Strange SOG Reinforcing

[msquared48]

Ran into a project on engineered fill from 1998 that has 7.5" and 6" SOG's. The steel is placed in the upper portion of the slab, 2" clear from the TOP of the slab. The 7.5" slab is cracking from the forklift loads. The slabs are both reinforced with #4@16 each way.

I would have expected a double matt of reinforcing here, both for strength and crack control. Has the thinking on slab reinforcing changed, or is this just a blow by the engineer?

I have never heard of placing the steel high in the slab, assumably only for crack control (which is too far apart anyway by my standards), when seeing high loads from forklifts. I am not surprised that the slabs cracked.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[rowingengineer]

Two layers for a slab that thick would be unusual in my local region.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

If it is a slab on grade, the only reason for the reinforcement is for shrinkage crack control. The location within the slab and the amount of reinforcing is subject to argument, but the standard is one layer. You are right...that amount of reinforcement does little good.

 

[shobroco]

Reinforcing should be in the top of a below grade slab with significant hydrostatic pressure under it, but that does not sound like your case.

 

[slickdeals]

M^2,

See attached a document from FORD's best practices that shows where the top bars are to be located for a slab on grade.

Oh..How do you upload attachments now?


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[Ron]

Odd pattern, no doubt. Have seen similar attempts using WWF, placing from the top into plastic concrete. Has to be for crack control; however, the location of the rebar near the top over-reinforces the top and gives a different stress distribution through the slab.

Since it is a SOG, I agree w/ hokie66 and RE...I would not expect to see two layers of reinforcement and the location of the rebar does nothing for flexure.....in fact, might be detrimental as I noted.

Don't know the forklift loading, but they can be very high, particularly with hard tire forklifts.

State of Washington DOT has a pretty good pavement slab analysis program, free to download. They have two actually....one is an FEA approach and the other is an elastic layer analysis approach. I have used elastic layer analysis for pavements for over 25 years...FEA for pavements is relatively new, although you could adapt almost any FEA to pavement slabs, provided you have the capability to select and use proper parameters for the analysis. The obvious control is the lateral stress at the bottom of the slab.

 

[JAE]

I guess I'm a little surprised by some of the responses.

Typically, for SOG, reinforcement has traditionally been held either in the center or in the upper 1/3, or as high as possible relative to the required top clear distance. Even with forklift loading.

The way I have always understood it, slab design (Westergaard, etc.) is sensitive to the subgrade modulus and with concentrated loads that develop tension in the concrete beyond cracking, the rebar is there only to keep cracks closed as tight as possible and thus should always be in the top.

The 0.0018Ag amount in ACI 318 is not intended for slabs on grade. I've seen many engineers use 0.0014Ag but never have read any research or justification for that number.

Other publications have suggested that for shrinkage cracking, rebar in the bottom is better (can't find the article right now but I've read it).

I would agree that #4 @ 16" o.c. in a 7 1/2" slab might be iffy. But I've seen dozens of 6" slabs with #4 @ 18" o.c. in the very top. the 18" spacing was for concrete puddlers to step between the bars.

 

[steellion]

I'm with JAE, I've always seen the reinforcement in the upper third of the slab thickness. For heavy vehicular traffic requiring a SOG of 8" or more, I use a top and bottom reinforcement layer.

 

[dik]

I normally place the reinforcing at the top... 2" cover to avoid the 1-1/2" sawcut...

That's where the cracks occur that can be seen... for frequent loaded heavy hard wheeled forklifts, depending on the sub-base... the slab may be a tad thin... and cracking may be extensive... in particular, for the 6" slab. 8" may have two layers, but with the light reinforcing ratio... neither would likely be of any benefit...

Dik

 

[Brad805]

From what is described thus far, I am with JAE. I suppose if we were talking very large forklifts, and a very low subgrade modulus I would be suspicious of the design, but I would also be interested in the subgrade preparation methods and if you have cored the slab to confirm the location of the bar. Poor subgrade prep is such a common problem.

Brad

 

[Ron]

For crack control, we try to have the rebar in the middle third of the thickness. For most slabs on grade, even forklift traffic, I do not use rebar. When you have a widely spaced rebar pattern, with the rebar near the surface, the stress distribution gets whacked out so that when the wheels cross over the rebar, the stress below is reduced and when the wheel gets to the middle of the wide rebar spacing, there is no help from the rebar, so the vertical plane where there is no superimposed rebar influence two things occur....the stress is higher and it is subject to load reversal, so fatigue comes into play.

Mike...what is the crack pattern?

My guess is the cracks are occurring about 4 to 6 inches from the rebar and perpendicular to the travel path. If the slabs have a lot of traffic, you might even be seeing a block pattern.

 

[msquared48]

Cracks are random mainly diagonal to the path of the forklifts. Confusing enough Ron?

I am calling in the original geotech to do a dynamic analysis for the subgrade as the client wants to relocate a rather large metal stamping press to another portion of the slab. 131K dynamic and 81K static. Rather large machine. I am just very nervous about the subgrade due to all the cracking seen in the slab. To me, if properly conpacted and designed, this should not happen. Presently the machine rests on a separate 2'thick matt footing showing no signs of any distress.

Perspnally, I would have used two layers of #4 at 12" OC in a staggered grid for this one. I just believe in giving the slab a chasnce to span soft spots in the subgrade. I do see the point about hydrostatic uplift though. It was not mentioned in the original geotech report, but there was a peagravel lens abot 2.5 to 5 feet down in an area close to the existing nit.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Are there joints in the slab? Is there distress at the joints due to the fork trucks? That is usually where there are problems.

Is this an external or internal pavement? If external, is pumping evident?

Is the cracking a problem? Joints are just straight cracks, and are usually more active than shrinkage cracks. If the cracks are being held together and are not moving vertically, then they are not detrimental, at least for the time being.

 

[msquared48]

Hokie:

These cracks are internal of any expansion joint cracking. The Slab is external.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

OK, but are the cracks a problem? Most times, the crooked cracks are not as much a problem as the straight (planned) ones.

 

[dik]

m^2...
YOu need to have a geotekkie provide info for dynamic loadings. I would normally cut out the footprint of the loaded area and cast a rather 'massive' concrete footing... many equipment manuals suggest that the mass of the foundation is 3x that of the equipment.

hokie...
the problem I've encountered with cracking is that the top free edge is continually loaded by wheel traffic (bigger problem with hard wheeled forklifts) and the free edge eventually crumbles. The advantage of forcing these locations can be that the joint can be detailed and filled with a suitable 'hard' edge restraining filler.

Dik

 

[msquared48]

Hokie:

I must have been tired last night. The slab is INternal.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Agree with dik that you need a separate footing for the press, similar to what it is on now.

So, as I understand it, the slab is performing adequately, and your only reason for concern is some cracking? Cracks don't necessarily indicate serviceability problems.