Carbon Fiber Reinforced Cement-based Composites Research Articles

Recent Advances in Properties and Applications of Carbon Fiber-Reinforced Smart Cement-Based Composites.

Under the strategies of low-carbon and environmental protection, promoting green technology innovation to achieve carbon neutrality in the construction field has become a universal goal. As the building material with the highest consumption, concrete has gradually begun to transform into a multi-functional and intelligent product. Therefore, the research on carbon fiber-reinforced cement-based composites (CFRCs) is of relative interest. It mainly uses carbon fibers (CFs) with high elasticity, strength, and conductivity to disperse evenly into the concrete as a functional filler, to achieve the intelligent integration of concrete structures and function innovatively. Furthermore, the electrical conductivity of CFRC is not only related to the content of CFs and environmental factors but also largely depends on the uniform dispersion and the interfacial bonding strength of CFs in cement paste. This work systematically presents a review of the current research status of the enhancement and modification mechanism of CFRC and the evaluation methods of CF dispersion. Moreover, it further discusses the improvement effects of different strengthening mechanisms on the mechanical properties, durability, and smart properties (thermoelectric effect, electrothermal effect, strain-sensitive effect) of CFRC, as well as the application feasibility of CFRC in structural real-time health monitoring, thermal energy harvesting, intelligent deformation adjustment, and other fields. Furthermore, this paper summarizes the problems and challenges faced in the efficient and large-scale applications of CFRCs in civil engineering structures, and accordingly promotes some proposals for future research.

Enhancing the shrinkage resistance and impermeability of cement-based materials using carbon fiber modified by in situ-grown nano-SiO2 and carbon nanotubes

To improve the shrinkage resistance and impermeability of carbon fiber reinforced cement-based composites (CFRC), the surface of carbon fiber (CF) must be modified to strengthen the bond between the CF and cement matrix. Therefore, a novel method for modifying CF using in situ-grown hybrid nano-SiO2 (NS) and carbon nanotubes (CNTs) is proposed herein to enhance its reinforcing effect. The results suggest that in situ-grown hybrid NS and CNTs can function synergistically and outperform in situ-grown single NS and CNTs in enhancing the reinforcing effect of CF. In situ-grown NS strengthens the chemical bonding between the CF and cement matrix via its seeding effect and pozzolanic activity. Meanwhile, in situ-grown CNTs enhances the physical bonding between the CF and cement matrix via its physical friction effect. More interestingly, the in situ-grown hybrid NS and CNTs function synergistically to enhance the reinforcing effect of the CF by strengthening the chemical and physical bonding between the CF and cement matrix. Hence, the in situ-grown hybrid NS and CNTs improve the shrinkage resistance and impermeability of CFRC more effectively compared with the in situ-grown single NS and CNTs.

Influence of Different Surfactants on Carbon Fiber Dispersion and the Mechanical Performance of Smart Piezoresistive Cementitious Composites

This experimental study presents the effect of different surfactants on micro-scale carbon fiber (CFs) distribution into carbon fiber reinforced cement-based composites (CFRC) in terms of flexural and compressive strength, stiffness, flexural toughness, and strain-sensing ability. Conducting a narrative review of the literature focusing on the fibers’ separation, this paper follows a methodology introducing a combination of mechanical and chemical carbon fibers dispersion, as well as the different mixing processes (wet or dry). Three types of surfactants: Carboxymethyl cellulose (CMC), cellulose nanocrystal (CNC), and superplasticizer (SP), were applied to evaluate the CFs distribution in the cement paste matrix. Compressive and flexural strength, modulus of elasticity, and ductility of the cement-based composites (CFRC) reinforced with 0.5 wt.% CFs were investigated by three-point bending and compressive tests; flexure tests were also conducted on notched 20 × 20 × 80 mm specimens using the Linear Elastic Fracture Mechanics (L.E.F.M.) theory. Moreover, the electrical conductivity and the piezoresistive response were determined by conducting electrical resistance measurements and applying compressive loading simultaneously. The results clearly reveal that the CFs/SP solution or the CFs’ dry incorporation led to a significant enhancement of flexural strength by 32% and 23.7%, modulus of elasticity by 30% and 20%, and stress-sensing ability by 20.2% and 18.2%, respectively. Although the wet mixing method exhibits improved mechanical and electrical conductivity performance, constituting an adequate strain and crack sensor, the authors propose dry mixing as the most economical method, in addition to the enhanced mechanical and electrical responses. The authors recommend an effective method for structural health monitoring systems combining an economical CFs insertion in cementitious smart sensors with great mechanical and self-sensing responses.

