Research On The Concrete Durability Due To Salinized Soil In The Western Region Of Jilin Province

Along with the wide application of concrete and increasing deterioration of the servingenvironment and resulted from the changes of temperature, humidity, water levels and thecorrosion of acid, alkali, salt, etc., the problems of premature aging and disease in theconcrete projects have been gradually drawn the attention by the field of civil engineering.The western region of Jilin province locates in the south-central Songnen plain and lyingto the north of Liaohe plain. Not only is it a typical season frozen area of northeast part ofChina, but one of the areas with the most serious soil salination.As the research subject in this paper, Da’an city of Jilin province situates in thehinterland of Songnen plain, it is not only a typical season frozen area of northeast part ofChina, but one of cities with the most serious soil salination in Jilin province. The saline soilin Da’an area belongs to carbonate (hydrogen) saline soil. With the bad physical and chemicalproperties and high PH value, the soil highly riches in salt and its composition is complex.Na+takes high proportion in its Cation, while HCO-3takes much of its Anion. In addition, theion species, such as Cl-, SO2-2++4, Mg, Ca2and K+，are fully complete. The adverseengineering natures of melt sinking and salt expansion of the salt soil cause road diseases likethe differential settlement of highway subgrade, expansion, road frost boiling and frostheaving damage, etc. Besides, the ions of CO2-3, HCO-3, SO2-4, Cl-affects the buildings, roads,bridges, dams, lateral canals, etc. Meanwhile, the harsh climate and many externalenvironmental factors (such as freeze-thaw cycles, dry-wet cycles, salt corrosion, etc.) result in damages and corrosion of the concrete materials and lead to degradation of concretestructure and threatening the duration and safety of such concrete structures.The domestic and overseas scholars have been studying the durability of concrete underdifferent environmental conditions, but their more attention is offered on the problems aboutconcrete durability under the effect of single factor like freeze-thaw cycle, chloride saltcorrosion, sulfate corrosion, and dual factor of salt corrosion and freeze-thaw. Study objectsare focused on the city roads and bridges by scattering the deicing salt, concrete engineeringcoastal areas and offshores or in the salt lake regions. Thorough studies are made onfreeze-thaw failure mechanism of concrete and chlorine salt erosion. While, few studies areengaged in concrete durability under the effect of complex factors in season frozen zones andlack of the corresponding literature and research achievements.In this thesis, the soil samples obtained from Da’an saline soil area are analyzed to definethe composition of soluble salt in order to make single and multi-salt erosion solution.Combining with the environment and climate features in the saline soil area of the west ofJilin, the thesis designs the test conditions of concrete durability. Considering the actualconstruction requirements of concrete projects and the engineering application of mixtureratio of concrete, the thesis designs five groups of mix ratio concrete specimens at the samestrength grade including normal concrete, fly ash concrete, air-entraining concrete, prismconcrete specimen with dimensions of40mm×40mm×160mm. The above five specimens areused to conduct four kinds of tests of salt solution soak, salt erosion and dry-wet cycling, salterosion and freeze-thaw cycling under the condition of multi-salt environment. The contrasttests are made as well under the condition of single salt environment on the basis ofevaluation norms of the mass and dynamic elastic modulus of concrete. Meanwhile, differentmeans such as physical and electron microscopic scanning, ultrasonic testing are used to studythe concrete behaviors under various test conditions including mechanism analysis,macroscopic phenomena, microcosmic structure, reduction laws of mass and dynamic elasticmodulus and influencing factors to concrete damages. Based on the testing results, the graycorrelation and rough set theories are applied to determine the correlation degree betweeninfluencing factors and concrete durability and the weight coefficients of factors. MATLABprogram is used to establish the evaluation models and suitable conditions, so that attenuation rule of dynamic elastic modulus of concrete can be evaluated under different environmentconditions.The main research content of this thesis is as follows:1. In actual engineering projects, many concrete structures are corroded by a variety ofsoluble salts because of lying long-term in the saline soil or ground water. In this thesis, thesalt soaking method is used to imitate the damage and changeable laws of concrete which iseroded by soluble salt over a long period of time. Test results show that the exposed part ofconcrete in multi-salt solution grows “mildew” due to the salt crystallization. Normal concreteand fly ash concrete are relatively serious, while air-entraining concrete shows better. It alsoshows that the lower air content, the smaller dosage of fly ash is, the higher multi-salt solutionconcentration, the more serious salt crystallization is. Little changes are found on