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This is a detailed
description of aggregate finishes and there advantages as they pertain
to the type of exposing mechanical or non-mechanical (sandblast, chemical retard,
etc...)
click here for the visual
color aggregate chart for all types
SANDBLASTED
Definition - This
finish is achieved by casting against a form surface that has
been painted with retarder which retards the set of the concrete
at its surface.
After the panel is
removed from the form, the retarder is removed by sandblasting.
The end result is a
panel with coarse aggregate exposed to the degree called for by
the design.
A. Reason for Use
1. Sandblasting
achieves an exposed aggregate finish as easily as possible.
2. Sandblasting allows
for the correction of many variations in exposure. This method
will achieve a more uniform surface.
B. Positive
Features
1. This method will
achieve a more uniform exposure of stone aggregate with minimum
effort.
2. Minor form
imperfections do not impair the final surface.
3. This method will
achieve a more uniform surface.
4. The variation in
color choices are endless.
C. Limitations
1. Any portion of the
panel poured in a vertical position will not show the same
concentration or positioning of aggregate as the flat surface.
(This problem might be corrected by sequential casting which
allows all surfaces to be cast flat.)
D. Expectations
1. Color consistency
is dependent on the consistency of the aggregate.
2. Consistency of
surface can be controlled reasonably well.
3. Sandblasting will
mute the finish and color of the aggregate to a degree,
depending on the hardness of the aggregate.
4. If a strong durable
aggregate is used, the color and texture of this surface would
not change over the years.
E. Summary
This finish should be
used where the beauty of the aggregate or a textured surface is
to be featured. The end result is a matte type finish, as
opposed to a brighter finish achieved with water washing.
II. EXPOSED
AGGREGATE - CHEMICALLY RETARDED AND WATER WASHED
Definition - This
finish is achieved by the application of a chemical retarder to
the surface of the form. The retarder prevents the matrix from
hardening at the surface of the panel to a specific depth,
controlled by the strength of the retarder. After curing
(normally overnight), the unhardened layer of matrix at the
surface of the panel is removed by a high pressure water washing,
thus, exposing the aggregate used in the concrete.
A. Reasons for Use
1. Exposed Aggregate
displays the natural beauty of the stone aggregate in its
natural colors and finish without damage.
2. It is an economical
finish to achieve.
B. Positive
Features
1. The aggregate is
not damaged or changed in this cleaning method.
2. Minor imperfections
in the form do not affect the final product.
3. This finish is
relatively easy to patch.
4. This finish tends
to distribute water run off more evenly, thus reducing the
streaking which appears on smooth surfaces.
5. The variation in
color choices are endless.
C. Limitations
1. The end result of
chemical retardation removed by water washing is controlled by
the retarder. Therefore, any variations in the exposure are not
as correctable as in sandblasting.
2. Vertical, radius or
complicated surfaces are difficult to cast with uniformity
because the retarder is subject to movement during casting on
these surfaces and variation in etch can result.
3. Water washing
can be
a seasonal activity for some producers in Northern climates.
D. Expectations
1. This finish will
display the aggregate in its natural beauty.
2. Control of mix,
slump, retarder and the time of exposure is essential since
there is very little correction allowed with the water washing method. Some variation will occur. Therefore, this should be
minimized by having as little contrast between matrix and
aggregate as possible.
E. Summary
If the bright, natural
colors of the aggregate are the prime concern, water washing is
the best way to achieve this result.
III. EXPOSED
AGGREGATE - SANDBLASTED
Definition - This
finish is achieved by casting concrete against a smooth hard
surface. After removal from the form, the finished surface is
sandblasted to remove the matrix and expose, as well as etch,
the coarse aggregate. The depth of the blast is determined by
the desired texture and the target color, as influenced by the
color of the matrix and the coarse aggregate.
The three different
types of exposures are:
Light Exposure - the
surface skin of cement and sand is removed just sufficiently to
expose the surface of the coarse aggregate.
Medium Exposure - a
further removal of the matrix exposes approximately the same
area of both coarse aggregate and matrix.
Deep Exposure - cement
and fine aggregate are removed to a depth where the coarse
aggregate becomes the dominant surface feature.
A. Reasons for Use
1. Sandblasted finish
can be used to achieve textured surfaces in which the coarse
aggregate is exposed, as well as etched by the blasting.
B. Positive
Features
1. Combining aggregate
size and color and matrix color with texture from the blasting
provides flexibility for the final finish.
2. Textured surfaces
tend to distribute water run off more evenly, thus reducing
streaking from normal weathering patterns.
