4. Cracking in Concrete

Cracks are physical manifestations of internal stresses or material weaknesses. They serve as direct pathways for aggressive agents, increasing permeability and reducing durability.

Cracking in concrete occurs when the material is subjected to internal stresses or weaknesses within its structure. These cracks act like pathways that allow water, salts, and other harmful substances to penetrate deeper into the concrete, which can lead to further deterioration and reduced lifespan of the structure. Essentially, cracks can compromise the integrity of concrete, making it more vulnerable to damage from environmental factors.

Imagine a plastic water bottle that's been squeezed too hard; it develops cracks that allow air to seep in. Similarly, when concrete cracks, it opens up paths for harmful elements to enter, much like air entering through the cracks of that water bottle, worsening its condition.

i. Classification of Cracks

• Structural Cracks: Due to load, settlement, thermal movement.
• Non-structural Cracks: Due to shrinkage, poor workmanship, environmental effects.

Cracks in concrete can be classified into two main categories: structural and non-structural. Structural cracks arise from factors like excessive loads or foundation settlement, which compromise the strength of the concrete. On the other hand, non-structural cracks occur due to less severe issues such as drying shrinkage or environmental changes, and while they may not threaten the structural integrity, they can still affect durability and appearance.

Think of structural cracks as major highways that have buckled under heavy traffic, necessitating immediate repairs to ensure safety. In contrast, non-structural cracks are like small cracks in a sidewalk—not immediately dangerous but should still be fixed to maintain a good appearance.

ii. Causes of Cracking

• Plastic Shrinkage: Occurs in fresh concrete due to rapid surface drying. Results in shallow cracks, often appearing within a few hours of placing.
• Drying Shrinkage: Long-term evaporation of moisture from hardened concrete causes contraction. Leads to distributed fine cracks on the surface.
• Thermal Cracking: Results from temperature differentials, especially in mass concrete. Heat of hydration and ambient temperature changes are key drivers.
• Creep and Load-Induced Cracks: Sustained loading causes deformation and possible cracking due to stress relaxation.
• Corrosion-Induced Cracking: Expansion of corroding steel reinforcement generates internal tensile stresses in concrete, causing it to crack and spall.
• Alkali-Aggregate Reaction (AAR): Alkalis in cement react with reactive aggregates, forming expansive gel. Cracks appear in random patterns with exudation of gel in severe cases.

Cracking can be caused by various factors:

• Plastic shrinkage occurs shortly after concrete is poured and exposed to air, leading to shallow, quick cracks.
• Drying shrinkage happens as moisture evaporates over time, resulting in fine cracks.
• Thermal cracking occurs due to temperature changes; for instance, the core of a large slab can heat up while the surface cools, causing stress.
• Creep refers to long-term deformation under constant load, which may lead to cracking.
• Corrosion of steel within the concrete generates internal pressure, leading to cracks as the steel expands.
• Finally, AAR is a chemical reaction that creates pressure in the concrete and leads to random cracking patterns.

If you've ever baked bread and noticed cracks form on the top as it rises and bakes, you can envision how plastic shrinkage works in concrete. Similarly, the way a sponge expands when wet but contracts and potentially cracks upon drying parallels drying shrinkage in concrete.

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