M. Jamshidi

Investigation on polymeric fibers as reinforcement in cementitious composites: Flexural performance

The objective of this article is to evaluate the effect of some synthetic polymeric fibers on the flexural behavior of cement-based matrix. On this basis, polypropylene (PP), nylon 6,6 (N66), and polyacrylonitrile (PAN) fibers were selected and characterized by optical microscopy, tensile test, aging in chemical solution, surface free energy testing, and scanning electron microscopy. The flexural behavior of fiber-reinforced cementitious composites was determined using a three-point bending test. N66 and PAN fiber reinforced composites had higher toughness and maximum flexural strength, which were in line with the results of surface free energy of the fibers.

Estimation of dynamic adhesion from static test results in the model cord–RFL–rubber system

Generally, nylon and polyester cords are used to reinforce rubber compounds. These composites are used in many sectors, such as tire and belt manufacturing. To increase adhesion performance a resorcinol–formaldehyde–latex (RFL) adhesive is applied on the cord, which bonds chemically to both cord and rubber and, thus, it improves both the thermodynamic work of adhesion and the loss function at the cord/rubber interface. Adhesion strength between the cord and rubber determines the performance of the system. So to study the performance of the cord–rubber system, adhesion strength must be evaluated. Cord–rubber adhesion strength can be evaluated in static and dynamic modes. The H-Pull (H-adhesion) test method is a static and relatively simple method that is usually employed to control raw material quality. Fatigue test is one of dynamic adhesion test methods that are used to determine the performance of cord–rubber interface. Some important factors such as cyclic stress and heat buildup are involved in this test procedure. To investigate the accuracy of the H-Pull test results, the cord–rubber samples were prepared using poly(ethylene terephthalate) (PET) cord and NR/SBR rubber. Then H-adhesion was determined at elevated temperatures. The adhesion strength was also evaluated in dynamic (fatigue) mode at different temperatures. Authors have proposed an equation to estimate dynamic adhesion from H-Pull test results.

Application of low modulus polymeric fibers in engineered cementitious composites

Nowadays, the advantages of staple fibers applied as reinforcement in cementitious composites are well known. The fiber to cement interfacial interactions influences mechanical properties of composites. Engineered cementitious composites are cement-based composites which are made of cement binder, small size sands, fillers, high modulus fibers, and supplementary cementing materials. They have improved tensile and flexural properties in comparison to normal concretes. To achieve these properties, high modulus fibers such as carbon, steel, and polyvinyl alcohol fibers have been used in engineered cementitious composites. In this research, low modulus polymeric fibers such as nylon 66, acrylic, and polypropylene were used as substitute of high modulus reinforcing fibers in engineered cementitious composite. The low modulus fibers were characterized carefully for physical–mechanical properties. The flexural behavior (flexural strength and flexural toughness) of the engineered cementitious composite specimens from this article was studied using a three-point bending test method. The results were compared to engineered cementitious composite containing polyvinyl alcohol. It was found that low modulus fibers caused considerable improvement in flexural behavior but results were lower than composites containing polyvinyl alcohol fiber. It was also found that these fibers are suitable choices for producing low price, acceptable performance engineered cementitious composites for usual applications in construction industry.

Mechanical performance and capillary water absorption of sewage sludge ash concrete (SSAC)

Disposal of sewage sludge from waste water treatment plants is a serious environmental problem of increasing magnitude. Waste water treatment generates as much as 70 g of dry solids per capita per day. Although one of the disposal solutions for this waste is through incineration, still almost 30% of sludge solids remain as ash. This paper presents results related to reuse of sewage sludge ash in concrete. The sludge was characterised for chemical composition (X-ray flourescence analysis), crystalline phases (X-ray diffraction analysis) and pozzolanic activity. The effects of incineration on crystal phases of the dry sludge were investigated. Two water/cement (W/C) ratios (0.55 and 0.45) and three sludge ash percentages (5%, 10% and 20%) per cement mass were used as filler. The mechanical performance of sewage sludge ash concrete (SSAC) at different curing ages (3, 7, 28 and 90 days) was assessed by means of mechanical tests and capillary water absorption. Results show that sewage sludge ash leads to a reduction in density and mechanical strength and to an increase in capillary water absorption. Results also show that SSAC with 20% of sewage sludge ash and W/C = 0.45 has a 28 day compressive strength of almost 30 MPa. SSAC with a sludge ash contents of 5% and 10% has the same capillary water absorption coefficient as the control concrete; as for the concrete mixtures with 20% sludge ash content, the capillary water absorption is higher but in line with C20/25 strength class concretes performance.

The effect of hybridization and geometry of polypropylene fibers on engineered cementitious composites reinforced by polyvinyl alcohol fibers

In the present study, the hybridization effect of polypropylene (PP) with polyvinyl alcohol (PVA) fibers in engineered cementitious composites (ECCs) was investigated. For this purpose, PVA reinforcing fiber was partially replaced by PP fibers with different circular, triangular, and trilobal cross-sectional shapes. The flexural behaviors of the ECCs containing hybrid fibers including first-crack strength, post-crack strength, and toughness were studied under three-point bending test. It was found that hybridization with nonround cross-sectional shape PP fibers had positive effect on the ductility but decreased the flexural strength of resultant ECCs. The greater reduction in toughness and post-peak strength was observed for composite containing circular PP fibers. The results indicated that partially replacement of PVA fibers with nonround cross-sectional shape PP fibers can be considered to be promising method to reduce the costs of ECC production along with attaining improved deformability.

