Carbon Fiber-reinforced Cement Research Articles

Physically simulating the dispersion of chopped carbon fibres in mortar

Carbon fibre dispersion exerts a critical influence on the characteristics of carbon fibre-reinforced cement mortars (CFRCMs). This study accordingly investigated the dispersion of chopped carbon fibres (CCFs) in mortar using physical experiments with a transparent simulated mortar system comprising colloidal silica and silica sand as replacements for cement and aggregate, respectively, and evaluated the microstructures and conductive performances of corresponding CFRCM specimens. In the simulated solutions, bundled CCFs in order were more prone to disperse into single fibres than clustered ones in disorder due to no physical fixation, colloidal silica prevented the formation of CCF clusters during stirring, and the shear force of moving sand dispersed but also broke CCFs. In the CFRCMs, sand aggregates reaggregated dispersed CCFs while larger sand particles increased CCF dispersion and improved specimen conductivity. Thus, decreasing the amount of CCFs clusters before dispersion, and a short stirring time in a single direction should be provided to prepare CFRCMs with well-dispersed CCFs. Meanwhile, a more dispersible aggregate and less formation of void should be also essential.

Mechanical Properties of 3D-Printed Carbon Fiber-Reinforced Cement Mortar

The 3D printing process is different from traditional construction methods of formwork casting due to the use of additive manufacturing. This study develops a suitable 3D-printed carbon fiber-reinforced cement mortar (CFRCM) considering the extrudability, fluidity, setting time, and buildability of the CFRCM. The difference in compressive strength and flexural strength between 3D-printed specimens and conventional cast specimens was investigated by varying the amount of carbon fiber added (carbon fiber to cement ratio, 2.5 vol.‰, 5 vol.‰, 7.5 vol.‰, and 10 vol.‰) and the curing times (7th day and 28th day). The results of the experiments indicate that the addition of 6 wt.% cement accelerators to the cementitious mortar allows for a controlled initial setting time of approximately half an hour. The fluidity of the CFRCM was controlled by adjusting the dosage of the superplasticizer. When the slump was in the range of 150 mm to 190 mm, the carbon fiber to cement ratio 2.5 vol.‰ could be incorporated into the cementitious mortar, enabling the printing of hollow cylinders with a height of up to 750 mm. Comparing the 3D-printed specimens with the traditionally cast specimens, it was found that the addition of a carbon fiber to cement ratio of 7.5 vol.‰, and 10 vol.‰ resulted in the optimal compressive strength and flexural strength, respectively.

Enhancing thermoelectric performance in carbon fiber-reinforced cement composites through boron doping

Although carbon fiber-reinforced cements (CFRCs) have the potential for ambient energy harvesting, their low thermoelectric performance restricts their practical application in construction. In this study, we investigate the possibility of using boron doped carbon fibers to improve the thermoelectric performance of cement composites. The electrical conductivity of a single carbon fiber was significantly enhanced, reaching a value of 272.1 × 102 S/cm despite the limited amount of boron doping. This was due to the formation of highly aligned, dense, thin, and long domains within the carbon fibers, which facilitated electron flow. The pristine carbon fiber's hydrophilic surface retarded the hydration reaction, while the boron-doped carbon fiber's hydrophobic surface had a diminished impact on the hydration products. The highest power factor of 0.02 μW/mK2 in cement composites was attained by incorporating boron doped carbon fibers, which exhibit outstanding electrical conductivity, into the cement matrix.

