Durability Test Research And Life Prediction And Evaluation Of Concrete Structural Materials In The Saline Environment Of Northwest China

Northwest China,located inland and blocked by high mountains,is at the end of the monsoon.With less rainfall and strong evaporation,it is the most widely distributed area of saline soil and salt lakes in China.In addition,the negative temperature weather in this area lasts for a long time and there are many freeze-thaw cycles.The concrete structures serving in this area suffer from very serious erosion and deterioration.Given the complex climate and harsh environment in this area,it is of great significance to find the damage factors which can best reflect the damage and deterioration of concrete structure materials and design the durability test to correctly reveal the deterioration law and erosion mechanism.At the same time,the traditional life prediction model established by using an erosion degradation mechanism is obviously not applicable in different environments,whereas the probabilistic reliability method is based on the macroscopic performance degradation data caused by numerous microscopic and mesoscopic damages,and uses the probability index as a criterion to judge the durability life.The resulting life value can directly reflect the probability of safety reserve or failure occurring.Based on this,this paper takes C50P,C50G,and C50FG concrete/reinforced concrete as the research object,and designs four types of durability tests,namely rapid freeze-thaw test of the corroded concrete,coupling test of the corrosive salt and freeze-thaw cycle,semi-immersion test of the corrosive salt,and field exposure test of saline soil.The deterioration law and erosion mechanism of the durability of three types of concrete structural materials in different erosion environments were studied through macroscopic,mesoscopic and microscopic levels and multiple indexes such as quality,strength,and corrosion current density.The correlation between the degradation indexes in different environments and the macroscopic durability life of concrete structural materials was discussed by grey theory.On this basis,durability degradation data were obtained,and the durability life of the concrete and reinforced concrete in four environments were predicted by the Weibull distribution model,Birnbaum-Saunders distribution model,Gamma random process model,Wiener random process model,and Copula function model.And the equivalent relationship between the indoor test and outdoor test was established by environmental factors.Meanwhile,durability life data were collected,and the reliability assessment of durability life（failure data）of concrete structure materials in different environments was evaluated by using different distribution models and parameter estimation methods.The main research conclusions and innovative results are as follows:1、In the rapid freezing-thawing environment after corrosive salt erosion,some data samples have stronger applicability to the Weibull distribution model,and concrete durability life obtained from some data samples is more accurate.The durability life of C50P,C50G,and C50FG concrete is 263.5 times,269.3 times and 280.3 times in SO₄^2-+CO₃^2-+Cl^-environment;268.3 times,279.5 times and 291 times in SO₄^2-+Mg²⁺+Cl^-environment;288 times,297.3 times and 309.3 times in CO₃^2-+Cl^-environment;281 times,301 times and 307 times in SO₄^2-+Cl^-environment;279 times,283 times and 293.3 times in SO₄^2-+CO₃^2-environment;295.2 times,310.3 times and 317.8 times in Mg²⁺+Cl^-environment;298 times,314 times and 325.8 times in SO₄^2-+Mg²⁺environment.2、The reliability curve based on the Birnbaum-Saunders model can well describe the damage and deterioration process of concrete under the coupling action of corrosive salt and freeze-thaw,which almost coincides with the reliability curve obtained by the Wiener random process.And the density curve based on a generalized algorithm can better reflect the deterioration characteristics of each concrete’s performance index in each stage of damage and deterioration and the change of speed reduction rate of the reliability curve.Based on Birnbaum Saunders,the durability life of C50P,C50G and C50FG concrete is 277.3 