Steel Fiber Reinforced Concrete Research Articles

The effect of using steel fibers reinforced concrete layer in compression zone on the flexural behavior of over‐reinforced concrete beams

AbstractDuctility is one of the most important requirements of RC design besides other requirements such as strength and stiffness. In recent years, concrete beams with a large amount of tensile reinforcement ratio (ρ) have been required in large projects, such as high‐rise buildings and bridges. Using high (ρ) increases the strength and stiffness of the beams and leads to a decrease the ductility. This paper aimed to investigate the effect of using steel fibers reinforced concrete (SFRC) layer in compression zone on the flexural behavior of over‐reinforced concrete beams. Partial replacement of concrete with SFRC layer in compression zone was used to avoid the brittle compression failure of concrete. For this purpose, 12 over‐reinforced concrete beams with a span of 1300 mm and a cross‐section of 120 mm × 180 mm were cast and tested under a three‐point loading. The first beam was used as a reference beam and others as strengthened beams using SFRC layer. The effects of thickness and length of strengthening layer, (ρ), volume fractions of steel fibers, and strengthening methods on flexural performance of tested beams were investigated. The flexural capacity, failure modes, crack patterns, load–deflection relationships, load–strain relationships, ductility index, and toughness were evaluated. The results showed that the suggested technique (SFRC layer in the compression zone) is an effective technique to increase ultimate load, ductility, and toughness by up to 35.72%, 121.01%, and 349.16%, respectively, compared to the reference beam. Furthermore, the failure mode changed from brittle to ductile failure. Also, one of the main advantages of this technique is allows to increase of (ρ) up to 8.55% (4.15ρb) without needing the compressive reinforcement. The results of this study indicate that the using SFRC layer in the compression zone is an appropriate and economical method to provide high flexural capacity and ductility over‐reinforced concrete beams.

Dynamic stress-strain relationship of steel fiber-reinforced rubber self-compacting concrete after exposure to high temperature

The purpose of this study was to investigate the dynamic compression characteristics of steel fiber-reinforced rubber self-compacting concrete (SFRSCC) after exposure to high temperatures by using the split Hopkinson pressure bar (SHPB) impact test. The study considered four parameters: strain rates of 55 s−1, 90 s−1, and 155 s−1; volume fractions of steel fibers of 0%, 0.5%, 1.0%, and 1.5%; volume fractions of rubber contents of 0%, 10%, 20%, and 30%; and temperatures of 25 ℃, 300 ℃, 600 ℃, and 900 ℃. Experimental results revealed that: (I) The toughening effect of rubber decreases after exposure to high temperature, which decreases the specimens' dynamic compressive strength and toughness index after high temperature with the increase of rubber content. (II) The dynamic compressive strength, toughness index, and dynamic increase factor (DIF) of the specimens increased with the increasing steel fiber content after exposure to the same temperature. (III) The dynamic compressive strength of the specimens decreased linearly as the temperature increased, and the toughness index initially increased and then decreased. However, the incorporation of steel fiber mitigated this reduction. (IV) The impact of strain rate on compressive strength, toughness index, and DIF-σ was more significant for specimens exposed to high temperatures than for those at room temperature Finally, a stress–strain curve prediction formula considering temperature, rubber content, steel fiber content, and strain rate of SFRSCC was proposed.

Effect of fibre diameter and tensile strength on the mechanical, fracture, and fibre distribution properties of eco-friendly high-strength self-compacting concrete

