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RE: minimum fc for Residential slab on grade

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Call out 3,000psi or whatever they want, but DESIGN for 2,50psi. Special inspection NOT required for slabs on grade per 1701.5.


I don’t think you can even get concrete less than 3,000psi unless maybe you mix it yourself.


-----Original Message-----
From: Dennis Wish [mailto:dennis.wish(--nospam--at)]
Wednesday, July 09, 2003 1:28 PM
To: light_framing(--nospam--at)
Cc: seaint(--nospam--at)
Subject: minimum fc for Residential slab on grade


I designed a single story residence (spec home but large) using a slab on grade which is typical in this area. Historically, we placed f'c=2,000 psi concrete for everything but where proprietary frames such as Hardy Frames are used. The typical soil in the area is silty sand with a bearing pressure of 1,500 psf and with increases allowed for depth and width.

I received a plan check correction based on the 1998 CBC and 1997  UBC. He quoted section 1921.2.4 which requires high strength concrete but has an exception that allows for concrete not less than 2,500 psi. This is generally required when inspection of the concrete is called for and not for simple slab on grades. The city has, during the 1997 UBC cycle allowed for f'c=2,000 psi concrete, but the contract plan check company requires 2,000 psi concrete so as not to force deputy inspection.


Are there any comments as how I should address the plan check correction since deputy inspection can be expensive and there are no problems with subsidence or expansive soils in the area.


Am I wrong on this one????? If so, it comes as a surprise.