Polyolefin fiber-reinforced concrete enhanced with steel-hooked fibers in low proportions

Over the past few years, polyolefin fiber reinforced self-compacting concrete has shown high performance in both fresh and hardened state. Its fracture behavior for small deformations could be enhanced with a small amount of steel-hooked fibers, obtaining a hybrid fiber-reinforced concrete well suited for structural use. Four types of conventional fiber-reinforced concrete with steel and polyolefin fibers were produced on the basis of the same self-compacting concrete also manufactured as reference. These concrete mixtures were manufactured separately with the same fiber contents being subsequently used for two more hybrid mixtures. Fracture properties, in addition to fresh and mechanical properties, were assessed. The research showed both synergies (with the two types of fibers working together in the fracture processes) and an improvement of the orientation and distribution of the fibers on the fracture surface.     

Mechanical properties of ultra-high-performance fiber-reinforced concrete: A review

A comprehensive investigation into the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC), considering various influential factors, is imperative in order to obtain fundamental information for its practical utilization. Therefore, this paper reviewed the early-age strength (or setting) development and mechanical properties of hardened UHPFRC. In connection with the latter, the effects of the curing conditions, coarse aggregate, mineral admixtures, fiber properties, specimen size, and strain-rate on the mechanical performance of UHPFRC were specifically investigated. It was obvious that (1) heat treatment accelerates the hydration process, leading to higher strength; (2) a portion of the silica fume can be replaced by fly ash, slag, and rice husk ash in mechanical perspective; (3) the use of deformed (hooked and twisted) or long straight steel fibers improves the mechanical properties at a static rate; and (4) high rate loading provides a noticeable increase in the mechanical properties. Alternatively, there are some disagreements between the results from various ‘size effect’ tests and the effectiveness of using twisted steel fibers at static and high rate loadings. Further research to reduce the production cost of UHPFRC is also addressed in an attempt to make its widespread use more practical.     

Low-velocity flexural impact response of fiber-reinforced aerated concrete

Impact response of fiber-reinforced aerated concrete was investigated under a three-point bending configuration based on free-fall of an instrumented impact device. Two types of aerated concrete: plain autoclaved aerated concrete (AAC) and polymeric fiber-reinforced aerated concrete (FRAC) were tested. Comparisons were made in terms of stiffness, flexural strength, deformation capacity and energy absorption capacity. The effect of impact energy on the mechanical properties was investigated for various drop heights and different specimen sizes. It was observed that dynamic flexural strength under impact was more than 1.5 times higher than the static flexural strength. Both materials showed similar flexural load carrying capacity under impact, however, use of 0.5% volume fraction of polypropylene fibers resulted in more than three times higher flexural toughness. The performed instrumented impact test was found to be a good method for quantifying the impact resistance of cement-based materials such as aerated concrete masonry products.     

Bond behavior of GFRP and steel bars in ultra-high-performance fiber-reinforced concrete

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data.     

Predicting the flexural behavior of ultra-high-performance fiber-reinforced concrete

To predict the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams including straight steel fibers with various lengths, micromechanics-based sectional analysis was performed. A linear compressive modeling was adopted on the basis of experiments. The tensile behavior was modeled by considering both pre- and post-cracking tensile behaviors. Pre-cracking behavior was modeled by the rule of mixture. Post-cracking behavior was modeled by a bilinear matrix softening curve and fiber bridging curves, considering three different probability density functions (PDFs) for fiber orientation, i.e., the actual PDF from image analysis and PDFs assuming either random two-dimensional (2-D) or three-dimensional (3-D) fiber orientation. Analytical predictions using the fiber bridging curves with the actual PDF or the PDF assuming 2-D random fiber orientation showed fairly good agreement with the experimental results, whereas analysis using the PDF assuming 3-D random fiber orientation greatly underestimated the experimental results.     

不同长径比聚丙烯纤维增强混凝土的力学特性

纤维长径比对混凝土力学性能影响显著,而长径比变化的实质是纤维直径和形态均发生变化,因此现有研究多是通过改变聚丙烯（PP）纤维（包含粗、细PP纤维）直径或截面形态设置长径比梯度,从而导致变量不唯一。本文对粗PP纤维（d=700 μm）和细PP纤维（d=80 μm）长径比对混凝土力学性能的影响进行了试验研究,分析了粗、细PP纤维增强混凝土的力学特性。结果表明:粗、细PP纤维增强混凝土坍落度随所掺纤维长径比增大而先降低后趋于稳定,抗压、抗弯、劈拉强度随所掺纤维长径比增大而呈现出先增大后减小的趋势,700 μm粗PP纤维最优长径比为42,80 μm细PP纤维最优长径比为200。此外,提出了宏观力学拟合计算理论用于分析粗PP纤维长径比对PP纤维增强混凝土抗弯强度的影响,以此来增强试验结果的预测性和可控性;对粗、细PP纤维在混凝土中的摩擦粘结机制进行了力学分析,掌握了影响摩擦粘结力的具体因素。      