The development of Steel Fibre-Reinforced Self-Compacting Concrete (SFR-SCC) is a complex task with significant practical implications for the construction industry. However, the absence of established design guidelines and recommendations, and the limited availability of design methodologies, present a significant challenge. Achieving the targeted rheological and mechanical properties while simultaneously minimising production costs requires a comprehensive understanding of the optimal blend of fibre type and quantity and the coarse aggregate content. This paper addresses the impact of steel fibre properties on the rheological and fundamental mechanical properties of eco-friendly high-strength self-compacting concrete (HSSCC), using 40% ground granulated blast furnace slag (GGBS) as a cement replacement. The research focused on 30 mm long hooked-end steel fibres with tensile strengths of 1345 MPa and 3070 MPa, and diameters of 0.55 mm and 0.38 mm, respectively. In addition, the study investigated the combined effect of coarse aggregate content and steel fibre type on the performance of eco-friendly HSSCC. The fresh properties of eco-friendly HSSCC were assessed using the slump flow and J-ring tests. Furthermore, mechanical and fracture properties such as compressive strength, splitting tensile strength, elastic modulus, four-point flexural strength, and three-point flexural strength on notched prisms were also evaluated. The distribution and alignment of steel fibres in the HSSCC were assessed using image analysis techniques, which showed that the steel fibre diameters were a crucial factor in the fibre dispersion. The results demonstrated that using steel fibres with higher tensile strength and smaller diameter significantly enhanced the splitting tensile strength, flexural strength, and fracture energy, compared to steel fibres with larger diameters and lower tensile strengths. Additionally, the study indicated that the elastic modulus and fracture energy of eco-friendly HSSCC reinforced with both types of steel fibre are highly dependent on the content of coarse aggregate used. The study highlighted that the coarse aggregate proportion and the steel fibre characteristics were two vital factors that influenced the rheological and mechanical properties of HSSCC and must be considered during the mix design process to optimise the desired properties while also minimising production costs.

Impact resistance of micro and macro crimped steel fibre reinforced self-compacting concrete with SCM

This paper aims to investigate the impact resistance of micro and macro crimped steel fibre-reinforced self-compacting concrete (SFRSCC) with the volume fractions of 0.25%, 0.50%, and 0.75%. A total of seven mixtures were investigated with different volume fractions of steel fibres considering in terms of fresh, mechanical properties and impact resistance. The total binder contains 60% cement and 40% supplementary cementitious materials (SCM) with a combination of 30% fly ash and 10% silica fume. The addition of steel fibre decreases the workability of SCC. However, the inclusion of SCM compensates and increases its workability. Mechanical behaviour of micro and macro steel fibre reinforced SCC outperforms the conventional SCC in terms of compressive strength, split tensile strength and flexural strength. The impact resistance of SFRSCC is determined through a modified testing procedure derived from the guidelines provided by ACI 544.2 R. It is observed that the volume fraction of both 0.75% micro and macro steel fibre has improved impact resistance with increase percentage of energy absorption of 180% and 240%. The relationship between UPV and impact test results found that the number of blows can be predicted using the UPV values from the regression equations. The results of the Weibull distribution (WD) and reliability analysis indicate that 80% (τ = 0.8) of the SFRSCC samples are reliable enough to survive 14 blows at first crack and 20 blows at failure crack. The incorporation of crimped steel fibres into the concrete matrix led to the even distribution of impact loading throughout the concrete specimen with the help of steel fibres, thereby improving the impact resistance of SCC.

Influence of Fiber Orientation on the Shear Properties of Steel Fiber-Reinforced Reactive Powder Concrete Beams

Steel fiber-reinforced reactive powder concrete (SFRPC) has good mechanical properties and is thus useful for engineering applications. Generally, the fibers in SFRPC are randomly distributed. To study the effects of the amount, length, and orientation of steel fibers on the shear strength of SFRPC beams, nonreinforced aligned steel fiber-reinforced reactive powder concrete (ASFRPC) beams were prepared using a large-scale electromagnetic field orientation device. Shear tests of specimens with a shear span ratio of 1.5 were performed under two fiber contents (1.0% and 2.0%) and three fiber lengths (20, 30, and 40 mm). The full-field strain during the loading process was obtained via the digital image correlation method, and the effects of fiber quantity and orientation on the shear strength of the beams were analyzed. The ASFRPC beams exhibited higher ductility but lower shear capacity than the SFRPC beams under the same conditions. The shear capacity increased with the quantity of steel fibers. According to the test results, the calculation formulas of the shear-bearing capacities of ASFRPC and SFRPC beams were established.

