Porosity & Permeability in Concrete

Concrete looks like a solid and impermeable material.

But if we could zoom into the concrete at the microscopic level, we would see a complex network of tiny pores and channels.

These pores are important because they provide pathways through which water, vapour, gases and dissolved ions can move.

Understanding this movement is fundamental to understanding concrete durability.

The interesting part is that water does not move through concrete in the same way under all conditions.

The mechanism changes depending on how wet the pore is.

Concrete contains pores of different sizes.

However, the presence of pores alone does not mean that water can easily pass through the concrete.

This brings us to two important terms:

Porosity

Porosity represents the amount of pore space within the material.

Permeability

Permeability describes how easily a fluid can pass through the material.

Therefore:

High porosity does not necessarily mean high permeability.

If many pores are isolated, water may enter them but cannot travel very far.

If the pores are interconnected, however, they can create a continuous pathway through the concrete.

Why interconnected pores matter

Consider two concrete specimens.

Concrete A

It contains many pores, but the pores are poorly connected.

There is plenty of pore space, but no continuous pathway.

Concrete B

It contains fewer pores, but they are interconnected.

Now there is a pathway for transport.

Therefore:

Pore connectivity can be more important than pore volume alone when considering permeability.

This is why engineers should avoid treating porosity and permeability as synonyms. The source explicitly makes this distinction.

Imagine one pore inside concrete

To understand transport, forget about the entire concrete structure for a moment.

Imagine that we can enter a single microscopic pore.

Depending on the moisture condition, that pore may be:

• almost completely dry,
• covered by a thin water film,
• partially filled with water, or
• completely filled with water.

The way water moves changes at each stage.
Refer the following image

Stage a: Water molecules stick to the pore wall

In a dry concrete pore, water molecules initially interact with the surface of the pore wall.

This process is called adsorption.

Think of the pore wall as a surface to which water molecules can stick.

At this stage, there is no significant bulk liquid flow.

The water is primarily associated with the pore surface.

This is represented as Stage (a) – Adsorption.

Stage b: Vapour diffusion

As moisture increases, the pore wall develops an adsorbed water layer.

The pore may still not contain enough liquid water for conventional liquid flow.

Water vapour can move through the pore.

This mechanism is called vapour diffusion and is shown as Stage (b).

A simple way to remember it is:

Dry pore → water sticks to surface → moisture moves mainly as vapour.

Stage c: Film transfer

With increasing moisture, the water layer on the pore surface becomes more significant.

Water begins to transfer through the developing liquid film.

This is represented as film transfer in Stage (c).

The important idea is that water is beginning to establish a more continuous liquid pathway.

Stage d: Surface creep

When the pore is still not completely filled, water can move along the pore surface while vapour transport can continue through the remaining space.

The document describes this stage as:

Surface creep + vapour diffusion.

This is shown as Stage (d).

Stage e: Partially saturated liquid flow

As moisture continues to increase, the pore becomes partially filled with liquid water.

Now liquid flow becomes increasingly important.

The pore contains both water-filled and unfilled regions.

This condition is represented as partially saturated liquid flow in Stage (e).

Stage f: Saturated liquid flow

Eventually, the pore becomes completely filled with water.

Now the situation becomes much simpler.

We have a continuous liquid pathway.

Water can flow through the pore under an appropriate pressure gradient.

This is shown as saturated liquid flow in Stage (f).

But how do ions move?

Now consider that the water inside the pore contains dissolved ions.

For example, imagine a chloride ion concentration that is high near the surface and lower deeper inside the concrete.

There is a concentration gradient.

The ions tend to move from the region of higher concentration toward the region of lower concentration.

This process is called: Ionic diffusion

Water flow is associated with a pressure gradient, whereas ionic diffusion occurs because of a concentration gradient.

Importantly, ionic diffusion can occur through liquid contained in partially saturated as well as fully saturated pores.

Why this is important for concrete durability

The movement of water and dissolved substances through concrete is directly relevant to durability.

If an interconnected pore network allows water and aggressive dissolved species to penetrate, they can travel deeper into the concrete.

Therefore, when we talk about durable concrete, we are not simply asking:

“How strong is the concrete?”

We also need to ask:

“How easily can water and potentially aggressive substances travel through its pore network?”

This is particularly important for structures exposed to water and aggressive environments.

The source notes the importance of absorption and permeability for water-retaining structures, watertight basements and durability assessment.

Reference:

Testing of Concrete in Structures by John H. Bungey, Stephen G. Millard & Michael G. Grantham