Composites under self-compression

Details Composites under self-compression Composites under self-compression

1998-02-13

2001-01-16

Weisberger, Rich (Department: 1774)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Noninterengaged fiber-containing paper-free web or sheet...

C428S292400, C428S293700, C428S294700, C428S295400, C428S299100

Reexamination Certificate

active

06174595

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A MICROFICHE APPENDIX, IF ANY
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, generally, to composite materials, and methods of making and using composite materials. More particularly, the invention relates to composite materials which are hardened and maintained under self-compression.
2. Background Information.
Common conventional fiber reinforced composites contain fibers such as glass, polymeric and carbon fibers in either continuous or chopped forms. The matrix materials are plastic resins such as the polymerization product of unsaturated polyester with styrene or the reaction product of amines with epoxy resin. The matrix material may also be a thermoplastic. Many other variations are known. Conventional composites are strong, impact resistant and lightweight. These properties are useful in the construction industry. In U.S. Pat. No. 4,316,925 carbon fiber composites are used to reinforce concrete. In some cases the fibers are tensioned to form the composite and align the fibers. For example in U.S. Pat. No. 5,114,653 polymeric fibers are used with an epoxy matrix to provide prestressed reinforcing rods for concrete. In U.S. Pat. No. 4,063,838 tensioned glass fiber composites are used to replace paint as a protective barrier for metal. In U.S. Pat. No. 5,362,545 wood is similarly protected.
Although a large number of conventional composites have been disclosed, the use of conventional composites in construction has been very limited. The reason is related to the costs of these materials relative to common construction materials such coated steel reinforcing rods, wood beams and molded or extruded plastic pipe, drains and the like. Also, conventional composites exhibit some serious deficiencies in their mechanical and chemical properties.
Construction materials usually need to be strong, rigid and inexpensive. Conventional composites usually have good strength but have limited rigidity. The rigidity is reflected in a material's modulus. The modulus of typical conventional composites is about 20 GPa (gigapascals) to 75 GPa, although some expensive exceptions exist. Steel, by comparison, has a modulus of about 210 GPa. Therefore it is often found that in order to obtain rigidity comparable to common materials the conventional composite must be used to an extent that the cost becomes one to two or more orders of magnitude, or more, higher than that of the common materials. An additional contributing factor is that although selected fibers may have a high modulus, the matrix resins have generally low moduli, typically from about 2 GPa to 5 GPa. The combination of fibers plus resin has substantially lower modulus than the fibers alone. Composites with costly very high fiber volume fractions thus need to be used to retain rigidity.
Another mechanical limitation of conventional composites is their failure mode. Steel, when highly stressed, exhibits ductile failure; thus steel continues to be load carrying at high stresses. Conventional composites usually exhibit brittle failure, therefore the load of the failed member is rapidly transferred to the remaining load carrying elements in the structure. This can lead to catastrophic failure.
Conventional composites also have limitations due to their chemical nature. Epoxy resins are photochemically reactive and thus need to be protected from outdoor light exposure. They also can be water sensitive. Polyester and vinyl ester resins, being esters, can be hydrolyzed in high pH environments, such as when embedded in concrete. Conventional composites, when used as coatings or barriers for common materials, also exhibit many of the problems of ordinary paint. Moisture can be trapped at the interface between the conventional composite and the common material and can cause blistering and corrosion. Gasses, such as hydrogen, can also cause bond failure at interfaces with steel.
To overcome the limitations of conventional composites, a large number of cementitious composites have been disclosed. Cementitious composites contain fibers and inorganic cement matrix materials such as Portland cement. More generally, inorganic cements have been classified as hydraulic cements, such as Portland cements, alumina cements, natural cements, pozzolans and slag cements and non-hydraulic cements such as gypsum and magnesium cement and the like. Hydraulic cements are water resistant when hardened. Some additional cements, not strictly belonging to these two groups are lime cement and various other silicate based cements. Cements usually contain water and chemical admixtures and often contain polymers and inert materials such as crystalline silica or other minerals. The corresponding composites belong to a class of composites called brittle matrix composites, because the neat matrix materials will exhibit fracture at very low strains, typically less than 0.1 percent. Other brittle materials include ceramics, glassy materials and some metals.
The cementitious composites, which have greatly improved ductility over the neat matrix, contain fiber volume fractions between 10 and about 30 percent. Volume fractions below about five volume percent may be useful for applications such as crack control, but they do not provide a marked improvement in strength and ductility over the neat matrix. Fiber volume fractions above about 30 volume percent are possible but they are more difficult to achieve due to the problem of mixing fibers with particulate containing matrix materials without excessive void formation; i.e. the problem of consolidation. By reference, the volume fractions of fibers in conventional composites is usually between about 10 and 70 volume percent.
Cementitious composites offer potential solutions to the limitations of conventional composites. The cements can be lower cost, often by an order of magnitude, than the resins of conventional composites. Further, cements usually have a very high moduli, typically from about 10 GPa to over 40 GPa. This offers the possibility of formulating composites with sufficient amounts of expensive materials such as fibers, to provide the required strength and ductility, while using much more of the low cost matrix material to provide the rigidity. That is, the structural element can be made as rigid as required, while retaining low cost, by making the element larger, as long as the fiber volume fraction remains high enough to provide good composite properties.
Cements have better weathering properties than resins since they are usually much less sensitive to light and water. Cementitious composites also can provide good barrier properties to common construction materials. The high pH of some cements can protect metals by passivation while the cementitious composite can provide strength and impact and abrasion resistance that is far superior to paint films. The cementitious composites usually are not complete barriers, thus they allow the slow passage of water and gasses which prevents damage to the interface of the composite and the surface of the material to be protected and/or reinforced, such as metals or treated wood. Cementitious composites can also provide protection and rigidity to certain plastic elements, such as polyolefin pipe and the like at low cost.
However, cementitious composites are severely limited by the low strength of the cements and by their brittle nature. Cements typically have low strength in compression and have extremely low strength in tension relative to matrix resins.
Another limitation is that it is difficult to mix fibers and cement pastes to obtain uniformity. It is much less difficult, for example, to saturate fibers with liquid resin. To reduce the mixing problem and to improve strength, short fiber cementitious mixes have often been made with excess water, then partially dewatered before use by a process of water removal related to filtration. Further improvements have been made by also including continuous fibers in the short fiber comp

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