Architectural precast concrete

Video of a precast concrete house frame

Architectural precast concrete is a concrete element that contributes to a structure’s architectural form or finished effect through application, finish, shape, color, or texture. This concrete is created by a mould, usually in a precast facility. This is type of concrete application is beneficial because the concrete is given the proper amount of time to cure, while concrete poured on-site is only allotted a small amount of time to set before construction on top of it must begin. Precast concrete comes in thousands of shapes and sizes, and can usually be custom ordered to match an architect’s needs. Hardware for connection to the structure may be structural steel shapes, bolts, threaded rods, and reinforcing bars. These connections are usually welded or bolted.

History

The Cathedral Notre Dame du Haut in France has the first documented modern use of architectural precast concrete. This concrete was only used for screen walls or as a fill in for cast-in-place concrete structures. The development of precast concrete for architectural purposes appeared around the 1920s but did not find popularity in the United States until after World War II ended. Surface finishing techniques for architectural precast concrete consisted of water washing, bush hammering, sandblasting, and acid etching. These treatments were originally developed for cast-in-place concrete, but became commonplace in the cast stone industry in the 1930s. John J. Earley and Earley Studios began the first work of exposed aggregate ornamental elements in 1932. The Baha’I Temple is one of the first architectural precast concrete projects in the United States. The panels consist of white concrete with quartz aggregate exposed. In 1938, administration buildings at the David W. Taylor Model Testing Basin were built using the Mo-Sai manufacturing technique. This was the first building to use these techniques, which allowed finishes with densely packed mineral aggregate and a minimum amount of cement and fine aggregate. John Earley collaborated with the Dextone Company of New Haven, Connecticut. Earley patented the idea of using step (gap)- graded aggregate to achieve uniformity and color control for exposed aggregate work. The Dextone Company used Earley’s ideas and refined methods that later became the standard operations for the Mo-Sai Associate. The Mo-Sai institute eventually included a number of licensed manufacturing firms. The acceptance of architectural precast concrete depended heavily on the amount of advertising activities, which highlighted the technical achievements of the product.

Manufacturing Process

Mo-Sai panels were usually 2 inches thick and up to 100 square feet. They could serve as a veneer or as form and facing for poured concrete. The panels were cast faced down, vibrated, and enforced with welded mesh. Coarse aggregate composed the face and the aggregate was usually either granite or quartz. The mixture usually consisted of 1 part fines to 7 parts of two or more sizes of coarse material. The aggregate to cement ratio in a damp mix was 5 to 1 parts. The backup mixes were made of washed concrete sand and crushed stone and could be placed integrally with the face material. After the cast had settled, it was pneumatically vibrated again to compact the material and draw excess moisture to the exposed back face, which was immediately evacuated with a hygroscopic material. Panels cured for twenty-four hours in the molds and were then removed to be stacked vertically on easels to cure for several days. Final finishings, such as acid etching, occurred before shipment. By 1949, Dextone made it possible to purchase polished concrete panels as well. In 1958 a new panel-casting method was introduced in the United States under the name shocked concrete (“Schokbeton”) with several franchise plants following closely behind. This process had been patented in Holland in 1932, and it concentrated on consolidating a no-slump concrete mixture by raising and dropping the form roughly 250 times per minute no more than 5/16 inch. The production of large precast panels is still relatively new, but small concrete elements have been produced in the past on drop tables, which is similar to this technique without the refined machinery.

Uses and Installation

Since the use of mobile cranes and materials-handling equipment was scarce, architectural precast concrete was difficult to use Also, metal and glass curtain-wall materials gave precast concrete immense competition. Precast concrete traditionally was slow to develop in popularity. With improved methods of production, better handling and erecting equipment and development of new techniques and materials, precast concrete saw a rise in popularity in the 1960s. Another factor in the developing reputation of precast concrete was in large part due to its variety of surface textures and patterns and exterior designs that could not be created by any other materials in such an economical means. Postwar buildings were pioneering projects, though they were limited in significance otherwise. Dormitories at the University of Connecticut, an eight-story building in Columbia, South Carolina, and a six-story office building in Miami, where the use of precast materials were used extensively to shape each of these buildings, as well as offering some decorative aspects. Another more notable use of architectural precast concrete is the Hilton Hotel in Denver, which used window wall panels fixed to a structural frame. The Police Administration Building in Philadelphia utilized the inherent structural characteristics of architectural precast concrete. This building had 5 foot wide, 35 foot high exterior panels, which supported two upper floors and the roof. It was made of an early model for blending precasting and post tensioning techniques in one structure.

Conservation

Conservation techniques used to architectural precast concrete have applications to historic structures. Investigations, both in the field and in laboratories, are necessary to determine the reason for the surface condition and how the remedies will effect on the concrete.

