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Review of ASM seminar on materials and building systems for industrialized housing
Meetings
Review of ASM Seminar on Materials and Building Systems for Industrialized Housing Reviewer: Albert G. H. Dietz* At the Annual Meeting of the American Society for Metals held in Cleveland in October, 1970, the Young Members section organized a seminar on the subject of Competitive Materials in Industrialized Housing. Organizers of the seminar were Professor Richard W. Hertzberg, Associate Professor, Department of Metallurgy and Materials Science, Lehigh University; and Seth R. Thomas, Manager, Metallurgy and Quality Control of Teledyne Rodney Metals, New Bedford, Mass. Participants in the seminar were Robert E. Philpot, Special Assistant to the Assistant Secretary of the United States Department of Housing and Urban Development; Professor Albert G. H. Dietz, Professor of Building Engineering in the Department of Architecture, Massachusetts Institute of Technology; Harold W. Belliston, Methods Engineer with the Portland Cement Association; John A. Stahl, Product Engineer with the Plastics Materials Market Development Division of the B. F. Goodrich Chemical Company and Harvey R. Geiger, Manager of Planning for the Industrialized Housing Division of Multicon Corporation. Mr. Philpot spoke of the growing needs for housing and the activities of the Department of Housing and Urban Development, especially its 'Operation Breakthrough program; Professor Dietz spoke generally of materials development for industralized building, gave some of the background in materials utilization, as well as in industralized building methods, and spoke of some of the problems and contraints in the industry; Mr. BeUiston spoke more specifically about cement and concrete in industrialized building; Mr. Stahl spoke about plastics and their emerging role in this field; and Mr. Geiger spoke about the contributions and potentialities of metals in industralized building. The following is a general summary of the principal points brought out in the seminar. The assistance of Professor Hertzberg and Mr. Thomas is gratefully acknowledged. * Professor of Building Engineering, Department of Architecture, Massachusetts Institute of Technology, Cambridge, Mass., U.S.A.
The Demand
Migration of rural populations to the cities, and outward migration from the central cities to the metropolitan suburbs during the '40s, '50s, and '60s of this century has created a vast demand for housing when coupled with the estimated 14 million new households to be formed in the next ten years. New housing is required, and replacement for deteriorated, abandoned and to-be-abandoned housing, especially in the central city areas, will require a doubling of the current rate of approximately 1.5 million housing units per year, to approximately 3 million units per year by the late 1970s. The Housing Act of 1968 recognized this situation by projecting a need of better than 27 million housing units during the decade 1968-1978. In its best year, the housing industry managed to produce approximately 2 million dwelling units. Many observers of the.housing industry believe that a rate of 3 million units per year cannot be achieved by the existing conventional industry and that a greatly stepped-up program of industrialization, that is, production of a larger proportion of the components of buildings in the shop, must be developed if anything like the objective of 3 million units per year is to be achieved. Furthermore, there will have to be a search for every available type of material that can be utilized, and combinations of such materials will have to be developed to make maximum efficient and economical use of their potentialities. As producers of metals and other materials are well aware, industrialization implies capital-intensive industry, and this, in turn, requires, for maximum efficiency and economy, steady production in sufficient volume over a long enough period of time to realize the potentialities in cost and time reduction sufficient to offset the carrying charges and overhead inherent in such processes. The volume of production represented by the housing market is attractive, but the conditions for steady large-volume production over a long enough period of time have not generally been present in the housing industry and, consequently, industrialization has not developed to any considerable extent. There are relatively few, even moderately large, organizations in the industry, and even these have had few opportunities to practice true industralization.
Materials Science and Engineering American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne-Printed in the Netherlands
226
MEETINGS
Operation Breakthrough The Department of Housing and Urban Development has recognized this situation and has instituted "Operation Breakthrough" in an attempt to encourage the development of new and more highly industralized processes for the construction of housing than have heretofore been common. It is also addressing itself to the problem of aggregating a sufficiently large market over a sufficiently long period of time to make the development of truly industrialized systems attactive and to encourage the building industry to organize along these lines. As a consequence of a country-wide competition involving hundreds of entrants, 22 proposed systems are being sponsored by Operation Breakthrough. They represent innovative ideas which show promise of being developed in a rather short time to test out the concepts represented by them. A second Operation Breakthrough program is attempting to search out newer, more radical ideas that will require a greater period of research and development to bring them to fruition. Heretofore, approximately 75 percent of all residential construction has been based on wood, particularly for single-family houses such as those found in suburban and rural areas. In Operation Breakthrough, 75 percent of the systems use concrete, metal, plastics or combinations of them. Considerable emphasis is being given to the "newer" types of materials and to improved combinations of all materials in conjunction with innovations in
!i!
structure and in production methods. At the same time, private industry, recognizing the potentialities of the very large market represented by housing, is moving in the direction of industrialization on its own accord and is experimenting with innovative ideas in the use of materials, production methods, and organization of the industry from raw materials production to the finished product.
Types of units At the risk of over-simplification, the various types of components being produced or being developed for industralized production may be characterized as big boxes or "modules", big panels or slabs, and frames and infill. The big boxes, in turn, may be subdivided into heavyweight and lightweight, and the same is true of the big slabs or panels. Associated with these structural subsystems are various attempts to produce the electro-mechanical subsystems of the building by industrialized processes.
Experience Abroad For a variety of reasons, industrialization of housing has proceeded much farther in Europe than it has in the United States or Canada. By and large, the most commonly employed systems in Europe are based on the big heavy panel, but there is also considerable use of frame and infill for other than housing applications, particularly educational, industrial, and commercial buildings. The big box approach is
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Fig. 1. Research house employing industrially-produced structural sandwich panels with plywood facings and foamed polystyrene cores. (Photograph from National Association of Home Builders.)