theappearance, mass and dynamic modulus of elasticity of the concrete soaking in water. But forthe concrete in multi-salt solution, the rate of mass loss shows the change from reduction togrowth. The greater multi-salt solution concentration, the faster mass loss of concrete is. Ratesof mass loss from large to small in turn are normal concrete, fly as concrete and air-entrainingconcrete. Relative dynamic elastic modulus of the concrete long-term soaking in multi-saltsolution appears growth at first and declines later. The higher multi-salt solution concentration,the faster decline of relative dynamic elastic modulus is. Both fly ash concrete andair-entraining concrete can improve concrete the resistance ability against salt corrosion,while the air-entraining concrete can mostly improve that.Through the contrast experiments on single salt soaking, the loss degree of the concretemass influenced by each salt ranks as NaCl> Na2SO4> NaHCO3> Water. The loss degree ofrelative dynamic elastic modulus of concrete influenced by each salt ranks as Na2SO4>NaHCO3> NaCl> Water.2. Dry-wet alternate effect is one of the important factors to affect the durability of theconcrete structures which are frequently suffering from the environment of changeabletemperature and humidity and under the condition of salt corrosion. The degree of concretedamage under the environment of salt corrosion is more serious than that in wet environmentpersistently. Through the tests of multi-salt or single salt erosion and dry-wet cycling, it isfound that the loss of concrete mass and dynamic modulus of elasticity is related to many factors such as types of salt, salt solution concentration, dry-wet cycle times, air content, thedosage of fly ash, etc. At the early stage of test, the mass of specimen increases to some extent.And then with the increasing times of dry-wet cycling, mass loss begins to speed up. Seriousconcrete spalling is found at20mm to40mm above the solution surface, and some specimenseven break at this part. The dynamic elastic modulus of concrete shows growth firstly andthen downward after reaching certain cycles. When the concentration of salt solution isexceeding26%, the dynamic elastic modulus of concrete does not show ascent stage, damageof concrete is faster and more serious. Both the air-entraining concrete and fly ash concreteare capable of improving the resistance to the damages of salt corrosion and dry-wet cycling.But the resistance of air-entraining concrete to salt corrosion and dry-wet cycling is betterthan fly ash concrete.Through the contrast tests of single salt and dry-wet circulation, the influence degree ofeach dissolved salt in the saline soil on the mass and dynamic modulus of elasticity ofconcrete ranks as NaHCO3> Na2SO4> NaCl> Water.3. By rapid freezing method, the thesis draws the following conclusion: under the actionof double factors of multi-salt erosion and freeze-thaw cycles, frost damage of concrete in saltsolution occurs much more serious than concrete in water, and the higher concentration of saltsolution is, the more serious damage of the concrete has. Introducing air-entraining agent intoconcrete does improve concrete resistance against frost damage, the higher the air content is,the better the concrete resistance against the salt frost is. For the low strength concrete, it issuggested to improve concrete resistance to the salt frost by increasing the air content. WhileFly ash concrete has not shown good frost resistance and salt frost resistance, even worse thanordinary concrete. Therefore it is unfavorable to use fly ash concrete into the concreteengineering where freeze-thaw and salt freezing damage easily happens.Each kind of single salt erosion solution has different influence on concrete resistanceagainst salt frost. Chloride salt mainly causes concrete spalling from the surface to inside.Sulfate salt makes concrete frozen and broken in the middle of the specimen, while concreteon the surface does not fall off seriously. The concrete damaged by soda salt combines thedamages features of chloride and sulfate salt, which concrete breaks in the middle part andseriously spalls on the surface. 4. Considering the particularity and rigors of the climate and environment in the westernsaline-alkali soil distribution in Jilin Province, the concrete engineering is extremelyvulnerable to environmental factors such as salt corrosion, freeze-thaw, dry-wet alternatingaction together.In this paper, concrete durability tests under the condition of multiple factors action suchas salt corrosion, freeze-thaw cycling, and dry-wet cycling are conducted. The test resultsshow: the mass loss and dynamic elastic modulus decay very quickly. Durability of concreteis closely related with its mixing ratio, the type of salt, its concentration of salt solution, andtimes of cycle. The resistance to salt corrosion, freeze-thaw, dry-wet action of air-entrainingconcrete is much better than fly ash concrete and ordinary concrete. The higher the saltsolution concentration, the more serious the concrete damage, and it obviously decreases thetimes of dry-wet action. The damage of concrete becomes even more serious with thedecreasing times of salt corrosion, freeze-thaw and dry-wet cycling.Concrete damage under the environment of multi-salt after freeze-thaw and dry-wetcycle