11.4.1. Carbonation

• Atmospheric CO₂ reacts with calcium hydroxide in concrete, reducing pH and compromising the passive layer on reinforcement.

When concrete is exposed to the atmosphere, the carbon dioxide (CO₂) from the air interacts with calcium hydroxide (Ca(OH)₂), which is a component produced during the hydration of cement. This reaction results in the formation of calcium carbonate (CaCO₃). As this reaction progresses, the pH of the concrete decreases, moving from a highly alkaline state (around pH 12-13) to a more neutral state (around pH 8 or lower). This reduction in pH negatively affects the protective oxide layer that surrounds the steel reinforcement bars (rebars) embedded in the concrete. Without this protective layer, the rebars become vulnerable to corrosion, which can weaken the entire structure over time.

Think of carbonation like rust forming on a bicycle. If you leave your bike outside in the rain without any protective coating, the metal parts can begin to rust when they react with moisture and oxygen in the air. In the same way, the protective layer around the steel in concrete can degrade due to carbonation, leading to 'rusting' or corrosion of the steel rebar inside the concrete.

The reduction in pH compromises the passive layer on reinforcement.

The passive layer is a protective film that forms on the surface of steel when it is embedded in concrete. This layer is essential for preventing corrosion. When carbonation occurs and the pH of the concrete decreases, this passive layer begins to break down. Once the passive layer is compromised, the steel inside the concrete can react with environmental factors like moisture and chlorides, leading to corrosion. Corroding steel expands in volume, creating internal stress and leading to cracks and spalling in the concrete structure itself.

Imagine a protective layer of wax on a car that prevents rust from forming on the metal underneath. If the wax wears off due to exposure to rain and sun, the metal will start to rust. Similarly, when carbonation lowers the pH, the protective oxide layer on the rebar breaks down, exposing the steel to rusting and corrosion.

Resulting corrosion products expand and cause cracking and delamination of concrete.

As the steel reinforcement begins to corrode due to the absence of a protective passive layer, the products of this corrosion (such as iron oxide) occupy more volume than the original steel. This expansion creates internal pressure within the concrete, which can result in cracking and sometimes even delamination, where layers of concrete separate and break away. This deterioration impacts the structural integrity and durability of the concrete element, making it less safe for use and potentially requiring costly repairs or reinforcement.

Think about a balloon filled with water. If you keep adding water to the balloon, it will eventually stretch and may even burst. Similarly, as the steel inside the concrete expands due to rust formation from carbonation, it exerts pressure on the surrounding concrete, leading to cracks and structural failure, much like the overstretched balloon.

--