Are you working with concrete in your new build or renovations? Do you want to minimise random cracking that affects most concrete structures? If so, you must consider and plan for concrete cutting of expansion joints. Even more important, one has to consider the ‘when’ of cutting control joints to ensure they effectively serve their purpose. It is not just a matter of taking a diamond saw blade to the concrete and making cuts. You ought to consider the time of cutting appropriately.
With this in mind, let’s explore how long you should let the concrete set before cutting.
What Are Control Joints?
To understand control joints and appreciate their importance, it’s critical to understand the nature of concrete. Wet concrete is usually fluid and easy to mould into any shape necessary. However, as the concrete starts to dry and cure, it’s subjected to enormous tensile forces. While concrete has enormous compressive strength, its tensile strength is about 8% to 12% of the compressive strength.
Concrete expansion joints are effectively planned cracking. The cuts give room for the slab of concrete to expand without developing random cracks that would otherwise develop.
Considerations Of Concrete Sawing Timing
There is no universal specific waiting time for concrete to cure to begin cutting. In practice, experts use a range of 6 to 18 hours. That’s because numerous elements influence the setting and curing process. Such elements include:
#1. The concrete mix design – Concrete designs that use curing accelerants tend to set and cure much faster than concrete mixes without. You can cut such concrete much earlier than a typical concrete slab.
#2. The prevailing weather conditions – Hot ambient temperature on a windy day allow concrete to cure faster than on a cool and still day.
#3. The moisture content of the concrete – The higher the moisture content, the longer it takes to set and cure.
The Timing For Concrete Cuts
The timing of concrete cutting will depend on the elements discussed above. Typically, the actual cuts should be made within 4 hours to 18 hours. In most cases, cutting concrete after 24 hours will produce the very same uncontrolled cracks that you’re trying to avoid by making expansion joints.
You have to time the cutting perfectly. The concrete should be cured enough that it does not ravel or spall. Ravelling happens when the concrete is not cured enough that the blade actually disturbs the aggregate materials rather than cutting through, resulting in a jagged edge. A jagged edge results in weak concrete. Moreover, the jagged edge makes resealing the expansion joint a tad more difficult.
However, when you wait too long, the concrete slab will have already experienced shrinkage stresses that create cracks. Consequently, the concrete will have developed numerous cracks due to being placed under enormous tensile pressure.
As such, waiting too long gives the concrete slab enough time for random shrinkage cracks to develop, rendering the expansion joints you create useless.
To determine the perfect timing to make cuts, you should establish a test area where you make test cuts. Only begin cutting the concrete as soon as the ravelling stops, i.e., the cut edge is straight and smooth.
The Type Of Blade Used In Concrete Sawing
While you must wait for the concrete to cure enough for the blade to make smooth and straight cuts, some blades allow contractors to work much earlier. Diamond saw blades, especially the wet blades that use sprayed water as a lubricant and coolant, require the concrete to be dry. Green blades can work on wetter concrete than the typical diamond saw blades. The only catch is that the blade suffers more wear and tear, making the process more expensive overall.
Note: While the timing of the sawing is a critical consideration, there are plenty of other considerations to have in mind when making expansion joints. For instance, you need to consider the depth of the cuts, the spacing of the cuts, and the cutting technique.
The best way you can determine how you need to wait and, therefore, when you should cut a slab of concrete is to make test cuts in an inconspicuous part of the slab.