Assessment of the crushing strength of concrete rings reinforced with synthetic fibers

Mechanical properties of fiber-reinforced concrete with adapted rheology

Concrete, as one of the most widely used construction materials, has a brittle behavior. Adding fibers with different types and contents would affect the ductility behavior and mechanical properties of concrete. Hence, an experimental study was conducted to investigate effects of type and content of polymer fibers on mechanical properties of fiber-reinforced concrete such as flexural strength, compressive strength, indirect tensile strength, and elastic modulus. In the present research, the concrete samples were made and, then, evaluated, using three different types of polymer fibers, including twisted, barchip, and fibrillated, with the contents of 0.2, 0.4, and 0.6 volume percentages, respectively. The results showed that by adding fibers to the concrete samples, the flexural and tensile strength was increased by 19.6–81.69% and 0.84–34.29%, respectively; besides, the addition of the fibers to concrete reduced the compressive strength and elasticity modulus by 4.57–26.32% and 12.48–37.08%, respectively. The concrete containing twisted and barchip fibers, despite the different types of fibers, had similar flexural performance.

Fiber reinforcement is currently most often used in floors, railway sleepers, prefabricated structural elements such as slabs, beams and tanks, and in small architecture elements. Designing elements or structures made of fiber-reinforced concrete requires knowledge of its basic mechanical parameters. In the case of concretes with metallic fibers, the literature can find many tests and standard guidelines regarding compressive, flexural, tensile strength and fracture energy. The properties of concretes with non-metallic fibers are slightly less recognized, especially concretes with new types of polymer fibers. Additionally, the lack of standardized methods of testing concrete with polymer fibers make their application much more difficult. In the article, the possibility of using the EN 14651 standard to assess the flexural tensile strength of concrete with the addition of 2.0 and 3.0 kg/m3 of synthetic fibers with different geometry and form was presented. There was a 5.5–13.5% increase in the flexural tensile strength depending on the mixture type. Moreover, in the case of fiber-reinforced concretes, the ductility was enhanced and the samples were characterized by significant residual flexural tensile strengths. Additionally, from the workability tests it was concluded that after the incorporation of fibers, the consistency class decreased by one, two or three. Nevertheless, the compressive strengths of concrete with and without fibers were very similar to each other, and varied from 58.05 to 61.31 MPa. Moreover, it was concluded that results obtained from three-point bending tests significantly differed from empirical formulas for the calculation of the flexural tensile strength of fiber-reinforced concretes with dispersed steel fibers present in the literature. As a result, the new formula determined by the authors was proposed for concrete with polymer fibers with a nominal fiber content ≤1.0% and slenderness of up to 200. It must be mentioned that the formula gave a very good agreement with studies presented in different literature positions. In addition, an attempt was made to evaluate the strengths of tested mixes in accordance with the Model Code 2010. However, it occurred that the proposed fiber-reinforced concrete mixtures would not be able to replace traditional reinforcement in a form of steel bars. Furthermore, in uniaxial tensile tests, it was not possible to determine the σ–w graphs, and received results for maximum tensile strength did not show the clear influence of fibers incorporation on concrete. Then, the fracture energy enhancement (from about 16 to 22 times) and dependencies: crack mouth opening displacement–deflection; crack mouth opening displacement–crack tip opening displacement; and crack tip opening displacement–deflection were analyzed. Finally, the results from flexural tensile tests were compared with measurements of the surface displacement field obtained through the Digital Image Correlation technique. It was concluded that this technique can be successfully used to determine the crack mouth and crack tip opening displacements with very high accuracy.

Influence of synthetic fibers on the performance of ultra-high performance concrete (UHPC) at elevated temperatures