is earlier and more seriously than the action of single freeze-thaw or dry-wet cycle. Theserious degradation of concrete performance is resulted from the cooperation of multi-salterosion, freeze-thaw, and dry-wet cycle. The various actions affect each other, and promoteeach other. While the damage of multifactor with salt erosion, freeze-thaw, and dry-wet cycleis not simple adding together.5. After analysis of the damage mechanism to concrete, salt erosion is an importantfactor to cause the degradation of concrete performance. The concrete demage caused by saltsincludes two aspects: chemical erosion damage and physical damage of salt crystallization.Under the condition of long-time soaking in multi-salt solution, the physical damage ofsalt crystallization is easier to implement in normal temperature environment. The saltschemical erosion to concrete is a long and slow process and can not complete within a shorttime, but it will be involved after long time.Under the action of double factors of salt corrosion and dry-wet cycle, physicalcrystallization damage of various salts is a major cause to the degradation of concreteperformance. Under the environment of harmful salts presence, salt corrosion and freeze-thawcycle have the positive effects to concrete, lowering both the freezing point and freeze-thaw damage. In addition, they can promote the development of crystallization pressure producedby salt crystals and osmotic pressure produced by salt concentration difference and ice-watersaturated vapor pressure difference. They can also make the negative effect of concreteexpansion and cracking. Continued salt erosion and freeze-thaw cycle exists overlappingdamage effects, which makes the negative effect greater than the positive effect gradually. Theperformance of concrete will deteriorate continuously, and eventually destroyed. While thesituation of concrete is more complicate under the action of salt corrosion, freeze-thaw, anddry-wet alternate action. All kinds of damage factors accumulate the concrete damage, and htconcrete damage effects of all kind of salts promote each other and influence each other, butcan not be regarded as the simple accumulation of each salt action.6．From the test results in this paper, the influence factors of concrete durability isclassified into:①external factors, such as dry-wet alternate action, freeze-thaw cycle, saltscorrosion, corrosion time, etc.;②internal factors, such as water-cement ratio of concrete, Porestructure of concrete, whether with mixed additives, and type of additive, etc. On the whole,environment condition factors have a great influence on the durability of concrete, that is, theworse of the environment determines the worse damage to concrete. Therefore, the harshenvironment determines the degree of concrete durability.7．By using the grey correlation theory, we can evaluate the various factors to influencethe durability of concrete, correlation degree between the various factors and mass loss ranksas freeze-thaw and dry-wet cycling times> dry-wet circling times> freeze-thaw cycling times>multi-salt concentration> air content> soaking time> fly ash content. In addition, thecorrelation degree between the various factors and dynamic elastic modulus decay ranks asfreeze-thaw and dry-wet cycling times> dry-wet circling times> freeze-thaw cycling times>air content> multi-salt concentration> fly ash content> soaking time. Among them, thevalue of associate degree between the air content and mass loss(γ=-0.738) or dynamic elasticmodulus loss(γ=-0.755) is minus, which shows that the influence of the air content on theloss of mass and dynamic elastic modulus is larger, the greater of air content is, the loss ofmass and dynamic modulus of elasticity is smaller. Therefore, in order to improve thedurability of concrete under the action of various environmental factors, the amount ofair-entraining agent in the concrete can be added more. By using the rough set theroy, we can determine the important degree of seven influencefactors to the loss of mass and dynamic modulus of elasticity, and the two rules are same. Theimportant degree ranks as freeze-thaw and dry-wet cycle, dry-wet cycle, freeze-thaw cycle,multi-salt concentration, air content, soaking time, fly ash content. The weight coefficients ofseven influence factors to the mass loss are0.2743、0.2012、0.1805、0.1273、0.0822、0.0813、0.0532respectively; and the weight coefficients of seven influence factors to the loss ofdynamic modulus of elasticity are0.2975、0.1913、0.1743、0.1158、0.0846、0.0703、0.0662respectively.In this paper, there are two norms, the loss rate of mass and dynamic elastic modulus, toevaluate the concrete damage. The value and change range of these two norms is so big thatthey are processed and compared through being put in the same coordinate system. TheDynamic elastic modulus are selected as the main evaluation index of concrete durabilitybecause it can more accurately and intuitively reflect the deterioration process of concreteperformance, timely and effective forecast failure behavior of concrete under the action ofdifferent environmental factors.The MATLAB program is applied to stablish the evaluationmodels and their application conditions, which can be used to evaluate and predict thedurability of concrete under different environmental condtions.