Exterior Slab Design 2

[SteelPE]

I have a project where I have been requested to provide additional services for design of a few exterior slabs. The slab is to be located on a project where I am the EOR. The project is located in the northeast and the slabs are to utilize radiant heat to reduce the need for clearing of the snow. From my understanding, the heat is only used when a snow storm is coming.

Slab one is 15’ wide x 60’ long and is to be located next to a garage. This is an area that I am not too worried about as the loadings are low and the slab is relatively small. Slab two is 95’ wide x 100’ long and is to support small aircraft (less than 42,000 lbs). I think I have the design pretty much covered using the book “Designing Floor Slabs on Grade” by Ringo and Anderson, however, I am a little concerned in regards to the details.

I intend to use a 5,000 psi air entrained concrete mix (to help against the freezing temperatures and the inevitable deicing chemical that might find their way onto the slab). I am also considering splitting the larger slab into 4 different sections roughly 42.5’x50’ to help deal with expansion and contraction (I might split it up even more as I move into the final design).

Is there any input one might have for a project like this?

This is a part of the project that was excluded in my initial proposal and I have gone “kicking and screaming” into.

 

[DaveAtkins]

I think your approach is sound. I recommend epoxy coated rebars (or wire mesh) if salt will be used on the pavement. Aircraft deicing chemicals may also be corrosive to steel reinforcing, but I don't know.

DaveAtkins

 

[EngineeringEric]

How thick is your slab for that 1/4 slicing dimensions? 8" 12" 16"? 42,000# isn't a huge load but you may want to consider utilizing dowels, for the exterior and exposed conditions you want ones that are coated and won't rust.

Are you providing top mats of rebar? Area of steel > 0.05?

One thing that comes to mind, if you are doing radiant heating (i have never done this exterior... Fun!). Anything you do now that has a cost increase will save the owner far more money when it comes to life-cycle and this is an expensive slab, you do not want to jack-hammer it if you get spalls or excessive cracking. Ounce of prevention type deal

 

[dik]

Are all slabs used for small aircraft, and how thick (maybe 8")? What are you planning to use for heat? electrical or hydronics? In my experience, electrical can be a little 'iffy' and may be prone to failure over time. Electrical heat tracing has improved over the last couple of decades.

I would divide your 15'x60' into 15'square panels. The 95'x100' would be divided into roughly 5x5 or 6x6 spaces. The 42.5'x50' would be divided into 3x3 panels.

I normally don't use dowels at joints, but, because of the electrical/mechanical components, I'd consider using dowels.

I'd use Dow Hy 60 or some type of extruded load bearing polystyrene (Type 4) under the slabs, and also 6 mil PEVB. between the insulation and compacted granular fill.

5000 psi may be a tad high, and would likely use 4000 psi, but, 5000 is better. Corrected: 30 MPa (what I would use) is actually about 4500 psi... so 5000 is on the mark.

Timing for sawcutting joints is critical. and I would fill the sawcuts with an appropriate joint filler to reduce water penetration.

Added: I have a problem with epoxy coated bars and would likely rely on a sealer with 2" concrete cover. You can also use HDG bars (a little pricier). You can tighten up any cracking with going to more than basic TS reinforcing. Spacing of sawcuts and timing will likely accommodate a regular rebar content. Your wheel loading is likely to be low and not much fatigue; loading may be 'channelised'.

Dik

 

[EngineeringEric]

I am surprised by the 6 mil VB for an exterior slab. I wouldn't be concerned with vapor transmission since more than likely there are no finishes. The lack of a VB will allow the slab to cure more evenly and reducing curling? Do you mind sharing some insight on that point dik?

 

[SteelPE]

We have sized the interior slab at 7" of which I was criticized about multiple times for being two thick. I have data from a few other projects where a 5" and 6" slabs were used successfully. The 5,000 psi mix is what we use for exterior applications when deicing chemical may be used and I believe is what is recommended in ACI. A 100 psi rigid is currently proposed under the slab with the contractor wanting to switch to 60psi rigid but that noise had been quieted slightly.

Any reinforcing in the slab is there for shrinkage and not strength (it is also being used for the radiant heat system). I am not in charge of selecting the radiant system, but I believe they are going in the opposite direction of the electrical system.

 

[dik]

The use of the PEVB is to minimise any moisture transmission from the soil to the slab and if the slab weren't heated, I would normally not use PEVB for an exterior slab.

Dik

 

[dik]

7" seems to be good with the added heating system; you want to minimise cracking. If the slab weren't heated 6" would likely be fine, but, you should reduce joint spaces, and, it will move a little more. You can eliminate the polystyrene, but, will bump up your heating system.

Added: 60 psi would likely work with your wheel loadings.

Dik

 

[SteelPE]

Some of these details are requirements of the project and are selected by others (polystyrene, radiant heating system, heating system type). The architect is very familiar with these types of projects and their requirements. I was brought in to work on the project because the engineer the architect uses is either retired or getting ready to retire. So some of the push back is coming from the architect (my client) not just the contractor.

 

[XR250]

I would not saw cut joints as you may inadvertently hit you mechanicals that may have floated up. Use keyway or some other method such as doing two checkerboard pours.

 

[dik]

You may be able to save a few bucks by going to a trapezoidal section with 5" at the unloaded edge and 7" in the loaded area... Section is easy to construct.

Added: XR250: with a little care, sawcutting should not be an issue.

Dik

 

[racookpe1978]

Be nice to be able to use some solar fluid/solvent banks for a part of the heat source. Unlike photovotalic (DC power) cells, solar water heating is practical in many areas of the country. It would reduce the need for lots of grid electric power almost all of the days, but would be be available as heat trace source on the few days when it is needed.

 

[KootK]

I almost did a similar project a couple of years ago. Looking at the typical detailing was quite daunting for me as a buildings guy. Thick slabs and lots of particular jointing that was unfamiliar to me. Frankly, I was grateful that the thing didn't come to pass. I did come across the resource shown below which was some help to my airport SOG edification. I do believe that the recommendations in that document are intended for a more serious level of airport traffic however. Runway etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK:
Great publication from 40 years ago... have used if for lots of runways and taxiways... really handy. I have a copy of the original calculations by Packard for his Hankel solution... I picked up a copy of his original paper and never had the time to 'sort it out'.

Have used PCA's 'Airport' program for numerous slabs for wheel loading and rack loading. Have a 5-1/4" disc with a copy on it, and have a couple of 5-1/4" drives, but no means of interface. Airport was a great program and one of the most useful ones.

I cut my original publication apart so I could scan it.

Dik

 

[KootK]

So... I accidentally posted some copyrighted stuff previously. My bad. It was old as dirt and googled so easily that I wrongly assumed that it was public domain. Anyhow, some good has come of it. In the process of trying to find a purchase link (fail) I stumbled upon some other stuff that looks kind of relevant. And it is public domain.

IPRF Best Practices
FAA ACI 150



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.