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Re: Air Ducts in Slabs-on-Grade ?

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Ken

The use of ducts for supply air or ventilation embedded in or under
concrete slab on grade construction is not that unusual.  We have a
group of elementary schools in this area that the HVAC system operates
through ducts installed this way.  A similar system is sometimes used to
provide waste heat beneath the slabs of on-grade freezers.

Do the existing ducts extend below the perimeter grade beam/foundation
stem wall or below the ground water table during the wet season?  If so,
sealing the ducts and rerouting the HVAC system may be the easiest way
to eliminate the majority of the existing problem.  If the duct work is
above the bottom of the grade beam and also above the water table
providing a good waterproofing membrane on the exterior face of the
foundation wall and installing a good footing drain system may be a
practical solution.

Leaving the voids in the slab unfilled may be problematic with a high
water table since the ducts may accumulate moisture and support the
growth of molds over time.  Therefore filling the voids with a pumpable
grout may be desireable.


"Caldwell, Stan" wrote:
>
> Ken:
>
> My post, way back whenever, probably involved ducts embedded within
> elevated, structural concrete floor slabs.  I have never heard of air ducts
> being cast into slabs-on-grade.   It seems to me that the types of problems
> that you are now experiencing could have been predicted by any competent
> structural engineer, even back in 1950.  Then again, single family homes
> didn't usually involve engineers back then, and very few do today.  Texas
> state law still does not require such homes to be designed by licenses
> professional engineers.  Our firm does not, and will not, do any residential
> work.
>
> Now then, my specific answers to your questions are:
>
> 1]      No.
>
> 2]      I recommend that you fill all of the voids 100% with high strength
> grout.  This can be done by pumping.
>
> 3]      This probably varies from company to company.  Have your lawyer read
> your insurance policy.  Perhaps you can hit the jackpot ... are there any
> signs of mold? <grin>
>
> I am forwarding this correspondence to the SEAINT Listserv.  Perhaps one of
> the 20,000 other subscribers can help you.  You can monitor the responses
> without subscribing by searching through the living archives at
> http://www.euken.com/group/seaoc/.
>
> Good luck!
>
> Stan R. Caldwell, P.E., F.ASCE
> Vice President
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Halff Associates, Inc.
> 8616 Northwest Plaza Drive
> Dallas, Texas  75225
> Phone:  (214) 346-6280
> Fax:  (214) 739-0095
> Email:  scaldwell(--nospam--at)halff.com
> Website:  http://www.halff.com
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> -----Original Message-----
> From: Ken Schaefer [mailto:kens(--nospam--at)bsapr.com]
> Sent: Monday, January 28, 2002 4:00 PM
> To: scaldwell(--nospam--at)halff.com
> Subject: in slab ducts
>
> While doing some searching on the internet, I came across bulletin
> board in which you had responded to someone asking questions about
> in-slab ducts.  If I'm mistaken, my apologies and just let me know.
>
> If not, I've got a couple questions for you.  I have a home in Fort
> Worth built in the 1950s.  Each room has at least two air returns
> along the baseboards.  All of the air returns, run through the slab.
> Evidently, they were some type of box form as there is nothing but
> concrete on all four walls of ductwork.
>
> We've had piers (18 ft deep) placed around the entire perimeter of
> the home to stabilize some movement.  In spite of this, there are
> evidently cracks in the slab the run all the way into the ducts.
> During heavy rains, this allows rain water to flow into the ducts.
>
> My questions to you are:
>
> 1) Have you seen this problem before?
>
> 2)  If so, can it be suitably resolved by sealing off all of the air
> returns (without filling the duct cavity with concrete or other
> material) and creating new in-ceiling air returns through out the
> house?
>
> 3) If in fact you are able to comment on any of this, might you have
> experience as to whether Insurance companies cover this type of
> structural failure?
>
> Thanks for you assistance.
>
> Ken Schaefer
> Blanchard Schaefer Advertising and Public Relations
> 1112 East Copeland  Suite 310
> Arlington, TX  76011
> Fax 817.860.2004
> kens(--nospam--at)blanchardschaefer.com
> www.blanchardschaefer.com
>
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--
++++++++++++++++++++++++++
Forrest T. Braun, P.E.
BBFM Engineers, Inc.
Ph (907)274-2236
Fx (907)274-2520
Anchorage, Alaska
http://www.bbfm.com
++++++++++++++++++++++++++

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