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Re: Slab on grade

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I am responding to the original post and several responses, but to save space
I am not copying the original text and responses into this note.

Regarding Mr. Horning's comments. First, polypropylene fibers are NOT
flexural reinforcement.  They are only shrinkage reinforcement.  Once the
concrete has shrunk the fibers provide very little value in flexure.  You can
call any supplier and talk to their engineer and they will tell you not to
count on fibers for flexural reinforcing.  If you need that type of
reinforcing then put the steel in.  One of the advantages of WWF is that it
does provide some, an admittedly low amount, of flexural reinforcement.  It
can help tie the slab together for minor cracks.  But its main purpose is
also for shrinkage stresses.  If you need flexural reinforcement, put it in,
not WWF.  Polypropylene fibers will temporarily leave a fuzzy surface on the
concrete, but it does wear off.  Polypropylene fibers are best used where WWF
would have a possible rusting environment, such as an outdoor exposure. 
Also, shrinkage reinforcing is usually placed about 2 inches down from the
top of slab.  Since you are putting #4 at 12 EW in the middle in the middle
for flexure, you will probably be okay.  But if your main intent is shrinkage
control, you may want to move it up slightly.

The suggestion of spacing your joints in feet at 3 to 5 ties the slab
thickness in inches is a good rule of thumb.  And, if you are going to put
control joints in the slab you should reduce the reinforcing at that location
to effectively weaken the slab.  This means you have to have planned the
joint layout as you lay out the reinforcing.

One critical item not mentioned is the thicken slab.  I am not sure why you
are doing it.  You said you are not attaching the slab to the foundation, so
why are you thickening it?  Why not just float it at the edge?  But, if you
do thicken it you have to adjust your jointing.  Thickening the slab creates
a point of resistance for the horizontal movement that occurs as the concrete
shrinks.  First, let's consider a typical concrete floor slab that is jointed
on all 4 sides.  How does the concrete shrink?  Shrinkage causes the
perimeter edge of the panel to move towards the center.  This is what opens
up the joints.  Therefore, the slab movement is greatest at the perimeter
edge and is zero in the middle.  Everything shrinks towards the middle.  Now
when you thicken a slab you create resistance to horizontal movement so that
the point of zero movement is now at the thicken slab and the greatest
movement is at the other edge parallel to the thicken slab.  You have
effectively doubled the size of the panel.  The thicken slab shrinkage behave
is the same as an ordinary panel twice its size.  So, the location of the
first shrinkage joint parallel to the thicken slab is 1/2 the spacing of a
typical joint.  Given your 27' x 105', I would locate the first joint 9 feet
from the thicken slab and then have an 18 feet panel to the center
construction joint.  I would space the transverse joints at 17 to 18 feet on

Curing a slab this large is very critical.  And, it doesn't matter if the
roof is on or not.  Curing should start immediately after finishing is
completed.  You should either wet cure it, or use a high solids curing

A 6 inch slab is quite substantial for such a light plane.  If you can limit
the area where you will park the plane and not put the dump truck or tractor
there, you might save some money by going to a 4 inch slab.

Someone previously mentioned expansive clays.  If you do have them in your
area then you must design for them, or you will have major problems.

One last item.  Concrete does shrink, and it does crack.  You have not failed
if you do find a crack.  Your goal is to either spread them out as micro
cracks which are not visible to the eye, or control where they occur, at the


Richard Lewis, P.E.
Missionary TECH Team

The service mission like-minded Christian organizations
may turn to for technical assistance and know-how.