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At what elevation to place reinforcement for a 6" Slab on Grade

At what elevation to place reinforcement for a 6" Slab on Grade

At what elevation to place reinforcement for a 6" Slab on Grade

(OP)
At what elevation  should steel reinforcement be placed in a thin, say 6", slab on grade? This may seem very straightforward, but I have seen different approaches to this. I have some equipment loads on my slab on grade that are significant but not excessive. Say I have a 15' x  9' slab on grade that is 6" thick.  I have turndowns all around the slab.  I only need a single layer of reinforcement, running each way.

Now, I would place the steel reinforcement at the bottom of the slab, with a callout like: #4@12" O.C. BOT. EA. WAY. I also place another piece of reinforcement for the side, bottom and the "angle" of the turndown. A thin strip of the top all around the slab is also reinforced.  That is how I would do it.

Even my structural software says to place it at the bottom.

Now, I have seen many people place the reinforcement at the TOP of the slab, with hooks at both ends that go into the turndowns, and leave the bottom of the slab, and the bottom and the "angle" of the turndown unreinforced.

There are some people who even place the reinforcement in the middle (i.e. Between the top and bottom) of the slab.

Where should the reinforcement rightly be placed?  Does the fact that it is a slab on grade make any difference? That is, soil pressure acting upwards against the slab.  Your comments and feedback, please.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

If there is only one layer of reinforcing, I would place it at the center, considering it is the intention of the reinforcing to enable the span to span any soft spots in the soil matrix where it can see both positive and negative moments.

Mike McCann
MMC Engineering

 

RE: At what elevation to place reinforcement for a 6" Slab on Grade

If it is a true slab on grade, then I would put the reinforcing in the center of the slab.  

In my opinion it is a complete judgment call.  Because it is only 6", your loads must not be extreeme and the reinforcement should be for temp and shrinkage & crack control, not bending.  

If the reinforcement is for bending, then you must meet the clearance requirements set forth for in ACI.  And if it is a slab on earth, you need 3" clear.

Also, it is always a good practice to call out for vapor barrier between the ground and the concrete.  This will help with clearance issue also.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

for exposure, 2" cover on bottom.  For an outside condition, you should have 1 1/2" cover above.  So really we are talking about 1 1/2 inches of play.  Tell the contractor to place it in the middle.  Unless you have decent onsite quality control, the rebar will probably be between 1/2" to 3" from sub grade.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

Most slabs-on-grade are designed as unreinforced sections, at least in my experience. I always specify that slab-on-grade shrinkage/temperature reinforcing be placed 2" clear from the top of slab. This is because you're trying to limit the visible widths of any cracks that may form. For a 6" slab I would use either a 6x6-w2.9xw2.9 welded mesh, or #4@18" o.c. each way. There is no need for hook bars. If you're using mesh, simply let it drape at the edges and provide 1#4 or 1#5 continuous at the bottom of the turndown in both cases.

This particular slab your looking at (15'x9')is small, but the aspect ratio is not good (ratio of length-to-width). I usually limit the aspect ratio to 1.5; yours' is 1.67. I suggest a 1/8"x1 1/2" sawcut at the middle of the 15' length, although the turndowns will help prevent cracking as well.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

2" from top of slab... visable cracks form on top so, best to reinforce in that area... also allows 1-1/2" deep x 1/4" wide sawcut.

Dik

RE: At what elevation to place reinforcement for a 6" Slab on Grade

By convention, middle third of the slab maintaining cover appropriate for the exposure.

Reinforcement in slabs on ground are not designed for flexure unless you are complying with ACI 318.  The posts above are correct that the section is designed as unreinforced and the steel is for T&S, to control the width and distribution of cracks.

In general, the deeper the steel, the deeper the cracks.  But, shallower steel is subject to greater moisture, oxygen, and chemical exposure (where it is exposed to these.)  The goal is to put the reinforcement deep enough to maintain protective cover and shallow enough to provide crack control at the top surface.  Deeper is not better.

If you intend for the slab to act as a flexural element, as in a mat or raft foundation, design it to ACI 318.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

Agree with TXstructural...If you have flexural stress in the slab, design accordingly. If you don't, you have no need to worry about it....the amount of steel for temperature control and shrinkage is insignificant.....it usually doesn't work anyway!  Set the control joint spacing to 12' or so each way and make them cut the joints the same day....they will b!tch about it, but stick to your guns.

For your protection as the design professional, specify the middle third of the slab...don't count on it being placed there, but if you don't have inspection responsibility at least that covers your liability.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

Welded wire reinforcement, or closely-spaced #3 bar, will provide optimum performance for crack control.  Specify and required that the reinforcement be properly supported in place and NEVER allow the reinforcement to be "pulled up", as this is prohibited by code and does not work.  A common approach is to specify that "steel reinforcement be properly supported in accordance with the current edition of CRSI Manual of Standard Practice or CRSI Placing Reinforcing Bars."

RE: At what elevation to place reinforcement for a 6" Slab on Grade

If the slab is supported on grade the place the reinforcement 1-3/4" from the top because that is the surface which you want to restrict the cracking. If you need the slab to distribute heavy loads then some bottom reinforcement will also be required.

RE: At what elevation to place reinforcement for a 6" Slab on Grade

None of 6" with slightly reinforced sog can withstand heavy load where presence of soft spot.
The Boussinesq approach or FEM can be used to estimate the local bending in such situation.  

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