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Re: Radiant heat/conc. floor

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I will move into a home which I've been building for the last year in about
3 weeks.  We used radiant heating in both the slab on grade, and in 1 1/2"
gypcrete over plywood/framed upper floors.  All the tubing was plastic (PEX,
oxygen permeable - a different brand than Wirsbo).

We reinforced the SOG with #4 @ 12" EW Ctrd in the 5-6in slab.  The
toughness of the slab was due to my own conservatism combined with the mix
of compacted fill & natural subgrade bearing.  Plastic tubes were tied on
top of the reinf'g (after its placement) at 9-10in intervals per the system
design grid.

Slab was, unfortunately placed on a hot day, approaching 90degF air temp by
11am and 100 degF later.  We didn't use fibermesh (wish I had now) but did
use a mid-range water reducer and kept W/C < .45 but had slump approx 6in.
I was there and we only added about 2gal water (10CY truck), mainly to
placate the flatwork crew.  Pumped mix, 3/4" max aggregate.

Slab has several cracks which have not opened to the point I would have
"structural" concerns.  Interestingly, these cracks did not become noticable
to the eye until we turned on the heating system, nearly 7 months after
placing the slab.  The cracks are definitely not related to the tubing
pattern (we laid the tubes ourselves) nor are they related to the gravity
loads bearing (a thickened portion of the slab carries columns/shearwalls).
They are stable and not growing.  I think the tubing heated and expanded the
top of the slab when it was 1st turned on more than the bottom due to
groundwater moisture present below (no heat during a record rainy winter)
and caused the slab to arch upward due to the expansion-induced curvature.

1.    If the tubing used is in the typical 1/2-5/8in range, thicken the slab
to 5in minimum and use fibermesh.  Center tubes as much as possible,
definitely DON'T keep them near the top.
2.    Use all the usual SOG design methods for a good, exposed industrial
floor (reasonable reinf'g, lower W/C ratio, DON'T use a pea-gravel mix even
if pumped, &etc.).
3.    Depending on geometry (interior corners) and aspect ratio, consider
using Zip-Rib for crack control joints.

Our success on the gypcrete portion has been good - limited cracking and
none associated w/ the tubes.  If you use ceramic tile, make sure to specify
an "anti-fracture membrane" over the gypcrete and to which the tile/thinset
directly bonds.  We used Laticrete products successfully (thick rolled on
coat, lay on fiber membrane, roll on coat immediately above membrane).  The
laticrete went down easily and is tough enough that cabinet crews did not
damage it when finishing, not installing, the cabinetry.

Good luck.  Radiant is by far the BEST heating system, although most
Americans won't pay for its higher cost.

Laurence Oeth, P.E.
Portland, OR

"Mark D. Baker" wrote:

> Does anyone have experience with a radiant heating system in concrete
> slab on grade? I have a client wishing to use this type of heating
> system. What precautions can be taken to ensure slab cracks not occuring
> at locations of pipes within the slab?
> Any takers on this?
> Mark