Doo-Yeol Yoo

Electrical Properties of Cement-Based Composites with Carbon Nanotubes, Graphene, and Graphite Nanofibers

This study was conducted to evaluate the effect of the carbon-based nanomaterial type on the electrical properties of cement paste. Three different nanomaterials, multi-walled carbon nanotubes (MWCNTs), graphite nanofibers (GNFs), and graphene (G), were incorporated into the cement paste at a volume fraction of 1%. The self-sensing capacity of the cement composites was also investigated by comparing the compressive stress/strain behaviors by evaluating the fractional change of resistivity (FCR). The electrical resistivity of the plain cement paste was slightly reduced by adding 1 vol % GNFs and G, whereas a significant decrease of the resistivity was achieved by adding 1 vol % MWCNTs. At an identical volume fraction of 1%, the composites with MWCNTs provided the best self-sensing capacity with insignificant noise, followed by the composites containing GNFs and G. Therefore, the addition of MWCNTs was considered to be the most effective to improve the self-sensing capacity of the cement paste. Finally, the composites with 1 vol % MWCNTs exhibited a gauge factor of 113.2, which is much higher than commercially available strain gauges.

Ultra-high-performance fiber-reinforced concrete: Shrinkage strain development at early ages and potential for cracking

This study investigates the effects of the concrete-ring thickness and steel fiber on the restrained shrinkage behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) using the ring test. To do this, three different concrete-ring thicknesses (tc = 20, 30, and 40 mm) and two different fiber contents (Vf = 0 % and 2 %) were considered. The test results indicated that the ring specimen without fiber exhibited shrinkage cracking after nearly 1.4 days because of its low tensile strength, whereas no shrinkage cracking was observed for the ring specimens with fibers during testing. A thicker concrete ring provided lower residual stress, relaxed stress, degree of restraint, and cracking potential. Therefore, it was concluded that using steel fiber and increasing the concrete thickness perpendicular to the tensile stress direction can improve the shrinkage cracking resistance of UHPFRC.

Hybrid Effect of Twisted Steel and Polyethylene Fibers on the Tensile Performance of Ultra-High-Performance Cementitious Composites

The hybrid effect of twisted steel (T) fibers with an aspect ratio of 100 and polyethylene (PE) fibers with four different aspect ratios of 400, 600, 900, and 1200 on the mechanical performance of ultra-high-performance cementitious composite (UHPCC) was investigated. This involved a total of 17 different sample types at an identical fiber volume fraction of 2% being made and subjected to compressive and tensile loads. Samples were made by replacing 0.5%, 1.0%, 1.5%, and 2.0% of T fibers with four different types of PE fibers. In addition, the pullout behaviors of fibers at cracked sections and the cracking behaviors of specimens were evaluated in order to determine the effect of the pullout mechanism of each fiber on the overall tensile performance. Test results indicate that the compressive strength decreased in proportion to the amount of PE fibers, regardless of their aspect ratio. The fiber hybridization had a great synergetic effect, successfully improving the tensile strength and strain capacity of UHPCCs; this effect was dependent on the aspect ratio of the PE fibers. Finally, the cracking behaviors were determined to be more related to the fiber type and pullout mechanisms than the tensile strength or strain capacity of UHPCCs.

Electrical and piezoresistive properties of cement composites with carbon nanomaterials

This study investigates the effect of nanomaterials on the piezoresistive sensing capacity of cement-based composites. Three different nanomaterials—multi-walled carbon nanotubes, graphite nanofibers, and graphene oxide—were considered along with a plain mortar, and a cyclic compressive test was performed. Based on a preliminary test, the optimum flowability was determined to be 150 mm in terms of fiber dispersion. The electrical resistivity of the composites substantially decreased by incorporating 1 wt% multi-walled carbon nanotubes, but only slightly decreased by including 1 wt% graphite nanofibers and graphene oxide. This indicates that the use of multi-walled carbon nanotubes is most effective in improving the conductivity of the composites compared to the use of graphite nanofibers and graphene oxide. The fractional change in resistivity of the composites with nanomaterials exhibited similar behavior to that of the cyclic compressive load, but partial reversibility in fractional change in resistivity was obtained beyond 60% of the peak load. A linear relationship between the fractional change in resistivity and cyclic compression strain (up to 1500 με) was observed in the composites with multi-walled carbon nanotubes, and the gauge factor was found to be 166.6. It is concluded that cement-based composites with 1 wt% multi-walled carbon nanotubes can be used as piezoresistive sensors for monitoring the stress/strain generated in concrete structures.

Fiber-Reinforced Cement Composites: Mechanical Properties and Structural Implications

Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea Department of Civil Engineering, (e University of British Columbia, 6250 Applied Science Lane, Vancouver, BC V6T 1Z4, Canada Department of Civil and Environmental Engineering, National Defense Academy, Yokosuka 239 8686, Japan Department of Civil and Environmental Engineering, University of Louisville, Louisville, KY 40292, USA Department of Civil Engineering, University of Victoria, 3800 Finnerty Road, Victoria, BC V8W 2Y2, Canada

Advanced Cementitious Materials: Mechanical Behavior, Durability, and Volume Stability

1Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea 2Department of Civil Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, BC, Canada V6T 1Z4 3Department of Civil and Environmental Engineering, National Defense Academy, Yokosuka 239 8686, Japan 4Department of Civil Engineering, Federal Centre for Technological Education of Minas Gerais (CEFET-MG), Av. Amazonas 7675, 30510-000 Belo Horizonte, MG, Brazil 5Department of Civil Engineering, University of Victoria, 3800 Finnerty Road, Victoria, BC, Canada V8W 2Y2