Techniques for designing laminated composites can be utilized for this purpose.

A method of analyzing and designing laminated composite-material wraps for columns, arches, domes, and other large reinforced-concrete structures involves an extension of composite-mechanics concepts and computational techniques developed previously for the analysis and design of the composite materials only. As used thus far, "composite materials" denotes polymeric matrices reinforced with polymeric or nonpolymeric fibers — e.g., epoxy reinforced with glass fibers. Wraps made of composite materials can be applied to reinforced concrete structures to repair them or as retrofits for reinforcement against loads that are expected to exceed original design loads.

Epoxy/Fiberglass Composite Would Be Wrapped around a steel-reinforced concrete column. Even though the composite overwrap would be only 0.5 in. (12.7 mm) thick, calculations indicate that it could prevent collapse in the event that concrete was subjected to double its rated load.

The method involves, among other things, recognition that reinforced concrete can also be regarded as a composite material and that a reinforced-concrete structure wrapped with a polymeric-matrix/fiber laminate can be regarded as a more-complex composite-material structure. The concrete can be regarded as a matrix, while the reinforcing steel bars embedded in the concrete can be regarded as fibers. Hence, the reinforced-concrete structure is amenable to the same finite-element analysis as that conventionally applied to polymer-matrix/fiber composites — of course, with appropriate modifications of the stiffness parameters and dimensions of the finite elements that represent the different constituent materials.

By analogy with laminated polymer-matrix/fiber composites, the overall laminate-wrapped reinforced-concrete structure can be treated computationally as a laminate that comprises (1) layers of concrete (matrix) only, (2) layers that contain both concrete and reinforcing bars (matrix and fibers), and (3) one or more layer(s) of the applied polymer-matrix/fiber laminate. Therefore, both without and with the composite wrap,the reinforced-concrete structure can be analyzed by use of general-purpose finite-element structural-mechanics computer programs and by composite-mechanics and progressive-structural-fracture computer programs developed previously for polymer-matrix/fiber composites. One can, for example, take advantage of all the features of the Integrated Composite Analyzer (ICAN) composite-mechanics program, which has been described previously in NASA Tech Briefs.

In some cases amenable to simplifying assumptions, the method can be practiced without need for a computer program. An example of such a case that illustrates the benefit of retrofitting is that of a composite overwrap to prevent or delay the collapse of a round concrete column reinforced with longitudinal steel bars. In this example, it is assumed that the concrete collapses to essentially a hydrostatic state when loaded to twice its rated ultimate load. Then using the dimensions indicated in the figure along with the associated material parameters and with a simple hoop-stress formula, one can calculate that collapse can be prevented (that is, the crumbled concrete can be contained) by an epoxy/fiberglass composite containing 50 volume percent E-glass fiber wrapped in one 0.1-in. (2.5-mm)-thick layer with fibers oriented longitudinally and in another 0.4-in. (10.2-mm)-thick layer with fibers oriented circumferentially.

This work was done by Christos C. Chamis and Pascal K. Gotsis of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 —8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16879.

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