mohammad zarrebini

Fiber strands due to their flexibility, high aspect ratio, cross-section varieties and degree of crystallinity are adequately strong to be used as reinforcement in composites such as concrete. Newly introduced fiber reinforced concretes (FRC) are the cementitious materials that exhibit reinforcing features in all directions. FRCs due to their interesting properties are enormously favored by civil and structural engineers. Natural and synthetic fibers can be employed in concretes, shutcretes and mortars. The interface between the added fibers and the cementitious matrix fundamentally influences the properties of the FRCs. Fibers are classified into hydrophobic and hydrophilic. The former fibers have negligible moisture absorbent capacity while exhibiting acceptable mechanical properties. Hydrophobic fibers are incapable of forming adequate adhesion with cementitious matrix. Properties such as low weight, strength parity in wet or dry conditions and inertness in acid or alkaline environments are among the salient properties of polypropylene (PP) fibers. PP as a hydrophobic fiber has gained wide acceptance as concrete reinforcement. The hydrophobicity of fibers, such as PP, has been always been disadvantageous for the use of these fibers in concrete structures. Treatments such as chemical surface modification imparts hydrophilic property to PP fibers. Thus the modified PP fibers can successfully adhere to concrete matrix. In this research melt-spinning technology as the most widely used manufacturing technique for production of the PP fibers was used. Pure and grafted anhydride maleic PP granules were used to produce both hydrophobic and hydrophilic PP fibers. The produced fibers were characterized according to relevant standards prior to be added to concrete samples at identical fiber volume fraction. The results pointed to the positive effect of the induced hydrophilic properties in the fibers as far as the fiber-matrix adhesion was concerned. The ability of the chemically modified fibers to absorb water when wetted with the moisture present in the concrete, greatly improved the adhesion of the added fibers with the concrete matrix. The effect of hydrophilicity of PP fibers on mechanical properties of reinforced concrete was investigated by comparing concrete samples prepared by modified and unmodified fibers. Results showed that in comparison to control concrete sample, addition of modified hydrophilic fibers to concrete enhances compressive, tensile and flexural strength of concrete by 11%, 43% and 75% respectively. It was found that compressive, tensile and flexural strength of concrete samples containing the chemically modified fibers were respectively higher by 5%, 8% and 5% in comparison to the concrete samples containing unmodified hydrophobic fibers. Addition of fibers is more effective in enhancement of flexural strength of resultant concrete. This is due to the fiber bridging phenomena that prevent both crack formation and propagation. Addition of fibers also improves load bearing capacity of the resultant concrete, which in turn leads to enhancement of flexural strength of the concrete. Results also showed that addition of hydrophobic polypropylene fibers leads to 66% increase in the flexural strength of the samples. The increase in flexural strength of the concrete samples containing hydrophilic fibers in comparison to the control sample was found to be 75%.

Energy absorption capacity is the salient property of concrete. In this work the effect of cross -sectional area of melt spun polypropylene fibers together with that of fibers diameter on behavior of fiber reinforced concrete was investigated. Variation on fiber dimensions was achieved by varying fiber spinning draw ratio and speed of melt feeding pumps. Variations in fiber diameter affect fiber specific area which in turn influences the adhesion of the fibers to the matrix in FRC. In order to evaluate the FRC behavior, the area under load versus displacement was studied. It was found that energy absorption capacity excessively increases as the cross-sectional area of the fibers decreases. The increase in the energy absorption was at least 6 times of that of reference specimen. It is while in the concrete mix containing the fibers with the least cross-sectional area, the increase in the energy absorption was 14 times the corresponding value in the reference mix. This was concluded to be due to the increase in contact area between the finer fibers and the matrix due to increase in number of fibers for a given fiber volume fraction in the concrete. Results also showed that the increase in energy absorption capacity of the FRC leads to higher workability of the concrete after appearance of the initial crack and ultimate failure of the concrete. Finally it was observed that load bearing capacity of the samples prior to disintegration was high.

