I am having concrete poured tomorrow on floor of pole barn. It is a 30 x 40 ft building with a 10X40 ft lean-to shed. Two 10 ft wide garage doors are located on gable end approach. Any thoughts on control joint placement? 5 inch thick in main building and 4inch thick in lean-to shed. Heavy steel sheets and rebar added at garage door openings. Any suggestions on control joint cuts. Mason will be making them Saturday am (day after the pour). Thanks in advance.
Slab control cut layout
- Thread starter WindyHill
- Start date
ConCretin
Well-known member
I got a little lazy and cut and pasted this from my Guide to Floor Slabs in the link below. Probably more info than you wanted; If you want to skip it, just cut your slab(s) into 10x 10 squares.
Crack control - The most common cause of cracking is shrinkage. Concrete shrinks as it 'drys' - as much as 1/2" in 100'. As your slab contracts, it is restrained by the ground, obstructions, reinforcing etc., which causes tension. Since this occurs when the concrete's inherent tensile strength is very low, cracks often result.
There are things you can do to control shrinkage. The most important is to limit slump. More water means more volume will be lost when the concrete dries and the more it will shrink. Placing in cooler temps, fibermesh, increasing aggregate size and proper curing will help but there is a practical limit to how big a slab can be built before shrinkage cracks become inevitable
Control joints (CJs) prevent ugly cracks you can see by creating a weak spot for the slab to crack where you can't. CJ's can be tooled into the slab while the concrete is plastic or saw cut after it hardens. Both should be cut at least 1/4 of the slab depth.
By definition, tooled joints are done early in the placement but it's vital that saw cuts be done in a timely manner too. Tension starts growing in your slab quickly as drying shrinkage occurs. If you don’t relieve that tension with CJs, the slab will do it for you in the form of ugly random cracks.
If your finisher will be tooling the joints, make sure his groover is deep enough. He'll have to cut the joints with a pole mounted groover while the concrete is still plastic to move the aggregate aside and re-groove to ensure the joint doesn't fill back in.
If you are cutting your joints, use an early entry or Soff-Cut saw and make your cuts immediately after finishing. These saws have a thin blade that turns into the floor and a plate that presses down on the concrete, which keeps the blade from pulling up the surface.
Plan your CJ layout and provide a sketch to your finisher. Recommendations for CJ spacing start at 8-12' but with some attention to overall crack control, you can spread them out to 15' or more. Keep the panels created by the CJs as square as possible with a maximum ratio of length to width of 1.5 to 1. If you are going to install a lift, most manufactures require a min distance from CJ's to the posts so take this into account.
Re-entrant corners ****. A re-entrant corner is an inside corner that projects into your slab. For example; a square hole creates 4 re-entrant corners. As the concrete shrinks, tension is concentrated at the corner as the slab tries to move in two directions. Cracking is almost inevitable. One obvious solution is to avoid square openings in your slab if possible - a round hole has no corners to crack. About the only other thing you can do is cut a diagonal control joint to the point of the reentrant corner.
When it comes to cracks, think of it this way - use fiber to minimize them, control joints to hide them and/or reinforcing to hold them together.
Crack control - The most common cause of cracking is shrinkage. Concrete shrinks as it 'drys' - as much as 1/2" in 100'. As your slab contracts, it is restrained by the ground, obstructions, reinforcing etc., which causes tension. Since this occurs when the concrete's inherent tensile strength is very low, cracks often result.
There are things you can do to control shrinkage. The most important is to limit slump. More water means more volume will be lost when the concrete dries and the more it will shrink. Placing in cooler temps, fibermesh, increasing aggregate size and proper curing will help but there is a practical limit to how big a slab can be built before shrinkage cracks become inevitable
Control joints (CJs) prevent ugly cracks you can see by creating a weak spot for the slab to crack where you can't. CJ's can be tooled into the slab while the concrete is plastic or saw cut after it hardens. Both should be cut at least 1/4 of the slab depth.
By definition, tooled joints are done early in the placement but it's vital that saw cuts be done in a timely manner too. Tension starts growing in your slab quickly as drying shrinkage occurs. If you don’t relieve that tension with CJs, the slab will do it for you in the form of ugly random cracks.
If your finisher will be tooling the joints, make sure his groover is deep enough. He'll have to cut the joints with a pole mounted groover while the concrete is still plastic to move the aggregate aside and re-groove to ensure the joint doesn't fill back in.
If you are cutting your joints, use an early entry or Soff-Cut saw and make your cuts immediately after finishing. These saws have a thin blade that turns into the floor and a plate that presses down on the concrete, which keeps the blade from pulling up the surface.
Plan your CJ layout and provide a sketch to your finisher. Recommendations for CJ spacing start at 8-12' but with some attention to overall crack control, you can spread them out to 15' or more. Keep the panels created by the CJs as square as possible with a maximum ratio of length to width of 1.5 to 1. If you are going to install a lift, most manufactures require a min distance from CJ's to the posts so take this into account.
Re-entrant corners ****. A re-entrant corner is an inside corner that projects into your slab. For example; a square hole creates 4 re-entrant corners. As the concrete shrinks, tension is concentrated at the corner as the slab tries to move in two directions. Cracking is almost inevitable. One obvious solution is to avoid square openings in your slab if possible - a round hole has no corners to crack. About the only other thing you can do is cut a diagonal control joint to the point of the reentrant corner.
When it comes to cracks, think of it this way - use fiber to minimize them, control joints to hide them and/or reinforcing to hold them together.
Weirth It
Active member
I had a slab poured for my 40' x 40' shop. The guy used concrete that had fiber mixed in and also used steel mesh sheets. Recommended I have control joints cut the next day. We chalked 2-lines running east to west and 2-lines running north to south, each 13'-apart which is perfect since inside dimension is 39' x 39'. Concrete cutter came the next morning and cut 1/8" x 1-1/4" on the chalk lines (chalk lines have to be painted with clear lacquer so they don't wash away). The slab cracked at every point where the control joint meets the stem wall and I am guessing down the entire length of the joint. No other cracks in the slab.
Even though it cost $500 for the concrete cutter, I think it is money well spent. Additionally, my control joint configuration can also be used to play large scale tic-tac-toe haha.
Even though it cost $500 for the concrete cutter, I think it is money well spent. Additionally, my control joint configuration can also be used to play large scale tic-tac-toe haha.