Multifunctional properties of carbon fiber integrated cement composite – A review and insights

This article examines the strength characteristics of carbon fiber-reinforced cement composite (CFRCC). The research explores the residual mechanical characteristics and microstructure of concrete reinforced with carbon fiber. The review also focused on the determination of the effect of CFRCC on the improvement of strength, microstructure, crystalline phase shift, and hydration products of the cement composites. The review indicates that the inclusion of carbon fibers can intensify the flexural and splitting strengths of CFRCC, but the compressive strength is magnified to a lesser extent. In addition to the initial elastic modulus and peak stress of carbon fiber-reinforced cement composite, a discernible strain-rate hardening effect was also observed. This indicates that carbon fiber-reinforced cement composite has a promising future as cement composite reinforcements. The review of the characteristics of the composites also suggests their applicability in tunnel building in the cement composite. It is determined that carbon fiber-reinforced cement composite has a stronger capacity to withstand impact loads, and further information about its features will be supplied. The scanning electron microscopy (SEM) result demonstrates that the single and most significant failure mode under stress is carbon fiber breaking and the pull-out of carbon fibers. Thus, the review reveals that carbon fibers have a limited but positive act on enhancing the mechanical properties of cement composites.

A Comparative Study on Electrical Resistivity and Compressive Strength Properties of Cement Composites Incorporating Conductive Materials

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.

Effect of Carbon and Steel Fibers on the Strength Properties and Electrical Conductivity of Fiber-Reinforced Cement Mortar

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.

Enhancing the mechanical and functional performance of carbon fiber reinforced cement mortar by the inclusion of a cost-effective graphene nanofluid additive

In this study, a low-cost and industrially scalable graphene nanofluid additive (GNA) was incorporated into carbon fibre-reinforced cement mortar (CFRCM) to enhance its functionalities. The results indicated that the synergistic effect of the carbon fibres and the GNA significantly improved many properties of the cement mortar. For the CFRCM modified with GNA (G-CFRCM), the maximum increases in the flexural and compressive strengths, 101% and 126%, respectively, which were much greater than those of the control, 25% and 45%, respectively, and the maximum increases in the thermal and electrical conductivities of the G-CFRCM, 36% and 68%, respectively, were much greater than those of the control, 27% and 19%, respectively. In addition, a surprising increase in the service life of the corresponding cathodic protection system with the G-CFRCM, more than 200% compared to that of the control showed, that this novel composite material is ideally suited for applications in high-efficiency cathodic protection systems.

Effect of plasma surface modification on pullout characteristics of carbon fiber-reinforced cement composites

Light but strong carbon fibers have been utilized as effective reinforcing lattices for cement matrices. However, to explore the intrinsic properties of carbon fibers within the cement matrix, the interfacial behavior between the fiber and matrix must be understood in terms of the bond strength as well as the interfacial failure mode. In the present work, we evaluated the pullout properties of carbon fiber bundles from a cementitious matrix after the fibers were modified via one of three plasma treatments using argon, nitrogen, or oxygen. The plasma-treated carbon fibers were characterized using Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy. The treatments introduced hydrophilic functional groups on the carbon fibers’ surfaces and increased their surface roughness. With an appropriate amount of hydrophilic groups on the carbon fiber surface, the rate of hydration was accelerated, which led to a denser cement structure surrounding the carbon fibers. In addition, the fibers’ surface roughness was critical toward improving mechanical interlocking between the fibers and cement matrix. The highest interfacial shear strength for the argon plasma-treated sample can be explained by the improvement in surface roughness without degrading the mechanical strength of the carbon fiber, as well as the modification of the surface from hydrophobic to hydrophilic.

Enhanced thermoelectric properties of carbon fiber-reinforced cement composites (CFRCs) utilizing Bi2Te3 with three doping methods

Bi2Te3 was incorporated in carbon fiber-reinforced cement composites (CFRCs) as thermoelectric-reinforced component via three different doping methods (uniformity volume mixing, gradient volume mixing, and gradient layer mixing), forming Bi2Te3 enhanced CFRCs. Seebeck coefficient and resistivity were measured for evaluating its thermoelectric properties. Scanning electric microscope (SEM) and three dimension video microscope (3-DVM) were applied to characterize the microstructure of prepared composites. SEM images showed that carbon fibers (CFs) overlapped on each other, resulting in a 3-D continuous electric conductive path. The introduction of Bi2Te3 enhanced the electric conductivity of the composites due to the quantum tunneling effect, which was beneficial to improve its Seebeck coefficient and the linearity and reversibility of the Seebeck effect. The thermoelectric performance of the composites was further improved by the synergetic effect of gradient effect and low dimension. Thermoelectric power (TEP) of the composite fabricated by gradient layer mixing method was enhanced as much as 10 times comparing to conventional CFRCs. The mechanism of the enhanced effect was also discussed.

