Fiber-reinforced Concrete Elements Research Articles

Fiber orientation and orientation factors in steel fiber‐reinforced concrete beams with hybrid fibers: A critical review

AbstractFiber orientation is of paramount importance for the design of fiber‐reinforced concrete (FRC) structural elements, because it markedly influences the postcracking properties of such material. For this reason, structural codes introduce orientation factors which aim to correlate the real mechanical properties of the structural element with the ones determined from standard beams. Although the need of considering fiber orientation in design codes is commonly accepted, the orientation factors are still based on a limited number of research studies, raising the need to better determine fiber orientation to improve the current standards and support the design process of FRC elements. In this research, a steel fiber‐reinforced concrete (SFRC) with a hybrid system of macro and microfibers is steered into a broad range of fiber orientations and cast into standard beams. Besides measuring the mechanical performance of these SFRC beams, three different methods for assessing fiber orientation are employed, namely electromagnetic induction, image analysis, and micro‐computed tomography. The comparison between the outcomes of the different methods provides detailed information about the accuracy and suitability of each method, considering the corresponding domain of applicability at structural level. Finally, a critical review of the most common 2D and 3D orientation parameters found in literature is performed, and the equations are adapted to account for the hybrid system of fibers.

Analytical calculation of the in-plane shear strength of fibre-reinforced concrete

Existing code prescriptions for determining the shear strength of fibre-reinforced concrete (FRC) elements are either empirical or rely on important assumptions that require further clarification. A mechanically based method is here proposed to determine the fibres and the shear reinforcement contributions to the in-plane shear strength of membrane elements. The method is derived from the comprehensive Cracked Membrane Model for FRC to determine in-plane shear strength of FRC panels and is suitably adapted to determine the shear strength of beam elements. The proposed methodology is compared to existing code formulations and validated against experimental results from 14 panels subjected to in-plane shear stresses and from 34 beams failing in shear. The average ratio between experimental and calculated shear strength is 1.00 for the panels and 1.11 for the beams, with coefficients of variation 11 % and 24 %, respectively. The proposed analytical methodology enables the determination of shear strength and failure modes in FRC elements, with or without shear reinforcement. Equations providing the fibre contribution and the compression field inclination are derived from mechanical considerations, emphasizing the key influencing variables. The proposed method can serve as a basis for code-like formulations for shear strength linked to the involved mechanics, but for which further levels of simplification are still required.

Comparative Analysis of Reinforced Concrete Structure with Traditional and Decoupled Masonry Infill Under Earthquake

Different types of concrete are invented in the past years, in order to improve it`s behavior for different loading and environmental situations. Among them, fibre reinforced concrete (FRC) is one of the most recently developed concrete types. Design procedures for the FRC elements are included in only a few existing code provisions. Among them, fib Model Code 2010 (MC 2010) provides the biggest amount of necessary information and recommendations in order to design FRC elements. Yet, a lack of the guidelines for the design of elements loaded with different combinations of bending moment and axial force is noticed in the existing code provisions. Therefore, in the scope of this paper, interaction curves for FRC are developed. The interaction curves are developed for both, ultimate limit state (ULS) and serviceability limit state (SLS). Furthermore, example of the use of such interaction curves is given. Comparison in the design of reservoir made of reinforced concrete (RC) and FRC is performed. It is shown that the use of FRC especial benefits design according to SLS, as FRC provides better behaviour regarding crack spacing and crack width of element, while developed interaction curves for SLS significantly decrease the time necessary for such a design.

Design of Fibre-Reinforced Concrete Structures

Different types of concrete are invented in the past years, in order to improve it`s behaviour for different loading and environmental situations. Among them, fibre reinforced concrete (FRC) is one of the most recently developed concrete types. Design procedures for the FRC elements are included in only a few existing code provisions. Among them, fib Model Code 2010 (MC 2010) provides the biggest amount of necessary information and recommendations in order to design FRC elements. Yet, a lack of the guidelines for the design of elements loaded with different combinations of bending moment and axial force is noticed in the existing code provisions. Therefore, in the scope of this paper, interaction curves for FRC are developed. The interaction curves are developed for both, ultimate limit state (ULS) and serviceability limit state (SLS). Furthermore, example of the use of such interaction curves is given. Comparison in the design of reservoir made of reinforced concrete (RC) and FRC is performed. It is shown that the use of FRC especial benefits design according to SLS, as FRC provides better behaviour regarding crack spacing and crack width of element, while developed interaction curves for SLS significantly decrease the time necessary for such a design.

