9.12. Volume Changes and Cracking

Causes of Volume Changes:

• Moisture movement
• Thermal expansion
• Chemical reactions
• External loading

Volume changes in concrete occur due to several factors, primarily influenced by moisture movement within the material. When concrete absorbs or loses water, it can expand or shrink, respectively. Thermal expansion refers to the increase in volume of concrete when it heats up. Various chemical reactions, including hydration and reactions with environmental elements, can also cause volumetric changes. Lastly, external loads applied to the concrete can induce stresses that affect its volume and shape over time.

Think of a sponge. When a sponge absorbs water, it expands to hold that moisture, similar to how concrete expands when it takes on water. Conversely, when the sponge dries out, it shrinks back down, just like concrete can shrink when it loses moisture.

Types of Cracks:

• Structural (due to overloads)
• Non-structural (shrinkage, temperature)
• Plastic and drying cracks

Cracks in concrete can generally be categorized into three types: structural cracks, which arise from excessive loading or stress beyond what the concrete can bear; non-structural cracks, which typically occur due to factors like shrinkage or temperature changes; and plastic and drying cracks, which happen when the concrete is still setting. Understanding these types allows for better assessment and remediation strategies during concrete construction and maintenance.

Imagine a bridge that cracks because of too many cars (structural cracks), or a sidewalk that develops small lines during a hot day because it expands too much (non-structural cracks). Just like how a wet towel can develop wrinkles as it dries, concrete can form plastic and drying cracks while it sets.

Control:

• Joints, reinforcements
• Controlled curing
• Use of fibers

Preventing cracks due to volume changes in concrete involves various strategies. One method is to introduce expansion joints that allow for natural movement without causing damage. Reinforcements such as steel bars help support the concrete and reduce the likelihood of crack formation. Controlled curing, ensuring the concrete maintains adequate moisture during the curing process, helps minimize shrinkage. Lastly, incorporating fibers into the concrete mix can enhance its resistance to cracking by providing additional tensile strength.

Consider how a well-designed bridge includes flexible joints that accommodate movement. Similar to how a well-hydrated plant can resist wilting, properly cured concrete stays resilient against cracking. Adding fibers to concrete is like adding extra support to a tower, making it less likely to topple under pressure.

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