Mater. Sci. Eng., 7 (1971) 225-233
MEETINGS
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Fig. 2. Precasting large concrete wall panels in heated molds for industrialized housing. (Photograph from MBM, Milan, Italy.)
used to a moderate extent. All of these approaches are being utilized or experimented with in the United States and Canada, and there is no basic difference, as far as the technology is concerned, among the systems employed on both sides of the Atlantic. Incidentally, there is also a good deal of activity in Japan, and to a varying degree in other parts of the world, such as Australia, Israel, and Latin America. Materials Wood
As indicated above, for single-family and small two-to-four-family houses, wood is one of the most important materials of construction. It is extremely versatile, familiar, widespread, readily available, and has a combination of properties highly favorable for construction for this kind of dwelling unit. It has lent itself readily to industrialization, and the great majority of prefabricated houses today are based on wood, as is the mobile home industry with its present annual production of 400,000 units, in itself equal to 25 percent of the housing market. Wood is used for all types, including frames, panels, and boxes. Its major drawback, of course, for dense, multi-family, high-rise or medium-rise residential units is its susceptibility to fire, and this effectively rules it out for many of the urban applications where
much of the demand during the next ten years will be concentrated. Here, however, there will continue to be major applications for wood, especially in non-structural or in semistructural components and in all manner of components such as cabinet work, floors, and finish. Concrete In Europe, concrete is, today, the most commonly employed material, and the large concrete panel forms the base of the most widely used industrialized systems. It can be, and is, utilized, however, also for large boxes and for frame plus infill types. The precast units may or may not be prestressed. Prestressing is common, particularly in floor panels spanning considerable distances. All three types of concrete-based systems are also employed in the United States. All of the normal types of cements can be, and are, employed for industrialization, but most common are general purpose and high early strength. Colors can easily be incorporated and many surface textures are available by the use of exposed aggregates of considerable variety, and other surface treatments such as sandblasting and chipping. If necessary or desirable, the surfaces of the concrete can be coated. Newer types of cements hold promise for industrialized processes. Among them are the expansive Mater. Sci. Eng., 7 (197l) 2~5-233
228
MEETINGS
Fig. 3. Large pre-cast concrete panels for industrialized multi-family housing. (Photograph from San-Vel, Techcrete.)
Fig. 4. Composite wall panels. Glass-fiber reinforced polyester outer shells, baked polyurethane finish, foamed concrete core, reinforced gypsum inner face, flexible epoxy-urethane bond, bitumen vapor barrier. Six panels attached to steel supporting frame erected at one time. (Photograph from Greater London Council.)
cements which expand slightly upon curing instead: of shrinking as most concretes do. This helps to overcome cracking and may, indeed, introduce some initial prestress into the steel reinforcement. A second type of cement makes it possible to regulate the speed of set, depending upon the requirements of the job. Evidently, for industrialization, the faster the cement sets, the sooner a slab or other component can be removed from the molds, and the greater the production that can be achieved with those molds. In this connection, a British development utilizes a press which squeezes out the excess water after a panel has been cast and densities it to such an extent that the panel can immediately be removed from the mold and set aside to cure. Polymeric materials have been added experimentally to concrete and cured in situ by highenergy radiation. The addition of polymers is designed to help overcome the disadvantages of concrete, mainly, brittleness and low tensile strength leading to its tendency to crack. Another approach to overcoming the disadvantages of concrete is the addition of strong fibers, both inorganic and organic, such a glass, asbestos, metal, and various types of polymeric fibers. Glass must be especially alkali-resistant for this purpose. It has long been known that the durability of concrete under exposure to various climatic conMater. Sci. Eng., 7 (1971) 225-233
MEETINGS
ditions can be increased by the entrainment of small percentages of air. A good deal of experimentation is going on with gas and air entrainment in various proportions. Light weight is achieved, indeed, by foaming the concrete, either by mechanically beating in air, or by chemical means such as the addition of aluminum powder, whose reaction with portland cement gives off hydrogen gas. The basic advantages of concrete are quite evident. It is highly fire resistant. It requires little or no maintenance under many conditions, its mass allows for good acoustical attenuation, it can be cast into a great variety of shapes including long spans, and it is generally economical in construction. The major disadvantages, as indicated above, are hardness, weight, brittleness, low tensile strength, and tendency to shrink and crack. Metals Steel is easily the most commonly used metal for structural purposes such as framing members, and including steel in sheet form for structural and semistructural applications. Metals, in general, are strong and tough and, therefore, can be used in effÉcient structural shapes such as framing members, or as thin sheets. This makes them relatively light in weight for construction purposes in spite of the high density of metals such as steel. Aluminum, the second most common building metal, is usually employed in sheet form for enclosure, roofing, and similar purposes or in the form of extrusions or other shapes for windows, doors, and similar applications. Copper, today, is mainly used for electrical applications but is also employed for roofing, rainwater conductors, and similar applications. Cast iron finds use in piping, plumbing fixtures and similar applications. Many other metals find use in metallic building products, either in their own right or as alloys, such as nickle or chromium in stainless steel. For industrialized processes, metal framing members, particularly steel, find their way into frame and infill systems and into components of lightweight boxes as opposed to the heavyweight boxes based on concrete. Steel is used as framing material for mobile homes, and aluminum is used for sheathing windows and doors, and other applications. Both steel and aluminum are widely employed for secondary components such as ducts and cabinetry, and these are joined by copper, lead, cast iron, and others for mechanical, plumbing and electrical subsystems. The principal advantages of the metals are their strength, toughness, wide range of other properties,
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and range of densities. Principal disadvantages are need for protection against high temperatures found in building fires, the fact that some of them may corrode and require protection, costs may or may not be favorable, and some limitations on the forms which can be achieved with framing members and sheet stock. These are usually linear, flat, or in simple curves. Not the least contribution of metals to industrialization is in the machinery and equipment that make it possible. Equipment such as the molds and the handling machinery in the shop, transport rigs, cranes, and the many other types of equipment needed to put the industrialized components in place, are metal. Without them, the whole concept would be impossible. Plastics Presently, the volume of plastics employed in building is modest, but there is great interest in their properties and potentialities, with a general belief that their uses in building will be more widespread in the future. Today, within their own domain, their properties are probably about as extensive and variable as those of the metals in their domain. There are some 20-30 basic types with a large number of variations made possible by copolymerization, additives, and other means of varying their properties. Although they are new, the history of their use in buildings is beginning to accumulate and some have histories of 20 years or more in building applications. Like concrete, plastics have no inherent shape of their own and have to be formed into final shape. For structural applications, the mechanical properties of unmodified plastics are modest. Strength is moderately good, but stiffness, as measured by elastic modulus, is often distressingly low. For semistructural applications, however, these attributes can frequently be overcome by the judicious employment of inherently stiff and strong shapes, such as domes, anticlastic surfaces, and a great variety of three-dimensional curved forms which are readily obtainable and which are inherently stiff and strong. The widely used bubble-shaped skylights, for example, take advantage of the easy formability of these materials to provide an efficient shape, from the standpoints of light transmission and resistance to loads such as snow and wind. For structural and semistruetural applications it is generally necessary to incorporate strengthening materials. Of these, by all odds the most commonly Mater. Sci. Eng., 7 (1971)225-233
