waterproofing concrete on metal decking 4

[sponcyv]

I have an exterior mezzanine that will have an occupied space below. This mezzanine needs to be completely watertight. The problem is that metal deck below the concrete and waterproofing above the concrete traps all the moisture of the slab inside. Has anyone ever had to waterproof an elevated concrete slab on metal decking?

 

[ajk1]

The reason for venting the deck is so that moisture from the concrete can escape thru the venting. The general principle is that concrete should be allowed to breathe and should not be totally encapsulated (as would be the case if you put a non-perforated metal deck on the soffit and a waterproofing membrane on top).

Where a metal deck is used on the soffit, it is required to be able to breathe when a waterproofing membrane is placed on the top surface -- see CSA Standard S413 (although this Standard is for parking garages, the general principle applies to other structures).

The issue of venting has nothing to do with whether the waterproofing membrane leaks or not. If it leaks, you are in trouble irrespective of whether the deck is vented or not. Acoustic deck has punched holes that may be sufficient for the venting, but the juice from the concrete may come thru the acoustic perforations, although that mat be acceptable. There are membrane people (such as Neoguard) who claim that their membrane is "breathable" but there are no standards for breathability, so I am not comfortable about it. A few years ago membrane manufacturers would not warrant the membrane if placed on slab on un-vented metal deck. Not sure what their current position is.

I have to run now but on the weekend I can send you the excerpt from CSA S413 that deals with this if you like.

If there are corrosive deicing chemical applied in winter, then the steel deck is even a worse idea.

The whole thing would be much better if it were a formed soffit and not a metal deck. Yes it is more expensive to form it, but you tend to get what you pay for.

Sounds like you have an unwise client if he/she says that leakage is acceptable. Don't become a prostitute by going along with something you know is wrong. The person telling you leakage is ok, will have disappeared when the leakage occurs and you will be left with the problem.

 

[KootK]

I would like to check out the S413 stuff. I have the doc if you can point me to a section.

The attached position paper from SDI actually seems to be in line with sponcyv's thinking.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sponcyv]

one other question that I had was what is the best practice for crack control for this mezzanine exposed to weather? I've read most of the threads related to this topic, but there doesn't seem to be a general consensus. This deck is 16'x24'. I'm planning on showing it without joints. I'm showing #4 bars @ 12" o.c. ea. way. Supports are at 4' o.c. The deck is 4" total thickness using 0.6 non-composite deck. I am also showing fibermesh in the mix 1.5#/cubic yard. #4's @ 12" provide a lot more than needed for flexure and temperature and shrinkage. I'm planning on specifying 0.45 max w/c ratio, 4,000 psi strength, 6" max slump, max 1" aggregate, 520 lbs minimum of cement per cubic yard, 6% +/- 1.5% air entrainment

 

[sponcyv]

ajk1 - I agree with you, but I also think the best thing for me to do is to clearly state that waterproofing is outside of my responsibility and that I recommend a waterproofing consultant be retained. I personally don't feel confident specifying waterproofing items and have not been asked to do so. I don't want to completely ignore the waterproofing issue. I just want to make it 100% clear that I'm not providing it, but I recommend they get consultation by a building envelope professional. I want to cover my rear and separate myself completely from the waterproofing. I shouldn't have to specify waterproofing. They need to hire an architect. That's my way of not being a prostitute.

 

[sponcyv]

Kootk - that's exactly what I read earlier today - the verbiage that I plan on using on my drawings is "if vapor impervious materials are applied to top of slab, metal decking must be vented".

 

[wannabeSE]

There are significant differences between Lightweight Insulating Concrete (LWIC) and structural concrete. It typically weighs less than 40 pcf and is specified for insulation and fire rating. Concrete on non-vented deck (form deck and composite) is commonly used on roofs and typical floors with impervious (and moisture sensitive) flooring. Some people believe the concrete moisture content goes down quicker with vented deck, so it is used in hopes that the flooring can be installed sooner. For lightweight structural concrete and hardrock, I leave the decision to use vented deck up to the contractor. It is means and methods. The specs allow it but don't require it. Vented deck is typically required for LWIC.

I am curious to see the CSA standard for parking garages. I wonder if the vented deck is required because the concrete can be flooded without any waterproofing. I did a little searching and here are a couple articles mentioning vented deck for use in parking garages. The second also recommends vented decks when the concrete is exposed to weather (I don't consider concrete covered by a roofing membrane to be exposed to weather)

http://msc.aisc.org/globalassets/modern-steel/archives/2009/02/2009v02_corrosion.pdf

http://eng.lacity.org/techdocs/technical_papers/Metal_Deck-AISC_NASCC_Cover.pdf

 

[MacGruber22]

I see nothing in the comments about the deck serving as a vehicular traffic surface, therefore you do not need a traffic membrane, only a waterproofing system.

If a true roofing waterproofing system with vented deck is not in the cards, I would consider the following:
1. Use heavy mesh instead of bars for better crack control
2. Require G120 galvanizing on the metal deck - never use G90 for exterior
3. Protect the top surface of the concrete with a 100% solids silane sealar. It is vapor permeable and will not cause problems over metal deck, unlike like a urethane traffic membrane. Lasts at least 5 years before needing re-applied. Cost about $0.80 per square footing in my neck of the woods.
4. f'c = 5 ksi min and w/c 0.38 with integral corrosion inhibitor along with proper air

Hope that gives some ideas

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

MacGruber22....it is a concrete deck over occupied space. A silane/siloxane sealer is insufficient for such purposes. There HAS to be a waterproofing system that meets the following qualifications:

1. The minimum deck slope shall be 1/4:12.
2. If an overburden or wearing surface is not to be installed, the waterproofing system must be approved by the manufacturer for use in vehicular and/or pedestrian traffic locations.
3. The waterproofing assembly must possess a Class A, Class B or Class C fire rating.
4. If any portion of the waterproofing membrane is to remain exposed, the waterproofing system shall be
ultra-violet resistant.
5. Flashings must be installed in accordance with the waterproofing manufacturer's published specifications.
6. The waterproofing system shall be flood tested in accordance with ASTM D 5957.
7. The flood test shall take place after installation of the waterproofing membrane and prior to the
installation of any above membrane components, wearing surface or overburden.
8. An approved testing lab shall provide written verification to the building official confirming that
the flood test was performed along with the results, prior to final inspection.

