Is Concrete Permeable? (ANSWERED)

Modern civilization is built on a foundation of concrete and steel.

And despite what most people think, concrete is a permeable substance that allows both water and gas to pass into and through it.

So, how is it possible that virtually every concrete structure we have built can withstand the onslaughts of moisture and gas without collapsing?

Is Concrete Permeable? (ANSWERED)

What Does Permeable Mean?

If a substance is permeable, it means that it is porous and will allow liquids or gasses to pass right through it.

Concrete allows both moisture and gas to pass through and is, therefore, a porous substance.

In fact, gasses such as Radon can pass through a concrete slab from the ground below.

The naturally occurring Radon gas is radioactive and causes lung cancer if breathed in.

Although it can disperse quite quickly outdoors, if it seeps through the foundations of buildings such as schools or residential homes, then it becomes a serious health hazard.

So, the permeability of concrete is a serious problem in construction and one which needs to be carefully addressed to avoid potential disasters.

Concrete Is a Porous Material

Concrete is very hard, yet its porous structure is at odds with its outward appearance.

To make concrete, one combines the following 3 ingredients:

  • Cement
  • Aggregate
  • Water

As the concrete cures, it dries out.

The water in the concrete mix initiates the chemical reaction that hardens the cement and as the water evaporates, it leaves little voids behind.

It is into these voids that water and gas seep, trying to find a way through.

If the concrete is between 4 and 6 inches thick, the tiny pathways left by the evaporating water provide a clear pathway for the water and gas to pass through.

Thicker concrete walls and slabs provide more of a barrier to water and gas, but it is still possible for moisture to find a way through as cracks form over time.

Permeability varies depending on the ratio of aggregate, water, and cement that is used to create the concrete mix.

How to Make Concrete

Concrete is a precise mix of cement, water, and various aggregates.

Depending on what the concrete is being used for, aggregates such as sand, gravel, or crushed stone, or a combination of these are added to the water and cement.

When the cement and water mix, they begin to harden and bond to the aggregate, which then becomes the substance we know as concrete.

A chemical reaction called hydration takes place when cement and water mix.

The resulting paste forms nodes and wraps itself around the aggregate, binding it into a solid rock-like material.

Unfortunately, as the water evaporates, it leaves behind a network of capillaries that draw water and gas through the concrete.

What is Cement?

Although the use of cement is widespread, it is not a naturally occurring substance.

Cement is a combination of the following materials:

  • Lime or calcium oxide – CaO
  • Silica – SiO2
  • Alumina – Al2O3
  • Iron – Fe2O3
  • Gypsum – CaSO4.2H20

Lime is obtained from limestone, chalk, calcareous rock, or shale, while silica comes from sand, old bottles, argillaceous rock, or clay.

Calcareous rock is a sedimentary rock that comprises mainly calcium from the remains of marine animals.

The argillaceous sedimentary rock contains either silt or tiny clay-sized particles.

The particles are less than 0.00246063 inches (0.0625 mm) in diameter.

It can also contain clay minerals.

The sedimentary rock Bauxite has a high Aluminum content together with Gallium.

The minerals Gibbsite (Al(OH)3), Boehmite (γ-AlO(OH)) and Diaspore (α-AlO(OH)), together with the two iron oxides goethite (FeO(OH)) and Haematite (Fe2O3), as well as Kaolinite (Al2Si2O5(OH)4) and tiny quantities of Anatase (TiO2) and Ilmenite (FeTiO3 or FeO.TiO2), make up the remainder of this complex component of cement.

Finally, gypsum is a mineral made up of hydrated calcium sulfate. It gives cement its grey color and has been found in the construction of the Pyramids in Egypt.

How Cement is Made

All the ingredients above except for gypsum are combined in precise chemical compositions depending on the type of cement that is required.

The mixture is then ground and blended by either a dry process or a wet process.

The resulting mix is then fired in a kiln at 2,600° F (1,427° C). The resulting pellets are called clinker.

Once the clinker cools, it is combined with the gypsum (which acts as a curing time regulator) and ground into an extremely fine powder.

What Determines the Permeability of Concrete?

Concrete’s permeability is determined by three factors:

1. Water to Cement ratio

The higher the quantity of water relative to cement present in the mix, the greater the permeability.

Using more water results in a lower level of compaction.

That means that the particles of cement and aggregate are less tightly bound together, leaving more voids through which water and gas can pass.

Because cement hardens through a chemical reaction with water, once the reaction is complete, a certain amount of water is left behind in the concrete.

It is then free to flow in whichever direction it chooses.

To achieve a low permeability close to zero, a ratio of 0.4 water to cement is required.

Additives called superplasticizers are used when manufacturers mix up very low permeability concrete.

2. Compaction of Concrete

When concrete is poured, a tool is used to vibrate the concrete in an attempt to remove air voids as well as excess bleed water.

This has the effect of reducing the number of pores, especially interconnected pores, through which moisture and gas can pass.

3. Curing of Concrete

Sufficient time is required for concrete to cure properly.

Often, construction teams will cover concrete with a damp cloth to prevent the surface from drying out in the sun.

This prevents water from getting trapped inside the concrete and prevents it from properly curing.

In critical areas like basements and flat roofs, additional waterproofing is required to prevent moisture and gas from penetrating the concrete.

You can judge the success of these waterproofing measures by the fact that boats and floating barges have been successfully constructed using concrete as well as dams and water reservoirs.

4. Other Factors that Affect Concrete Permeability

As it gets older, cracks develop which affect the permeability of the concrete.

The size of aggregates used and the addition of additives together with the quality of water used affect concrete’s ability to withstand moisture and gas encroachment.


Concrete is a strong, versatile material that is flawed by being extremely porous.

However, with the development of modern additives and manufacturing processes, it has overcome its permeability to become a vital structural product.

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