Alva Peled

Pultruded Fabric-Cement Composites

The use of reinforcement in thin cement-based elements is essential to improve the tensile and flexural performance. The reinforcements can be either short fibers or continuous reinforcements, in a fabric form. Practical use of fabric-cement composites requires an industrial, cost-effective production process. The objective of this study was to develop the pultrusion technique as an industrial, cost-effective production method of prefabricated thin-sheet fabric-reinforced cement composites. Woven fabrics made from low-modulus polyethylene and glass meshes were used to produce the pultruded cement composites. The influence of fabric type, cell opening, application of pressure during casting, and cement-based matrix modification were examined. The tensile strength and ductility of the pultruded fabric-cement components were found to be relatively high, exhibiting strain hardening behavior even for fabrics with low modulus of elasticity. The best performance was achieved from glass fabric composites with a high content of fly ash. The mechanical properties were significantly affected by the matrix formulation, rheology of the matrix, and the intensity of the pressure applied after the pultrusion process. The promising combination of fabric reinforcement in cement composite products using the pultrusion process is expected to lead to a new class of high-performance fabric-cement composite materials.

Geometrical characteristics and efficiency of textile fabrics for reinforcing cement composites

One of the most efficient ways to obtain a high performance cementitious composite is by reinforcement with continuous fibers. Production of such composites can readily be based on the use of textile fabrics, which are impregnated with cement paste or mortar. The present paper discusses the bulk properties and geometrical characteristics of textile fabrics that should be considered in order to predict the performance of cement composites reinforced with fabrics. Geometrical characteristics are the nature of the basic reinforcing unit in the fabric (yarn) and the various geometries by which these yarns are combined together in the fabric (weft insertion warp knitted, short weft warp knitted, and woven fabrics). It was found that the geometry of a given fabric could enhance the bonding and enable one to obtain strain hardening behavior from low modulus yarn fabrics. On the other hand, variations of the geometry in a fabric could drastically reduce the efficiency, resulting in a reduced strengthening effect of the yarns in the fabric relative to single yarns not in a fabric form. The improved bonding in low modulus yarn was found to be mainly the result of the special shape of the yarn induced by the fabric. Therefore, in cement composites, the fabrics cannot be viewed simply as a means for holding together continuous yarns so that they can be readily placed in the matrix.

Fabric structure and its reinforcing efficiency in textile reinforced cement composites

In polymer matrices reinforced with fabrics, the effectiveness of the reinforcement is reduced when the yarns do not maintain a straight geometry. In cement composites, this concept may not be adequate since the nature of the interaction between the cement matrix and the fabric and its individual yarns is more complex, as concluded from pullout tests. The present paper discusses the bulk properties and geometrical characteristics of textile fabrics that need to be considered in order to predict the performance of cement composites reinforced with textile fabrics. It was found that the geometry of a given fabric could enhance the bonding and enable one to obtain strain hardening behavior from low modulus yarn fabrics, due to the special shape of the yarn induced by the fabric. On the other hand, variations of the geometry in a fabric could drastically reduce the efficiency, resulting in a lower strengthening effect of the yarns in the fabric, relative to single yarns not in a fabric form. Therefore, in cement composites the fabrics cannot be viewed simply as a means for holding together continuous yarns to be readily placed in the matrix, as is the case in composites with polymer matrix.

Analysis of the impedance spectra of short conductive fiber-reinforced composites

The presence of small amounts of short conductive fibers in a composite of finite matrix conductivity results in the subdivision of the one matrix impedance arc into two separate low and high frequency arcs in the complex impedance plane. These features are attributable to a “frequency-switchable” interfacial impedance on the fiber surfaces, rendering them insulating at DC and low AC frequencies, but conducting at intermediate frequencies. A combination of physical simulations (single wires in tap water) and pixel-based computer modeling was employed to investigate the roles of fiber pull-out, debonding, and orientation on the impedance response of fiber-reinforced composites. The ratio of the low frequency arc size to the overall DC resistance (γ-parameter) is sensitive to pull-out and/or debonding, especially when a fiber just barely makes contact with the matrix. The γ-parameter is also quite sensitive to fiber orientation with respect to the direction of the applied field. Ramifications for the characterization of cement, ceramic, and polymer matrix composites are discussed.

Electrical impedance spectra to monitor damage during tensile loading of cement composites

Conductive fibers can reinforce concrete and monitor damage leading to the development of smart material. This research studied the correlation between the electrical and mechanical properties of cement composites reinforced with conductive carbon fibers. The tensile behavior and impedance behavior of extruded and notched composites with a fiber volume fraction of 0.5 and 3% were examined; mechanical load and electrical field were applied longitudinally. The crack growth of these composites during loading was observed and analyzed by digital image correlation. Impedance spectroscopy (IS) measurements were made under loaded and unloaded conditions to address the effect of specimen geometry, the manufacturing process, and the effect of fiber volume fraction. Using these IS measurements, along with numerical computation, the bridging area of the fibers could be extracted quantitatively from the tensile measurements. It is shown that such methods can be useful to elucidate the role of the reinforcing fibers during fracture.

