COLUMN BUILDING SCIENCES
issue with sand layers under concrete slabs becoming saturated with
water and unable to dry down. The
initial tile setting using modified
thin set works as the thin set can dry
downwards into the concrete slab
if the slab is not excessively wet. We
get a bond. But over time as the sand
layer attempts to dry upwards the
thin set to tile interface becomes
saturated and we lose the bond due
to the hydrolysis reaction.
How does all this apply to thin
"real" stone and thin manufactured
stone veneer failures? Both thin
"real" stone and thin "manufactured" stone veneer failures are
occurring because the polymer
modified thin set mortar bond is
failing due to a hydrolysis reaction due to high alkalinity driven
by water being concentrated at the
mortar to stone or mortar to manufactured stone veneer interface.
More rainwater exposure, more risk.
The water cannot dry or redistribute itself into the "real" stone
because the stone is relatively
impermeable. The water cannot dry
or redistribute itself into the "manufactured" stone veneer because the
manufactured stone is back-coated
("sealed") to prevent excessive water
absorption out of the thin set. The
issue is desiccation of the polymer
modified thin set mortar leading
to not establishing a bond.# So a
"porous" not real stone is made to
behave like a non-porous real stone
with the back-coating (Photo 9).
Much redistribution also cannot occur inward with both the
thin real stone and the thin
PHOTO 7 Floor Tile. Installation over an impermeable
anti-fracture membrane must be done with unmodified thin set. Evaporation or drying cannot occur
downward into the concrete slab through the antifracture membrane. Since tiles are also waterproof
and vapor closed evaporation or drying cannot occur
upwards through the tile. No evaporation. Therefore,
only unmodified thin set can be used. If a modified
thin set is used, a full bond does not occur.
manufactured stone veneer
because of the change to OSB
sheathing (much lower inward
vapor transmission) and a lack of
drainage or the use of rigid insulation (no inward vapor transmission) and a lack of drainage.
So why the bond failure predominately at the veneer to thin set
interface and not a bond failure at
the thin set to stucco interface? The
veneer to thin set interface is wetter
and stays wetter longer than the thin
set to stucco interface. The stucco is
more absorptive than both the thin
stone veneer and the manufactured
stone veneer and some inward
redistribution occurs. We got more
inward redistribution when we had
plywood sheathing and gypsum
sheathing rather than OSB and foam
sheathing.
So we get a weak bond or a failed
bond or we never got a bond at the
thin set to veneer interface. But
what "pushes" to stone off the wall?
Osmosis (Figure 2). Yup. We get it in
PHOTO 8 Tile in Shower. Installation of tile over an
impermeable waterproof membrane must be done
with unmodified mortar. Evaporation cannot occur
through the waterproof membrane into the tile
backer board and evaporation cannot occur outwards through the shower tile into the shower area.
Therefore, only unmodified thin set can be used. If a
modified thin set is used, a full bond does not occur.
PHOTO 9 Coating on Back of Manufactured Stone
Veneer. Manufactured stone is back-coated
("sealed") to prevent excessive water absorption
out of the thin set. The issue is desiccation of the
polymer modified thin set mortar leading to not
establishing a bond. So a "porous" not real stone is
made to behave like a non-porous real stone with
the back-coating.
#Back
in the day, bricklayers would dip the bottom surface of bricks into a pan of water
before placing the bricks on a mortar bed. This was to prevent the "kiln-dried" brick from
sucking too much water out of the mortar bed-desiccating the mortar bed-resulting in a
cold joint rather than a chemical bond. Ah, the old days. When was the last time you saw
this happen on a job site?
O C T O B E R 2 0 19
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