3. Textured surfaces
tend to forgive minor surface imperfections when observed at
normal viewing distances.
4. Damage is more
easily repaired on textured surfaces than on smooth finishes.
5. The surface of the
coarse aggregate is etched at the same time that it is being
exposed by the blasting operation.
C. Limitations
1. Sandblasting
hardened concrete is time consuming and expensive. The deeper
the blasting, the higher the cost.
2. Damage to
sandblasted surfaces, while easier to repair than smooth
surfaces, is more difficult to repair than chemically retarded
surfaces.
3. Air voids and
uniformity of the aggregate density are difficult to control on
vertical and sloped returns.
4. Soft aggregates
tend to erode at the same rate as the matrix and sometimes
cannot be used.
5. The exposure is
influenced by the size of the sandblast sand and the skill of
the sandblasting technician. Operating techniques must remain
the same throughout the project to insure uniformity.
D. Expectations
1. Good color
uniformity can be achieved if care is taken in selection of the
raw materials which contribute to the color of the product.
Contrasting matrix and coarse aggregate colors should be avoided
if uniformity of color is desired.
2. The color of the
finish will progressively change as the depth of the blast
changes. The color will initially reflect the color of the
matrix. As the coarse aggregate is exposed, the color will be
influenced by the color of the coarse aggregate.
E. Summary
This finish is widely
used for light and medium exposure. Labor expense increases for
deep etches.
IV. SMOOTH - AS
CAST
Definition - Concrete
is placed against hard, smooth form work to achieve a smooth
"as cast" finish on the pre-cast element.
A. Reasons for use
1. This method shows
the natural beauty of concrete without trying to simulate any
other building product.
2. A very clean
definition of profile can be achieved with this surface.
3. If the surface is
to be painted, this finish provides an excellent surface, while
keeping cost. to a minimum.
B. Positive
Features
1. Cost is reduced by
eliminating additional finishing steps after removal from the
form.
2. Sharp lines at
intersecting planes can be achieved with this finish.
3. The smoothness of
this finish will self-clean accumulated dirt more readily than
an etched surface. However, streaking may remain.
C. Limitations
1. Resulting surface
of concrete will mirror the surface of the form it is cast
against, showing even minor imperfections of the form.
2. Since color is
basically controlled by the cement, variations between the
pieces, and within pieces will be more pronounced.
3. The surface is
difficult to repair.
4. Air voids show more
(generally) on this surface.
5. Surface crazing
(fine spider lines) will develop on this surface.
6. Some shadowing can
occur with this finish. "Shadowing" is aggregate or
reinforcing showing through the surface.
7. Different form
surfaces will affect surface color.
D. Expectations
1. Some color
variation can be expected with this finish, but the variation
can be minimized with careful selection of mix ingredients,
combined with close control of water/ cement ratio and mixing
time.
2. All vertical and
some horizontal surfaces will have voids created by entrapped
air or small water pockets. These voids can be minimized with
proper mix design and casting procedures.
3. Use of white cement
will achieve a greater uniformity of color than gray cement.
Allowable color variation in gray cement is enough to cause
noticeable color differences in precast panels.
E. Summary
Successful completion
of a project with this finish requires considerable involvement
prior -to signing a contract, including visiting existing
projects. Mockups should be made representing configurations
involved in the project. Mockups should be cast for approval for
acceptance of color variation, quantity of voids, and overall
appearance of architectural design by the client.
This type of finish is
strongly discouraged.
V. SMOOTH FINISHES
- LIGHT SANDBLAST
Definition - This
finish is achieved by casting concrete against a smooth hard
surface. After removal from the form, the element is given a
light sandblasting. This light sandblasting will remove the
cement skin from the surface. The resultant finish is a smooth,
sand-textured surface.
A. Reasons for Use
1. Smooth cast,
lightly sandblasted precast can achieve an appearance very close
to natural stone.
2. The smoothness of
this surface will self-clean accumulated dirt more readily than
an etched surface. However, streaking may remain.
3. It is possible to
achieve a clean definition of profile with this surface.
B. Positive
Features
1. This is a common
finish within the capacity of most precast producers.
2. Light sandblasting
is good way to remove the appearance of the "poured"
concrete look.
3. It minimizes
crazing by removing the cement skin at the surface of the
concrete.
4. It reduces color
variation by removing the cement surface, allowing the sand to
control the color to a greater degree.
C. Limitations
1. Smooth surfaces
show imperfections of surface more readily than on more deeply
etched surfaces.
2. Damage to smooth
flat surfaces is more difficult to repair.
