بررسي پارامتري خوردگي موضعي آرماتورها براي تيرهاي بتن مسلح مطابق تحليل آماري

Effects of Local Corrosion on Steel Bars of Reinforcement Concrete Beams using Statistical Analysis

خوردگي ميلگردها در بتن يك فرايند پيچيده بوده كه موجب كاهش سطح مقطع فولاد و زوال مقاومت سازه بتن آرمه مي گردد. در اين مقاله خواص آماري پارامترهايي مانند ضريب انتشار يون هاي مخرب، نرخ خوردگي و زمان شروع خوردگي براي خوردگي موضعي آرماتور تيرهاي بتن آرمه ارزيابي شده است. از توابع توزيع احتمال مختلف مانند گاما، نرمال، لوگ-نرمال، گامبل و وايبول جهت برازش پارامترهاي خوردگي استفاده شده و بهترين تابع توزيع احتمال پارامترهاي خوردگي با استفاده از آماره مربع كا انتخاب گرديده است. بر اساس تحليل آماري پارامترهاي خوردگي، اثر خصوصيات آماري متغيرهايي نظير مقاومت فشاري بتن، پوشش آرماتورها، مقدار ثابت و مورد انتظار يون مخرب بر زمان شروع خوردگي با استفاده از روش شبيه سازي مونت كارلو بررسي شده است. در انتها ميزان كاهش آرماتورهاي وابسته به زمان در تيرهاي بتني براي چهار قطر مختلف ميلگرد (شماره هاي 8، 12، 16 و 20) مطالعه شده است. نتايج حاكي از آن است كه اكثر پارامترهاي خوردگي از تابع توزيع احتمال لوگ-نرمال پيروي مي كند و پوشش آرماتورها نقش بسزايي در كاهش خوردگي ميلگردها دارد. همچنين خواص آماري سطح مقطع خورده شده به قطر و زمان خوردگي وابسته است بطوريكه افزايش زمان و كاهش قطر موجب كاهش بيشتر سطح مقطع آرماتور مي گردد.

Corrosion of steel bar has a complex process which leads to the reduction of cross section of steel bars and degradation of concrete structures. Corroded steel bars are among the most important issues in durability of concrete in the marine structures. Important effects of corrosion are seen in the damages made by corroded concrete structures including all kinds of structural and non-structural damages. Structural damages are more important because of the reduction of the safety factor of the structure in response to the applied external loads. These failures include reduction of bars’ cross-section and changes in the mechanical behavior of steel. Corrosion includes two processes: (1) corrosion initiation and (2) corrosion propagation. The corrosion initiation is related to the time when corrosive ions reach to the surface bars and chemical features are activated. Propagation is related to the time during which the structure loses its capability, subject to the loss of the cross-sectional area of reinforced steel bars, reduction of bond, and crack initiation and propagation. The predicted models of life-service of a reinforced concrete structure should contain two processes of corrosion. In new structures, the initiation time of corrosion and insurance for the whole time of corrosion initiation time is more important. Whereas, controlling the corrosion propagation must be regarded for existing structures. In this paper, statistical characteristics of the chloride diffusion coefficient, corrosion initiation time and corrosion rate including the best probability distribution function and its parameters are investigated based on Monte Carlo simulation of pitting corrosion data. The distribution function parameters of the corrosion variables i.e. the chloride diffusion coefficient, corrosion initiation time and corrosion rate were calculated using the Maximum likelihood method based on mathematical pitting corroded model. The model contains corrosion initiation and corrosion propagation processes. The probability density functions such as: Gamma, Gumbel, Lognormal, Normal, and Weibull were used in the statistical analyses of corroded pitting parameters. The best probability distribution function was selected using chi-square method. The Lognormal distribution function obtained the best probability function for the threshold chloride concentration, the corrosion initiation time and the corrosion rate. The corrosion initiation time depends on four basic random variables such as: compressive strength of concrete, concrete cover, threshold chloride concentration and surface chloride concentration. Thus, statistical effects of these random variables on corrosion initiation time are parametrically investigated using 10000 Mont Carlo simulations. It is obvious that increasing the concrete resistance leads to an increase in the corrosion initiation time and standard deviation of the density function. The concrete physical and mechanical characteristics are effective variables on the corrosion initiation time but the threshold chloride concentration and the surface chloride concentration are insensitive variables on the mean of corrosion time; however, they lead to significant changes in standard deviation of the corrosion time. Finally, Various bar diameters such as: Φ8, Φ12, Φ16 and Φ20 were investigated in time-dependent area of the corroded steel in concrete beams. Amount of the cover is important variable in corroded cross section of bars. Also, increasing bar diameter and decreasing corrosion period led to a reduction in the rate of the bars’ cross section.