Cement-based composites are generally non-conductors with high electrical resistivity, but they can be used as conductors by incorporating conductive materials. Recently, research has been actively conducted to develop high-conductive fiber-reinforced cement mortar (FRCM) due to increased interest in multifunctional cement mortar required by the market today. Thus, the present paper investigated the effect of the developed conductive FRCM containing carbon and steel fibers on the fresh and mechanical properties as well as electrical conductivity. The performance of conductivity FRCM was studied based on flow, unit weight, air content, three-point flexural, compression, and electrical resistance tests. In addition, their performance was also compared and reviewed with plain mortar (PM). Furthermore, the surface shape and element components of the developed conductive FRCM fracture surface were analyzed using a scanning electron microscopy (SEM) and an energy dispersive X-ray spectrometer (EDS). The results showed that the addition of steel fibers slightly decreased the relative flow value, whereas the incorporation of carbon fibers is very disadvantageous in terms of fluidity due to the fiber ball phenomenon. The unit weight of mixture containing carbon fibers was somewhat decreased, whereas the changes in the amount of air contents were relatively insignificant regardless of the fiber volume fractions. The flexural strength of conductive carbon fiber-reinforced cement mortar (CFRCM) and steel fiber-reinforced cement mortar (SFRCM) was significantly improved compared to that of PM. The compressive strength of CFRCM decreased significantly as the volume fraction of carbon fibers increased. Overall, even if the steel fibers were added up to 1.25%, the effect of improving the electrical conductivity of SFRCM was insignificant. In the case of the CFRCM used in this study, it was found that the percolation threshold existed between 0.3% and 0.4% fibers, and it was thus thought that the optimum dosage of carbon fiber should be secured by at least more than 0.4% in terms of electrical conductivity. Therefore, the most important factor for the electrical conductivity effect was found to be carbon fiber, whereas the effect of steel fiber was insignificant.

To improve the interfacial bond properties of the carbon fiber coated with a nano‐SiO₂ particle in a cement paste matrix, the present study proposed a method of coating nano‐SiO₂ particles on the surface of the carbon fiber by the chemical reaction of a silane coupling agent (glycidoxypropyltrimethoxysilane, GPTMS) and colloidal nano‐SiO₂ sol in an alkaline environment. To verify whether a nano‐SiO₂ particle was effectively modified on the surface of the carbon fiber, the surface morphology, chemical composition, and chemical structure were characterized and analyzed by several techniques such as the scanning electron microscope (SEM), energy‐dispersive spectrometer (EDS), and Fourier‐transform infrared spectroscopy (FT‐IR). Nano‐SiO₂ particles were entirely covered and uniformly distributed on the surface of the carbon fiber, resulting in the formation of a thin layer of nano‐SiO₂ particles. A thin layer of nano‐SiO₂ particles reacted with Ca(OH)₂ to form a calcium‐silicate‐hydrate (C‐S‐H) gel, which is most helpful to increase the form between the fiber and the matrix. In addition, a pull‐out test of the tow carbon fibers was performed to verify the effect of the new surface modification method on the interfacial bond properties of the carbon fiber embedded in the cement paste matrix. The experimental results showed that the frictional bond strength of the carbon fiber coated with a nano‐SiO₂ particle was significantly increased compared to the plain carbon fiber. These results were expected to improve the interfacial bonding force of hardened cement paste from the formation of the C‐S‐H gel produced through the chemical reaction of nano‐SiO₂ particles coated on the surface of the carbon fiber with Ca(OH)₂. In particular, it was confirmed that the carbon fiber‐reinforced cement paste (CFRCP) specimens coated with a nano‐SiO₂ particle and silica fume which replaced 10 wt.% of cement by mass showed the highest pull‐out resistance performance at 28 days of age. The new surface modification method developed in this study can be very beneficial and helpful in improving the interfacial bond properties of CFRCP.

Fiber-reinforced cement mortar (FRCM) has been widely used since it has many advantages compared to plain mortar (PM), and various fibers are highly applicable as repair and reinforcement materials for concrete. In the present paper, an experimental study was planned to investigate the properties, such as flexural performance (flexural strength and toughness), compressive strength, and impact resistance of mono fiber-reinforced cement mortar (MFRCM) containing only steel fiber (SF) or carbon fiber (CF), as well as hybrid fiber-reinforced cement mortar (HyFRCM) containing different combinations of SF and CF. The fiber content was used in five levels (0.0, 0.25, 0.5, 0.75, and 1.0%) at a total volume fraction of 1.0% by volume. The results show that HyFRCM containing 0.75% SF and 0.25% CF improved compressive strength, flexural strength, and impact resistance compared to MFRCM and other HyFRCM, resulting in a synergistic effect of hybrid reinforced fibers. It is noted that, in the case of HyFRCM containing 0.5% SF and 0.5% CF, the flexural strength was slightly lower, but the highest flexural toughness was obtained, which led us to judge that the result shown in this investigation can be the optimal fiber combination to improve toughness and energy absorption capacity.

