Steel Fiber-reinforced Concrete Structures Research Articles

The basis for ductility evaluation in SFRC structures in MC2020: An investigation on slabs and shallow beams

AbstractThe paper presents a synthesis of an extensive experimental campaign on linear and two‐dimensional steel fiber reinforced concrete (SFRC) structural elements carried out to check the ductility requirements aimed at guaranteeing limit analysis approaches for the computation of ultimate load‐bearing capacity of SFRC structures; special attention is devoted to the role of the degree of redundancy of the structure. In particular, full‐scale shallow beams and slabs reinforced with steel fibers (with or without conventional longitudinal reinforcement) were tested in two different laboratories: the Politecnico di Milano (PoliMI) and the University of Brescia (UniBS). In this experimental campaign, two different fiber contents and fiber types were considered. The experimental investigation, carried out within the activities to support Annex L of Eurocode 2, was fundamental also for developing the design rules included in the fib Model Code 2020 and allowed to formulate conclusions regarding optimization of the mix design, ductility, and design prediction at the ultimate capacity.

Design of Steel Fibre Reinforced Concrete Structures According to the Annex L of the Eurocode-2 2023

The Annex L of the new Eurocode – 2 gathers -for the first time- provisions for the design of Steel Fibre Reinforced Concrete (SFRC) structures. This response to the necessity of the construction sector for provisions and regulations of this composite material within the European space and this Annex is supported by the solid background derived from consistent research and experiences carried out along the last two decades. The existence of these design provisions could both favour competitiveness and enhance sustainability performance within the construction sector since new concrete reinforcement alternatives could be considered while ensuring the structural reliability level accepted for traditional reinforced concrete structures. This paper is aimed to cover -not exhaustively- all those features concerning SFRC included into the EC-2 and complement those -when considered necessary- with scientific literature.

Research and mechanism analysis on dynamic compressive behavior of steel fiber reinforced concrete

This paper describes an investigation of the influence of steel fiber on the dynamic compressive behavior of steel fiber reinforced concrete (SFRC) at different strain rates, and reveals the mechanism of its change from the macro and meso levels. Meanwhile, a suitable dynamic constitutive model for SFRC is proposed. Four kinds of steel fiber contents (0 %, 1 %, 2 %, 3 %) and four kinds of high strain rates (40 s−1–220 s−1) were considered in the Split Hopkinson pressure bar (SHPB) test. Based on the designed tests, the failure pattern, the stress–strain curve, and the mechanical characteristic parameters of the SFRC under different loading conditions were obtained. The effects of steel fiber content and strain rate on the dynamic mechanical properties of concretes were analyzed. It is indicated that there are great differences in the failure pattern of the SFRC under different loading conditions. On the one hand, with the increase in the strain rate, the specimen is broken more completely. On the other hand, the reinforcing effect of steel fibers is obvious and as the steel fiber content increases, the crushing degree of the sample decreases significantly. The dynamic compressive strength of the SFRC is most sensitive to the strain rate and reaches its maximum value when the fiber content is 1 %. However, the variation of the peak strain showed no obvious regularity. The reinforcement mechanism of steel fibers was analyzed via the CT scanning technology. The results show that the main factors affecting the mechanical properties of SFRC are fiber content and porosity, but the fiber distribution is almost unaffected. The SFRC dynamic constitutive model was proposed by improving the HJC dynamic constitutive model. In the proposed model, the influence of steel fiber on strength was introduced and the influence term of strain rate was adjusted. The proposed model was verified by applying it to the numerical simulation of the dynamic compression performance of SFRCs and it shows good applicability. The presented research provides a theoretical basis for calculation and analysis of SFRC structures under extreme dynamic loads.

Numerical Analysis of Corrosion Reinforcements in Fibrous Concrete Beams

Abstract This paper offers a finite element method (FEM) to simulate the behavior of steel fiber reinforced concrete (SFRC) beams with corrosion of the longitudinal reinforcement using the ABAQUS package. This work was undertaken with the concrete damaged plasticity model (CDP). The expansion of corrosion product was utilized to represent the steel-concrete boundary to study the behavior of SFRC beams. Three beams with three volume fractions of steel fiber (0.8 %, 1.2 %, and 1.8 %) and three reinforced concrete (RC) beams with and without stirrups were created and tested under four-point loading to assess the shear capacity of beams. Corrosion of rebars at one of the RC beams that does not contain shear reinforcements will be studied. The crack patterns and load deflections of these beams were compared with experimental results found by the authors. The conclusions of this analysis will be valuable in considering the structural behavior of SFRC structures with uniform steel bar corrosion using FEM. Analytical results showed that the suggested model is qualified in better simulation and in accuracy of numerical and experimental results. The differences between analytical and experimental results were less than 8 % for load carrying capacity and 14 % for deflection; these differences are also satisfactory within the limits of the engineering conclusion.