The effect of graphene oxide grafted carbon fiber on mechanical properties of class G Portland cement

As described in the research, CFs were reinforced by graphene oxide (GO) and prepared by grafting-to technology and this new graphene oxide modified carbon fiber (GO-CF) showed excellent adhesion properties to cement material, which was with excellent mechanical properties and analyzed by infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). To deeply explore the reinforcement mechanism of graphene oxide to CFs, not only were scanning electron microscopy analysis and X-ray diffraction (XRD) analysis conducted, but triaxial strain-stress mechanical performance tests, a dynamic simulation to subsurface operation environment, were also utilized to confirm the mechanical effect of GO-CFs to cement. The results showed that GO-CF reinforced cement (GOCFRC) exhibited excellent compressive strength performance with GO of only 0.4% BWOC content. And cement compressive strength increased to over 1.18 times that of pure cement after 14-day curing period. Above all, the new kind of reinforced cement material showed superior toughness compared to pure cement. Its flexural tensile strength increased by 38.81% in 3-day curing period, 38.65% in 7-day curing period, and 41.76% in 14-day curing period. Furthermore, it displayed excellent triaxial stress-strain toughness compared to pure cement: Young’s modulus increased to 150.9%, ultimate stress increased to over 121%, and ultimate strain increased to about 267%. Excellent compatible interface adhesion performance of new carbon fiber was detected through SEM analysis. Via this research, the application mechanism of GO reinforced cement material was improved, and a new type of carbon fiber reinforced cement (CFRC) with higher mechanical performance was developed.HighlightsGraphene oxide (GO) was used for the surface properties modification of carbon fiber.Graphene oxide (GO) can significantly increase the mechanical behaviors of carbon fiber.Graphene oxide-carbon fibers (GO-CFs) can significantly increase the mechanical behaviors of cement.The influence that GO-CF had on the mechanical integrity of the cement was discovered (SEM).The reinforcing mechanism that GO-CF had on the oil-well cement was discussed in detail.

Preparation and piezoresistive properties of carbon fiber-reinforced alkali-activated fly ash/slag mortar

In this study, a novel type of conductive mortar was proposed and prepared by alkali-activated fly ash/slag (AAFS) combined with carbon fibers. The workability, mechanical properties, electrical resistivity and piezoresistive properties of carbon fiber-reinforced AAFS (CF-AAFS) mortar were investigated and compared with carbon fiber-reinforced cement mortar (CF-CM). It was found that CF-AAFS exhibited much better workability and significantly higher compressive strength (>66 MPa at 28 days) than CF-CM with the same carbon fiber content. With 0.2 vol% carbon fibers, the electrical resistivity of CF-AAFS mortar decreased from 3313 Ω·cm to 165 Ω·cm at 28 days, reaching the percolation threshold. AAFS mortar with only 0.1 vol% carbon fiber content exhibited good piezoresistivity under both monotonic and cyclic compression. Therefore, CF-AAFS was regarded as a promising material in the use of self-sensing structure in construction.

Crack detection of a HPCFRCC thin plate using electrical resistivity method

Electrical resistivity measurement has been used as a test for studying the durability-related properties of cement composites for decades. Applications of electrical resistivity measurement namely health monitoring and crack detection are of great importance for increasing the service life of structures. The former is the capability of cement composite to sense its own strain (piezoresistivity) and the latter is based on the irreversible change of resistivity in the presence of crack. In this study, at first the effect of different fractions of silica fume and carbon fiber on piezoresistivity of cement composites were evaluated by monitoring the variations of resistivity under compression. Secondly, a high performance carbon fiber-reinforced cement composite (HPCFRCC) thin plate specimen was subjected to four-point bending while the variations of resistivity were recorded so that the zone at which the initiation of crack occurs can be detected. Results showed that both compressive strength and water absorption are highly affected by silica fume content while flexural strength slightly changed by increasing silica fume. While the addition of carbon fiber increased water absorption, it marginally affected mechanical strengths. Piezoresistivity test showed the ability of carbon fiber as a functional filler and the facilitated dispersion of fibers in high concentration of silica fume particles. It was confirmed that when carbon fiber content increases to more than 0.4 volumetric percentage, conductivity of specimen substantially increases and required sensitivity from piezoresistivity test is obtained. Crack detection test showed that when cracks are formed the planar surface resistivity of thin plate significantly changes. Thus, the formation and propagation of cracks can be monitored using electrical resistivity measurement in self-sensing cement composites.

