Durability And Service Life Design Of RC Members Exposed To Chloride Salt-Frost Damage

Concrete is the most important structural material in Civil Engineering, Concrete's durability andservice life are major scientific and technological issues concerned by both international and domesticengineering. Domestic and foreign research on the durability of the de-icing salt on concrete mainlyconcentrated in the anti-erosion performance under salt freezing circumstance, viz.one side' spallingproperties under freeze-thaw cycles in the de-icing salt solution. However, the freeze-thaw damage ininternal structure, chloride ion concentration, steel corrosion and changes of structural capacity havenot been systematically estimated. Therefore, the present research on the durability and service life ofstructural concrete is incomplete. In this study, based on the field research in the main airportexpressways and overpasses in the cold region of China, systematically experiments were carried outincluding various rapid freezing and thawing durability tests in ordinary concrete with gas and no gas,with admixture and no admixture, high strength concrete and high performance concrete in3.5%NaClsolution (de-icing salt). The freeze-thaw durability exposure test was also simultaneously performed inthe natural outdoor winter-cold environment. Therefore, the laws of the changes of relative dynamicelastic modulus, the process and mechanism of erosion and damage, the mechanism of diffusion ofchloride ions, bending resistance and deformation laws are presented. Furthermore, the bearingcapacity model of reinforced concrete and service life prediction model under the action of salt freezingwere established, which will provide a theoretical reference for the durability design in engineeringconcrete structures. The main contents and results in this paper are as follows:1. By visual inspection and nondestructive testing in some overpasses in Shenyang, some seriousdamage related to salt freezing durability, such as coarse aggregate exposure, bare steel corrosion andstirrups rust and breakage can be easily found, even in some overpasses repaired and reinforced severaltimes. Lack of strict construction control results in uneven thickness of concrete cover, inducingrelatively larger deviation. Because of carbonation, salt freezing and other environmental factors, thealkaline and density of concrete decrease. This results in accelerated steel corrosion and concretedamage. The chemical and microstructure analysis on the on-site concrete and the products of steelcorrosion indicate that the chloride ion concentration inside the structural concrete which is affected bythe erosion of rain is high. Besides, in the protective layer, the chloride ion concentration does notcomply with the Fick diffusion law. Therefore, the surface effects should be taken into account in thechloride diffusion model for life design and relative parameter should be modified. A linear relationship was found between the total and free chlorine ion concentration in structural concrete; the calculatedchloride binding capacity in overpass concrete is only0.0687, indicating that chloride salts play a keyrole in the corrosion damage of steel reinforcement in concrete.2. The rapid salt freezing tests on concrete structures and specimen show that the anti-salt freezingperformance of C30is poor. This damage results from the erosion on the surface of concrete. Thedeclining speed of relative dynamic elastic modulus will be delayed as the result of the restriction effectof reinforcement steel bar. The anti-salt freezing capacity of high performance concrete increasesdramatically; However, too much fly ash addition will significantly reduce the anti-salt freezingperformance of high performance concrete; Air-entrained concrete specimens with silica fume showhigher anti-salt freezing performance, but the corresponding concrete structures show seriousdeterioration due to the macro expansion of internal shrinkage cracks. And this deterioration will not beimproved as the strength grade increases. Therefore, for concrete with silica fume, the anti-salt freezingperformance of specimen will not reliably reflect the anti-salt freezing capacity of reinforced concretestructures.3. Low vacuum scanning electron microscopy (SEM) was used to observe the structure andmorphology of bubbles in hardened concrete. The characteristic parameters of bubble structure werecalculated by the matching image analysis software. The results show that the air content in concretewith good salt freezing tolerance should be increased to more than5.0%. For high performanceconcrete with mineral admixture, the bubble spacing required for good anti-salt freezing capacity haverelation to strength grade, i.e, when strength level is lower than the C50strength grade, the averagespacing must be less than250μm, while when the strength level increases to more than C60, theaverage spacing can be increased to700μm.4. The salt freezing damaged thickness of concrete structures was studied by ultrasonic levelmeasurement and a correction method reflecting the influence of reinforcement on the ultrasonic testresults. A new concept for comprehensive description of salt freezing damage in concrete structure wasproposed. The thicker the damaged layer of concrete and the lower speed of sound propagating in thedamaged layer, the greater the degree of salt freezing damage.5. When the mechanical properties of salt freezing damaged concrete structures were studied, itwas found that the stress state of cross section under initial load still met the plane section assumption.Therefore, the existing reinforced concrete structure theory can be used to establish the structuraldesign models and formulas under the action of salt freezing. After various rapid salt freezing circleswere conducted, the salt freezing damage and loose peeling phenomenon occured inside the concrete and in the surface, respectively. And cracking points are vague in elastic stage, the correspondingbending stiffness and capacity limits also decrease, the midspan deflection increases correspondingly.6. Based on plane assumption in Material Mechanics and the calculation methods in Design ofConcrete Structures(GB50010-2010) on flexural strength and stiffness and deflection, the salt freezingparameters are also introduced. The relative dynamic elastic modulus, thickness of salt freezing damageand steel corrosion rate were proposed to be design parameters of durability in the salt freezingenvironment. The design formula of reinforced concrete structures was also established, consideringthe effect of salt and freezing. The calculated values from the formula are in agreement with theexperimental values, which can be applied to the durability design for the actual engineering structures.7. According to the degradation of bearing capacity and the reliability theory, a three-stage servicelife theoretical model (Induction period, deterioration and expiration period) was established on theconcrete structures under the action of salt and freezing. Based on chloride ion diffusion, the changes ofrelative dynamic elastic modulus and the concrete surface's denudation degree, the basis of service lifecalculation in the first stage can be established. Viz. with the service time prolonging under the actionof salt freezing, when the concrete surface erosion begins and inside,the freeze-thaw damageoccurs,and the free chloride ion concentration on the surface of reinforcement start to arrive atthreshold value to rust, the bearing capacity of concrete structures begin to decay, then thecorresponding time is the end of t1period; Based on the facts that the degradation of bearing capacityof concrete induced by the decrease of mechanical properties such as compressive strength and the steelbeginning to rust, the basis of service life calculation in the second stage can be established. viz. Whenthe surface of structural concrete spall greater than1500g/m2, the internal relative dynamic elasticmodulus decrease to80%, and steel corrosion rate is over6%, the main material of concrete structures(concrete and steel) reaches the durability of the material damage limit value, the corresponding time isregarded as the service life time in the second stage (t2). The calculated actual bearing capacity ofconcrete structures declines to the limit design load, the corresponding time is regarded as the servicelife time in the third stage (t3).8. Based on the systematic experiments on durability and three-stage theoretical service life modelestablished in this paper, various bearing capacity deterioration curves and service life of mixtures withgas and no-gas, with the admixture and noimineral admixture ordinary concrete, high strength concreteand high-performance concrete under the action of the salt freezing were respectively established. Thecalculated results of C30concrete service life match the one form the practical engineering.9. By the use of the three-stage theoretical service life model established in this paper, thedurability of the main concrete bridge structures in typical salt freezing environment was designed to provide a numerical example. The results were also compared between the conventional design anddurability design. The results show that a feasible design method is present for the durability design ofconcrete structure in the salt freezing environment with the life expectancy of50or100years.