Crack-bridging degradation and evolution in SFRC structural beams under variable amplitude flexural cyclic loading

A few studies have demonstrated a degradation in the performance of the steel-fibers in bridging cracks over the lifetime of steel-fiber reinforced concrete (SFRC) structures but only considered for constant amplitude cyclic loading. However, SFRC structures in practice are subjected to variable amplitude loading, thus their crack-bridging would be influenced by the preceding loading history. This concern has so far remained unexplored. This study presents experimental and analytical flexural cyclic responses of SFRC structural beams under constant and variable amplitudes. The crack-bridging stresses are evaluated using a proposed inverse method based on sectional analysis calculations. Besides, the maximum rebar strain level is correlated with the crack-bridging degradation law, aiming at reflecting the beam’s macroscopic response. The results show that crack-bridging stress increases if the fatigue load level increases, indicating that new fibers play a role in bridging cracks along the crack length. Further, decreasing the fatigue load level leads to an instant decrease in crack-bridging, which is followed by a stabilization as the pullout stress applied to the bridging fibers is reduced. Finally, a diagram of crack-bridging degradation and evolution is proposed regarding the maximum rebar strain, which is supposed to be a valuable tool in the SFRC structural design.

Numerical simulations of blast responses for SFRC slabs using an orthotropic model

This paper proposes a material model of steel fiber reinforced concrete (SFRC) subjected to dynamic loadings on the basis of an orthotropic concrete model. The effects of strain rate and fiber reinforcement are taken into account in the material model of SFRC from the uniaxial stress-strain relation to the triaxial strength envelope. The compressive and tensile strains as failure criteria are developed by reflecting the dynamic material properties, respectively. In particular, the post-cracking response according to steel fiber addition is investigated to improve the accuracy of the blast analysis for SFRC structures, and the proposed model is implemented as a user defined material model in LS-DYNA. The numerical results are compared with the experimental data in terms of damage pattern and average diameter of the damaged area for RC and SFRC slabs under explosions. The plastic based concrete models of K&C and CSCM are additionally compared to predict the behavior of SFRC structures. Furthermore, parametric studies are conducted with the proposed model to investigate the effects of reinforcement spacing and steel fiber on improving the structural resistance of SFRC slabs.

A strain rate dependent nonlinear elastic orthotropic model for SFRC structures

A strain rate dependent orthotropic model for steel fiber reinforced concrete (SFRC) structures subjected to impact or blast loads is proposed in this paper. Based on the previously introduced orthotropic model for plain concrete, an improved strain rate dependent material model for SFRC is developed to account for the effect derived from the addition of steel fibers in the concrete matrix and to cope with the multiaxial strain rate dependent behavior of SFRC. In addition to verification of the constructed multiaxial strength envelope of SFRC through correlations with experimental data, improved failure strains in the uniaxial stress-strain relation are also introduced to minimize the mesh size dependency in the numerical results. Finally, the proposed orthotropic model is incorporated into LS-DYNA as a user defined material model. Numerical analyses were performed for experiments of RC and SFRC slabs under projectile impact and blast loading, and the obtained results show that the addition of the steel fibers provides a considerable improvement in the structural resistance and the proposed model effectively simulates the structural responses of SFRC structures while minimizing the mesh size dependency in the numerical results.

Modelling and experimental validation of flexural tensile properties of steel fiber reinforced concrete

In this paper, a four-linear model for tensile σ-ε relation is proposed to analyze and evaluate the residual bending performance of steel fiber reinforced concrete (SFRC) in flexure. The four-linear model for tension is categorized into pre-cracked stage, cracking moment, crack propagation stage and failure stage. The strength of SFRC after cracking has been fully considered at the cracking moment and crack propagation stage in this model. Using axial force and bending moment equilibrium principles, the expression of each parameter in the σ-ε relationship is established. The proposed σ-ε relation is verified experimentally. It showed that the proposed σ-ε relation fairly predicts the load–deflection behavior of SFRC prisms. Further, the proposed σ-ε relation of SFRC is optimized by a simplified analytical approach in which the value of each key point is expressed by residual flexural tensile strength, and could be applied to predict the mechanical properties of SFRC structures.

Study of Combined Multi-Point Constraint Multi-Scale Modeling Strategy for Ultra-High-Performance Steel Fiber-Reinforced Concrete Structures.

