Study Of The Durability Of UHPC Beams Under Chloride Ion Erosion

As a new type of concrete material,UHPC has been gradually applied to long-span bridge structures in recent years,which has excellent mechanical properties and durability.However,in some environments,the bridge structures are severely corroded by chloride ions,and the durability of UHPC remains to be further studied.Based on the existing research,the chloride ion diffusion model under multi-factor coupling is established first,and then the UHPC chloride permeation test is carried out.Finally,the durability service life prediction is carried out by using the test data and the bridge engineering example.The following work is done:(1)Based on the diffusion process of chloride ions in saturated concrete,temperature,humidity,concrete degradation,load,combination of chloride ion is all taken into account,and the chlorine in UHPC under multi-factor is established by using Fick’s second law.Combining with relevant test data,the ion diffusion model is verified,which compared with the diffusion model in hardened cement mortar and the diffusion model considers the influence of time,and sensitivity analysis of the parameter of the model is carried out.The results show that,compared with the other two models,the model used in this paper is closer to the experimental data,and can predict the distribution of chloride ions in UHPC more accurately and effectively.Sensitivity analysis of the model parameters showed that the diffusion coefficient,degradation coefficient,temperature and chloride ion combination coefficient have significant effects on the chloride ion diffusion of UHPC,while the load effect coefficient has little effect on it.(2)The compressive strength and chloride ion corrosion resistance tests of UHPC were carried out using UHPC test blocks with various water-binder ratio and attapulgite content.The results show that when the attapulgite content is set,the compressive strength of UHPC decreases as the water-binder ratio increases;when the water-binder ratio is set,the compressive strength of UHPC decreases as the attapulgite content increases.When the water-binder ratio is 0.18,however,adding 5% attapulgite in UHPC will increase its compressive strength.The increase in water-binder ratio causes the surface chloride ion concentration to increase and the diffusion coefficient to decrease;the increase in attapulgite causes the surface chloride ion concentration to decrease and the diffusion coefficient to increase.However,when the waterbinder ratio is 0.18,5% attapulgite doping causes the surface chloride ion concentration to decrease.In this experiment,the best values of water-binder ratio and attapulgite content in UHPC were 0.18 and 5%,respectively.(3)The UHPC test block was subjected to 1D,2D,and 3D chloride ion diffusion tests at various times;the test results confirmed the applicability of the chloride ion diffusion model proposed in this paper in the UHPC anti-chloride ion erosion study,and according to Fick’s second law,the diffusion coefficient and the surface chloride ion concentration are obtained by fitting;the test results are entered into the COMSOL software for solution,and the test data is validated.The results show that the chloride ion concentration in the range of 0-5 mm from the surface of the UHPC test block increases with time under the action of chloride ion erosion.Compared with the 1D erosion,the chloride ion concentration under the action of 2D and 3D erosion obviously increase;the chloride ion concentration in the range of 5-10 mm is small,and the increase is not obvious with time;the chloride ion concentration after 10 mm is very small and hardly changes.The diffusion coefficient gradually decreases with time,and the surface chloride ion concentration gradually increases.The distribution of chloride ions in UHPC in different dimensions is calculated by COMSOL,and the calculation results are basically consistent with the experimental data;when UHPC is corroded by 2D and 3D chloride ions,the concentration of chloride ions on the surface is higher,and the ion diffusion depth is also greater.(4)The durable service life of UHPC box girder under chloride ion erosion was calculated,and the conditions of unloaded,loaded and cracked were considered.First,the COMSOL software is used to calculate the durability service life of UHPC box girder under the action of1 D and 2D chloride ion erosion,and the durability service life prediction formula considering the diameter of the steel bar and the thickness of the protective layer is established when the 2D chloride ion is diffused,the influence of different parameters on it is analyzed.Secondly,the durability service life of UHPC box girder under load is calculated,and the two cases of no cracks and cracks are considered respectively.The results show that when the critical chloride ion concentration is 0.05% and 0.14%,the durability service life under the action of onedimensional chloride ion is 37 years and 109 years,respectively,and the durability service life of UHPC under the action of chloride ion is much longer than that of ordinary concrete.Compared with the 1D erosion effect,the durability service life of UHPC box girder under the effect of 2D chloride ion erosion is reduced,and with the increase of the critical concentration value,the difference in the durability service life also gradually increases.When the critical concentration is 0.05% and 0.20%,the difference is 7 years and 130 years.Before the bottom of the box girder cracks,the time required for the chloride ion concentration on the surface of the steel bars to reach the critical value decreases as the stress level increases,which is reduced by nearly 50% as compared to the unloaded state.The time it takes for the chloride ion concentration on the surface of the steel bar on the tension side of the bottom plate to exceed the critical value is significantly reduced in the cracked condition of the box girder bottom plate,and its durable service life is less than 10 years.