Effects of Woven Fabric Geometry on the Bonding Performance of Cementitious Composites: Mechanical Performance

Abstract The effect of the geometry of woven fabrics on the bond between monofilament polyethylene yarns and cement matrix was studied in the present work. The fabrics were all plain weave, with varied fills density: 5, 7, or 10 fills per cm; the warps’ density was kept constant at 22 warps per cm. The interfacial bond was evaluated by pullout tests. To characterize the influence of the fabric’s geometry on bond performance, the influence of different parameters of the fabric’s geometry that may affect bond were separated: (1) pullout of a single crimped yarn untied from the fabric to characterize the influence of the shape of the individual crimped yarn; (2) pullout of a single yarn from free fabric (not embedded in the cement matrix); and (3) pullout of a yarn from a fabric embedded in the cement matrix. Straight yarns were also tested for comparison. It was found that the woven fabric provided a considerably better bond to the cementitious matrix than the bond of a single straight yarn. The crimped geometry of the yarn in the fabric was found to have a significant influence on increasing the bond between the woven fabric and the cementitious matrix.

Parameters related to fiber length and processing in cementitious composites

Effects of fiber length on the tensile and flexural performance of cast and extruded PVA fiber reinforced cement composites were investigated. Microstructural characterization, image analysis, and statistical tools were used to study the influence of processing and fiber length on fiber-matrix bond, fiber dispersion and fiber orientation in the composites. In the extruded composites, shorter fibers improved the performance. In the cast composites, longer fibers gave the best performance. This contradictory trend was found to be a result of differences in fiber failure mechanism, fiber distribution and fiber orientation. Microstructural observations indicated a strong matrix-fiber bond for the extruded composites. Statistical quantification of image analysis indicated a better distribution and alignment of shorter fibers in extruded composites.RésuméLes effets de la longueur des fibres sur les performances de tension et de flexion des composites moulés et des composites extrudés, à base de ciment renforcé et de fibres d’acétate de polyvinyle, ont été étudiés. La caractérisation microstructurale, l’analyse d’images et des outils statistiques ont été utilisés pour étudier l’influence du traitement et de la longueur des fibres sur le lien fibres-matrices, la dispersion et l’orientation des fibres dans les composites. Dans les composites extrudés, des fibres plus courtes ont amélioré les performances. Dans les composites moulés, des fibres plus longues ont engendré de meilleures performances. Cette tendance contradictoire s’est avérée être le résultat de différences dans le mécanisme de rupture des fibres, la distribution des fibres et l’orientation des fibres. Les observations microstructurales ont indiqué un lien matrice-fibres important pour les composites expulsés. La quantification statistique de l’analyse d’images a montré une meilleure répartition et un meilleur alignement des fibres plus courtes dans les composites extrudés.

HIGH CONTENT OF FLY ASH (CLASS F) IN EXTRUDED CEMENTITIOUS COMPOSITES

This paper presents mechanical properties and durability of extruded fiber-reinforced cement composites that contain a high percentage of Class F fly ash. The fly ash was examined as a replacement for cement in a plain cement matrix and composites containing polyvinyl alcohol, glass, acrylic (PAN), polypropylene, or cellulose fibers. Accelerated aging was used to study the effect of fly ash on the durability of the different systems. It was shown that fly ash improves the flexural performance of the extruded composite. The extent of this improvement varied with fiber type. PAN, glass, and cellulose fibers were most affected by the use of fly ash. The greatest improvement in both flexural strength and ductility was obtained for 28-day moist-cured composites containing PAN fibers when as much as 70%, by volume, of the cement was replaced with fly ash. In this case, a five-fold increase in toughness and an approximately 20% improvement in strength were observed for steam-cured composites.

ENHANCED BONDING OF LOW MODULUS POLYMER FIBERS-CEMENT MATRIX BY MEANS OF CRIMPED GEOMETRY

Abstract Low modulus polymeric yarns (fibers) can be used as primary reinforcement in thin sheet cement products if their bonding to the matrix can be made sufficiently high. Straight yarns usually have a low bond due to the hydrophilic nature of the yarn and its low modulus which does not allow for sufficient clamping stresses to develop and enable effective frictional bond. In the present paper the potential of modifying the shape of the yarn to achieve a crimped geometry was studied to enhance its bond resistance. Such geometry can be achieved in the production of individual yarns and it is the geometry which exists in woven fabrics. The crimped yarns investigated here were obtained by untying yarns from woven fabrics. The fabrics were produced especially for this work to achieve controlled geometry of the yarn, which was characterized in terms of the wave length and amplitude of the crimped shape. The bonding performance was characterized by pull-out tests of the crimped yarns. The crimped shape enhanced the bonding considerably, and the pull out resistance was found to be a linear function of the product of the wave amplitude of each crimp and the number of waves (i.e., yarn length/wave length) along the yarn. The dominant bonding mechanism was mechanical anchoring.

Atomic Force Microscopy Examinations of Mortar Made by Using Water-Filled Lightweight Aggregate

This paper discusses the results of a research program that studied the influence of internal curing on the microstructure of mortars. Internal curing uses water-filled lightweight aggregates (LWAs) to supply additional water to the cement paste as it hydrates, thereby enabling an increase in the degree of hydration. The increased hydration can result in the densification of the microstructure. In particular, the densification occurs at the interfacial zone around the LWAs. The current work attempts to obtain a better understanding of the beneficial effects of internal curing on the basis of experimental observations of the microstructure and the nanostructure. The objective of this study was to examine the differences and the similarities that exist at both the micro-scale and the nanoscale of conventionally cured mortars and internally cured mortars. The specimens were tested at different ages to examine the influences of the internal curing over time. Water sorption, scanning electron microscopy, and scanning atomic force microscopy were used for this study. It was found that LWAs can be used for internal curing to provide a greater degree of hydration in a small region around the aggregate interface, which results in a microstructure that is more dense and more homogeneous and that contains less calcium hydroxide.