Compared with normal strength concrete (NSC), ultra-high-performance steel fiber-reinforced concrete (UHPFRC) shows superior performance. The concrete damage plasticity (CDP) model in ABAQUS can predict the mechanical properties of UHPFRC components well after calibration. However, the simulation of the whole structure is seriously restricted by the computational capability. In this study, a novel multi-scale modeling strategy for UHPFRC structure was proposed, which used a calibrated CDP model. A novel combined multi-point constraint (CMPC) was established by the simultaneous equations of displacement coordination and energy balance in different degrees of freedom of interface nodes. The advantage is to eliminate the problem of the tangential over-constraint of displacement coordination equation at the interface and to avoid stress iteration of the energy balance equation in the plastic stage. The expressions of CMPC equations of typical multi-scale interface connection were derived. The multi-scale models of UHPFRC components under several load cases were established. The results show that the proposed strategy can well predict the strain distribution and damage distribution of UHPFRC while significantly reducing the number of model elements and improving the computational efficiency. This study provides an accurate and efficient finite element modeling strategy for the design and analysis of UHPFRC structures.

Probabilistic Studies on the Shear Strength of Slender Steel Fiber Reinforced Concrete Structures

Shear failure is a brittle and undesirable mode of failure in reinforced concrete structures. Many of the existing shear design equations for steel fiber reinforced concrete (SFRC) beams include significant uncertainty due to the failure in accurately predicting the true shear capacity. Given these, adequate quantification and description of model uncertainties considering the systematic variation in the model prediction and measured shear capacity is crucial for reliability-based investigation. Reliability analysis must account for model uncertainties in order to predict the probability of failure under prescribed limit states. This study focuses on the quantification and description of model uncertainty related to the current shear resistance predictive models for SFRC beams without shear reinforcement. The German (DAfStB) model displayed the lowest bias and dispersion, whereas the fib Model 2010 and the Bernat et al., model displayed the highest bias and dispersion. The inconsistencies observed in the resistance model uncertainties at the variation of shear span to effective depth ratio are a major cause for concern, and differentiation with respect to this parameter is advised. Finally, in line with the EN 1990 semi-probabilistic approach for reliability-based design, the global partial safety factors related to model uncertainties in the shear resistance prediction of SFRC beams are proposed.

Impedance-based structural health monitoring applied to steel fiber-reinforced concrete structures

The goal of the present contribution is to test the effectiveness of an impedance-based structural health monitoring (ISHM) methodology to detect damage in steel fiber-reinforced concrete structures. For this aim, a piezoceramic sensor coated with epoxy is used. A prismatic concrete specimen was evaluated for the case in which the damage condition was obtained from compression testing. The impedance signatures were measured both before and after the compression testing for the specimens placed inside an environmental chamber. An optimization procedure was applied to compensate for the temperature effects that appear on the impedance signatures. For illustration purposes, the coated piezoelectric transducer was initially bonded to the surface of an aluminum beam to detect damage induced by a loosening bolt. An impedance analyzer was used to measure impedance signatures in the frequency range of 40–70 kHz for the procedure with the aluminum beam and 50–80 kHz for the prismatic steel fiber concrete specimen. An in-house portable impedance meter (SySHM impedance meter) was used to measure the impedance signatures. The results encourage the use of the ISHM approach for health monitoring of steel fiber-reinforced concrete structures.

Application of the design code for steel fibre reinforced concrete into finite element software

Steel fibre reinforced concrete (SFRC) is more and more used as construction material. Well-known are applications for industrial floors. Nevertheless, this material is also widely used in structural concrete elements where it is especially effective to cover crack widths in combination with standard conventional reinforcement. Currently there is no general part in Eurocode 2 [1] for buildings regarding steel fibre concrete. There are various standards and recommendations for the calculation of the steel fibre contribution. One of the most applied publications is the German guideline DAfStb [2] which was used as a basis for the numerical analysis outlined in this paper. Thus, company SCIA N.V. together with Bekaert N.V. as producer of DRAMIX® fibres developed a special tool for designing and checking SFRC structures. This tool was built in into finite element software SCIA Engineer and based on the German guideline. To gain a fundamental understanding of the structural behaviour, we have also developed non-linear FEM models, which contain a built-in module for SFRC structures. Numerical simulation results are compared to experimental performance of SFRC slab on piles of the Nanhui project in Shanghai in terms of ultimate load and crack pattern.