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MEETINGS
Fig. 5. Polyvinylchloride overlay on wood window sash and frame. (Photograph fromAnderson,B. F. Goodrich ChemicalCo.) employed is high-strength glass fiber. Fibers of this type have tensile strengths ranging in the vicinity of 500-700,000 psi at one-third the density of steel, and elastic moduli of about 10 million, comparable to aluminum. These reinforced plastics, usually based on polyester resins for building applications" because of ease of handling and relatively low costs, are light, tough, strong, and considerably stiffer than unreinforced plastics, but still possess relatively modest moduli of elasticity compared, for example, to steel or aluminum. They begin as masses of glass fiber and barrels of liquid resin. They have to be shaped into final form. The most efficient structural forms are shells, folded plates, corrugations, and other non-planar and non-linear forms, plus stiffeners such as ribs, hat sections, and so forth. Because of their toughness, they can be used in thin sections and these are frequently highly translucent, as contrasted with other structural materials. For industrialized structures and enclosure, they lend themselves favorably to use in panels and boxes, especially if curved or otherwise shaped, rather than as linear framing members, for which they compete
unfavorably with traditional materials such as metal and wood except for special uses. Their corrosion resistance, lightness, colorability, range of properties from complete transparence to complete opacity, toughness, and ready formability recommend them for a variety of uses. Major uses of plastics are in nonstructural major and minor components of building. There is a great deal of interest in the all-molded bathroom, and there have been some notable uses of such bathrooms, employing reinforced plastics, acrylics, polyvinylchloride, and others. Plastics are standard materials for floor covering and are widely used for interior and exterior wall covering. High-pressure decorative laminates are standard materials for counter tops, furniture, facings on doors, and wall covering. Polyvinylchloride is used for windows, either in its own right or combined with metal and wood. It is also used along with polyethylene, polypropylene, and a variety of copolymers for piping or related uses. Hardware components employ nylon and other plastics. The list can be extended more or less indefinitely. Finally, the polymeric materials form the basis for sealants, gaskets, and caulking compounds, as well as the highstrength waterproof engineering-type adhesives for waterproof plywood and many other engineering applications, including those in building. Composites
The foregoing materials all have their advantages and disadvantages from the building standpoint. No one of them is perfect or it would be the only material employed. There is a growing tendency to use them in combination in the form of composites, and to take advantage of the "synergistic" effect of such combinations whose properties are frequently superior to or even entirely different from the properties of the individual materials acting alone. Composites may be classified in a variety of ways, but frequently are considered to be: Fibrous, fibers in a continuous matrix, Particulate, particles embedded in a continuous matrix, Laminar, layers of materials bonded together. Some writers like to add a fourth type of composite: skeletal, in which an impregnant interpenetrates voids within a skeleton; and a fifth, or flake, in which fiat particles are embedded in a continuous matrix of a cement, such as portland cement combined with water. Mortar for masonry is an obvious corrolary of concrete. Both of these can be modified Mater. Sci. Eng., 7 (1971) 225-233
MEETINGS
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Fig. 6. Translucent structural sandwich with glass fiber reinforced polyester facings bonded to aluminum grid core. (Photograph from Kalwall.)
by the addition of polymeric materials. As a matter of fact, high-strength mortars with polymeric additives make it possible to preassemble masonry-based panels, such as brick or concrete block, to be hoisted into place, and make it possible, also to provide panels of this type considerably thinner than are required with standard unmodified mortars. Fiber-based composites have already been mentioned in the form of glass-fiber reinforced plastics. Fibrous additions to concrete have also been mentioned. Fiber-reinforced materials appear to be destined for increasing use in the future, generally, including industrialized building components. laminar materials are widespread. The highpressure decorative laminates have already been mentioned. Laminar metals such as alclads are commonly employed. Thermostats depend for their behavior on laminated metal. There are many other instances, including lead-coated copper, and so forth. From the building standpoint, one of the most interesting laminates is the structural sandwich, in which two relatively thin, hard, strong, high-density facings are combined with a relatively thick core of low-density material to provide a panel having many of the attributes of an I beam. The facings provide the strength and internal couple or moment to resist external forces, and the core provides the resistance to shear, as well as stabilizing the facings. In a building panel, the facings are also expected to provide resistance to exterior weathering and inte-
rior wear and tear, as well as colors, textures, and other attributes. The core provides thermal insulation, and the combination core and facing provides resistance to sound transmission. Many materials can be used for facings including metals, wood-based materials, plastics, concrete, cement-asbestos board, and other composites, and the same is true of the cores, including plastics foams, honeycombs, foamed concrete, metallic grids, and wood-based and fiber-based building boards. An interesting instance of the use of composites is provided by the high-rise flats of the Greater London Council. These were developed according to performance requirements, rather than materials specifications. The requirements were for resistance to wind, acoustical attenuation, low heat transmission, zero flame spread, two-hour resistance to standard fire penetration, minimum thickness, minimum weight, and minimum maintenance. The response was a composite wall panel consisting of a glass fiber reinforced polyester outer shell with an external facing of baked-on polyurethane, a lightweight foam concrete infill bonded to the shell with a flexible adhesive, and an internal face of gypsum reinforced with fiber and wire, bonded to the concrete foam core with a bitumen binder which also acted as a vapor barrier. This composite panel met all of the requirements set forth by the Greater London Council at 15 percent of the weight, and one-third the thickness of standard masonry or precast concrete components. Mater. Sci. Eng., 7 (1971)225-233
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MEETINGS
Fig. 7. Proposed method of filament-winding large boxes for industrialized housing. Filament-wound facings in plastic matrix on insulating structural lightweight core. (Photograph from Progressive Architecture.)
