in cement pastes were compared,including the effects of dilution,cohesion,chemical,Ca(OH)2 orientation optimization,micro fiber and filling.Compared with aragonite CW,calcite CW presents more significant effect in nucleation and lubrication.(2)Micro composition,microstructure and mechanical properties CW reinforced cement paste(CWRC)were studied after high temperature exposure.It was found that CW could still play its role as micro enhancement below 600?,and the mechanical properties of CWRC was improved after heating at high temperature.The optimum CW mass content is 10%for compressive strength and 30%for flexural and splitting tensile strength.Pore structure,interface elements and nano-mechanics results showed that below 600?,microstructure of CWRC after high temperature heating didn't decline significantly.Phase analysis implied that from 400 to 500?,CW transformed from aragonite to calcite phase;at the same time high temperature steam environment accelerated pozzolanic reaction of mineral admixtures and the continuoued hydration of unhydrated cement particles in cement paste.The combined effect of above two is important reason to improve the microstructure and macroscopic mechanical properties of CWRC after high temperature exposure.(3)A good state of fresh cementitious composites is a prerequisite to obtain excellent mechanical properties after hardening.This thesis studied the rheology and workability of fresh MSFRC.In rheological test,larger hysteresis loop area of mortar was associated with small size of CW.With the increase of fiber reinforcing index(RI.the product of fiber aspect ratio and volume fraction),the flowability of MSFRC decreased gradually.However,the flowability of MSFRCs were improved to a certain extent due to "fiber gradation effect" when incorporated CW.Based on the theoretical model of loosen packing density,the rheological RI thresholds of single PVA fiber and steel-PVA fiber-CW multi-scale hybrid fiber systems were given as 1.0 and 2.0,respectively.Workability prediction model was proposed with the consideration of elastic modulus of fibers in the hybrid fiber cementitious composites.(4)Macro morphology,mass loss,and ultrasonic velocity changes of MSFRC were investigated in the condition with/without reinforcement after high temperature exposure.Steel-PVA hybrid fibers could effectively avoid the burst of mortar at high temperature.The incorporation of multi-scale fiers with CW could further restrict or even avoid the crack development at high temperature.Below 800?,the introduction of CW did not significantly increase the mass loss and did not decrease the ultrasonic velocity of MSFRC.The quantitative relationships between MSFRC mass loss percentage,ultrasonic velocity and temperature were established,providing a methord for the simple and quick performance evaluation of MSFRC fire after high temperature.(5)The direct tensile and flexural properties of MSFRC were studied after high temperature exposure.The presence of CW could significantly improve the flexural strength and toughness of steel-PVA hybrid fibers reinforced mortar after high temperature heating.Hybrid effect analyses between CW and steel-PVA hybrid fibers indicated that below 500?,1.5vol.%steel fiber+0.5vol.%PVA fiber+3vol.%CW presented a stable positive hybrid effect in terms of flexural strength and toughness.Quantitative relationship between MSFRC flexural strength,toughness and RI was explored.Moreover,quantitative relationship between MSFRC direct tensile strength,flexural strength,toughness and temperature was proposed as well.Compression properties and the compressive damage constitutive model of MSFRC were studied after high temperature exposure.The multi-scale hybrid fibers could improve peak compressive stress and strain energy of mortar,and effectively improved the compressive properties of mortar after high temperature heating.The prediction models of compressive peak stress,elastic modulus,peak strain,strain energy after high temperature heating were proposed,respectively.Compressive stress-strain damage constitutive model was established considering the influence of temperature and CW volume fraction.(6)Multi-scale structure and systematic mechanism analysis of MSFRC were performed before and after high temperature exposure.Generation and development of cracks could be prevented by introducing steel fiber,PVA fiber and CW in cementitious composites at different scales,resulting in higher strength and toughness.No obviously weak interface transition zone was found on the surface of a single CW because the small micron sized CW and its chemical effect in cementitious composites.CW could improve the density of steel fiber and PVA fiber surface interface through physical and chemical interaction.Multi-scale fiber system was demonstrated effective improvement on the internal structure and appearance morphology of cementitious composites and their mechanical properties before and after high temperature exposure.In conclusion,based on these positive effects of multi-scale fiber system on microstructure and initial crack strength of cementitious composites,as well as the significant inhibitory effect on high-temperature burst and thermal cracks,MSFRC is recommended as a concrete protective layer with fire and high temperature resistance in engineering,such as underground structure.