10.8
Contracción por carbonatación
El CO_2 atmosférico penetra en los poros del hormigón y, en presencia de humedad, forma ácido carbónico, que reacciona con el hidróxido de calcio pres…
En presencia de humedad, el dióxido de carbono atmosférico que ingresa a la superficie del concreto se transforma en ácido carbónico, que reacciona químicamente con el hidróxido de calcio en el cemento hidratado para formar carbonato de calcio y agua.
El carbonato de calcio producido durante la reacción se deposita en los poros del hormigón, mientras que el agua liberada ayuda a la hidratación del cemento sin reaccionar.
Esto da lugar a la contracción del hormigón, conocida como contracción por carbonatación.
La carbonatación comienza en la superficie del hormigón y se desplaza lentamente hacia el interior.
Su progreso depende de la permeabilidad del hormigón, el nivel de humedad, el contenido de dióxido de carbono y la humedad relativa del aire ambiente.
A medida que avanza la carbonatación, la naturaleza alcalina de la pasta de cemento hidratada se neutraliza.
En consecuencia, si el oxígeno y la humedad penetran en el hormigón, es probable que se produzca la corrosión del refuerzo de acero incrustado en él.
Comúnmente, para evaluar el grado de carbonatación en el concreto, una superficie recién rota se rocía con fenolftaleína.
Las áreas que se vuelven rosadas revelan la presencia de hidróxido de calcio libre, lo que indica la ausencia de carbonatación, mientras que las áreas que conservan su color original indican carbonatación.
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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.