Strain rate effect on interfacial bond behaviour between BFRP sheets and steel fibre reinforced concrete

Numerous studies have shown that using steel fibre reinforced concrete (SFRC) and retrofitting with Fibre-reinforced polymer (FRP) composites can improve the strength and ductility of RC structures against impact and explosive loadings. The interface between FRP and concrete has been identified as one of the weakest parts of the FRP strengthened structures subjected to dynamic loading, with debonding failure usually observed as the primary failure mode. In order to properly analysis and design of FRP strengthened reinforced concrete (RC) structures, it is important to understand the dynamic bonding strength between FRP and concrete. An experimental investigation regarding to the dynamic interfacial bond behaviour between basalt fibre (BFRP) sheets and SFRC is carried out in this study. Concrete prisms were made of short steel fibres with three volumetric fractions (i.e. Vf = 0.5%, 1.0%, and 1.5%) to improve the tensile strengths. To achieve different strain rates, the loading velocities varied from 8.33E-6 m/s, 0.1 m/s, 1 m/s, 3 m/s, to 8 m/s. Experimental results show the bond strength and bond-slip were sensitive to strain rate. The loading rate changed the debonding failure modes from concrete substrate failure to interfacial debonding. In addition, the shear resistance of the interface increased with the fibre volume under both quasi-static and dynamic loadings. Based on the testing data, an empirical bond-slip model, incorporating the volumetric fraction of steel fibre and strain rate, is established for FRP-strengthened SFRC structures.

Model for the compressive stress–strain relationship of steel fiber-reinforced concrete for non-linear structural analysis

A model for non-linear calculations of steel fiber-reinforced concrete (SFRC) elements in compression is proposed and described technologically. The same curve produced by Eurocode 2 (EC2) is used until compressive strength, although non-dimensional variables refer to fiber concrete parameters. Beyond the peak, we use a parabola that is derived so that the average energy consumption of the material equals this same mean energy obtained from a database comprised of 197 tests. Estimates for the compressive strength of SFRC and the corresponding critical strain are also provided by correlation with the database using the surface response methodology. The total energy consumption of SFRC in compression is approximately four times higher on average than the corresponding energy of the base plain concrete, which is included in the model. Therefore, it can considerably improve the modeling of ductility in SFRC structures.

Pull-out behaviour of spiral-shaped steel fibres from normal-strength concrete matrix

Reinforcing concrete with steel fibres has been proven being able to improve the properties of concrete such as strain capacity, impact resistance, energy absorption and tensile strength because the discrete steel fibres act as dispersed reinforcement in mitigating crack propagation. A number of laboratory tests in recent years have indicated the superiority of spiral geometry of steel fibres as compared to other fibre types in terms of ductility, crack controllability, dynamic strengths and energy absorption. Understanding fibres’ pull-out behaviour is essential to evaluate the properties of steel fibre reinforced concrete (SFRC). The pull-out behaviour of straight, hooked-end and twisted steel fibres have been investigated by many researchers. However, there has been no study that focuses on pull-out behaviour of spiral fibres in the literature yet. The pull-out behaviour of steel fibre with spiral geometry from concrete matrix is experimentally investigated in the present study. In addition, steel fibres with hooked ends were also prepared and tested for comparison. The spiral fibres with various geometric characteristics such as pitch length, embedment length and coil diameter were pulled out from concrete matrix at different rates. Test results showed several peak loads before reaching a final maximum pull-out force. Based on the experimental results, the influences of embedment length, pull-out rate, coil diameter and pitch length were investigated. Through analysing the testing results, combining hooked-end fibres and spiral fibres was proposed as a hybrid steel fibre reinforcement technique in order to further improve the performance of SFRC materials and structures. Laboratory tests in this study proved the effectiveness of this technique in enhancing the capacity of the reinforced concrete structure.

An efficient method to derive statistical mechanical properties of concrete reinforced with spiral-shaped steel fibres in dynamic tension

Steel-fibre-reinforced concrete (SFRC) has been recognised as an effective solution to resist impact loading on structures. The reliable application and efficient design of SFRC structures depends on the knowledge of its mechanical properties. Since many important factors, including the locations and orientations of fibres and aggregates in concrete and the material properties of concrete matrix, are intrinsically random, the mechanical properties of SFRC present a high level of randomness. To accurately quantify them, effective statistical techniques are indispensable. Using traditional statistical techniques, a large quantity of data, from either experiments or numerical simulations, are needed to derive the correlation between the mechanical properties and the random factors. However, both ways are time-consuming and costly. Therefore, very little information regarding the statistical mechanical properties of SFRC can be found in the current literature. In this study, a kernel-based nonparametric statistical method is proposed to derive the statistical mechanical properties of SFRC with limited number of data. The behaviours of SFRC with randomly distributed spiral-shaped fibres and aggregates under impact loading are simulated using commercial software LS-DYNA. The simulation accuracy is validated by the experimental results. The influences of various volume fractions of fibres on dynamic increase factor (DIF) of the tensile strength of SFRC specimens under dynamic loadings at different strain rates are quantified through a prediction model obtained from kernel regression. The results demonstrate that the proposed method is able to estimate the DIF value of SFRC based on the tensile strength and strain rate, and to derive the statistical mechanical properties of SFRC.

