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Re: construction joint spacing for SOG

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Speaking fundamentally and without specialty in high tech SOG's:

For a 6" slab, using F=1.5 (as per general rec.) and fs =30,000 psi allowable 
rebar stress, #3@12 allow for an L=59 ft. According to the subgrade theory 
then, this is the amount of rebar required to keep shrinkage cracks small and 
tight (not prevent them) in a slab with construction joint spacing at about 
60 ft. apart. The #3 would stop at the construction joints, where smooth 
dowel bars would be used so as to allow shear transfer without in-plane 
shrinkage restraint of the slab. Note that this amount of rebar gives 0.15% 
reinf ratio, which is comfortably in excess of the 0.10% sometimes 
recommended as required minimum rebar in a SOG. If small, tight cracks are 
not objectionable then, I don't see that control joints are needed in between 
these construction joints.

If small, tight cracks are not OK because they are visible or may degrade 
with use, then control joints (with depth at least 1/4 slab thickness), which 
act as stress risers, should be added with the purpose to force the shrinkage 
crack locations to be at the control joints. For the 6" slab, with 3/4" and 
larger aggregate, the spacing of the control joints would be 15 ft. apart 
according to info in the PCA "Concrete floors on Ground". BUT I am not sure 
if it is really necessary, at added expense, to stop the #3 rebar at each 
control joint and provide a smooth dowel bar. With control joints at this 
relatively close spacing, will not the shrinkage cracks still form in the 
joint even though the #3 continues through the slab at these locations ? What 
does experience show ? I agree that it is conservative to stop the slab rebar 
at control joints, and add a smooth dowel bar, and I agree that this may 
increase the chances of shrinkage cracks forming at control joint locations. 
But it would seem that the restrained shrinkage forces are so large that the 
presence of a #3@12 in the middle of the slab has little effect in altering 
shrinkage crack location for relative close control joint spacings as 
determined from the long standing criteria (joint spacing in ft = 2 to 3 
times the slab thickness, depending on agg size and concrete slump). Again, 
what's the experience ?   

______________________________ Reply Separator _________________________________
Subject: construction joint spacing for SOG
Author:  seaint(--nospam--at) at Internet
Date:    01/04/2001 11:19 AM

We are designing a slab on grade for an industrial occupancy with moderate 
fork truck loading. We have used the PCA-type approach as outlined in Ringo 
and Anderson's book. We have used the Subgrade drag equation for the basis 
of calculating steel area in slab.
        F L w
As = --------------
        2   fs
The value of L is the slab length between free ends which we have assumed 
is the distance between construction joints. We have detailed a formed key 
at the construction joint with no steel running through. The slab is 
thickened at these locations to account for the effects of loading at a 
non-continuous edge. In our case we were using 10M (little bigger than #3) 
at spacings to suit the value of "L".
1.  We design thickness for loading on the interior of the floor slab. What 
about the control joint locations. Would not a sawcut of 1/4 depth be 
treated as a dowelled joint therefore causing the slab to require 
thickening at all control joints? Because of this you would end up treating 
the entire slab as being partially unsupported because the suggested 
tapering for thickness changes is 1:10.
2.  At free ends we used slab thickening with a formed key with no steel 
running through. Any comments? The frequency of the construction joints is 
almost dictated by a reasonable amount of reinforcing. This seems to be 
construction joints that are a little too close (70' to 100'). Any thoughts? 
3.  I just received a call from a manufacturer of steel fibres. They sound 
great but I have not used them before. The most important thing in this job 
is too have NO cracking as this is replacing a cracked floor that was 
constructed not too long ago. Has anybody had any negative experiences 
using the steel fibres? They are suggesting the total replacement of the 
steel with the steel fibres.

David Handy, P.Eng.