From: James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)
Date: Thu, 4 Jan 2001 14:17:10 -0500
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)seaint.org 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 = --------------
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.