3. Minor imperfections
of the cast show more quickly on this surface than on deeper
etched surfaces.
4. Variations in color
show more quickly on smoother surfaces and are more difficult to
control depending on the depth of sandblasting.
5. Air voids,
especially in the vertical portion of any cast, show more on
smooth flat surfaces.
D. Expectations
1. All returns
(turning vertically from a horizontal position while being cast)
will have air holes and can be grouted if they are
objectionable.
2. Use of white cement
will achieve a greater uniformity of color than gray cement.
Allowable color differences in gray cement are enough to cause
noticeable color differences in precast panels.
3. Sand is a very
important ingredient in a light sandblast finish, since it is
the predominant surface.
E. Summary
This finish can be
used to achieve a natural stone type finish. It will give you
excellent detail, but it is subject to variation in color, as is
seen in natural stone.
VI. SMOOTH - ACID
ETCH
Definition - This
finish is achieved by casting concrete against a smooth hard
surface. After removal from the form the element is allowed to
harden to a uniform hardness. The product is then washed with an
acid solution and scrubbed to remove the cement surface to a
sand surface level. The result is a smooth sand textured
surface.
A. Reasons for Use
1. Acid etching
produces a surface closely resembling natural stones such as
limestone, brownstone and sandstone.
2. The
"pre-weathered" surface will remain consistent for a
very long period of time.
3. Detail is not
damaged with acid etching.
4. Exposed sand
retains more "sparkle" with acid etching than with
sandblasting.
B. Positive
Features
1. A finer sand
texture can be achieved than with sandblasting.
2. Brighter, deeper
colors are achieved with acid etching.
3. Retention of detail
is best achieved with acid etching.
4. Acid etching
minimizes surface crazing by removing the cement skin on the
surface.
C. Limitations
1. The method of
applying and removing acid over a larger area is difficult to
achieve with great consistency.
2. This surface is not
as easy to patch as deeper etches. (This does not apply to minor
"bug" holes which are fairly easy to grout and
refinish.)
3. Imperfections in
the form show more readily than on deeper etches.
4. Return, or vertical
casts will have air-voids. If these are objectionable they can
be grouted and rewashed.
5. Acid etching can be
a seasonal activity in Northern climates with some producers.
D. Expectations
1. As stated above,
surfaces cast vertically will have minor air voids. These can be
grouted and rewashed if they are objectionable.
2. The color and
texture of sand used is very important with acid etched
elements. It is an important ingredient in controlling the
appearance and color of the precast element.
E. Summary
This finish is used to
achieve a bright, sparkling natural stone look. It is used
successfully on smaller trim work such as sills, lintels, belt
course and similar decorative elements. It is not recommended
for large panel work.
VII. FORM LINERS
FINISH
Definition - This
finish is achieved by the use of plaster, "rubber,"
grained wood, rope or most other material as a liner in the
casting form to impart a particular finish to the face of the
panel.
A. Reasons for Use
1. Unlimited effects
or textures can be achieved by use of liners.
2. This method can be
combined with sandblasting, etc. to achieve even greater variety
of finishes.
B. Positive
Features
1. Form liners are
extremely versatile.
C. Limitations
1. Commercially
available liners are fixed sizes. Joints from piece to piece are
difficult to hide.
2. The surface of the
panel is usually difficult to repair.
3. One piece liners of
plaster or "rubber" are expensive to produce.
4. Each different
liner material has its own strengths and weaknesses. The
material and desired effect should be coordinated with the help
of a pre-caster.
D. Expectations
1. Since the detail of
this finish is limited only by the imagination of the architect,
it is hard to generalize on expectations.
2. It is particularly
important to spend all the necessary time even prior to bidding
in making samples to be sure the desired result is achievable in
a satisfactory manner.
E. Summary
Use of liners opens an
unlimited number of options on finish. However, use of this type
of finish requires close coordination between the architect and
a pre-caster to be sure the desired end result will be
achievable.
VIII. TOOLED
FINISHES
Definition - This
finish is achieved by casting concrete against smooth or
specially textured or patterned form work. After removal from
the form, the hardened surface is treated mechanically to create
the desired effect. "Fractured Fin" and "Bush
hammered" are two types of finishes which employ tooling.
A. Reasons for Use
1. This method is used
to achieve a very special effect.
2. It is also used to
obtain a finish that cannot be achieved by other means.
B. Positive
Features
1. Consistency (or
lack of consistency if desired) is controlled by the craftsman
that finishes the panel.
2. Tooling is an
effective way to achieve "broken stone" finishes.
