How to specify
Architectural concrete
how to get what you specify
BY IVER JOHNSON, PRESIDENT
GUNNAR I. JOHNSON & SON INC.
GENERAL CONTRACTORS
MINNEAPOLIS, MINNESOTA
that remained below the construction joint can still be
seen. The more prominent thin horizontal stain or discoloration seen just above the construction joint is discussed under Discoloration. The most reliable way to
avoid this kind of paste leakage is to design walls so
forms need not be lapped over board patterned surfaces,
as discussed under Architectural and form design.
o designers, architectural concrete is a magnificent art form. To specifiers, it is a stimulating
challenge to their interpretive powers. For cont ra c t o r s, this art form provides perhaps their
most outstanding opportunity to display pure craftsmanship on a truly monumental scale.
But as in any complex undertaking, all must work together to ensure that the final results meet everyone’s
e x p e c t a t i o n s. Each one must understand which options the others have at every step from conception to
completion.
We have been able to cooperate with designers and
specifiers to overcome several pitfalls common to most
architectural concrete projects. These are:
Discoloration
T
• Scabbing from grout or paste leakage
• Color variation
• Bugholes or entrapped air
Scabbing from grout or paste leakage
Photo 1 shows one of the most common flaws that designers have to contend with in architectural concrete.
The cause is obvious. The deck form was overlapped and
clamped directly to the outside of the wall. Grout leaked
through the form joint between the deck and wall.
Photo 2, a closeup of the same area, is a graphic reminder that no matter how tightly you clamp, it is impossible to prevent grout leakage through a simple wood
form joint for architectural concrete. Specifiers must remember that architectural concrete surfaces are neither
smooth nor straight (Figure 1). The specifier should require that joints between concrete surfaces and forms
be sealed by calking or gaskets. This is simple for the
contractor to do and it greatly enhances the quality of
the finished product.
At the same time everyone should remember that,
even when such form-to-concrete joints are calked or
gasketed, pressure on the form can cause bulges that
disturb a seal. Photo 3 shows a properly calked joint that
failed at some spots because of bulging. The photo
shows that the scabs have been removed, but scab stains
Photo 3 also shows a second problem common to architectural concrete—discoloration. The dark ve rt i c a l
board mark at the upper right of the picture has a somewhat unusual cause. Years ago, our local supplier told
us that such discoloration was caused by sugars in the
wood which dissolved in concrete during the first pour.
Ever since, we have made it a practice to wash new
forms and replacement boards with a cement paste before they are used the first time. This dissolves the sugars
and eliminates the problem. We strongly recommend
that such washing be specified where high quality concrete finishes with minimum color variation are desired.
Also in Photo 3, leaking paste which formed the scabs
below (now removed) took water from above the joint,
hence discoloration. This characteristic stain is evident
the full length above the construction joint.
Photo 4 is a closeup view of the surface shown in Photo 3. It illustrates discoloration from water loss. Note the
dark area around the bugholes and the thin dark ve rt ical line marking the location of the seam between form
boards. Though no significant grout was lost, the color
change from disproportional water loss is evident. Tight
Figure 1. Designers must
remember that patterns in
architectural concrete walls
make it impossible to secure
overlapped wood forms tightly
enough to produce the
continuous contact necessary
to prevent grout or paste
leakage.
Photo 1. Scabbing from grout or cement paste leakage.
Photo 4. Closeup view of concrete surface on right side of
Photo 3.
Photo 2. Closeup view of scabbing shown in Photo 1.
Photo 5. Color variation between lifts of concrete in a wall.
Photo 3. Stains and discoloration above and below
construction joint in architectural concrete wall.
Photo 6. Architectural concrete design with strong
horizontal elements.
seams eliminate this problem.
Photo 5 illustrates a typical problem for which there
appears to be no simple cure. Notice the very dark color
of the last lift of concrete placed in this wall. Such discoloration can be caused by a number of different factors, including
• different water-cement ratio of concrete in the last lift
• long delay in placing the last batch, thus permitting the
form to heat or dry out in the sun
Whatever the cause, differences in color between lifts
is beyond the control of the specifier and often beyond
Photo 7. Closeup view of surface shown in Photo 6.
Figure 3. Details of form
bulkhead design featuring
rubber gasket seal versus
commonly used design.
Figure 2. Details of form used to produce board marked
surface with strong vertical and horizontal rustications (see
Photos 6 and 7).
that of the contractor. Designs with strong horizontal elements and very rough surfaces will help conceal these
almost inevitable defects.
Architectural and form design
Photos 6 and 7 show a project where all parties cooperated during the design phase and produced a system
that reduced or totally eliminated the defects discussed
thus far. Furthermore, this construction system was fast
and economical. The project averaged two stories a
week and the form cost was amortized over 23 reuses.
Taken from about 30 feet, (1)* Photo 6 shows the strong
3-inch-wide(2) h o ri zontal rustication strip alternating at
about 1- and 3-foot (3) spacings. These help mask color
variations between lifts.
These feature strips served another—perhaps more
important—function. They provided an adequate bearing surface to clamp formwork for each story. This
means formwork did not have to lap over the rough
board textured wall face.
