Spacing of control joints in slabs?

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Author Message David G. Axt, CCS, CSI ,SCIP Senior Member Username: david_axt Post Number: 1906 Registered: 03-2002 Posted on Thursday, April 07, 2022 - 11:38 am:    It seems the spacing of (crack) control joints in concrete slabs is more of an art than a science. I cannot find any definitive standard (ASTM or ACI) that indicate spacing. The best I can find is an article from PCA: https://www.cement.org/learn/concrete-technology/concrete-construction/contraction-control-joints-in-concrete-flatwork#:~:text=Joints%20are%20commonly%20spaced%20at What do you indicate in your specifications? David G. Axt, CCS, CSI, SCIP Specifications Consultant Axt Consulting LLC Nathan Woods, CSI, CCCA, LEED AP Senior Member Username: nwoods Post Number: 851 Registered: 08-2005 Posted on Thursday, April 07, 2022 - 12:01 pm:    I typically have 5" thick, 4,000 PSI slabs, and specify joints at 12 to 15 feet on center. The spacing is less critical than the shape. If your joint pattern results in a rectangle, shrinkage cracking will turn it into a square. Its much better to have as square a shape as possible. Another important factor is how you relive corners. I add joints at ALL reentrant corners, and where there are steps in the slab, or other major influences on how the slab will move as it cures. I also am very specific about controlling moisture in the slab. I do not allow the old fashioned method of 2" of sand under the slab. Instead, I cast the slab directly against the vapor barrier, and I minimize the water/cement ratio to .45 typically. There are additional controls related to surface treatments to retard the curing process to reduce curling, but that is all pretty typical stuff. Phil Kabza Senior Member Username: phil_kabza Post Number: 702 Registered: 12-2002 Posted on Thursday, April 07, 2022 - 05:33 pm:    What Nathan said. Phil Kabza FCSI CCS AIA SpecGuy Specifications Consultants www.SpecGuy.com phil@specguy.com J. Peter Jordan Senior Member Username: jpjordan Post Number: 1176 Registered: 05-2004 Posted on Friday, April 08, 2022 - 11:08 am:    I think Mr. Woods' comments are on target, especially with regard to configuration. I would add, however, that the spacing depends on the mix design, especially with water to cement ratio. Incidentally, these comments about configuration apply to portland cement plaster (it's concrete with finer aggregate). J. Peter Jordan, FCSI, AIA, CCS, LEED AP, SCIP David G. Axt, CCS, CSI ,SCIP Senior Member Username: david_axt Post Number: 1907 Registered: 03-2002 Posted on Monday, April 11, 2022 - 02:27 pm:    Do you have any specific standards that I can put in my specifications? David G. Axt, CCS, CSI, SCIP Specifications Consultant Axt Consulting LLC Nathan Woods, CSI, CCCA, LEED AP Senior Member Username: nwoods Post Number: 852 Registered: 08-2005 Posted on Monday, April 11, 2022 - 03:02 pm:    David, maximum joint spacing is almost always in the structural general notes, usually sheet S0.1 or similar. If you are looking for an ACI guideline, ACI 302.1 section 8.3.12 Saw-cut joints talks at when (timing related to curing status), method, and how deep to cut, but is silent on spacing. Nathan Woods, CSI, CCCA, LEED AP Senior Member Username: nwoods Post Number: 853 Registered: 08-2005 Posted on Monday, April 11, 2022 - 03:05 pm:    I should note, I am a big proponent of "early-entry" sawcutting, where you get on the slab in the first few hours and put in the saw cuts PRIOR to the concrete shrinkage process as the water bleeds out of the slab during the curing process. The proprietary term for this is "Soff-Cut" and requires a specific type of blade and cutting tool. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 972 Registered: 10-2007 Posted on Tuesday, April 12, 2022 - 06:23 pm:    I have found it difficult to layout crack control joints that meet the proposed criteria. In addition, if the slab reinforcing is continuous at the joint, it prevents the joint from opening up. As a result, I have adopted an alternate approach. Since concrete inevitably has cracks the secret is not to eliminate them but rather to make sure the cracks are small, so they are not objectionable. To do this I have typically used a 5" slab with #5 reinforcing at 12" OC EW. The reinforcing limits the size of any cracks. In addition, the concrete mix should use a water reducing admixture to help minimize the amount of water. For slabs on grade, I specify a low slump. I am not a fan of w/c ratio since that often results in more cement and more water which results in more shrinkage. I can hear the objections about the amount of reinforcement but in my experience, I had no push back and I have not received reports of undesirable cracks. Make your life easier.