Evaluation of carbon fiber dispersion in cement-based materials using mechanical properties, conductivity, mass variation coefficient, and microstructure

Uniform dispersion of carbon fibers is key to improving the properties of carbon fiber reinforced cement-based composites (CFRC). For the sake of high reliability, several methods were applied to measure the dispersed state of carbon fibers in the matrix. The mechanical and electrical properties of CFRC are assessed directly, while the mass variation coefficient, SEM, XRD, and BET appraise the dispersity indirectly. Experimental results indicate that the high fiber content is beneficial to the promotion of toughness and conductivity of CFRC, while the uniformity becomes enhanced by the higher dispersant content; the carboxymethyl cellulose sodium (CMC) has outstanding dispersion effect compared with other additives. Moreover, the ductility of CFRC is more applicable for estimating the fiber dispersion, which eliminates the matrix interference, and the strength is considerably affected by the matrix defects. Finally, the fiber content has an impact on the macro-level properties of the composites, but the hydration products of mortar are hardly influenced by the dispersants.

High-throughput design of fiber reinforced cement-based composites using deep learning

As the combinatorial space of a composite is virtually infinite and cannot be explored completely, a deep-learning method was proposed for high-throughput fiber-reinforced cement-based composites (FRC) design. First, a deep hierarchy network was developed to measure the relationship between the experimental variables and the FRC properties. A gradient-based high-throughput method based on the deep hierarchy network was then proposed to design FRCs, which were expected to have one or more certain properties. At last, a fine-tuning method was employed to guarantee its transferability for all types of FRCs. The results showed that the proposed method was able to design cement-fiber-water-curing-aging systems for carbon fiber reinforced cement-based composites (CFRCs). The fine-tuning method could transfer the CFRC model to design other FRCs. Thus, the proposed method showed promise for releasing the composite material property optimization from labor-consuming and low-efficiency laboratory tests.

A new method for CF morphology distribution evaluation and CFRC property prediction using cascade deep learning

This work presents a deep-learning method to characterize the carbon fiber (CF) morphology distribution in carbon fiber reinforced cement-based composites (CFRC), predict the CFRC properties, and measure the contributions of different CF morphology distribution directly using X-ray images. Firstly, the components of CFRC in slices of X-ray images were segmented and identified using a fully convolutional network (FCN). Then the CF morphology distribution evaluation were conducted based on the results of the FCN. At last, the prediction of CFRC properties was realized using a cascade deep learning algorithm and CF morphology distribution results. The results showed that the FCN provided more reasonable segmentation results for each component in CFRC than traditional methods. CF clustered areas and CF bundles increased sharply with the increase of CF content, while uniformly dispersed CF areas showed the opposite trend. The cascade deep learning provided a method to predict the CFRC properties (e.g. resistivity and bending strength) using X-ray scanning images, which could also quantificationally measure the contributions of different CF morphology distribution to properties of the CFRC. Therefore, the proposed method could be regarded as a nondestructive and effective test for CFRC property evaluation.

Characterization of carbon fiber distribution in cement-based composites by Computed Tomography

Carbon-fiber-reinforced cement-based composites (CFRC) have been considered as an innovatively functional civil engineering material for self-monitoring buildings, snow-melting pavement and smart pavement. However, the distribution of carbon fiber can significantly influence the performance of CFRC, especially their mechanical and electrical properties. The main purpose of this research is to explore the effect of carbon fiber distribution on mechanical and electrical properties of CFRC. For this purpose, the components in consecutive slices of Computed Tomography (CT) images were identified and segmented by using their grayscale thresholds. The typical morphology of each component in micro-graph of fracture surface was classified. The classification was used to verify the results identified by their grayscale in CT images. Indexes of planar distribution (Pd), spatial distribution (Sd) and volume factor were designed and calculated using 3D reconstruction model of the specimen. The resistivity, bending strength, bending fracture energy and compressive strength of specimens with different carbon fiber contents were tested. Finally, the grey entropy correlation analysis (GECA) was conducted to determine the effect of fiber distribution on the performances of CFRC. Results showed that four components contained in CFRC are the sub-mm pores, highly clustered area, clustered area and fiber sparse area. The volume fractions of pores (VP), highly clustered area (VH), and clustered area (VC) increase with the increase of carbon fiber content while the sparse area (VS) shows the opposite trend. The Pd and Sd increase with the increase of carbon fiber content. Sd has greater influence on the bending strength and toughness of CFRC. Volume factor plays a more important role on compressive strength and electrical resistivity of CFRC.