Mechanical performance of fiber-reinforced concrete and functionally graded concrete with natural and recycled aggregates

This study aims to assess the mechanical performance of functionally graded concrete (FGC) in comparison to conventional plain cement concrete (PCC) and fibre-reinforced concrete (FRC). For this purpose, five concrete mixes were prepared, defined in eleven patterns, including PCC as a control mix, two FRC mixes, and eight combinations for FGC mixes. The hooked-end steel fibres and recycled thin polyethylene terephthalate (PET) strips from used bottles were utilized as fibres in 0.75 % of the volume of the mix in the FRC and FGC mixes. The natural aggregates were replaced by 15 % by weight with recycled plastic aggregates (RPA) and recycled concrete aggregates (RCA) in the FGC mixes. The mechanical performance of all specimens was assessed by compressive, split tensile and flexural strength tests, whereas the concrete quality was evaluated by the ultrasonic pulse velocity (UPV) test. The results revealed that the FRC mix containing steel fibres has 50.6 % higher compressive strength, while PET-FRC exhibited 42.4 % lower strength than the PCC. The best combination of the FGC mix is RCA + SFRC (H), which showed 27.6 % more compressive strength than PCC. The steel fibre-reinforced concrete (SFRC) mix has the highest split tensile strength, which is 47.5 % higher than the PCC. The best combination of FGC mixes is SFRC + PET-FRC (J), yielding 29.3 % higher tensile strength than PCC. On the other hand, both fibres contribute to the flexural strength enhancement of FRC. The FGC mix (K) yielded the highest flexural strength, with PET-FRC and SFRC in the upper and bottom layers, respectively, giving 40 % more flexural strength than the PCC specimen. The UPV values depicted that fibres and recycled aggregates in FGC specimens enhanced the quality of concrete. The highest value of UPV was recorded in the mix PCC + SFRC (G) containing PCC and steel fibres, which almost reached the upper limit of the recommended UPV values.

Non-destructive testing method of fiber content in steel fiber reinforced concrete based on magnetization loss

Steel fiber reinforced concrete (SFRC) has been widely used in construction engineering due to its superior crack resistance and flexural capacity. Its mechanical properties mainly depend on the distribution of fibers, and the uneven distribution will reduce the structural performance. Therefore, this paper proposed a fast non-destructive testing method for steel fiber content evaluation based on magnetization loss. This method could solve the problem that the traditional methods were difficult to detect in situ. It is found that the higher the steel fiber content, the greater the magnetic loss and electrical loss of the fiber under sinusoidal excitation, resulting in the changes of the excitation coil impedance. Based on this phenomenon, the impedance difference modulus and quality factor evaluation index of the excitation coil are proposed, which can be used to accurately estimate the internal fiber content of concrete in-situ detection. The feasibility of the method is verified by experiments, which is of great significance to the quality control in the manufacturing process of SFRC.

A novel analytical framework for assessing the impact response of SFRC beam

This paper presents a novel model for predicting the impact response of steel fiber reinforced concrete (SFRC) beams. The model utilizes the principles of energy conservation and impulse-momentum theorem to calculate the maximum reaction force and peak impact force. A tensile behavior model for simulating the concrete behavior is proposed considering the effect of volume fraction of steel fibers and the effect of strain rate on the concrete properties. Additionally, the conventional beam theory, along with a cross-section-layered approach, is used to express the total beam's reaction forces vs deflection relationship. Afterwards, the model calculates the maximum midspan deflection of SFRC beams subjected to impact loading by applying the principle of conservation of energy and considering the effect of strain rate. The proposed model is compared with 121 impact tests, and the results show that the model can estimate the maximum reaction force of SFRC beams with acceptable accuracy.

A quantitative investigation on the fragmentation performance of SCDA in cracking steel fiber reinforced concrete

