slab on grade and saw cuts

[purdue87]

i am a concrete contractor and have been asked to form and pour a 6" slab on grade for a municipal project where there will be dumpsters (debris and green waste) sitting on the slab. about 1,500sqft. the slab is designed with a 4000psi mix.

the question is: the slab has been designed with a mat of #4 rebar AND saw cuts. i don't think i have ever seen a slab with a mat of rebar that is also saw cut. is this normal? thanks.

 

[frv]

We do it all the time.

 

[Ginger]

I would have thought that saw cuts would be fine provided that the reinforcement is stopped either side of the saw cut. The saw cut is obviously there for stress relief and will generate a crack through the entire depth of the section. If reinforcement passes through the cut area then I would expect to see a protective sealant to prevent water and detritus entering the crack and corroding the reinforcement.

 

[hokie66]

It is indeed normal to saw cut slabs on grade, regardless of the amount of reinforcement, unless the slabs are posttensioned.

The saw cuts are an attempt to force the restraint shrinkage cracking to occur in a straight line rather than randomly.

Having said that, your question is a particularly good one for a slab being used as you describe. There will be a lot of debris on the slabs, and this dirt, grit, whatever, is bound to find its way into the joints unless the joints are well sealed and maintained, which in itself is unlikely.

When the joints are full of debris and the slab gets hot in the summer, enough force can build up to cause the slabs to slide, opening up the joints further, and on and on. If there are only slabs involved, that may not be an issue, but the slabs can move enough to damage perimeter elements. So while that should not be your problem if you do as you are told, it would be worth mentioning to your customer.

 

[LCruiser]

It also depends on whether it's inside or outside. As mentioned, when it's cold detritus will get inside the cracks, then when it's warm the concrete can expand and spall. From what it sounds like, I'd use epoxy coated rebar and forget the sawcuts for longevity.

 

[miecz]

I see no need for saw cuts in a 1500 sq. ft. slab with #4s. I think they do more harm than good, as the #4s would distribute the shrinkage to many small cracks. The owner should be made aware that the joints will need constant maintanence.

 

[dik]

Sawcutting time is critical... should be 6 to 8 hours after finishing slab and earlier if a soft cut type saw is used. Sawcutting is IMHO, more important than the rebar...

Dik

 

[csd72]

The reinforcement is there to try to minimise cracks between the joints.

 

[dik]

csd72... agreed... but the timing and spacing is far more critical than the rebar.

 

[csd72]

True

 

[dochardnose]

Stop the steel 2 inches from the edge of each planned control joint and each construction joint. Install load transfer devices (dowels at construction joints and dowel baskets at control joints). Flat plate or diamond shaped dowels will be best. This will allow horizontal movement but will pick up the loads verically.
Fill the joints with a semi-ridgid epoxy or polyurea based joint sealant.

If the steel is allowed to run through the joints it will add restraint and increase the potential for random cracking. The joints will then become nothing but maintenance issues.
Refer to the American Concrete Institutes document ACI 302.1R-04.Guide to Construction of Slabs.

 

[abusementpark]

We generally only call for the slab to be saw cut when we are using WWF as the main reinforcement. I thought if you have a traditionally reinforced slab, that reinforcement provided will be enough to minimize the temperature and shrinkage cracking or keep the cracks small??