Nonlinear semi-numeric analysis and simplified analysis of polymer fiber-reinforced concrete prisms under three-point bending test conditions

This paper presents a semi-numeric nonlinear analysis and a simplified analysis for evaluating the load-displacement behavior of polymer fiber-reinforced concrete elements under three-point bending test conditions. The considered elements were notched and unnotched prisms. The nonlinear semi-numeric analysis was based on the moment-curvature relation, plastic hinge approach, and virtual work method. The simplified analysis assumed multilinear load-displacement behavior of the prisms under three-point bending test conditions. It included four variants for the input tensile strength properties and crack widths considered for the serviceability and ultimate limit state. The notched polymer fiber-reinforced concrete prism three-point bending test results from a prior study were the basis for evaluating the simplified analysis, which was subsequently compared with the nonlinear semi-numeric analysis outcomes from the same investigation. Additionally, the simplified analysis and the nonlinear semi- numeric analysis were used to determine the load-displacement behavior of unnotched polymer fiber-reinforced concrete prisms with heights ranging from 25 mm to 250 mm. The results indicate that the simplified analysis provides a practical and efficient method for estimating the behavior of polymer fiber-reinforced concrete structures under three-point bending test conditions, yielding relatively accurate results with minimal computational effort compared to the more exact nonlinear semi-numeric analysis.

Digital design and fabrication of clay formwork for concrete casting

This paper investigates the potential of clay extrusion as formwork for casting customized and building-scale fiber-reinforced concrete elements. Customizable shapes are produced using clay as cheap, sustainable, and easily demoldable formwork, extending its printable height limit. The coupled incremental clay 3D printing and concrete casting process allows the layered casts to start curing and reduces the hydrostatic pressure from concrete. The concluding case study, the Cocoon, demonstrates the method’s capability to achieve building-scale height, integrate openings, and create complex surfaces. The introduced method seeks to challenge techniques and materials for 3D-printed formworks, demonstrating the ability to reduce the environmental impacts of concrete construction without compromising the complexity and time efficiency of bespoke elements.

A smeared crack formulation for simulating fracture of fibre-reinforced concrete by means of a trilinear softening diagram

This study presents a smeared crack model for reproducing the fracture behaviour of FRC that is based on an embedded crack formulation used lately with this material. In both cases, a trilinear softening diagram allows reproducing the post-peak behaviour correctly. The model proposed here differs from the one on which it is inspired, since it is a smeared crack model based on the crack band concept where the material damage is isotropic and controlled by a damage factor ranging from 0 to 1, while the original model is an embedded crack model that allows fracture in three directions per element. This new formulation overcomes some limitations of the embedded crack model, that could only be used with triangular elements with an only integration point, since it can be used with triangular and quadrilateral elements with any number of integration points. Unlike previous models, developed for commercial codes, this model is developed in OOFEM, a free finite element code developed at the Czech Technical University in Prague (Czech Republic) and Chalmers University of Technology (Sweden), thus providing the general public with an advanced tool that allows reproducing fracture in structural elements made with fibre-reinforced concrete elements. This work shows that the proposed formulation provides similar results to those obtained with the model on which it is inspired under different situations: mode I fracture, size effect analysis and, finally, modes I and II mixed fracture.

Программа и результаты экспериментальных исследований характеристик фибробетона, изготовленного по механической технологии создания агрегированного ориентированного фибрового армирования

Statement of the problem. The pace of development of modern construction creates the need to develop new technologies that make it possible to impart special properties to classical building materials, allowing them to expand the range of their application. The developed technology of mechanical distribution and orientation along the fiber fibers acting in the structure of tensile stresses in the body of linear fiber-reinforced concrete and fiber-reinforced concrete elements can provide more efficient use of fiber-dispersed reinforcement by including more fibers in the work of the section. It is necessary to develop a program of experimental studies of the strength and deformation properties of fiber-reinforced concrete in order to test the technology of laying fiber-reinforced concrete mixture developed by the authors.
 Results. A program of experimental studies of the strength and deformation characteristics of fiber-reinforced concrete, manufactured using the mechanical technology for creating aggregated oriented fiber reinforcement, is presented. The results of experiments carried out in accordance with the developed program are also presented.
 Conclusions. The presented program of experimental studies of the characteristics of fiber-reinforced concrete, made according to the technology developed by the authors for the mechanical creation of aggregated oriented fiber reinforcement, became the basis for experimental studies, the purpose of which was to test the technology developed by the authors for the mechanical laying of a fiber-reinforced concrete mixture with a uniform distribution of fiber fibers in a concrete matrix, as well as obtaining new data on the mechanical and deformative properties of such fiber-reinforced concrete.

