Expansion joint sealant: why you need it, and how to choose it

What is an expansion joint?

An expansion joint is a created joint between two sections of a concrete floor slab, which gets narrower when the panels of the slab expand, due to an increase in ambient temperature (imagine an outside hard-standing in the heat of summer).

This is different from a contraction joint (or ‘control joint’), which opens and gets wider as the concrete contracts while curing.

The terms ‘expansion’ and ‘contraction’ therefore refer to the movement of the concrete, rather than the movement of the joint.

However, the term ‘expansion joint’ is sometimes used in both scenarios – probably because the concrete which expands when it gets warm also contracts when it cools.  The principles of sealing the joint are similar in both cases.

Joints are usually a metal joint (armoured joint) built into the floor during the concrete pour; or a saw-cut joint (induced joint) cut into the finished floor.

Why seal an expansion joint?

• Joint arris protection.  Exposed edges of concrete slabs are vulnerable to damage caused by vehicle wheels passing across the joint, particularly at 90⁰, and particularly when the wheels are small and hard.  Repeated traffic can cause the joint arris to spall, which makes it harder for vehicles to cross smoothly and safely, and causes premature vehicle damage.
• To prevent chemical infiltration.  If there is a risk of chemicals permeating into the ground through the joint gap, then sealing it is essential.
• Health & safety.  Open joints can attract dirt and debris, which in some buildings can be unhygienic and unattractive.  Unsealed joints can also be a hazard for pedestrians – eg, in DIY stores where customers may be wearing high heeled shoes, or sandals, or there may be children whose small feet could trip on the joint.

When should you use expansion joint sealant?

The nature of a joint is that its width will change with movement in the concrete.  When a floor is freshly poured, the joint will initially be narrow, but will increase in width over several weeks and months as the concrete continues to cure and contract.

However, it is wise to seal the joint as soon as possible, as unprotected joints will be subject to the risk of damage as soon as the building is brought into use.

This means that you may need to seal a joint more than once during its lifetime, initially with a narrow flexible seal that will expand as the joint opens, and then with a harder permanent joint sealant once the joint has widened.

For true expansion joints, which will widen and shrink due to heat movement, ensure that the sealant is flexible enough to accommodate the expected joint width changes.  If the expansion joint is being used externally (which is likely), ensure the joint sealant is suitable for external use.

How should you seal an expansion joint?

Concrete floor joints can be deep, and it is not necessary to fill the whole depth of a joint with sealant.  Where the joint is wide enough, push a foam backer rod into the joint first, to provide a solid base on which to place the sealant, to stop the sealant falling down into the joint and being wasted.

Some joint sealants are ‘gun-grade’, supplied in easy-to-use cartridges (eg, Roc Joint PU920/925).  Others are pre-formed PVC profiles which can be pushed or lightly hammered into the joint gap (eg, Roc Plast 55).  Others are resins which can be poured into joints (eg, Roc Joint PU12).

For more information on our expansion joint sealants, please browse our Joint Sealants section, or ask us for guidance.