External finished concrete expands and contracts upon wetting and subsequent drying out. This is most significant where large areas of concrete are concerned and in areas where concrete is bounded by other structures such as walls or buildings. If provision is not made for such movement, pressure cracks can appear. These can be deep enough to penetrate through the entire thickness of the concrete, resulting in structural failure.

Movement can be accommodated by the inclusion of expansion joints at 12-metre intervals, sealed with a material that expands and contracts with the concrete but is waterproof, durable and will not allow dirt and grit, and so on, to get into the joint. Similar provision is made where a concrete slab butts up to an existing wall or structure; this type of joint tends to be referred to as an ‘isolation joint’.


Fig. 48 Roll of polyethylene expansion joint foam.

The materials for expansion and isolation joints vary, but the most common in a domestic setting are rolls of polyethylene foam. This comes in various widths to suit different concrete slab thicknesses and is positioned against inside faces of the formwork before the concrete is placed.

Longer-lasting expansion joints for more critical applications can be made with various gun-grade or pourable Polyurethane – or poly-sulphide-type compounds. These have a high modulus of elasticity, so they can stretch and shrink more than normal sealers. The packaging is often labelled with ‘Hi-Mod’ or similar. A 12mm open joint is formed in the concrete by way of a length of softwood or polystyrene foam, which is then removed when the concrete has set. The joint is then filled with the sealer compound as per the manufacturer’s instructions; care must be taken not to smear or stain the concrete surfaces as the sealant is very sticky. During installation, it is advisable to protect the surfaces of the concrete on either side of the joint by masking it off with tape or pieces of timber. Once complete, the treated joints are covered with a board until the compound has cured, to prevent it being affected by foot traffic.