Slab on grade vapor barrier performance

Archive of Older Topics Archive - Specifications Discussions #4
21

The thickness of the slab has a huge impact on drying time. Much of the water of convenience will be a very long time coming out from the deep horizons of the concrete when it is allowed to dry in only one direction.

One very confusing aspect of the CaCl test is that it only reads the moisture being emitted from the top half-inch or so of the concrete. On a well cured slab, the top can be forced dry while the interior of the slab is very wet. Examining a failure and finding high vapor emissions at a later date is readily explained by moisture redistribution or equilibration through the slab.

For this reason I always use RH readings carefully harvested from several locations to get a good picture of the true ‘potential’ of the concrete slab to cause problems later on.

In my former life I trouble-shot (shooted?) flooring failures for a medical facility builder that provided a lifetime warranty on their buildings. You can imagine the importance placed on finding the causes and cures for flooring failures when you have a lifetime warranty on those floors. Oh the stories I can tell…

Now I am a consultant and mitigation contractor. I am Koester certified, and I cringe at all those spray and pray systems. LOL

Oh, I recommend a two to three day moist cure. Longer than that and the cement matrix begins sealing the pores of the concrete so thoroughly that the drying process is impeded, less than that and the concrete surface may be left weak. With a low water/cement ratio and the use of less cement and a well graded aggregate, we can get strong concrete that will dry to required RH and MVER in a very short time, say 3-4 months for a 4" slab.

22

Excellent thread.
So now we know to limit the cement content and the water-cement ratio, use a good vapor retarder with no blotter course, wet cure for 2 to 3 days, and test using Relative Humidity, not just calcium chloride.

Now let’s continue…

With LEED, we have the inclusion of fly ash, granulated slag, etc as replacement pozzolans to offset the amount of Portland cement in our mix. How does this affect our efforts to decrease the amount of cement in the mix?

Regarding curing, sealing, densifying, etc., what happens after the initial 2-to-3 days of wet cure? Do we opt for silicate technologies for concrete being left exposed (no floor coatings or coverings)? What about at floors with finishes? There is a whole industry (see http://www.4specs.com/s/09/09-6105.html) of manufacturers of systems, including reputable systems, that claim to be able to reach that elusive 3 pound threshold. Do we give up and just include it in the project or do we include a square foot allowance and require unit prices, or do we approach this in some other manner (like make it the Contractor’s problem)? As an aside, I prefer to have an independent agency test the floor specifically to prevent the probability of having the wolf guard the henhouse. I too have heard of contractors who have made sure the concrete ‘failed’ just to ensure extensive remediation.

Surface prep: How important is the inclusion of light shot-blasting in the spec to remove the fracture plane that is often a problem even if the pH and moisture content are okay?

Obviously I have my bias; curious to hear from each of you.

23

The best publication for this issue is ACI 302.2R “Guide for Concrete Slabs that Receive Mosture-Sensitive Flooring Materials.” That document references reseach that found that a 6-inch slab takes twice as long to dry as a 4-inch slab, and an 8 inch slab 2.8 times as long as 4-inch. In addition, slabs drying in two directions take about twice as long as those drying in one direction. W/C ratios also affect drying time, though less dramatically than other factors. Finally, keeping the slab dry after curing by keeping rain and other sources of construction water off the slab is critical, as the drying time needs to “restart” after re-wetting. Though it will dry faster after a rewetting event, overall progress is significantly slowed.

We are specifying significant levels of control of the environment that the contractor must use, specifically for the purpose of promoting drying. We will be trying a method where the contractor must a provide moisture mitigation membrane if the drying does not reach the necessary levels by the time finish flooring is installed. A matter of 2 to 4 months of properly protected concrete should be enough according to research studies cited in the ACI document.

I do not believe that making the flooring contractor “own” the mitigation membrane is the correct approach, unless the owner wants to pay for it whether needed or not. This is because the flooring contractor has zero control over the drying before they arrive. Making the moisture mitigation membrane a requirement only if the floor slab is not dry when needed gives a financial incentive to take the appropriate steps to get the slab dry.

