Search

This study aims to investigate the effects of a calcium sulfoaluminate (CSA) based expansive agent (EA) on the tensile behaviors of ultra-high-performance concrete (UHPC) reinforced with straight and twisted steel fibers. For this, five different CSA EA amounts ranging from 0% to 8% were considered. Because the major purpose of adding a CSA EA into a cementitious matrix is to reduce the amount of shrinkage, its implication on the free-shrinkage response of UHPC was also evaluated. The test results indicate that approximately 7% and 10% smaller shrinkage strains were obtained in UHPC matrices with 6% and 8% CSA EAs than that of plain UHPC, respectively. Most of the tensile parameters, including the tensile strength, strain capacity, and g-value, of UHPCs with straight and twisted steel fibers were deteriorated when including a CSA EA and increasing its amount, except for the tensile strength of straight fiber specimens. Thus, the CSA EA needs to be carefully applied for UHPC mixture, despite its positive effect on reducing the amount of shrinkage strains by greater than 6%. There was no clear trend regarding the effects of the amount of CSA EA on the number of fibers located near the localized crack of ultra-high-performance fiber-reinforced concrete (UHPFRC), and the unit number of fibers in UHPFRC specimens with 2% (by volume) of 0.3-mm diameter steel fibers ranged from 25 to 35 ea/cm2. The tensile strengths of UHPC reinforced with straight and twisted steel fibers were both improved when increasing the number of fibers, although the former showed a much higher rate of increase and degree of correlation than the latter. The strain capacity and g-value of UHPC with straight and twisted fibers showed only marginal correlations with the number of fibers regardless of the amount of CSA EA applied.

This study investigates the implications of carbon nanomaterial type and amount on the electrical properties of cement paste. For this, five different nanomaterials, i.e., carbon nanotube (CNT), carbon fiber (CF), graphite nanofiber (GNF), graphene (G), and graphene oxide (GO), and two different volume fractions of 0.5 and 1% were considered. In addition, the self-sensing capacity of the cement composites with nanomaterials was evaluated under cyclic compressive loads. Test results indicate that the conductivity of plain cement paste was improved by adding carbon nanomaterials. In most cases, the conductivity of the composites was reduced by an increase in curing age and a decrease in nanomaterial amount, except for CF. The composites with CNTs exhibited the best self-sensing capacity regardless of volume fraction (vf), and the order of self-sensing capacity of the composites at a vf of 1% was CNT > GO ≈ GNF > G. The composites with 0.5 and 1 vol% CFs were determined to be not appropriate for a sensor measuring compressive behaviors. The gauge factor of the composites incorporating 1 vol% CNTs was obtained as 77.2–95.5.

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.

This study examined the feasibility of using multi-walled carbon nanotubes (MWCNTs) in cement paste for piezoresistive sensors. Two types of MWCNTs with different carbon content (>90% and >99%) were incorporated into cement paste at 1% by weight of binder (1 wt%). Plain cement paste and cement composites including 1 wt% graphite nanofiber (GNF) and graphene (G) were also considered for comparisons of the electrical conductivity. The test results indicate that the MWCNTs more effectively improved the conductivity of the cement paste than GNF and G. In addition, composites with MWCNTs with lower flowability had less resistivity than those with higher flowability. The size effect in electrical resistivity was observed in the cement pastes with and without the carbon-based nanomaterials, but it was mitigated by incorporating MWCNTs in the cement paste. The stresses and strains under cyclic compression and monotonic tension were well simulated by the measured fraction change in the resistivity of the composites with 1 wt% MWCNTs. The gauge factor (GF) for the composites with 1 wt% MWCNTs was higher than that of commercially available strain gauges, and it was affected by the loading condition: a higher GF obtained under compression than under tension.

