Properties of ceramic fiber reinforced cement composites

Mechanical properties and preliminary durability of ceramic fiber reinforced Portland cement composites tested with wet-hot accelerating method were investigated. The results showed that the flexural strength of mortar could be increased obviously by adding ceramic fiber into it, but the effect of the flexural reinforcement was influenced by various factors, including fiber length, fiber content and kinds of matrices; the preliminary durability of ceramic fiber in ordinary Portland cement tested with wet-hot accelerating method was much better than that of alkali-resistant (AR) glass fiber. The mechanism of the durability of ceramic fiber in ordinary Portland cement is also discussed.     

Cracking processes in steel fiber reinforced cement paste

Fracture patterns produced when a crack advanced from a notch in cement paste specimens reinforced with steel fibers were studied by SEM methods. The specimens were small compact tension specimens with precast notches that could be wedge loaded within the SEM chamber in a moist environment. Steel fibers were positioned either in an array of five parallel fibers spaced 2 mm apart and across the expected crack propagation path, or else were in random orientation on the plane being observed. The cracks induced by wedge loading were found to be geometrically complex and certainly could not be described as simple straight cracks as assumed in various models. On intersection with fibers oriented perpendicularly to them, the cracks tended to displace laterally and branch into a number of microcracks; on intersection with fibers at less than perpendicular angles, the tendency was for the crack to change course and run parallel to the inclined fiber. Often at perpendicular intersections the crack appeared to be arrested in the matrix 10 to 40 μm ahead of the actual fiber interface, and then produced a “pseudo-debonding crack” parallel to the fiber but some distance away from the actual interface. These cracking patterns are considered to be influenced by the microstructure of the cement paste near the interface, which is clearly different from that of the bulk cement paste.     

The effect of interfacial microstructure on the interfacial strength of glass fiber/polypropylene resin composites

In this study, a new method to form resin droplets on fibers has been developed, and samples for the single fiber pull-out test were prepared using this method. The effects of microstructure of polypropylene (PP) resin and the microstructure of interface between the glass fiber and PP resin on the interfacial strength have been investigated. In addition, the influence of the microstructure of the interface on the interfacial strength of glass fiberreinforced PP composites have been discussed. It has been found that in the pull-out test, the transcrystallinity formed at the interface between the glass fiber and PP resin improved the interfacial strength when no spherulites developed in the PP matrix. On the other hand, it has been found that when the spherulites were well developed in the PP matrix, the transcrystallinity formed at the interface reduced the interfacial strength. Finally, rapid cooling has been shown to improve the interfacial strength between the fiber and resin in the crystalline polymer matrix composites. © 1994 John Wiley & Sons, Inc.     

炭纤维增强水泥基复合材料(CFRC)的电磁性能

炭纤维增强水泥基复合材料(Carbon Fiber Reinforced Cement Composites,CFRC)是新发展起来的一种电磁屏蔽材料,它是防止电磁污染的防护性功能材料之一。本文阐述了炭纤维增强水泥基复合材料的制备成型工艺;分析了炭纤维掺入量和长度、水灰比和密实成型制备工艺、炭纤维分散性、养护龄期、外加剂、炭纤维表面化学气相沉积(CVD)处理等因素对CFRC力学性能、导电性能、压敏性能及电磁性能的影响。合适的炭纤维掺入量和长度、炭纤维的均匀分散、合理的水灰比和炭纤维表面处理是影响CFRC导电性能和电磁性能的主要因素。CFRC对电磁波的屏蔽效果除利用屏蔽效能从反射电磁波角度衡量外,亦可从吸收电磁波角度利用反射率进行评价。     

碳纤维和钢渣水泥基复合材料导电性及损伤识别研究

在水泥基复合材料中参入碳纤维和钢渣,以研究复合水泥基材的导电性能,结果表明,碳纤维能够显著改善水泥基材的导电性,碳纤维体积率为1.2%时,其导电性能最佳,且碳纤维对试件导电稳定性有显著影响;在碳纤维水泥基材中参入钢渣并不会显著影响试件导电性,会显著影响导电稳定性;抗压和抗折试验结果表明利用导电性判断构件是否损伤(开裂)及损伤程度在试验上是可行的。     