We’ll see if it flies.

24

We specify a 15 mil “vapor barrier”, recognizing that moisture from the ground takes years to make itself manifest in the slab.

We currently use 2" sand under the slab, but that cannot be saturated or problems will arise. That is the hard one - keeping the sand at the proper moisture level for working but not too wet. Contractors here don’t seem to want to eliminate the sand claiming an impact on bleed off time and finishing.

We reduce water/cement ratio to 0.48 with a 14 day wet cure, or 0.42 with a 7 day wet cure.

We have been specifying 4,000 psi for interior slabs as a way to densify the concrete, but have experienced curl, likely as a result of this. We may back that down to 3,000 psi.

We will not specify spray-on or in-mix silicate products to reduce vapor emissions.

We do specify topical vapor emission products as a matter of course, usually as an allowance so that the money and time is in the project to deal with potential problems. Testing determines the outcome. Sometimes we see low test levels, sometimes we don’t. We make the Owner choose whether or not to accept the risk of not installing the vapor emission product.

This is our current “best guess” as to how to handle the problem.

Scott Parish

25

OK I’m jumping into the fray as a first time poster, but a “lurker” on the forum for the last couple of months…and to top it all off I’m Canadian, not American so there are some differences in our respective construction environments.

A couple of things have struck me from reading through this post (and I will admit I skimmed a few of the responses rather quickly) so I will offer them up as points of consideration:

  1. If the project is has a high water table or high moisture content within the native soil conditions then, as mentioned earlier, there are sheet vapour (yes, I know; there is no “u” in vapor…what can I say I’m a Crazy Canuck) retarders on the market that offer much high perm ratings than off the shelf clear poly (8, 10, or 15mil). Although the costs for these products (such as Vapor-Mat by W.R. Meadows or Perminator by Grace Construction Products) is significantly higher than clear poly, there performance is generally much better and offer the benefit of a reinforced polyester scrim to reduce the likelihood of punctures during concrete placement. Also, with any underslab sheet vapour retarder a key to the overall efficacy of the product is the treatment of the joints.

  2. I don’t believe anyone has discussed the inclusion of a water reducing admixture to the concrete design mix which allows the workability required during placement, but aids in removing the (as identified) “troublesome” water trapped in the slab after placement and during the curing stage in an accelerated fashion. These are common place in our area. Euclid has different products available to assist in the regard.

  3. Including the provision for dehumidification during the curing stage prior to placement of floor coverings. This is also commonplace in my area and greatly assists in getting the “troublesome” water out of the slab.

  4. If you happen to be in an area where the fall/winter/spring is less than conducive to construction activities such as curing concrete (yeah I mean copious amounts of COLD and SNOW eh?) then you may also want to address the type of temporary heaters the Contractor uses. Although most Contractors like to use direct-fired propane-powered unit heaters (affectionately called Salamanders in my area; no I have no idea why), these operationally cost-effective devices also spew large amounts of water vapour into the air, which makes the removal of “troublesome” water in concrete slabs more difficult to remove without the use of dehumidification.

A combination of the above generally works quite well for our projects, a lot of which are institutional projects (schools, long term care facilities etc) which tend to have a lot of resilient floor finishes.

26

Paul, Don’t worry, I think we can handle some stray 'u’s here and there!

We don’t have as much cold in Massachusetts as much of Canada, but we nevertheless prohibit salamanders due to the moisture given off, as well the fact that they send other noxious combustion gas products into the space. Also, one of the requirements of our Division 01 for controlling the environmental moisture is to use desicant drying, if necessary, to accelerate the drying process.

27

Jouhn - thaunks for the waurm welcoume!!

I can be a bit of a smarta$$ sometimes. After 20 years in a couple of architecture practices (maybe if we practice enough we will eventually get it right??) and some retail design & construction experience one could almost say I have become significantly jaded and “curmudgeonly” to fit in well (hopefully) on this forum!

You can also expect to hear some bad jokes (?) and potentially wry comments from me as well. But in our line of work, how else would we maintain our sanity???

