Benefits of Cutting Joints in Concrete Slabs Following Placement
The American Concrete Institute (ACI) publishes recommendations on proper saw-cutting of contraction joints in flatwork (ACI 360R-06). This includes cutting with conventional wet-cut walk-behinds saws, dry-cutting with walk-behind saws and the use of early entry saws as the best method for control of random cracks.
Saw-cutting joints has become the norm for most concrete slabs for both residential and commercial floors. Some of the reasons for the adoption of saw-cutting joints are:
Saw-cutting joints has become the norm for most concrete slabs for both residential and commercial floors. Some of the reasons for the adoption of saw-cutting joints are:
- Saw cuts are often less noticeable on the final floor, creating a more attractive surface.
- Saw cut joints leave a smoother transition between slab panels, eliminating a potential trip hazard.
- They leave a better riding surface for forklifts on industrial floors.
- Saw cut control joints are easier to maintain, as they are thinner and require less joint filling.
Early Entry Saws
The real advantage to these saws is concrete crack control. Ultra Early Entry™ saws allow contractors to cut control joints before stress fractures begin to form in the concrete and direct any cracking to the joint. This process is often specified by architects or engineers, and particularly in critical applications and super-flat floors. These saws can be used as soon as the concrete is hard enough to walk on, which also increases production and eliminates the need to come back the next day.
Walk-Behind Saws
Conventional walk-behind saws can be used to cut control joints using either wet or dry cutting methods, and cutting typically takes place between 4 and 12 hours after slab finishing. This method may require deeper cuts to meet design specifications and may result in cracks that run away at an angle from the saw kerf. One downside from conventional cutting versus early entry is cleanup. Slurry created by wet cutting may require special handling to meet environmental regulations for disposal, and dry cutting may create hazardous silica dust. Most conventional saws do not have sufficient dust collection ports to meet the Occupational Safety and Health Administration compliance requirements.
Common Tips to Increase Production and Quality
Early Entry Saw Cutting
- Know your slab thickness:
Control joints cut with conventional saws are recommended to be cut to a depth of at least 1/4 of the slab thickness. Early entry can typically be cut to a 1" depth for slabs up to 9" thick. This may be a significant advantage in cutting speed, but for slabs that will require joint filler, this may considerably reduce the amount of filler needed. - Watch your setting times:
Since you’re dealing with fresh concrete, often placed outdoors, timing is everything when scheduling your early entry saw cut of a control joint. Technical experts, such as the Portland Cement Association (PCA), offer a more descriptive guideline - “joints should be sawed as soon as the concrete will withstand the energy of sawing without raveling or dislodging aggregate particles.”
For most job conditions, sawing occurs about 5 hours following the initial placement. For a more practical approach, many contractors rely on the “heel” test. They place their boot on the slab, then wait to begin their saw cut when they don’t see the impression on the concrete left by their boot heel. - Check blade alignment:
After you take the saw off the trailer and before placing it on the slab, make sure the blade is aligned in the skid plate slot. A misaligned blade can cut into the skid plate causing damage to the machine and creating a joint not perpendicular to the slab’s surface. - Check used blades:
Inspect used blades to make sure none of the diamond segments are overheated and glazing over. - Check blade block plungers:
Saws such as the Husqvarna Soff-Cut® Ultra Early Entry™ line use a skid plate to control chipping or spalling of the joint while cutting green concrete. The blade block enclosure works to keep pressure on the skid plate. Plungers should be able to move up and down freely allowing proper pressure of the skid on the concrete surface! - Break in new saw blades:
When first using a new blade, saw at about half of the normal forward speed for 100 lineal feet. This break-in time extends blade life and you’ll get faster cuts in the long run. - A New blade means new skid plate:
Change the skid plate with every new blade. If starting out the day with a used blade and skid, inspect the bottom surface of the skid to make sure there is no damage from the previous job. - Find the right speed for that day:
Each slab will dry differently, so use your experience to find the optimum forward travel speed for that placement. The blade must cut as freely as possible, not putting too much pressure on either side of the joint. On most saws, if you feel a pull to the left, you’re going too slow, and pulling to the right indicates you need to slow down. - Protect your joint:
Don’t walk on saw joints while cutting and keep others off the slab until final set occurs. When cleaning up cuttings, be sure to move in the same direction/path as the joints. Sweeping across the joint could damage the collar. You should use joint protectors at saw joint intersections and at saw drive wheel locations. - Double check the required saw cut depth:
Check with the engineer before saw cutting to ensure that the saw cut depth meets the structural specifications. There could be differing thicknesses not observable from the surface. Several ACI documents suggest that a good rule of thumb is to cut the joints 1/4 to 1/3 the slab thickness. If the joint is too deep, aggregate interlocking will not be sufficient to transfer loads. If the saw cut is too shallow, random cracking might occur.
Using Concrete Saws During Repairs on Slabs and Walls
Concrete saws are also used to remove hardened concrete and masonry to make a repair. You should have the right fleet of ring saws, power cutters, and concrete chainsaws to remove the concrete on all types of projects. When approaching a new saw cut, be prepared.
- Check for any buried rebar or conduit:
Prior to any cut, verify that there are no buried utilities or structural steel embedded in the cutting zone. There are hand-held ground penetrating sensors that can identify materials buried up to 8 to 12” in the slab or wall.
- Adopt the proper cutting technique and select the appropriate concrete saw:
Concrete chainsaws are the right tool for entry cuts such as windows and doors. Walk-behind saw are appropriate for pavement tear out. Ring saws work well when cutting openings in gutters and other below grade repairs. And power cutters allow you to make deep cuts using the step method. - Match the blade to the project:
Determine if you are wet cutting or dry cutting. Match blade diameter to joint depth. Check for reinforcement in the cutting zone. If you discover that the cutting zone is heavily reinforced with large diameter rebar, use blades with soft metal segment bonds. - Start each cut carefully:
Push down to allow the blade to reach the required depth. Then start moving the saw, following the chalk line. For best results, cut in long, steady lines rather than short cuts. - Keep your blades sharp:
Cutting concrete with a dull blade increases the chances of fracturing. A dull blade also causes the saw to work harder, possibly overheating the saw, which can shorten its lifespan. And dull blades increase the amount of silica dust created during sawing. - Protect your blade:
Don’t allow the saw to twist or bind the saw blade. And don’t let the blade spin in the joint or cut, as this will increase wear.