Using of polymeric fibers for reinforced concrete structures has significantly developed in recent years. Polymeric fibers start their contribution in the behavior of concrete members after cracking. In order to a careful investigation into post-cracking behavior need to apply fracture mechanic concept (growth of cracks) in reinforced concrete members. For this purpose, in this research 14 concrete prisms (100×100×400 mm in dimensions) with 35 mm notch depth (at the center of tensile side) for four-point flexural strength test were fabricated. Fiber composition, geometry and hybridization percent were varied in these samples. Derived outputs illustrated that macro polypropylene (PP) fiber has no significant effect on concrete ductility, whereas it leads to increase the flexural strength. But micro polyester (PET) and PP fibers have more effective performance during forming cracks in concrete members. PET and PP fibers have a more suitable function during concrete cracking and the samples containing these fibers have no significant drop in their bearing while the cracking is started. In addition, samples reinforced with PP and PET fibers indicated that by increase in macro fibers, the flexural strength were increased where as ductility indices decreased. In general, samples reinforced with %60 of PP macro fibers and %40 PET micro fibers have the best performance.

Needle-Punching process is a mechanical method by which entanglement of fibrous webs can be enhanced. Fiber entanglement together with fabric weight can be held responsible for the variations in the mechanical properties of nonwovens. In this work, mean fiber orientation angle and fabric weight measurements were used for prediction of bending and tensile properties of needlepunched fabrics. Samples of needled nonwoven fabrics in four groups with different fabric weights were produced using appropriate carding/cross-lapping machines. The samples of each weight group were needled at various punch densities and needle penetration depths. The orientation of the fibers in the samples was determined using Radon transform method. Bending stiffness and breaking strength of the samples in both machine-direction and cross-machine direction were determined using two simply supported beam system and strip method, respectively. The results indicated that an increase in the amount of punch-density generally leads to variation in orientation angle of the fibers in the samples. It was observed that an increase in the amount of punch-density led to an increase in mean fiber orientation angle in all samples along the machine-direction. This is due to the displacement of the fibers by the needling process. Additionally, fiber orientation pattern of the samples enormously changed due to the changes in needle penetration depth. Multiple regressions based on least square method were used to estimate the mechanical properties of needlepunched fabrics. The results pointed to the paramount importance of the degree of fiber orientation and fabric weight as two influential factors controlling the mechanical properties of needle punched fabrics.

This paper reports on the effect of fiber crimp frequency on sound absorption capability of staple polypropylene nonwoven batts. Stuffer box was used to impart crimp to spun tow. Crimping of the tow renders the fibers the required textile applicability. In this work, polypropylene batts composed of staple fibers with linear densities of 9, 14, and 18 denier were employed. Three crimp frequency namely low, medium and high were imparted to fibers of each denier group. Impedance tube method with sound frequencies in the range of 250 – 4000 Hz was employed to measure the sound absorption coefficient of the batts. The results showed sound absorption properties of fibrous batts were affected by fiber crimp frequency, fiber fineness and web thickness. The highest sound absorption coefficient for the 9, 14 and 18 denier batts was 78.90, 77.14 and 71.18, respectively. The crimp frequency of the fibers making these batts was 1.9, 2.3 and 3.6 crimp per cm, respectively. It was found that higher crimp frequency along the fibers of the batts lead to higher sound absorption capacity. Finally, the highest sound absorption coefficient was recorded for the web with finest fibers and highest crimp frequency.

Random-velour needling technology is a modified version of conventional needling process. Properties of the random-velour needled fabric are controlled by the structural alteration that occurs during random-velour needling, is due to re-orientation of fibers within the pre-consolidated fibrous assembly by special fork needles. This interaction results in creation of a dual structure, comprising base and pile layers. In this work the effect of needling parameters and fiber characteristics on force exerted on the fork needle was investigated. The effect of principal parameters on total average force «Frms» exerted on individual fork needle was determined using an Artificial Neural Network (ANN) modeling and the error percentage of absolute average of predicted tests data was also calculated. Significance percentages of input parameters on «Frms» was indicative of the similar influence of fiber characteristics and needling parameters on «Frms». Results manifested the importance of punch density and barbed needle penetration depth during initial consolidating needle-felting operation. Result of the neural network assessment testified that the network in general was capable of mapping input and output parameters.