I wasn’t worried to much about being accepted because I am a Canadian as it seems ya’ll have accepted at least one other Canadian, albeit ex-pat, (Wayne Yancey) around here…which also gave me hope as he is also a fellow rider (I dislike the term biker as it usually conjures up negative stereotypes of the less-than-savoury type). Do any other current or former Canucks hang out here?

I’m sure you will all be pleased to know my current choice of ride is American, so I did my part to support your economy!

28

Scott,
I’m curious as to your location, since you mention common use of sand beds by contractors.

I think that including an allowance is making the owner take the risk of excess slab vapor emissions, because she is being asked to pay for it (if needed) regardless of whether the contractor met contractual obligations to properly schedule the project, protect the slab from moisture, use appropriate concrete mixes and curing methods, or other steps within their control. Contractor’s should make provisions to deal with slab moisture based upon available research into the subject, contract requirements, and their own experience. They are in the best position to know what the most cost-effective methods are to meet the project conditions. If the schedule allows a slow dry–great. If some dessicant drying is needed in a few areas, they can plan for it. If they know that the schedule is aggressive and neither of these will work, they can forget all the other stuff and go with the epoxy membrane. (In that case, maybe they use a cheaper concrete finish, too, knowing they’ll shot-blast anyhow.)

I do not believe that excess concrete slab moisture is an unforeseen condition.

29

John:
We are in Visalia, CA.

We tried the tack you are suggesting…making the contractor responsible…but without real success. Why? Because when the contractors did everything we asked them, they still could not achieve the 3 pounds needed, most of the time. This is just not an exact science with a cause/effect relationship that can be counted on every time.

And if we are going to place all the risk on the contractor and force it down their throats, then the owner will pay a premium. And…the finger will get pointed at the designer as well…and we’ve had enough of that.

So yes, the owner should pay for it and if they don’t want to pay for it, they should take the risk - not the designer or contractor.

30

I received a message from our local Stego rep, Lance Escue, about a week ago. As I attempt updating our Office Masters from MF95 to MF04, and being currently in Div 07, I sought his advice on the subject. Here is what he sent me.

Dennis,

Thank you for speaking with me last week about the most current under slab vapor barrier specifications. Please consider updating your master specification to include (editable word copies of 11 mil, 16 mil, and 31 mil composite attached for your use).

Sorry about all the attachments but I wanted you to have the documents on file we ask that all your specified vapor barrier manufacturers provide. When you specify true ISO 9001 certified manufacturers they will be able to provide all of these.

We look forward to the opportunity of presenting the AIA/Sustainable Design Seminar when convenient for you.

AIA Program # 91188 - Controlling Water Vapor Moisture Intrusion through Concrete Slab-On-Grade 1 CEU Sustainable Design

M W Escue Company, manufacturers’ representative for Inteplast Group, would like to ensure that you have our updated specifications for under slab on-grade vapor retarders, and we, respectfully, request your consideration to be included in your Master Office Specifications.

The attached specifications (VB250 & VB350) each include a total of six (6) true manufacturers of vapor retarder products designed for “under slab on grade” use. These specifications offer everyone a broad selection of excellent quality, well-known and recognized, and readily available manufacturers’ products in our industry; and allows for competitive bidding.

We are pleased to announce that the ASTM committee updated their ASTM E 1745 standard this year for Under Slab Vapor Retarders.

The updated listing is ASTM E 1745-09 and the major change was the minimum water vapor permeance (WVP) changed from 0.3 to 0.1

We, respectfully, request that the specifications include the updated reference to ASTM E 1745-09, Class A (the highest level of protection) and reference 03300 and 07260 (which many manufacturers of underslab vapor retarders have adopted).

Attached is a copy of our suggested “under slab on grade vapor retarder” specification(s) and they are in word format so you can edit as needed:

Barrier-Bac VB-250 (11 mil) Membrane - 11 mil (over the 10 mil minimum per ACI-302) WVP 0.020 / WVT 0.006
Barrier-Bac VB-350 (16 mil) Membrane - 16 mil (over the 10 mil minimum per ACI-302) WVP 0.009 / WVT 0.003
Barrier-Bac VBC-350 (31 mil) Composite Membrane - 31 mil (for project conditions which require greater concrete adhesion to the membrane)

Included in the Guide Specifications (except the VBC 350 Composite Membrane) are six (6) manufacturers in the products section we have limited the products to companies that actually manufacture vapor retarder products in the USA.

