Control joints in shop slab??

[Tractor Seabee]

Tractor Seabee,
Your post was informative thank you for posting that. Hopefully cracks will be "controlled" in my slab, if not oh well it was a pretty cheap chance to take. Since you seem to know a lot about concrete. What is your suggestion for a house slab? Overkill rebar. post tension with several beams? Several people tell me a lot of rebar is the way to go and several people have said post tension, I like the theory behind post tension. All these different opinions have made this a difficult decision? What would you suggest?

Use an engineered solution for re-bar in the foundation. Go with fiber reinforced 4,000# concrete for the floor and cut those control joints. All those "good ol boy" (the guys who say "don't tell me how to do concrete, I've been doing it for 40 years) will only cost you money with no value added. Where you live beware of hot weather curing requirements, letting the top dry out fast will give you a not desirable end product. Make sure spray membranes are compatible with floor covering adhesives. Moisture migration from below can be a problem but probably not in Texas. If you do do use moisture barrier use at least 10 mil plastic. thinner will probably be destroyed during concrete placement.

Retirement has dulled my brain on the name of the ACI and Concrete Contractors Association literature and I just packed it all away in temp storage while moving. I will rack my brain some more.

Ron

 

[Tractor Seabee]

The soft cut or green cut saw really works. But around here not to many contractors have one set up. I wonder if it is because they don't want to wait the extra time for the concrete to cure enough that the wheels on the saw don't leave a mark.

Probably 2 reasons: The saws cost a lot more and do not get enough hours on them for contractors doing residential, light commercial work, (they should be able to rent one) and they are one of those non-believers in modern technology. Could be a combo of both. Avoid those guts even though they are cheap. There really is not a lot of wait time and doing now avoids a return trip which is useless anyway. Remind them about who is paying the bill.

Ron

 

[Tractor Seabee]

Definitely saw in control joints. For a 4" slab the max spacing would be 8' oc with 3/4" or smaller aggregate and 10' oc for larger than 3/4". The spacing changes because the larger aggregate will shrink less. The joints should be cut 1/4 of the slab thickness or 1" in this case. The joints should come off any reentrant corners because there will be stress concentrations at the corners.

Because of the expansive soils in Texas many will do post tensioned in that region. I will say that making modifications to a post tensioned slab is very difficult. If you have plans to remodel a kitchen or add onto the building, I'd suggest going with conventional reinforcement.

If you have expansive soils, be sure to have the contractors grout the gravel filled trenches. This will prevent the water in the gravel from running under the building and causing the soil to swell.

Regarding the ability to open up the spacings or even omit the joints... with the modern macro fibers I'm seeing some incredible joint spacings. Just did a 8" slab with a heavy dose of macrofiber, 3,000psi (lower strength shrinks less), water reducer and 1.5" aggregate and those joints were set at 60' oc!

Cord, you would probably find it well worth the money to attend the World of Concrete and take in some of those seminars that discuss current technology in concrete slabs on grade. Yes I have seen the large slabs you discuss and that proves the new ideas have merit; but, I doubt the contractor doing the OPs work would ever get there, much less pay for the engineering that proceeds such work. First place, he would not have the proper equipment or even know what to rent or operate it properly. Engineers design concrete not Architects; normally, there are exceptions due to specialty training.such as ACI standards which are now in the structural codes by reference.

Ron

 

[Redlands Okie]

Post tension floors are a REALLY great product. One bad thing on post tension is your pretty well eliminating any penetration of any kind in the slab in the feature for risk of damaging the cables.
Lots of rebar works fine also.

 

[kevin37b]

Tomorrow is too late for cutting expansion joints. The concrete will have already begun to crack but it will be months before you see it. Tomorrow's cut lines will help some, but 2 down the middle will mostly be for aesthetics. They should have been cut early this morning. I would have cut your floor into approximately 12' squares.

Exactly , needs to be softcut as soon as you can walk on it .

 

[Tractor Seabee]

For those needing a good reference the following link will get the Contractor's Guide to Quality Concrete Construction. It is the Concrete Contractor's Bible by their industry association and endorsed by ACI. There is also a Supplement to this third edition not available on Amazon but direct from their Organization. ACCA and ACI both have on-line libraries on their websites.

Ron