A better application of Soundless Chemical Demolition Agents (SCDAs) in fragmentation is based on our knowledge on their expansive characteristics. In this study, an integration of experimental and modeling approaches was developed to investigate SCDA expansion and corresponding cracking processes of Steel Fiber Reinforced Concrete (SFRC). The expansive properties of the selected SCDA were first estimated by free volume expansion (FVE) tests and expansion pressure (EP) tests. Then, cracking experiments were performed on 150 mm cubic SFRC blocks with a predrilled hole, and the cracking process was monitored by using Acoustic Emission (AE) and Digital Image Correlation (DIC) techniques. Generally, the cracking process can be divided into three phases, termed micro-cracking phase, macro-cracking phase, and failure phase. It was observed the hole diameter and fiber content exert significant influences on the cracking process. The crack initiation time was shortened as the hole diameter increases, while it was prolonged with increasing fiber content. The cracked area ratio was further proposed and measured to assess the cracking performance of SCDA. The ratio increases from 2.73% to 8.293% as the hole diameter increases from 16 mm to 26 mm, while it decays from 9.75% to 1.823% as the fiber content increases from 0 to 2.4%. After verified with the experimental data, a modified Concrete Damaged Plasticity (CDP) model was adopted to extend the fragmentation of SFRC from the centimeter scale to the meter scale. The simulations were conducted on SFRC models with various hole diameters, spacings, and fiber contents. The simulated results show the fragmentation performance is related to both hole parameters (e.g., hole diameter, spacing, etc.) and SFRC properties (including elastic modulus, fiber content, etc.). In particular, the cracked area (DamageT > 0.9) varies directly with hole spacing, diameter, and fiber content. Built on the simulated results, an empirical equation was formulated to quantitatively evaluate the fragmentation ability of individual holes. In addition, extra simulations were conducted to assess the efficiency of SCDA in cracking PC/SFRC beams of various sizes. The obtained cracked area tends to decrease first and then approach a constant value with increasing beam sizes, indicating the existence of a size effect, which is primarily due to variations in concrete restraints provided by beams of different sizes. This work quantitatively evaluates expansive properties of SCDA and SCDA-induced cracks, and thus provides a reference for optimizing the field application parameters.

Effect of Contents, Tensile Strengths and Aspect Ratios of Hooked-End Steel Fibers (SFs) on Compressive and Flexural Performance of Normal Strength Concrete

This study is a part of the study to simplify the reinforcing details of reinforced concrete (RC) structural members by substituting the conventional reinforcement with hooked-end steel fibers (SFs). This paper investigates the effects of SF strength, dosage and aspect (l/d) ratio on the compressive and flexural behaviors of normal strength concrete with specified compressive strength of 30 MPa. In this study, hooked-end SFs of high strength (2000–2400 MPa) and normal strength (1100–1200 MPa) were used with three l/d ratios of 64, 67 and 80. Hooked-end SFs were incorporated with three dosages of 20 kg/m3 (0.25 vol.%), 40 kg/m3 (0.50 vol.%) and 60 kg/m3 (0.75 vol.%). Eighteen steel fiber reinforced concrete (SFRC) mixes were mixed. To evaluate the compressive and flexural performance of each SFRC mixture, three SFRC cylindrical and prismatic specimens for each mixture were manufactured and tested, respectively. The test results that the inclusion of hooked-end SFs had little effect on the compressive strength, while it improved the toughness of concrete. Hooked-end SFs were also found to be effective in enhancing the flexural performance of concrete. The dosage and properties (strength and l/d ratio) of SFs significantly affect the residual flexural tensile strength (fR1 and fR3) at serviceability (SLS) and ultimate limit state (ULS) defined in fib Model Code 2010 (MC2010).

Machine learning-driven predictive models for compressive strength of steel fiber reinforced concrete subjected to high temperatures

Steel-fiber-reinforced concrete (SFRC) has emerged as a viable and efficient substitute for traditional concrete in the construction industry. By incorporating steel fibers into the concrete mixture, SFRC offers enhanced crack resistance, improved post-cracking performance, and effective stress transfer. To optimize cost and time in the construction sector, the application of machine learning (ML) methods is now prevalent for accurately estimating concrete characteristics. Accordingly, the present study focuses on utilizing novel ML techniques that include adaptive neuro-fuzzy inference systems (ANFIS), artificial neural networks (ANN), and gene expression programming (GEP) to predict the compressive strength (CS) of SFRC at elevated temperatures. For developing the ML-based models, 307 experimental records were acquired from the published studies conducted between 2000 and 2022. The models' accuracy was assessed using multiple statistical indices. The developed models provide excellent performance with a correlation coefficient (R) value of 0.962 for ANN, 0.998 for GEP, and 0.968 for ANFIS models. Overall, the GEP model provided higher accuracy and less error compared to ANN and ANFIS models. Moreover, the SHapley Additive exPlanation (SHAP) technique was used to interpret the outcomes of the ML-based model predictions. The combined SHAP value of cement content, temperature, and water-to-cement ratio accounts for 80.7% of the total SHAP value across all features. In addition, the SHAP analysis revealed that the actual temperature holds greater significance compared to the heating rate when considering its impact on compressive strength. The comparison of the developed model with the multivariable regression (MLR) method provided that the ML-based models have more prediction accuracy than traditional prediction methods. The proposed models provide designers and builders with an efficient and versatile tool for evaluating attributes, enabling accurate predictions of the CS of SFRC under high temperatures in construction applications.

