Laying Concrete Block
Concrete blocks were used as the building material for
the new bankers in the masonry shop.
Once the outline was squared and scribed, the floor was prepped with a
damp sponge. Mortar was taken from the
mortar board and placed on the block outline with a trowel. The tip of the trowel was used to furrow the
mortar, which provided an even bed for laying the blocks. A Portland cement mortar of one –part cement
to two and one –quarter parts sand, was used.
The purpose of the sponge was to establish a moist area for the block
and mortar to bond. When laying blocks,
it is important to place the block as precisely as possible to prevent excess
movement on the mortar bed. Movement of
the blocks can disrupt the bonding between the block and mortar, and render the
bond useless. Another key aspect of
laying block is not to set it before it has been leveled and checked for proper
positioning. The mortar joint provides a
cushion for adjusting the block, and if the block is set to quickly, this
cushion is lost and further movement nullifies the bond.
Upon completion of the first course,
it is used as a guide for the remaining courses. The tops of the blocks are soaked with a
sponge and mortar is applied. Buttering
the ends of the blocks is imperative when locking blocks in to form mortar
joints. One of the most common mortar
joint dimensions is the 3/8” joint. The
same process is repeated until the desired height is reached. As courses of blocks are laid, a full block
is placed across the mortar joints.
Levelness should also be checked across the tops and sides of blocks as
each successive goes up.
Three Coat Process of Applying Plaster on Blocks
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| Leveling Plaster with a Straightedge |
The first step in laying plaster on concrete blocks is
to wet down the blocks with water. This
creates suction that aids the plaster in adhering to the block. If there are curves and/or corners in the
area being plastered, it is helpful to set up forms of some kind to help
establish the desired thickness. Strips
of wood can be nailed into the block using masonry nails, or holes can be in
metal panels to be held in place by screws.
Drilling holes in the block before inserting the screws reduces the
chances of damaging the masonry units and makes the job much less
difficult. Using forms when plastering
not only help set the thickness, but help in the leveling and smoothing process
as well.
The scratch coat is the first coat,
and when combined with the brown coat, becomes part of what is known as the
base coat. The main purpose of the
scratch coat is to form an adhesive bond or mechanical bond, depending on the
type of lathing being used (Van Den Branden and Hartsell 258 and 259). It is essentially the foundation for the
structure of the plaster wall (Van Den Branden and Hartsell 259).
A one to two and one –half mixture
of lime, sand and water was used for the scratch coat on the concrete
bankers. The lime mortar was placed on a
board, known as a “spot,” to keep it more readily available. A hawk was used to transport and carry a
supply of mortar. The key to applying
plaster is to cover the entire area as quickly as possible, so that the chances
of areas drying out and not bonding with wet plaster are greatly reduced. When the plaster is nearing the desired
thickness, traditionally 3/8” for the scratch coat, a featheredge or slicker is
used to level the surface. In order to
get the best result, the slicker should be pulled up from the bottom of the
plastered area with direct pressure and moved in a side to side fashion up the
length of the plaster. Dips and gouges
revealed by this process should be filled in and the process is repeated.
The final step is where the “scratch” coat gets its
name. Once the surface is smoothed and
leveled, a tool called a scarifier or scratcher is used. The scratcher is pulled horizontally across
the plaster in a slight wave –like manner.
This process creates points of attachment for the brown coat. It is important that the scratch coat is
still moist when scratched.
In the three –coat plaster process,
the brown coat is the next phase. The
scratch coat is sprayed with water and the application process is
repeated. The scratch and brown coats
both have the same mixture ratio. The
plaster is spread in using the same method as the scratch coat, minus the
scratching. Once the brown coat has had
a little drying time, a sponge float can be used to gently scrub down the
surface to smooth and fix imperfections before the finish coat is applied. The sponge float also gives the plaster a
textured appearance.
The finish coat is the final step in the three coat
plaster process. Traditionally, the
finish coat is applied to form a thickness of 1/8” or ¼”. The surface of the second coat should be
allowed to set for a period of twenty four hours and then dampened with water
before the finish coat is applied. In
order to obtain the best finish, the finish coat needs to be moist cured for
twenty for hours after application. If
Portland cement plaster is used as a finish coat, it should not be less than ¾”
thick to avoid cracking (Van Den Braden and Hartsell 283). The finish coat makes or breaks the aesthetic
beauty of the plastering. A smooth and
dense base coat is imperative when applying the finish coat. The base coat provides the bond and strength
on which the finish coat will adhere. A
poorly established base coat will lead to the finish coat weakly bonding or
cracking (Van Den Braden and Hartsell 283).
Works Cited
Van
Den Branden, F. and Hartsell, Thomas,
L. Plastering Skills. IL: American Technical
Publishers, Inc, 1983.
Print.
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