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RE: Slab-on-grade control joint question

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While the plastic shrinkage occurs shortly after the concrete is placed the
drying shrinkage occurs mush more slowly.  Maybe 20 percent of the drying
shrinkage occurs in the first month.  Thus the reinforcement will attempt
to resist the drying shrinkage.

Once the concrete has set up any shrinkage in the concrete will induce
tension in the steel.  According to the theory of crack control joints all
of the shrinkage in aprox 20' of concrete will occur at the saw cut.  If
the total shrinkage is 0.25" then the strain in the reinforcing steel that
passes through the joint will be quite large but only a couple of inches
away the strain in the reinforcing is assumed to be small.  This does not
make sense.  I do not believe that these crack control joints work as
planned except when there is little or no reinforcing crossing the joint.

Realizing that for most slabs small well distributed cracks are not a
problem I propose bumping up the steel area somewhat and not worry about
crack control joints.  

If you are worried about plastic shrinkage I propose that you specify good
curing.  To assume that plastic shrinkage is controlled by the crack
control joints assumes that the concrete (green) is strong enough to drag
the slab enough to open up the joint.  But plastic concrete has little
tensile capacity.

Mark Gilligan



As Gerard pointed out, reinforcement does not really do much to prevent
cracking due to shrinkage.  Since most shrinkage in concrete occurs very
rapidly after the placement of the concrete, the concrete is not "solid"
enough to allow the reinforcment to "bond" with the concrete allowing the
reinforcement to provide "resistance" to the shrinkage.  Keep in mind as
well that the concrete in the slab will cure at different rates through
the slab thickness as the moisture can escape at different rates
throughout the slab thickness.  Since the top of the slab is exposed to
the air, it will be the layer that cures the fastest (thus the reason that
wet curing with a mist system is usually the best method...prevents the
top surface from drying faster than the "core" of the slab).  Thus, the  
shrinkage cracks will form at the top of the slab where there is no
reinforcement (reinforcement is typically supposed to be either at
mid-depth or 1/3 depth from the top, but not right at the top).  The
result is that the shrinkage cracks will typically form very shortly after
the placement of the concrete and most reinforcement will not
slow/prevent/stop these cracks.  Some argue that polypropelene (sp?)
fibers will slow/prevent shrinkage cracks but that is highly debatable.

As Gerard pointed out, WWF (or WWR as it is now called), rebars, and steel
fibers are mainly for temperature reinforcement.  These types of
reinforcement will also help with the much longer term effects of
shrinkage, but don't do much at all for the initial shrinkage of the
concrete right after placement.

As pointed out in recent posts, the best way to minimize shrinkage cracks
is proper curing, appropriate control joint (or weakened plane joints as
Gerard calls them) and construction joint locations, appropriate mix
design considerations (i.e. aggregate size), and good placement/finishing

The point is that using something like a Soff-Cut will weaken the top
surface to "pick" where the shrinkage crack will occur.  The fact that is
does not cut through the reinforcment does not matter too much as that the
reinforcement is at a level in the concrete that more than likely is still
somewhat "fluid" so the reinforcement will not restrain the movement too


Ypsilanti, MI

On Sun, 4 Aug 2002, Cliff Schwinger wrote:

> Since the reinforcing steel is providing the "real" resistance to
> cracking, will a weakened plane (created via a Soff-Cut machine) really
> create enough weakening in the slab (with continuous reinforcing across
> the control joint) to ensure that the crack will occur at the joint and
> not at some other location.
> I have a feeling that if the reinforcing is laying on the bottom of the
> slab it will not be as effective in restraining cracks and the
> "soff-cut" joint will be sufficiently weak to cause the shrinkage crack
> to occur there. But if the reinforcing is properly chaired to within 1"
> clear of the top of the slab, I have a feeling that the "soff-cut" joint
> not that much more of a "weakened plane" than anywhere else on the slab.
> Cliff
> > cracking and are not deep into the slab. In a weakened plane joint,
> the
> > main requirement is to not soff-cut the slab too late. The reinforcing
> > is the same as everywhere else in the slab - no cutting is necessary.

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