2.1. Types of Transport Mechanisms

Permeation

• The movement of fluids under pressure through the pore system.
• Governed by Darcy’s Law: $Q = \frac{k A \Delta h}{L}$ where:
  • $Q$: discharge,
  • $k$: permeability coefficient,
  • $A$: cross-sectional area,
  • $\Delta h$: hydraulic head,
  • $L$: length of the flow path.

Permeation refers to how fluids move through a material. In this case, it involves fluids moving under pressure through the interconnected pores within concrete. Darcy’s Law provides a formula that describes this movement, where 'Q' represents the amount of fluid that flows, 'k' indicates how permeable the material is, 'A' is the area through which the fluid is passing, 'Δh' is the difference in height causing the fluid to move, and 'L' is the length of the path the fluid travels. Understanding this concept helps predict how well concrete can resist water and other fluid pressures.

Think of permeation like water flowing through a sponge. When you squeeze a sponge, water is forced out through its pores. Similarly, when pressure is applied to concrete, water can move through its pores. The easier the water can flow, the more permeable the concrete is, just like a sponge with bigger holes allows more water to escape quickly.

Diffusion

• Movement of ions or molecules due to concentration gradients.
• Relevant for gases like CO₂ or chloride ions entering concrete.

Diffusion is the process by which ions or molecules move from areas of high concentration to areas of low concentration. In the context of concrete, this is significant because gases like carbon dioxide (CO₂) or chloride ions can penetrate the material over time. This process affects the durability of concrete as the ingress of harmful substances may lead to corrosion of the reinforcing steel and other degradative reactions.

Consider diffusion as the way a drop of food coloring spreads in a glass of water. Initially, the coloring is concentrated in one spot, but over time it spreads out evenly throughout the water. Similarly, when carbon dioxide in the air comes into contact with concrete, it slowly moves into the material until it affects the entire section that is exposed.

Capillary Suction (Absorption)

• Suction of water due to surface tension in the capillaries.
• Common when concrete is exposed to water intermittently.

Capillary suction occurs when water is drawn into the tiny pores (capillaries) of concrete due to surface tension. This is particularly evident when concrete is intermittently wet, such as during rain or flooding. The smaller the pores, the greater the suction effect, which can lead to water being absorbed into the concrete, potentially leading to damage like freeze-thaw cycles or increased permeability.

Imagine how a paper towel absorbs spilled liquid. The fibers in the towel create small gaps that draw in liquid through capillary action. Similarly, when concrete becomes damp, its tiny pores can draw in water, which may cause problems if the moisture leads to expansion with freezing temperatures or other damaging reactions inside the concrete.

Wick Action

• Occurs when water moves along reinforcement or interfaces due to pressure or gravity.

Wick action refers to the phenomenon where water travels along the surface of materials, particularly along reinforcing bars or interfaces in concrete structures. This can happen due to pressure differences or gravitational forces. Wick action can lead to moisture accumulating in areas where it can cause harmful effects, such as corrosion of the steel reinforcement. Understanding this is essential for designing concrete to minimize water ingress and protect structural integrity.

Think of wick action like a candle wick drawing up wax to keep a flame alive. Just as the wick pulls liquid wax towards the flame, water can be drawn into concrete through fine channels created by reinforcing bars, which can lead to moisture-related issues if not managed properly.

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