Dispersion of carbon fibers and conductivity of carbon fiber-reinforced cement-based composites

Abstract Dispersion of carbon fibers in the cement matrix remains a hot topic in the preparation of carbon fiber-reinforced cement-based composites (CFRC) because it affects greatly both the mechanical and electrical properties of the composites. In this work, a new dispersant hydroxyethyl cellulose was used with the aids of pre-dispersion by ultrasonic wave to realize the uniform distribution of chopped carbon fibers in the cement matrix. The fracture surface of the prepared CFRC was observed by scanning electron microscopy, the elemental distribution was investigated by energy dispersive spectroscopy, and the components was analyzed by X-ray diffraction. Influences of carbon fiber lengths and contents, water/cement weight ratio, molding process, curing time, and silica fume content over the conductivity of the CFRC composites were studied. The mechanism of conductivity was discussed. Results shown that the electrical resistivity intended to decrease with the increasing of carbon fiber contents. The mass fraction 0.6% of carbon fibers was a turning point. The concentration of hydroxyethyl cellulose between 1.66% and 1.86% was mostly beneficial for the dispersion of carbon fibers. The resistivity was increased first and decreased then with the increase of water/cement ratio. When the CFRC sample was prepared by the vibrating pressing method, the resistivity of the sample was reduced far greatly than that of the sample by the vibrating method. The incorporation of silica fume into the CFRC composites exerted not only a good effect on the dispersion of carbon fibers, but also increased the density of the composites to further influence the conductivity of the CFRC.

CVD Treatment of Carbon Fibers and Evaluation of Their Dispersion in CFRC

Carbon-fiber-reinforced cement-based (CFRC) composites is a promising functional material which can be used both in the military and civil fields against electromagnetic interference. However, it is essential to make carbon fibers dispersed uniformly during the preparation of CFRC. In this work, short carbon fibers were treated through Chemical Vapor Deposition (CVD) process at high temperature between 900°C and 1200°C under the protection of diluted nitrogen gas N2 to modify the surface of carbon fibers to further strengthen the bonding between carbon fibers and cement matrix. Natural gas (98% CH4) was used as a precursor. It was decomposed to produce an uneven layer of pyrocarbon that was deposited on the surface of carbon fibers. CVD-treated carbon fibers were pre-dispersed by using ultrasonic wave. Both hydroxyethyl cellulose (HEC) and silicon fume were used as dispersants and as admixtures. They helped CVD-treated carbon fibers distribute uniformly. The mass fraction of HEC was around 1.78% in the aqueous solution. Four methods, namely, the simulation experiment (SE) method, the scanning electron microscopy (SEM) method, the fresh mixture (FM) method, and the electrical resistivity measurement (ERM) method were, respectively, applied to evaluate fiber dispersion degree. Each method indicated its own advantages and disadvantages and it therefore catered for different conditions. Of the four evaluation methods, the SE method was the most convenient way to determine the pre-dispersion state prior to the preparation of CFRC composites. This method was helpful for predicting the subsequent dispersion state of carbon fibers in the cement matrix because it economized a large quantity of raw materials and time.

Dispersion of carbon fibers in cement-based composites with different mixing methods

Mixing methods can significantly influence dispersion of carbon fibers in cement-based composites. In this work, pre-mixing method (carbon fibers were added before cement) and after-mixing method (carbon fibers were added after cement) were adopted to prepare carbon-fiber-reinforced cement-based composites (CFRC). X-ray computed tomography (CT) was used to obtain the cross-sectional images of successive layers in the composites. Scanning electron microscope (SEM) was adopted to capture microstructure images of the composites. The CT images were continuously identified through their grayscale peaks. The sub-mm pores were separated from carbon bundles to improve the accuracy of the identification results. Two-dimensional (2D) dispersion characteristics of two different mixing methods were quantitatively analyzed by extracting pixels area of each component. Meanwhile, three-dimensional (3D) models were established by means of optimized CT images to obtain the volume fraction of carbon fiber bundles. The results show that the average areas of uniformly dispersed area, which obtains through pre-mixing method, are higher than those with after-mixing method. The dispersion effect of pre-mixing method is superior than that with after-mixing method. In addition, the volume fraction of carbon fiber bundles in specimens prepared through pre-mixing method was lower than that prepared by after-mixing method. Thus, the contrast resolution of X-ray CT system can recognize materials with over 0.3% density difference in CFRC; and the pre-mixing method is recommended in the preparation of carbon-fiber-reinforced cement-based composites.