Constraints There are many constraints to the widespread adoption of industrialized procedures for housing. These have strong effects upon the choice of materials. A major constraint is unfamiliarity on the part of the building industry with the new developments in materials and composites, and on the other hand, unfamiliarity on the part of industries not now associated with building respecting the pecularities and requirements of the building industry, as these, in turn, are constrained by many other factors outside the industry, itself. Government policy. A major factor, which has added to uncertainty in the building industry over the years, has been government policy, particularly economic and financial policy. Housing is highly dependent upon the availability of plentiful funds at moderate interest rates to encourage the production of housing at economic rents or carrying Charges. Government policy has been far from consistent in this respect, and has led to gyrations and
and rapid rises and falls in construction volume which have militated strongly against the introduction of capital-intensive and expensive industrialized processes. Codes. Building codes are frequently cited as major stumbling blocks and obstacles to the greater use of industrialization. They are frequently characterized as archiac and behind the times, and to a degree, this is true. On the other hand, many building codes are inherently sensible, if, perhaps, too conservative. The major problem with building codes is not that they are arbitrary or capricious, but that they are different. Frequently, relatively minor differences among codes in adjacent cities or municipalities within logical marketing areas are such as to make it difficult or impossible to produce, in large volume, the standardized components necessary to the success of industrialization. Furthermore, some of the loudest critics of building codes are the individuals who have a new product or process to sell which they have not sufficiently developed and proved to convince building code officials that they should accept the new and untried device. As new codes are written by the national organizations attempting to promulgate uniform codes, and by municipalities and States as they revise their codes, the trend is toward "performance" rather than "specification" codes. Instead of specifying details of construction, the performance expected of buildings is set forth, and the method of achieving it is left largely to the designer and builder. The Greater London Council case cited above is an example of performance requirement, although not by a municipal code. Although this approach requires a much higher degree of sophistication on the part of the drafters and the code officials, it does open the door to innovations, including innovations in materials. Labor. Labor is also often criticized as being a major obstacle to the introduction of industrialization or other new approaches in building. Here, again, to a degree, this is true, not so much for the often-quoted featherbedding techniques or opposition to innovation of labor, but because of the traditional crafts organization of building labor. This introduces questions of jurisdiction, of who is going to have possession of the new device, when such a new device or component or material is introduced. It is not so much labor opposition to the introduction of new ideas as rivalry among different crafts to obtain control. Mater. ScL Eng., 7 (1971) 225-233
MEETINGS
A similar problem arises when work normally accomplished on the site is transferred to the shop. A particularly bad situation arises if the shop is, for example, nonunion and the site is unionized. The nonunion product of the shop is likely to be refused at the site by unionized labor. A different problem arises when both are unionized, but the question arises who will organize and have jurisdiction over the union in the shop. It makes little sense to have many different crafts represented in the shop and the question is rather, which one of the different crafts will have jurisdiction over all labor in the shop, no matter what crafts are involved. Certifyin9 innovation. There is no clear procedure for the introduction of innovative ideas into the building industry in the United States. There is no central accepted and recognized certification board which examines new ideas and issues certificates or other generally accepted evaluations valid throughout the industry. This makes rapid acceptance difficult, considering the fact that there are approximately 50-60,000 home builders; 20,000, or more, registered architects; some 70-80,000 registered engineers working in the general building field; and hundreds of thousands of materials dealers and others involved, not to mention the building code officials and financial people who must also be convinced. Industrial 9aps. When a composite comes along that incorporates a variety of different materials, there is no good industrial setup for its production. The metals people are not much interested in a composite based mainly on concrete, and one that incorporates four of five different materials may have a hard time finding an industry to produce it. Management. Organization. One of the major lessons to be learned from the European industrialized housing industry is that the successful ones are the ones that are well organized and managed. There is no magic in any of the European systems that either is not or cannot be reproduced in the United States--all of them are, to a greater or lesser extent. The industry must find ways of organizing itself, however, to accept the basic idea of industrialization, which calls for close control all
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the way from raw materials production to the finished product, including the fabricating shop, transportation from shop to site, site organization, and the myriad feeders that lead into this mainline of the production stream. Public acceptance. Finally, and possibly paramount, is public acceptance. The new industrialized dwelling may be the best or the lowest cost ; but if it does not meet with public approval, it will fail. It is highly important, therefore, for the producer to understand the needs and prejudices of the market. Designers must be intimately acquainted with needs and prejudices and must learn how to design for industrialized processes at the same time that they meet the public requirements.
Conclusion Operation Breakthrough, mentioned above, is not only attempting to promote and encourage innovative technology, it is also addressing itself to the various constraints enumerated here. It is trying to aggregate the market. It is working with local code officials to find ways and means of surmounting differences and local obstacles. It is working with local zoning authorities to find the necessary land for large-scale development of aggregated market. It has set up its own evaluation team utilizing the National Bureau of Standards as a testing facility or the supervision of testing, and it has organized a technical evaluation team to assist it in its evaluations of innovative ideas and the setting up of standards for them. From the foregoing, it should be evident that if and as industrialization of housing production develops, major opportunities exist for conventional and "new" materials. The demand, if realized, is so great that all materials must be utilized, alone or in combination, to provide the needed volume at reasonable cost, and industrialization must play a significant role. Those materials that adapt best, will have the edge over the others. Received January 18, 1971.
