Minimizing cracks in an exposed slab on grade 2

[ccap]

I'm designing an exposed slab for a church and I'd like to minimize cracks as much as possible. I'm using expansion joints and saw-cut joints throughout the slab in a decorative pattern. The soils engineer recommends the slab be 4" thick with #3 bars 12" oc ew over 4" crushed aggregate over 2" sand over 10 mil mb over 2" sand. This design seems more suited for a slab with a floor covering. Can anyone provide further tips?

 

[Ron]

The subgrade you have noted should be adequate to support the concrete floor, but the constructability of it is difficult. It will be likely be displaced during construction causing a greater potential for cracking.

I would use a consistent subgrade layer...coarse sand or similar as a capillary barrier, with vapor barrier below the sand.

A 4-inch thick slab is hard to control in thickness, thus the potential for "random" cracking is greater. Go to a 5-inch thick slab and specify a maximum of 5-3/8" and a minimum of 4-3/4". Make sure your control joints are spaced no more than 12-1/2 feet apart (you could go to 15 feet for other applications, but for exposed aesthetic slab, I would stay with 12-1/2 feet). Make sure the joints are saw-cut into the slab within 8 hours of placement and that they are at least 1-1/4 inches deep. Don't wait until the next day! Cracks will have already occurred if you wait. They'll show up later.

You likely will not need expansion joints, just isolation joints. All other joints should be control joints, though if you are placing a lot of concrete, you might need construction joints. Know the difference, but more importantly, make sure the contractor knows the difference.

 

[boo1]

I was thinking the same things as Ron stated above, about the subgrade and slab thickness. He said it much better than I could. But will try to identify some of problems areas I see in the field that lead to cracking:

Excess water in the mix
Concrete does not require much water to achieve maximum strength. But a wide majority of concrete used in residential work has too much water added to the concrete on the job site. This water is added to make the concrete easier to install. This excess water also greatly reduces the strength of the concrete. To often I hear a concrete crew tell the batch driver, "add 20 gallons of water". Allow no water to be added on site and /or test specimines to be taken.

Rapid Drying of the concrete
Also, rapid drying of the slab will significantly increase the possibility of cracking. Specify a curing top coat to be installed the day of placement or inaccordance with manufactures instuctions.

Improper strength concrete poured on the job
Concrete is available in many different strengths a 3000 or 3500 psi mix should be specified.

Lack of control joints
Control joints help concrete crack where you want it to as Ron indicated

Reincorcement installed wrong
Often I see the reinforcement installed at the bottom of the slab. Specify spacers and chair height to hold reinforcement at mid-slab height.

 

[jheidt2543]

Just a few more thoughts:

I don't see the need for a vapor barrier on an exterior slab, it is just an unnessary added step. For interior slabs definitly, but not exterior.

#3 bars @ 12" seems excessive in an exterior slab unless you have some type of soil problem we don't know about. There are miles of exterior sidewalks with no reinforcing. However, since you want to limit cracking in your decorative slab, use 6x6-10/10 mesh (sorry for the old designation, I can never remember the new mesh numbers).

Also, for an exterior slab, add an air entraining agent. The added air content provides some added workability when placing the concrete and helps in the slab's freeze/thaw durability.

Lastly, if you are in an area that has freezing temps, you might try to find a concrete aggregate that is low in "chert" content or "chert free". Chert is a type of stone that absorbs water and when it freezes will split, thus causing the "popouts" you see in sidewalks and driveway surfaces.

Best of luck.

 

[Focht3]

I'll agree with everything - until I reached jheidt2543's comments on the vapor barrier and rebar. We don't know ccap's client, site, soil conditions or prevailing weather. The vapor barrier doesn't cost much, and would allow the slab to be enclosed at a future date. (This might also explain the various layers that "seems more suited for a slab with a floor covering.&quot

And while #3's on 12 inches may seem excessive, it isn't that expensive, either. Local practice may have found that the "heavy" reinforcement works well to minimize cracking.

 

[ccap]

Thanks to everyone who's replied. Let me offer more information. The slab will be inside a church, exposed (no flooring), jointed and saw-cut in a decorative pattern, stained and sealed. It will be constructed on a site in Orange, CA (i.e. doesn't freeze), on soils that the geotech says are fine to coarse, gravelly, generally clayey sands... with no ground water within 50'... 70% relative density and high shear resistance... and not likely to be affected by liquifaction. Does this change anyone suggestions?

 

[Ron]

ccap...no, the suggestions still apply. I agree that rebar is excessive, as your clayey sand "movement" is not going to be substantial considering your low water table. Putting your vapor barrier below a sand layer will mitigate moisture issues with the floor.

 

[Focht3]

I concur with Ron - vapor barrier appropriate for stained and sealed slab inside a church. Given the additional information, though, the rebar does seem a bit much...but this may be a code-driven issue. While I am a licensed civil engineer in California, I can't answer the "why" of the slab reinforcement. It may be a code requirement; it may be needed to stiffen the floor for earthquake conditions; or it may simply be too conservative. To get an idea of the seismicity of Orange, CA follow the following link:

http://gmw.consrv.ca.gov/shmp/html/pdf_maps_so.html

You can download the current Alquist-Priolo Seismic Hazard maps for southern California from this page.

 

[FSS]

One other note to follow the excellent comments above. Be careful with your control joints. Your explanation of a "decorative pattern" for your joints has me wondering how uniform their layout is. Sharp angles and the like which break up the slab into non-uniform and/or uneven narrow shapes can do more harm than good. Just follow sound jointing methods (as your contractor should already know about).

 

[BantrelStructural]

I have never much liked mesh or light reinforcing for slabs because even despite careful chairing prior to the pour, as a concrete hose is dragged over the reinforcing, or the team of concrete placers marches over it, it usually ends up down in the sand layer, or worse yet part down in the sand, and the opposite side pivoting over the chair ends up poking through the top surface. On that note, mesh does have a slight advantage because it can more easily be re-lifted just as the concrete placement approaches.

Now my experience is more on industrial or commercial projects requiring heavy duty floor performance where it was quite reasonable to use fairly heavy reinforcing that can support construction loads (#5s or 15M bars minimum). This is of course not reasonable for the application in question here.

As far as the need for reinforcement, I would suspect that unless the floor is acting as a diaphragm that there would be no code requirement for reinforcing (but I am not familiar with California Seismic requirements). If you read slab-on-grade literature from PCA, with proper joint control there is no real need for reinforcing steel, although it does have the added advantage of providing some bridging strength over small voids cause by soft spots or poorly compacted backfill over utilities or near foundations. The primary reason for reinforcing steel is to provide crack control for shrinkage/creep, particularly if jointing &/or mix design &/or installtion quality is not well controlled, but then only if it is installed in the proper location (i.e. mid slab for a single layer, within proper cover distances for top & bottom steel).

An alternate that you might want to consider is fibre reinforcement (but because of the exposed nature of the surface, use polypropylene rather than steel fibres). And like any specialty product, the contractor should be familiar with its use. Most suppliers that I have dealt with are willing to provide a manufacturer's rep to assist the contractor in proper use of their product.

Hope this helps...