Mortar
Mortar
is the bonding agent that holds masonry units together. One key characteristic of mortar is its high
compressive strength, which gives it good load –bearing qualities. If the mortar is to be effective, it must be
durable, form a strong bond between masonry units, and resist moisture
penetration. Moisture penetration is
mortar’s worst enemy. Another important
factor when considering which type of mortar to use is its strength. Stronger mortar is not always the best
choice. The strength must meet the
requirements of the job and fit the needs of the masonry units. When the bond between the mortar and masonry
unit is too strong, stress will be relieved through the masonry unit rather
than the mortar joints. This will lead
to cracks and accelerated deterioration of the masonry units (Kreh).
Lime
Mortar: the ingredients needed for lime mortar are as follow: sand,
hydrated lime and water. A good all
around mortar ratio is 1 to 2 and one –half.
This means that two and one –half units of sand will be added for every
one unit of hydrate lime. Hydrated lime
derived from the slaking of quicklime. The result is a dry powder that only
needs water added and a soaking period in order to be used. When mixing mortar, it is essential to use
the same measuring device for both so that equal proportions are obtained. After the lime and sand are dry mixed, water
is added until the desired workability is reached. Lime mortars can be reconstituted by adding
water if drying occurs.
Natural
Cements: these are cements that are
derived from naturally sedimentary rocks such as gypsum and limestone (Van Den
Branden and Hartsell 88-97). The
production of natural cement begins with the calcination, or burning, of the crushed
rock in kilns. The clinker is then
ground and processed to form the cementitious material (Kreh and Van Den Braden
–Hartsell). These products can then be
combined with water to form natural cement.
An aggregate is often used in the mixture process as well, but the
amount varies depending on the application.
Modern
Cement: the most common type is
Portland cement. Portland cement was
discovered by Joseph Aspdin in 1824.
Aspdin was an English stonemason who was trying to find a way to produce
mortar that hardened when water was added.
The resulting cement was named portland because it was very similar to
the natural stone found on the Isle of Portland (Kreh 114). Portland cement –hydrated lime mortars
contain a mixture of Portland cement, hydrated lime, sand, and water. The compressive strength is one of the best
attributes, but because of this, it is very important to know the strength
required for each job. Masonry mortar
containing Portland cement is very different from Portland cement concrete
(Kreh 114). The charts provided explain
the differences and ratios of each.
Portland
Cemen
(Chart
content comes from Kreh 114-115)
|
Type I
|
General
–purpose cement that is used in pavements, sidewalks, reinforced concrete
bridge culverts, and masonry cement.
|
|
Type II
(Modified Portland Cement)
|
Hydrated at a
lower temperature and generates heat at a lower rate than Type I. Usually used for large piers, heavy
abutments, and retaining walls.
|
|
Type
III (High –early strength Portland)
|
Achieves
full strength much faster than Type I.
Type III reaches full strength in 1 to 3 days, and is often required
for jobs when freezing is expected.
|
|
Type
IV (Low –heat Portland)
|
Type IV
cures and gains strength slowly. Used
for dams, bridges, or areas that large amounts of concrete are needed.
|
|
Type
V (Sulfate –resistant Portland Cement)
|
Type V
is intended to be used only in conditions where the cement is exposed to high
sulfate action. This cement also
reaches full strength slower than normal Portland cements.
|
Portland Cement –Hydrated Lime Mortars
Type
M: has a greater compressive strength and more durable than other
types. It is used for masonry that is
below ground and of lower grade such as foundations, retaining walls, and
manholes. Type M is high frost resistant
qualities and can withstand pressure forced up from the ground. 2500 psi.
Type
N: most often used mortar type. Type N has medium –strength and good for use
above grade masonry that is exposed and requires high compressive strength for
lateral loads. Prebagged cement mortars
are similar to Type N and are used on chimneys, parapet walls, and exposed
exterior walls. 750 psi.
Type
S: reinforced and standard masonry
that requires flexural strength. 1800 psi
Type
O: low strength mortar used on non
load-bearing interior walls. It can be
used as a load-bearing mortar when constructing a solid masonry wall that is
not to experience severe weather or carry extreme loads. 350 psi. (Kreh
121-22).
Chart content found in Kreh’s Masonry Skills
on page 123.
|
Mortar Type
|
Parts by Volume of Portland Cement
|
Parts by Volume of Hydrated Lime
|
Sand, Measured in a Damp, Loose Condition
|
|
M
|
1
|
1/4
|
Not
less than 2 -1/4 and not more than 3 times the sum of the volumes of cement
and lime used.
|
|
S
|
1
|
1/2
|
Not
less than 2 -1/4 and not more than 3 times the sum of the volumes of cement
and lime used.
|
|
N
|
1
|
1
|
Not
less than 2-1/4 and not more than 3 times the sum of the volumes of cement
and lime used.
|
|
O
|
1
|
2
|
Not
less than 2 -1/4 and not more than 3 times the sum of the volumes of cement
and lime used.
|
Mixing Mortar
Mixing mortar by hand is a common
practice on the job site when a large amount of mortar is not needed. The first step in mixing mortar is gathering
all of the ingredients (usually Portland cement, lime, sand and water) needed
so that they are readily available. The
most efficient method to manually mix mortar is in a mortar box. If a wheeled cart is available, place the box
on top of it to ease the transportation of the mortar once it is mixed.
Once all of the preparation work is
completed, spread the required amount of sand evenly into the mortar box. Depending on whether a Portland cement –lime
mortar or a simple lime mortar is being made, spread the needed amount of the
binding material evenly over the sand. A
hoe is then used to dry blend the mortar mix until it is of a uniform
consistency. An even mix is generally
indicated by have one consistent color in the mortar box. The most effective and efficient method of
using the hoe is by taking short, choppy strokes to pull the mix from one side
of the box the other.
After dry mixing the mortar, the
mixture is pulled to one end of the mortar box to form a pile. This creates a space to add water. If possible, use the same measuring device
that was used for the aggregate and binding material to add the water to the
mixture. The amount of water need
depends on the materials being used and the consistency best suited for the
job. It is always better to add water
slowly to avoid drowning the mortar. The
process used for dry mixing is also used to blend the water into the mixture.
A simple test to determine if the
mortar is mixed well is using a trowel to pick up a small amount of
mortar. If the mortar sticks to the
trowel blade when turned upside down, it shows that the mortar has good workability,
adhesiveness and is ready for use. Fat
mortar is indicated by having too much binding material in the mixture. The mortar will cling to the trowel and is
not easily worked by the mason. Lean
mortar on the other hand, is mortar lacking enough binding material and will
not adhere to the trowel or masonry unit.
Sources
Van
Den Branden, F. and Hartsell, Thomas,
L. Plastering Skills. IL: American Technical Publishers, Inc,
1983. Print.
Kreh,
Richard T. Masonry Skills 5th
Edition. NY: Delmar Learning, a
division of Thompson
Learning, Inc, 2003. Print.
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