Mater. Sci. Eng., 7 (197l) 225-233
Review of ASM Seminar on Materials and Building Systems for Industrialized Housing Reviewer: Albert G. H. Dietz* At the Annual Meeting of the American Society for Metals held in Cleveland in October, 1970, the Young Members section organized a seminar on the subject of Competitive Materials in Industrialized Housing. Organizers of the seminar were Professor Richard W. Hertzberg, Associate Professor, Department of Metallurgy and Materials Science, Lehigh University; and Seth R. Thomas, Manager, Metallurgy and Quality Control of Teledyne Rodney Metals, New Bedford, Mass. Participants in the seminar were Robert E. Philpot, Special Assistant to the Assistant Secretary of the United States Department of Housing and Urban Development; Professor Albert G. H. Dietz, Professor of Building Engineering in the Department of Architecture, Massachusetts Institute of Technology; Harold W. Belliston, Methods Engineer with the Portland Cement Association; John A. Stahl, Product Engineer with the Plastics Materials Market Development Division of the B. F. Goodrich Chemical Company and Harvey R. Geiger, Manager of Planning for the Industrialized Housing Division of Multicon Corporation. Mr. Philpot spoke of the growing needs for housing and the activities of the Department of Housing and Urban Development, especially its 'Operation Breakthrough program; Professor Dietz spoke generally of materials development for industralized building, gave some of the background in materials utilization, as well as in industralized building methods, and spoke of some of the problems and contraints in the industry; Mr. BeUiston spoke more specifically about cement and concrete in industrialized building; Mr. Stahl spoke about plastics and their emerging role in this field; and Mr. Geiger spoke about the contributions and potentialities of metals in industralized building. The following is a general summary of the principal points brought out in the seminar. The assistance of Professor Hertzberg and Mr. Thomas is gratefully acknowledged. * Professor of Building Engineering, Department of Architecture, Massachusetts Institute of Technology, Cambridge, Mass., U.S.A.
The Demand
Migration of rural populations to the cities, and outward migration from the central cities to the metropolitan suburbs during the '40s, '50s, and '60s of this century has created a vast demand for housing when coupled with the estimated 14 million new households to be formed in the next ten years. New housing is required, and replacement for deteriorated, abandoned and to-be-abandoned housing, especially in the central city areas, will require a doubling of the current rate of approximately 1.5 million housing units per year, to approximately 3 million units per year by the late 1970s. The Housing Act of 1968 recognized this situation by projecting a need of better than 27 million housing units during the decade 1968-1978. In its best year, the housing industry managed to produce approximately 2 million dwelling units. Many observers of the.housing industry believe that a rate of 3 million units per year cannot be achieved by the existing conventional industry and that a greatly stepped-up program of industrialization, that is, production of a larger proportion of the components of buildings in the shop, must be developed if anything like the objective of 3 million units per year is to be achieved. Furthermore, there will have to be a search for every available type of material that can be utilized, and combinations of such materials will have to be developed to make maximum efficient and economical use of their potentialities. As producers of metals and other materials are well aware, industrialization implies capital-intensive industry, and this, in turn, requires, for maximum efficiency and economy, steady production in sufficient volume over a long enough period of time to realize the potentialities in cost and time reduction sufficient to offset the carrying charges and overhead inherent in such processes. The volume of production represented by the housing market is attractive, but the conditions for steady large-volume production over a long enough period of time have not generally been present in the housing industry and, consequently, industrialization has not developed to any considerable extent. There are relatively few, even moderately large, organizations in the industry, and even these have had few opportunities to practice true industralization.
Materials Science and Engineering American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne-Printed in the Netherlands
226
MEETINGS
Operation Breakthrough The Department of Housing and Urban Development has recognized this situation and has instituted "Operation Breakthrough" in an attempt to encourage the development of new and more highly industralized processes for the construction of housing than have heretofore been common. It is also addressing itself to the problem of aggregating a sufficiently large market over a sufficiently long period of time to make the development of truly industrialized systems attactive and to encourage the building industry to organize along these lines. As a consequence of a country-wide competition involving hundreds of entrants, 22 proposed systems are being sponsored by Operation Breakthrough. They represent innovative ideas which show promise of being developed in a rather short time to test out the concepts represented by them. A second Operation Breakthrough program is attempting to search out newer, more radical ideas that will require a greater period of research and development to bring them to fruition. Heretofore, approximately 75 percent of all residential construction has been based on wood, particularly for single-family houses such as those found in suburban and rural areas. In Operation Breakthrough, 75 percent of the systems use concrete, metal, plastics or combinations of them. Considerable emphasis is being given to the "newer" types of materials and to improved combinations of all materials in conjunction with innovations in
!i!
structure and in production methods. At the same time, private industry, recognizing the potentialities of the very large market represented by housing, is moving in the direction of industrialization on its own accord and is experimenting with innovative ideas in the use of materials, production methods, and organization of the industry from raw materials production to the finished product.
Types of units At the risk of over-simplification, the various types of components being produced or being developed for industralized production may be characterized as big boxes or "modules", big panels or slabs, and frames and infill. The big boxes, in turn, may be subdivided into heavyweight and lightweight, and the same is true of the big slabs or panels. Associated with these structural subsystems are various attempts to produce the electro-mechanical subsystems of the building by industrialized processes.
Experience Abroad For a variety of reasons, industrialization of housing has proceeded much farther in Europe than it has in the United States or Canada. By and large, the most commonly employed systems in Europe are based on the big heavy panel, but there is also considerable use of frame and infill for other than housing applications, particularly educational, industrial, and commercial buildings. The big box approach is
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Fig. 1. Research house employing industrially-produced structural sandwich panels with plywood facings and foamed polystyrene cores. (Photograph from National Association of Home Builders.)
