Waterproofing a precast garage deck for occupancy below

[JAE]

I've got a project where a precast double tee parking garage is intended to serve as a "roof" over an occupied space below. The question I have is how to properly detail the deck to be waterproof.

The two ideas I've seen are:
Option 1
1. On the double tees, place a 2 to 3 inch concrete topping as a composite structural slab over the tees to serve as a diaphragm.
2. Over this topping, place a waterproof membrane with a thin protection board (1/4" perhaps).
3. Over the waterproofing, place a layer of rigid insulation (4")
4. Over the insulation place a 4 to 5 inch thick concrete wearing surface - basically a pavement of sorts with sawed joints.

Option 2
1. On the double tees, place a 2 to 3 inch concrete topping as a composite structural slab over the tees to serve as a diaphragm.
2. On the topping place a layer of rigid insulation (4")
3. Over the insulation place a 4 to 5 inch thick concrete wearing surface - basically a pavement of sorts with sawed joints.
4. On the wearing surface place a traffic bearing waterproofing system (Autogard by Neogard or similar).

Are there any problems with the above systems and are there any other methods that others have used for this situation?

Thanks,

 

[dik]

I've used Option 2 with a 2-1/2" topping over the DT's and 4" topping on top. The 2-1/2" topping had control joints tooled into it between the DT's and caulked with a polyurethane.

Dik

 

[JAE]

dik - no insulation?

 

[JAE]

Also - what sort of waterproofing did you use?

 

[Brad805]

If you put enough rebar in the topping to minimize the crack width, you could look at internal waterproofing the concrete with Kryton or Xpex. I also learned about another newer internal admixture where they offer guarantees with the product. The literature is not in the office right now, but I can get it if you are interested.

Another idea is using a torch on membrane. A sloped insulation package and SBS package may be more cost effective than pouring the sloped topping if you can develop the diaphragm forces with welded connections.

Brad

 

[msquared48]

I've never done either, but one thing strikes me nere.

With the option #1, should a leak occur, your only option to seal the leak is to add another entire membrane of the same type as #2 on top of the slab.

Should it happen to #2, it clould be spot sealed with the same material.

I don't like the membrane as the wear surface against vehicles, but for ease of maintenance should a leak occur, it is more convenient.

One question I would have of the sealant is how well does it withstand tire squirm on a hot day?

Mike McCann
MMC Engineering

 

[jh2002]

You might want to consider a split slab system with a drainage layer sandwiched between the waterpoofed structural slab and the wearing surface. I believe this type of system is common for plaza decks, but I would imagine there are drainage composites that are capable of handling vehicular loads.

 

[Ron]

Mike has hit on the difficulty of membrane waterproofing on traffic surfaces. If the membrane leaks, you have to remove the topping to get to the leak.

I would consider, rather than a 4 or 5 inch topping, using pavers over the membrane. I would insulate from the underside of the double tees and not worry about having to deal with compressible insulation under traffic. Using the pavers allows the membrane to be repaired if necessary without undue expense.

As for a membrane, the better waterproofing is achieved by hot asphalt or coal tar; however, double tees tend to move a lot, so built-up membranes are less tolerant of movement. Single ply systems are tolerant of movement, but they are not as good for long term waterproofing as asphalt based systems. In this situation, I would probably go with a high quality membrane (say a scrim reinforced hypalon), carefully detailed and with rigorous inspection requirements. You could also consider a high quality urethane system such as Vulkem 351. In both cases I would use backer rod and sealant in all double tee joints.

DO NOT depend on concrete admixtures for waterproofing. The don't work well enough for such applications.

 

[dik]

Sorry was referring to concrete only... Option 2 has 4" rigid insulation...

Dik

 

[dik]

I had to dig up a project folder... I was thinking of a Tremco product and Ron called it Vulkem 351 is the one I've used a couple of times and it is excellent. I've not used an ethafoam backer bar, but with an actual joint (some sort of mechanical anchorage for lateral). I've used alterating plates to prevent vertical differential movement, has worked OK so far... I've used the polyurethane caulk with a tooled joint and it works very well. Not much individual movement. In addition, I use 'real' expansion joints... you can either install a cheap one, or alternatively, an expensive one that works. I also generally install a stainless steel 'gutter' with the expansion joint...

Dik

 

[JAE]

jh2002 - the detail we've looked at is similar to one by Carlisle (attached).

