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#3s - second try

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I didn't see this come through - and got a message that it was undeliverable - so I'll try it again.
 
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As a second aside,  I probably would not specify # 3s at 12 in. for a slab.   As a general rule,  bars either need to be stiff enough to support workers walking on them, or they need to be spaced far enough apart so workers can step through them.
 
#4s at 12 are often de rigueur for elevated slabs, among other reasons because the math is real easy during layout.   But #3s are really not stiff enough to support some of the individuals one is likely to see at a construction site.
 
#3 at 18 are pretty common in a lot of slabs on ground  - this comes out to 0.1% steel for a 6 in. slab.  The point of such a low steel percentage is not to allow wider joint spacing than an unreinforced slab,  but to prevent any faulting or opening at any cracks that might occur.
 
However this 0.1% is an empirical number.  As are all the joint spacing numbers. 
 
Basically, the ACI 360 discussion is along the lines of "Well, what does everyone think this number should be?".  One guy sticks up his hand and says,  "We've tried 30t joint spacings and with our aggregate and our cement,  it's too much,  we get a lot of cracking."  Then someone else sticks up his hand and says "We can do 30t, but only if it is a 5 in. slab."  Etc.
 
Say there are 60 people at the meeting.  That means there are 120 hands.  And 120 opinions.
 
Gail Kelley