Deterioration

Water movement, chemical deposits, erosion, plant growth, freeze-thawing cycles, trapped moisture, metal corrosion, delamination, and spalling can cause deterioration of architectural precast concrete. When concrete walls are installed vertically, it is unlikely that it will reach the saturation point, which can create freeze-thaw expansion pressures. The lack of air entrainment in concrete that was cast before 1940 should not be a concern. Horizontal sections where precipitation can accumulate can result in freeze-thaw damage. Over time, concrete that maintains moisture that also has metals with inadequate cover may corrode and cause the concrete to spall. If water infiltrates architectural concrete cladding through either panels or joints, corrosion of the connections can occur. It is important to evaluate the structure’s condition to discover the amount of extend of deterioration and possibly discover the cause. A systematic review of service records and the original structure of the design will be the only way this information can be retrieved. If the records are incomplete, observation and good judgment should be applied in planning the field investigation. The program should include visual examination, non destructive testing to identify concrete, connection and sealant failures, and a collection of a specimen for laboratory testing if necessary.

Conservation Techniques

As with most conservation projects, it is suggested that a sample or test piece be acquired to understand how the materials will react. Cleaning can remove harmful soil and preserve the physical integrity of the architectural precast concrete’s surface. Even after testing, a small inconspicuous area should be cleaned first, then evaluated before further cleaning should be attempted. Allowing the test area to dry for at least one week should be given to fully understand the damage the cleaning could cause. Cleaning should always been attempted with the least aggressive method that will remove dirt and stains. Cleaning by means of dry scrubbing with a stiff nylon brush or moist scrubbing (with water) is the least abrasive cleaning manor. Also using low-pressure misting or intermittent moderate to high pressure washing is another alternative. Chemical cleaning, such as detergents or acids, should be used as the manufacturer suggest. Using sand blasting, industrial baking soda, or other abrasives can dull or erode the cement atmosphere and are generally advised against.

Repair techniques and materials for restoring architectural precast concrete are selected based on several factors, such as final finish, size and location of damaged area, temperature conditions, age of components, or surface texture. Trial mixes are required to determine the exact quantity needed to make appropriate repairs. Making a series of gradations from light to dark colored trials pieces on the same day is the easiest way to identify what mix is appropriate. A minimum of seven days is suggested to allow the test pieces to cure properly. 14 days followed by normal drying time can bring the wait period to a total of twenty-eight days.Curing and ultraviolet bleaching takes time to set and cure and could affect the finished coloring of the product. There should be moist-cured for a minimum of 3 days to prevent cracking, although 7 days is ideal. Materials with different densities will cure at a faster rate than surrounding concrete and are exposed to curing environments different from the existing component, which may require a color altering to match the adjacent areas. Epoxies and other bonding agents may be required for larger repairs. Latex bonding agents are not ideal with repair materials because of the difference in texture and appearance. Bonding agents should be neatly applied and be allowed to dry until tacky before repair mix is applied.

When repairing a crack, is it important to identify the size of the crack. Cracks between 0.003 to 0.250 inches with a depth no more than 12 inches can be repaired by pressure injecting a low viscosity, high-modulus, 100 percent solid two component epoxy. It should be capable of bonding to wet surfaces and the color should be as closely matched to the original color to not be obtrusive. A silane or dilute solution of an acrylic sealant will minimize the moisture’s migration into the repaired concrete and it should not stain the concrete.

When a connection I corroded, new galvanized, epoxy coated, or stainless steel plates can be installed with expansion or chemical anchors inserted into drilled holes in the hardened concrete It is of the utmost importance that the holes be straight, proper size in diameter, deep enough, and cleaned out. The bolts should be tightened to the recommended torque and may require a pneumatic impact wrench. When placing expansion anchors in predrilled holes, expanding the anchor in the direction of the edge should be avoided or cracking may occur.

In some areas, such as urban and industrial zones, sealers may be applied to improve the concrete weathering ability. The sealer can also reduce the absorption of moisture and minimizes the wet-dry cycle. Applying the sealer properly is easy if the manufacturer’s instructions are followed, there is a qualified operator and extensive pretreatment also helps. The sealer’s application should be limited to proper timing, temperature, and the concrete’s moisture contact. Consulting the manufacturer of the sealer and sealant is strongly suggested before application. Coatings should be applied only after repairs and cleanings have been completed.

Replacement

The use of elastomeric materials can produce accurate form liners for replacing components. Photogrammetric surveys and original construction documentation will aid in the proper replacement of these materials. New components can be developed to match the size and details using accurate models.

References

• Freedman, Sidney. "Architectural Precast Concrete." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 108-12. Print

Examples

One of nine columns at the Bahai House of Worship. This building was created using architectural precast concrete (Wilmette, Il, 1920).   Architectural precast concrete wall veneer in the style of bricks