Research On Preparation And Permeability Of UHPC Based Fiber Distribution

Ultra-high Performance Concrete(UHPC)is a new high-tech concrete based on the ultra-high strength and high density,which has a characteristic of high specific strength,low resource consumption and high durability.The application of UHPC in civil engineering in 21 st century not only meets the requirements of light and enhanced structure,large span and low-carbon green,but has a promising potential for petroleum,nuclear power,marine and military engineering facilities that are under high erosion environment,hence UHPC is regarded as the core of future concrete technology development.Extensive studies have been conducted on initially improving the mixing proportion design,basic properties and preparation methods.It was mostly supposed that the evaluation of mechanical properties of UHPC was with regards to lab prepared specimens where steel fibers dispersed uniformly and randomly in mortar matrix,and of the permeability with regards to sound specimens just considering single factor.Evidently,results received from ideal supposition were not reliable enough to evaluate the loading capability and durability property of UHPC structure.In order to obtain better security,global stability and durability of actual UHPC structure,the relationship between rheological behavior of fresh concrete and steel fiber distribution was studied in this paper to take control of the fiber dispersion inside the matrix and to provide theoretical support for UHPC preparing.Additionally,the influence of mechanical loading damage on transport property of UHPC was investigated to provide design basis for predicting service life.The main research contents and results are as follows:(1)The relationship among water to binder ratios,dosages of superplasticizer,fresh UHPC properties and mechanical property was investigated through mixing proportion test.The effects of mixing proportion on rheological behavior of fresh UHPC mixtures and strength changes as well as loading failure of specimens were analyzed.It can be indicated from the results that yield stress decreased with the higher water to binder ratio and superplasticizer dosage,which improved flowability of fresh UHPC mixture.As for the plastic viscosity of fresh UHPC mixture,it was decreased by the higher water to binder ratio and little influenced by the variation of superplasticizer dosage.Besides,the additional superplasticizer dosage might cause negative influence on strength growth at a low level of water to binder ratio,by contrast improve the flowability and air content at high water to binder ratio which enhanced the strength.It can be also found that added fibers enable to reduce the flowability and entrapped air content as well as to enhance the splitting tensile stress and flexure stress.(2)By using section image processing(a destructive method),steel fiber dispersion in UHPC specimens containing different mixing proportions was analyzed and quantified,considering the rheological behavior of fresh UHPC mixtures.The fiber distribution influenced by different water to binder ratios,superplasticizer dosages and fiber volume fraction was investigated and measures of controlling the distribution by rheological parameters were presented.It was concluded that the distribution of steel fibers along casting depth varied with the water to binder ratio,superplasticizer dosage and fiber volume fraction,which mainly depended on rheological parameters of fresh mixture,especially the yield stress.The yield stress and plastic viscosity could increase the difficulty of fiber dispersion during mixing procedure when they were high,and might lead to a remarkable segregation during casting period when they were low.Additionally,the optimal yield stress of fresh matrix ranged among 900~1000Pa,700~900Pa and 400~800Pa respectively for UHPC mixtures with 1%,2% and 3% fiber volume fraction.(3)By using X-ray CT imaging technology(a non-destructive method),the internal microstructure of cement concrete could be visualized and the actual contents of steel fibers and air bubbles were quantitatively obtained.Based on corresponding mechanical property tests,the correlation of rheological behavior,steel fibers and air bubbles distribution and splitting tensile strength was analyzed.According to the experimental results,the lower yield stress of fresh mixture resulted in the higher tendency of steel fiber sedimentation and air bubble upwards movement,indicating a worse local mechanical property and degraded homogeneity along casting depth in UHPC.It can be also confirmed that appropriately increasing yield stress and adding steel fibers enable to improve the homogeneity of fibers and air bubbles,and further enhance the homogeneity and stability of UHPC mixtures.(4)As for the UHPC specimens subjected to pre-cracking treatment by splitting tensile test,the crack development after treatment under different stress levels and the permeability of UHPC specimens before and after treatment were obtained and evaluated.The influence of the fiber volume fraction and pre-cracking loading level on transport property of UHPC specimens was then analyzed.Besides,the calculation model for permeability of pre-cracking UHPC specimens was established.The results revealed that the ductility of UHPC was remarkably improved by fiber incorporation.The lateral displacement and lateral residual displacement in terms of UHPC after the same level of pre-cracking loading were increased with additional fibers.Besides,the more steel fibers had a lesser effect on the deformation recovery.The gas permeability values of sound specimens were generally less than 0.81×10-19 m2,whereas that of the fiber adding UHPC discs generated a 2~5 orders of magnitude growth due to the lateral residual displacement after pre-cracking.In addition,the higher addition of steel fibers effectively delayed the initiation and suppressed the extension of the main crack,which was beneficial to UHPC transport properties.(5)In the sorptivity test,the capability of undamaged and after pre-cracking UHPC samples to absorb and transmit liquid was characterized,and the influence of fiber volume fraction and load level was analyzed.The respective advantages of gas permeability and sorptivity to assess penetrating quality of UHPC were compared.The sorptivity of undamaged UHPC samples ranged from 10.5 to 12.5 g/(m2·min0.5),which was improved by additional steel fibers.The sorptivity of samples after pre-cracking treatment increased with the higher splitting load level,confirming the initial flaws,cracks and pores tended to develop with the loading damage of the matrix and to gradually connect,which was influenced by both amount and width of cracks in the matrix.For describing the degrading process of UHPC permeability,the sorptivity seemed more sensitive to the low deformation damage for UHPC than the gas permeability and the gas permeability was found to be a better indicator to describe the durability degradation of UHPC after exposure to a high level of deformation.