Measuring effective interfacial shear strength in carbon fiber bundle polymeric composites

Adhesion in composite materials is often quantified using the single fiber fragmentation (SFF) test. While this method is believed to provide accurate values for the fiber–matrix interfacial shear strength (IFSS), these may not accurately reflect the macroscopic mechanical properties of specimens consisting of tows of thousands of tightly spaced fibers embedded in a resin matrix. In these types of specimens, adhesion may be mitigated by fiber twisting and misalignment, differences in the resin structure in the confined spaces between the fibers and, most importantly, by any incompleteness of the fiber wetting by the resin. The present work implements fiber band fragmentation (FBF) testing to obtain effective interfacial shear strengths, whose values reflect the importance of these factors. The fiber fragmentation in these specimens is tracked through the counting and sorting of acoustic emission (AE) events occurring during the tensile testing of the specimen and yields the average critical fiber fragment length. AE results, in conjunction with stress-strain data, show that fiber breakage events occur at acoustic wavelet amplitudes substantially greater than those generated by fiber/matrix debonding. Kelly–Tyson analysis is applied, using the measured critical fiber fragment length together with known values for the fiber diameter and tensile strength to yield the effective IFSS. FBF tests are performed on carbon fiber/poly(vinyl butyral) (PVB) dog-bone fiber-bundle systems, and effective IFSS values substantially lower than those typically reported for the single fiber fragmentation testing of similar systems are obtained, suggesting the importance of multi-fiber effects and incomplete fiber wetting.     

Some statistical considerations of steel fiber composites

Because of the decisive influence of the fabrication method on the strength and durability of fiber composites, the design of fiber concrete elements should be based on statistical considerations of their properties. The paper presents a statistical analysis of compressive and flexural strength results of both plain and steel fiber concrete. Fiber concrete results in compression and flexure show larger standard deviations and greater departure from normal distribution. However, there is no need to test more specimens than that required for plain concrete, although the method of compaction should be specified for flexural strength design. Fibers increase compressive strength, and the difference observed from that of plain concrete is not due to chance. No relation exists between maximum load in flexure and the debonding behavior of the fibers.     

The ultrasound‐based interfacial treatment of aramid fiber/epoxy composites

The quality of interfacial adhesion of aramid/epoxy composites affects the mechanical performance of the material, and thus there is a need to improve the condition by using the ultrasound‐based interfacial treatment. To do so, an ultrasonic transducer has been developed and evaluated under various operational conditions when it is installed in the winding system. It has demonstrated several key characteristics such as low power, high amplitude (more than 80 μm), and continuous working (more than 8 h) without water‐cooling. Subsequently, experiments were carried out to determine the mechanical performance of the polymer material with and without ultrasound treatment, showing that the ultrasonic treatment has improved the interfacial performance up to 10%, compared with those without any ultrasound‐treatment. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08 µW m⁻¹ K⁻² and higher thermoelectric figure of merit 3.11×10⁻³, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407 eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (10¹⁴–10¹⁶ cm⁻³) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.     

Electromagnetic interference shielding reaching 70 dB in steel fiber cement

An electromagnetic interference (EMI) shielding effectiveness of 70 dB at 1.5 GHz has been attained in cement paste that contains 0.72 vol.% stainless steel fibers of diameter 8 μm and length 6 mm. The shielding is primarily by reflection. The material exhibits electrical resistivity 16 Ω cm. The presence of sand essentially does not affect the shielding effectiveness. The fibers remain effective in the presence of steel rebars. For comparison, the shielding effectiveness of a solid piece of stainless steel is 78 dB at 1.5 GHz.     

