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Fracture resistance of micro-fiber reinforced cement composites

University of British Columbia

The addition of very fine fibers of steel and carbon has proven to be one of the most effective ways of improving the strength and toughness of Portland cement-based materials. It is now recognized that these properties are directly related to the capability of the material to resist crack propagation. In the present study, a contoured double cantilever beam (CDCB) testing apparatus was developed to assess the crack growth resistance of fiber reinforced composites. This property was obtained in terms of fracture toughness versus effective crack length curves (KR-curves) using concepts of non-linear fracture mechanics. The influence of the different mix design parameters on the crack growth resistance of both steel and carbon fiber reinforced composites was determined. The parameters studied included both the content and the type of fibers, the content of silica fume, the content and the gradation of the sand and the length of the carbon fibers. Moreover, the flow number of the fresh mixes, compressive strength and modulus of elasticity were measured. Among the parameters investigated, the fiber content in the mix was found to be the most effective in enhancing the properties of the composites; improvements in the load carrying capacity, specific fracture energy, fracture toughness and crack growth resistance were observed with an increase in the fiber volume fraction. These improvements were also found to be more pronounced in the cases where carbon fibers were used rather than steel fibers. The length of the carbon fibers, however, did not seem to have an influence on the properties of the composites. Silica fume additions only appeared to have an influence when introduced into carbon fiber composites. An addition as low as 5% seems to be sufficient to eliminate all agglomerations of fibers demonstrating the efficiency of the silica fume only as a workability aid and that greater silica fume additions are not justified. Sand additions appeared to be an excellent means of improving both the tensile strength and the crack growth resistance of plain mortars. In that sense, it can be mentioned that the fracture toughness of mortar containing a sand/cement ratio of 2 was only 12% smaller than the fracture toughness of paste reinforced with 2% of steel fibers. On the other hand, sand did not have a significant influence on the crack growth resistance of fiber reinforced mortars. Reducing the cement content by the addition of sand in producing fiber composites would therefore reduce both shrinkage as well as the cost without affecting the efficiency of the fiber. However, additions of sand seemed to have a detrimental effect on the air content of the composites which led to reductions in the compressive strength. This effect was proportional to the sand content and was more pronounced in the case of steel fibers.

Thesis/Dissertation

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2009-01-30

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http://hdl.handle.net/2429/3986

Civil Engineering

University of British Columbia

UBCV

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