Joints Used in Floor Construction

Construction Joints

• A horizontal construction joint, commonly called a cold joint, can take the form of a keyed joint or a doweled joint. A keyed joint is a method of fitting two sections of the concrete floor slab together like a jigsaw puzzle. A doweled joint is where the two sections of concrete are separated by a small space but joined by a reinforcing steel rod inserted into both to link them. The space allows the slabs to expand and contract without cracking under the pressure, and the steel dowels ensure even distribution of weight across both sections of slab.

Expansion Joints

• In expansion or isolation joints, a resilient material such as foam rubber, plastic or cork is inserted between the concrete flooring and other parts of the building structure, such as walls, driveways, poles or other obstructions. This gives the concrete floor section space for expansion without creating pressure against the solid obstruction. This prevents the cracking that occurs when slab movements are restrained in any way.

Contraction Joints

• A contraction joint, also called a dummy joint, is a gap created in the concrete after pouring it. The gap is located at a carefully selected point, which enables it to expand and contract without causing the pressure that will make it crack randomly. To create the gap, a plastic strip called a zip-strip is inserted between two sections of concrete and removed after the concrete hardens. Alternatively, grooves are made in the partially dried concrete with a metal tool. This makes the concrete thinner and weaker in those areas, enabling it to expand without encountering resistance.

Sawed Joints

• A sawed joint is a contraction joint cut into the concrete with a power circular saw. These joints have sharp, straight edges, as opposed to the grooved edges of contraction joints made with a grooving tool. The cuts are made using a saw fitted with a reinforced abrasive blade and flowing water to cool the concrete while it is cut. Builders commonly cut the joint again before the concrete hardens completely to ensure that it has not bonded during the curing process.