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How to obtain predictable
architectural concrete
Evaluating the critical factors
BY JAMES M. SHILSTON
PRESIDENT ARCHITECTURAL CONCRETE CONSULTANTS
A DIVISION OF GENERAL PORTLAND INC.
T
he lack of predictability of architectural concrete surfaces is
f ru s t rating both to contractors and
architects. Many designers have attributed poor results to inexperienced contractors, and contractors
have justifiably claimed that what
the architect wanted was not the
same as he had indicated in the
contract documents. To have a reasonable anticipation that results
will be as plan ned, the contractor
needs to isolate, understand and
utilize the keys for obtaining the
finish objective. He should also verify that the keys have been properly
treated in the architect’s office.
Tabulated guidelines
The table provided here reflects
the degree of influence which various details in the construction
process have on architectural concrete finishes. A rating of 4 indicates
that the degree of influence is low.
When such a rating is given the construction methods normally required for a good structural concrete project are sufficient. A rating
of 1, howe ve r, indicates the degree
of influence is high and careful control of the construction process or
details is critical to achieving good
results in architectural concrete.
Ratings 2 and 3 are relative intermediate levels of influence.
This table is intended as a general
guide only. Since project details
vary, the rating for a specific finish
can also vary by one digit from project to project. While the chart may
indicate a critical rating of 1, under
certain conditions this might
change to 2. If the level were
changed all the way from 1 to 4 it is
probable the architect and contractor would lose control over the results and have to accept the consequences.
There is no attempt in this chart
to relate the importance of one detail to another. For example, it is obvious that the form rigidity for a
smooth as cast nonabsorptive
formed surface is not nearly as important to the total effect of that surface as is the cement color even
though both details are given a rating of 1.
tive were set for Seattle, Pittsburgh,
Boston or Atlanta, it would impose
additional cost because most local
standard sands tend toward values
that are too cool. The numbers indicate only the level of concern
which should be directed to each
detail.
It would be possible to prepare a
specification which would require
the amount of control for a rating of
1 for each item. But were this done,
the cost of the execution of architectural concrete would be prohibitive.
To achieve results within reasonable
economics, the architect must recognize points on which to be very
strict and those on which to be lenient.
Ratings and costs
An analysis of the classifications
for form rigidity for as-cast surfaces
can serve as a basis for explaining
the meaning of the table. There are
four different levels of control for
four finishes. The absorbent form
for these purposes would be conventional plywood or individual
b o a rd s. The nonabsorptive forms
would be steel, fiberglass-reinforced
plastic, polyvinyl chlorides or other
plastics, or elastomerics. Following
are the reasons why the four levels
The ratings do not necessarily reflect the relative cost or difficulty of
getting a quality product or detail
treatment even with a rating of 1.
Local practices or materials may
routinely provide what is wanted
even in structural concrete. The rating of 1 for fine aggregate color for a
light abrasive blast finish serves as
an excellent example. If the objective is to achieve a warm value with
a light abrasive blast, it would not
add to the cost in St. Louis, Memphis, Houston or Baltimore because
the local standard concrete sands
will produce such results. On the
other hand, if the same finish objec-
Understanding the table
The information in the table that
accompanies this article appeared
in the May 1973 issue of Architectural Record.
modulus. Impact-hammered finishes are not affected by the concrete
mix. Mix design techniques in the
American Concrete Institute Standard 211 are useful for applications
from thin shells to footings but not
for all architectural concrete mixes.
Admixtures are important to architectural concrete mixes for workability and for minimizing the possibility that lift lines will occur due to
earlier set in warm weather. Consistency control is obviously important
to architectural concrete. Some new
mixer trucks cannot discharge low
slump concrete, so it is important to
assure that mixers used are adequate to the job. A good design
should not be changed to meet the
limitations of a mixer.
Boarded, absorptive forms
have an effect on both color
and texture that is different
from the effect of smooth,
nonabsorptive forms.
Forms
are important for form rigidity for
good architectural results: The rating of 1 is given for smooth nonabsorptive formed surfaces because
the concrete cast against such a surface will tend to be uniform in color
and, for most forms, somewhat
glazed. Imperfectly plane surfaces
from dimpled or bellied forms
would therefore become accentuated when light strikes the surface at
angles that produce shadows.
A rating of 2 is given to the
smooth absorptive form because
the variations in absorption will
cause some variations in color of the
finished concrete surface thus minimizing the effect of the va ri a t i o n s
in plane.
A 3 rating is given to the textured
nonabsorptive form because texture
becomes a further accentuation of
surface va ri a t i o n s, minimizing the
effects of bulging and dimpling. The
restriction of 3 is given because the
textured surface frequently relates
to board forms which, if not rigid,
show a tendency to spring the joint
between two pieces of adjacent
forms and allow some leakage at
these intersections. The honeycomb at the leakage points with the
surrounding darker lines would
usually be objectionable. These surface imperfections open the concrete to moisture penetration and m
ay ultimately cause spalling and
rusting of reinforcing steel.