Preparation of carbon fiber reinforced cement-based composites using self-made carbon fiber mat

The carbon fiber mat was first manufactured in the laboratory by means of the following procedure: dispersing chopped carbon fibers in the dispersant solution with the concentration of 0.4%, separating carbon fibers from the dispersant liquid using the thin gauze, and depriving the dried fiber mat from the gauze. The mat was then used to prepare carbon fiber reinforced cement-based composites (CFRCs) directly. Once the mat was mixed with water and cement, the adhesion between fibers would fail because of the water-solubility of the dispersant (also worked as the binder in the dry mat), leading to the mat unwoven and fiber dispersion into the cement matrix. To develop this method, different types of dispersant and carbon fibers were combined to prepare carbon fiber mat. Effective dispersion of carbon fibers in cement matrix has been obtained. Further, the applicability of this method has been proved to prepare CFRCs containing coarse aggregate, varying concentrations of carbon fibers, and carbon black. It was shown that the percolation threshold of CFRCs was about 0.6–0.9vol.%, and CFRCs containing 0.3vol.% of carbon fibers and not less than 0.6vol.% of carbon black possessed low resistivity and good mechanical properties.

Effect of carbon fiber dispersion on the mechanical properties of carbon fiber-reinforced cement-based composites

The preparation of carbon fiber-reinforced cement-based composites involved two-step dispersions of carbon fibers. Both steps affected greatly the mechanical properties of the composites. With the aid of ultrasonic wave, a new dispersant hydroxyethyl cellulose was used to help fiber dispersion in the first step. The fracture surface of the composites was observed by scanning electron microscopy. The distribution of major elements was analyzed by the energy dispersive spectroscopy and the composition was analyzed through X-ray diffraction. The flexural strength, tensile strength, modulus, and compression strength were measured. Results showed that the distribution of major elements varied with the variation of the fiber dispersion status. The compressive strength increased by 20%, the tensile strength was 2.4 times that of the material without carbon fibers, the modulus increased by 26.8%, whereas the flexure stress decreased by 12.9%.

Effect of chemical vapor deposition treatment of carbon fibers on the reflectivity of carbon fiber-reinforced cement-based composites

Short carbon fibers were treated through isothermal chemical vapor deposition technology at temperatures between 900 and 1200 °C. The fiber surface and the fracture surface of the carbon fiber-reinforced cement-based composite were observed by scanning electron microscopy. The influence of the fiber dispersion on the mechanical properties of the composites was investigated. The reflectivity of the composites against electromagnetic waves was measured in the frequency range of 8.0–18.0 GHz for carbon fiber contents of 0.2, 0.4, 0.6, 0.8, and 1.0 wt% by weight of cement. Results showed that before treatment, the reflectivity increased by 76.6% after the fiber content was over 0.4%. The minimum value was −19.2 dB and almost 98% of the incident wave was absorbed. After treatment, the reflectivity decreased by 66.7% and all the values were always above −10 dB. The minimum data was −8.1 dB and 72% of the incident wave was absorbed.

Smart Properties of Carbon Fiber Reinforced Cement-based Composites

In this study carbon fiber reinforced cement-based composites (CFRC) are fabricated by compressing, and the smart properties of the composites are studied. The results show that there exists a remarkable negative temperature coefficient (NTC) and a positive temperature coefficient (PTC) in the composite. The resistivities of the composites formed under 10 MPa are more sensitive to temperature than that of the composites formed under 2 MPa, and the critical temperature zone of the composites formed under 10 MPa (100-130°C) is narrower than that of the composites formed under 2 MPa (50-100°C). The fractional change in resistance of CFRC shows obvious one-to-one correspondence with cyclic load. The amplitude of the fractional change in resistance of the composites formed under 10 MPa is obviously larger than that of the composites formed under 2 MPa, which is more suitable for the application of important buildings such as bridges, dams, and so on to realize the real-time, dynamic and health monitoring, and damnification evaluation.

Characteristics of resistivity-temperature for carbon fiber reinforced concrete

The resistance response to temperature change of carbon fiber reinforced cementbased composites (CFRC) is reported, which shows some outstanding phenomena of positive temperature coefficient (PTC) of resistance and negative temperature coefficient (NTC) of resistance during the temperature rising. The influences of carbon fiber, cementbased matrix and thermal cycles on the characteristics of temperatureresistivity for the system were also discussed. Because of the special characteristics for temperatureresistivity, carbon fiber cement based composites can be useful in structure with the function of alarm for fire.