Steered fiber orientation: correlating orientation and residual tensile strength parameters of SFRC

Adding steel fibers to concrete improves the post-cracking tensile strength of the composite material due to fibers bridging the cracks. The residual performance of the material is influenced by fiber type, content and orientation with respect to the crack plane. The latter is a main issue in fiber-reinforced concrete elements, since it significantly influences the structural behavior. The aim of this research is to develop a tailor-made composite material and casting method to orient fibers in order to optimize the performance of the material for structural applications. To this aim, a mechanized concreting device that induces such preferred fiber orientation is designed and fabricated. It uses vibration and a series of narrow channels to guide and orient fibers. A composite with oriented fibers is produced using a hybrid system of macro and micro fibers and high-performance concrete. From the same concrete batch, specimens are cast both with and without the fiber orientation device, obtaining different levels of fiber orientation. Three-point bending tests are performed to measure and compare the residual tensile strength capacities with standard specimens cast according to EN 14651. Elements with favorable fiber orientation show a significant increase in residual tensile strength with respect to the standard beams. Finally, computed tomography and an electromagnetic induction method are employed to better assess the orientation and distribution of fibers in the beams. Their results are in good agreement and enable to link the residual tensile strength parameters with fiber orientation.

Experimental and numerical investigation on the resistance of steel fiber-reinforced heavyweight concrete slabs against rigid projectile impact

This study presents an inclusive experimental and numerical investigation of the resistance of steel fiber-reinforced heavyweight concrete under the impact of a high-velocity rigid projectile. Thus, concrete specimens were made using hooked-end steel fibers at 0%, 0.5%, 1%, 1.5%, 2%, and 2.5% volume fractions to assess the physicomechanical specifications of heavyweight concrete containing iron ore as fine and coarse aggregates. Also, the compressive and splitting tensile strengths, modulus of elasticity, stress–strain relationship, bulk density, and water absorption capacity of heavyweight concrete specimens were compared to those of the equivalent normal-weight concrete. The penetration tests were conducted on concrete slabs fabricated with normal weight and heavyweight concrete mixes to examine the specimens’ failure mode, penetration depth, and spalling volume. The results revealed that normal-weight concrete's compressive and splitting tensile strengths were more susceptible to adding steel fibers. It was also shown that increasing the density and fiber content significantly affected the impact resistance and residual velocity of fiber-reinforced concrete elements. Heavyweight concrete provided better life safety measures against the impact load than normal weight specimens, while the repair and reuse of heavyweight concrete components were feasible. Moreover, numerical modeling was performed using LS-DYNA finite element software, and the predictions were compared to the proposed experimental results and the analytical relations in the literature. There was good agreement between the results.

A model for predicting the splitting bearing capacity of Fiber Reinforced Concrete elements under partially loaded areas

AbstractConcrete elements are frequently subjected to partially loaded areas; a typical example is represented by precast tunnel segments. In fact, during the excavation process, the hydraulic jacks of the boring machine exert, on the last assembled ring, high forces concentrated on small areas with respect to the size of ring joint. Similarly, when lining is embedded in ground, high hooping forces are transmitted through longitudinal joints, which present a contact zone smaller than the segment cross section. In both cases, spreading these loads into tunnel segments result in tensile transverse stresses which may cause splitting cracks. In the last decades, the growing desire to find more economic and sustainable solutions has driven enormous efforts in the tunnel industry to find new design solution such those based on Fiber Reinforced Concrete (FRC) or a combination of FRC and traditionally steel rebars. Research studies have already demonstrated the ability of fibers in controlling splitting crack phenomena. However, there is a lack of knowledge on reliable analytical approaches to quantify these benefits. The main aim of this paper is to develop a new model for determining the bearing capacity of FRC elements under partially loaded areas when splitting failure occurs. The analytical model herein reported allows a good prediction of the maximum loads exhibited by FRC prisms tested under high concentrated loads, whose data were selected from the literature.

МЕХАНИЧЕСКАЯ ТЕХНОЛОГИЯ СОЗДАНИЯ НАПРАВЛЕННОЙ ОРИЕНТАЦИИ ФИБР В ЛИНЕЙНЫХ ФИБРОБЕТОННЫХ И ФИБРОЖЕЛЕЗОБЕТОННЫХ ЭЛЕМЕНТАХ

Current trends in modern construction dictate the need to establish the possibility of developing a mechanical technology for the uniform distribution of fiber fibers in the body of linear fiber-reinforced concrete and fiber-reinforced concrete elements, which would ensure more efficient inclusion of the largest amount of fiber into the element due to its orientation along the tensile stresses acting in the structure. Current trends in modern construction dictate the need to establish the possibility of developing a mechanical technology for the uniform distribution of fiber fibers in the body of linear fiber-reinforced concrete and fiber-reinforced concrete elements, which would ensure more efficient inclusion of the largest amount of fiber into the element due to its orientation along the tensile stresses acting in the structure.

