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Introduction
This paper is on how polymers will affect construction and building materials in the future. A general description of a polymer follows. Polymers are made up of monomers which are small molecules. The monomers are joined together chemically in a repeating linking process called polymerization. The resultant polymer consists of many of these long stands joined together, in fact polymers may consist of many thousands of monomers. Obviously, there are numerous possible configurations of monomers to produce many different polymers with a wide variety of properties. For the purpose of this paper the term polymer will be used in a generic sense although specific polymers may be referred to.
When the initial plastics were produced they were fairly brittle, had low strength and durability and couldn't handle temperature extremes well. About all it had going for it was that it was light and was cheaper to make than metals. In the years since then polymers have been engineered to overcome these initial shortcomings.
In the future polymers will be used in many fields to replace or in conjunction with existing materials. Polymers have higher strength to weight ratios than metals in tension and a much higher ductility and these properties will continue to increase with technology. They are more elastic than metals, able to withstand larger deflections without becoming permanently deformed or failing. In the instance where failure does occur it is with plenty of warning due to large deflections and distortion of the polymer.
Polymers as main load bearing members
Polymer strength and testing
Several material manufacturing companies have designed and produced polymers that they wish to market as structure material. However, polymers have received very little structural testing, especially fiber reinforced polymers(FRPs). A major weakness with using polymers as structural members is their internal structure. Due to the way the monomers are joined together polymers are extremely well at bearing longitudinal tension loads but are relatively weak in their capacity to bear longitudinal compression, lateral and torsion loads. For some polymers the lateral and torsion bearing ability is only 20% of the longitudinal bearing ability. Another problem is that polymers are not covered in structural codes and specifications. Attention needs to be turned to the structural uses of polymers so that the necessary testing and code revisions may be done. Most existing testing on polymers has been done by the manufacturers and is set to their criteria, which is not in accordance with what is required by structural members. Some of the problems with polymer strength when used as load bearing structural members is the affect that temperature has on polymer ductility and also there is a significant difference in polymer behavior with respect to the amount of strain as the result of loading. Polymers will need to be able to function fairly uniformly within an acceptable temperature range of around - 70 to at least several hundred degrees Celsius.
Polymer resistance to fire
Untreated plastics subjected to a flame will melt, losing its' shape and stength and give off toxic fumes. Aromatic polymers are extremely flamed retardant, but expensive enough to be impractical for extensive use. Several other modern efforts at flame retardancy result in harmful emissions. A new method has been developed in which a polymer can be treated by having a flame applied to it which creates a charred layer which will then resist burning. The problem with this is oxidation of done on FRPs due to enhanced strength. Using this process perhaps nonflammable strands of kevelar, asbestos or other materials could be bonded to the polymers to flameproof them. Continual work is being done in developing new polymers and it is certain that with improved chemical composition completely nonflammable polymers will be developed in the near future.
Of major importance is the emission toxic smoke from burning polymers. While often discussed in terms of lethality, many emissions can incapacitate-an equally lethal effect if surrounded by fire. In addition some polymers give off respireable toxic particles as well as fumes. As yet it has not been possible to create a polymer that does not give off toxic fumes when burned. This will certainly need to be addressed before polymers can be used a major portions of the material in buildings. The problem can be minimized by having polymers with low ignitability, flammability and flame spread. Low heat transference to surrounding materials is also desirable. In the future new polymers or coating processes will be developed to solve this problem.
Earthquake material
With the recent earthquakes in California it has become obvious that what had been thought to sufficient, as an earthquake code was deficient. Although some modern buildings withstood the quakes remarkably well, other structures such as the freeway overpasses were demolished. This is slabs (usually concrete) that make up the road will experience tension on the bottom. However in an earthquake they may experience tension on the top. This can be accounted for by putting rebar in the top as well as the bottom of the slabs. But the problem still exists that large deflections may occur in a structure with long unsupported stretches like in an overpass.
Polymers will be an ideal solution for this problem, in that they can take much larger deflections without sustaining permanent damage. Due to the ease in molding plastics shock-absorbing pockets with damping ability could be built into plastic slabs. Or polymers could be used with concrete creating a composite with higher flexibility. Polymer rebar could replace steel thus negating encased in a thin polymer coat. Polymers are also ideal for repairing structure damages in earthquakes as will be discussed below.
Polymers used to improve existing materials
Polymer increased durability
Polymers can and are being used in asphalt mixes to increase durability, but with mixed results. In several instances roads using polymer enhanced asphalt needed to be resurfaced within a period of one year of the original paving. However quite a few other projects have shown a tremendous improvement in durability. The main problem is that polymers can not be treated simply as any other asphalt; special care and procedures need to be followed to make sure proper bonding takes place. As this is a fairly new process optimum polymers and their ratio to the rest of the mix hasn't been found. Another benefit to this process is the ability to use industrial waste by-products as the polymer.