Entities which out-source and/or import and function only as marketers to the industry have NOT been listed.

By using this specification, your firm will not only protect itself from liability by meeting current governing standards and industry recommendations, but you will also be providing a high level of protection for your clients and the people who will occupy the buildings you design. The most important aspect of this specification is the performance based characteristics. The (Water Vapor Permeance) WVP-0.020 / (Water Vapor Transmission) WVT-0.006 (gr/hr-ft2) for Barrier Bac VB250 11 mil vapor retarder and WVP- 0.009 / WVT-0.003 (gr/hr-ft2) for Barrier Bac VB350 16 mil vapor retarder defines the low WVP permeance and the reference to ASTM E 1745-09, Class A emphasizes the puncture and tensile strength characteristics of products. The Inteplast Group Barrier-Bac product characteristics meet and/or exceed certain other competitive vapor retarder products on the market today. (WVT is not listed in or part of ASTM E 1745-09; however, several entities do state this data; so, we have elected to do so as well). Barrier-Bac vapor retarder membranes exceed ASTM E 1745-09, Class A.

It is important to remember that Inteplast Group is the manufacturer and their Barrier-Bac manufacturing facility in Lolita, TX is an ISO 9001:2008 registered/certified facility. (Just one aspect of having this certification is the manufacturer must produce the same consistent/continual quality assurance stated in their Product Data Information with each and every production). There are several entities that market, private label, out source plastic membrane for below slab application and they have no manufacturing facilities. It is believed some even import into the U.S. Likely, importers have no true means to verify the proper quality control/assurance of the products they are promoting and selling? Recommend that you always specify that material be manufactured in the United States and accept no “marketers” products. (If you have concerns, simply ask for a “Certificate of Origin”).

If you have any questions regarding the suggested specification, Inteplast Group, Ltd - Barrier-Bac and its supplementary products, the ISO 9001:2008 Certification, or any information in general about below-slab moisture protection, please do not hesitate to contact M. W. Escue Company, Inc:

You can request anytime by contacting us at 901-861-5502, barrierbac@mwescue.com, or underslab@mwescue.com

Lance Escue
lance@mwescue.com
Cell: 901-277-9077
Office: 901-861-5502
Fax: 901-861-5510

31

Hmmm.. by specifying 11 mil, 16 mil, and 31 mil, aren’t you eliminating Stego?

32

about ten posts up, there is a comment about using fly ash as an additive for LEED points. Fly ash in concrete will slow the curing time (and the more fly ash the slower); in the northwest there isn’t a note in the building code that allows for slower break strengths just because you use fly ash. Therefore, in fly ash mixes, most of the time it isn’t substituted for cement (the point of the LEED credit) but is used along with the same cement content. For the most part, if you want to have the slab workable, you don’t want to use more than 25%fly ash in the mix, or the mix will be too “sticky” to trowel.
I have typically used a 12 or 15 mil crosslaminated barrier, with absolutely NO sand course (the SoCal engineers still want that, as do the Arizona ones); and appropriate protection and saw cuts and haven’t had any moisture problems in slabs – ground level or elevated – except once when there was a plumbing leak and the contractor was complaining about the dampness of the slab and how it was ruining his schedule. After he discovered the leak, he had a lot more problems than just his schedule.

33

I believe that the emphasis on ISO 9001 is inappropriate. ISO 9001 simply means that the manufacturer has adopted a number of practices and standards related to manufacturing of the product and has been audited to verify that they are being followed. It does not guarantee that the product will be perfect nor should it be inferred that non-ISO 9001 firms do not produce a quality product.

My perception is that if ISO 9001 were to be specified as a requirement the competition would not be able to comply. The focus should be on the reputation of the manufacturer supplemented by testing of product delivered to the project if there is a significant concern.

