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Slab on grade

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Subject: Re: Slab on grade
Author:  James F Fulton at CCMCED01
Date:    10/8/98 9:41 AM


For control joints, there is a rule of thumb that says space control 
(contraction) joints in feet at between 2 to 3 times the slab thickness in 
inches, for a 6" slab this amounts to 12' to 18' joint spacing. 

Your proposed rebar of #4@12 for the 6" slab gives a reinf. ratio [.2 sq 
in/(12x6)] of .28%. I think you could back off from that some, say to #4 at 
14" (.24%) or even maybe #4@16" (.2%). On location in slab, some refs say 
locate at 1/3 thickness from the top, which is 2" clear from the top, but I 
think at mid depth is OK for the 6" slab. You must understand that whether 
the LOADED slab cracks or not depends more on the subgrade soil conditions ( 
its modlus of subgrade reaction) and concrete tensile strength than the 
reinforcement. Since concrete tensile strength is important, we always 
specify 4000 psi strength concrete. The slab should receive a 7 day minimum 
cure, preferably a wet cure with sprinklers or saturated burlap, rather than 
spray with curing compound.

This slab design should be acceptable for shrinkage crack control and for the 
plane load you gave, but a loaded dump truck could cause the slab to crack 
structurally, depending on the MSR for the soil. PCA has a long used 
publication and calc procedure to check/design the grade slab for wheel 
loads. I would not plan on "no cracks".

If the clay is highly plasitc, many times the soils engineer will recommend 
that it either be lime stabilized or be removed and replaced with compacted 
backfill or even a lean clay if available. The excavation depth plus      
subbase of 18" is specified at our Houston area plant with their clay. So if 
a 4" gravel subbase is used, then about 14" of high plastic clay removal is 
needed. Your clay may not be as bad.


______________________________ Reply Separator 
_________________________________ Subject: Slab on grade
Author:  seaint(--nospam--at)seaint.org at Internet Date:    10/8/98 8:42 AM


I am writing this not as an engineer, but as an owner/contractor.  My 
experience, both as SE and contractor, is exclusively with steel.  All I know 
about concrete is that it cracks.

I am acting as engineer/contractor/owner/banker/inspector/anything-else- 
anyone-can-think-of on a 54'x105' hangar.  (I've always heard that anyone who 
acts as their own attorney is employing a fool, does the same apply to 
engineers?  It's fun wearing all the hats!)  

I plan the SOG to be 6" 3000 psi concrete with #4@12" both ways in the 
middle, and a 12"x12" thickened edge, tapered to normal slab thickness at 
2:1, on 4" clean gravel.  The footings are not cast integrally with the slab. 
 Soil is well compacted red clay common to eastern Tennessee. Use will be for 
planes less than 3200#, plus dump trucks, backhoes, and any other toys my 
wife acquires.  The finisher is just that, a finisher.  He knows nothing 
about the stuff we're supposed to worry about - rebar placement, crack 
spacing, mix, etc.  Three questions:

1)  How often should I cut control joints.  I will form and pour the slab in 
two pours on seperate days, each 27'x105'.  I want the minimum number of 
joints, but I also don't want any random cracks.  Suggestions?

2) What about fibers?

3)  Is there anything in the above that jumps out at those experienced with 
concrete and shouts "This guy is an idiot"?  Anything I should do different?  
Again, suggestions?

By the way, if there are any pilots out there comfortable on 1900' of grass, 
The 100 Aker Wood private field is at N85d46' W84o46'.  Drop in someday.

TIA

Phil Hodge
phil(--nospam--at)joistdesign.com