Study On Preparation Technology And Mechanical Properties Of UHPFRC

Ultra-high performance fiber reinforced concrete（UHPFRC）is a new cementitious material with ultra-high strength,high toughness and high durability.It has been nearly thirty years since the concept was put forward.After such a long time of research,many achievements have been made.However,the preparation process of UHPFRC is relatively complex,and the selection of raw materials and cure method are very curial.So far,there is still no clear feasible and relatively mature theoretical mix design model.When preparing UHPFRC,it is often necessary to rely on working experience to try out an optimal mix ratio.Therefore,a detailed and complete preparation scheme is particularly important.From how to select the materials to the final successful preparation of UHPFRC,it has great research value and application prospect to clarify the specific trial process.Based on the summary and analysis of the existing theoretical model of UHPFRC mix proportion,according to the Dinger-Funk design model,under the premise of only considering the physical accumulation,and according to the particle size distribution of raw materials,the internal material reaches the closest accumulation state.The theoretical mix proportion is verified and optimized by the mix proportion of the trial test,so as to obtain the optimized optimal mix proportion.Through fluidity,compressive strength,flexural strength and other mechanical properties test and microstructure test,the influence of raw materials and admixture content on material properties was analyzed.Under the condition of hot water curing,a super high performance concrete with glass powder was successfully prepared.The research results have good reference value for promoting the engineering application of UHPFRC.The main research contents and conclusions are as follows:（1）The development process of the theoretical mixture ratio model is sorted out,and the influencing factors of the theoretical mixture ratio design model of UHPFRC are analyzed.Combined with the central effect hypothesis,the mixture ratio design process in this paper is proposed.The effect of fly ash,silica fume and glass powder replacing part of cement on the performance of UHPFRC was studied.The optimal mix ratio of cementitious materials was obtained as follows:cement:fly ash:silica fume:glass powder=1:0.15:0.30:0.1.The additives and aggregates were studied.The results showed that the UHPFRC prepared with water-binder ratio of 0.18,sand-binder ratio of 1.1,water reducer content of 3%and defoamer content of 0.3%had the best performance.After high temperature curing for 2 d,the compressive strength could reach 134.2 MPa.（2）The effects of different types of fibers on the mechanical properties of UHPFRC were studied.The results show that the compressive strength of UHPFRC is reduced by adding basalt fiber,and the fluidity is greatly reduced.The flexural strength of UHPFRC is only improved.When steel fiber is added,the compressive strength and flexural strength can be greatly improved,but the fluidity will be greatly reduced when the content is too high.When the content is 3%,the maximum compressive strength of UHPFRC can reach 210MPa,and the maximum flexural strength is 57.4MPa,but the fluidity is only 127mm at this time.However,when the steel fiber content is less than0.5%,the fluidity of UHPFRC will be greatly improved,and the end-hook steel fiber is better than the straight steel fiber.（3）The influence of different steel fibers on the mechanical properties of UHPFRC was studied.The results show that increasing the proportion of flat steel fibers significantly helps to improve the compressive strength of specimens.When the straight steel fiber is 1.5%and the end hook steel fiber is 0.5%,the compressive strength of UHPFRC can reach 176MPa and the flexural strength is 44.4MPa.When basalt fiber and steel fiber are mixed,the strength of the specimen is smaller than that of the specimen with only steel fiber.However,when basalt fiber is fixed,the strength improvement effect of end-hooked steel fiber is better than that of straight steel fiber.When the three fibers are mixed,the influence of fibers on the strength of the specimen is not obvious.On the whole,with the decrease of basalt fiber content,the compressive strength of UHPFRC increases,and the maximum compressive strength can reach162.4MPa,and the maximum flexural strength can reach 32.5MPa.（4）The effects of different nano-materials on the mechanical properties of UHPFRC were studied.The results show that with the increase of nano-materials content,the fluidity of the specimen decreases,the lowest is 177mm,and the compressive strength and flexural strength of the specimen increase,the highest are143.8MPa and 28.8MPa,respectively.The improvement effect of nano-Si O₂was higher than that of nano-CaCO₃.（5）The microstructure of the specimen was observed by electronic scanning microscope.The results showed that the hydration degree of UHPFRC was reduced after the incorporation of basalt fiber and steel fiber.However,there were many pores in the structural plane of the former,and the microstructure was worse.Although the latter would also reduce the hydration degree of cement and other cementing materials,and reduce the production of C-S-H gel,steel fiber and cement matrix would be more closely cemented together to form a more dense network structure.In addition,compared with the microstructure of nano-materials,it was found that nano-Si O₂was more likely to aggregate C-S-H gel and form a large blocky gel structure due to the internal needle-like structure.Compared with nano-CaCO₃,the microstructure was better.