Influences of molding processes and different dispersants on the dispersion of chopped carbon fibers in cement matrix

Chopped carbon fiber-reinforced cement composite (CFRC) has become one of the potential smart materials in recent years. The key process of preparing this kind of composite at the early stage is how to disperse carbon fibers evenly into the cement matrix to achieve CFRC composite with good properties. In this study, a three-step mixing process and a six-step mixing process for the preparation of CFRC were suggested, respectively. The fracture morphology was observed by scanning electron microscopy. The influence of mass fractions of three common dispersants methyl cellulose (MC), carboxymethyl cellulose sodium (CMC) and hydroxyethyl cellulose (HEC) on the dispersion of short carbon fibers in water under certain temperature was investigated prior to the manufacture of CFRC. The correlation between mass fraction and viscosity was discussed, and the dispersion effect was assessed from the structure of dispersants and molding process. A hypothetical capsule theory was proposed to reasonably explain the dispersion effect of MC, CMC, and HEC. The experiments showed that pre-dispersion by ultrasonic vibration improved the dispersion of carbon fibers greatly. The dispersivity of carbon fibers was closely related to the category and mass fractions of dispersants. With the same mass fraction of dispersants at a certain temperature, the dispersion effect was in order of HEC > CMC > MC. Meanwhile, when the mass fraction of HEC was between 0.6 and 0.8 wt% by weight of cement and the mass fraction in the aqueous solution was between 1.65 and 1.80 wt%, carbon fibers dispersed most ideally and distributed further uniformly in the cement matrix.

Portland cement paste with aligned carbon fibers exhibiting exceptionally high flexural strength (> 100 MPa)

Here, we introduce a nozzle injection technique for carbon fiber-reinforced cement paste leading to unidirectional alignment of cement-embedded short carbon fibers that follow the movement direction of the guided nozzle. In comparison to non-reinforced cement pastes, this novel material exhibits a tremendous increase of its flexural strength upon admixing and aligning 1 to 3 percent (by volume) of chopped carbon fibers. Cement pastes containing carbon fibers aligned in the stress direction thus acquire high compressive and flexural strength values at the same time. Mechanical tests prove the material to withstand flexural loads larger than 100MPa in conjunction with a deflection hardening behavior resembling that of high performance fiber-reinforced cementious composites at relatively low fiber volume content. Insights into the preparation, fiber alignment, rheology and the fracture behavior of this material are presented in this study.

Compression sensibility of magnetic-concentrated fly ash mortar under uniaxial loading

The electrical conductivity, compression sensibility, workability and cost are factors that affect the application of conductive smart materials in civil structures. Consequently, the resistance and compression sensibility of magnetic-concentrated fly ash (MCFA) mortar were investigated using two electrode method, and the difference of compression sensibility between MCFA mortar and carbon fiber reinforced cement (CFRC) under uniaxial loading was studied. Factors affecting the compression sensibility of MCFA mortar, such as MCFA content, loading rate and stress cycles, were analyzed. Results show that fly ash with high content of Fe3O4 can be used to prepare conductive mortar since Fe3O4 is a kind of nonstoichiometric oxide and usually acts as semiconductor. MCFA mortar exhibits the same electrical conductivity to that of CFRC when the content of MCFA is more than 40% by weight of sample. The compression sensibility of mortar is improved with the increase of MCFA content and loading rate. The compression sensibility of MCFA mortar is reversible with the circling of loading. Results show that the application of MCFA in concrete not only provides excellent performances of electrical-functionality and workability, but also reduces the cost of conductive concrete.