Size and geometry dependent tensile behavior of ultra-high-performance fiber-reinforced concrete

This research investigated direct tensile stress versus strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) with various sizes and geometries. The UHPFRC in this research contained 1% macro twisted and 1% micro smooth steel fibers by volume. The effects of gauge length, section area, volume and thickness of the specimens on the measured tensile response of the UHPFRC were experimentally discovered. The different sizes and geometries of specimens did not generate significant influence on the post cracking strength of UHPFRC whereas they produced clear effects on the strain capacity, energy absorption capacity and multiple cracking behavior of UHPFRC. The strain capacity, energy absorption capacity and the number of multiple micro cracks within unit length obviously decreased as the gauge length, section area and volume of UHPFRC specimens increased. In contrast, as the thickness of the specimen increased, different tendency was observed.     

Mechanical behavior of hybrid steel-fiber self-consolidating concrete: Materials and structural aspects

This work presents the preliminary results of an experimental investigation on the mechanical behavior of self-consolidating concrete reinforced with hybrid steel fibers in the material and structural scale. Straight and hooked end steel fibers with different lengths and diameters were used as reinforcement in fiber volume fractions of 1.0 and 1.5%. In the fresh state the concrete was characterized using the slump flow, L-box and V-funnel tests. To determine the effect of the hybrid reinforcement on the plastic viscosity and shear yield stress a parallel plate rheometer was used. Following, the mechanical response was measured under tension and bending tests. In the flexural test, the movement of the neutral axis was experimentally determined by strain-gages attached to compression and tensile surfaces. Furthermore, the mechanical response of the material under bi-axial bending was addressed using the round panel test. During the test the crack opening was measured using three linear variable differential transformers (LVDT’s). The cracking mechanisms were discussed and compared to that obtained under four point bending and direct tension. The obtained results indicated that the fiber hybridization improved the behavior of the composites for low strain and displacement levels increasing the serviceability limit state of the same through the control of the crack width. For large displacement levels the use of the longer fibers led to a higher toughness but with an expressive crack opening. Due to its structural redundancy the round panel test allowed the formation of a multiple cracking pattern which was not observed in the four point beam tests. Finally, the obtained material’s properties were used in a nonlinear finite element model to simulate the round panel test. The simulation reasonably agreed with the experimental test data.     

Effect of cryogenic temperature on the flexural and cracking behaviors of ultra-high-performance fiber-reinforced concrete

This study investigates the flexural and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) before and after exposure to cryogenic temperatures for liquefied natural gas (LNG) storage tank applications. Normal concrete (NC), which has been used to make LNG storage tanks in Korea, was also considered for comparison. In order to evaluate the cracking resistance of NC and UHPFRC, several edge-type slabs were fabricated and tested by restraining their thermal deformation. Four-point bending tests were also performed to estimate the flexural performance before and after cryogenic cooling. Test results indicate that UHPFRC exhibited higher resistance to microcrack formation under these conditions. UHPFRC also showed substantially better flexural performance, both before and after exposure to cryogenic cooling, compared to NC. In addition, the microcracks in UHPFRC that were induced by the pre-cracking load were suddenly and effectively filled with calcium carbonate (CaCO₃), which was formed by a chemical reaction between melting water and calcium ions. This was verified by energy dispersive X-ray spectroscopy analysis. CaCO₃ formation resulted in enhanced flexural performance, including higher strength, deflection capacity, and energy absorption capacity, as compared to the virgin UHPFRC specimens without any cracks.     

The mechanical properties of oxyfluorinated hybrids of glass fibre and polypropylene fibre

The mechanical properties of a low-cost system comprising orthophthalic polyester resin reinforced with hybrids of glass and polypropylene fibres were investigated. The fibres were oxyfluorinated to overcome the poor surface adhesion properties of polypropylene. Interlaminar shear tests, Izod-type impact tests and tensile tests were considered. It would be expected that increasing polypropylene fibre content corresponds with a decrease in mechanical properties due to the poor properties of polypropylene. Oxyfluorinated laminates containing approximately 25% and 50% polypropylene in the warp direction were, however, found to exhibit significant improvements in interlaminar shear strength, in peak shear stress under impact loading as well as in impact resistance over untreated glass fibre laminates. Scanning electron microscope images show that the reason for this improvement is that the interfacial bond between the polypropylene fibres and the resin is strengthened to such an extent that failure occurs within the polypropylene fibres rather than at the interface.     