Ron - individual deck pavers on a parking garage? Does that work? Seems like the pavers I've seen are for pedestrian plazas for foot traffic only.

dik and Ron - I'll check out the Vulkem.

 

[Brad805]

Ron, I assume the internal waterproofing problems you have seen relates to the deformation of the double tees?

Not sure why I thought a torch on made sense. We do not do parking structures. Sorry.

Brad

 

[Ron]

JAE...interlocking concrete pavers are fine for vehicle traffic use. Check out this link...

Paver Guide Specs

Brad805...a modified bitumen membrane has superior waterproofing capability and I specify them. For this application, it would require no attachment of the membrane within about 12 inches of each DT joint to allow strain relief in the membrane from the DT movement. This makes it a bit more difficult to work with in this application.

 

[JAE]

I'm not sure I can put the insulation below the double tees but I suppose it could be an option.

Ron - the pavers you linked to are fine (I was thinking of pedestal mounted 2 ft square type pavers with air below).

The double tees WILL have a concrete topping on them and probably have to have joints created at the flange joints as cracks will most likely occur along those edges. So the waterproofing would be on top of the bonded DT topping. So if you use a hot asphalt waterproofing are we talking about probable tearing along those joints (vs. a membrane with a 12" setback on the attachment)?

 

[Ron]

JAE...less of an issue with a good topping on the DT. I would still use a slip sheet over the probable crack locations, even if only a narrow one (6-8"). One solution for this is to use a crack isolation membrane under the modified bitumen membrane, as the slip sheet. This allows everything to be fully adhered.

Also, I like hot mopped modified bitumen over torch applied. Torch applied requires a lot more skill to properly apply, and when applied directly to primed concrete, the concrete becomes such a heat sink that the tendency for debonding is greater.

I am a fan of asphalt or coal tar roofing/waterproofing over synthetic membranes.

 

[ToadJones]

this post has grabbed my attention...I don't work with this type of thing at all.

Why is there so much concrete used in the system?

Are the DT's not capable of forming a diaphragm without a composite slab?

 

[dik]

I've not done it, but you can use 'pavers' for large aircraft, too... have several papers on this.

Dik

 

[SteelPE]

I worked on a similar project a number of years ago. I have a 15,000 square foot basement with parking above. The parking was on grade and I was paranoid about having to support loading from Fire Trucks.

We decided on using a composite steel beam and deck system (the column spacing below was not critical as the space was used for storage). A membrane was then placed above the composite slab with a protective board and then a 3" concrete topping/wearing surface.

In this instance, applying the membrane on top was not an option.

I don't remember the name of the product used as the membrane. Is was a field applied coating. I remember there were some issues during construction as the GC wanted to apply the material before the concrete had time to cure.

 

[SkiisAndBikes]

A couple comments about the waterproofing systems...

With option 1, I would look towards a hot applied rubberized asphaltic type membrane. A single ply system would be problematic over a DT system, however a two ply system with spun bonded polyester fabric reinforcement embedded in the system would perform well. Bakor 790-11 and Hydrotech 6125 are two examples of systems used widely on bridge decks and parking structures. Thicknesses will have to be controlled to ensure the membrane does not exceed the recommended thicknesses or else the topping may start to 'move/float' under vehicular start/stop loads. Detailing of joints, etc.. is important. Others are correct when they state that any waterproofing repairs will require local removal and replacement of the concrete topping. At some point in the distant future, the entire topping would have to be removed and replaced once the membrane wears out. That may be 30+ years from now, but should be a consideration. You will need to think about drain locations to take away the water from the top of the membrane ... drain locations are usually a weak spot in the membrane system and leak first. A thin type waterproofing system, if there is slope to the deck, might not need drains ... not sure of your situation related to slope of the deck.

With option 2, there are many thin type waterproofing systems. I am a fan of polyurethane type systems. Vulkem by Tremco is one product, there are many others. These systems are very durable to vehicular traffic and generally their specifications include beefed up specifications for heavy traffic areas such as turn aisles, ramps, entry/exit booths, etc.. They can easily last 10years plus (I have seen 25+), however, depending on how the membrane is wearing, the top surface can be lightly shotblasted and recoated as a maintenance item. Replacement does not require removal of the topping. Surface preparation is paramount!! The longevity of the thin system is dependentant on a good bond to the concrete topping.

Pavers can support very heavy traffic loads (buses/trucks) if the correct product is specified. I have seen them used often over podium slabs above occupied space, however, never in a multistory parking garage.