Research On Preparation And Critical Properties Improvement Of Decorative Ultra-High Performance Concrete

With the rapid development of urbanization and the continuous improvement of living quality,concrete constructions were required more in aesthetic and durability performance.Ultra-High Performance Concrete（UHPC）provided sufficient strength and durability,matched the requirements on high durability,large size,thinness and lightness,and play an important role in long-span bridge engineering,blasting-resistant engineering and radiation engineering.However,ordinary UHPC lacking good appearance,and the problems,such as high material cost and large autogenous shrinkage,limited its application.Therefore,this thesis researched on the preparation of decorative UHPC（DUHP）,and improved the key properties like appearance,photocatalys and autogenous shrinkage properties.DUHPC was prepared with large volume fraction of mineral admixture,and the workability,strength and appearance of DUHPC with different minerals ratio was studied,and the importance of factors was revealed by ANOVA method to set the optimal mix proportion.Nano-Ti O₂（NT）was selected to develop impermeable and photocatalytic properties of DUHPC.The pore structure,micro-mophology,hydration products were characterized by NMR,SEM,TG and other methods to revealed the development mechanism of NT in DUHPC.Cellulose fiber（CF）with internal curing effect was used to prepare low shrinkage DUHPC,and the influence of CF on autogenous shrinkage of DUHPC were studied by non-contact shrinkage test.The adsorption effect of CF,pore structure and mircrostructure were characterized by Zeta potential,FTIR,NMR and SEM to revealed inhibition mechanism of CF on autogenous shrinkage of DUHPC.The results were:（1）With different ratio of mineral admixtures,the compressive strength of DUHPC was ranged from 160 to 190 MPa,with the whiteness higher than 67,which performed good mechanical and aesthetic properties.W/B had the most significant effect on the fluidity and strength,followed by SF,and LP and MK were the least affected.The best ratio of DUHPC was W/B=0.16,MK:SF:LP=1:3:6.（2）NT increased the strength of DUHPC,but the increase range was declined with the addition of NT.When NT was 1%,the water absorption rate of DUHPC was the lowest,and the capillary adsorption transfer rate was 1.07×10^-3mm·s^-1/2,which was 20.7%lower than that of the control group.The main reason was that NT promoted the reaction between WPC and Ca CO₃,and the generated gel material and calcium aluminate refined the pores.However,more NT deteriorated the workability of matrix,resulting in more initial microcracks and pores.So that the water absorption rate of DUHPC gradually increased and exceeds that of the control group.（3）The photocatalytic efficiency of DUHPC significantly increased with the increase of NT content.When the content of NT was 10%,the photocatalytic efficiency tended to be saturated,which was about 15 times higher than that of the control group.However,when the content of NT was 13%,the adsorption amount of DUHPC on rhodamine B increased by about 12%.And the unhydrated WPC reacted to form Ca CO₃,which hindered the contact between UV light and NT.Resulting in decrease in the growth rate of photocatalytic efficiency when the content of NT exceeded 10%.（4）CF had little effect on working performance of DUHPC.CF improved the compressive and flexural strength of DUHPC.When CF content was 0.9 kg/m3,the compressive and flexural strength of DUHPC reached 156.3 MPa/42.5 MPa at 28d.Autogenous shrinkage of DUHPC was mainly concentrated in the early period,accounting for about 70%of the entire cycle.The release effect and bridging effect of CF significantly reduced the autogenous shrinkage of DUHPC.When CF content was0.9 kg/m3,the autogenous shrinkage of DUHPC was reduced by 65%compared with the reference group.（5）Early autogenous shrinkage of DUHPC was divided into six stages.The shrinkage of matrix in the first two stages was mainly due to the initial hydration heat shrinkage of matrix.After WPC entered the hydration acceleration period,the internal humidity of the matrix decreased,resulting in matrix was shrinkage in the third stage.When the CF content was 0.9 kg/m~3,humidity of matrix was 42%higher than that of the reference group,and the autogenous shrinkage of the matrix was 524×10^-6.With the hydration of WPC,the internal temperature of the matrix continued to increase,and the gypsum reaction and the CO₃^2-dissolved by LP reacted with the aluminum phase to generate expansion in the matrix.Which resulted in large stress in the matrix and short-term expansion of the matrix in the fourth stage.With the hydration reaction of WPC,gypsum and aluminum in the matrix were gradually consumed and the amount of expansion was gradually reduced.And the temperature of matrix was gradually decreased Which made the matrix still shrink in the latter two stages.When the CF content was 0.9 kg/m~3,the internal humidity of the matrix decreased the least due to its water release effect,which was about 16%higher than that of the reference group,and the minimum autogenous shrinkage of the matrix was 68×10^-6.Through Kelvin’s law and Yang-Laplace equation calculation,it was found that CF reduced the capillary negative pressure of DUHPC which decreased the autogenous shrinkage of DUHPC from the root.The calculation shown that the simulated value of DUHPC autogenous shrinkage was in good agreement with the measured value,but the simulated value of autogenous shrinkage of DUHPC after the fourth stage was larger than the measured value due to the expansion of the matrix in the IV stage.