Multilevel Computational Model for Failure Analysis of Steel-Fiber–Reinforced Concrete Structures

AbstractA multilevel modeling framework for the failure analyses of structures made of steel-fiber–reinforced concrete (SFRC), which allows researchers to follow the effects of design parameters such as fiber type, distribution, and orientation from the scale of fiber–matrix interaction to the structural behavior, is proposed. The basic ingredient at the level of single fibers is an analytical model for the prediction of the pullout response of straight or hooked-end fibers. For an opening crack in a specific SFRC composite, the fiber bridging effect is computed via the integration of the pullout response of all fibers intercepting the crack, taking anisotropic fiber orientations into consideration. For the finite-element analysis of the failure behavior of SFRC structures, interface solid elements are used to represent cracks. The softening behavior of opening cracks is governed by cohesive tractions and the fiber bridging effect. The use of an implicit/explicit integration scheme enhances the computatio...

Analysis of Steel Fiber-Reinforced Concrete Elements Subjected to Shear

In this paper, a rational analysis procedure is presented for modeling the shear behavior of steel fiber-reinforced concrete (SFRC) elements. In the development of the analysis procedure, the Disturbed Stress Field Model (DSFM), based on the Modified Compression Field Theory (MCFT), is modified by implementing constitutive models for SFRC, which are derived from the Diverse Embedment Model (DEM). For the contribution of steel fibers, a local stiffness matrix for fibers has been developed separately from those for concrete matrix and conventional reinforcement. The composite element stiffness matrix for an SFRC element with conventional reinforcement is then derived by superposing the three local stiffness matrixes. In the element stiffness matrix, the effect of shear slip at a crack is also taken into account by considering the resistance due to steel fibers against shear stress on crack surface. Through comparisons with the test results of SFRC panels previously reported in the literature, it is shown that the actual shear behavior of SFRC panels are accurately predicted by the proposed analysis procedure, not only for the shear strength but also for the shear strain at the failure. Through implementation into finite element analysis programs, the analysis procedure devel oped in this paper can be useful in the modeling of SFRC members and structures also containing conventional reinforcement.

Compressive Behavior of Fiber-Reinforced Concrete with End-Hooked Steel Fibers.

In this paper, the compressive behavior of fiber-reinforced concrete with end-hooked steel fibers has been investigated through a uniaxial compression test in which the variables were concrete compressive strength, fiber volumetric ratio, and fiber aspect ratio (length to diameter). In order to minimize the effect of specimen size on fiber distribution, 48 cylinder specimens 150 mm in diameter and 300 mm in height were prepared and then subjected to uniaxial compression. From the test results, it was shown that steel fiber-reinforced concrete (SFRC) specimens exhibited ductile behavior after reaching their compressive strength. It was also shown that the strain at the compressive strength generally increased along with an increase in the fiber volumetric ratio and fiber aspect ratio, while the elastic modulus decreased. With consideration for the effect of steel fibers, a model for the stress–strain relationship of SFRC under compression is proposed here. Simple formulae to predict the strain at the compressive strength and the elastic modulus of SFRC were developed as well. The proposed model and formulae will be useful for realistic predictions of the structural behavior of SFRC members or structures.

SFRC exposed to high temperature: Hot vs. residual characterization for thin walled elements

An experimental investigation based on unnotched four point bending tests is here presented. It aims to check the high temperature resistance of thin slabs made of steel fiber reinforced concrete (SFRC). The application of fire recommendations to SFRC structures requires knowledge of the tensile strength decrease due to high temperature exposure. Very few data are available in the literature, but useful information could be obtained in “residual” conditions, i.e. by testing the specimen at room temperature after a suitable thermal cycle. The paper discusses the influence of the testing conditions (hot vs. residual) on the tensile mechanical behavior of the material, the evaluation of self-stresses induced in the samples by the test procedure adopted and the variation of post-cracking tensile properties at SLS and ULS according to the design approach proposed by Model Code 2010.