Display devices that can visually present various bio-signals are a core future technology required for skin attachable wearable electronics. Quantum dot light emitting diodes (QD-LEDs), which can be used as high performance display devices, exhibit many advantages such as a narrow bandwidth, high color purity, high environmental stability, easy and fast processing, and a high brightness at low voltage. In this study, we report on the fabrication of a stretchable QD-LED array for the visual display of body movement and skin temperature signals from skin-attached sensors. After forming an array of a rigid NOA63 islands, an array of CdSe/ZnS QD-LEDs is fabricated on top of that island array and electrically connected using liquid metal Galinstan interconnections. Finally, the array of QD-LEDs on the NOA63 islands is transferred onto a stretchable elastomer substrate of Ecoflex/PDMS. Of particular importance, for stable performance of the QD-LEDs under deformation, in replacement of the conventionally used ITO, a flexible and highly transparent electrode of Au grid/Ethylene glycol doped PEDOT:PSS is used. Such a design architecture can minimize the strain applied to the QD-LEDs by concentrating the strain onto the soft elastomer film substrate and the Galinstan interconnections. As a result, the fabricated QD-LED array exhibits stable operation under both 50% uniaxial and 30% biaxial strains. After attachment of the stretchable QD-LED array onto skin, the extent of knee bending, and changes in skin temperature are displayed as changes in the pattern of the QD-LED array. This work demonstrates the potential application of our stretchable array of QD-LEDs for easy and daily monitoring of health conditions through a visual display.

This study experimentally analyzed the self-sensing performance of the tensile behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) incorporating four different types of conductive powders: carbon black (CB), nickel powder (NP), graphite powder (GP), and steel slag powder (SP). The conductive powders were incorporated by weight ratios of 5% and 10% of quartz powder (QP) which is used as a mechanical filler of UHPFRC. The results showed a decrease in tensile strength with an increase in the incorporated amount of CB, NP, and GP and an increase in tensile strength according to the increase in the incorporated amount of SP from 5% to 10%. The results of microstructure analysis by EDS mapping showed that, when incorporating 5% of CB and GP, which have a lower density than QP, conductive powders were sufficiently distributed, enabling both direct contact between conductive powders and the tunneling effect. In the case of NP and SP, which have a higher density than QP, at least 10% of NP and SP must be incorporated to achieve self-sensing performance. The tensile stress/strain versus time responses of UHPFRC with conductive powders were simulated using the measured FCRs. The pre-peak tensile behaviors were also well simulated using empirical equations based on the obtained FCR by nonlinear regression analysis. The predicted pre-peak tensile behaviors had smaller errors compared to the experimentally obtained tensile behaviors. In the case of the post-peak, only tensile stress was well simulated. As a result, 10% of NP and 5% of GP had sufficient self-sensing performance for UHPFRC with conductive powders.

As the buildings and infrastructures are considered as the most crucial target for developing an energy zero smart city, the concept of the net zero energy structure (NZES), which can harvest renewable energy and offset the energy consumption, has been considered as an integrated solution. Herein, a strategy for harvesting and storing energy using cement-based conductive composite (CBC) with carbon fiber fillers as structural energy material is suggested. Simultaneous consideration of electrical and mechanical properties of CBC results in the optimal concentration as its electrical percolation threshold (~ 1 vol%) to take sufficient long-range electrical conductivity while minimizing the deterioration of the mechanical property. The CBC introduced triboelectric nanogenerators, CBC-TENGs, are developed to demonstrate the harvesting various renewable energies, such as the wind, biomechanical, and blue energies, and the practical using of it. Moreover, the CBC-capacitor, which employs CBC as the core element of the capacitor, is fabricated to show the potential of CBC as a member in the structural capacitor. Given that cement is the most ordinary but indispensable one among the constructing materials, CBC has a great potential in the development of the NZES and in realizing the energy zero smart city based on the excellent functionalities of CBC.

This study aimed to improve the interfacial bond and tensile performance of ultra-high-performance concrete (UHPC) containing steel fibers by modifying the fiber surface using an ethylenediaminetetraacetic acid (EDTA) electrolyte solution, cold gas plasma, and nanosilica (SiO2) coating. The test results indicate that the surface roughness of the steel fiber significantly increased because of the chelate effect through the EDTA electrolyte solution and the deposited nanosilica particles. The root mean square roughness, R q , of the plain fiber sample increased by approximately two and nine times owing to the EDTA treatment and nanosilica coating, respectively. The roughened surface was more effective in enhancing the pullout resistance than the increase in surface energy, and the nanosilica coating further increased chemical adhesion and frictional shear stress, significantly increasing bond strengths and pullout energy, owing to the densified interface by the pozzolanic reaction. The tensile performance of UHPC containing steel fibers could be improved by the chelating effect of the EDTA electrolyte solution and nanosilica coating. The EDTA-treated fiber sample with nanosilica coating thus provided the best tensile performance with a tensile strength of 20.3 MPa, strain capacity of 0.99%, and g -value of 159.1 kJ/m3.

