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 Copyright © 1979, The Aberdeen Group All rights reserved
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