Orientation Of Steel Fibers Research Articles

Electrical resistivity measurements to determine the steel fiber content of concrete

AbstractAlthough steel fiber reinforced concrete is becoming increasingly popular in the construction industry, its application is currently limited to certain areas. One reason is the lack of an economical non‐destructive testing method to determine the content, distribution, and orientation of steel fibers in fresh and hardened concrete. However, these parameters are decisive for the assessment of the static performance of a building component. In this article a new test method is proposed, which is based on the electrical conductivity of concrete. Using a two‐electrode experimental set‐up, measurements of the electrical conductivity were done on hardened concrete cubes with defined fiber content and a correlation between electrical conductivity and fiber content could be identified. Within the scope of extensive test series, the fiber content and the age of specimens were varied, and several identical series were produced and observed to ensure statistically verified results. At this stage of research the test method based on electrical conductivity measurement provides reliable results on the fiber content of preconditioned concrete cubes. Based on the results a new model was developed to quantify the fiber content of the examined cubes. This model is based on the estimation of the relative conductivity, the so‐called increase of conductivity. Undesired influences caused by aging, respectively hydration in the young concrete age were eliminated, by relating the conductivity of fiber reinforced concrete to an unreinforced concrete. Further investigations with focus on the influence of concrete composition and other specimen sizes are going to be conducted as next steps for the development of an in situ test setup for the diagnosis of concrete structures.

Study on dispersion uniformity and performance improvement of steel fibre reinforced lightweight aggregate concrete by vibrational mixing

Steel fibres (SFs) easily cluster at high dosages during mixing, which directly limits the performance of concrete. Recently, vibrational mixing has provided a novel perspective to overcome the defects of ordinary mixing. However, the influence of vibration effects on steel fibre reinforced lightweight aggregate concrete (SFRLAC) has not been investigated. In this paper, SFRLAC was prepared based on ordinary mixing and vibrational mixing, respectively. Different volume dosages of SFs (0%, 0.5%, 1.0%, 1.5% and 2.0%) were selected as variables. The flowability test, the dry density test and the mechanical properties test were used to characterise the macroscopic performance of the concrete. After the macroscopic test of SFRLAC, the distribution and orientation of SFs were quantified by integrated digital image processing (DIP) technology. The interfaces of the SFs and the cement matrix were characterised by environmental scanning electron microscopy (ESEM). The obtained results revealed that vibrational mixing could improve flowability. Meanwhile, it can increase the compressive strength and flexural toughness of SFRLAC continuously after SFs volume exceeds 1.0%. Moreover, the technology was conducive to dispersing SFs at high dosages and made the orientation more consistent than the ordinary mixing. Notably, a dense paste film appeared on the fibre surface, suggesting a significant improvement in the load capacity. Thus, vibrational mixing can be used to prepare the reinforced concrete with high SFs contents.

Tunning matrix rheology and mechanical performance of ultra-high performance concrete using cellulose nanofibers

Abstract With the growing demand for sustainability and rapid development of nanotechnology, nanocellulose materials extracted from natural plants have attracted great attention. The incorporation of nanocellulose materials leads to a change in viscosity and yield stress on ultra-high performance concrete (UHPC). Rheological parameters affect the mechanical strength and steel fiber distribution of UHPC significantly. Therefore, it is essential to adjust the matrix rheology within an appropriate range through nanocellulose materials. This study aims to propose a novel method to optimize steel fiber distribution and mechanical properties of UHPC by adjusting the matrix rheology with cellulose nanofibers (CNFs) suspensions. The relationship among CNF concentration, steel fiber distribution, and the mechanical properties of UHPC was established. Test results showed that the failure mode of UHPC containing CNFs changed from single cracking to multiple cracking, accompanied by 11–23% enhancement in tensile strength. With the increase of CNF concentrations, the probability density distribution functions of steel fiber orientation showed the trend toward the distribution with a larger inclination. The addition of CNF suspensions effectively reduced the number of steel fibers settling to the bottom of the specimens. Scanning electron microscopy analyses demonstrated that the nanoscale reinforcement by CNFs was conducive to improving the mechanical properties of UHPC.