This study investigated the feasibility of using waste liquid crystal display (LCD) glass powder as a partial replacement for the silica flour that is typically used as a filler in ultra-high-performance concrete (UHPC). In this study, 50% of the silica flour was replaced with LCD glass powder, and its effects on the interfacial bond properties of two steel fiber shapes—circular and triangular—were evaluated. The test results indicated that the LCD glass powder effectively enhanced the bond between the steel fibers and the UHPC, which created a denser interfacial zone; this could be attributed to the filling effects of the LCD glass powder and secondary hydrates. The use of the LCD glass powder increased the pullout energy by 15–40% and the average bond strength by 16%, along with formation of additional scratches on the fiber surfaces. For the same matrix, as compared to the circular steel fibers, the triangular steel fibers yielded superior pullout resistance owing to their higher intrinsic fiber efficiency ratio. Thus, the highest average bond strength and pullout energy of 11.2 MPa and 744 mJ could be obtained from the UHPC mixture containing the LCD glass powder and triangular steel fibers.

A stretchable polyaniline nanofiber temperature sensor array with an active matrix consisting of single-walled carbon nanotube thin-film transistors is demonstrated. The integrated temperature sensor array gives mechanical stability under biaxial stretching of 30%, and the resultant spatial temperature mapping does not show any mechanical or electrical degradation.

Cement-based sensors with hybrid conductive fillers using both carbon fibers (CFs) and multi-walled carbon nanotubes (MWCNTs) were experimentally investigated in this study. The self-sensing capacities of cement-based composites with only CFs or MWCNTs were found based on preliminary tests. The results showed that the percolation thresholds of CFs and MWCNTs were 0.5–1.0 vol.% and 1.0 vol.%, respectively. Based on these results, the feasibility of self-sensing composites with four different amounts of CFs and MWCNTs was considered under cyclic compression loads. When the amount of incorporated CFs increased and the amount of incorporated MWCNTs decreased, the self-sensing capacity of the composites was reduced. It was concluded that cement-based composites containing both 0.1 vol.% CFs and 0.5 vol.% MWCNTs could be an alternative to cement-based composites with 1.0 vol.% MWCNTs in order to achieve equivalent self-sensing performance at half the price. The gauge factor (GF) for that composite was 160.3 with an R-square of 0.9274 in loading stages I and II, which was similar to the GF of 166.6 for the composite with 1.0 vol.% MWCNTs.

This study examined the electrical and self-sensing capacities of ultra-high-performance fiber-reinforced concrete (UHPFRC) with and without carbon nanotubes (CNTs). For this, the effects of steel fiber content, orientation, and pore water content on the electrical and piezoresistive properties of UHPFRC without CNTs were first evaluated. Then, the effect of CNT content on the self-sensing capacities of UHPFRC under compression and flexure was investigated. Test results indicated that higher steel fiber content, better fiber orientation, and higher amount of pore water led to higher electrical conductivity of UHPFRC. The effects of fiber orientation and drying condition on the electrical conductivity became minor as sufficiently high amount of steel fibers, 3% by volume, was added. Including only steel fibers did not impart UHPFRC with piezoresistive properties. Addition of CNTs substantially improved the electrical conductivity of UHPFRC. Under compression, UHPFRC with a CNT content of 0.3% or greater had a self-sensing ability that was activated by the formation of cracks, and better sensing capacity was achieved by including greater amount of CNTs. Furthermore, the pre-peak flexural behavior of UHPFRC was precisely simulated with a fractional change in resistivity when 0.3% CNTs were incorporated. The pre-cracking self-sensing capacity of UHPFRC with CNTs was more effective under tensile stress state than under compressive stress state.

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.