The development of ultra-high-performance fiber-reinforced cement-based composites (UHP-FRCCs) was motivated by the need for a new and versatile material with high energy absorption capacity. With its excellent cracking resistance and consequent long life, UHP-FRCC is suitable for use in seismic design applications. The present study proposes a material design concept based on a multi-scale fiber-reinforcement system. In this approach, long, thick macrofibers are blended with short, thin mesofibers and microfibers. Such a combination of macrofibers, mesofibers, and microfibers is expected to enhance the mechanical properties of the composite under tension. However, the ductility of cement-based composites reinforced solely by microfibers is largely unknown. Therefore, in this study, the authors assessed whether microfiber improves the ductility of UHP-FRCC in two series of experiments. Wollastonite, which is a needle-shape mineral, is employed as a microfiber, and two different types of steel fibers are used as meso- and macrofibers. First, the enhanced toughness was evaluated in three-point bending tests on notched mortar beams. Second, the influence of wollastonite microfiber on the mechanical properties of the blended multi-scale fiber-reinforcement system was evaluated in uniaxial tension tests. Blends of macrofibers, mesofibers, and wollastonite microfibers exhibited strong reinforcement characteristics. The results indicate that the ductility of composites reinforced with wollastonite microfibers is highly dependent on the microfiber contents and type of fiber used and that blending of micro-, meso-, and macrofibers produces a highly ductile UHP-FRCC. Thus the material design concept based on the multi-scale fiber-reinforcement system proposed in this paper was shown to be effective in increasing the ductility of UHP-FRCC, even under uniaxial tension.
In the present study, non-evaporable water (NEW) and degree of hydration (DOH) of cement paste in the presence of silica nanoparticles (SNPs) and silica fume (SF) have been investigated. The incorporation of SNPs increased the NEW content due to the formation of more amount of hydration products. On the basis of NEW content, DOH was also calculated at different time intervals and it was found to increase from 28% to 85% in SNPs modified cement, while with the addition of SF increment was 74% at 56 days of hydration. In addition, capillary porosity of SNPs incorporated cement paste was also calculated and observed ~75% reduction as compared to plain cement. These results attributed that the SNPs refines the pore structure as a result of pozzolanic reaction and formation of additional calcium-silicate-hydrate (C-S-H). FTIR results show the appearance of bridging silicate tetrahedral (Q2), characteristic peak at 970 cm-1 and a hump at ~1108 cm-1 due to the formation of polymerised C-S-H. The microstructure studies through SEM revealed that the SNPs refined the pore structure of the cement paste leading to denser microstructure as a result of more polymerized C-S-H gel formation, desirable for high strength and durability.
The diagonal application of Carbon Fiber-Reinforced Polymer fabrics on hollow clay tile infill walls has been qualified as an efficient rehabilitation method for deficient reinforced concrete frames. However, majority of the experimental studies were conducted on 1/3-scaled RC frames and the effect of specimen scaling has not been questioned. In the current study, the results of an experimental campaign on 1/2-scaled RC frames are presented. Test specimens are grouped in two series having two different aspect ratios. In each series, there are two RC frames having hollow clay tile infill walls with/without CFRP reinforcement. The results of 1/2-scaled specimens are compared with the experimental results obtained from 1/3-scaled frames. In addition, the numerical model which were developed by the authors for 1/3-scaled frames are employed for modeling 1/2-scaled specimens and the results were assessed by comparing with the test results.
A novel electrodeposition method in which silicate ions are key materials, has been applied on mortar samples. The microstructures of electrodeposited mortar were observed using scanning electron microscopy (SEM), X-ray diffraction instrument (XRD), mercury intrusion porosimeter (MIP) and differential thermo-gravimetric analysis (DTG). The durability of electrodeposited mortar was evaluated by examining the resistance to carbonization, sulfate attack and chloride diffusion. The results indicate that the method can not only refine the pore structure, but also coat the surface of mortar. In comparison with the control mortar, the carbonization depth at the same carbonization time and the reduction in the compressive strength after the identical time of exposure to sodium sulfate solution for the electrodeposited mortar are decreased. In addition, the chloride diffusion coefficient is remarkably decreased by approximate three times. Based on these, the electrodeposition method is promising for the application in upgrading the durability of real concrete structure.
Superplasticizers (SPs) are widely used in various concrete made in Japan to decrease the amount of water required for cement mixing. The use of hot-weather concrete is increasing. The number of interstitial cement phases, such as the aluminate and ferrite phases, will be increased to treat waste and byproducts in the cement industry. In such cases, early hydration will affect cement phase fluidity. Therefore, chemical admixtures, especially SPs, are gaining importance. It has been reported that changes in concrete fluidity with time are affected by SP molecular structure. However, the mechanisms of fluidity change is still not completely clarified. It is therefore necessary to study the interaction between cement and SPs. This study focuses on the mechanism of SP action, especially during early hydration. Polycarboxy-late-based SP showed better fluidity retention than naphthalene sulfonate-based SP. This difference in fluidity-retaining performance is related to the amount of SP adsorbed per unit area. The fluidity reduction of cement paste with naphthalene sulfonate-based SP is attributed to a decrease in residual SP concentration in the liquid phase.
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