10.8
Contração por carbonatação
O CO_2 atmosférico penetra nos poros do concreto e, na presença de umidade, forma ácido carbônico, que então reage com hidróxido de cálcio no cimento…
Na presença de umidade, o dióxido de carbono atmosférico que entra na superfície do concreto se transforma em ácido carbônico, que reage quimicamente com o hidróxido de cálcio no cimento hidratado para formar carbonato de cálcio e água.
O carbonato de cálcio produzido durante a reação se deposita nos poros do concreto, enquanto a água liberada auxilia na hidratação do cimento não reagido.
Isso resulta na contração do concreto, conhecida como encolhimento por carbonatação.
A carbonatação começa na superfície do concreto e se move lentamente para dentro.
Seu progresso depende da permeabilidade do concreto, do nível de umidade, do teor de dióxido de carbono e da umidade relativa do ar ambiente.
À medida que a carbonatação avança, a natureza alcalina da pasta de cimento hidratada se neutraliza.
Consequentemente, se o oxigênio e a umidade penetrarem no concreto, é provável que ocorra corrosão da armadura de aço embutida nele.
Normalmente, para avaliar a extensão da carbonatação no concreto, uma superfície recém-quebrada é pulverizada com fenolftaleína.
As áreas que ficam rosadas revelam a presença de hidróxido de cálcio livre, indicando a ausência de carbonatação, enquanto as áreas que mantêm sua cor original indicam carbonatação.
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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.