Numerical analysis of the shear behavior for steel fiber reinforced concrete beams with corroded reinforcing bars

Steel fiber reinforced concrete (SFRC), which improves mechanical properties after cracking, became an interesting material for reinforced concrete structures. However, there is limited research on the performance of SFRC beams in corrosive environments. This paper concerns numerical simulations to validate and evaluate the impact of design-oriented parameters contributing to the shear behavior of SFRC beams corroded under chloride attack. The finite element analysis (FEA) results were compared with the experimental data for both non-corroded and corroded SFRC beams. To minimize the differences, the comparison between the proposed models and experiment showed a close agreement on the load–displacement relationship, failure mode, and cracking pattern. After conducting parametric investigations, the numerical results reveal that the shear reinforcement ratio insignificantly affects the shear strength of corroded SFRC beams. In contrast, concrete compressive strength, shear span-to-depth ratio, and corrosion degree of tensile longitudinal reinforcements significantly impact the shear strength of corroded SFRC beams.

Computational Fracture Evolution Analysis of Steel-Fiber-Reinforced Concrete Using Concrete Continuous Damage and Fiber Progressive Models.

The process of concrete cracking is a common problem because the first micro-cracks due to the loss of moisture may appear even before the concrete is loaded. The application of fracture mechanics allows for a better understanding of this problem. Steel-fiber-reinforced concrete (SFRC) samples with a notch were subjected to a three-point bending test, and the results for crack energy were used to analyze the concrete's material properties. In this paper, an experimental and numerical analysis of SFRC with rapid changes in the force (F) crack mouth opening displacement (CMOD) curve (F-CMOD) is presented. In order to obtain the relevant F-CMOD diagrams, three-point bending tests were carried out with non-standard samples with a thickness equal to one-third of the width of standard samples. For analysis purposes, crimped steel fibers were adopted. A probabilistic analysis of the most important parameters describing the material in question, such as peak strength, post-cracking strength, crack mouth opening displacement (CMOD), fracture energy, and the post-cracking deformation modulus, was conducted. The tests and the analysis of their results show that the quasi-static numerical method can be applied to obtain suitable results. However, significant dynamic effects during experiments that influence the F-CMOD curves are hard to reflect well in numerical calculations.

A Finite Element Approach to Evaluate and Predict the Shear Capacity of Steel Fiber-Reinforced Concrete Beams

Adding steel fibers to a concrete matrix enhances the shear capacity of reinforced concrete beams. A comprehensive understanding of this phenomenon is essential to evaluate engineering designs accurately. The shear capacity of Steel Fiber Reinforced Concrete (SFRC) beams is affected by many parameters, such as the ratio of the shear span to the effective depth of the SFRC beam, the compressive strength of concrete, the longitudinal reinforcement ratio, volume fraction, aspect ratio, and the type of fibers. Therefore, to cover the influence of these parameters on the shear capacity of SFRC beams, 91 beams from previous studies, divided into 10 groups, were considered in the current study. Two approaches have been used to predict the shear capacity of SFRC beams. The first approach used 7 predicting equations derived from previous studies and the second one used finite element analysis (ANSYS software) to simulate the 91 beams. Despite the many approaches to simulate the structure elements, there is no reliable approach able to simulate satisfactorily 91 SFRC beams as this study does. The log file of ANSYS software was used to simulate and calculate the shear strength capacity of the beams. The results show a reasonable agreement with the experimental tests. The extracted results were much closer and more realistic than those obtained by the predicting equations. Also, the χ factor (squared value of experimental shear capacity to the predicted shear capacity) of the ANSYS software results is 97%, while the closest proposed equation gives 91%.