C. Limitations
1. Tooling is one of
the most expensive of the pre-cast finishes.
2. Variations due to
more than one craftsman working on the panel can show up with
this type of finish.
D. Expectations
1. This method allows
the architect to use some very different type surfaces, such as
"split rib," and "broken stone" faces.
2. This is the best
way to achieve surfaces closely resembling hand tooled natural
stone faces.
E. Summary
Tooled surfaces allow
a great variety of surfaces on pre-cast panels. However, it is an
expensive surface and it is very dependent on the caliber of the
artisans.
The result from tooled
finishes is much more individual in nature than many of the
other more standard finishes.
IX. NATURAL STONE
VENEER FINISH
Definition - Natural
stone cladding or veneer finish is achieved by placing natural
stone (limestone, granite, marble) pieces into a form and
casting concrete behind it to achieve a large pre-cast panel
having a natural stone face.
Its purpose is to
achieve a natural stone face on the building, while using the
efficiency of pre-cast concrete.
A. Reasons for Use
1. To provide a
natural stone face while using the efficient methods of pre-cast
panels.
2. Stone-faced
pre-cast
panels are far superior to individually set stone in achieving a
water tight surface.
3. Time and money are
saved since the panels can be produced while the structure is
being erected.
B. Positive
Features
1. Use of natural
stone bonded to pre-cast will achieve a safer, stronger
application of the natural stone to the face of the building.
2. Panels can be
prepared prior to completion of the structure of the building.
3. Damage to the stone
is minimized since the handling is all done on the ground.
4. Many smaller stone
panels are incorporated into each pre-cast panel achieving
efficiencies of manufacturing and erection.
C. Limitations
1. Some complicated
involved shapes do not lend themselves to casting natural stone
into pre-cast panels.
D. Expectations
1. Natural stone on
pre-cast will look like field set or frame set stone.
E. Summary
Natural stone set on
pre-cast is an excellent option to achieve a natural stone
building with greater efficiency, and achieve a far superior end
result (safety, water tightness, etc.)
Almost without
exception, time and money will be saved with this method.
X. BRICK FACE
FINISH PANELS
Definition - This is a
pre-cast panel with a cavity cast in, or a plate cast in if the
brick runs to the bottom of the edge so that the brick can be
set in the panel after its removal from the form.
Thin brick or tile can
also be set into a form and the concrete is cast behind it. In
this case, the comments on natural stone panels would apply more
closely.
A. Reasons for Use
1. This is a method of
achieving a brick building with the efficiency of pre-cast
concrete.
2. It is more
economical to lay brick at the ground level as opposed to
working from a scaffolding.
3. Panels can be
prepared while the structure is being erected.
B. Positive
Features
1. This is an
economical way to achieve an essentially brick building.
2. This is a superior,
water tight exterior when compared to a conventional set brick
building.
C. Limitations
1. Extreme care must
be taken in design and detailing to assure proper coursing of
brick when panels are installed in the building.
D. Expectations
1. This method will
deliver a brick building incorporating pre-cast trim that is
equal or superior to a conventionally set brick and trim
building.
E. Summary
Brick set into
pre-cast
will achieve equal or superior workmanship and economy over
conventionally set brick.
The end result will be
a more watertight building
Preparation of the
panel while the structure is being erected will save time and
money.
Xl. CAST STONE -
DRY TAMP FINISH
Definition - This
finish is achieved by ramming moist zero slump concrete against
smooth rigid formwork until the product is densely compacted and
ready for removal from the form. After curing, the product can
be hand rubbed or acid etched.
The result is cast
stone, an architectural pre-cast concrete masonry product which
simulates natural cut building stone and other masonry
materials.
A. Reasons for Use
1. To simulate other
masonry materials, i.e., Limestone, brownstone, unpolished
granite, etc.
2. Dry tamp guarantees
that the finish will have no "bug holes."
3. Economical
repetition of form use is achieved when fabricating smaller
elements.
B. Positive
Features
1. This finish
produces a fine grained texture, with no coarse aggregate
exposed, to simulate natural stone.
2. Due to lower mix
water content, finer aggregates can be used.
3. Vertical surfaces
will always have the same finish as horizontally cast surfaces.
C. Limitations
1. The process is
limited to smaller "mason set" elements because
consistency of finish is difficult to control in larger panels.
2. Manufacturing
tolerances are difficult to control in large elements.
3. "L"
shaped and "U" shaped returns and elements not having
flat backs are both difficult and costly to produce.
