Slab on Grade Value Engineering 1

[ladisirt]

I have a slab on grade design/specification for a church application that the CM is questioning from a cost savings point. The slab is 4 inches of concrete with 6x6W1.4x1.4 mesh over 6 inches of free draining granular material. The CM is requesting that the mesh be omitted and the stone thickness be reduced to 4 inches. I am not concerned about loading on the slab, so the unreinforced option may be feasible. I have control joints spec'd at 12 feet on center per ACI 360 recommendations for normal shrinkage concrete. I also have specifications for the gradation of the aggregate, workability factor, mortar fraction, etc, to help control the quality of the mix and reduce curling/shrinkage. The architect is nervous about an unreinforced slab, so I would like to hear some success stories from the design community before I allow the change.

I have equal/greater uncertainty about the drainage aggregate thickness. There is no recommendation from the geotech report, and I have never used less than 6 inches. But given the application I am willing to entertain the 4 inch thickness if that is all that's needed.

Thanks in advance,
Bob

 

[Ron]

Bob,
The wire is not reinforcing. Your slab is unreinforced without regard to the wire layer. It is there to hold the cracks that will occur, tightly closed. Don't omit that, but make them place it right so that it is in the middle third of the slab thickness. If you must delete the wire, then use a fiber-enhanced concrete mix...my choice would be steel fiber, not polypropylene fiber.

As for the "free draining" material, it is actually more of a capillary barrier than a drainage layer. If you need drainage immediately beneath a building floor slab, you have more problems than that layer will solve. There shouldn't be a problem with dropping the thickness down as long as the fineness modulus of the material is more than about 2.8 to 3.

I like your joint spacing. Make sure they cut the joints the same day the concrete is placed. Don't compromise on this.

With regard to mix controls, be sure that the coarse aggregate is as large as practicable. Don't let them slip a small aggregate, pump mix in on you. Yes, it's easier to place and will be just as strong, but will likely have several times more shrinkage and requires more water in the mix, so you'll have vapor migration issues a bit longer that can affect floor coverings.

Unless you are very comfortable with mix design development and analysis, don't play with the mortar fraction and other finer points of mix design. Give them some general parameters and have them submit a mix design to meet those parameters, with a proven record of service in the area.

 

[geoman110]

I generally agree with Ron’s comments, particularly in regard to the “free drainage layer”. However, in respect to removing the wire mesh, I have done several lightly loaded slabs (for commercial retail) consisted of 4” plain concrete (not wire mesh) over 8” of granular base. The granular base has some structural effect in regard to load distribution, particularly at the joints. The granular base can be reduce to 6” thick if the subgrade is very competent. I personally do not recommend reducing to less than 6”.

Ultimately, it depends on the slab loads. If there are really light loads (similar to retail stores) and the subgrade is competent, you can ignore the wire mesh. However, you have to follow Ron’s recommendations in regard to joint spacing and time of cut and so on. If the subgrade is very competent, you can reduce the thickness of bedding to 6”. I afraid to go less than 6”, at least for capillary reasons.

 

[ToadJones]

I recommend applying a spray type curing compound immediately after finishing....especially outdoors.

 

[msucog]

Making sure the subgrade, all utility trenches and fill is well-compacted one of the primary things you need for a successful slab. Some go to fibermesh mix to omit the wire. I prefer the wire and heavier gauge properly supported (not pulling up during the pour). As far as the underslab aggregate layer, my recommendations would depend on where groundwater is located. If it's much deeper than subgrade, I'd consider omitting that layer. However, usually the poly is thin and not sufficient so omitting the moisture break is not a good idea if you're not using heavier poly. Also, my comments have not taken in to consideration loading, soil types or other subgrade/structural aspects. And as with most construction, independent third party testing/evaluation is crucial.

 

[OHIOMatt]

I have quite a bit of experience with un-reinforced concrete slabs. The key for success is to have tight quality control over the entire process. Here are a few points to consider. While I would like to say that this will lead to crack free concrete, I am a realist, but the below will greatly reduce your problems.