In the future the requirements and processes of polymer-asphalt blends will be refined to maximize the potential of this composite. As the savings from a lessened need for repaving accumulate, perhaps even a thin layer of FRPs could be laid on top of the road surfaces. The process may be extended to include all typical plastics so that all the plastic waste from individuals can be used instead of being put in a landfill.
Recycled polyethylene terephthalate (PET) can be used to create polymer concrete providing an increase in strength and at the same time a substantial reduction in weight. The polymer concrete (PC) has very low water permeability and can be used as overlays to protect existing conventional concrete structures. Since PCs have low cure times (setting overnight) they are ideal for use in repairing damaged structures.
Polymers as a repair material
Polymers can be used to supplement the strength of deteriorating conventional members and to prevent additional deterioration. The application may be done by wrapping the member completely in the polymer so that it is encased or by injection into cracks/voids of the members. Application is already being tested and implemented on the first, but injection will require a lot of testing. Adhesiveness and strength will be the primary criterion to test. Different polymers will bond better with materials than others so a guideline and tables for optimum bonds will have to be established. A portable high-pressure system will need to be developed for application of the polymer fill.
Future Impact
Design
Because of the inability of polymers to support loads other than tension loads there is a limit to their usefulness under existing design methods. While it may be possible to find ways to link monomers together laterally to create a homogenous property like polymer in the future a more plausible solution to this problem would be to change design methods. The way floors are laid out in buildings now is they are placed over joists which channel the load to main load bearing members. While nothing will change in respect to the main load bearing members, the floors may be supported by hanging them from the ceiling by polymer rods. This will use the excellent tension qualities of polymers to reduce the amount of material required to support the floor. The polymer tension members could be secreted in walls or the architect could leave them in the open and touch them up for aesthetic value.
Polymers will be used to increase ductility in structures that are in high earthquake zones. This will shift the trend from more and more reinforcement of structures to flexible structures that to some extent can move with the earth. For structures with high polymer content, damaged to the structure whether as a result from an earthquake or some other source will be easier to repair. It will not be necessary to completely demolish damaged structures, but to refuse of add polymer fill to the damaged areas.
Construction
Another change may be the widespread use of precast members in structures. Polymer concrete will make the members lighter and more durable for moving. This will simplify design so that instead of having to design from scratch there can be sections of structures such as whole rooms already prefabricated that only need to be joined together. This will be ideal for prison construction as well as low cost apartment housing. This will drastically cut down on construction costs due to reduced labor cost. Labor is by far the highest cost in construction and depending upon the project can run as high as 70% of total cost.
A major revolution in pouring concrete could come about. Right now the highest cost in pouring concrete is the formwork. In the future this will be greatly simplified and quickened by using polymer molds. These molds will be non removable and once joined together the concrete will be poured into them For concrete slabs the polymer forms will also replace rebar thus having a dual purpose. Concrete column construction will become much easier, and standard polymer forms will be sold in material warehouses. With enough advance notice given nonstandard forms could be constructed for any desired concrete shape.
Labor costs will be further reduced due to extremely short curing times for polymer concrete. Curing times will continues to drop until only a few days are required for the concrete to set sufficiently. This drastic reduction in construction cost will create a boom in the construction business as more firms and companies will be able to afford to construct new buildings. For those who already can afford construction costs they will have more and larger buildings built.
Environmental Aspects
Plastic waste will no longer be a problem. Plastic has an extremely long life and will still exist when most other materials in a landfill have deteriorated and decomposed. If there is large enough demand for building products that can be made out of recyclable plastics, material industries will construct a collection system for household polymer waste. Most people are environmentally conscious and would be willing to spend a little extra time sorting their garbage for recycling if convenient recycling centers were available. Although there would be a high initial cost for house to house pick up of waste plastics this would be a mostly one-time cost and would quickly pay for itself. Thus in the future plastics will be in useful material and structures instead of littering the environment.
References
Aklonis-Macknight.(1983). Instroduction to Polymer Viscoelasticity. New York: John Wiley & Sons, Inc
Bicerano, Jozef. (1993). Prediction of Polymers Properties. New York: Marcel Dekker, Inc.
Feldman, Dorel. (1989). Polymeric Building Mateials. New York: Elsevier
Roulin-Moloney. (1989). Fractography and Failure Mechanisms of Polymers and Composites. New York: Elsevier
Various Authors.(1990). Fire and Polymers. Washington, DC: ACS Advisory Board.
Various Authors (1994). Proceedings of Materials of ASCE Conference. Chicago: ASCE
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