Influence Of Gansu Localized Materials On Basic Properties Of UHPC And Its Action Mechanism

Ultra-high performance concrete,as a new generation of cement-based material born in the late 20th century,has superior mechanical properties and durability compared with traditional concrete,and low carbon environmental protection and sustainable,so it has a bright engineering application prospect under complex environmental conditions in Gansu Province.In order to promote the popularization and application of UHPC in the province,this paper uses machine-made sand and attapulgite,a local material unique to the province and abundant in reserves,to prepare UHPC.The specific work and results are as follows:（1）The effects of calcination at different temperatures on physicochemical properties of Gansu attapulgite were studied through experiments.The results showed that the color of Gan su attapulgite gradually deepened with the increase of calcination temperature,and it showed deep red at 750℃.The volume of attapulgite gradually decreases with the increase of calcination temperature.When the calcination temperature reaches more than 600℃,partial sintering occurs,but the proportion of sintering is not large.The dehydration process of Gansu attapulg ite can be roughly divided into five stages,and the mass loss at each stage is 3.5%,2.06%,2.63%,3.14%and 0.38%,respectively.The mass loss at 750℃ has reached 97%of the final loss.The average particle size,diameter distance and consistency of Gansu attapulgite decrease first and then increase with the increase of calcination temperature,and the overall distribution is relatively uniform.The main minerals contained in Gansu attapulgite are palywisite,SiO₂ and dolomite.With the increase of calcination temperature,palywisite gradually decomposes,a nd the reactivity of attapulgite continuously increases.The diffraction peak disappears at 600℃.（2）The effects of calcination of Gansu attapulgite at different temperatures on the working and mechanical properties of UHPC were studied through experiments.The results show that:Under the same dosage,the hydroscopicity and colloid of attapulgite at 500℃ calcination are the lowest,and the fluidity of UHPC slurry is the highest,but it is still much higher than that of silica fume particles.When 10%is added,the fluidity has decreased significantly,and t he fluidity is 88.98%of the reference group.Secondly,when the content of attapulgite was 30%,the difference between the compressive strength of UHPC test blocks in the 500℃,600℃ and 750℃ calcined groups was less than 5%,especially the compressive strength of the 500℃ test group was only 1.22%less than the base group.Gansu attapulgite can enhance the flexural and splitting tensile strength of UHPC.With 20%attapulgite content and 500℃ and 600℃ calcination,the flexural strength of experimental groups increased by 13.45%and 2.82% compared with raw attapulgite,respectively,and the splitting tensile strength increased by 8.14%and 6.05%compared with the reference group,respectively.Finally,in each test group,adding attapulgite calcined at 400℃ and 500℃,UHPC has the highest tension/compression ratio and folding/compression ratio,and the best toughness and ductility.（3）On the basis of the test data,multiple linear regression analysis and GA-BP neural network were used to establish the compressive strength prediction models of machine-made sand UHPC.The research results showed that:The average errors between the predicted and measured values of 7d and 28d compressive strength of machine-made sand UHPC using BP neural network optimized by genetic algorithm are only 6.43%and 2.51%,which are much smaller than the traditional multiple linear regression analysis model.At the same time,it is found that the water-binder ratio has the greatest influence on the early and late strength of machine-made sand UHPC,and the cember-sand ratio,mineral admixture content and steel fiber content also have obvious influence on the compressive strength of UHPC.（4）On the basis of Fick’s second Law,a chloride ion diffusion model was established considering the influence of multiple factors,and the durability service life of UHPC composite beam of a highway and box beam of high-speed railway in Gansu Province was calculated and analyzed.The research results show that:The service life of different types of concrete structures varies greatly.AT-UHPC and UHPC structures have excellent chloride ion erosion resistance when the protective layer thickness is the same.When the thickness of the protective layer of NC and HPC structures is increased from 40mm to 60mm,their service life is only increased by about 20 years.The UHPC structure only needs about 35mm of protective layer thickness,which can reach 100 years of service life under the harsh environment in the west.At the same time,the distribution of chloride ions in the top and bottom of UHPC box girder under load is basically consistent with the strain of the box girder,which is significantly affected by the shear lag effect.The time for chloride ion concentration at the corner of box girder bottom plate to reach the critical concentration under load is 83.29%without considering the load a ction.Therefore,when calculating the durability life of UHPC box girder,the influence of load action and section geometry on the chloride ion diffusion performance of UHPC should be fully considered.n geometry on the chloride ion diffusion performance of UHPC should be fully considered.