Degradation mechanisms of natural fiber in the matrix of cement composites

The degradation mechanisms of natural fiber in the alkaline and mineral-rich environment of cement matrix are investigated. Cement hydration is presented to be a crucial factor in understanding fiber degradation behavior by designing a contrast test to embed sisal fibers in pure and metakaolin modified cement matrices. In addition to durability of sisal fiber-reinforced cement composites determined by means of flexural properties, degradation degree of the embedded fibers is directly evaluated by proposing a novel separation approach. The results indicate that, by reducing alkalinity of pore solution, metakaolin effectively mitigates the deterioration of natural fiber. By combining results of thermogravimetric analysis and microstructure, the alkali degradation process of natural fiber, which consists of hydrolysis of lignin and hemicellulose, stripping of cellulose microfibrils and deterioration of amorphous regions in cellulose chains, is visually presented. Two new concepts of mineralization mechanism, calcium hydroxide (CH)-mineralization and self-mineralization, are also proposed and quantitatively characterized.     

Studies of jute fiber composites with polyesteramide polyols as interfacial agent

Polyesteramide polyols have been synthesized by melt condenstion using a mixture of alkanolamines, polyethylene glycols, and dicarboxylic acids/anhydrieds, and the behavior or resin samples as interfacrial agents in unidirectional as well as random composites of jute/epoxy and jute/polyester has been evaluated. Mechanical properties of these composties with or without interfacial agents have been determined along with the effect of water uptake on such properties. The incorporation of polyesteramide polyol (PEAP) resins as interfacila agents has been found to significantly improcve the mechanical properties of jute fiber composites. It has also been found that increasing the hydroxyl value of PEAP results in a better bonding of the composities up to a certain optimum limit of hydroxyl value beyond which the molecular weight of the interfacila agent as well as its bonding strenght decreases. Use of PEAP resin of optimum hydroxyl value and molecular weight also significantly improves the water resistance capacities of jute/epoxy composites.     

The transfer of stress through a steel to concrete adhesive bond

This paper describes part of a programme of research aimed at investigating the potential for strengthening reinforced concrete beams in shear by means of externally bonded steel plates. This may be a useful strengthening technique following the assessment of older bridge and building structures designed to outdated codes of practice. In order to produce a design guide for such shear plate bonding, a method for determining the anchorage length needs to be devised. By measuring the strain distribution in a steel plate adhesively bonded to a concrete block, the shear stress distribution within the adhesive and the effective anchorage length can be determined. A series of 15 experimental tests have been conducted to investigate the transfer of stress through a steel-concrete adhesive bond. The experimental programme was supported by theoretical and finite element analysis. The shear stress in a steel-concrete adhesive bond was found to be distributed exponentially, peaking at the loaded end of the specimen. For the specimens used, the stress distribution was distributed over a length of up to 155 mm for serviceability loads.     

Effect of molecular weight and fiber diameter on the interfacial behavior in glass fiber/PP composites

In this study, the effects of fiber diameter, molecular weight of the matrix polymer, and interfiber spacing in glass fiber-reinforced polypropylene composites were investigated on the interfacial microstructure. The influences of the surface state of the fiber and the heat-treatment condition on the interfacial morphology and the spherulitic formation process in the matrix were also investigated. Consequently, it was found that both the fiber diameter and molecular weight of the polymer significantly influence the thickness of the transcrystalline layer. Also, as the interfiber spacing becomes smaller, the spherulites in the matrix polymer are not seen to be formed between the transcrystalline layers developed on the glass-fiber surface. In addition, the radius of the largest spherulites in the matrix polymer was found to be about the same as the thickness of transcrystalline region and to largely depend on the holding time at the crystallization temperature and cooling condition (or rate). © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 67:1191–1197, 1998     

Enhanced thermoelectric effect of carbon fiber reinforced cement composites by metallic oxide/cement interface

Thermoelectric power of carbon fiber reinforced cement composites was firstly enhanced efficiently by metallic oxide microparticles in the cement matrix. The absolute Seebeck coefficient of these composites increased steadily with increasing metallic oxide content and achieved 4–5 folds of the original one. The largest absolute thermoelectric power of +100.28 µV/°C was obtained for the composite with 5.0 wt% Bi₂O₃ microparticles. The carrier scattering of the interface between oxide microparticles and cement matrix is probably attributed to the Seebeck effect enhancement.     

