Topping Slab for Strength 1

[CSFlanagan]

My client is considering purchasing an existing 25,000 sf shell building. Building was constructed as a shell building by a local municipality. The building upfit was started by a prospective tenant, who then closed down. The building has never been used. The 4" thick floor slab has radiant heat below. Existing slab reinforcement is unknown. Soils in our area are dense clay and provide good bearing. The Under-slab radiant heat system is unknown, but these are typically placed on a stone sub-base, so it is reasonable to assume the sub-base of the slab was well compacted. Client does not intend to use the radiant heat.

My client wants to use the building for light storage, with forklifts (5000 pound capacity) and rack storage. Building eave is only 15 feet, so racks won't be too large or tall, and stored materials will be light.
Option 1 is to demo the floor slab and pour a new floor, probably 6" thick reinforced.
Option 2 is a topping slab, probably another 4" thick.
Option 3 is to use the floor as-is unless & until floor cracks become a problem. Then remove/replace as necessary.

CLient is only asking an opinion at this point. Concrete coring and subsurface investigation could be performed.

 

[LittleWheels]

A cover meter survey might be quick and helpful if you need to assess the slab's structural capacity. I have found Hilti Ferroscan to be reasonably accurate, particularly in such a thin slab, but there are plenty of other manufacturers.

 

[SwinnyGG]

I'd GPR it to determine bar sizes/layout - a very inexpensive process compared to a topping slab or ripping it all out.

Your option 3 is the cheapest now, but will really suck if they have to move all their stuff out later to place the slab.

 

[dik]

On a cohesive soil, a 4" slab is next to useless for anything but a residential slab... Bonding a topping is tricky and may fail. I'd go for a new 6" or 8" or whatever he needs for his use. On some plastic clays, hydronic heating can cause desiccation of the soil and lead to shrinkage.
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

I had a similar situation a few years ago -
We did the following:
1. Took cores to verify slab thickness throughout areas where light storage racks and forklifts would go.
2. Took soil samples and had a geotech provide a subgrade modulus (k value)
3. Reviewed the forklift the owner wanted to use and extracted wheel/load/axle information.
4. Analyzed the slab for the rack loads and wheel loads per PCA slab design methodology.
5. Found it didn't work (5" slab) for the forklifts and cycled through various forklifts (lighter and lighter) until one was found that worked.
6. Wrote a report outlining our findings on the lighter forklift but....
7. Cautioned the owner that the PCA method was somewhat theoretical and that slab cracking could occur despite our findings and....
8. The owner opted to remove and replace some areas of the slab with thicker slabs where high traffic volume (more fatigue) would occur.

 

[phamENG]

I'm guessing there are joints in the floor? You need to know how (and if) they're doweled together or you'll get separation and eventually damage to the concrete from the fork truck wheels.

Another thing to consider is the flatness of the existing floor. Some racking systems and fork trucks require a certain flatness to operate correctly or even safely. You probably won't be dealing with one of those, but it may be worth checking.

Using it until major maintenance is needed is not a good idea. Racking is expensive not only to buy but also to assemble and disassemble. Where will the owner keep is goods while the slab is being ripped out? No...better to rip it out and put in what's needed now.

If you want to go the testing route, talk with a geotech and see how much of the slab needs to be removed for them to check the subgrade. That's what's really important. If the subgrade is no good, a 4" slab isn't going to do squat no matter how many bars and dowels you have in there.

 

[CSFlanagan]

Thanks for the replies and opinions everyone. You've pretty much confirmed what I suspected.
I've recommended to the Owner that he budget a new floor for the building in his offer to the present owner. There are just too many variables and unknowns on the existing slab and anticipated use.
Thanks!!

 

[dik]

JAE... I like your procedure.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik