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tech info
Architectural Precast Concrete
As published in Construction Canada Magazine 1994 by Bruce D.Taylor The intent of this article is to provide a general review of architectural precast concrete, its uses, finishes, aesthetic design and specification considerations, and overall wall assembly performance. Uses and Material Characteristics An architect has a great many construction products from which to select when giving thought to what materials will best personify the building design. Architectural precast concrete is a material used to clad the exterior building envelope, where each building design can be a custom creation, reflecting desired aesthetic expressions through colours, textures and physical sizing of precast components. One must think of a material, initially fluid in nature, with the ability to assume any design form from the mould into which concrete is poured. The subsequent curing, finishing and site installation ultimately provides a wall assembly which could be lean and slick, or strong and massive or perhaps very ornate and sculptured emulating detailed stone work found in architecture in previous centuries. Precast can be considered a plastic material in its uncured stage, with infinite shapes, sizes and panel configurations. However most budgets for buildings are finite, which will cause a designer to seek a compromise. The more complicated the precast, the more expensive the precast wall. This is not to say that a complicated design is not feasible, but due consideration must be given to repetition of similar precast units. The more units which can be cast from a single mould reduces the initial cost of the mould and change-over costs to adapt an initial mould to a new precast shape are also reduced. Flat, repetitious precast wall assemblies are the most cost effective, with shaped spandrel panels and column covers increasing the costs, and complicated and/or curved non-repeating precast wall assemblies found at the other end of the cost spectrum. Precast concrete panels have significant weight considerations. Do consider panel sizes for shipping and erection. Many precast manufacturers have developed specialized transportation trailers to ship large units. Consult with manufacturers in your region to determine the practical limitations. In urban areas large mobile cranes are readily available to make long difficult lifts of precast panels. However in remote areas, specialized cranes may not be readily available. Reducing the size of panels is a simple solution to making use of available equipment. Once again, consult a local precast manufacturer. Early involvement of precast technical expertise can save a designer time in the development of a practical precast concrete building envelope. The development of proper working drawings from the original design drawings will provide the bidders with definitive information on what to bid and allow them to incorporate cost saving precast designs into the bid, benefitting the client. Precast Wall Assemblies as Part of the Building Envelope The technology of modern construction has changed. The success of various wall assemblies, the failure of others, and more stringent codes has led designers to pay more attention to the development of “tight buildings”. Manufacturers of air barrier systems, vapour barrier systems, high performance insulation, and metal stud support assemblies have been successful in the development, marketing and selling of their products. These products are used in conjunction with one another to provide a weather tight, insulated building envelope. In this type of wall assembly the architectural precast is merely a veneer providing a “rain screen” for those materials behind. Only the exterior of the precast joints are caulked to act as an initial moisture block, and to provide a finished appearance. Base joints are left open to allow weeping of moisture. Air circulation in the air cavity between the back of the precast and the insulation is achieved by introducing small gaps or plastic weeper tubes in the caulked joint, allowing any condensation that may develop to dissipate. The need to have the air/vapour barrier on the warm side of the wall assembly has put the precast manufacturers to task figuring ways to attach several tons of concrete onto the building structure without disrupting/puncturing the building envelope (air/vapour barrier, insulation and support system). On low rise buildings this can be achieved by transferring gravity loads to the foundation wall and having only simple lateral connections penetrate the air/vapour barrier/insulation assembly (or make the lateral connection at the parapet). On high rise construction the attachment of precast becomes more complicated with need for openings in the air vapour barrier/insulation assembly to allow for the connection of the precast panel to the structure. Special attention is required to ensure the proper closure and sealing of these openings after the precast connections have been completed and prior to the start of the interior finishes. Modified rain screen assemblies have been used for many years and are still being successfully used today. Simply put, modified rain screen is the development of the previously described rain screen principles, but within the confines of the precast joints. The back face of precast joints are caulked tight developing the air/vapour barrier and the exterior joints are caulked but allow for the exchange of air through the introduction of weep holes and breather openings. The result is an air chamber within the precast joint, vented to the outside. Insulation is applied to the back surface of the precast completing the wall assembly with the interface of the insulation and precast being considered the air/vapour barrier. The weak link in this system is ensuring a proper air seal is achieved and maintained in areas where the interior joint is not accessible from the interior and must be sealed from the exterior. The air/vapour barrier seal is only as good as the workmanship. Other considerations for the use of architectural precast concrete wall panels include sound transmission. A four inch thick precast concrete panel weighs 50 pounds per square foot. The mass of precast provides a high density building envelope with an effective way to minimize sound transmission. Precast concrete offers a designer the ability to incorporate fire rated wall assemblies as part of the building envelope where proximity to property lines, or local codes, deem the need for fire walls. Interior fire separation walls in large warehouses have been fabricated from precast to achieve the necessary fire resistance rating. The long span characteristics of precast concrete simplify structural design and time of construction. Finishes As a designer develops a general building facade design, consideration for the actual colours and texture of the precast components should be made. This decision process may not be as easy as it seems. Each texture and finish has their own distinct advantages, limitations, and disadvantages. Some finishes and textures may not be practical with the panel design. Panels with deep returns may preclude exposed aggregate finishes where panel returns (the vertical portion of the panel when being cast) will not have the same dense aggregete finish found in the approved sample. The cement colour, the type of sand, and the coarse aggregates all provide their specific characteristics to the overall finish of the completed panel. Many precast manufacturers have a ready supply of one foot by one foot by one inch samples depicting the infinite variety of colours and textures available. The selection of some finishes may be limited or costly due to the availability of raw materials in that particular area. The finishing of precast panels is accomplished via various techniques. Exposed aggregate finishes are achieved by coating the form into which the concrete will be poured with a concrete retarder. The retarder arrests the curing/hardening of the concrete which comes in contact with it to a depth determined by the strength of the retarder. Once the panel has cured and is stripped from the mould, the panel is moved to a wash area where high pressure water removes the uncured matrix (cement and sand) leaving the coarser aggregates in place and imbedded in hardened concrete. Sandblasting removes the cement sand matrix by abrasive removal, a result of the impact of sand on the panel surface. Coarse aggregate exposure will not be as pronounced, with a greater percentage of matrix showing than found in exposed aggregate finishes. The amount of sandblasting can be reduced to a light dusting which can imitate the finish of limestone or other natural stone products. Smooth as cast finishes have been used where a subsequent, on site, finish is applied, such as paint or textured coating. A site applied coating after precast erection may offer a designer a greater freedom with colours. The drawback may be the required maintenance of the paint or coating system. A comparative cost analysis of both a site applied finish and a factory applied precast finish should be made. Interesting patterns can be achieved in precast concrete panels through the use of form liners. These liners are fabricated with a variety of textures such as sandblasted wood, rough sawn lumber, both small and large ribbed patterns, and running coarse brick. Acid etching of panels removes the concrete matrix to expose the sand. The resulting finish can look like many of the natural stone finishes. Bush hammering of panels with the use of mechanical tools spalls a layer of concrete and aggregates at the surface of the panel. The result is a slightly roughed texture with the matrix and aggregates exposed. The above described finishes deal with achieving the desired aesthetics through the actual finishing of the precast concrete panels. The advent of granite, stone and brick faced precast concrete panels has allowed architects to incorporate the natural beauty of these materials onto the face of one large precast panel. Installation of many smaller granite/stone panels is achieved with the installation of only one precast panel. (John Kooy has provided an interesting discussion on this subject with comparisons to other pre-assembled wall systems in Construct Canada September/October 1992). Design Considerations when specifying Architectural Precast Concrete Wall Panels Problems have arisen when a designer has used two or more different products for the building veneer (i.e. composite metal panels and precast concrete panels), without defining the extent of responsibility for the air/vapour barrier/insulation behind the different products. With composite metal wall panels, it is generally the trade practice for the sub-contractor to supply and install a complete insulated wall assembly, whereas trade practices within the precast industry are such that the building envelope behind the precast panels falls into another trade contractor ’s responsibility. Close attention is required to ensure a proper marriage of the two systems. Another area which is neglected in the bidding documents is the delegation of responsibility, in the correct trade specification, and provision of proper details to enclose pockets in masonry, stud walls or concrete shear walls from the inside of the building once the precast panels have been erected and connections completed. This is becoming more of a problem with the increasing use of total rain screen where a complete insulated wall assembly is in place prior to the precast being installed. Eliminate cost extras by specifying the finish and materials, or selecting and specifying a specific manufacturer ’s sample, in the bid documents. Do have designers and engineers consider where connections will be made and accommodate for this in both the structural and architectural design. Are slabs and/or beams strong enough to support the gravity loads of the panels? Conclusion The busy times and high prices of the 80 ’s gave cause for the development of new types of wall systems, some mimicking the aesthetics of materials which have been in use for years, and in some cases centuries (masonry). The firm establishment of these products and systems in the construction market place in the 90 ’s may now change with the surplus capacity (reduced prices) of competing wall systems such as masonry, curtain wall assemblies and architectural precast concrete wall system. Architectural precast concrete wall panels have provided a successful means to clad buildings for more than 40 years with little or no maintenance proving the soundness and integrity of this type of building material. Reference Publications available: 1. Architectural Precast Concrete Design Manual 2. Architectural Precast Concrete Colour and Texture Selection Guide 3. CPCI Metric Design Manual For further general industry information, please contact the Canadian Precast/Prestressed Concrete Institute, 196 Bronson Avenue, Suite 100, Ottawa ON KlR 6H4, Phone (613) 232-2619. |
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