Origin of the thermoelectric behavior of steel fiber cement paste

Carrier scattering dominates the origin of the Seebeck effect in steel fiber cement. The scattering sites include the fiber-matrix interface, which is like a pn junction, since the fiber and cement paste have opposite signs of the absolute thermoelectric power. The scattering results in positive and negative values of the absolute thermoelectric power, depending on the fiber content.     

Improvement of interfacial properties in bismaleimide composites using functionalized graphene oxide grafted carbon fiber

A hierarchical reinforcement, which was used to improve the interfacial properties of bismaleimide (BMI) composites, was prepared by grafting functionalized graphene oxide (GO) onto a carbon fiber surface. The GO and carbon fibers were first functionalized separately to create interactional functional groups on their surfaces. The grafting process was then realized by an amidation reaction of the amine and acyl chloride function groups formed on GO and carbon fibers, respectively. The surface groups of functionalized GO and carbon fibers were identified by an X‐ray photoelectron spectroscopy (XPS). The resulting reinforcement was further characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and dynamic contact angle analysis. Experimental results showed that the functionalized GO were successfully grafted onto the carbon fibers surfaces and significantly increased the surface energy of carbon fibers. The study also indicated that the prepared hierarchical reinforcement could significantly improve the interfacial adhesion of resulting BMI composite. POLYM. ENG. SCI., 58:886–893, 2018. © 2017 Society of Plastics Engineers     

The effect of loading rate on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present work, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) under the loading rate of 1, 10, and 100 mm/min are studied by experimental methods. The interfacial mechanical properties of hybrid composites are determined by monotonic and cycle loading-unloading single-lap-shear tests. The experimental results reveal that interfacial shear strength increases with loading rate, while the shear stiffness shows insensitive to loading rate. A regression function is built to describe the variation of interfacial shear strength with loading rate. The cyclic loading-unloading curves of hybrid composites samples indicate that loading rate effects on the interfacial nonlinear behavior of hybrid composites are considered by affecting plastic deformation. In addition, scanning electron microscopy and digital image correlation observations reveal the failure mechanisms of overmolded hybrid composites. The failure behavior of overmolded hybrid composites is mainly CFRT laminate failure for all cases. The evolution of non-uniform strain fields indicates that the fracture of overmolded thermoplastic composites may initiate at the edges and spread out to the far fields.     

Interfacial phenomena in steel fibre reinforced cement I: Structure and strength of interfacial region

This paper describes a study of the structure, composition and strength of cement paste near the cement-steel interface of an embedded wire. Scanning electron microscopy of paste and mortar specimens shows that the interface does not change appreciably for curing times greater than a week and suggests enrichment of the interface with Ca(OH)₂. X-ray diffraction provides evidence that this enrichment amounts to 20% to 40% of the concentration of Ca(OH)₂ in the bulk of the cement paste. The strength of the cement paste, deduced from indentation hardness measurements, shows that there is a marked decrease in strength within 0.75 mm from the wire surface. This appears to be a general feature of cement paste composites prepared with vibration.     

Dependence of interfacial strength on the anisotropic fiber properties of jute reinforced composites

The upsurge in research on natural fiber composites over the past decade has not yet delivered any major progress in large scale replacement of glass fiber in volume engineering applications. This article presents data on injection‐molded jute reinforced polypropylene and gives a balanced comparison with equivalent glass reinforced materials. The poor performance of natural fibers as reinforcements is discussed and both chemical modification of the matrix and mercerization and silane treatment of the fibers are shown to have little significant effect on their level of reinforcement of polypropylene in comparison to glass fibers. A hypothesis is proposed to explain the poor performance of natural fibers relating their low level of interfacial strength to the anisotropic internal fiber structure. POLYM. COMPOS., 31:1525–1534, 2010. © 2009 Society of Plastics Engineers