Where to Draw the Line
Have you ever wondered why, amidst the beautiful stamp design of a concrete floor, there are long straight lines spaced somewhat randomly across the concrete? It is not so obvious if the design is made up of squares and rectangles. But if the slab is supposed to look like round stones or some other shape that involves curves, a line cutting through the surface can be somewhat confusing.
Don’t worry those lines aren’t random. They weren’t added willy nilly. They play a very important role. If your concrete contractor didn’t explain it and you didn’t have the guts to ask ‘what the heck are those for’, now is your chance to find out.
The Short Answer
The lines are called Control Joints and Expansion Joints. They are the most widely used method to control cracking in concrete. Contractors place them in the concrete surface at predetermined locations to create weakened planes where the concrete can crack in a straight line or beneath the surface.
The Control Joint
A control joint is used to reduce the occurrence of shrinkage-related cracking. A fresh concrete mixture is a fluid mass, as it hardens it shrinks because the water in the concrete is evaporating. As the water evaporates, it leaves behind large voids between the solid particles. These empty spaces make the concrete weaker and more prone to cracking. These cracks almost always happen at reentrant corners (corners that point into the slab) or with circular objects in the middle of a slab (pipes, plumbing fixtures, drains, and manholes). Since concrete cannot shrink around a corner, stress will cause the concrete to crack from the point of that corner.
The control joints placed in the concrete will open up as the concrete slab gets smaller allowing the shrinkage pressure to spread through the joint.
The Expansion Joint
Heat causes concrete to expand. When concrete expands, it pushes against anything in its way (a brick wall or adjacent slab for example). When neither has the ability to flex, the expanding force can be enough to cause concrete to crack.
Expansion joints are used as a point of separation (or isolation), between other static surfaces. Typically made of a compressible material, expansion joints must act as shock absorbers to relieve the stress that expansion puts on concrete and prevent cracking. They are placed before the concrete is poured, extend the depth of the slab and stretch its full width.