Crack Free Concrete Made With Nanofiber Reinforcement
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2011-05-10
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Alternative Title:Mechanical Properties and Nanostructure of Cement-Based Materials Reinforced With Carbon Nanofibers and Polyvinyl Alcohol (PVA) Microfibers
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TRIS Online Accession Number:01342003
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Edition:Final Report
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Abstract:Failure in cement based materials is a gradual multi-scale process. When loaded, initially short and discontinuous microcracks are created in a distributed manner. These microcracks coalesce to form large macroscopic cracks, known as macrocracks. Fibers bridge cracks and transfer the load, delaying the coalescence of cracks. Due to the multi-scale nature of fracture, the influence of fibers in reinforcing cement based materials, mainly depends on the scale of reinforcement. Macrofibers (typically defined as fibers with diameters greater than 500 µm [0.02 in]) can improve post-peak toughness by bridging macrocracks. Fine microfibers (typically defined as fibers with diameter less than 50 µm [0.002 in]) on the other hand, bridge the microcracks which delay the process by which the microcracks coalesce to form macrocracks. However, cracks in cement based materials initiate from the nanoscale where microfibers are not effective. The development of fibers at the nanoscale has opened a new field of research within concrete. Previous research by the authors of this work on reinforcing cementitious materials using nanofibers, such us multi wall carbon nanotubes (MWCNTs), has shown that the flexural strength and stiffness of cementitious matrices can be significantly increased by adding very low concentrations of homogenously dispersed carbon nanotubes (as little as 0.025% by weight of cement)1. Nanoimaging of the fracture surfaces of cement nanocomposites have shown that MWCNTs reinforce cement paste by bridging nanocracks and pores, indicating that the addition of MWCNTs can enable the control of the matrix cracks at the nanoscale level2. The nanoindentation results have indicated that MWCNTs can strongly modify and reinforce the nanostructure of the cementitious matrix by increasing the amount of high stiffness C-S-H and reducing the nanoporosity3-4. Besides the benefits of reinforcement, autogenous shrinkage results have shown that MWCNTs can also have beneficial effects on the transport properties of cementitious materials5. Recently in the field of fiber reinforced concrete there has been much enthusiasm for the development of hybrid fiber systems where two or more types of fibers are combined. Hybrid fiber systems are a promising approach for making efficient use of fibers with the intent of conferring the best performance characteristics of each of the constituent fiber types to the composite material6. Recent work at ACBM has shown that fiber hybridization effectively enhances the material performance. The mechanical properties of cementitious composites were improved by using fibers of varying sizes and moduli. Use of hybridization was also shown to enhance the mechanical performance of extruded high performance fiber reinforced cement composites. Peled et al.7 as well as Cyr8 examined the effect of combining low modulus polypropylene microfibers (PP) with high strength and high modulus glass or polyvinyl alcohol (PVA) microfibers in extruded composites. They found that the addition of PVA microfibers to glass/PP hybrid composite significantly increases strength and toughness. Lawler et al.9-11 reinforced concrete and mortar with steel macrofibers and steel or PVA microfibers. The macrofibers enhanced the postpeak performance, increasing the composite ductility.
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