МЕХАНИЧЕСКАЯ ТЕХНОЛОГИЯ СОЗДАНИЯ ПЛИТНЫХ ФИБРОБЕТОННЫХ И ФИБРОЖЕЛЕЗОБЕТОННЫХ ЭЛЕМЕНТОВ С НАПРАВЛЕННОЙ ОРИЕНТАЦИЕЙ ФИБР В ДВУХ НАПРАВЛЕНИЯХ

A modified mobile installation has been developed for creating slab fiber-reinforced concrete and fiber-reinforced concrete elements with a directional orientation of the fibers in two directions according to the proposed mechanical technology. The presented mechanical technology for creating slab fiber-reinforced concrete and fiber-reinforced concrete elements will make it possible to provide a mutually perpendicular orientation of the fibers in various layers of the structure, which will allow for the most complete inclusion of each fiber fiber into the work of the element. Also, due to the use of a mobile installation for laying fiber-reinforced concrete mix, specially designed and modified to work with slab elements, a uniform distribution of fibers in the body of concrete will be mechanically ensured.

The Behavior of Hybrid Fiber-Reinforced Concrete Elements: A New Stress-Strain Model Using an Evolutionary Approach

Several stress-strain models were used to predict the strengths of steel fiber reinforced concrete, which are distinctive of the material. However, insufficient research has been done on the influence of hybrid fiber combinations (comprising two or more distinct fibers) on the characteristics of concrete. For this reason, the researchers conducted an experimental program to determine the stress-strain relationship of 30 concrete samples reinforced with two distinct fibers (a hybrid of polyvinyl alcohol and steel fibers), with compressive strengths ranging from 40 to 120 MPa. A total of 80% of the experimental results were used to develop a new empirical stress-strain model, which was accomplished through the application of the particle swarm optimization (PSO) technique. It was discovered in this investigation that the new stress-strain model predictions are consistent with the remaining 20% of the experimental stress-strain curves obtained. Case studies of hybrid–fiber–reinforced concrete constructions were investigated in order to better understand the behavior of such elements. The data revealed that the proposed model has the highest absolute relative error (ARE) frequencies (ARE 10%) and the lowest absolute relative error (ARE > 15%) frequencies (ARE > 15%).

Improvement of Strength and Strain Characteristics of Lightweight Fiber Concrete by Electromagnetic Activation in a Vortex Layer Apparatus

The relevant problem of choosing effective materials for enclosing structures is compliance with the requirements of increased thermal resistance, reduced mass of buildings and structures, and reduced material consumption, labor intensity, and construction costs. These requirements are satisfied by structures made of lightweight fiber-reinforced concrete, which are the subject of attention of many scientists and engineers. One of the most rational requirements for industrial use is the activation of untreated components of the concrete mixture. This article is devoted to studying the influence of the activation of fiber-reinforced concrete elements in the vortex layer apparatus on concrete strength and structural characteristics. The effect of the raw component processing time of the concrete mixture on the strength and deformation characteristics of the lightweight fiber-reinforced concrete was studied. The optimal processing time for the cement–sand mortar in the VLA-75-85s was determined. It was shown that the activation of the vortex layer in the apparatus leads to an increase in strength from 27% to 61% and an improvement in the deformation characteristics of lightweight fiber-reinforced concrete by up to 12%. Furthermore, it was found that the use of activation in VLA leads to an increase in the coefficient of constructive quality for all experimentally determined strength characteristics of lightweight fiber-reinforced concrete by up to 27%.

