slab on grade
slab on grade
(OP)
There is a small area (about 50 square feet of an existing slab on grade that is only 2" thick and is underlain by sand base which seems reasonably well compacted. The slab on grade in the remaining area of the building (which was built in 1966) is 6" thick. I believe the 2" thick slab was placed about 25 years ago, probably unknown to the architect at the time, when underground ducts were re-routed when an addition was built onto the building. The room in which this 50 square feet of 2" slab occurs is used as an accounting office, the office is about 20 feet square and had vinyl tile on it. It has performed satisfactorily to-date. A recent flood required removal of the vinyl tile, and when I chain dragged the concrete surface I identified this area of thinner slab by the very hollow sound when the chain passes over it (as well as by the change in appearance from the adjacent slab). However when we drilled an exploratory core hole, we did not find any signifcant void under the slab, but we did find that the slab was only 2" thick.
Questions:
1) To-date there has been only a desk on the 2" slab area, but the owner wants to have the flexibility to place filing cabinets anywhere in this room. Should we take out the 2" slab at this time, or do you think that a 2" slab is sufficient to support filing cabinets? I have no confidence that the Westergaard equations work in this case.
The cost of replacing it with a 6" slab matching the remainder of the slab in the building, including some dowels, is $3000.
I figure that the filing cabinets contents probably would not exceed about 20 pounds per foot. So 4 drawers, each 24" deep would be about 160 pounds plus the weight of the cabinet would be about say another 100 pounds = 260 pounds. Does not seem like that much weight.
2) Are there other reasons to replace the 2" slab. I have never heard of a 2" slab on grade.
Questions:
1) To-date there has been only a desk on the 2" slab area, but the owner wants to have the flexibility to place filing cabinets anywhere in this room. Should we take out the 2" slab at this time, or do you think that a 2" slab is sufficient to support filing cabinets? I have no confidence that the Westergaard equations work in this case.
The cost of replacing it with a 6" slab matching the remainder of the slab in the building, including some dowels, is $3000.
I figure that the filing cabinets contents probably would not exceed about 20 pounds per foot. So 4 drawers, each 24" deep would be about 160 pounds plus the weight of the cabinet would be about say another 100 pounds = 260 pounds. Does not seem like that much weight.
2) Are there other reasons to replace the 2" slab. I have never heard of a 2" slab on grade.






RE: slab on grade
The performance of such a thin slab would be based upon the stiffness of the underlying subbase. The weight of the proposed cabinets may or may not be reflected in cracks depending on the actual subgrade modulus.
Even if the subbase was good, I'd still want to warn the owner that heavier loads could result in settlements and flexure/cracking in the slab.
RE: slab on grade
RE: slab on grade
For such a small area I'd recommend it to the owner.
RE: slab on grade
If required, you may also refer to ACI 360 - Guide to design of slabs on ground
RE: slab on grade
Dik
RE: slab on grade
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: slab on grade
That said it will probably be OK - BUT maybe NOT. Sit down and explain it to the owner and tell him that it HIS call - and get it in writing.
Or tell him it is OK for "light office loads - BUT he can try anything he wants.
What is the worst that can happen - he replaces it now or maybe later?? Same price
RE: slab on grade
For $3k, I'd replace it.
RE: slab on grade
I have told them they have a choice, but it would be nicest to replace it and they agree, and things are now proceeding that way.
Thanks for all the comments that helped me make up my mind. It is a place of worship and I am a member there, so it is not a question of them suing me for anything (LOL), as I have been quite open on it all. Thanks again.
RE: slab on grade
RE: slab on grade
For 50 sq ft, the contractor could easily do the job in one day even without a jackhammer.
There isn't even a yard of concrete there for a new 6" slab.
So literally there is a few hundred dollars in materials.
Get some more quotes and fix the floor for the owner.
I poured a 1400 sq foot portion of my driveway for less!!!! (only 5" thick).
RE: slab on grade