times,290.7 times and 301 times in SO₄^2-+CO₃^2-+Cl^-environment;285.7 times,295.3 times and 307.7 times in SO₄^2-+Mg²⁺+Cl^-environment;308 times,333 times and 340.7 times in CO₃^2-+Cl^-environment;315.7 times,336.3 times and 351.3 times in SO₄^2-+Cl^-environment;296 times,306.3 times and321.3 times in SO₄^2-+CO₃^2-environment;323.7 times,351.3 times and 364.7 times in Mg²⁺+Cl^-environment;333.3 times,363.7 times and 380.3 times in SO₄^2-+Mg²⁺environment.3、Gamma random process model can well describe the damage and deterioration process of continuous cumulative corrosion of reinforcement bar by corrosive ions in a semi-immersion environment.The reliability function and density function of the Gamma&B-S model obtained by B-S distribution approximation are simpler,easier to calculate,and more applicable to engineering,and have little difference from the reliability curve obtained by the complete Gamma model.Based on Gamma&B-S model,the durability life of C50P,C50G,and C50FG reinforced concrete in SO₄^2-+CO₃^2-+Cl^-environment is 83 months,97 months and 106 months;128 months,139 months and 144 months in SO₄^2-+Mg²⁺+Cl^-environment;106 months,131months and 140 months in CO₃^2-+Cl^-environment;155 months,169 months and 183 months in SO₄^2-+Cl^-environment;138 months,149 months and 152 months in SO₄^2-+CO₃^2-environment;392 months,472 months and 634 months in SO₄^2-+Mg²⁺environment;235months,258 months and 282 months in Cl^-environment;604 months,670 months and 684months in SO₄^2-environment.4、In the semi-immersion environment of corrosive salt,the degradation trajectory of each index of the Wiener stochastic process model based on the maximum likelihood method and common law is different,but the reliability curves of the two parameter estimation methods based on different degradation indexes have the smallest difference and overlap around 0.4～0.5reliability.Based on the Wiener model,the durability life of C50P,C50G,and C50FG concrete is 139.4 months,150.2 months and 157.3 months in SO₄^2-+CO₃^2-+Cl^-environment;148.1months,157.2 months and 166.1 months in SO₄^2-+Mg²⁺+Cl^-environment;237.7 months,252months and 269.2 months in CO₃^2-+Cl^-environment;194.7 months,201.9 months and 213.8months in SO₄^2-+Cl^-environment;170.3 months,178.8 months and 190.3 months in SO₄^2-+CO₃^2-environment;184.8 months,193.1 months and 207.3 months in SO₄^2-+Mg²⁺environment;331.4 months,355.2 months and 398.6 months in Cl^-environment;267.1 months,288.4 months and 313.5 months in SO₄^2-environment.5、In the field exposed environment of saline soil,the damage and deterioration of the buried end of concrete are stronger than that of the exposed end,and the erosion resistance of concrete with mineral admixtures is better than that of ordinary concrete.Based on Copula function,the degradation process of quality index and dynamic elastic modulus index is well combined.And the combination of Gamma based stochastic process and Wiener based stochastic process in the same index is also well realized.Finally,the complete durability life of C50P,C50G and C50FG reinforced concrete in a full buried environment is 395 months,420months and 435 months,and the median life is 352 months,379 months and 391 months.The median durability life of C50P,C50G,and C50FG concrete in the half-buried environment is221 months,262 months and 321 months,and the complete life of C50P,C50G and C50FG concrete is 350 months,426 months and 565 months.6、The environmental factors in the rapid freezing-thawing environment of corroded concrete are the largest.The environmental factors based on the median life of C50P,C50G,and C50FG concrete are about 133～151,150～175 and 177～206 respectively.Compared with the rapid freezing-thawing environment of corroded concrete,the environmental factors based on the median life of three types of concrete are about 19.09～23.91,20.05～27.04 and23.47～31.99 respectively.The environmental factors of the three types of concrete in the corrosive salt semi-immersion environment are not only smaller,but also the difference of environmental factors in different corrosive salt environment is obvious.The environmental factors in the three-salt semi-immersion environment are about two times that in the single-salt semi-immersion