Concrete expansion joints explained

If you are looking for information on concrete expansion joints, we’ve got everything you need to know below. Read on to learn:

• What a concrete expansion joint is and why they’re important
• Where to place concrete expansion joints
• The most suitable concrete expansion joint fillers for different applications
• How to install expansion joints
• Tips from our experts on planning and placing joints for concrete

Let’s get started.

What is a concrete expansion joint?

A concrete expansion joint is a small gap which allows concrete to expand and contract as and when the temperature changes. They’re also known as concrete control joints.

A concrete expansion joint forms a break between the concrete and other parts of a structure to allow movement without causing stress, which could otherwise lead to cracking. It’s important to remember that concrete is most susceptible to cracking after being poured.

Why do you need concrete expansion joints?

All concrete (from home use concrete to reinforced concrete) shrinks slightly as it dries. Then, when it’s set, it expands or contracts depending on the temperature. Concrete expansion joints prevent cracks from forming as a result of concrete expanding or contracting. Correct use of joints allows for the right amount of movement in the concrete, making them core to the overall strength of a structure.

Specialist advice for placing concrete in cold weather

Where do cracks in concrete commonly occur?

If concrete is exposed to heat it expands, and if it’s exposed to cold, it contracts. But if this pressure has nowhere to go, it has to go into the concrete itself. That’s what causes cracking, and it’s most common in concrete foundations (like the perimeter or corners of a building), slabs, and driveways.

For detailed information about choosing the right materials for building foundations, read: The ultimate building foundation guide.

Where are concrete control joints needed?

Where there have been consecutive concrete pours or where concrete is laid within an area bordered by walls, buildings, or objects, then concrete expansion joints are imperative. If several construction joints are needed or you’re installing mass concrete (thicker than 0.5m, usually), make sure your joints are designed and specified by a structural engineer.

For information on preventing cracking in mass concrete, read our technical article on thermal management.

Concrete expansion joints for paving

If you have laid a concrete slab with a surface area exceeding 6m2, you need to incorporate expansion joints.

Control joints for infrastructure

For large-scale infrastructure projects, like bridges or commercial buildings, control joints are necessary to prevent the formation of cracks - and maintain the structure’s resilience.

Using concrete expansion joints for roads

Concrete roads need to be able to respond to changing temperatures. Control joints give the concrete enough room to expand or contract while limiting the risk of cracking.

Understand which type of concrete is best suited to your application by taking a look at all our concrete products.

What is the right concrete expansion joint material to use?

An expansion joint filler is the material you need to use when laying concrete and preventing cracks. These fillers come in two forms: fibreboard filler and foam filler. Foam filler is used in structural steel applications, whereas fibreboard filler is suited for concrete applications.

Fibreboard filler

Fibreboard filler is used to create expansion joints for concrete walls, pavements, and suspended slabs. It’s also used as a filler for concrete infrastructure, from runways and roads to bridges.

What happens if you don’t put control joints in concrete?

Concrete without expansion joints is at higher risk of cracking. Even a small crack can cause serious implications further down the line, so it’s important to reduce the likelihood of them forming.

Without concrete joints, there’s more chance of dead-load and live-load stress leading to cracks and structural instability, too. Control joints also prevent moisture issues, which can compromise the overall safety of a structure.

If your concrete has cracked, we’ve outlined what you need to do in our technical article: Has your concrete cracked?

How to install concrete expansion joints

You can install concrete expansion joint filler before or after the concrete is laid. In the first instance, a flexible material is inserted along the length of the joint before the concrete is poured. Alternatively, once the concrete has set, grooves can be cut in the concrete. This will control where the concrete will crack, leaving a neat saw cut at the surface. From here, you can add joint materials where required, but care must be taken to ensure the correct depth is achieved.

How to prepare for a concrete pour

Tips for using concrete expansion joint filler to place joints

Below, our experts have shared their top tips for using control joints when laying concrete slabs.

• Place joints around 40 times the slab thickness apart. For a slab which is 100mm thick, the joints should be placed around 4,000mm (4 metres) apart.
• Make sure joints are cut deep enough. They need to be at least a quarter of the thickness of the slab. For a 100mm slab, cut the joints at least 25mm deep.
• Cutting joints after the concrete has been poured? Don’t leave it too long! If the joints aren’t cut within 12 hours after you’ve poured concrete, you’re increasing the risk of it cracking.
• For invisible joints, place them under walls. If you want neat and tidy joints that can’t be seen, putting them under walls means they’ll be hidden from view.
• Joints are most effective when the aspect ratio of the slab is kept to 1:1. For example, a 5m x 5m slab. If the slab is narrower than it is long, it’s possible to increase this to a maximum aspect ratio of 1:1.5, so 2m x 3m for instance. It’s important not to leave the placing of concrete expansion joints to chance, so enlist the services of a structural engineer if you need to place several joints.

If you need further guidance or advice around placing joints or choosing filler for your concrete project, please get in touch.

Help with concrete expansion joints

Planning and placing concrete expansion joints is a complicated business and one you need to give careful consideration to. Creating the right gaps, at the right spacing and filling them with the correct material is vital to protecting the integrity of your concrete drive, slab, or road. The best thing to do if you’re in any doubt is to speak to an expert structural engineer who’ll be able to make sure you get it right. For further information about concrete expansion joints, please fill in the form below.

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