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What is thinset mortar, dryset
mortar or drybond mortar?
Thinset mortar is a blend of cement, very finely graded
sand, and a water retention compound that allows the
cement to properly hydrate. Tile set by the thinset
method is adhered to the substrate with a thin layer
of "thinset" cement. The terms thinset cement,
thinset mortar, dryset mortar and drybond mortar are
synonymous. This type of cement is designed to adhere
well in a thin layer - typically not greater than 3/16th
thick. For example, a 3/8" notch trowel will produce
a 3/16th inch thick coating after the tiles are pressed
in to the cement. While very minor adjustments in height
can be made, this method is not appropriate for adjusting
the level or flatness of a surface - rather the tile
will follow the plane of the substrate.
The American National Standards Institute (ANSI) defines
the properties of thinset mortar in the A118.1 specification.
What is medium bed thinset?
Medium bed "thinset" mortars can be used
to adjust for slightly larger variations in the substrate
than can be accommodated with thinset mortar. They are
also used with large, heavy or thick tiles, where a
thicker setting bed and a coarser aggregate may be required
to achieve a flat installation and to support the weight
of the tile while the cement is curing.
What is a thick-bed installation,
mortar bed installation, or thick-set installation?
Thick-bed installations are based on the traditional
method of packing a mortar bed over a surface before
installing the tile. The tile is adhered to the mortar
bed either while the mortar bed is green (just beginning
to dry) or after the mortar bed has cured. The mortar
bed may be reinforced with wire and either set over
a cleavage membrane (that allows the mortar bed to "float"
free of the substrate) or bonded to the substrate -
hence the use of the terms "floating mortar bed"
or "bonded mortar bed". For wall applications,
metal lath is mechanically anchored to the substrate
and the mortar locks into the metal lath as it cures.
The terms thick-bed installation, mortar bed installation,
and thick-set installation are synonymous.
What are the advantages
of a mortar bed?
In the case of the floating mortar bed, the tile layer
is unaffected by minor cracking and movement in the
substrate. This can be very important in applications
over concrete where cracking in the concrete could result
in cracking in the tile. It is also very important in
tile installations over structural slabs (not on grade)
or other structures where vibration and deflection can
be expected (as occurs in some exterior walls).
Mortar beds also allow for the following:
1. Level out unevenness in the substrate
2. Create an ideal surface to which the tile can be
bonded
3. Incorporate slope in the tile layer if needed (e.g.
slope to a drain)
4. Reinforcement of the substrate (usually relevant
in wood framing applications)
5. Allow radiant hydronic tubing to be installed
6. Protect metal, PVC, or CPE waterproof pans
As bonded mortar beds do not float over the substrate,
they do not offer protection from cracking or movement
in the substrate. However, they do offer the same advantages
listed above and in many cases can be safely installed
in a thinner layer than a floating mortar bed.
[Note: Anti-fracture membranes used in thinset or bonded
mortar bed applications can also protect the tile layer
from most kinds of cracks in the substrate.]
What are the advantages
of thinset installations?
Thinset applications are less expensive and typically
faster to install than mortar bed applications. However,
as the tile is bonded directly to the substrate, any
variation or movement in the substrate can affect the
tile. Also, there are many types of polymer-modified
thinsets on the market allowing the specifier the opportunity
to match the thinset properties with the project requirements.
In many mortar bed installations, the mortar bed will
be allowed to cure and then polymer-modified thinset
will be used to bond the tile.
What is latex-modified
thinset?
Thinset cement, to which polymers have been added, is
commonly called latex-Portland cement mortar. In fact,
this term is a bit of a misnomer. The original polymers
used to modify thinset were based on latex and the term
originates from their use. Today, there are over 10,000
polymers considered by cement chemists when formulating
their products. Polymers such as EVA, PVA, SBR, and
others are all commonly used in the industry. Many of
these polymers are acrylics and not latex chemicals.
The use of these polymers allows specific properties
to be imparted to the cement - commonly, freeze/thaw
resistance, improved flexibility, and improved adhesion.
There are also polymers used to make the cement waterproof
or sufficiently elastic so that it acts like an anti-fracture
membrane.
The American National Standards Institute (ANSI) set
minimum performance levels for latex-Portland cement
mortars in the A118.4 and the A118.11 specifications.
What is the desired compressive
strength for a mortar bed?
There has been quite a bit of debate regarding this
subject in the tile industry. Originally, mortar beds
were intended as a leveling and load dispersing layer
over a subfloor that met the standard L/360 criteria.
As such, compressive strengths could be very low (less
than 1000 lbs. and high sand to cement mixes were commonplace.
Also, this method allowed the floor to packed relatively
quickly with short curing times.
Over the years, grout and mortar manufacturers have
developed richer mixes with greater compressive strengths.
Typically, these mixes also require more liquid to develop
their maximum strength. In some applications, these
stronger mixes are used over insulation and provide
structural rigidity.
However, longer curing times and mortar bed curling
(from uneven drying) have also resulted. The debate
in the industry stems from differing opinions on whether
the greater compressive strength is desirable over slab
on grade construction or whether it provides no benefit
(and could be detrimental due to longer cure times,
increased shrinkage, and the possibility of curling).
There is also debate regarding the merits of greater
compressive strength when using mortar beds over various
suspended slab systems.
What is a cleavage membrane
or slip sheet?
These terms are synonymous and refer to the sheet used between the mortar bed and the substrate. This sheet prevents the mortar bed from bonding to the substrate and allows it to "slip" if there should be movement in the substrate. Page 30 of the 1999 ANSI A108 standard lists many types of slip-sheets.
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