10.8
Karbonatlaşma Büzülmesi
Atmosferdeki CO_2 betonun gözeneklerine nüfuz eder ve nem varlığında karbonik asit oluşturur, bu da daha sonra hidratlanmış çimentodaki kalsiyum hidro…
Nem varlığında, beton yüzeyine giren atmosferik karbondioksit, shidratlı çimentodaki kalsiyum hidroksit ile kimyasal olarak reaksiyona girerek kalsiyum karbonat ve su oluşturan karbonik aside dönüşür.
Reaksiyon sırasında üretilen kalsiyum karbonat beton gözeneklerinde birikirken, salınan su reaksiyona girmemiş çimentonun hidratasyonuna yardımcı olur.
Bu, karbonatlaşma büzülmesi olarak bilinen betonun büzülmesine neden olur.
Karbonatlaşma betonun yüzeyinde başlar ve yavaşça içe doğru hareket eder.
İlerlemesi betonun geçirgenliğine, nem seviyesine, karbondioksit içeriğine ve ortam havasının bağıl nemine bağlıdır.
Karbonatlaşma ilerledikçe, sönmüş çimento hamurunun alkali doğası nötralize olur.
Sonuç olarak, oksijen ve nem betona nüfuz ederse, içine gömülü çelik donatının korozyonu meydana gelebilir.
Genellikle, betondaki karbonatlaşma derecesini değerlendirmek için, yeni kırılmış bir yüzeye fenolftalein püskürtülür.
Pembeye dönen alanlar, karbonatlaşmanın olmadığını gösteren serbest kalsiyum hidroksitin varlığını ortaya çıkarırken, orijinal rengini koruyan alanlar karbonatlaşmayı gösterir.
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Q1: What chemical reaction causes carbonation shrinkage in concrete?
Atmospheric carbon dioxide enters concrete pores and forms carbonic acid in the presence of moisture. This acid reacts with calcium hydroxide in the hydrated cement, producing calcium carbonate and water. The calcium carbonate deposits in the pores, causing concrete volume to contract, while the released water aids hydration of unreacted cement.
Q2: How does carbonation progress through concrete?
Carbonation starts at the concrete surface and moves inward slowly. Its progression rate depends on the concrete's permeability, moisture level, atmospheric carbon dioxide content, and relative humidity. Water-filled pores slow carbon dioxide diffusion, so concrete shielded from rainfall but exposed to moist air experiences more significant carbonation than periodically rain-rinsed concrete.
Q3: What are the effects of carbonation on concrete properties?
Carbonation slightly reduces concrete permeability as calcium carbonate fills pores and slightly enhances strength through continued cement hydration. However, it neutralizes the alkaline nature of the cement paste. When carbonation reaches reinforcement steel, the loss of alkalinity allows moisture and oxygen to penetrate, significantly increasing the risk of steel corrosion.
Q4: Why does carbonation increase the risk of reinforcement corrosion?
The alkaline environment of hydrated cement normally protects embedded steel from corrosion. Carbonation neutralizes this alkalinity, removing the protective barrier. If oxygen and moisture then penetrate the concrete and reach the reinforcement steel, corrosion becomes likely, compromising the structural integrity of the concrete member.
Q5: How is carbonation depth measured in concrete?
A freshly broken concrete surface is sprayed with phenolphthalein indicator solution. Non-carbonated areas containing free calcium hydroxide turn pink, while carbonated areas retain their original color. This color contrast reveals the carbonation depth and helps assess how far the carbonation process has advanced into the concrete.
Q6: What factors influence the rate of carbonation in concrete?
Carbonation rate is controlled by concrete permeability, moisture content, atmospheric carbon dioxide levels, and ambient relative humidity. Lower permeability slows carbon dioxide diffusion. Higher moisture content in pores also reduces diffusion rates. Environmental conditions with moderate humidity and higher CO2 concentrations accelerate carbonation compared to very wet or very dry conditions.
Q7: How does carbonation differ from other forms of shrinkage in concrete?
Carbonation shrinkage results from a chemical reaction between atmospheric CO2 and cement paste components, causing volume reduction and potential reinforcement corrosion. Unlike drying shrinkage, which occurs from moisture loss, carbonation is driven by atmospheric carbon dioxide penetration and can occur even in moist environments, making it a distinct deterioration mechanism.