Mater. Sci. Eng., 7 (1971) 225-233
MEETINGS
227
Fig. 2. Precasting large concrete wall panels in heated molds for industrialized housing. (Photograph from MBM, Milan, Italy.)
used to a moderate extent. All of these approaches are being utilized or experimented with in the United States and Canada, and there is no basic difference, as far as the technology is concerned, among the systems employed on both sides of the Atlantic. Incidentally, there is also a good deal of activity in Japan, and to a varying degree in other parts of the world, such as Australia, Israel, and Latin America. Materials Wood
As indicated above, for single-family and small two-to-four-family houses, wood is one of the most important materials of construction. It is extremely versatile, familiar, widespread, readily available, and has a combination of properties highly favorable for construction for this kind of dwelling unit. It has lent itself readily to industrialization, and the great majority of prefabricated houses today are based on wood, as is the mobile home industry with its present annual production of 400,000 units, in itself equal to 25 percent of the housing market. Wood is used for all types, including frames, panels, and boxes. Its major drawback, of course, for dense, multi-family, high-rise or medium-rise residential units is its susceptibility to fire, and this effectively rules it out for many of the urban applications where
much of the demand during the next ten years will be concentrated. Here, however, there will continue to be major applications for wood, especially in non-structural or in semistructural components and in all manner of components such as cabinet work, floors, and finish. Concrete In Europe, concrete is, today, the most commonly employed material, and the large concrete panel forms the base of the most widely used industrialized systems. It can be, and is, utilized, however, also for large boxes and for frame plus infill types. The precast units may or may not be prestressed. Prestressing is common, particularly in floor panels spanning considerable distances. All three types of concrete-based systems are also employed in the United States. All of the normal types of cements can be, and are, employed for industrialization, but most common are general purpose and high early strength. Colors can easily be incorporated and many surface textures are available by the use of exposed aggregates of considerable variety, and other surface treatments such as sandblasting and chipping. If necessary or desirable, the surfaces of the concrete can be coated. Newer types of cements hold promise for industrialized processes. Among them are the expansive Mater. Sci. Eng., 7 (197l) 2~5-233
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Fig. 3. Large pre-cast concrete panels for industrialized multi-family housing. (Photograph from San-Vel, Techcrete.)
Fig. 4. Composite wall panels. Glass-fiber reinforced polyester outer shells, baked polyurethane finish, foamed concrete core, reinforced gypsum inner face, flexible epoxy-urethane bond, bitumen vapor barrier. Six panels attached to steel supporting frame erected at one time. (Photograph from Greater London Council.)
cements which expand slightly upon curing instead: of shrinking as most concretes do. This helps to overcome cracking and may, indeed, introduce some initial prestress into the steel reinforcement. A second type of cement makes it possible to regulate the speed of set, depending upon the requirements of the job. Evidently, for industrialization, the faster the cement sets, the sooner a slab or other component can be removed from the molds, and the greater the production that can be achieved with those molds. In this connection, a British development utilizes a press which squeezes out the excess water after a panel has been cast and densities it to such an extent that the panel can immediately be removed from the mold and set aside to cure. Polymeric materials have been added experimentally to concrete and cured in situ by highenergy radiation. The addition of polymers is designed to help overcome the disadvantages of concrete, mainly, brittleness and low tensile strength leading to its tendency to crack. Another approach to overcoming the disadvantages of concrete is the addition of strong fibers, both inorganic and organic, such a glass, asbestos, metal, and various types of polymeric fibers. Glass must be especially alkali-resistant for this purpose. It has long been known that the durability of concrete under exposure to various climatic conMater. Sci. Eng., 7 (1971) 225-233
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ditions can be increased by the entrainment of small percentages of air. A good deal of experimentation is going on with gas and air entrainment in various proportions. Light weight is achieved, indeed, by foaming the concrete, either by mechanically beating in air, or by chemical means such as the addition of aluminum powder, whose reaction with portland cement gives off hydrogen gas. The basic advantages of concrete are quite evident. It is highly fire resistant. It requires little or no maintenance under many conditions, its mass allows for good acoustical attenuation, it can be cast into a great variety of shapes including long spans, and it is generally economical in construction. The major disadvantages, as indicated above, are hardness, weight, brittleness, low tensile strength, and tendency to shrink and crack. Metals Steel is easily the most commonly used metal for structural purposes such as framing members, and including steel in sheet form for structural and semistructural applications. Metals, in general, are strong and tough and, therefore, can be used in effÉcient structural shapes such as framing members, or as thin sheets. This makes them relatively light in weight for construction purposes in spite of the high density of metals such as steel. Aluminum, the second most common building metal, is usually employed in sheet form for enclosure, roofing, and similar purposes or in the form of extrusions or other shapes for windows, doors, and similar applications. Copper, today, is mainly used for electrical applications but is also employed for roofing, rainwater conductors, and similar applications. Cast iron finds use in piping, plumbing fixtures and similar applications. Many other metals find use in metallic building products, either in their own right or as alloys, such as nickle or chromium in stainless steel. For industrialized processes, metal framing members, particularly steel, find their way into frame and infill systems and into components of lightweight boxes as opposed to the heavyweight boxes based on concrete. Steel is used as framing material for mobile homes, and aluminum is used for sheathing windows and doors, and other applications. Both steel and aluminum are widely employed for secondary components such as ducts and cabinetry, and these are joined by copper, lead, cast iron, and others for mechanical, plumbing and electrical subsystems. The principal advantages of the metals are their strength, toughness, wide range of other properties,
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and range of densities. Principal disadvantages are need for protection against high temperatures found in building fires, the fact that some of them may corrode and require protection, costs may or may not be favorable, and some limitations on the forms which can be achieved with framing members and sheet stock. These are usually linear, flat, or in simple curves. Not the least contribution of metals to industrialization is in the machinery and equipment that make it possible. Equipment such as the molds and the handling machinery in the shop, transport rigs, cranes, and the many other types of equipment needed to put the industrialized components in place, are metal. Without them, the whole concept would be impossible. Plastics Presently, the volume of plastics employed in building is modest, but there is great interest in their properties and potentialities, with a general belief that their uses in building will be more widespread in the future. Today, within their own domain, their properties are probably about as extensive and variable as those of the metals in their domain. There are some 20-30 basic types with a large number of variations made possible by copolymerization, additives, and other means of varying their properties. Although they are new, the history of their use in buildings is beginning to accumulate and some have histories of 20 years or more in building applications. Like concrete, plastics have no inherent shape of their own and have to be formed into final shape. For structural applications, the mechanical properties of unmodified plastics are modest. Strength is moderately good, but stiffness, as measured by elastic modulus, is often distressingly low. For semistructural applications, however, these attributes can frequently be overcome by the judicious employment of inherently stiff and strong shapes, such as domes, anticlastic surfaces, and a great variety of three-dimensional curved forms which are readily obtainable and which are inherently stiff and strong. The widely used bubble-shaped skylights, for example, take advantage of the easy formability of these materials to provide an efficient shape, from the standpoints of light transmission and resistance to loads such as snow and wind. For structural and semistruetural applications it is generally necessary to incorporate strengthening materials. Of these, by all odds the most commonly Mater. Sci. Eng., 7 (1971)225-233
