If your new to laying concrete or it’s been a while since you’ve laid your last slab you might need a refresher on what control, contraction and a concrete expansion joint are. You also will need to know if they are needed, where they are needed and when and how to make them. Getting familiar with these joints will ensure the integrity of your slab and protect the surrounding structures along with ensuring any unsightly cracks occur where you want them.
Why does concrete crack?
Concrete, like any material expands when it gets hotter and contracts (shrinks) when gets cooler. Also, when concrete is freshly laid it is full of water. As this water dries out and the concrete hardens the concrete will also shrink. As a masonry material, concrete doesn’t handle this expanding and contracting very well and it cracks. To prevent this special joints need to placed in the concrete to allow the concrete to expand and contract and also control the points where the concrete will inevitably crack.
What is a concrete expansion joint?
A concrete expansion joint is used to prevent a concrete slab from impacting surrounding structures when it expands or contracts. These are also known as isolation joints as they completely separate a section of concrete from other slabs of concrete or surrounding structures such as walls and posts.
You often see a noticeable concrete expansion joint on pathways where some rubber or tar is placed in between longer sections of path. Expansion joints can also be seen on bridges where metal teeth are used to fill the gap between the bridge and the road slabs.
What is a concrete contraction joint?
A contraction joint is a cut made part way through a slab of freshly poured concrete at regular intervals. This cut is designed to control where cracks occur during both the setting of the concrete and as the concrete contracts when gets cold. These are also known as control joints. The purpose of making the cut partially through is to create a point of weakness so that the concrete will crack in a specified area rather than a jagged unsightly creak in a prominent position.
Contraction joints can be seen as smaller lines on pathways in between the expansion joints.
The remainder of this article will focus on the cutting of control joints.
How to cut control joints in freshly poured concrete
Control joints are needed at regular intervals to ensure cracks will not appear in between control joints. The distance between joints need to be made no more than 25 times the thickness of the slab. So if the slab is 10cm thick the control joints need to be made every 2.5 metres.
On any sections with inside corners (also known as re-entrant corners) control joints should be added on these known stress points. See below:
The depth of a control joint is usually a quarter of the slabs thickness. So the same 10cm slab should have a control joint cut which is 2.5cm deep.
When to make the cuts
Cuts should be within 12 hours of the concrete being poured otherwise it risks cracking.
How to make the control joint cuts
The quarter thickness cuts in the slab can be made in freshly poured concrete using a grooving tool. However, a concrete saw can be used when the concrete is hard enough that is won’t disturb the concrete surface, usually after 6 hours.
Getting help making contraction or control joints in concrete
Making sure your control joints are cut right often requires a professional concrete cutter to do the job right the first time.
Mastercut Concrete have been cutting concrete in Sydney for over 20 years and have a fleet of skilled and mobile concrete cutters to cut the necessary contraction and control joints in your concrete slab soon after pouring. Our staff are equipped with a range of top quality machinery and hardware perfect for the job. Contact 02 9755 5505 for a free consultation and quote or complete an online quote request.