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What is the capillary tendency of concrete?

Elden Lindamood | Posted in Green Building Techniques on

I was at a seminar the other day and the presenter, who is a civil engineer and self-proclaimed building science expert, made a statement that caused me concern. He was contending that a vapor barrier is not necessary under a concrete slab on grade because in order to get vapor transmittance through the slab, the vapor pressure would have to be greater than the compressive strength of the concrete (i.e. 4000 psi concrete would need greater than 4000 psi of vapor pressure to move vapor through it). I would like more opinions on that please.
He also dismissed the need for a capillary break for similar reasons but moved on to other topics before explaining it much. I am assuming (which I shoudn’t) that he supposes a course aggregate base to relieve any capillary action before it reaches the slab, but I am not sure. In the end he specifically said a capillary break between the footing and the foundation wall was pointless. Comments on that are welcomed also.
So what I would really like to know is how prone to capillary action is concrete, and is it affected at all by the design strenght of the concrete?
Lots of questions in there actually, but opinions (or science) would be appreciated.

Elden in Duluth.

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  1. user-282515 | | #1

    Development of Discontinuous Capillary Porosity in Concrete and Its Influence on Durability -

    From the Introduction: The task of civil engineers is to design these structures. But should civil engineers care that premature deterioration may make them unreliable before the end of their intended service life? The costs to repair or replace structures are significant. During their academic careers, civil engineers learn: the details of structural design, how to calculate loads and stresses, and how to use various codes and handbooks to determine the proper sizes of beams and columns. But structures are not only subjected to physical loads. They must also withstand the environment in which they are placed. Structures are exposed to freezing conditions, chlorides in the form of
    deicing salts or sea water, and aggressive chemicals in groundwater among other substances that can cause deterioration. Engineers must not only think in terms of structural failure or collapse,but also in terms of failure to perform as intended (serviceability). The duty of the civil engineer is to design structures that will perform for their intended design life without major repair or replacement. Clients are often concerned with costs of a project, and rightfully so. But often for small additional costs in materials and construction practices, significant cost savings can be realized over the design life of a structure. The benefits of life-cycle cost analysis must be
    communicated to the client.

    Strength and durability can be related to the pore structure in concrete. Voids in porous materials are filled with water or air. As such, the voids are not capable of carrying load. As the
    volume of pores increases, the load carrying capacity, or strength, decreases. As well, since the pores carry the substances that deteriorate concrete, as the volume of voids increase, the permeability markedly increases. The ease of fluid ingress can be directly related to the volume of pores. Therefore, the likelihood of damage increases.....

    Having poured more than my fair share of concrete, I can safely say water will always try to find a way through. To not spend a little money to install a vapor barrier under concrete (6 mil poly) is an intentional act to undermine durability.

  2. Expert Member
    Michael Maines | | #2

    In theory, if a high-strength concrete is used and it is perfectly cured, it would be waterproof. A company called Superior Walls pre-casts their walls this way, and although I don't have first-hand experience with their product it makes sense from an engineering point of view. If there are no pores for the water or water vapor to move through, then capillary action or vapor intrusion cannot occur.

    In the real world of homebuilding though, high strength concrete is rarely used, it's poured too wet, and forms are stripped MUCH too quickly for the cement to fully hydrate. All three of those things create pores, or tiny holes in the concrete. If those holes are connected, or as Grant suggests, if they become connected over time due to deterioration of the concrete, you will have channels for water intrusion.

    I've never used a capillary break between the footing and the wall, but it is not a bad idea. We use interior and exterior perimeter drains, properly located, and a capillary break between the floor slab and the wall, for the full basements we typically build. We also deal with stormwater at the roof or grade level so it is not contributing to damp soil around the foundation. If the water table is high, we use an exterior waterproofing system to keep water away from the concrete walls.

  3. Summers 44 | | #3

    Grant has hit on some key points regarding mix and design. Unforunately the deletrious affects to a concrete structure that is not stable is widely overlooked in construction and results in degradation to the concrete itself or coatings that are applied.
    A vapor barrier of 6 mils is a good choice, however they are subject to punctures and openings for plumbing that compromise even the best Vapor barrier installation. Ever see a vapor barrier on the side of a slab on Grade?
    Can't count the number of flooring problems we have been called to fix because moisture is wicking up thru a slab. Air conditioned interior space with a relative humidity of 70% or less draws moisture into a slab where it collects on the top surface. This moisture will cause adhesives to loose a bond or collect in quantities enough to cause bubbles under an expensive epoxy coating. I have seen continuos rubber flooring systems [as in hospitals] where maintenance would cut the rubber flooring with a razor knife to soak up water that condenses under the flooring. An operating room floor with duct tape to close an opening is not sanitary or pretty!
    All concretes have a gel pore/capillary system that is formed as water evaporates out to the surface. This also becomes THE path for a host of contaminates to enter or a way for a concrete internal chemistry to migrate out to a surface. Efflorescence on a surface is nothing more than internal chemistry reacting with the atmosphere. Carbonation, High Chloride contents and loss of the passivating layer on re-bar [forming corrosion cells] are all a result of the capillary system being open. An 8000 pound + mix with all the additives and superplasticisers is very dense but still has porosity to it.
    Millions are spent on concrete reconstruction/repair every year because the extra effort to protect it was not taken.
    Short answer is: After placement, waterproof your concrete and fill the voids that allow transmission of anything into or out of your concrete. You can avoid problems by taking measures to protect it from the onset. Otherwise you will be facing repairs way before design life is acheived. Glenn

  4. Riversong | | #4

    That civil engineer should have his/her license revoked.

    While a typical 8" thick concrete wall will have a perm of about 0.4, it is hardly impermeable to water vapor. But, far more importantly, concrete has very high capillarity (theoretically, can lift water 6 miles!).

    A sub-slab membrane serves as a capillary break, a vapor retarder, and a radon barrier. A capillary break between footing and wall, as well as between wall and slab edge, is critical for a livable and durable structure. And, of course, a capillary break between concrete wall and wooden sills or beams (in pockets) is standard practice for good reason.

  5. Elden Lindamood | | #5

    Thanks for all the great replies and resource links. They are all very much appreciated.

  6. London Floor Sanding | | #6

    I won't repeat anything the other, quite well informed commentators before me have. But I will reiterate that the speaker describer by the OP is not the expert he proclaims himself to be. A sub-slab membrane is an absolute necessity for many more reasons than moisture resistance. It was a good idea for you to check this online rather than taking one 'experts' opinion. Cheers!

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