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Fig. 5. Polyvinylchloride overlay on wood window sash and frame. (Photograph fromAnderson,B. F. Goodrich ChemicalCo.) employed is high-strength glass fiber. Fibers of this type have tensile strengths ranging in the vicinity of 500-700,000 psi at one-third the density of steel, and elastic moduli of about 10 million, comparable to aluminum. These reinforced plastics, usually based on polyester resins for building applications" because of ease of handling and relatively low costs, are light, tough, strong, and considerably stiffer than unreinforced plastics, but still possess relatively modest moduli of elasticity compared, for example, to steel or aluminum. They begin as masses of glass fiber and barrels of liquid resin. They have to be shaped into final form. The most efficient structural forms are shells, folded plates, corrugations, and other non-planar and non-linear forms, plus stiffeners such as ribs, hat sections, and so forth. Because of their toughness, they can be used in thin sections and these are frequently highly translucent, as contrasted with other structural materials. For industrialized structures and enclosure, they lend themselves favorably to use in panels and boxes, especially if curved or otherwise shaped, rather than as linear framing members, for which they compete
unfavorably with traditional materials such as metal and wood except for special uses. Their corrosion resistance, lightness, colorability, range of properties from complete transparence to complete opacity, toughness, and ready formability recommend them for a variety of uses. Major uses of plastics are in nonstructural major and minor components of building. There is a great deal of interest in the all-molded bathroom, and there have been some notable uses of such bathrooms, employing reinforced plastics, acrylics, polyvinylchloride, and others. Plastics are standard materials for floor covering and are widely used for interior and exterior wall covering. High-pressure decorative laminates are standard materials for counter tops, furniture, facings on doors, and wall covering. Polyvinylchloride is used for windows, either in its own right or combined with metal and wood. It is also used along with polyethylene, polypropylene, and a variety of copolymers for piping or related uses. Hardware components employ nylon and other plastics. The list can be extended more or less indefinitely. Finally, the polymeric materials form the basis for sealants, gaskets, and caulking compounds, as well as the highstrength waterproof engineering-type adhesives for waterproof plywood and many other engineering applications, including those in building. Composites
The foregoing materials all have their advantages and disadvantages from the building standpoint. No one of them is perfect or it would be the only material employed. There is a growing tendency to use them in combination in the form of composites, and to take advantage of the "synergistic" effect of such combinations whose properties are frequently superior to or even entirely different from the properties of the individual materials acting alone. Composites may be classified in a variety of ways, but frequently are considered to be: Fibrous, fibers in a continuous matrix, Particulate, particles embedded in a continuous matrix, Laminar, layers of materials bonded together. Some writers like to add a fourth type of composite: skeletal, in which an impregnant interpenetrates voids within a skeleton; and a fifth, or flake, in which fiat particles are embedded in a continuous matrix of a cement, such as portland cement combined with water. Mortar for masonry is an obvious corrolary of concrete. Both of these can be modified Mater. Sci. Eng., 7 (1971) 225-233
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Fig. 6. Translucent structural sandwich with glass fiber reinforced polyester facings bonded to aluminum grid core. (Photograph from Kalwall.)
by the addition of polymeric materials. As a matter of fact, high-strength mortars with polymeric additives make it possible to preassemble masonry-based panels, such as brick or concrete block, to be hoisted into place, and make it possible, also to provide panels of this type considerably thinner than are required with standard unmodified mortars. Fiber-based composites have already been mentioned in the form of glass-fiber reinforced plastics. Fibrous additions to concrete have also been mentioned. Fiber-reinforced materials appear to be destined for increasing use in the future, generally, including industrialized building components. laminar materials are widespread. The highpressure decorative laminates have already been mentioned. Laminar metals such as alclads are commonly employed. Thermostats depend for their behavior on laminated metal. There are many other instances, including lead-coated copper, and so forth. From the building standpoint, one of the most interesting laminates is the structural sandwich, in which two relatively thin, hard, strong, high-density facings are combined with a relatively thick core of low-density material to provide a panel having many of the attributes of an I beam. The facings provide the strength and internal couple or moment to resist external forces, and the core provides the resistance to shear, as well as stabilizing the facings. In a building panel, the facings are also expected to provide resistance to exterior weathering and inte-
rior wear and tear, as well as colors, textures, and other attributes. The core provides thermal insulation, and the combination core and facing provides resistance to sound transmission. Many materials can be used for facings including metals, wood-based materials, plastics, concrete, cement-asbestos board, and other composites, and the same is true of the cores, including plastics foams, honeycombs, foamed concrete, metallic grids, and wood-based and fiber-based building boards. An interesting instance of the use of composites is provided by the high-rise flats of the Greater London Council. These were developed according to performance requirements, rather than materials specifications. The requirements were for resistance to wind, acoustical attenuation, low heat transmission, zero flame spread, two-hour resistance to standard fire penetration, minimum thickness, minimum weight, and minimum maintenance. The response was a composite wall panel consisting of a glass fiber reinforced polyester outer shell with an external facing of baked-on polyurethane, a lightweight foam concrete infill bonded to the shell with a flexible adhesive, and an internal face of gypsum reinforced with fiber and wire, bonded to the concrete foam core with a bitumen binder which also acted as a vapor barrier. This composite panel met all of the requirements set forth by the Greater London Council at 15 percent of the weight, and one-third the thickness of standard masonry or precast concrete components. Mater. Sci. Eng., 7 (1971)225-233
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Fig. 7. Proposed method of filament-winding large boxes for industrialized housing. Filament-wound facings in plastic matrix on insulating structural lightweight core. (Photograph from Progressive Architecture.)