Effect of fresh concrete compression technique on pre- and post-heating compressive behavior of steel fiber-reinforced concrete: Experiments and RSM-based optimization

Despite the many advantages of fresh concrete compression technique for improving the performance of hardened concrete under compression, no study has so far addressed various behavioral aspects of this type of concrete in different conditions and scenarios such as fire, which may occur during the service life for a structure made up of this concrete type. Here, the compressive performance of compressed concrete with steel fibers was addressed before and after experiencing elevated temperatures considering key variables including the volume quantity of steel fibers, diameter-to-thickness ratio of steel tube (mold), capacity of the reference concrete, and exposure temperature. To reach this goal, 60 concrete cylinders were tested under axial compression, and the results including the compressive strength, elastic modulus, strain at peak stress, toughness, axial stress–strain graph, and weight loss were assessed together with the results of a visual inspection, failure mode, and microstructure of concrete. The results demonstrated that although the fresh concrete compression technique notably improved the mechanical features of concrete, especially the compressive strength by up to 100 % at the ambient temperature, adding steel fibers, lowering the water-to-cement ratio (w/c), and using a tube with a thinner wall as the mold lowered the efficiency of this technique in improving the compressive performance. In addition, the amount of increase in concrete strength due to compressing the fresh concrete had a linear relationship with the amount of excess water removed from it. The positive impact of the fresh concrete compression technique on the concrete strength at high temperatures was significant. In this regard, the compressive strength reduction of the compressed concrete was 20 % lower than that of the uncompressed concrete. However, the fresh concrete compression technique was not very effective against the deterioration of the modulus of elasticity caused by high temperatures. Ultimately, the response surface method (RSM) was used to reach an optimum solution for the design variables that could maximize the compressive capacity of fibrous compressed concrete per different target temperatures.

Study on the penetration capability of GPR for the steel-fibre reinforced concrete (SFRC) segment based on numerical simulations and model test

The SFRC segment is gaining attention in shield tunnels due to their better mechanical properties. However, the existence of steel fibres poses a greater difficulty when attempting to utilize ground penetrating radar (GPR) for nondestructive quality inspection compared to conventional reinforced concrete segments. This paper presented a refined numerical model of SFRC segments with gprMax software, to investigate the penetration characteristics of GPR. The numerical model incorporated four different steel fibre contents, namely 0%, 0.5%, 2.5%, and 5%. The model also included an internal void and a surface metal plate to compare and analyze the waveform characteristics of GPR with the normal sections of the segment. The orthogonal test of numerical simulation investigates the imaging characteristics, power attenuation patterns, and imaging features of three different GPR frequency gradients: 200 MHz, 500 MHz, and 1000 MHz. This analysis helps in selecting the appropriate detection frequency based on the observed energy attenuation patterns and imaging characteristics. Then, the model test was conducted at the selected frequency to study the penetration performance of GPR. The research results indicate that the numerical model established in this paper demonstrates good effectiveness in studying the waveform characteristics of steel fibre-reinforced concrete under various conditions. The numerical model established in this paper realized the study of the penetration characteristics and imaging features of different steel fibre contents, GPR frequencies, and detection targets. Analysis of waveform characteristics, power attenuation, and imaging results indicates that GPR at 500 MHz exhibits better detection performance for SFRC segments. The results of the numerical simulation were verified through model tests of real SFRC segments, demonstrating that 500 MHz has good penetration performance for SFRC segments. This paper also proposed a calculation method for equivalent relative dielectric constant (ERDC) based on the model test results and validated its rationality through the analysis of numerical simulation results and real GPR waveforms.