Reliability-based assessment of the partial factor for shear design of fibre reinforced concrete members without shear reinforcement

Fibre reinforced concrete (FRC) is increasingly used for structural purposes owing to its many benefits, especially in terms of improved overall sustainability of FRC structures relative to traditional reinforced concrete (RC). Such increased structural use of FRC requires safe and reliable models for its design in ultimate limit states (ULS). Particularly important are models for shear strength of FRC members without shear resistance due to the potential of brittle failure. The fib Model Code 2010 contains a model for the shear strength of FRC members without shear reinforcement and the same partial factor accepted for RC structures is accepted for FRC elements. This approach, however, is potentially on the unsafe side since the uncertainties of some design-determining mechanical properties of FRC (i.e., residual flexural strength) are larger than those for RC. Therefore, in this study, a comprehensive reliability-based calibration of the partial factor γc for the shear design of FRC members without shear reinforcement according to the fib Model Code 2010 model is performed. As a first step, the model error δ is assessed on 332 experimental results. Then, a parametric analysis of 700 cases is performed and a relationship between the target failure probability βR and γc is established. The results demonstrate that the current model together with the prescribed value of γc = 1.50 does not comply with the failure probabilities accepted for the different consequences of failure of FRC members over a 50-year service life. Therefore, changes to the shear resistance model are proposed in order to achieve the target failure probabilities.

Experimental Tests of Fiber-Reinforced Concrete Slabs and Comparison of Deformations Using 3D Graphs

Abstract The presented article deals with experimental testing of fiber-reinforced concrete slabs using a specialized testing device, the so-called Stand. The experimental testing involved a total of three slabs with different fiber dosing. Dosing was chosen of 25, 50, and 75 kg/m3. In the case of fiber dosing of 75 kg/m3, the concrete recipe was changed with regard of fiber dosing. The test equipment allows vertical loading of the elements and, together with the installed track sensors and computer technology, records the vertical deformations. The deformation of the slabs was evaluated using 2D deformation sections in the slabs of mounted sensors and using 3D deformation sections with the use of interpolation of intermediate values. The article is supplemented by a test evaluation of the mechanical properties. These were concrete compressive strength, modulus of elasticity, and tensile strength. The mechanical properties were also tested and evaluated for plain concrete (dosage 0 kg/m3). By evaluating the mechanical properties and deformations, it is possible to observe differences in the behavior of fiber-reinforced concrete elements for different degrees of fiber reinforcement. The article evaluates the positive effect of fibers on the load capacity and the deformations of slabs.

Sectional analysis of the flexural creep of cracked fiber reinforced concrete

AbstractThis paper presents a sectional analysis of cracked fiber reinforced concrete (FRC) under flexural loading. The approach considers three separate loading conditions: monotonic, cyclic, and creep bending loads. Different parts of the approach are experimentally calibrated and validated with tests on polypropylene FRC. In the monotonic regime, numerical optimization is used to find the optimal stress–strain relation of FRC to capture the experimental bending behavior as closely as possible, while satisfying the horizontal force and bending moment equilibrium. To model the cyclic behavior, the constitutive law is extended by introducing a tension‐only scalar damage function. The agreement between the predicted and measured response is again very good, and the measured upwards shift of the neutral axis during unloading is predicted as well. Finally, the flexural creep response of a cracked FRC beam is simulated by using uniaxial creep data. The approach predicts the time‐dependent crack widening under sustained flexural loading and can be used to check SLS criteria in the design of FRC elements.

Systematic Review on the Creep of Fiber-Reinforced Concrete.

Fiber-reinforced concrete (FRC) is increasingly used in structural applications owing to its benefits in terms of toughness, durability, ductility, construction cost and time. However, research on the creep behavior of FRC has not kept pace with other areas such as short-term properties. Therefore, this study aims to present a comprehensive and critical review of literature on the creep properties and behavior of FRC with recommendations for future research. A transparent literature search and filtering methodology were used to identify studies regarding creep on the single fiber level, FRC material level, and level of structural behavior of FRC members. Both experimental and theoretical research are analyzed. The results of the review show that, at the single fiber level, pull-out creep should be considered for steel fiber-reinforced concrete, whereas fiber creep can be a governing design parameter in the case of polymeric fiber reinforced concrete subjected to permanent tensile stresses incompatible with the mechanical time-dependent performance of the fiber. On the material level of FRC, a wide variety of test parameters still hinders the formulation of comprehensive constitutive models that allow proper consideration of the creep in the design of FRC elements. Although significant research remains to be carried out, the experience gained so far confirms that both steel and polymeric fibers can be used as concrete reinforcement provided certain limitations in terms of structural applications are imposed. Finally, by providing recommendations for future research, this study aims to contribute to code development and industry uptake of structural FRC applications.

Calculation update of strength of expanded clay fiber-reinforced concrete elements with mixed reinforcement

The paper deals with the methodology for calculating the strength of the expanded clay fiber-reinforced concrete elements with mixed reinforcement with the most complete view of the post-yield properties of prestressed and non-stressed rebars, as well as the influence of prestressing on the properties of the expanded clay fiber-reinforced concrete.