D. Expectations
1. Cast stone looks
very similar to any natural cut building stone except polished
granite or marble.
2. The color of fine
aggregate plays a predominant role in controlling the appearance
of dry tamp cast stone.
E. Summary
This finish is used
successfully in masonry applications and is exemplary where
small trim pieces are desired and natural stone could similarly
be used. (i.e. sills, lintels, coping, belt course and various
trim elements.
It is not normally
recommended for large panel work.
Not all producers are
equipped to produce this finish.
XII. MATERIALS -
PORTLAND CEMENT
Definition - The
cement used to make concrete is a combination of a variety of
sands and stone containing calcium, silica, aluminum, magnesium
and other elements.
The combination of raw
materials is fired in a kiln to produce a "clinker."
This clinker is then pulverized to a required fineness to become
cement powder.
The sands and stones
which contain the necessary raw material to make cement are not
necessarily the same colors from area to area.
Therefore, cements can
come in colors from white to dark gray, depending on the area
from which raw materials are drawn.
It is this resultant
variation in color which allows the pre-caster to use cement as a
controlling factor in coloring the final product.
However, the
complicated process of making cement and the variations in the
natural materials used to make cement make it just about
impossible to have absolute uniform consistency from one
"grind" of cement to the next.
A. Effect of Cement
on Finish
1. Cement is a
dominant factor in controlling color in "as cast"
finishes since the outer surface of the panel is in large
percentage cement particles.
2. One brand and type
of cement should be used throughout a project to minimize the
variation in final color.
3. Variation in
production (slump, mixing time, weather, humidity, cold or heat
of the day) will affect the color of the panel as well as
variation in the cement itself. Therefore, it is strongly
recommended that the surface of the panel always receive one of
the surface finishing techniques described in this publication
to achieve better uniformity.
4. White and
gray cements can be blended together to achieve any shade of color
that falls between the two cements.
5. White cement is
much more consistent in color than gray cement.
B. Summary
1. Cement is the main
coloring agent in finishes with little or no exposure.
2. Some variation in
color will result on smooth surfaces using grey cement.
XIII. FINE
AGGREGATE
Definition - The fine
aggregate used in concrete is usually a natural sand, or a
manufactured sand made by crushing quarried stone, passing a #4
screen (roughly 3/16" or less).
The consistency of
color is in direct relationship to the consistency of color
within a sand bank, or the rock from the stone quarry.
Fine aggregate
combines with cement paste and water to make a matrix which
fills all voids and ties the coarse aggregate together in the
concrete.
A. Effect of Sand
on Finish
1. Sand particles are
larger than cement particles, so in finishes which remove the
cement skin but are not deep enough to expose the coarse
aggregate, sand is a major factor in controlling color. This is
especially true with white cement.
2. Sand is available
in infinite varieties of colors and is generally very reliable
in controlling consistency of color.
3. Light colored fine
aggregates combined with light colored coarse aggregates will
produce a more uniform colored surface than will light and dark
combinations.
B. Summary
Fine aggregates are
very important in the coloring and texture of finishes such as
acid etched and light sandblasted finishes.
Fine aggregates are
generally a reliable control of color.
XIV. COARSE
AGGREGATE
Definition - The
coarse aggregate which provides the mass or body of the concrete
normally consists of natural gravel or pebbles manufactured by
crushing quarried stone.
The function of coarse
aggregate is to provide the mass or body to concrete. But, with
the advent of exposed aggregate finishes, the coarse aggregate
has become a means of achieving many effects in finish and
appearance.
A. Effect of Coarse
Aggregate on Finish
1. Coarse aggregate
comes in an unlimited variety of colors, shapes and sizes. Use
of coarse aggregate to affect visual appearance allows a wide
range of choice to the architect.
2. Coarse aggregate
becomes the dominant ingredient in controlling color once
exposure passes the level of sand-textured panels.
3. Texture as well as
color can be controlled by the choice of the coarse aggregate.
B. Summary
By removing the matrix
on the face of a panel and allowing the coarse aggregate to
show, the architect can have a wide range of colors, textures
and contrasts at his disposal to achieve the visual effect he
seeks.
Coarse aggregates
provide an effective means for control of texture and color.
XV. COLORING
PIGMENTS
Definition - Coloring
pigments are available for use in precast concrete. They are
usually natural or synthetic iron oxides which meet the latest
ASTM requirements (C-979).
Coloring pigments are
added directly to the mix so that color is dispersed throughout
the batch of concrete. Coloring pigments are normally used to
achieve colors which are difficult to get with available sand
and/or cement colors
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