34

Regarding the statement that there isn’t a provision in the code that allows slower strength if fly ash is used:

ACI 318-05 Section 5.1.2 States “… If other than 28 days, test age for f’c shall be indicated in design drawings or specifications”

A 56 day test age is regularly specified when using slag and fly ash as well as when very high strength concrete is used in highrise buildings.

The down side is that you wait longer to find out if you had a bad batch of concrete. Never the less there is no reason not to specify the longer periods.

35

I’ve had success with convincing the soils & structural engineers on my projects in SoCal not to use sand. I specify a minimum 15 mil Stego or equivalent product.
I just tell them this is what I’m doing and send them the references from ACI 302.1R
Arizona engineers need to read the Green Sheet on MVE published by the Phoenix CSI chapter a few years ago. They’re the reason it was created.
The Stego rep was in my office a while back with the same line presented to Dennis - the problem the way I see it, is, if we specify it their way, no one else can comply

36

To Nathans’ comment…I posed the question back to Lance…His response is as follows…

All of the other approved manufacturers listed are 10 or 15 mil products.

The minimum stated in spec is 10 mil or 15 mil - its just that Barrier Bac is made to 11 mil or 16 mil (price is still the same as, or in many cases, lower to distributor than the 10 or 15 - explained later).

Only TRUE manufacturers are listed.

Since Inteplast Barrier-Bac is a total vertically integrated manufacturer (wells in the gulf - 3 of their own (not leaking), resin plant - Baton Rouge, and film manufacturing plant - Lolita, TX) they are probably the most competitive of all the products listed and less than Stego to the Distributors.

Stego is not a manufacturer - they are a marketing company that purchases film stock from a bag manufacturer in Yakima, WA - they do a great job on specifications; however, they do not actually own any plants or equipment nor do they actually manufacturer any products.

The 31 mil (membrane + geotextile fabric) product has few competitors; however is priced lower than the known available products.

The 31 mil is for job specific needs - shifting soils, under gym floors, post-tension concrete, polished concrete, terrazzo floors, etc.

I know the spec is demanding but it just covers true manufacturers of the under slab vapor barriers.

If you need to keep Stego in the spec - please just add Barrier-Bac VB-250 (11 mil) and Barrier-Bac VB-350 (16 mil) as others listed.

You could remove all of the references to ISO Certification, Certificate of Origin, Current Testing Data, etc.

37

The allowance of a longer curing time in the ACI does not mean that the all local building codes (AHJ) will allow a longer curing time. I’ve worked in many jurisdictions that have set break times and do not allow longer curing times with the exception of very high strength concrete.
I also had success in calling out the lack of sand barrier in SoCal, Arizona and Florida, but it took a number of terse conversations along the lines of “we have the license and you don’t” before they got the message.

38

Anne

I would suggest that when the AHJ is not allowing the longer test age that they are not enforcing the building code as adopted but rather are enforcing what they are used to doing. The code is clear that this is an option given to the applicant.

ACI 318 is a reference standard in the IBC and thus is a code requirement.

There is an endemic problem with building officials and plan checkers who do not understand that they have an obligation to enforce the code and cannot enforce requirements that were not legally adopted. In these situations you could force their hand but generally the decision is made to accommodate them. The result is that the building official and staff think they have a right to impose their preferences and the design professionals come to accept this as normal.

39

For slabs on grade the default capillary break in the new 2010 “California Green Building Standards Code” consists of a vapor barrier in direct contact with the slab and on top of a 4" aggregate base. (Section 4.505.2)

You can download the document from http://www.documents.dgs.ca.gov/bsc/documents/2010/Draft-2010-CALGreenCode.pdf.

ICC will shortly be publishing the code.

40

Let’s say an Architect wants to eliminate the vapor barrier altogether and use a surface applied product intended to block or reduce MVT. Does anyone know if these products are acceptable to IBC as an “other approved equivalent method” for retarding vapor transmission? don’t ask me why… I have sent them ACI publications and recommended against ommitting the barrier, but that is what they want to do (per the structural engineer’s recommendation)