I am enjoying the image of a crew of demolition laborers jackhammering and removing a concrete slab in order that the architect may inspect the condition of the immediately underlying sheet of plastic. This should go really well.
Isn’t that what slab jacking is for (haha)?
I understand the motivation for providing a membrane and I would specify one if for no other reason than to not be accused of being the cause of a floor failure.
I did spend some time trying to understand what is the problem that we are supposedly trying to prevent.
I am simply suggesting that we really do not understand the need for a membrane. When is it critical and when is it not. Sure we can focus on the failures but what about the “successes” that occur even when “good practice” has not been followed. There are too many successes associated with projects that did not follow what is considered good practices.
I am suggesting that the narrative is flawed and that instead of science driving our practices they are shaped by a need to give us cover in case there is a problem.
The soil under concrete slabs on grade always contains some moisture, even when the water table is much lower than the slab. Without a vapor barrier this moisture can slowly diffuse up into the concrete, generating alkaline deposits on its surface and interfering with adhesion of flooring materials. The degree to which this will happen is difficult to predict, so the practice for many years has been to assume that soil moisture will diffuse into the concrete if nothing is done about it, and to proactively place a moisture-impermeable membrane below the concrete to prevent it. Doing so on the positive side of the concrete will be much more effective than any negative-side remediation. This practice is based on scientific understanding of the processes of moisture diffusion in concrete, but each application involves a situation that is so difficult to characterize, that “better safe than sorry” is the prudent approach.
When I moved back to the Mainland in 1997, from an academic environment to professional practice, Stego was already pitching their vapor barrier as superior to the 6-mil polyethylene sheet (tradename “Visqueen”) in durability. No one seemed to care much about permeability until the “black mold” crisis that developed around 2000. Flooring adhesives had moved to water-based formulations which proved problematic under the conditions that Ms. Eckhardt described. Many design professionals (architects and engineers) began to migrate to a vapor retarder that was not only durable but was also much more resistant to water vapor transmission. Despite Stego’s aggresive marketing pitch, there are several manufacturers out there that offer comparable products. I still hear of issues with “wet” slabs, but I think this is caused by too much water in the concrete rather than moisture vapor transmission.
The “6-mil polyethylene vapor barrier” was still a requirement in the IBC through 2006 if I remember correctly even though it was not only “flimsy” under construction conditions, but also not really much of a vapor retarder (permeance of 0.230 for 6-mil and 0.140 for 10-mil).
I developed a separate Division 07 section for this stuff which I always include. Although most structural engineers have caught on, 10 years ago, their general note to provide a 6-mil polyethene vapor barrier would cause a conflict with my section which would trigger an RFI
Greta: If the problem is alkali-related, that can be mitigated by using pozzolanic supplementary cementitious materials that consume lime generated by portland cement hydration.
jpjordan- nice post…
The RFI was answered " Follow the specification that states to use the most stringent specification…"??? Correct!!!
I am half kidding…
Another good topic for a Post- How many RFI’s are average for a $certain dollar amount project?
I have been seeing 10 RFI’s per $1M… so you can imagine how many are on a $60M healthcare project!!!
Do the tests used measure the rate of diffusion through the concrete or do they measure the moisture in the concrete voids from a multiplicity of sources? My research led me to believe the latter
Yes there is some diffusion but are the test results a realistic measure of the diffusion.
You can get different test results if the salt is finely ground.
When dealing with moisture problems and membranes under the slab only believe half of what you hear.
I think there is also a strong dose of “an ounce of prevention is worth a pound of cure,” even if the actual forces are not fully understood. In this situation, the initial cost of of efforts to limit moisture migration is substantially less that the cost to fix the problem (if it occurs) after occupancy. This is similar to the thinking behind using stainless steel brick ties lest corrosion damage them. The fix is super expensive compared to the upcharge for stainless vs. galvy.