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Sulfate Attack on Concrete - Chapter 3

Concrete deterioration - NGUYÊN NHÂN CHỦ YẾU CỦA SUY THOÁI BÊ TÔNG Sự suy giảm của bê tông có thể được gây ra bởi hóa chất và các quá trình vật lý, hoặc sự kết hợp của họ. Các cơ chế hóa học bao gồm lọc các thành phần dán, cacbonat của canxi hydroxit và CSH, hư hỏng dán tiếp xúc với hóa chất tích cực (axit, hoá chất nông nghiệp, sunfat), sự ăn mòn của cốt thép, và tổng hợp các phản ứng kiềm- | 3 Concrete deterioration 3.1 PRINCIPAL CAUSES OF CONCRETE DETERIORATION Deterioration of concrete may be caused by chemical and physical processes or their combination. Chemical mechanisms include leaching of paste components carbonation of calcium hydroxide and C-S-H paste deterioration by exposure to aggressive chemicals acids agricultural chemicals sulfates corrosion of steel reinforcement and alkali-aggregate reactions. Physical or mechanical causes of concrete deterioration are represented by abrasion erosion cavitation and most important by freezing and thawing cycles. Most chemical mechanisms of deterioration involve damage to the cement paste matrix but deterioration of the paste-aggregate interface the aggregate itself or the pore structure often accompanies the paste deterioration. Sulfate attack the subject of this monograph is a form of chemical mechanism. A common denominator of most mechanisms of deterioration is access to concrete of moisture. Without water with mechanical damage such as abrasion being an exception no mechanism of deterioration can proceed. Examples of such processes are of both chemical e.g. alkali-silica reaction corrosion of reinforcement sulfate attack and physical e.g. freezing-thawing nature. The general factors that accelerate or retard chemical damage of concrete are summarized by ACI as follows Table 3.1 . When examining concrete deterioration one has to take into consideration the local microclimate the concrete is exposed to. The concrete surface durability may depend on the local temperature and humidity thus an important consideration is exposure or lack of exposure to rain direct sun etc. It should be remembered that field concrete in contrast to laboratory-made specimens is usually exposed to more than one mechanism of deterioration such situation may lead to synergistic interactions resulting in increased rate of deterioration. If combined mechanisms are operative it may be often difficult to clearly identify the .