Measurement and modelling of conduction in carbon fibre-cement composites

The present work investigated the conductive performance of carbon fibre-reinforced cement under different frequencies of alternating current, involving fibre fractions both below and above the percolation threshold. The electrical conductivity of carbon fibre-reinforced cement changes according to the frequency of the alternating current. Tests on samples with fibre fractions below the percolation threshold show similar change pattern in the impedance–frequency curve. The trend of decreasing impedance with increasing frequency becomes less marked as the carbon fibre content increases. An equivalent circuit model of carbon fibre-reinforced cement has been proposed to describe the electrical performance based upon the experimental results. Moreover, using linear regression the correlation coefficients were analysed to show the rationality of the model. Results indicate that the resistance under alternating current is lower than that under direct current. The capacitance increases with increasing fibre content.

Stress-Sensitive Property of Carbon Fiber Reinforced Cement under Cyclic Load

In this paper, changes of resistivity of carbon fiber reinforced cement (CFRC) under cyclic load at different stress levels (15%, 35% and 55% of the failure stress) were studied. Experimental results showed that resistivity decreased with the increasing stress and increased with the decreasing stress under cyclic compression load. The resistivity of CFRC was reversible and cyclic under cyclic load at low stress level. When stress increased to a certain value, the decreasing rate of resistivity with increasing stress slowed down. When stress increased further, CFRC specimen resistivity cannot return to the initial value, showing a certain irreversible behavior. The feasibility of CFRC used in testing concrete fatigue was also discussed based on the conducting mechanism of CFRC.

Modeling of conductivity in carbon fiber-reinforced cement-based composite

Carbon fiber-reinforced cement is a multifunctional composite material owing to its conductive behavior. This article presents a model for quantitative analysis of the conductive mechanism. The model is based on the concept that the electronic conduction dominates when the composite is in dry state and is a combination of ohmic continuum conduction and tunnel transmission conduction. Therefore, Ohm’s law and tunneling effect theory are employed in the modeling process. Validity of the model has been confirmed by comparison of the calculated and measured results. Furthermore, the model is applied in simulating the strain sensing characteristics. Numerical results are shown and compared with measured data obtained under compressive loading. The agreement between theoretical and experimental results provides further support to the model.

Study on Electrical Properties of Carbon Fiber Reinforced Cement Paste with CCCW

In this work, CCCW (cementitious capillary crystalline waterproofing materials) was added in CFRC (carbon fiber reinforced cement paste). Variations of electrical conductivity of CFRC with fiber contents (0.2%~2.0% by the weight of cement) and CCCW (3% by the weight of cement) were studied. The results showed that the resistivity of CFRC containing CCCW versus the concentration of carbon fiber curves had typical features of percolation phenomena. The percolation threshold was 1.2 wt.%. The relationship between resistance and compressive stress was repeatable in CFRC with carbon fiber content of 1.2 wt.%. The resistance decreased in nearly linearity to the compression stress during loading, and increased during unloading.

Current distribution in reinforced concrete cathodic protection system with conductive mortar overlay anode

The current distribution was studied in a designed three-layer reinforced concrete cathodic protection (CP) system, with carbon fibre reinforced cement (CFRC) as the conductive mortar overlay anode. The influence of steel bars’ initial corrosion state, concrete resistivity and magnitude of impressed current density on the current distribution was discussed, respectively. Testing results show that the initial corrosion rate of steel has a great effect on the protection current distribution. In addition, there exists a threshold for corrosion rate, beyond which the uniformity of protection current distribution worsens markedly. Higher concrete resistivity not only deteriorates the current distribution, but also has an adverse effect on the rebars far away from the anode. However, the magnitude of impressed current density only affects the uniformity of current distribution to some extent when the rebars are corroding. This study could be useful to the cathodic protection of reinforced concrete structures in field condition.