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Stone Failures-Why? Article #6
Article #1, Article #2, Article #3, Article #4, Article #5, Article #6, Article #7, Article #8, Article #9, Article #10, Article #11, Article #12, Article #13, Article #14

Failure Number 6†††††† Concrete Slabs Without Discontinuous Capillary Pore Structure.

California was in a draught in the late 1980ís. When the rainstorms started up December 1990 and continuing through the El Nino storms of 1999, we have investigated hundreds of moisture related stone flooring problems and failures.

We have taken the seminars from the American Concrete Institute on Concrete Slabs and Trouble Shooting Concrete.These seminars do not address the solutions to moisture wicking, percolating, migrating and transmitting up through concrete slabs, except by how to install concrete correctly.

I have pursued the educational session at the Construction Specifications Institute Annual Convention on Concrete Problems and Solutions.This was June 29, 1996 in Denver, Colorado. Mr. Koneval spoke on specifications, specifying needs and wants, super flat floors, concrete slab design, with achieving at least 4000 psi. Cracking and curling of concrete are the predominant problems related to improper water to cement ratios.

I attended the San Diego Chapter American Concrete Institute chapter program on troubleshooting concrete slabs. I specifically asked how much more it costs to make the concrete more impermeable to reduce the moisture wicking, migrating, percolating and transmitting through concrete slabs. Response was 15 to 20 cents a square foot or about $200 per 1000 square feet. The concrete contractors stated they want to densify the concrete in institutional, commercial and residential construction, but the majority of builders and general contractors do not want to pay to install concrete correctly.

Uniform Building Code only requires a 2500-psi concrete slab.The purpose of the concrete slab is to keep vermin out and having a foundation to support walls of the building.

I researched the National Association of Corrosion Engineers and received the response to reduce moisture and vapor migration, wicking, percolating and transmitting through concrete slabs where floor coverings will be installed, the concrete needs to be a maximum ratio of 0.45 pounds of water per pound of cement, and wet cover cured.

July 17, 1996 we attended the Academy of Textiles and Flooring Course on Solving Moisture Related Flooring Failures On Concrete Foundations.

We learned creating the discontinuous capillary pore structure in the concrete slab is the key for successful floors to reduce the moisture and/or vapor emission migration, wicking, percolating and transmitting up through the concrete slab. This was the same information as received from National Association of Corrosion Engineers in working with low water to cement ratio of 0.45 pounds of water per pound of cement, with wet cover curing the concrete slab, will create a discontinuous capillary pore structure in the concrete slab.

In essence, a 2500-psi concrete slab will serve as a sponge.Moisture will collect in the top gradient ĺ inch of the concrete slab. When the temperature changes above the concrete slab, moisture or vapor emission will migrate, wick, percolate and transmit up out of the concrete slab.Moisture migrates and moves toward cool temperatures. Vapor emissions migrate and move toward heat.Heating and air conditioning can exacerbate these conditions in closed buildings.

Theme is too much water added during the mixing and pouring of concrete, weakens the concrete strength and increases the permeability of the concrete slab.

Other influences include capillarity flow, hydrodynamic pressure, hydrostatic pressure, efflorescence, sub-efflorescence, and barriers on the sides of and beneath the concrete slab.

The International Conference of Building Officials publishes the Concrete Manual.

The intention of the Concrete Manual is all concrete slabs placed to where adjacent landscaping may be installed, are required to have waterproofing or damp-proofing to prevent capillarity flow of moisture and/or vapor transmission into the slab through the edges of the concrete slab.

Hydrodynamic means pressure pushing moisture or vapor emissions.

Hydrostatic is moisture or vapor at rest, seeking equal level in adjacent area when a path of travel is created.

Efflorescence is the migration of certain crystalline compounds, which are deposited, with the loss of water. A deposit of soluble salts or calcium carbonate, usually white in color, is the symptom of the moisture migration through the substrate.

Sub-Efflorescence is efflorescence deposited below the surface.Salts deposited can be an aggressive force with moisture or vapor emission expanding the salt and creating 6000 to 8000 psi force causing blistering, shaling and deterioration of the surface materials or adhesives.

The barrier placed underneath the concrete slab is to assist in curing of the concrete slab and reduce the migration of salts up through the concrete slab. American Concrete Institute has removed all references to 4-mil or 6-mil polyethylene sheeting under concrete slab as an acceptable barrier due to deterioration exposed to sunlight, punctures, and consumed by micro organisms.

Once a concrete slab has gone through the 28-day cure time, it takes at least one month of cure time for each one inch of concrete.As an example, a 5-inch concrete slab requires 6 months of cure time before the concrete slab is dry.

If you are interested in more information, along with how to test for moisture in concrete slabs, The Academy of Textiles & Flooring in Whittier, California has courses on Vapor Emission Testing.

We use the Tramex Concrete Moisture Encounter as a quick indicator of moisture and/or vapor emission within the assembly and within the top gradient of the concrete slab.

Most floor covering assemblies are not recommended to be installed over concrete slab with greater than 3% or equivalent greater than 3 pounds per thousand square feet per 24 hours using the calcium chloride dome test. This is further support for using a waterproof membrane bonded to the concrete slab with rapid setting mortar prior to installation of natural stone floor.Marble Institute of America recommends always using a water barrier over the concrete slab prior to the installation of stone.

Other notes include:

Do not install a waterproof membrane over an unvented concrete slab. Unvented concrete slabs used as roof decks with membranes installed, will fail in three years.

This is where I come back to the government regulations on VOCís.

Type I solvent based mastic was eliminated in California in 1986 as suitable bonding adhesives.Therefore the Type II and multipurpose mastics are used for installation of anti-fracture membranes in California by the floor covering installers.The failures of the water-soluble mastics with slip-sheet assemblies are extensive.The failures are exacerbated especially when the concrete slab is not scarified prior to installation to remove paint, drywall mud, over spray of stains applied to cabinets during construction.

Calcium chloride used to speed the setting of the concrete slab during installation, once cured, promotes and increases the moisture and vapor emission migration through the concrete slab at a higher rate.

A builder in San Diego, that I have testified against in trial, is performing extensive testing through their quality control program on concrete slab construction.Results are densifying the concrete slab reduces moisture and vapor emission problems with floor coverings.

Cost to repair floor failures is $10-30 a square foot and cost to construct the concrete slab with a discontinuous capillary pore structure is estimated at 20 cents a square foot.

We encourage the professional tile and stone contractor to offer to the buyer the choice of using anti-fracture waterproof membranes bonded to the scarified concrete slab with rapid setting mortar to assist in reducing moisture and vapor emission migration, wicking, percolating and transmitting through concrete slabs from affecting the stone floor assembly.


This article is part of a series of articles on Stone Failures (Dec. 2000) by Greg Mowat

Forensic Tile Consultants
9541 Vervain Street
San Diego, CA 92129-3523
(858) 484-8118, Fax 484-8302

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