environment,and the environmental factors in the chloride environment are the least.For reinforced concrete in a semi-immersion environment,the environmental factor in the sulfate environment is the least.7、In the rapid freezing-thawing environment after corrosive salt erosion and semi-immersion environment of corrosive salt,the micro parameter that has the greatest correlation with the macroscopic durability life of concrete is porosity,followed by spectral area,and finally permeability,while the pore diameter that has the greatest correlation with the macro durability life is 0.1～1μm,followed by>1μm,and finally<0.1μm.In the coupling environment of corrosive salt and freeze-thaw cycle,the correlation between each mesoscopic parameter and the macroscopic durability life of concrete is spectral area,porosity and permeability from large to small,while the correlation between each pore diameter and the macroscopic durability life of concrete is>1μm,0.1～1μm and<0.1μm from large to small.8、In the rapid freezing-thawing environment after corrosive salt erosion,the correlation between each index and the macroscopic durability and life of concrete in descending order is strength,mass,elastic modulus and porosity;In the coupled environment of corrosive salt and freezing-thawing cycle,the correlation in descending order is dynamic elastic modulus,strength,mass and porosity;In corrosive salt semi-immersion environment,the correlation in descending order is porosity,elastic modulus,strength and quality respectively.With the increase in the erosion cycle,the correlation between the degradation indexes and the macroscopic durability life of concrete becomes higher and higher,that is,the correlation between the later test data and the concrete durability life is higher than that of the earlier data.9、In two types of freezing-thawing environments,the damage and degradation effects of corroding salts on concrete from strong to weak are SO₄^2-+CO₃^2-+Cl^-,SO₄^2-+Mg²⁺+Cl^-,SO₄^2-+CO₃^2-,CO₃^2-+Cl^-,SO₄^2-+Cl^-,Mg²⁺+Cl^-,SO₄^2-+Mg²⁺.In the semi-immersed corrosive salt environment,the damage and deterioration of different salt solutions to steel bars from strong to weak are SO₄^2-+CO₃^2-+Cl^->CO₃^2-+Cl^->SO₄^2-+Mg²⁺+Cl^->SO₄^2-+CO₃^2->SO₄^2-+Cl^->Cl^->SO₄^2-+Mg²⁺>SO₄^2-,and the damage and deterioration of concrete from strong to weak are:SO₄^2-+CO₃^2-+Cl^->SO₄^2-+Mg²⁺+Cl^->SO₄^2-+CO₃^2->SO₄^2-+Mg²⁺>SO₄^2-+Cl^->CO₃^2-+Cl^->SO₄^2->Cl^-.In addition,the concrete is more sensitive to sulfate than the chloride,while the reinforcement is more sensitive to chloride than sulfate.10、The most accurate parameter estimation results were obtained by the maximum likelihood method for reliability life assessment based on the Weibull distribution.The median rank method and look-up table method can greatly simplify parameter estimation by transforming and equivalent substitution of the distribution function.However,the scale parameter values obtained by these two methods have little difference,while the estimated value of the shape parameter has agreat difference.Meanwhile,among the four types of median rank methods,the parameter estimates of（N_i-0.5）/n and（N_i-3/8）/（n+1/4）median rank methods are most close to the maximum likelihood method.And among the three kinds of look-up table methods,the best linear invariant estimation method is the closest to the maximum likelihood method.11、The failure data obtained through the accelerated test can be applied to the Weibull distribution model,Gamma distribution model,Normal distribution model,Lognormal distribution model,Logistic distribution model,and Loglogistic model.However,the two types of Normal models have the best applicability,followed by the two types of Logistic models,and the logarithmic model and non-logarithmic model have equal applicability to life data.The applicability of the Weibull and Gamma models is relatively poor.Meanwhile,the 3-parameter model of each model has better applicability than the 2-parameter model.And the reliability curves of the 2-parameter model and the 3-parameter model have obvious turning points at the transitions of the first and second stages,while the 2-parameter model is smoother at the transitions of the first and second stages.