Figure 2 illustrates the form detail used to produce the
surface shown in Photos 6 and 7. Exterior formwork is
in place and anchored to the previous pour. The form
face consisted of 3⁄4-inch-thick(4) (plus or minus 1⁄8 inch)(5)
by 11⁄2-inch-wide(6) rough sawn boards screwed into a 3⁄4inch(4) plywood backup. The detail at the upper right of
the figure shows a horizontal cross section which also indicates the 3⁄4-inch-thick(4) t ra p ezoidal wood strip that
produced vertical rustications. These were spaced on 8inch(7) centers. Small gaps between the rough sawn
Figure 4. Vibration and revibration schedule developed by
Gunnar I. Johnson & Sons to reduce bugholes in
architectural concrete.
boards sealed themselves with cement paste during the
preliminary grout wash of the new wood discussed earlier. These caused no water-loss problems.
The detail at the lower right of Figure 2 shows perhaps
the most important element of the forming system. The
top of each wall section has a 3-inch(2) rustication built
into it. The bottom of the form has a rubber bar gasket
(of 50 Shore A durometer hardness) permanently affixed.
This rubber is soft enough to create an excellent seal to
the concrete below yet firm enough to reproduce the
proper angle at the bottom of each pour. (Vertical rustication strips were cut at 60 degrees(8) to mate perfectly
with the 30-degree-angle(9) on the rubber bar.)
The construction joints on these walls showed very little scabbing. The use of four 3-inch-wide(2) h o ri zo n t a l
rustications per 8-foot(1) wall section made the construction joints almost invisible and at the same time did
much to minimize the visual effect of color va ri a t i o n s
from one lift to the next.
Figure 3 shows the difference between the bulkhead
design used on this project (upper detail) and a typical
design commonly used (lower detail). With the designers’ consent, bulkheads were revised to form a point [almost] instead of the typical 90-degree(10) square. No rm a lly, the formwork for a 90-degree(10) bulkhead requires a
loose 8-inch(7) section be secured to 2- by 4-’s.(12) Not only
is securing difficult, it is almost impossible to calk joints
at y in Figure 3 within the confines of an 8-inch(7) space.
Howe ve r, with the alternative bulkhead design actually used, the plywood end pieces were permanently secured to the main wall form and sealed to one another
with a vertical rubber bar gasket. With this bulkhead design, joints at x in Figure 3 are permanently calked and
the form ties provide sufficient pressure for a tight seal
with the gasket.
Photo 8 shows the project at the 18th-floor level.
These were expensive forms to build. Howe ve r, their extensive reuse and the 2-floors-per-week average that was
maintained more than justified the initial expense.
Revibration
A few comments about vibration and revibration in
architectural concrete are in order. The generally recommended practice for vibrating concrete is one lift at a
time, with the vibrator head penetrating 6 inches(13) into
the previous lift. Our experience has shown that this is
not sufficient for quality architectural concrete work.
Figure 4 illustrates the revibration schedule our firm has
developed over the years. Each penetration of the vibrator extends completely through the previous lift and 6
inches(13) into the third lift below. This practice substantially reduces bugholes at the interface of each lift.
Normally, pours are scheduled at one half hour per lift.
In such cases, the final vibration is done one half hour after the last pour is completed. We have found that maintaining the same time lapse between vibrations in the
last lift is important for minimizing bugholes.
It must be stressed that revibration, while very effective, must be used with discretion. It places greater stress
on formwork and form ties. Also, if the formwork leaks,
revibration will cause the loss of greater amounts of cement paste.
In short, forms which are tight, properly calked and
designed to handle additional stress will work with a
consistent revibration schedule to produce excellent
results.
Concrete mix for exposed aggregate
Photo 9 shows an exposed aggregate stairway built
recently by our company. It illustrates how gap-graded
c o n c rete can be used to produce an aggre g a t e - ri c h
s u rf a c e.
This particular stairway used a design mix with a ratio of coarse to fine aggregate of 8 to 2. Cement content was 7 bags per cubic yard .(14) This mix pro d u c e s
4000-psi(15) concrete with excellent pro p e rt i e s. When
sandblasted, it reveals a dense distribution of coarse
aggregate particles.
Summary
There are at least five areas in which specifiers can definitely influence the quality of architectural concrete
surfaces. These require
• that joints in formwork be sealed by calking or gaskets
• that concrete in forms be double-vibrated
• that formwork be built and sealed rigidly enough to remain sealed when double-vibrated
• that formwork not be lapped over uneven architectural concrete surfaces (This generally requires help from
designers.)
• that an 8-to-2 ratio of coarse to fine aggregate, with a
proportionally greater cement content, be specified for
structural exposed-aggregate concrete
Photo 9. Exposed aggregate
concrete stairway built with
gap-graded mix reveals dense
distribution of coarse
aggregate after sandblasting.
Metric equivalents
(1) 10 meters
(9) 1⁄8’ radian
(2) 75-millimeter-wide
(10) 2.5-meter
(3) 0.3- and 0.9-meter
(11) 1⁄2‘ radians
(4) 19-millimeter-thick
(12) Nominally 50- by 100-millimeter lumber
(5) 3 millimeters
(13) 150 millimeters
(6) 40-millimeter-wide
(14) 390 kilograms cement per cubic yard
(7) 200-millimeter
(15) 28-megapascal
(8) 1⁄3‘ radians
PUBLICATION #C790006
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