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