Finally, the 4 rating is given the
textured nonabsorptive form because such a form would be fairly
large and not subject to the potential amount of leakage found between more numerous individual
pieces of board.
It is obvious that the nonabsorptive formed textured surface is the
easiest for the contractor to achieve
and therefore the most predictable.
Concrete mixes
With regard to color of concrete
mix ingredients it can be seen that,
as depth of aggregate exposure becomes greater, major changes develop in the relative importance of
each of the three main ingredients.
Also, for deeper aggregate exposure
by abrasive blasting more attention
must be given to the aggregate gradations. The radical change in fine
aggregate gradation over the span of
the four abrasive blast finishes is attributable to the probable need to
change to a gradation outside the
ASTM C 33 lower limit for fineness
The quality of forms must be better when concrete is to remain in
the ascast condition than when a
heavily distressed texture is to be
produced. As form quality increases, the architect is wise to design in
such a manner as to facilitate reuse.
In the table the line labeled Reuse
Limitation indicates a relative need
for limiting the reuse of forms due to
deterioratio applies mainly to wood.
The finer the finish, the greater the
control needed in the forming material. Whereas any imperfection in
forms for a brush-blasted surface
will be telegraphed to the concrete,
properly reconditioned scars in
forms for concrete to be heavyblasted or jackhammered will not
be visible after the finishing process.
Under the general heading of Butt
Joints, three primary classifications
are considered. The reference to
Butt Joint Location relates to the
presence of butt joints at places other than behind rustications. Every
butt joint is a potential leakage
point at which discoloration that
cannot be removed by distressing
can occur. When tape cannot be
used because tape deformation is
visible on the finished surface, great
care must be given to the location
of form butt joints. On the other
hand, when there is reason to use
Gap gradings are not standard in the
ACI 211 mix design method but
require special mix design techniques
to achieve this result.
tape, some butt joints can be allowed because leakage cannot occur when the joint is properly covered. A more practical alternative to
tape is covering the butt joint with a
g ro oved rustication. In the finer textures this is the only solution to a
planned treatment of butt joints in
f o rm w o rk if the variable lines they
produce in the concrete are objectionable.
Tightness of forms is one key to
high quality results. Leakage can
cause a considerable amount of
honeycomb as well as more bugholes in the finished concrete surface near the top of a section. There
is more latitude to control of tightness of forms in some cases because
bugholes are at least minimized by
the finishing technique. While bellying forms are practically always objectionable to some degree in architectural concrete, the need for
design strength of the form increases in importance for other reasons.
If concrete mix retarders are used to
minimize the potential occurrence
of lift lines, difficulty can be encountered during construction if the
forms are not strong enough to take
a full hydrostatic head. Many structural concrete forms are designed
for six to seven feet of hyd ro s t a t i c
head and, if the concrete is fluid to
a greater height, the forms will fail
under the load. Stripping control is
very important for as-cast surfaces
and those on which planned projections from the concrete could be accidentally broken off.
equipment failure. In no case
should the concrete be moved horizontally by vibrators; it must be
placed as close as possible to its final position.
Release agents
Consolidation
Release agents, when improperly
applied, cause as much variation in
the color of as-cast arc h i t e c t u ra l
concrete finishes as any other factor known. As texture becomes
more pronounced, the ultimate influence of the release agent becomes less important. It is always
desirable to make certain there is no
laitance or buildup of product on
the form surface before the release
agent is applied.
Consolidation of arc h i t e c t u ra l
concrete is one of the most important though frequently neglected
steps in the construction. If the vibrator operator’s work is not done
properly all of the fine architectural
planning will be of little importance.
The recently issued American Concrete Institute Standard 309 covering consolidation of concrete is a
great step forward but there is still a
long way to go before the man in the
field can recognize why a particular
type of vibrator has been selected
and understand how it should be
used. A vibrator is more than a device for consolidating concrete. It is
also a device for internally mixing
two lifts of concrete and preventing
lift lines.
Form ties
Form ties significantly influence
the visual effect of arc h i t e c t u ra l
c o n c re t e. They are placed in the
forms on a pattern, which should be
consistent with the type of form design. The effectiveness of treating
surfaces by patching tie holes is
questionable. Small ties are less
conspicuous than large ones but
have less holding capacity. Cone
type ties have been used by many
architects as an expression of the
pattern. There another systems but
probably more important than the
tie system itself is assurance that the
tie is properly placed to prevent disfiguring leakage.
Concrete placement
A rc h i t e c t u ral concrete must be
placed not poured. Only with the
jackhammered finish is a rating of 4
given for any placement detail. This
rating allows the use of a pump.
