11.3. Types of Concrete Deterioration

11.3.1 Chemical Attack

• Sulfate attack: Sulfate ions from soil or water react with hydrated cement products, forming expansive compounds like ettringite.
• Acid attack: Acids dissolve calcium hydroxide and other cementitious compounds.
• Alkali-Aggregate Reaction (AAR):
  • Alkali-Silica Reaction (ASR): Reactive silica in aggregates reacts with alkalis in cement, producing an expansive gel that causes cracking.

Chemical attacks on concrete are severe threats that can compromise its integrity. Sulfate attack occurs when sulfate ions from soil or water interact with the hydrated products in cement, leading to the formation of ettringite, which expands and can cause cracking. Acid attack happens when acidic substances dissolve essential components like calcium hydroxide, weakening the concrete structure. Another significant risk is the Alkali-Aggregate Reaction (AAR). In this case, silica present in aggregates reacts with alkali hydroxides in cement, creating an expansive gel which puts pressure on the concrete, ultimately resulting in cracking.

Imagine having a sponge full of water. If you put that sponge in a dish with vinegar (an acid), the vinegar starts to dissolve parts of the sponge. This is similar to what happens in concrete during an acid attack. The sponge represents the concrete, and the vinegar is the acid that damages it. Similarly, if you add salt (sulfate ions) to the sponge, it starts to expand, leading to bursting if the sponge cannot accommodate the increase in volume, just like the sulfate attack damages concrete.

11.3.2 Physical Weathering

• Freeze-Thaw Cycles: Water within pores freezes and expands, creating internal stresses and spalling.
• Abrasion and Erosion: Surface wear due to mechanical action, especially in hydraulic and industrial structures.

Physical weathering of concrete primarily occurs through freeze-thaw cycles. When water seeps into the tiny pores of the concrete and freezes, it expands. This expansion creates significant stress within the material, which can lead to spalling, or pieces breaking off the surface. Abrasion and erosion refer to the wear and tear caused by mechanical action, often seen in hydraulic structures or areas with heavy foot or vehicle traffic. These forces can wear down concrete surfaces, reducing their durability.

Consider a bottle of soda that is sealed and put in the freezer. As the liquid freezes, it expands, and if there's not enough space in the bottle, it will crack and leak. Similarly, when concrete traps water in its pores, the freezing process can crack it apart due to pressure. Think about a rough road that feels uneven after many vehicles have passed over it. Over time, the constant rolling can wear away the surface, just like how abrasion affects concrete in busy areas.

11.3.3 Corrosion of Steel Reinforcement

• Carbonation and chloride penetration depassivate the protective oxide film on reinforcement bars, leading to rusting.
• Corrosion products expand and cause cracking and delamination of concrete.

Corrosion of steel reinforcement within concrete is a critical issue that can severely affect concrete structures. Carbonation occurs when carbon dioxide from the atmosphere penetrates the concrete and reacts with calcium hydroxide, lowering the pH. This process disrupts the passive protective layer on steel rebar, making it vulnerable to corrosion. Chloride penetration, often from deicing salts or marine environments, also causes this protective film to break down, leading to rust formation. The expansion of corrosion products can crack and flake off the surrounding concrete, known as delamination, weakening the overall structural integrity.

Think of a rusty bicycle. When it rains, water and moisture seep into the bike's metal parts, leading to rust. If that rust continues to develop, it creates flakes that prevent smooth movement and can even cause parts to break. Similarly, when moisture and carbon dioxide infiltrate concrete, they can cause the embedded steel to corrode, leading to cracks and potentially dangerous structural failures.

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