THESIS
2017
xx, 280 pages : illustrations ; 30 cm
Abstract
Conventional cementitious materials are highly brittle. Research on fracture of composites has led to the development of Strain-Hardening Cementitious Composites (SHCCs, also known as Bendable Concrete), which exhibit tensile strain-hardening behavior up to several percent strain, accompanied by the formation of multiple fine cracks. However, the high material cost of SHCCs, which mainly comes from the short random fibers, limits the wide applications of the material. Therefore, partially or even totally replacing the commonly-used polyethylene (PE) or polyvinyl alcohol (PVA) fibers by other cost-effective fibers is critical for the practical applications of SHCCs. In addition, the utilization of hybrid fibers in an appropriate combination in SHCCs has attracted extensive attention due...[
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Conventional cementitious materials are highly brittle. Research on fracture of composites has led to the development of Strain-Hardening Cementitious Composites (SHCCs, also known as Bendable Concrete), which exhibit tensile strain-hardening behavior up to several percent strain, accompanied by the formation of multiple fine cracks. However, the high material cost of SHCCs, which mainly comes from the short random fibers, limits the wide applications of the material. Therefore, partially or even totally replacing the commonly-used polyethylene (PE) or polyvinyl alcohol (PVA) fibers by other cost-effective fibers is critical for the practical applications of SHCCs. In addition, the utilization of hybrid fibers in an appropriate combination in SHCCs has attracted extensive attention due to their potential benefits compared to mono fiber reinforcements.
Due to the limitations of existing studies, current comprehension of what engenders the optimal hybridization of fibers in achieving the best performance remains inadequate. The present thesis attempts to address this challenge by systematically investigating hybrid-fiber SHCCs (HySHCCs) at multiple length-scales, through the application of micromechanical modeling, experimental evaluation and finite element simulation methodologies.
To lower the material cost and further improve the mechanical properties, HySHCCs with PVA and steel fibers was explored. A green matrix with ultrahigh-volume fly ash (UHVFA) was first developed. Then the interactions between steel and PVA fibers were experimentally evaluated and theoretically modeled at the single-fiber and single-crack levels. In addition, a series of SHCCs with different fiber combinations were comprehensively tested under compression, tension, bending and shear, while both the bending and shear behaviors were numerically simulated. According to the results, though HySHCCs generally show lower ultimate tensile strain than PVA-SHCCs, the crack control ability, elastic modulus, compressive strength, flexural strength, flexural toughness and shear performance are superior.
To lower the material cost and enhance the greenness, while maintaining adequate mechanical performance, HySHCCs with PVA and recycled polyethylene terephthalate (PET) fibers were investigated. A functional UHVFA matrix for waterproofing applications was first developed. Then the PET fibers were designed and surface treated according to processing, mechanical and durability requirements. Afterwards, the crack-bridging relation and the overall tensile performance of HySHCCs were theoretically modeled. Furthermore, the tension performance of a series of HySHCCs after standard 28-day curing and accelerated aging was experimentally evaluated. According to the results, satisfactory mechanical performance can be achieved even when 50% of PVA fibers are replaced by PET fibers in SHCCs.
With the above activities, this doctoral research not only provides deep insights on the design of SHCCs for construction applications by comprehensively considering the trade-off between mechanical performance, material cost and environment impact, but also proposes new strategies for the recycling of solid wastes (including fly ash and PET materials) in the construction industry.
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