When installing a concrete base, screed, or applying a Cemcrete coating in your home or project, it is of upmost importance to remember that there are some rules of nature that apply.
Although concrete is revered for its impressive hardness and high compressive strength, it is weak in tension (it does not stretch or bend without breaking). However, it does still move (it shrinks and expands) and will need space to do so. If it is tied to another structure, or even to itself, these movements create restraint which causes tensile forces and invariably leads to cracking.
In order to alleviate this restraint, the concrete needs to be allowed to freely shrink as it dries (this is referred to as internal restraint; when one part of a slab shrinks more than another, or shrinks in a different direction), or expand and contract with temperature changes. This is where joints come in…
There are various types of joints that are used in the construction of concrete slabs and screeds. Construction joints are placed at the very beginning stages of the building process when the concrete base is being poured. The slab very rarely can be placed / poured all at once, and thus joints are used to separate each section from each other. Expansion and isolation joints are used to allow independent movement between adjoining structures.
Cemcrete overlays (such as RenoCrete and CreteCote) can be installed as a seamless finish, however joints are still absolutely essential in the substrate below and should be honoured straight through to the final decorative finish. If not honoured and the decorative cement coating is applied over the joint, a crack may form in its place.
A control joint can also be referred to as a “planned crack”, that will allow for movement in the screed / topping caused by temperature changes and drying shrinkage. Therefore, if the screed does crack, you will want to have an active role in deciding where the screed will crack and allow the cracks to form in a straight line that you have created with a joint, rather than having cracks forming at random. This type of joint will be crucial for the successful application of Cemcrete’s Colour Hardener. Control joints are recommended in areas such as the narrowing of a passage or between two columns. They can also be used to section off floor panels to a size the application team can manage within a specific time frame.
Proper timing and depth of the control cut is essential. If you wait too long, the slab will crack where it wants to rather than where you want it to. And if the joint is not cut deep enough it will not create the plane of weakness needed to guide the crack.
Joints don’t need to be plain and boring; they can be created with other decorative materials for different styles and aesthetics, such as wooden, copper, tile, brass or other inlays.
Take the time to plan these joints correctly, and the end result will be worth it.