This study aims to explore the impact of different fibers on the explosive spalling resistance, residual compressive strength, residual flexural tensile strength, and mass loss of self-compacting concrete (SCC) after heating to elevated temperatures. The fiber types, fiber contents, and target temperatures are selected as the tested variables. Steel fibers (SF), polypropylene fibers (PF), and their combination are used to improve the fire resistance of SCC. Then, the specimens are tested after exposure to different target temperatures varying from 200 °C to 1000 °C and cooling down to room temperature naturally. The obtained results reveal that PF can effectively prevent explosive spalling in the SCC. Moreover, it was found that the temperature of 400 °C is a critical temperature for the residual compressive strength of fiber-reinforced SCC. Regarding the flexural tensile strength, the temperature of 200 °C was found to be a changing point of the SCC after heating to elevated temperatures. A sharp decrease in the compressive strength is observed when the target temperature exceeds 400 °C. For temperatures below 400 °C, the compressive strength is enhanced slightly with the increase of the target temperature. Moreover, incorporating a fiber cocktail is an effective way to improve the explosive spalling resistance and residual mechanical properties in the case of fire. Finally, the regression analysis was performed and the proposed correlations between the residual strengths and elevated temperatures are compared with equations in the literature. Good accuracy of the proposed correlation is achieved for fiber-reinforced SCC.

To enhance the low strain capacity and brittleness of concrete, several researchers have already reported on the mechanical properties of rubberized concrete, which lead to reduced environmental concerns, direct cost reduction, low unit weight, high toughness, and improved absorption of impact. However, to overcome the drawbacks of low flexural strength and the low stiffness of rubberized concrete and to improve the crack resistance, steel, or synthetic (polypropylene) fibers, or both, were added into the rubberized concrete in this study. Based on the flexural performance test according to ASTM C1018 and ASTM C1609, this study investigates the combination of crumb rubber, steel or synthetic fibers, or both, in zero-slump concrete mixtures used in the dry-cast production method for concrete pipes. A series of concrete mixes were examined with the variation of the fiber volume fraction (Vf) (steel: 0.17 % and 0.33 %/synthetic-polypropylene: 0.17 %, 0.33 %, and 0.52 %), and crumb rubber with different replacement ratios of 3 % to 20 % (by volume) for sand (fine aggregate) in the concrete mixture. Seventeen rubberized mixtures were developed by incorporating different dosages of steel and polypropylene fibers and their combinations. The influence of fiber type, dosages of fibers, and rubber are analyzed on the basis of experimental results. Results indicate that the effect of hybrid reinforcement using both steel and polypropylene fiber in rubberized concrete is considerable in terms of increase in both peak load and toughness. The specimen of hybrid fiber-reinforced rubberized concrete (HYFRC) with 44 lb/yd3 (0.33 %:Vf) of steel fiber, 8 lb/yd3 (0.33 %: Vf) of polypropylene fiber and 3 % of crumb rubber (replacement with fine aggregates) showed higher peak load, modulus of rupture, and toughness than other mixtures. However, excessive replacement of rubber into the specimens had a negative effect on the flexural properties (strength and toughness).

Mechanical performance and post-cracking behavior of self-compacting steel-fiber reinforced eco-efficient ultra-high performance concrete

The use of self-compacting concrete (SCC) reinforced with fibers has great potential in the precast concrete industry as the concrete can be delivered straight into the moulds, without any vibration or compacting effort. Similarly, it has the potential to replace traditional steel reinforcement depending on the design requirements. Novel synthetic fibers have recently become available in the market, but still, limited information is available on the performance of SCC reinforced with such fibers. This paper investigates the use of twisted-bundle macro-synthetic fiber in self-compacting concrete. Three different concrete mixtures with fiber dosage of 4, 6, and 8 kg/m3 were produced in large scale batches, and their performance was compared in terms of slump-flow, compressive strength, split tensile strength, modulus of elasticity, and flexural strength. Moreover, a comprehensive evaluation of the post-cracking residual strength is presented. It was found that the mixture with 4 kg/m3 fiber content has the most satisfactory flowability, whereas 8 kg/m3 mixture achieved the highest residual flexural strength. Based on the observed post-cracking behavior, a simplified stress-crack opening constitutive law is proposed. Since the fiber dosage affects the residual flexural strength, a factor related to fiber content is recommended while determining the ultimate residual flexural strength.