 

[MacGruber22]

For some reason I was envisioning an open mezzanine. Completely agree with Ron.

"It is imperative Cunth doesn't get his hands on those codes."

 

[sponcyv]

MacGruber22 - I was planning on specifying rebar because there is a better chance that it will not just be laid down on top of the deck and actually be placed in the desired position. For additional crack control, I am adding fibermesh which has performed well historically on jobs that I've been involved in. I'll specify the G120 decking as you suggested. I've never seen a mix with 0.38 w/c ratio. This is the bare minimum required for hydration of concrete from what I read.

Ron - I completely agree. I'm showing 1/4"/ft slope on my drawings and as you mentioned previously, I'm assuming no responsibility as it is out of my scope. I'm clearly stating that my drawings do not consider waterproofing or fire rating or anything relating to other disciplines. I'm clearly stating that my drawings only cover structural items. Honestly, I don't care if the occupied space below gets wet. It is not my responsibility to waterproof the deck and I'm clearly making the owner and contractor aware.

 

[MacGruber22]

Sponcyv - we use 0.38 regularly for high moisture exposure. Low w/c ratio's like this are used in combination with high range water reducers. It seems you need all the durability you can get.



"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

MacGruber22...agree completely with your last post.

 

[MacGruber22]

Thanks, Ron.

On sort of a side note, Sponcyv, I would also consider using this project as an opportunity to start learning how to specify drainage and waterproofing systems - specifically for parking and plaza structures. Competent structural engineers are perfectly capable of performing that task for common construction, and for small projects (such as the one you describe) it is a cost-saver for the client.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

MacGruber22...agree again. I routinely design waterproofing systems for structural protection. Sometimes a lot more difficult than the structural design!!

 

[MacGruber22]

Thanks, Ron. Yes, it often is more complicated!

Speaking of client complications, I am assuming that the contractor/owner has shot down a formed concrete slab for this mezzanine, sponcyv? If not I would say that you recommend that over stay-in-place centering, because of being exterior. They may rather form it up than order G120 deck, which is not in stock. On the plus side, in order for a membrane manufacturer to release their warranty, they need to approve the substrate. The manufacturer can run some vapor transport calculations and specify the percentage of venting required for a non-vapor permeable soffit. Always lean on the material manufacturer's for their vast specialized experience.

"It is imperative Cunth doesn't get his hands on those codes."

 

[sponcyv]

McGruber22 and Ron - I value your insight and appreciate you taking the time to answer these tough questions. McGruber22 - do you have an example of the concrete mix with 0.38 w/c ratio that you have been using? Perhaps a concrete mix submittal? Being that water intrusion issues are so litigious, I'd rather not learn the hard way. I would like to learn how to specify them, but don't want to bite off more than I can chew especially for this project. The contractor has not shot down formed concrete. I can run that by him.

 

[sponcyv]

McGruber22 - if going with cast in place slab on temporary forms, supported by steel beams, would you do one slab pour for a 24'x16' area with no control joints? I was not planing on doing composite beam action. How do you detail concrete slab supported by steel beams? I won't have headed studs. Is there a need for any mechanical connection between steel beam and concrete slab?

 

[MacGruber22]

sponcyv - I will have to take a look in the project folders, but LaFarge concrete is our go-to supplier for high-performance concrete. It may be quicker for you to just call your local LaFarge rep and ask for it. They are very responsive. I don't think you need 0.38 if you go with a formed slab, then only ACI minimums of 0.40 (chloride attack) 0.45 (no chloride) for exterior. Regardless, I would require an integral corrosion inhibitor. LaFarge tends to use this Grace product:

https://gcpat.com/construction/en-ca/Documents/DCI-20J_DCI.pdf

My thoughts on the other items - No control joints - just be generous with your crack control steel and provide good details. Provide standard detail for construction joint in case their pour is interrupted for some unforeseen reason. Yup - no need for composite beam there. Non-composite beams are designed with the slab cast against the side of the compression flange all the time without studs, to prevent positive moment LTB. When the spans get long, you need to consider a cross-bracing for fascia beams to the interior beams for wind and generally stability. Obviously, when diaphragm transfer is an issue for a larger structure, some sort of mechanical connection is required. See the attached - it is a repair detail but you will get the idea. I show the repaired slab cast against the existing beam top cover plate (don't ask why that cover plate was there). Remember, if you take your moment diagram, scale it down by 98%, and then divide by the lever arm between the flanges, you have the required top flange brace force per foot. The value is always surprisingly smaller than you think you need.

"It is imperative Cunth doesn't get his hands on those codes."

 

[sponcyv]

Thank you MacGruber22! I appreciate the advice.

 

[KootK]

Where a metal deck is used on the soffit, it is required to be able to breathe when a waterproofing membrane is placed on the top surface -- see CSA Standard S413 (although this Standard is for parking garages, the general principle applies to other structures).

I am curious to see the CSA standard for parking garages.

Just following up on this.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.