4.2. Chemical Deterioration

Sulfates in soil or water react with calcium compounds in concrete to form ettringite, causing expansion and cracking. Use low C3A cement and pozzolans to resist.

Sulfate attack occurs when sulfate ions, often present in soil or groundwater, react with the calcium compounds in concrete. This reaction forms a substance called ettringite, which expands, leading to cracks in the concrete. To prevent this damage, it is crucial to use specific types of cement that have a low amount of tricalcium aluminate (C3A). Additionally, incorporating pozzolanic materials such as fly ash can also enhance resistance to sulfate attack.

Think of sulfate attack like a sponge that expands when it soaks up water. If you place a sponge in a bucket of water, it swells up and can eventually break if it expands too much. Similarly, ettringite expands within the concrete, causing it to crack and deteriorate. So, choosing the right materials in concrete is critical to prevent this expansion from happening.

Reaction between alkalis in cement and reactive silica in aggregates produces gel that absorbs water and swells. Control by limiting alkali content or using non-reactive aggregates.

Alkali-Aggregate Reaction (AAR) is a chemical reaction that can happen in concrete when alkalis from cement interact with reactive silica present in certain aggregates. This reaction produces a gel that absorbs moisture and expands, potentially causing significant structural damage over time. To mitigate this issue, concrete producers can either limit the amount of alkali in the mix or use aggregates that do not react with alkalis, thus preventing the problem from occurring.

Imagine mixing baking soda and vinegar. When they combine, they react vigorously and produce a lot of bubbles and foam, which expands. In a similar way, the reaction of alkalis and reactive silica in concrete creates a gel that expands over time, leading to cracks. Just as bakers carefully select ingredients to avoid unwanted reactions, concrete engineers must carefully select materials to prevent AAR.

CO₂ from the air reacts with calcium hydroxide in concrete to form calcium carbonate. This reduces pH and allows reinforcement corrosion. Controlled by increasing concrete cover and reducing permeability.

Carbonation happens when carbon dioxide (CO₂) from the air reacts with calcium hydroxide in concrete, transforming it into calcium carbonate. This process lowers the pH level of the concrete, which can make it more susceptible to corrosion of steel reinforcements inside. To combat carbonation, engineers can increase the thickness of the concrete cover over the reinforcements and reduce the permeability of the concrete, which helps keep harmful substances from penetrating.

Imagine a metal object left outside in the rain. Over time, it oxidizes and rusts due to exposure to moisture and air—similar to how concrete can degrade through carbonation. A thick layer of paint protects the metal just like adding extra concrete cover protects the steel inside the concrete from corrosive elements in the environment.

Chlorides from de-icing salts or seawater cause reinforcement corrosion. Use pozzolans, epoxy-coated bars, and surface sealers.

Chloride attack occurs when chlorides, often from de-icing salts used on roads or from seawater, penetrate the concrete and reach the steel reinforcement bars. This can lead to corrosion of the steel, which expands and causes cracks in the concrete. To protect against chloride attack, it is beneficial to use supplementary materials like pozzolans that help lower permeability, epoxy-coated reinforcement bars that resist corrosion, and surface sealers that create a barrier against chloride ingress.

Consider how salt from the ocean can corrode a metal boat over time if it's not properly maintained. Similarly, chloride can corrode the steel inside concrete, leading to structural problems. Just like boat owners apply protective coatings to keep saltwater at bay, engineers use specific materials and techniques in concrete to safeguard steel reinforcements from chlorides.

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