Is Concrete damp proof? It depends – Learn more

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is concrete damp proof

Concrete is the most widely used construction material in the world. It is extremely versatile and easy to work with. This means it can be used in a wide variety of applications.

Everything from a small concrete slab to sit a shed on, to huge industrial structures, such as dams and bridges.

Standard concretes are quite porous, and this means they are not damp proof. However, there are types of concrete available that are watertight. This is achieved by adding additional waterproofing solutions when mixing.

Why is standard concrete not damp proof?

When standard concrete is mixed a combination of water, cement and aggregate (sand and gravel) is used. There are many different grades of concrete, each type will have a different mixture of the required ingredients.

This means different ratios of cement to aggregate, different grades and sizes of aggregate, etc. The type of concrete you use will depend on the job you are doing, and the characteristics you require from the finished concrete.

Water is a vital ingredient, as it binds the other materials together. Once the materials have been combined, the concrete will start to go off. The process will start within a couple of hours of mixing. This means it is essential that you start to lay the concrete as soon as possible.

Usually, the concrete is easy to work with, due to the amount of water it contains. In fact, the amount of water used when mixing, is more than is required for hardening.

The reason so much water is used, is the concrete would be unworkable with less liquid, it would also go off much faster.

As the mix starts to cure and harden, this excess water works its way to the surface of the concrete and evaporates away.

As the water travels through the concrete, it creates a network of small capillaries. These capillaries result in the concrete being quite porous.  

Small capillaries act like a sponge, this means water can wick up into the concrete when it comes in to contact.

In many cases this is not an issue. However, if you are trying to keep water out, or even hold water in, this may be an issue.

How do you stop damp coming through concrete?

In many instances standard concrete (not watertight) will be used in combination with some form of membrane to provide a waterproof barrier.

A good example of this, is residential property in the UK. When the foundations are created, concrete is used.

There are several different types of foundation. The one that is used will depend on a variety of factors, relating to the soil type, and the property itself.

Two of the most common types are strip foundations and raft foundations.

A strip is a shallow foundation, that is poured into the ground in channels. Once the concrete sets, the main load bearing walls are built off the concrete strips. These are more commonly used where the soil already has good load bearing capacity.

A raft foundation is a concrete slab that sits under the entire area of the building. This will be more common where soil pressure is low, and the load needs spreading over a larger area.

Both types will employ some form of waterproof membrane, to stop rising damp entering the property, via capillary action in the concrete.

A slab of concrete will usually have a damp proof membrane, either directly below it, or above it. If it is above, there will be a screed on top. Either way, this will then overlap into a DPC (damp proof course) placed in the mortar joint, at approximately 150mm above external ground level.

What this means, is even though the concrete is not damp proof. Water will not be able to travel past the DPM or DPC.

However, these can fail over time. When they do, rising damp can be a problem if the waterproof barrier is breached

If this does happen, there are various ways it can be repaired.

For the DPC (in the mortar joint between your brickwork, a common fix is damp proof injections. This forms a new barrier, by spreading through the capillaries the water would use to rise into the property.

When the issue is with the DPM, then solutions such as tanking slurry, or an epoxy resin can be used. Both are quite easy to apply with a brush or roller, respectively.

How damp-proof concrete works

A standard concrete is adequate for many jobs. However, there are certain construction projects that need to be watertight. If this is the case, a plastic membrane is not the best, or even easiest option.

This could include structures like:

  • Swimming pools
  • Dams
  • Reservoirs
  • Water or sewage tanks
  • Underground structures such as tunnels and car parks
  • Bridges stood in water
  • Etc.

In any of these instances and many others, a fully waterproof system will be required.

To achieve this, the concrete has waterproofing additives added to the mix, these additives create a watertight finish in two main ways:

  1. It replaces a portion of the water used when mixing the concrete. This means less liquid will leave the concrete, resulting in less and smaller capillaries.
  2. The capillaries that remain are sealed with the waterproofing additive.

The result is a completely watertight concrete structure. It is worth noting, that this is not only surface level damp proofing, like a membrane. This process damp proofs the entirety of the concrete structure.

If you were to split the concrete and then pour water on the exposed internal surface, the water would settle on its surface and there would be zero absorption.

One of the leading companies when it comes to concrete damp proofing, is Sika®. You can watch a useful video below of how their products and entire process works.

Conclusion

As you can see, concrete in its standard form is not waterproof. It is quite porous. However, there are several solutions to stop water penetrating through.

This can be additional protection via physical and chemical membranes, as well as additives that can turn your porous concrete, into a completely waterproof building material.