Constraints There are many constraints to the widespread adoption of industrialized procedures for housing. These have strong effects upon the choice of materials. A major constraint is unfamiliarity on the part of the building industry with the new developments in materials and composites, and on the other hand, unfamiliarity on the part of industries not now associated with building respecting the pecularities and requirements of the building industry, as these, in turn, are constrained by many other factors outside the industry, itself. Government policy. A major factor, which has added to uncertainty in the building industry over the years, has been government policy, particularly economic and financial policy. Housing is highly dependent upon the availability of plentiful funds at moderate interest rates to encourage the production of housing at economic rents or carrying Charges. Government policy has been far from consistent in this respect, and has led to gyrations and
and rapid rises and falls in construction volume which have militated strongly against the introduction of capital-intensive and expensive industrialized processes. Codes. Building codes are frequently cited as major stumbling blocks and obstacles to the greater use of industrialization. They are frequently characterized as archiac and behind the times, and to a degree, this is true. On the other hand, many building codes are inherently sensible, if, perhaps, too conservative. The major problem with building codes is not that they are arbitrary or capricious, but that they are different. Frequently, relatively minor differences among codes in adjacent cities or municipalities within logical marketing areas are such as to make it difficult or impossible to produce, in large volume, the standardized components necessary to the success of industrialization. Furthermore, some of the loudest critics of building codes are the individuals who have a new product or process to sell which they have not sufficiently developed and proved to convince building code officials that they should accept the new and untried device. As new codes are written by the national organizations attempting to promulgate uniform codes, and by municipalities and States as they revise their codes, the trend is toward "performance" rather than "specification" codes. Instead of specifying details of construction, the performance expected of buildings is set forth, and the method of achieving it is left largely to the designer and builder. The Greater London Council case cited above is an example of performance requirement, although not by a municipal code. Although this approach requires a much higher degree of sophistication on the part of the drafters and the code officials, it does open the door to innovations, including innovations in materials. Labor. Labor is also often criticized as being a major obstacle to the introduction of industrialization or other new approaches in building. Here, again, to a degree, this is true, not so much for the often-quoted featherbedding techniques or opposition to innovation of labor, but because of the traditional crafts organization of building labor. This introduces questions of jurisdiction, of who is going to have possession of the new device, when such a new device or component or material is introduced. It is not so much labor opposition to the introduction of new ideas as rivalry among different crafts to obtain control. Mater. ScL Eng., 7 (1971) 225-233
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A similar problem arises when work normally accomplished on the site is transferred to the shop. A particularly bad situation arises if the shop is, for example, nonunion and the site is unionized. The nonunion product of the shop is likely to be refused at the site by unionized labor. A different problem arises when both are unionized, but the question arises who will organize and have jurisdiction over the union in the shop. It makes little sense to have many different crafts represented in the shop and the question is rather, which one of the different crafts will have jurisdiction over all labor in the shop, no matter what crafts are involved. Certifyin9 innovation. There is no clear procedure for the introduction of innovative ideas into the building industry in the United States. There is no central accepted and recognized certification board which examines new ideas and issues certificates or other generally accepted evaluations valid throughout the industry. This makes rapid acceptance difficult, considering the fact that there are approximately 50-60,000 home builders; 20,000, or more, registered architects; some 70-80,000 registered engineers working in the general building field; and hundreds of thousands of materials dealers and others involved, not to mention the building code officials and financial people who must also be convinced. Industrial 9aps. When a composite comes along that incorporates a variety of different materials, there is no good industrial setup for its production. The metals people are not much interested in a composite based mainly on concrete, and one that incorporates four of five different materials may have a hard time finding an industry to produce it. Management. Organization. One of the major lessons to be learned from the European industrialized housing industry is that the successful ones are the ones that are well organized and managed. There is no magic in any of the European systems that either is not or cannot be reproduced in the United States--all of them are, to a greater or lesser extent. The industry must find ways of organizing itself, however, to accept the basic idea of industrialization, which calls for close control all
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the way from raw materials production to the finished product, including the fabricating shop, transportation from shop to site, site organization, and the myriad feeders that lead into this mainline of the production stream. Public acceptance. Finally, and possibly paramount, is public acceptance. The new industrialized dwelling may be the best or the lowest cost ; but if it does not meet with public approval, it will fail. It is highly important, therefore, for the producer to understand the needs and prejudices of the market. Designers must be intimately acquainted with needs and prejudices and must learn how to design for industrialized processes at the same time that they meet the public requirements.
Conclusion Operation Breakthrough, mentioned above, is not only attempting to promote and encourage innovative technology, it is also addressing itself to the various constraints enumerated here. It is trying to aggregate the market. It is working with local code officials to find ways and means of surmounting differences and local obstacles. It is working with local zoning authorities to find the necessary land for large-scale development of aggregated market. It has set up its own evaluation team utilizing the National Bureau of Standards as a testing facility or the supervision of testing, and it has organized a technical evaluation team to assist it in its evaluations of innovative ideas and the setting up of standards for them. From the foregoing, it should be evident that if and as industrialization of housing production develops, major opportunities exist for conventional and "new" materials. The demand, if realized, is so great that all materials must be utilized, alone or in combination, to provide the needed volume at reasonable cost, and industrialization must play a significant role. Those materials that adapt best, will have the edge over the others. Received January 18, 1971.
Mater. Sci. Eng., 7 (197l) 225-233