Mechanical properties of ceramic fiber-reinforced concrete under quasi-static and dynamic compression

The research herein is made on the quasi-static and dynamic mechanical properties of ceramic fiber reinforced concrete (CRFRC for short) through the adoption of a hydraulically-driven testing system as well as a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. As test results have turned out, such quasi-static properties as compressive strength, splitting tensile strength and flexural strength of CRFRC increase with the rise in the volume fraction of fiber. Within the strain range of 20–120 s⁻¹, the effect of the axial strain acceleration on the dynamic strength of CRFRC could be ignored. Therefore, the dynamic increase ratio (DIF) derived from SHPB tests can truly reflect the dynamic enhancement of CRFRC. The dynamic strength, critical strain and specific energy absorption (SEA) of CRFRC are sensitive to the strain rate. The addition of ceramic fiber to plain concrete can significantly improve its properties—dynamic strength, critical strain and energy absorption. And also, an analysis is conducted of the mechanism for strengthening and toughening the concrete.     

Thermo-mechanical properties of fiber reinforced raw perlite concrete

The effect of hooked steel, wavy steel and polypropylene fibers on the thermo-mechanical properties of raw perlite aggregate concrete was investigated. In order to determine the effect of fiber ratio on the thermo-mechanical properties of 100% raw perlite concrete, 0.25%, 0.75%, 1.25%, and 1.75% fiber ratios were used by volume of the sample and also, 350 kg/m³ cement dosage and 3 ± 1 cm slump were used. When compared to the control sample that contains no fiber, (1) with the increase of steel fiber ratio in the mixtures thermal conductivity (TC), unit weight, splitting-tensile strength, and flexural strength of concretes increased, (2) with the increase of steel fiber ratio in the mixtures compressive strength of concretes decreased, and (3) with the increase of polypropylene fiber ratio in the mixtures TC, unit weight, compressive strength, splitting-tensile strength, and flexural strength of concretes decreased.     

Special manufacturing and characteristics of basalt fiber reinforced hybrid polypropylene composites: Mechanical properties and acoustic emission study

Basalt fiber reinforced, polypropylene matrix hybrid composites were manufactured in the process of carding, needle-punching and pressing. Hemp, glass and carbon fibers were applied besides basalt fiber in these composites. In order to achieve a sufficient interfacial adhesion, the fibers were treated with the reaction mixture of maleic acid anhydride and sunflower oil. The hybrid effect in these composites was examined as a function of fiber content and fiber combination. The strength properties of hybrid composites improved owing to surface treatment and this was proven by mechanical tests and microscopic analysis, as well. Acoustic emission methods revealed that there is a correlation between the physical parameters of sound waves that occurred during failure and the mechanical properties.     

Correlation of constitutive response of hybrid textile reinforced concrete from tensile and flexural tests

This papers addresses the disparities that exist in measuring the constitutive properties of thin section cement composites using a combination of tensile and flexural tests. It is shown that when the test results are analyzed using a simplified linear analysis, the variability between the results of tensile and flexural strength can be as high as 200–300%. Experimental results of tension and flexural tests of laminated Textile Reinforced Concrete (TRC) composites with alkali resistant (AR) glass, carbon, aramid, polypropylene textile fabrics, and a hybrid reinforcing system with aramid and polypropylene are presented. Correlation of material properties is studied analytically using a parametric model for simulation of flexural behavior using a closed form solution based on tensile stress–strain constitutive relation. The flexural load carrying capacity of TRC composites is computed using a back-calculation approach, and parameters for a strain hardening material model are obtained using the closed form equations. While the parametric model over predicts the simulated tensile response for carbon and polypropylene TRCs, predictions are however consistent with experimental trends for aramid and glass TRCs. Detailed discussion of the differences between backcalculated and experimental tensile properties is presented. Results can be implemented as average moment–curvature relationship in the structural design and analysis of cement composites.     

