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What is reflective
cracking?
Whenever tile is bonded to concrete, cracks occurring
in the concrete can cause cracks in the tile layer -
this is often called "reflective cracking".
What about tiling over
control joints?
The Tile Council of North America Handbook recommends that
control joints in concrete carry through the tile. Clearly,
this is an industry-approved, nearly foolproof, and
very safe way of making sure that movement in the control
joints does not cause a reflective crack.
Due to a lack of consensus in the industry regarding
competing anti-fracture products and the standards of
performance for such, TCNA does not, at present, recommend
a method for tiling over control joints with an anti-fracture
membrane. This is not to say this process will not work
- it can if the right products and methods are used
and the slab does not continue to curl.
If the slab does curl at the control joint (which is
not uncommon), any curling that occurs after tiling
may damage the tile.
Can you tile over
control joints using an anti-fracture membrane?
Some manufacturers have proprietary products where
they will guarantee a tile installation (when their
products are used in accordance with their methods)
over control joints, so long as vertical deflection
does not occur. However, expansion joints must be used
in the tile layer.
It is a frequent misconception that anti-fracture membranes
allow you to eliminate movement joints - they do not.
There always must be soft joints in the tilework to
allow for expansion and contraction.
Typically the joints in the tile installed over an
anti-fracture membrane must be placed near the joints
in the concrete but not necessarily directly in line
with the control joints.
What about tiling
over small cracks in the concrete?
Even small shrinkage cracks in concrete can be dimensionally
active; continued curing of the slab can cause these
cracks to expand or propagate. This type of cracking
can be easily avoided in the tile layer - either by
installing the tile on a mortar bed set over a cleavage
membrane (method F111 for example), or by installing
the tile over a crack isolation membrane using a thinset
method.
In the mortar bed installation, the mortar bed is not
bonded to the concrete - rather it is isolated from
cracks in the concrete by the cleavage membrane. This
allows the tile to "float" over the concrete.
In the thinset installation, a crack isolation membrane
is bonded to the concrete. Tile is bonded (with thinset)
to the surface of the membrane.
What is an anti-fracture membrane?
The internal make-up of this membrane is such that movement
in the concrete is not directly transferred to the tile.
Although the membrane is bonded to the concrete and
the tile to it, the membrane stretches where needed
to prevent or reduce force transference. These membranes
are either trowel applied or sheet applied. In many
cases multiple components or steps are part of the system.
Performance varies also - it is important to check with
the crack isolation membrane manufacturer regarding
their installation instructions and intended use.
Can roofing felt or scribing
felt be used as an anti-fracture membrane?
Some contractors have used felt paper as an inexpensive
type of anti-fracture membrane. Unfortunately, this
type of installation generally does not provide suitable
bond strength between the tile and the floor, does not
hold up to moisture, and can promote fungal growth.
What are the uses of crack
isolation membranes?
Products made specifically for crack isolation are sold
for everything from isolating the tile from shrinkage
cracks to tiling over control joints to protecting the
entire floor from potential cracking in the concrete.
When used to cover the entire floor, many manufacturers
will warrant the entire floor installation, including
the cost of replacing and installing new tile.
As noted previously, it is a frequent misconception
that anti-fracture membranes allow you to eliminate
expansion joints - they do not. There always must be
soft joints in the tilework to allow for expansion and
contraction. Guidelines for expansion joint placement
are given in the Tile Council of North America Handbook. The
exact placement of expansion joints is a function of
many items including exposure to sunlight and the range
of high to low temperature, moisture, aging of the concrete
(where relevant), structural movement, expected loading,
and other design criteria. The manufacturer's recommendations
should also be consulted and followed.
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