In the present study, SiO₂ particles were coated on the surface of carbon fibers by means of chemical reaction of silane coupling agent (glycidoxypropyl trimethoxysilane, GPTMS) and colloidal SiO₂ sol to improve the interfacial bonding force between fibers and matrix in cement matrix. The surface of the modified carbon fibers was confirmed through a scanning electron microscope (SEM). The mechanical properties of SiO₂‐coated carbon fiber mortar and uncoated carbon fiber mortar with different fiber lengths (6 mm and 12 mm) and fiber volume fractions (0.5%, 1.0%, 1.5%, and 2.0%) were compared and analyzed. The experimental results show that the flow values of the carbon fiber mortar were greatly disadvantageous in terms of fluidity due to the nonhydrophilicity of fibers and fiber balls, and the unit weight decreased significantly as the fiber volume fractions increased. However, the air content increased more or less. In addition, regardless of whether the fibers were coated, the compressive strength of carbon fiber‐reinforced mortar (CFRM) composite specimens tended to gradually decrease as the fiber volume fractions increased. On the other hand, in case of the SiO₂‐coated CFRM composite specimens, the flexural strength was significantly increased compared to uncoated CFRM composite specimens and plain mortar specimens, and the highest flexural strength was obtained at 12 mm and 1.5%, particularly. It can be seen that the new carbon fiber surface modification method employed in this study was very effective in enhancing the flexural strength as cement‐reinforcing materials.

This paper aims to evaluate the resistance performance of the vinyl acetate ethylene polymer cement (VAEPC) composite and the polyvinyl alcohol fiber‐reinforced cement (PAFRC) composite against a low‐velocity impact in varying temperature. Their impact resistance performances are analyzed and compared with plain mortar after 28 days of age. Low‐velocity impact tests were carried out under the various room temperatures of −70°C, 70°C, and 140°C. Also, an INSTRON CEAST 9350 drop‐tower system has been used to get the impact load, fracture energy, and displacement of the specimens while loading low‐velocity impacts. From these tests, the failure pattern, shape, and strength of each test specimen were evaluated for the VAEPC, the PAFRC composite, and the plain mortar. Those test results showed that the flexural strength of both the VAEPC and the PAFRC composites has increased compared to that of the plain mortar. However, the compressive strength of the PAFRC composite decreased slightly after 28 days, while its flexural strength increased by 24.4% compared to that of the plain mortar. In addition, the drop test results show that PAFRC composite specimens have the highest impact fracture energy compared to other specimens at −70°C, 70°C, and 140°C, whereas plain mortar specimens have their lowest. This is because the PVA fiber included in the PAFRC acts as a bridge to suppress crack propagation and to improve energy absorption performance, which helps it resist relatively better against impact. It is also known that while the VAEPC composite and the plain mortar were destroyed in a form of being perforated, the specimens of PAFRC composite were observed to some extent to suppress the perforation failures. Therefore, under a load of low‐velocity impact, the resistance performance of the VAEPC composite and the plain mortar was proven to show brittle fracture behavior, while the PAFRC showed ductile fracture behavior in virtue of PVA fiber reinforcement which improved its flexural performance. According to the SEM observation which followed the tests, the PAFRC composite as a fiber‐reinforced material of the hydrophilic material was found to show the most excellent interfacial bond adhesion compared to the other composite and the plain mortar. The PAFRC composite manufactured in the study has been proven to be very useful as a reinforcement material in both high and low temperature environments.

Conductive cement composites have attracted high attention in recent years due to the possibility of achieving multifunctional materials. The usual approach has been to incorporate steel fiber, carbon fiber, graphite, and carbon nanotube/or carbon nanofiber into the cement matrix. In this article, an experimental comparative study was conducted on the electrical resistivity and compressive strength properties of cement composites incorporating carbon fiber (CF) and steel fiber (SF), which are conductive materials. The electrical resistivity of the conductive fiber-reinforced cement composites (FRCCs) was measured using the four-probe method, and compressive strength was conducted based on the KS L ISO 679 and ASTMC109/C109M test methods. Their performance was compared and analyzed with plain cement composites (PCCs). Afterwards, the fracture surfaces of the conductive SFRCCs (steel fiber-reinforced cement composites) and CFRCCs (carbon fiber-reinforced cement composites) were also analyzed using a scanning electron microscope (SEM). The experimental results showed that, in all specimens, the electrical resistivity gradually increased as the curing ages elapsed, while the electrical resistivity decreased significantly as the fiber volume fractions increased. Overall, the addition of SF up to a fiber volume fraction of 1.25% did not significantly affect the electrical resistivity of cement composites. In contrast, however, the electrical resistivity of CF decreased slightly even at a low dosage (i.e., 0.1–0.3%), and thereafter, it was significantly decreased. The percolation threshold of the conductive CFRCC incorporating CF used in this experiment was 0.4%, and this seemed to be the optimal CF dosage that can greatly improve the conductive cement composites while maintaining the maximum performance of compressive strength. In addition, the compressive strength of conductive SFRCC was higher than that of PCC, whereas in the case of conductive CFRCC, it showed a greater tendency to decrease significantly as the fiber volume fractions increased. We expect the results of SEM image analysis to be useful as basic data for establishing microstructure mechanisms of reinforcement fibers in the cement matrix.