Effect of different fiber combinations and optimisation of an ultra-high performance concrete (UHPC) mix applicable in structural elements

Mechanical properties of ambient cured high strength hybrid steel and synthetic fibers reinforced geopolymer composites

Mechanical properties of fiber-reinforced concrete containing waste porcelain aggregates under elevated temperatures

The construction industry requires concrete with adequate post-cracking behavior for applications such as tunnels, bridges, and pavements. For this reason, polypropylene macrofibers are used, which are synthetic fibers that fulfill the function of providing residual strength to concrete. In this study, an experimental plan is carried out to evaluate the bending behavior of concrete reinforced with polypropylene fibers using the four-point bending test according to ASTM C1609. Three fiber dosages (3.6, 7.2 and 10.8 kg/m3) and three fiber lengths (40, 50, and 60 mm) were used. The use of macro polypropylene fibers increased the post-cracking behavior of concrete. In addition, based on the experimentally obtained results and available literature data, a multivariable equation was developed to predict the concrete toughness as a function of the volume, slenderness, and modulus of elasticity of the fibers. A Pearson’s correlation coefficient, r of 0.90, showed a strong correlation between the developed equation and the experimental data. From this equation, it was possible to determine the participation of the following parameters in calculating toughness. The participation or weight of the fiber’s modulus of elasticity on the concrete’s tenacity is 26%, the volume of the fiber is 39%, the slenderness is 19%, and the reinforcement index is 16%.

Impact and durability properties of alccofine-based hybrid fibre-reinforced self-compacting concrete

Problem. As a major industrial solid waste, iron tailings not only occupy a lot of agricultural land, but also pollute the environment. It is necessary to study the effect of adding different concentrations of aqueous cationic latex and different lengths of basalt fibers to the composition of crushed stone-sand mixtures of iron tailings and cement on the parameters of flexural ultimate tensile strength and the characteristics of the modulus of elasticity calculation. Goal. The effect of additives of aqueous cationic latexes of different concentrations and basalt fiber of different lengths in the composition of crushed stone-sand mixtures of iron tailings with cement on the flexural tensile strength and the elastic modulus as design characteristics was investigated. Methodology. The first stage of the study was to determine the effect of the content of two cationic latexes on the values of the tensile strength in flexion and the elastic modulus of the material from CSM-40 made of iron tailings reinforced with cement. The content of Butonal NS 198 and Butonal 5126 latexes in the composition of CSM-40 was 0, 3, 5, and 10 % by weight of the optimal amount of water at the maximum density of the mixture with cement. At the second stage, the influence of the length of basalt fibers at their content of 0.05 % by weight of the crushed stone and sand mixture on the values of the flexural tensile strength and the elastic modulus of the material from the cement-reinforced iron tailings CSM-40 was studied. It has been established that the addition of cationic latexes of the Butonal series to the composition of crushed stone-sand mixtures of iron tailings with cement causes a moderate increase in the values of the flexural tensile strength and the elastic modulus, compared to the material without the addition of latexes. Experimentally, a more significant increase in the values of the flexural tensile strength and elastic modulus of fiber-reinforced materials made of crushed stone-sand mixtures of iron tailings with cement was found compared to materials modified with cationic latexes. Conclusions:1.When the latex content is 5% (equivalent to the critical concentration of micelle formation in aqueous solution), the flexural tensile strength can be increased by a maximum of 14-15%. Under similar conditions, the elastic modulus value increases by 12-18%. 2.The addition of 0.05% (by weight) basalt fiber to the crushed stone and sand mixture of rust-colored quartzite reinforced with cement improves the bending strength and elastic modulus values compared to the non-fiber material. The largest increases in flexural tensile strength (18.5%) and elastic modulus (27.1%) are for materials with basalt fiber lengths of 18 mm.

Influences of moisture content and loading rate on flexural toughness were experimentally studied for fiber reinforced shotcrete (FRSC) with steel fiber or macro synthetic polypropylene fiber. According to the four-point bending test method specified in ASTM C1609 and Chinese standard CECS 13, the flexural toughness of specimens after drying for 0h, 16h, 24h and 72h in condition of (20±2)°C and (60±5)% relative humidity was tested at a loading rate of 0.05 mm/min. For specimens after drying for 24h and 72h, flexural toughness was tested at loading rates of 0.05 mm/min, 0.10 mm/min, and 0.20 mm/min respectively. With the moisture content decreasing, the flexural toughness T100,2.0, first-peak flexural strength, and residual flexural strength at prescribed deflections of FRSC exhibited decreasing tendency. The specimens with 0.5 vol% of steel fiber showed higher T100,2.0 value than that with 0.9 vol% of macro synthetic fiber. The residual strength and flexural toughness of FRSC increased with the increase of loading rate.