I am renovating a 42' x 60' concrete shell building to use for my printshop.

I plan to insulate it well, including 2" foam (Foam Control brand) insulation under new concrete slab. I am looking for advice on whether to use 2# foam rated at 25psi compressive strength or to go with the heavier 2-1/2# material rated at 40psi.

Currently my heaviest press weighs 8,000 pounds with about 4' x 5' footprint. I will be bringing in paper skids weighing up to 2500#

I called a local engineer (Wheeling WV area) who scoffed that "no one insulates under slabs" and said he couldn't help me.

The price difference doesn't seem excessive, so I'm inclining to go to 40psi for the heavy load areas for peace of mind unless consensus is that 25 is ample.

JMRPress,
Since the floor will be subjected to heavy loads you might want to consider post-tensioning the slab along with a modest amount of rebar on the edges to resist the compression.  Rebar alone may not keep the slab from cracking since rebar usually comes into play after the slab has cracked.  Post-tensioning uses 270,000 psi lubricated and sheathed cables that are usually strectched in stages to 33,000 psi after a period of time.  With this system you may also want to consider using a monolithic slab and footer.  Talk to some knowledgeable people in your area about how to do the slab and footer to avoid cracks in the concrete.

Thank you both for the good input.  I assumed I should put insulation under slab because several nearby friends who have been into efficient building for years have done it.

I  plan to put 10” insulation on the interior block walls, made up of 2” foam (board or spray) and an additional 8” of cellulose or blown fiberglass.

Water is surprisingly close to the surface in the shop location, less than 5 feet.  (There is a shallow dug well along one wall.)

It sounds like my worry over the foam density choice is misplaced, that I should instead be more concerned about the concrete design & reinforcing.  I was planning 4” to 5” (more in the heavy load areas) with the common 5” (9 or 10 gauge) grid wire mesh reinforcing.  

When we are actually printing, the temperature will need to be maintained in the 70-75 F range for proper ink functioning, though at other times it isn’t critical.  I was initially planning to just pour a cap on the old very uneven and oil soaked floor and only insulate the perimeter.  But as I have heard about the passive house standard I have been inspired to aim higher, so I am currently planning to remove the old floor so as not to loose headroom. 

I imagine post tensioning is a big production, I hope not necessary for my shop, but I’ve heard it can be quite effective. 

Please do not rely upon wire mesh to eliminate cracks.  Structurally, wire mesh is not strong enough to prevent cracks.  Wire mesh has to be in the top one-inch of a 4" thick slab to have any effect at all.  And then the effect of the wire mesh would be to hold the cracks tightly together.  Of course, installing mesh at the top of the slab is almost impossible since the crew will be walking on the un-anchored mesh while installing the concrete.

A typical 4" thick 3000 PSI concrete slab may not be stong enough to withstand the stresses and strains generated by the heavy presses and forklifts.  As a minimum, I would specify a 6" thick 4000 PSI concrete slab with a low water-to-cement ratio.  A mid-range water reducing additive will make the concrete spread easier and help reduce cracking during initial curing.

For the slab I suggest that you price unbonded post-tensioning versus 13mm (#4) rebar.  (Price post-tensioning on a 48 inch grid versus rebar on a 24" grid.)

If you do decide to use rebar instead of post-tensioning then price basalt rebar versus steel rebar.  For properties of basalt rebar see:  http://www.withconcrete.com/index_files/RockRebar.htm

If there is a chance that the heavy presses will vibrate enough to disturb other operations, then consider placing a separate footer under the presses.  Isolation joints in the concrete slab around the presses may also be needed to reduce vibration to the other areas.

Most structural engineers that I have dealt with are not into insulation.  However, it might save you some money and trouble by consulting a structural engineer about the footer and slab construction for your special operation.  All we can do in this forum is to make suggestions based on our own experience.  Since we can not possibly understand all of the problems and requirements of a job we have never seen, we should defer to local experts.

Jonr,
Agreed.  Post-tensioning is getting to be more common with residential slabs in areas with prairie soils (Gumbo Clay) to reduce and control cracking.  Oftentimes post-tensioning can be less costly than using a lot of rebar.

Posted By JMRPress on 11 May 2010 04:26 PM
Is Manual-J a program for calculating/predicting building performance and balancing insulation? No I haven't.

Thanks for the input about concrete and reinforcing. Good to know about post tensioning as a way to use fewer resources.

How much space around a slab is required for the post tensioning jacks?  I presume the perimeter jack gap would have to be poured after post tensioning the main slab.

Thanks for explaining ACCA Manual-J

JMRPress,
Only your local post-tensioning company would know how much space would be needed for their jacks and working room.

If the four steel posts are at the perimeter, then there should be no interference with post-tensioning the slab.  However, if the posts are within the interior area, then the steel tensioning cables can be placed in such a way to either miss the posts or to curve around them.  Regardless where the posts are located you may need to install isolation joints in the concrete around each post so they can expand and contract without cracking adjacent concrete.