Concrete mixes for pumping must
usually be different from those for
architectural concrete, and pumps
that require mixes of roughly 50 percent coarse and 50 percent fine aggregate are not recommended.
When a pump can handle an architectural concrete mix design with a
low water-cement ratio there should
be no objections to its use. If used,
there should be alternative placing
techniques available in the event of
Steel
Setting of reinforcing steel is generally thought of as a field problem.
In architectural concrete construction, howe ve r, it should be the responsibility of the architect to plan
the sizes of bars and reinforcing
steel placement details so that the
work can be accomplished without
detriment to the surface finish. Improperly placed reinforcement can
create such a mass of metal as to
make effective workmanship in the
field impossible; also, the steel can
be so close to the surface that rusting, and eventually spalling, will occur. Support systems such as chairs
for metal in beams must be provided but their type must be chosen
with regard for their visual effect.
Finishing
The type and condition of finishing equipment and the choice and
timing of techniques have a major
influence on finish results. Heavy
work must be performed with sufficiently heavy equipment. Care must
be given to timing because finish-
ing ease is related to the strength of
the concrete surface. If much mortar is to be removed it is obvious that
the work should be done as soon as
practical after casting. In impact
hammer work the only requirement
is that the concrete be strong
enough to prevent coarse aggregate
particles from being knocked from
their sockets, thereby creating
bugeyes. Timing and different types
and gradations of abrasives used in
blasting have a major influence on
the results. Large abrasive particles
will generally attack large aggregate
particles. Fine particles create more
dust problem, to which there is increasing objection. When chemical
retarders are used several should be
evaluated so as to choose the one
best for the particular concrete mix
and job conditions. Tool condition
and sharpness are important. A
bushhammer operator will use approximately one tool per 30 to 40
square feet of surface hammered.
Hammering with dull tools is very
expensive because little work is accomplished and the finish has little
character. Finishing of as-cast surfaces, if any, usually involves only
cleaning with a product such as a
masonry cleaner. Construction dirt
and the natural efflorescence of
portland cement concrete may have
to be removed. This is a good opportunity for a bid deductive alternate if
the cleaning is not necessary.
General table usage
The table will be of value in helping the designer determine the best
finish for the construction conditions of a particular project. The architect should first study the locale
where the work is to be perf o rm e d
and evaluate forming know-how,
contractor techniques, ready mixed
concrete and precast concrete facilities, and the overall quality of work
completed in the past. Should he
find there are great limitations concerning delivery of special mixes, he
should avoid any design requiring a
heavy abrasive blast finish because
it would place a big demand on a
special concrete mix. A finish objec-
tive more closely related to a textured nonabsorptive form would
take better advantage of the capabilities.
If there is a shortage of carpenters, quality of workmanship is poor
and there is not much repetitive
forming on the project, the finish
that would be most easily achieved
would be one that is forgiving of
form deficiencies. A jackhammered
texture, though expensive, usually
provides a surface that is more forgiving of forming variations than
any other. Here, money should be
saved in concrete mix controls,
placement techniques and forming
so that funds will be available for
finishing.
The table has also been prepared
in the form of a punch-card checklist for attachment to job files.
Good practice
If the contents of this table are
kept in mind, the following practices can be reasonably incorporated into an architectural concrete
project:
• Section sizes and reinforcing steel
details can be designed to facilitate constructability. There must
be adequate space to work and to
place concrete.
• The occurrence of construction
joints, which are almost impossible to conceal, can be articulated;
in most cases the engineer will
want these joints in the center
span of beams.
poseful variation via texture is
more readily noted than the incidental variation normally obtained in the usual concrete construction process.
• Reinforcing steel details can be
double checked to make sure that
adequate space is available for
casting. This can be the difference
between a reasonably achievable
project and an impossible one.
• A clear statement of what is wanted can be made in the specifications. If form butt joints should
occur only behind rustications,
the specifications must so state.
Since it is critical that forms remain tight under the hydrostatic
head of the concrete and the
movement of the vibrator, there
must be a specially pre p a re d
specification for the gasketing of
corner joints.
• Remember that arc h i t e c t u ra l
concrete is a highly refined concrete which must receive as much
additional attention as would
millwork compared to rough carpentry. Though the structural requirement will always govern for
that quality, construction planning and details are extremely
critical when architectural results
are wanted.
• Locally available aggregates can
be tried, with some flexibility of
results being achieved by selectivity of the cement. Howe ve r, for
most conditions, the ready mix
producer can supply architectural concrete with greater ease and
at less cost by using special cement rather than special aggregates.
• Nonabsorbent, sufficiently textu
red forms can be chosen wherever possible for walls that are to be
left as cast. Smooth, as-cast walls
with no variation are difficult if
not impossible to achieve. Pur-
PUBLICATION #C730363
Copyright © 1973, The Aberdeen Group
All rights reserved