What are Control Joints in Concrete?
- Unfortunately, due to shrinkage that takes place when concrete cures, cracks in concrete can only be controlled and not prevented. Control joints are the lines that are cut or tooled into the concrete when the concrete is placed to aid in controlling this tendency to crack by creating weakened planes within the slab. A well thought out control joint layout is important to prevent visible cracks throughout a concrete slab, by identifying problem areas before the concrete hardens.
Control Joint Depth:
- Control joints are cut or tooled a quarter of the slab thickness as shown in figure below. For example, if the slab is 4” thick, the control joint will be 1” deep. By creating this weak point within the slab, the concrete will tend to crack at the joint rather than at the surface in undesirable locations.
Control Joint Spacing:
- As a general rule of thumb, control joints should be spaced at two to three times the thickness of the slab in feet. For example, if you have a slab that is 4 inches thick, you would want to space your control joints at approximately 8 to 12 feet on center.
Creating Control Joints (Tool vs Sawcut):
- There are two ways to create control joints: sawcutting & tooling.
- Sawcutting is performed after the concrete has hardened enough to walk on safely. A saw is used to cut the joints into the concrete and you are left with a sharp clean joint. With sawcut control joints, it is very important for your concrete contractor to accurately judge when it is the correct time to sawcut the concrete. Too soon and the concrete will chip, along the newly cut joint. Too long and the concrete will begin to form shrinkage cracks in undesirable locations. As a general guideline, the concrete should harden to the point that you can no longer leave an impression with your fingernail but no longer than 24 hours after the pour. It is important to note that this timing can vary depending on the outside temperature, the location of the concrete (shaded or sunny), and the mix design used.
- Tooling is performed by running a grooving tool through the concrete while the concrete is still wet and results in a wider joint as shown in the figure above. Tooled joints are more commonly found in sidewalks, driveways, and exterior concrete projects.
- Overall, selecting tooled or sawcut control joints is a preference of ease and aesthetics.
Control Joint Layout:
- When planning your control joint layout, be sure to work with an experienced concrete contractor, architect, or engineer. A poorly planned control joint layout leads to unwanted concrete cracking and an unprofessional finish. Careful consideration must be taken to balance the aesthetic look you want to achieve with a functional layout that minimizes cracking in the concrete slab. For example, a couple of problematic areas that often lead to concrete cracking are sharp corners and embeds such as drains.