I have always been a little wary of the subgrade drag approach, but there
isn't much else to hang your hat on. Your F factor should probably be pretty
generous to account for the restraint created by the thickened edges, and
you can set your fs low, but sometimes adding more steel just aggravates the
cracking problem, since it creates more resistance against the shrinking
concrete. A good guide on the interrelationship between reinforcement
levels, member thickness, and concrete tension stress levels is contained in
PCA's circular on the design of circular R/C tanks.
I would put a double layer of polyethylene sheeting under your slab as a
slip sheet and reconsider those thickened joints. I would use formed rather
than sawn joints and start your pours from the center of the building and
work out--the idea being to give each pour the best chance of relieving its
thermal shrinkage as possible before the next pour, and minimizing restraint
on the greenest side of the joint.
I would also insist on a very well controlled wet curing process, do
everything you can to keep the concrete cool and the heat of hydration down
(lower f'c and less cement, Type II cement). You could consider a
post-tensioned slab if cracking is extremely critical, but I don't know if
that is feasible in your situation.
I don't know how effective steel fiber will be. I would expect it to be
pretty helpful from the perspective of very good distribution, but have no
direct experience. Plastic fibers will not prevent shrinkage cracks,
although they may mitigate their size.
From: David Handy [mailto:dhandy(--nospam--at)trg.ca]
Sent: Thursday, January 04, 2001 8:20 AM
Subject: construction joint spacing for SOG
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.