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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 paper presents an improved numerical model for the reliability analysis of corrosion initiation due to the combined action of carbonation and chloride ingress. The coupled mechanism of carbon dioxide, chloride ions, heat and moisture is considered in the model. The probability of corrosion initiation is evaluated by means of a single limit state function in which the critical concentration of chloride ions depends on the degree of carbonation. In addition, the coupled diffusion coefficients are interpolated linearly depending on the degree of carbonation in order to simplify the structure of the model without compromising accuracy.

This study investigated the effects of carbon fiber (CF) length, electrode spacing, and probe configuration on the electrical conductivity of cement composites. Accordingly, 57 different types of samples were prepared, considering three different CF lengths, five different CF contents, three different electrode spacings, and two different probe configurations. This research found that the influence of CF length on the electrical resistivity of cement composite depends electrode spacing. For the cement composite with wide electrode spacing of 40 mm, its resistivity decreased as increasing CF length as in the previous study. However, when the electrode spacing is 10 mm, which is narrow (10 mm), the resistivity of the cement composite rather increased with increasing CF length. The results implied that when an electrode is designed for the cement composite incorporating CF, the CF length should be short compared to the electrode spacing. The percolation threshold of CF measured by the two-probe configuration was 2% or more. This is higher than that measured by the four-probe configuration (1%). At a lower CF content than 2%, the two-probe configuration gave higher resistivity of the cement composite than the four-probe configuration. However, the difference coming from the different probe configurations was marginal as increasing the CF content.

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.

This study aims to investigate the grindability and performance of glass-blended cements depending on the grinding method and glass type. Two grinding methods, intergrinding and separate grinding, and two types of waste glass, liquid crystal display glass (LCDG) and soda-lime glass (SLG), were considered. To study grindability, the milling characteristics of the coarse waste glass and fine glass-blended mixtures were investigated. The four types of glass-blended cements produced by the two grinding methods and the two types of waste glass were evaluated on particle size distribution, chemical composition, heat flow of hydration, hydration behavior, strength development and mitigation in alkali–silica reaction (ASR) expansion. It was found that the intergrinding method was more cost-efficient in producing glass-blended cements than the separate grinding method. Intergrinding of the glass-blended mixtures resulted in finer cement and coarser glass fractions. The interground glass-blended cements showed faster hydration reactivity but slightly lower strength and mitigation in ASR expansion than did separately ground glass-blended cements.

The waste glass sludge is a waste produced in the glass industry. It is in a dust form and disposed with water. In the disposal process, various cohesive agents are incorporated in order to precipitate the glass particles efficiently. In this paper, we investigate the pozzolanic reaction of the waste glass sludge incorporating precipitation additives experimentally. The consumption of calcium hydroxide, the setting time and the compressive strength and the pore structure were tested for two different types of the waste glass sludge depending on whether precipitation additives were used. It was found that the waste glass sludge incorporating the precipitation additives had a higher pozzolanic potential than the reference waste glass sludge without precipitation additives.

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.

In this study, we developed a cement-based wireless sensing system for self-monitoring of concrete infrastructures in a railway environment. The cement-based sensor was developed by inserting electrodes into cement composites incorporating 1.0 vol% multi-walled carbon nanotubes (MWCNTs) as conductive fillers. We designed and fabricated a wireless signal transmission module that can be mounted/dismounted directly on/from the cement-based sensor and a wireless reception module that receives and saves wireless sensor signals. The wireless transmission/reception module was applied to the cement-based sensor, and sensing response during the cyclic compression test was compared with that in a wired environment. The sensing response and the gauge factor in the wireless environment were found to be very similar to those in the wired environment. The loss rate of wireless transmission data was tested according to the distance between the transmitter and the receiver to determine applicability in an actual railway environment. Based on the test results, the distance of stable wireless transmission was evaluated for the open space, railway station, and railway sleeper. When the developed cement-based wireless sensor is applied to the railway sleeper, the wireless sensing system is expected to be used up to 3.5 years when the battery needs to be replaced.

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.