Expansion joints explained: movement, sealants and joint systems

01Why buildings need joints

Long slabs and roofs expand in the heat of the day and contract at night; concrete shrinks as it dries; different parts of a building settle and deflect differently. Without joints, that movement becomes cracks in random places; joints concentrate it in planned lines — which then have to be sealed against water.

• Expansion and isolation jointsSeparate adjoining structures or elements so each can move on its own.
• Control (contraction) jointsSet where shrinkage cracking happens, in a straight line.
• Construction jointsStops between pours, not for movement.

02Anatomy of a sealed joint

Joint faces

Sound, clean, dry concrete edges, primed where the sealant needs it.

Joint filler

A compressible board that forms the gap in the pour.

Backer rod

A closed-cell foam rod that sets the sealant depth and gives it a rounded back.

Sealant

An elastomer bonded to the two faces — polyurethane, polysulfide or silicone, chosen for movement, exposure and chemicals.

Joint waterproofing tape

A flexible strip across the joint that keeps the waterproofing system continuous.

Cover plate or profile

Protects the joint where there is traffic.

03Two-sided adhesion, not three

A sealant can stretch only if it is free to change shape. Bonded to the two faces and to the bottom of the joint as well, it cannot — and it tears at the first opening. A backer rod or bond-breaker tape under the sealant prevents that third bond. The sealant depth is set relative to the joint width by the manufacturer’s rules: too deep and it cannot flex, too thin and it tears.

04Choosing the sealant

05Joints in traffic areas

In car parks and podiums, wheels load the joint edges and the seal. Joints there use edge protection, elastomeric or metal cover profiles and a waterproofing tape beneath, designed for both the movement and the traffic.

06How joints fail

• Adhesive failureThe sealant peels off a face: dirty, damp or unprimed edges.
• Cohesive failureThe sealant tears through its body: a joint too narrow for the movement, or three-sided adhesion.
• Hardening and crackingWith age and UV the material loses its elasticity.
• Edge spallingConcrete edges break under traffic when they are not protected.
• Broken tape continuityAt the ends and junctions of joints.