What strength concrete for foundation?

[bob_mp]

Hi Guys,
Getting close on the foundation for the new (21x44) garage. The digging is done and the forms are going up. Pretty psyched.

I am going with a 6" slab, 4000 PSI concrete, #4 rebar on 10" centers, on chairs, over Stegowrap. The concrete contractor says overkill. Perfect.

I asked them about upgrading the foundation concrete strength up from 2500 PSI. He said it was completely unnecessary.

Any thoughts? I doubt it would be expensive. Are there any pros or cons to upgrading the foundation strength? I'm located in the Bay Area, CA, so freezing is not an issue.

Thanks for the help.

Bob

 

[KELLHAMMER]

3000 psi is typical for most residential work. It usually doesn't add alot to the cost of the concrete to increase to 3500 or even 4000. Unless it is specifically specified by my engineer the 3000 psi concrete is what we use and it is fine.

 

[Red05GT]

We've always used 3000 psi as a standard baseline. If soil type looked marginal, we
would upgrade to 3500 or 4000. We've even added fiber for additional strength. Tell
your concrete guy you'll pay for the crete upgrade, 9-10 yds @ $5-10 per yd, $ 45-100.
Cheap insurance, just sayin.

 

[kbs2244]

The rim only has to support the building.
And, on a PSI basis, they are pretty light.
(Remember that in the shop you are going to be concentrating weight onto small spots. On the foundations it is spread up and down the wall.)
I would go with the pro.

 

[retrobuilder]

You are using #4 rebar (1/2") at close 10" centers. Are they crossed or just one way? If crossed you should tie them at where they cross.

You can go higher psi but the large rebar is likely why the guy advised you can get by at 2500. We use higher rating in GA.

 

[brownbagg]

i like 3000 but 4000 in a gargae is better because of dropping things, hammering, working, it help keep the concrete from chipping. but just because you buy 4000, dont let the finisher throw the water to it or you end up with 2500 at a price of 4000

 

[Shocker]

My slab is very similar to yours. I specified 4000 PSI. Necessary if you are going to install a lift.

 

[PetesPonies]

I always used a 6 bag mix. I have never had a problem with dropping things, hammering on the concrete or installing a surface mount auto lift.

 

[K'ledgeBldr]

Instead of speculating on what is best- or relying on answers from across the country that may or may not have any similarities to the Bay Area, why not ask the AHJ's their requirements? Is there any special quake requirements for the build?

 

[bob_mp]

Instead of speculating on what is best- or relying on answers from across the country that may or may not have any similarities to the Bay Area, why not ask the AHJ's their requirements? Is there any special quake requirements for the build?

There are no special requirements on the concrete; the approved plans specify 2500 PSI concrete.

My architect specified 6" walls, for more interior space. I was thinking of upgrading in part because of the narrower width.

The bar is running in both directions. I will check that it it gets tied at the crosses. Great call.

Thanks!
Bob

 

[joes169]

Are you planning on running Abram's tanks over the slab?

4000 psi concrete is viable, as it has less to do with the load on it and more to do with the quality of the surface. Higher strength mixes will wear far better.

The #4 rebar on 10" centers is ridiculous IMO, and the 6" depth is likely overkill as well..........

 

[Red05GT]

I didn't think your concern was the slab, but the footer. My concern in bay area, CA
would be the weaker footer concrete cracking which in turn would crack the stem wall
and on into the slab if not properly isolated with perimeter expansion joint material. I
would run a couple of # 4 rebars in the footer for sure.

 

[Falcon67]

Here's a link to our local AHJ details. Easy doc to review with lots of detail. We have expansive soils here and no frost line, but YMMV in comparison. You don't have a freezing problem, but you have an earth turns to jello problem, occasionally. This is the doc I used as a guide, although I live in a nearby city.

http://www.abilenetx.com/buildinginspection/doc/concrete_foundations.pdf

More to your Q
- All concrete shall be of a design mix to have a 28-day compressive strength of 3,000 p.s.i.

 LAP splices shall be in accordance with the following:
 LAP #3 reinforcing steel (3/8” diameter bars) minimum 15”
 LAP #4 reinforcing steel (1/2” diameter bars) minimum 20”
 LAP #5 reinforcing steel (5/8” diameter bars) minimum 25”
 LAP #6 reinforcing steel (3/4” diameter bars) minimum 30”
 LAP #7 reinforcing steel (7/8” diameter bars) minimum 35”
 Anchor bolts – spacing 6’ 0” O.C. (On Center) maximum. 12” from corners. ½” X 8” minimum size bolts.
 Provide chairs or other suitable supports for slab and beam reinforcing.
 All reinforcing except #3 reinforcing steel shall conform to ASTM grade 60-type steel. #3 reinforcing steel
may be grade 40-type steel.
 Minimum slab thickness to be 4” and minimum reinforcing to be #3 bars at 18” B.W. (Both Ways)

 

[willymakeit]

I would follow your AHJ ecommendation. 2500 psi is your break compressive strenght. Most concrete anymore exceeds this at a 7 day break. There is no way to know for sure without a 7,14, and 28 day break if you have a need to know. This involves having a engineering firm do air, slump test and test cylinders.
I dont think 4000 psi is excessive for floors. I personaly like 3000 or more for walls.
Your local building dept may have a standard guide line.

 

[bob_mp]

Are you planning on running Abram's tanks over the slab?

Interesting - that's exactly the question the concrete sub asked. Planning on a 2 post lift.

I ended up going with 3500 PSI for the foundation. It's probably overkill. It wasn't particularly expensive, though.

Thanks for the help guys.

Bob