Experimental study of the thermo-mechanical properties of recycled PET fiber-reinforced concrete

This work presents an experimental study of thermal conductivity, compressive strength, first crack strength and ductility indices of recycled PET fiber-reinforced concrete (RPETFRC). We examine PET filaments industrially extruded from recycled PET bottle flakes with different mechanical properties and profiles. On considering a volumetric fiber dosage at 1%, we observe marked improvements in thermal resistance, mechanical strengths and ductility of RPETFRC, as compared to plain concrete. A comparative study with earlier literature results indicates that RPETFRC is also highly competitive over polypropylene-fiber-reinforced concrete in terms of compressive strength and fracture toughness.     

Mechanical properties of hybrid fabrics in pultruded cement composites

This work concerns the tensile properties of cement-based hybrid composites manufactured as: (i) sandwich composites that combine different layers of single fabric types; and (ii) hybrid composites, made from several yarn types within the same fabric. Hybrid combinations of low-modulus fabrics of polyethylene (PE) or polypropylene (PP) and high-modulus AR glass or aramid fabrics were prepared by the pultrusion process and tested in tension. Influence of pultrusion direction on the results was one of the parameters studied. It was found that hybrid composites made from PE and AR glass sustain strains better than 100% AR glass composites, and are stronger than a single PE fabric composite. A hybrid fabric composites made with combination of high strength–high cost aramid and low stiffness–low cost PP yarns performed better than a single aramid fabric composite relative to their reinforcing volume contents. Results show that making hybrid composites is an attractive option for cement-based elements. The performance of hybrid fabric composites is also influenced by the arrangement of fabric layers in the laminates. Composites with brittle and relatively strong fabrics (glass) at the mid-section and ductile fabrics (PE) near the surfaces of the composite performed better in tension than composites with the opposite arrangement.     

Reduction of fire spalling in high-performance concrete by means of superabsorbent polymers and polypropylene fibers: Small scale fire tests of carbon fiber reinforced plastic-prestressed self-compacting concrete

High-performance concrete (HPC) is prone to explosive spalling when exposed to fire, which may lead to failure of the concrete elements. Polypropylene fibers (PP) are often added to HPC, as upon their melting they create channels through which water vapor is evacuated, preventing the build-up of high vapor pressures. In self-compacting HPC (HPSCC), the amount of PP fibers needs to be limited in order to keep the self-compacting properties, which may reduce the fire resistance.In this paper, a novel strategy to reduce fire spalling in HPSCC is illustrated, based on adding small particles of superabsorbent polymers (SAP) during mixing. The SAP end up as empty macropores, similar to air voids, in the HPSCC matrix. The PP fibers-SAP voids system percolates at a lower fiber loading than the fibers alone, allowing maintenance of the self-compacting properties while reducing substantially the fire spalling. In particular, in this paper it is shown how addition of SAP is able to reduce fire spalling in thin-walled HPSCC slabs prestressed with carbon fibre reinforced plastic reinforcement.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Constitutive modeling of reinforced concrete and prestressed concrete structures strengthened by fiber-reinforced plastics

A batch of constitutive models for steel reinforcing bar, prestressing tendon, concrete and fiber-reinforced plastic are proposed for the nonlinear finite element analysis of reinforced concrete structures, prestressed concrete structures, reinforced concrete structures strengthened by fiber-reinforced plastics and prestressed concrete structures strengthened by fiber-reinforced plastics. These material models have been tested against series of experimental data and good agreements have been obtained, which justifies the validity and the usefulness of the proposed nonlinear constitutive models.     

Mechanical, durability and microstructural characteristics of ultra-high-strength self-compacting concrete incorporating steel fibers

Few researches are carried out in the Gulf area to study the feasibility of producing UHSC using available local materials with the inclusion of steel fibers, and investigate its properties and durability. Local available materials and the inclusion of steel fibers with different volume fractions are investigated to produce UHSC. Different mechanical properties are evaluated (compressive strength and splitting tensile strength). Durability of the concrete in high sulfate and high temperature condition (i.e. resembling Gulf environment) is evaluated. Also, chloride permeability, bulk chloride diffusion and electrical resistivity are evaluated. Test results indicate that local material can produce UHS–FRC. The ductility of the concrete is greatly improved by the incorporation of steel fibers and increases as the fiber volume increases. Chloride permeability, bulk chloride diffusion and electrical resistivity are affected by the volume fraction of steel fibers. The inclusion of steel fibers did not have significant effect on the durability of the concrete in the sulfate environment. Microstructural investigations of UHS–FRC concrete were also performed. The microstructural investigations shed some light on the nature of interfacial bond of fibers and the cement paste and its effect on its mechanical and fracture properties.