Experimental study on recycled steel fiber-reinforced concrete under repeated impact

Abstract Recycled steel fiber comes from the waste produced by machining. Adding recycled steel fiber into concrete can significantly enhance the toughness of concrete. In order to study the impact toughness of recycled steel fiber-reinforced concrete, the drop weight repeated impact experiment method was used to study the performance of recycled steel fiber-reinforced concrete under repeated impact load. Four kinds of recycled steel fiber-reinforced concrete samples with different volume contents were designed and made, and the loading impact experiments under five working conditions were carried out. Taking the drop weight and drop height as changing parameters, the corresponding blow counts of the first crack and sample failure under the impact of the drop hammer are recorded, and the ductility coefficients of different samples are calculated. The results show that the impact resistance of the sample decreases significantly with the increase of the drop weight and drop height. With the increase of recycled steel fiber content, the impact toughness of the sample increases obviously. The impact toughness of recycled steel fiber-reinforced concrete under standard loading conditions is the best.

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.

Combined corrosion and inclination effects on pullout behavior of various steel fibers under wet-dry cycle deterioration

The interfacial bond behavior of corroded fibers in cracked steel fiber reinforced concrete (SFRC) is critical for understanding its structural deterioration performance, while available research focused on aligned straight or hooked-end fibers. This study investigated the combined effects of corrosion and inclination on the pullout performance of various steel fibers bridging artificial cracks exposed to wet-dry cycles of chlorides. Three deformed steel fibers, i.e., hooked-end (H), crimped (C), and milled-cut (M) fibers, with three angles of 0°, 30°, and 45° were examined under both uncorroded and corroded conditions. After corrosion, the average bond strength of H fibers was enhanced due to increased roughness and compact rust layer if the high inclination-induced fracture was avoided, while that of C and M fibers was decreased at various angles because of loose and porous rusts. The highest bond strengths of corroded H, C, and M fibers were discovered at 30°, 30°, and 0°, as a result of the combined mechanisms of snubbing effect, matrix spalling, or corrosion-induced fracture. The ultimate displacement of all fibers increased with angles due to increased matrix spalling area and fiber plastic deformation. The inclination effect of corroded fibers was more significant on energy absorption capacity than average bond strength, regardless of fiber type. A degradation model that describes the snubbing and matrix spalling effects, as well as the corrosion-induced deterioration and fracture was developed, based on which the critical inclination angle was found to decrease with increasing embedded length and decreasing tensile strength.

Long-term mechanical performance of high fluidity fiber reinforced concrete modified by metakaolin

To clarify the long-term strength and toughness of metakaolin (MK) and steel fiber (SF) modified concrete with higher fluidity and water/binder ratio, a series of tests including slump tests, compression tests, splitting tests, digital image processing and Scanning Electron Microscope (SEM) tests were performed on MK-SF concrete cured for 7–360 days. Results reveal that the slump of fresh concrete decreased with an increase in the MK and SF replacement rates. Moreover, the impact of MK on the slump of steel fiber reinforced concrete (SFRC) was more pronounced when combined with a lower water/binder ratio, resulting in increased viscosity. At the pre-peak stress region of the strain-stress curve, the compressive strength fc, tensile strength ft, Young’s modulus Ec, elastic modulus E0, and tensile strain at peak stress εt-max of high fluidity MK-SF concrete increased with increasing MK and SF admixing ratio, regardless of curing age. Notably, the coupling effects of MK and SF became more prominent after long-term curing. Without MK incorporation, the effects of SF and curing time on the above indices were relatively implicit. At the post-peak stress region of strain-stress curves, there existed a residual stage. The inclusion of MK significantly improved the long-term residual strength and strain of SFRC. Additionally, the toughness index Mc, which represents the total area of the compressive strain-stress curve containing both the pre-peak and post-peak regions, also exhibited substantial development with curing time, primarily attributed to the incorporation of MK and SF. The coupling of MK and SF led to a transformation of the concrete failure mode from brittle to ductile. Regression analysis reveals that a linear equation adequately described the long-term relationships of fc-ft, fc-Ec, fc-E0, fc-Mc, and fc-εt-max in MK-modified SFRC. Based on the testing data, a relative strength or toughness index λ and a new generalized hyperbola model were proposed to predict the long-term mechanical behavior mentioned above. Through crack morphology and microstructure analysis, the distinct roles of MK and SF in the composite material were examined.