Analysis Of Performance And Stressing State Of UHPC With Waste Tyre Fibers

Ultra-High Performance Concrete（UHPC）is a kind of ultra-high strength cement-based composite material with high toughness and excellent durability.However,in the practical application of UHPC,its popularization and application are limited due to the high cost and complex production process.Therefore,reducing the cost is the key to the current research and application of UHPC.In this paper,waste tyre steel fiber and polymer fiber are used to replace steel fiber and polypropylene fiber respectively to reduce the cost of UHPC,and macroscopic performance,microscopic performance and bending test are conducted,and then the structural stressing state theory is applied to investigate the evolution of the stressing state of UHPC plates in bending tests,and the main research contents are as follows.（1）The effects of waste tyre fibers on the macroscopic properties of UHPC are investigated by testing the fluidity,compressive strength,flexural strength and dynamic elastic modulus of UHPC with different fiber replacement rates.The results show that:The fluidity of UHPC decreases with the increase of the replacement ratio of waste tyre steel fiber.Waste tyre steel fibers significantly affect the compressive strength at 3-7 days and the flexural strength and strength growth rate at 7 and 28 days for groups with a replacement rate greater than 50%.The fluidity of UHPC decreases with the increasing replacement rate of waste tyre polymer fiber.The waste tyre polymer fiber can significantly affect the compressive strength at 3,7and 28 days and the flexural strength at 3,7 and 28 days.Waste tyre polymer fibers significantly affected the compressive strength at 28 days and the flexural strength at 3,7 and 28 days.The dynamic elastic modulus of UHPC decreases more with the increase of replacement rate for both waste tyre steel fiber and polymer fiber.（2）Scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）and thermogravimetry differential thermal analysis（TG-DTA）are used to investigate the micro properties of UHPC.The results show that:The SEM results show that the hydration products will be interconnected with the increasing age,making the matrix more uniform and dense.Through the EDS of the control group,it can be found that the density of matrix can be determined by the color of elements.The XRD results of the control group show that with the continuous hydration process of cement,the diffraction peaks of C₂S and C₃S become less obvious,while the diffraction peaks of C-S-H and calcium zeolite become obvious.The results of TG-DTA of the control group showed that the first heat absorption peak and weight loss of the curve became more and more obvious with the age,but the second and third heat absorption peaks and weight loss became less and less obvious.（3）Seven UHPC plates with different fiber replacement rates are made to investigate the influence of waste tyre fibers on the bending properties of the plates.The results showed that:The addition of waste tyre steel fiber can enhance the toughness of UHPC board before entering the yield stage of reinforcement,but in the failure stage,it shows more and more obvious brittleness with the increase of replacement rate.The polymer fiber of waste tyre can significantly affect the polypropylene fiber to limit the expansion and extension of micro-cracks at the initial loading stage.With the increase of replacement rate of waste tyre polymer fiber,it shows obvious brittle failure characteristics,and its bearing capacity becomes smaller and smaller after entering the failure stage.（4）The stressing state of UHPC plate is characterized by the sum of GSED,and then the M-K discriminant method is used to identify the branch points and failure load points in GSED’sum value trend diagram of seven plates,and then the stressing state is divided into three stages:linear elastic stage,elasto-plastic stage and failure stage.In addition,the sudden change characteristics before and after the characteristic load are verified by the stressing state mode diagram,which reveals the evolution law of the stressing state of UHPC plate.Finally,the design load determined according to the regulations is compared with the design load determined according to the method in this paper to verify the necessity of taking the failure load as the design load.The results showed that:With the increase of replacement rate of waste tyre steel fiber,the elastoplastic branch point load and failure load decrease continuously,and the elastoplastic branch load becomes insensitive to it gradually when it increases to a certain degree.The elastoplastic branching point load and failure load decrease with the increase of polymer fiber replacement rate.The design load M_u determined by the regulations still has a large safety margin compared to the design load M_{u Q} determined by the method in this paper,which will cause material waste.