Creep Behavior and Mechanical Properties of Reactive Powder Concrete at High Temperature

Author:Muhammad Abid

Supervisor:hou xiao meng

Database:Doctor

Degree Year:2019

Download:21

Pages:177

Size:10821K

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This research work was conducted to investigate the creep behavior along with other important mechanical properties of fiber reinforced reactive powder concrete at high temperature.Reactive powder concrete(RPC)is an advanced generation of concrete with ultra-high strength,remarkable durability and excellent toughness.The improved performance of RPC is mainly because of microstructural development.Over the last two decades due to the superior performance of RPC at ambient temperature,it has been widely used in civil,nuclear and industrial structures,around the world.The knowledge of concrete structures under high temperature is of great interest in nuclear engineering applications and in safety evaluation against fire in civil constructions.Therefore,this research work about temperature dependent creep and mechanical properties of RPC will be very beneficial for updating some design codes such as ACI 216R-89,ASTM and Eurocode etc.This study includes a state of the art review phase,experimental phase,analytical phase and finite element modeling phase.In the state of the art review phase,the experimental results about the deformational,mechanical and thermal properties of RPC at and after exposure to high temperature are compared with traditional types of concrete and code provisions.It has been found that very limited work has been carried out about the temperature dependent strain and mechanical properties of RPC at high temperature.The quality of concrete after high temperature is usually assessed by non-destructive testing(NDTs).However,the comparative study among the residual mechanical properties and NDTs are very scarce.Furthermore,the review shows a lack of experimental data about microstructural changes due to coupled thermo-mechanical loading.The systematic investigations are still missing in state of the art.This research work focuses on the impact of optimum dosage of steel,PP and hybrid(steel and PP)fibers on creep behavior along with other important mechanical properties of RPC at elevated temperature.The types of RPC with steel,PP and hybrid are hereafter referred as SRPC,PRPC and HRPC.The experimental phase focused on determining the temperature-dependent creep behavior,important mechanical properties at and after exposure to high temperature and microstructural analysis after high temperature.Temperature-dependent creep is further characterized into free thermal strain(FTS),short-term creep(STC)and transient strain(TS),based on different thermo-mechanical regimes.Varying heating and loading schemes were considered such as steady-state and transient thermo-mechanical conditions.The target temperatures considered for steady-state thermal conditions are 120,300,500,700 and 900℃.The result shows that STC increases with increasing stress level and higher target temperature.The increase in STC becomes too obvious above the transition stage of quartz aggregate.Furthermore,HRPC and SRPC have significantly higher STC than PRPC and other traditional types of concretes.The evolution of FTS and TS was quite slow below 250℃.However,at high-temperature significant increase in FTS and TS were observed.Furthermore,increasing stress level and the addition of steel fibers results in high TS.Overall,the performance of PP fiber was better than the steel and hybrid fibers on the creep behavior of RPC.It is found that STC for 3 hours can reach up to 10~33 times of one year ambient temperature creep for different stress levels and target temperatures.Furthermore,the STC of RPC is significantly higher than NSC and HSC owning to high amount of cementitious material being used,quartz aggregate and steel fibers etc.The changes in mechanical properties of RPC at and after exposure to high temperature were also studied.The mechanical properties considered are cubic compressive strength,axial compressive strength,split-tensile strength,flexural strength,elastic modulus,peak strain and stress-strain curve.The quality of RPC was also assessed after exposure to high temperature by NDTs mainly including ultra-sonic pulse velocity test(UPV)and resonance frequency test(RF).The hot-state strength starts to decrease gradually with increasing temperature.The results showed that the relative compressive strength of RPC is much lower than that of NSC under 300?C,but higher above 300?C.This will result in under reinforced RPC beams failure due to light reinforcement during fire.On the other hand,the residual strength increase from room temperature to 300℃.However,above 300℃,it’s decreasing gradually.There is no sizable difference between the hot and residual strength of RPC above 300℃.The fluctuating pattern in temperature induced creep strain and mechanical properties has been assessed at the micro level by SEM,XRD,MIP,TG and DSC analyses.The Result of TG/DSC curve shows the evaporation of free and gel water near 100-250℃,the decomposition of CH and C-S-H hydrates near 500℃and the decarbonation of calcite near 700-900℃.The predominant hydrates C-S-H,CH,C3A,C2S,C3S and calcite were identified within 25 to 35~οfrom XRD analysis.It is evident that the peaks of C-S-H,CH,C3A were reduced gradually above 500℃.The gradual rise in STC at increasing temperatures is attributed to these physical and chemical changes take place at elevated temperature.The predominant hydrates C-S-H,CH,C3A,C2S,C3S and calcite were identified within 25 to 35~οfrom XRD analysis.It is evident that the peaks of C-S-H,CH,C3A were reduced gradually above 500℃.The transformation of quartz sand occurred about 570℃,which increases the STC significantly.Furthermore TS increases due to the dehydration of C-S-H gel about 500℃under the influence of loading.Moreover,the conversion of portlandite into calcium oxide changes the porous network and consequently increasing TS.The porosity of RPC increasing gradually with high temperature.The micrographs show that RPC has very dense structure up to 300℃due to the formation of secondary hydrates such as xonotlite and tobermorite.However,above300℃the cracks become obvious,the cement hydrates dissolved,quartz transformation occurred and pores increases significantly.The analytical phase of this research combined,analyzed,and elaborated upon the results from the experimental phases.This phase included developing predictor equations for the creep behavior and other mechanical properties of RPC at high temperature.The proposed equations will be useful in fire resistance design calculation of RPC structures.The finite element modeling phase focused on developing a 3D model of RPC structures that can accurately capture the thermal and structural response under fire considering the effect of temperature-induced strains.The model shows that the effect of temperature-induced transient strain significantly increasing the axial deformation of the structural members,neglecting it will result in an underestimated design for resisting the fire exposures.