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Concrete Precast Waterproofing

JasonQ | Posted in Construction Techniques on September 27, 2006 11:12am

Gotta good one here…in the house I’m building, there’s a 2-car garage with a precast hollow-core concrete floor.  The area beneath will be used as a woodshop space.  (For those unfamiliar w/ what I’m talking about, I’ve attached a photo.)

Naturally, this can’t be driven on as-is, and will have an additional poured concrete layer atop it after the joints are grouted with concrete.

Now…here’s the problem, and my question.  Problem is – I’m concerned that in the winter time, snowmelt will eventually work its way through the slab, which is a bit thinner than the usual garage floor, and through the joints, thence dripping onto my tools, lumber, etc.  How can I prevent this?  If the damned thing leaks, it’ll be useless as a shop space, and will become a $10,000 storage room.

A coworker has a similar setup, and he put in a “tub” of EPDM roofing membrane after the joints were grouted, then had the slab poured atop.  The concrete flatwork guy on the project, who happens to be my father-in-law, says it’s a bad idea to do this, as the slab will not mechanically bond to the hollow-core, thus making it more prone to bad sorts of cracking, since it’s pretty thin (he said about 2.5″ where it meets the driveway.)

I bounced the EPDM idea off the architect (my dad), and he echoed the FIL’s concern.

Could I seal the joints with some sort of concrete sealant, then grout and pour?  Problem with that is that if the sealant ever gives out, I’m back to Square One.

I’m at a loss.  I don’t know enough to argue successfully with my father-in-law, and he’s not the kind of guy who listens anyway – you can’t tell him ANYTHING.  Any suggestions?

Jason

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Replies

  1. bustaduke | Sep 28, 2006 03:48am | #1

    Don't know if you will have the same problem as we had.

    We used the same concrete precast floors you have in a four story prison here and the top floor ceiling (roof area) rained all the time.

    It wasn't raining because of a roof leak, it was raining because the temperature between the inside and the outside was different and the hollow cores were filling up with condensation.

    The temperature inside was cold around 68 with the temperature outside in the upper 90's with high humidity.

    I'm still not sure how they fixed the problem, I think they increased the spray foam roof membrane to stop it.

    1. McMarky | Sep 28, 2006 08:00am | #2

      I don't know if it is important to have the slab actually poured against the pre-cast.  The slab is not structural, and if a connection was important, then some studs would have been called out.  I would consult a masonry waterproofer, and possibly an engineer, no offence to you father and father-in-law

  2. User avater
    zak | Sep 28, 2006 08:37am | #3

    Have you looked into xypex?  http://www.xypex.com

    I used xypex as an admix for a few concrete roofs, up to about 15'x30'.  It seems to be working as advertised- no leaks through the roof, although condensation is an issue if there isn't adequate insulation/venting. 

    You basically add xypex to the water before the concrete is mixed, then treat as normal, cure it really carefully.

    I've heard of people skim coating cement/xypex on top of existing surfaces too. 

     

    zak

    "When we build, let us think that we build forever.  Let it not be for present delight nor for present use alone." --John Ruskin

    "so it goes"

     

    1. Stray | Sep 28, 2006 04:01pm | #4

      Standing water is your only enemy with snow melt.  A good epoxy paint coat on the garage floor/topping slab should be sufficeint IF you pour the slab with a nice slope to drain (or to the door).  Sure, you'll have to re-do this evey once in a while as it wears away.

      With snow melt probably comes melted road salt as well (depending on your location) and that aint good for your concrete slab or precast anyway.  Slope to drain will get this away from your nice new concrete. 

      I would think placing a rubber dam between the two will also trap moisture under/within your topping slab.  Is there any chance the garage above the shop will get down to freezing temperatures?

      EPDM might work for a while, but without being protected from abrasive damage between the slabs, the thermal cycles and differential expansion rates will essentially cut it apart and compromise it.  If you REALLY want to do this, you'd need to go for a waterproofing membrane SYSTEM, designed for commercial roofing applications.  They have friction mats that protect the membrane from damage.  This will be pricier than just buying a sheet of rubber to throw down though.  And I'd still be leery of allowing the top slab to be saturated with the water and salts.

       

       Ithaca, NY  "10 square miles, surrounded by reality"

  3. Lansdown | Sep 28, 2006 04:07pm | #5

    There are waterproofing systems for just that application, I see it spec'd all the time around here for parking garage structures. I can't remember the names off hand, if I find them I'll post back. You also might want to contact the pre-cast manufacturer for suggestions too.

  4. User avater
    Gene_Davis | Sep 28, 2006 06:34pm | #6

    My house has a heated shop underneath a two car garage.  Same situation as you . . . the absolute assurance of waterproofing is a mandate.

    We coated the "topping" slab with a Sonneborn elastomeric system.  Both primer and topcoat went on at 80 mils wet, using a notched squeegee, pole-mounted, then backrolling.  The product is a two-part moisture-curing urethane, and when mixed, is the consistency of heavy acrylic paint.  Overnight cure for the primer, then the topcoat.

    Topcoat is available in various colors.  We used a mocha tan.  To give the topcoat some grab and render it slip-proof, we sprinkled on by hand, between the squeegee and the backrolling, a light coating of granulated rubber.  It had the consistency of coarse sand.  The backrolling mashes it in and wets all the particles.

    The cured membrane is thick and elastomeric, sufficient to bridge any kind of cracking that can occur.  With the 160 total wet mils of membrane, plus the embedded rubber grit, the floor has a distinct spring to it.  Drop a crescent wrench on it, and it lands with a soft thud, and does a tiny bounce.

    This stuff is typically used in parking garages and commercial terrace decks, and is not inexpensive.  But it sure is worth it.

    Conipur¯ II Deck Coating System

    View Image

    Conipur¯ II Deck Coating System

    Conipur¯ II Deck Coating System is a fluid-applied polyurethane waterproofing coating. Conipur¯ II uses a fast-setting two-component reactive curing mechanism. It has a very low odor and is VOC compliant. Conipur¯ II Deck Coating System is composed of: Conipur¯ 78 Primer, a two-component polyurethane-based adhesive primer. Conipur¯ 265-Z Base Coat, a two-component fast-curing polyurethane base coat with outstanding mechanical properties including excellent elongation. Conipur¯ 275 Top Coat, a two-component fast-curing aromatic polyurethane topcoat with outstanding mechanical properties, including high tensile strength, and excellent tear and abrasion resistance. Conipur¯ 285 Top Coat, a two-component aliphatic 100% solids, fast-curing topcoat that provides superior performance and UV resistance.



    Edited 9/28/2006 11:36 am ET by Gene_Davis

  5. User avater
    BillHartmann | Sep 28, 2006 06:48pm | #7

    I have a similar garage.

    But mine is solid, poured in place. It is well sloped and has never shown a drop of water or even moist spot showing below.

    I see that you already have the preceast in place. But if I was doing one today I would seriously look at using the ICF forms for the slab.

    Lots of ehat is lost through that concrete ceiling.

  6. dendrone | Jun 24, 2012 10:36am | #8

    MY hollowcore project

    Jason,

    I am currently doing a project with Hollow Core Panels. Wanting to waterproof the underside like you did and was wondering the longterm outcome.....

    Mine is a deck project and will be exposed to the elements from the outside. Most of it will be covered by extended roofing above, however, wind driven rain and the end of the deck will be fully exposed. This will create a challenge for keeping the below workshop dry.

    Any help is greatly appreciated.

    Dennis

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