Cutting Contraction Joints.

Although it is impossible to guarantee against cracking, the chance of random uncontrolled cracks is greatly reduced by the introduction of contraction joints. The correct timings, locations and spacings of these joints are vitally important to ensure that you get the most benefit from any joints that are introduced.

Contraction Joints (or Crack Control Joints) are cut into the concrete, in straight lines. Wherever it can be done, they will be cut along the natural line of the pattern. Joints are cut as soon as possible, but in any instance within 24 hours or sooner…..the sooner the better. (There will be exceptions to this in very cold conditions when the cuts may damage the surface). The concrete is cut to between 1⁄3 and 1⁄2 of slab depth. Never full slab depth.

Saw cut contraction joints will always be used most often, however there are instances when the concrete is very likely to crack before the saw cut is introduced (even if it is cut within 24hours). In these instances, soft cuts can be introduced. These are cut into the concrete either before it’s coloured, (using a groover), or during the printing process (using a cutting bar). Although these types of joints won’t normally be used there are instances when a slab will be likely to crack before a normal saw cut is made, unless a soft cut is introduced.

Generally, whether using saw cuts or soft cuts, based on a min 100mm thick concrete slab with fibre reinforcement, the guide for joint placement is:

• Wherever practical, install a joint from a high stress area such as a corner or a restriction.
• The maximum allowable distance between joints is twice the width of the slab. i.e. a 2:1 length to width ratio.
• No individual piece of concrete, when cut, should exceed 20m2.

Note:

Be aware that although 1,2 or 3 metre wide areas would require a joint at least every 2, 4 or 6 metres respectively, this does not apply to a 4m wide slab, since based on the above 2:1 rule it would be every 8m and since 4m x 8m = 32m2, this would then exceed the 20m2 rule. Therefore a joint would need to be cut at every 5m (not 8m) so that the 20m2 rule is not broken, and since it’s all about reducing the chance of cracking, on a 4m wide area, Dr Concrete would cut a joint in every 4m. This would not adversely affect the aesthetics of the job, however it would help to reduce the chance of cracking yet further since the slab shape would be square and slab size would be under 20m2.

Simple eh?! Corners & restrictions + Max 2:1 + No bigger than 20m2.

Next Dr Concrete will be a few tips on how to further reduce the chance of uncontrolled random cracking.

Dr Concrete