While fresh concrete is easily moulded into just about any shape due to its fluid and plastic nature, hardened concrete is known to crack easily.
This is due to the level of shrinkage involved in the process as well as due to temperature fluctuations.
These cracks, when left unattended, can be unsightly and, where deep and wide enough, can adversely affect the load-bearing capabilities of the cured concrete surface.
Control joints can be defined as a form of intentional cracks made in the concrete to accommodate any changes in the volume of the concrete resulting from drying shrinkage or fluctuations in temperature.
These control joints make it possible for contractors to control where cracking will occur, if it does, as well as its orientation – in a straight line and not at random.
To have any success in preventing the occurrence of random cracks in concrete by cutting control joints it’s important to understand the following:
It is important to cut control joints into concrete at the right time.
Sawing control joints into the concrete after cracks have started appearing is a waste of time as the damage is already done.
To ensure that cracks do not start appearing before the control joints are sawed into place, it is important to avoid delaying the process unnecessarily.
The best time to start cutting control joints into concrete is after the poured concrete has dried enough for you to saw through without dislodging or ravelling the particles of the poured mixture.
In most cases, control joints should be cut into concrete after it has had 6 to 18 hours of drying time.
This process should never be delayed for more than 24 hours.
It is important to note that the nature of the concrete mixture, as well as the prevailing temperature, have an influence on how long concrete takes to dry.
That is why the timing of the concrete control joint cutting process is crucial.
Control joints should be properly placed all over the concrete surface to have any real chance of preventing the occurrence of random and unnecessary cracks.
The length, measured in feet, of the space separating control joints, should be about twice or thrice the depth of the concrete slab, measured in inches.
This simply means that for a standard 4-inch deep slab, a control joint should be cut after every 8 to 12 feet.
In terms of depth, concrete joints should be a quarter of the slab thickness.
Again, using the standard thickness of a concrete slab, 4 inches, the cut made should be one inch deep.
Any deeper and you might end up weakening the slab; on the other hand, making a shallower cut may mean that it does not have the ability to control cracking.
When planning where to cut control joints in concrete, it is also important to ensure that they are properly placed to prevent entrant corners.
Entrant corners usually create areas of weakness on the concrete slab.
To ensure that the concrete cutting professional does a perfect job, contractors should clearly mark the concrete surface, if possible.
Concrete saws are effectively used to accurately cut control joints into concrete slabs once the poured concrete has had enough time to dry.
This is to prevent the saw blade from chipping away the edge abutting it.
When dealing with freshly poured concrete, grooving tools are used to cut control joints.
Concrete usually cracks as it dries, due to shrinkage.
Control joints are used to ensure that these cracks do not affect the appearance or strength of concrete slabs.
These must, however, be properly planned and made to achieve the desired results.