| Movement
Joints, Expansion Joints or Control Joints
Why are movement joints
needed?
Recognizing that tile is a facade, movement joints
are needed to eliminate stresses that can occur between
the substrate and the tile due to differing amounts
of expansion and contraction.
Where should movement joints
be placed?
The Tile Council of North America's TCA Handbook recommends
allowing for expansion and contraction in every tile
installation. In small rooms, a gap at the perimeter
of the room (often hidden by baseboard or shoe molding)
is sufficient. For larger areas, the movement joints
will be visible.
We can not specify the exact location nor frequency
of movement joints as there many site related conditions
that must be addressed - however, we do offer guidelines
in Detail EJ171 in the TCA Handbook. It is especially
important to note for interior installations, movement
joints are placed more frequently when moisture or direct
sunlight is expected. For exterior installations, the
range of temperature from summer highs to winter lows
must be considered.
Why do rooms with more
sunlight need more movement joints?
The intent of the guideline regarding sun exposure
is to recognize that areas that get warm (or wet) may
experience greater amounts of differential expansion.
If the areas exposed to sunlight are warmer than surrounding
areas, movement joints should be used more frequently.
If the tile surface is not appreciably affected, no
accommodation is needed in the joint spacing.
Only the area subjected to increased temperature needs
to have movement joints more closely placed, not the
entire floor if elsewhere the floor is an even temperature.
What other things should
be considered when determining spacing for movement
joints?
Many things can subject the tile layer to shear forces
in addition to temperature and moisture. The following
is a partial list:
· Continued curing of the concrete substrate
can put the tile in compression
· Deflection and vibration of the substrate -
particularly with suspended slabs
· Seismic activity
· Changes in the plane of the substrate
· Location of weight-bearing columns
· Type of tile or glass
From what is a movement
joint made?
Movement joints are filled with material that allows
for contraction and expansion. For floor applications,
urethane, neoprene, or polysulfide are most often used
in traffic areas and silicone sometimes where traffic
is not a concern. Traffic areas require a sealant with
a shore hardness of 35 or greater.
Tented Tile
What causes tile to tent?
Tile heaved off the floor, or tented, and sometimes
cracked is often a sign that movement joints were not
used sufficiently. For tile over concrete, the curing
of the concrete places the tile under compression.
Why do installations
tent after a number of years?
How long it takes tile to tent is directly a function
of at least three variables - the rate of concrete shrinkage,
the shear strength of the thinset, and any expansive
forces applied to the tile layer (heat for example).
When the tile is poorly bonded, the tile can tent very
quickly. If there is a strong bond, often the grout
will compress significantly before the tile will lose
its bond. Of course the type of tile is important as
well - thinset has a harder time bonding to porcelain
than most other tile. At the other extreme, I have seen
a saltillo installation where the tile did not tent
but rather spalled as the thinset and grout were stronger
than the tile.
When tile fails with a loud report, this certainly
indicates that a good bond was present. Only when the
shear force exceeds the strength of the bond, will the
tile let go. Frequently, either the tile or the concrete
will be without thinset residue - as if the thinset
was not applied correctly originally. Usually, if the
tile is tenting years after the installation, this was
not the case. Had the thinset not been applied correctly
originally, the tile would have tented long before.
Rather, it is important to consider that the cleavage
plane will usually occur at the thinset transition -
either the bond to the concrete or the bond to the tile,
depending on the relative permeability and exact composition
of each.
Hence, it is common to see one surface or the other
sheared clean of thinset. Even in "explosive"
failures where the tile cracks and "jumps"
off the floor, usually one surface is free of thinset.
Clearly a good bond had been established.
With organic bonding agents and some of the polymer-modified
thinsets, continued shear forces degrade the bond over
time. So even when tile tents without an explosive report,
the original installation may have had sufficient adhesive.
In summary, every installation should allow for movement.
Properly designed installations, where expansion and
contraction do not create shear forces, should have
no problem for the tile to stay well adhered.
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