Polymeric fibers pull-out behavior and microstructure as cementitious composites reinforcement

Bonding strength is an important factor which affects the performance of fiber reinforced cementitious composites. The present work describes the bonding strength between three polymeric fibers (polypropylene (PP), nylon6,6 (N66), and acrylic (AC)) embedded in a cement paste. Also, the pull-out behavior of fibers from cement matrix along with their tensile behavior was studied. The specimens were tested after 7, 14, and 28 curing days with cement to water ratio of 0.5. It was found that although the higher bonding strength to the cement matrix was achieved for AC and PP fibers, the energy absorption capacity of the fibers during composite fracture is obtained for fiber with lower bonding to the cement matrix (N66). Scanning electron micrographs were used to characterize the fiber surface before and after the pull-out tests.

Study on temperature dependence of loss function in a model cord–adhesive–rubber system

Energy dissipation (loss function) measurements for characterizing adhesion using the H-pull test have been applied to the tire cord/rubber composite system and the relationship between the loss function and test temperature was also investigated. Test samples consisted of commercial nylon 66 tire cord, commercial resorcinol–formaldehyde–latex (RFL) adhesive system and NR/SBR rubber compound. The H-adhesion of cord/rubber samples decreased with increasing test temperature. The surface tensions of RFL-coated cord and rubber were determined using Young and Fowkes equations. Also the interfacial tension was determined using the harmonic-mean equation. Then the thermodynamic work of adhesion (W) was determined using these data. Finally the loss function (ϕ) of the adhesive was determined using the Gent equation. When the cord–rubber interface was heated, the loss function decreased. We have proposed a Weibull model equation which describes the temperature dependence of the loss function in a model cord/RFL/rubber system.

Ductility improvement of cementitious composites reinforced with polyvinyl alcohol-polypropylene hybrid fibers

This paper investigated the hybridization effect of two chemically different fibers on the flexural behavior of cementitious composite. On this basis, polyvinyl alcohol and polypropylene fibers (polyvinyl alcohol/polypropylene fiber ratios: 75/25%, 50/50%, 100/0% and 0/100%) at different fiber volume fraction contents (1.2% and 2%) were considered as the variables. This study is especially focused on the influence of fiber types and their hybridization on composite deformability. The composite samples are subjected to the three-point bending test after 28 days of curing. The results showed that the fibers increased the flexural strength and ductility of cement matrix. It is revealed that the toughening improvement mechanism of polyvinyl alcohol and polypropylene fibers in cementitious composites are extremely different. Hybridization of polyvinyl alcohol and polypropylene fibers caused no significant improvement on flexural strength, but the strain capacity of composite under flexural load was increased. Finally, it was observed that the replacement of polyvinyl alcohol fiber with 25% volume fraction of polypropylene fibers can be considered as an important method for improving ductility of engineered cementitious composites.

Correlation between water permeability of latex-modified concrete (LMC) and water diffusion coefficient of latex film

Cement-based materials are generally known as weak materials in flexure and tension in comparison with compression. Polymers are used in cement-based materials to improve their flexural and tensile behaviors. The composite is called as polymer-modified concrete/mortar. Furthermore, polymers decrease permeability of water into cementitious matrices. Polymers are usually used as admixtures in concretes in form of latexes. Latexes are water-based polymers, which are consistent with water-based concrete matrices. On this basis, these kinds of products are called latex-modified concretes (LMCs). However, it has been found that chemical composition, particle size distribution, molecular weight, physical/mechanical properties of latexes affect performance of modified concretes. In this investigation, six latexes in three categories (acrylic, SBR and polyvinyl acetate) were used as concrete admixtures. They were characterized for chemical composition (by FTIR analysis), minimum film formation temperature, pH, glass transition temperature (Tg), particle size and particle size distribution to evaluate the effect of each property on LMC performance. Due to the formation of latex film in the microcracks and pores of concrete microstructure, it was suggested that diffusion of water into films controls permeability of whole concrete structures. On this basis, the diffusion coefficient of the latex films subjected to water was measured using a new method (continuous FTIR analysis). Capillary water absorption test was performed on concrete specimens to verify validity of the suggestion. It was found that there is a correlation between capillary water absorption of LMCs and water diffusion coefficient of latex films.

Investigation on pullout behavior of different polymeric fibers from fine aggregates concrete

Nowadays, the advantages of staple fibers applied as reinforcement in cementitious composites are well known. The interaction of fiber to cement matrix affects on the mechanical properties of composites. In this research, bonding of three types of polymeric fibers (acrylic, polypropylene, and nylon 66) to a fine aggregates concrete has been studied. Single fiber pullout test was carried out to investigate the bonding and fiber–matrix adhesion. Afterwards, scanning electron microscopy was performed to characterize pulled out fibers surfaces. The effect of water-to-binder (w/b) ratio on the fiber–matrix bonding was also assessed. It was found that the pullout strength of acrylic fiber is more than polypropylene (29%) and nylon 66 (10%) fibers. It was also found that the pullout energy of polypropylene and nylon 66 fibers is 75% higher than the acrylic fiber. The best result of bonding strength was obtained at w/b ratio of 0.25. It was concluded that it is possible to improve the mechanical properties of concrete using low price polymeric fibers.