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Slab on grade

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Phil,

Check out Concrete International Magazine, July, 1998, for diamond plate 
dowels.  [PNA, Inc., www.pna-inc.com]  This appears to be a lot easier than 
drilling holes in forms and embedding greased dowels.  I don't know what the 
cost difference would be, but the labor would sure be easier.

Also, check out ACI Committee Report 302.1R, "Guide for Concrete Floor and 
Slab Construction."  This recommends the maximum spacing of contraction 
joints to be, in feet, 2 to 3 times the slab thickness in inches.  For this 
recommendation, 42' would be much too long;  don't even think about 105'.  
Use wet curing --- do not use spray-on membrane curing.  Proper use of wet 
curing will also help prevent slab curling at construction joints.

Instead of gravel, I prefer to use clean concrete sand.  Compacted with a 
vibratory plate compactor, it can become as hard as concrete and will help 
eliminate capillary action.  Not using a visqueen barrier will also help 
prevent curling as, during curing, slab moisture can progress downward as 
well as upward.  If visqueen is used, it is all the more important to use wet 
curing of the concrete. 

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

P.S.  It's good to see another DOS user, if for nothing else, to share the 
jabs of Bill Allen and Stan Caldwell.  DOS LIVES! (www.caldera.com for DR-DOS 
and OPEN DOS as well as OPEN LINUX.)

Phil Hodge wrote:

. > Joints:  I will use smooth greased dowels at the cold joint up the
. > middle.  16" long 1/2" rods @ 12" sound right?  I'll also stop half
. > the rebar at the saw cut joints.  I plan on getting on the slab the
. > following morning to cut joints.  Thanks for the reminder on all
. > those points.  The building will have 42' wide doors, so a 21'
. > modularity works out fine, and advance planning is no problem.  Of
. > course, I'd rather go with 42' spacing.  Mark Gilligan questioned,
. > and Dick Horning responded, with a brief discussion of joint
. > spacing versus rebar.  Dick - how much more steel would I need for
. > 42' spacing?  How about 105'?  Is Jim Kestner's formula applicable
. > over a wide range, or only for "reasonable" spacing.  It looks a
. > little too linear to cover loooong spacing.  Also, shouldn't fs be
. > 60,000 if I'm using Gr 60 steel?
. > 
. > Base:  The base, and sub-base, are what exists.  I can, and am,
. > moving it around some, but I'm stuck with what I have.  It's clay,
. > but appears to be non-expansive.  Three years of playing with the
. > stuff, plus charts showing expansive clay regions of the country,
. > both indicate that is not a problem.  It's well compacted, and all
. > the soft spots found during compaction have been removed and
. > backfilled.  I can add 4" of clean gravel, but my understanding is
. > that's to prevent capillary action drawing water up through the
. > slab, not to help the base any.  I'm willing to spend the money for
. > the gravel if it'll help anything, but would prefer to not waste
. > it.  Same thing for visqueen - I see it spec'ed a lot, but if
. > moisture is not a serious problem isn't it a waste of money?  Or
. > does visqueen reduce the subgrade drag coefficient, and thus the
. > tendency to crack as it shrinks?  What is an appropriate subgrade
. > coefficient in the formula from Jim Kestner?
. > 
. > Curing:  The slab will go in before the building, but I can easily
. > sprinkle the slab for a week after pouring.  I hope to pour it this
. > winter, so high sun and temperature shouldn't be a problem.  We can
. > do things like that in Tennessee, doesn't make me miss previous
. > homes in Pennsylvania and Minnesota at all.
. > 
. > Fibers:  The manufacturers all make their limitations very clear -
. > I ask only about using them in addition to the steel, not instead. 
. > They increase the cost of the material and finishing labor about
. > 10%.  Burning off any loose fibers after curing is no problem, and
. > they might even degrade due to solar radiation by the time we get
. > the roof on.  But is the $1,000 additional cost worth it?  If it's
. > the difference between visible surface cracks versus not, than it
. > is.  But I can add 30% more steel for the same cost, in place. 
. > Recommendations?  By the way, my costs will vary significantly from
. > "real" jobs due to way I value my play time.
. > 
. > ACI stuff:  Yeah, I should read all that stuff.  But I don't design
. > SOG at work, and this is supposed to be fun.  It's a lot more fun
. > to get out there on a sunny day and play in the mud than it is to
. > sit in my office and read.
. > 
. > Thickened slab:  Richard Lewis raised a very good point.  I've seen
. > thickened edges more often than not.  I've also seen the slab
. > thickened at all the joints, which would seem to defeat the whole
. > joint.  The only reason I planned to thicken my edge is because the
. > edge of any slab is weaker than the center.  Adding a little more
. > steel should do the same.  So I think I'll go with an additional
. > couple of perimeter bars, and keep the thickness constant.
. > 
. > Usage:  We plan on living at the 100 Aker Wood, with all of our
. > attendant toys, for the rest of our lives.   Hopefully another 40
. > years or so.  With that in mind, I'm not going to try and save a
. > little money now by limiting the areas the heavy equipment can
. > operate.  For now I know the first bay will always house my main
. > plane, but the $420 I'd save by using a 4" slab in that one bay
. > would be likely to backfire some day down the road.
. > 
. > Runway, and other airplane loads:  The loads from a plane in my
. > category, 3200# or less, are significantly lower than what any
. > paved runway is designed for.  The runway is grass, and the tires
. > leave less of an imprint on a wet runway than do the horses or
. > small dogs, about the same as a large dog or person.  When the
. > ground is dry (as it has been all summer), there are no marks at
. > all.  Acceleration is all through the propeller, not drive wheels,
. > and I rarely brake when I land.  So there are essentially no
. > horizontal forces on the runway.  None of this is true at even a
. > normal general aviation airport that might handle light jet
. > traffic, to say nothing of a commercial airport.  The loads to the
. > hangar floor will be roughly equivalent to a car.
. > 
. > Fun:  Lots of it.  I've found I am much more conservative in many
. > of my design assumptions on my own buildings than those for other
. > clients.  It's easier to explain to the owner the cost/benefit
. > trade-offs when I'm wearing all the hats.  There is also a much
. > higher cost of mistake.  On real jobs for which I get paid, if I
. > make a mistake it can cost me some money, I can lose my license, go
. > out of business, etc.  All relatively minor compared to having to
. > live with the mistake.  Joking, or course, but only partially.  On
. > the steel super-structure for this building I'm using an open-web
. > trussed three dimensional rigid frame.  Not the most economical way
. > to do it, but the most fun.  Except I was more nervous designing it
. > than anything I've designed in the past 30 years.  
. > 
. > Richard Lewis pointed out, correctly, that I have not "failed" if
. > there is a crack in the slab.  Cracks in concrete are right up
. > there with death and taxes.  On a "real" job he'd be right, and I
. > wouldn't worry.  On this job, where my goal is to have a slab with
. > no extra cracks, I will have failed in that aspect.
. > 
. > One of the neat aspects of building for myself is I get to prove,
. > to myself, what sort of engineer I am.  I guess that's what makes
. > me more nervous.  The proof of the pudding is in the tasting, so
. > I'm out there eating pudding, and loving it.
. > 
. > Phil Hodge
. > phil(--nospam--at)joistdesign.com
. >