Slab thickness for new shop

[Dirtymilkman]

The higher the psi rating the whiter and brighter the floor will be. You want bright. 

[davehummell]

I had the guy finish my floor smooth and I had him seal it I can cleanup spills real easy my shop was designed for a machine shop not a mechanic's shop but I keep a bay open to work on my junk 

[hicrop10]

Our old equipment shop where we would bring in d9 dozers and twin engine scrapers that weighed 100,000 had a foot thick slab and we never had a problem,matter a fact it was poured in 1963 and it still hasn’t been replaced.

[JoeH]

9 hours ago, Dirtymilkman said:

The higher the psi rating the whiter and brighter the floor will be. You want bright. 

Higher PSI also means more brittle, less forgiving to slab flexing.  But again, sub grade prep and rebar are King.  What concrete can hold on one square inch dirt can only hold on one square foot. (Give or take).

Steel rebar and slab thickness help to disperse point loads (i.e. bottle jacks) across a larger footprint underneath the slab.  Worth looking at concrete load flex diagrams to understand how concrete is stressed.  When a load is put on the slab, the top half of the slab will be in horizontal compression and the lower half will be in horizontal tension.  Having rebar in the top and bottom halves of the slab would make a 3000 PSI slab good enough.   Concrete has a lot of compressive strength, but not much tensile strength.  The rebar in the lower half is what gives the slab tensile strength during flex to prevent cracking.

[Dirtymilkman]

I say, if you pour it thick enough with rebar, a little void underneath won't matter. Poly underneath makes a HUGE  difference in my area in the summer for sweating. My main shop I finally just put a/c in it because of moisture issues. Little bit of poly would've been the difference. 

[JoeH]

To that point I delivered concrete to a house years ago. Plumbers had to trench the septic out through the existing garage, so they cut a 1 ft wide strip out of the garage floor.  The entire garage floor was floating 6 inches!! The dirt had settled away from the slab, leaving the whole garage floor floating!

Homeowner was an older gentleman, had been parking his car in there for years.  His response?  "This'll be the next homeowners problem, not mine!"  They finished the septic repair and filled in the trench, leaving the rest of the floor floating!

[JoeH]

On 1/11/2020 at 5:00 PM, ivanuke said:

Yep I'm in Houston, we rarely see freezing temps. I don't think we got any this past few years. Maybe a day or two. The majority of my property is covered with 4-6" of crushed concrete that my trucks are parked on. I was planning on bringing in select fill as a base for the slab and compact it. Would it be a good idea to add a layer of crushed or would it not make a difference.

How finely crushed are we talking about?  6 inch chunks or 1 inch gravel sized? Or a mixture?  Larger chunks could create pressure points, as they don't readily give way to movement like gravel does.

[davehummell]

In the spring if it's really wet I run 2 dehumidifiers. The floor will be damp  and I can't have the machines getting rusted up. When I was running the shop two shifts I had heat all the time but I am retired and cant afford to heat it when I'm not doing anything in it. I had rebar and mesh put in the floor. I have no idea what psi concrete was used the guy had a local concrete company and I rebuilt his front axle and resealed the backhoe valve package on his john deere loader backhoe. I payed less than a 1000.00 for 3 concrete truck loads and 5 guys to finish my floor I offered to help and was told I would be in everyone's way just stand back and watch.

[ivanuke]

10 hours ago, JoeH said:

How finely crushed are we talking about?  6 inch chunks or 1 inch gravel sized? Or a mixture?  Larger chunks could create pressure points, as they don't readily give way to movement like gravel does.

1" crushed concrete.

Would you recommend I do 6" slab, with rebar on top and bottom or wire mesh top and bottom? I do have 20 ton bottle jacks but I always put a 1/4" 1x1ft steel plate underneath. In houston there is A LOT of humidity, I want to have the shop insulated but I don't know if that would keep the floor from sweating. Should I invest in a vapor barrier?

[JoeH]

Vapor barrier under the concrete only prevents ground moisture from soaking up through the slab. It doesn't prevent condensation from forming on a cold floor with warm humid air above it.

I wouldn't do any less that 6".  If there are parts of the garage that won't/can't have trucks parked on them (shelving/tool areas/etc) you can do one layer of rebar in the middle to save $.  For truck lanes/parking areas I'd do the rebar at the 2 and 4 inch depths in the concrete. Metal closer than 2 inches to the surface of the concrete can rust & pop the concrete off. You see this on older bridge underpasses frequently.

[1958 F.W.D.]

On 1/11/2020 at 8:57 PM, Bullheaded said:

And do yourself a favor. Put down poly vapor barrier and a layer of rigid insulation under your pad. Your floor will stay warm and dry. Makes a huge difference. No damp cold floor to lay on.

6 mil vapor barrier and slab insulation required by most building codes for any accessory building over 1000 S/F 

[harrybarbon]

put the plastic down on top of the base, concretor must not add water to the premix concrete in the agitator before or after it is discharged from the barrrel (it weakens the mix design strength and breaks the concrete curing process) VIBRATE THE CONCRETE as it is placed and no cement dust to be spread when finishing the concrete - in time it will crumble and peel - follow this process and you will get a strong dry concrete slab - 6 inches is minimum and 8 inches is extra strength for your long term safety. If you can get the guys to have a slight fall into the center and out towards the front opening, you can pressure wash the slab and the water will run out the front - or to a proper sludge pit drain - this depends on your local authority re washing out workshop floors etc 

 

 

 

[Mack Technician]

8"

[mrsmackpaul]

5 inch is what Im doing

As always we all have a opinion based on what we feel is right

A few things you must know is things like how reactive the soil is, the climate, how wet the area is and how dry it gets

Does it freeze, apparently this causes the ground to shift quite a lot 

I know of one slab only 3 inches thick for semi trailers gross 42  tons and its still there 40 years later 

So there's is lots to consider and what works for one part of the country wont work in another

Do your research and dont listen to old wives tails 

I have seen a lot of sheds were tracked type earthmoving gear is repaired that have train line cemented in level with the floor so it the tracks arent walking on the cement as such

 

Paul

Edited January 27, 2020 by mrsmackpaul