1. Resist the urge to specify a concrete with too much cement. A 3,500 or 4,000 psi mix with properly graded aggergate is usually sufficient. Consider allowing fly ash and water reducers in order to reduce the total amount of cement. More cement means more water which means more shrinking.

2. Properly prepare the aggregate base. Make sure that it is smooth and well compacted. A smooth base or a poly over the base will reduce the friction between the bottom of the slab and the base. This will help reduce the number of random cracks. As noted above, be consistant with the elevation. I typically spec +0 - 1/4" as a tolerence for the base. Do not use a 2" sand layer, as it will rut with traffic during placement. Properly isolate the slab from any footings. If you end up going with a 4" base, set the top of any interior footing at 4" below bottom of slab.

3. Watch corners, column isolation joints, etc. 12' is reasonable for control spacing, but less (say 10') would be even better. Make sure that the cuts are made as early as possible with an early entry saw.

4. Cure the slab with water and curing sheets (no burlap) if possible. If water is not possible use a chemical cure with a minimum of 25% solids. This should occur as soon as the saw cut is made. If it is hot and or windy, consider the use of an evaporation retarder.

Welded wire is placed in slabs to hold cracks tight. The method above is based upon the idea that you try to prevent them to the greatest extent possible.

Micro fibers are placed in the concrete to prevent or reduce plastic shrinkage cracking. This occurs as the concrete surface crusts and shrinks as the concrete is still plastic. With an evaporation retarded and proper cure, this issue is significantly reduced.

Finally, I ask you to consider this. If the concrete cracks, what are the results? Typically, if the floor is covered, the results are very minor. If you have a consistant base, there should be no vertical separation so there should be no real trip hazzards etc.

I will say this, if the GC actually follows the methods above, it will not likely result in a cost savings over what he bid. WWR is not too expensive.

 

[ladisirt]

Guys,
Thanks for all the advice. My mix design, slab cure, and cut procedures all align nicely with these recommendations. I am holding fast on the 6" aggregate layer and allowing removal of the mesh.

 

[geoman110]

OHIOMatt, I agree with all of your recommendation and it is in conformance with my personal experience with the plain concrete slab on grade. Applying, curing compound and surface hardener also helps a lot. However, I am struggling with your following comment:

”A smooth base or a poly over the base will reduce the friction between the bottom of the slab and the base. This will help reduce the number of random cracks.”

If you reduce the friction between the concrete and granular (by placing a poly over the granular), does not it increase the chance for warping of the slab. I am a geotechnical and it is more a structural issue. However, this is what I have heard from the structural engineers in the projects that we worked together and consisted plain concrete slab-on-grade.

 

[OHIOMatt]

geoman, you are correct. The poly can lead to an increase in the potential for slab curl. The moist cure reccomeded helps to conteract that issue. Also the tigher spacing of the control joints will limit the magnitude of the upward curl.

For the slab in question, a small amount of curl will likely not even be noticed.

If you had a slab that was loaded heavier, such as with a fork lift, curl would be much more of a concern as you would run the risk of breaking the corners as well as other issues.

 

[geoman110]

Thanks for the reply. If I understand it correctly, there are two sources of curl:
(1) due to reduced friction between slab and base (happening under load anytime during the service life of the slab); and
(2) caused by difference in rate of cure between top and bottom (happening during the first 14 days and can be controlled by moist cure of the top).

 

[OHIOMatt]

Actually, the thought behind reducing friction is to help keep tensile stresses lower in the concrete. If, as the concrete shrinks, it is free to do so with little resistance from the base, the stresses would be lower, which should translate to fewer cracks.

 

[Mickney]

4" inches of an Aggregate Base Course stone should be fine provided the soil subgrade is stiff, properly evaulated by the special inspections firm and passes a proofroll. You are talking more about a "static slab on grade" versus a warehouse slab on grade with fork lift traffic.