A stereological approach to estimation of fibre distribution in concrete

Steel fibre reinforced concrete (SFRC) continues to gain popularity as a sustainable construction material with enhanced strength and durability properties. These mechanical properties are dependent on the distribution and orientation of steel fibres achieved during casting, the determination of which is challenging. Three-dimensional X-ray micro-CT scanning is currently the only technique that yields the true three-dimensional (3D) dispersion of fibres, but the technology is applicable only to small samples at laboratory settings. In this study, a mathematical framework is developed to utilize 2D probing, like X-ray projectional radiography, which is more practical for in situ nondestructive characterization of fibre dispersion in SFRC members. For the first time, fibres are treated as finite-sized inclusions, developing the appropriate mathematical formulations consistent with published research in the field of stereology and practically applying them to digitized X-ray projected images of SFRC and UHPFRC. Stereological principles were employed to establish the volume fraction of fibres (3D property) from their areal fraction in images. The method was developed and tested for both X-ray slices and projected images. Tests were performed on UHPFRC cores as well as SFRC dogbones with different nominal fibre dosages. It is shown that accurate estimation of fibre volume fraction and their spatial distribution can be robotically achieved from 2D imaging using the developed algorithms. The influence of parameters such as fibre geometry and thickness of structural members on the applicability of the stereological equations and accuracy of estimated results is investigated, with a comprehensive discussion of the stereology approach.

Investigation on Fracture Behavior of Cementitious Composites Reinforced with Aligned Hooked-End Steel Fibers

Aligning steel fibers is an effective way to improve the mechanical properties of steel fiber cementitious composites (SFRC). In this study, the magnetic field method was used to prepare the aligned hooked-end steel fiber cementitious composites (ASFRC) and the fracture behavior was investigated. In order to achieve the alignment of steel fibers, the key parameters including the rheology of the mixture and magnetic induction of electromagnetic field were theoretically analyzed. The results showed that, compared with SFRC, the cracking load and the ultimate load of ASFRC were increased about 24–55% and 51–86%, respectively, depending on the fiber addition content. In addition, the flexural tensile strength and residual flexural strength of ASFRC were found to increase up to 105% and 100%, respectively. The orientation of steel fibers also has a significant effect on energy consumption. The fracture energy of ASFRC was 56–70% greater than SFRC and the reinforcement effect of hooked-end steel fiber was higher than straight steel fiber. The fibers in the fracture surface showed that not only was the number of fibers of ASFRC higher than that of SFRC, but also the orientation efficiency factor of ASFRC was superior to SFRC, which explains the improvement of fracture behavior of ASFRC.

Non-Destructive Multi-Method Assessment of Steel Fiber Orientation in Concrete

Integration of fiber reinforcement in high-performance cementitious materials has become widely applied in many fields of construction. One of the most investigated advantages of steel fiber reinforced concrete (SFRC) is the deceleration of crack growth and hence its improved sustainability. Additional benefits are associated with its structural properties, as fibers can significantly increase the ductility and the tensile strength of concrete. In some applications it is even possible to entirely replace the conventional reinforcement, leading to significant logistical and environmental benefits. Fiber reinforcement can, however, have critical disadvantages and even hinder the performance of concrete, since it can induce an anisotropic material behavior of the mixture if the fibers are not appropriately oriented. For a safe use of SFRC in the future, reliable non-destructive testing (NDT) methods need to be identified to assess the fibers’ orientation in hardened concrete. In this study, ultrasonic material testing, electrical impedance testing, and X-ray computed tomography have been investigated for this purpose using specially produced samples with biased or random fiber orientations. We demonstrate the capabilities of each of these NDT techniques for fiber orientation measurements and draw conclusions based on these results about the most promising areas for future research and development.

A New Method to Determine the Steel Fibre Content of Existing Structures—Evaluation and Validation

The in-situ measurement of the content and orientation of steel fibres in concrete structures is of great importance for the assessment of their specific mechanical properties, especially in the case of repair. For existing structures, the actual fibre content as well as the orientation of the fibres, which is based on many factors such as casting or compacting direction, is typically unknown. For structural maintenance or rehabilitation, those factors have to be determined in order to apply meaningful structural design calculations and plan necessary strengthening methods. For this reason, a new method based on the analysis of drilling cores of concrete structures has been established. The newly developed non-destructive test setup used in this research consists of a framework for cylindrical specimens in combination with an LCR meter to determine the electrical resistance of the fibre reinforced concrete. In combination with a suitable FEM model, concretes with fibre contents up 80 kg/m3 were analysed to derive a first model to assess the actual fibre content of steel fibre reinforced concretes. After a calibration of the literature’s equation by use of an adjusted aspect ratio for the analysis of drilling cores, the estimation of the fibre content is possible with high accuracy for the tested material combination. The results show that the newly developed test method is suitable for the rapid and non-destructive structural diagnosis of the fibre content of steel fibre reinforced concrete based on drilling cores using electrical resistivity measurements.

Fracture properties of steel fiber reinforced concrete: Size effect study via mesoscale modelling approach

Steel fiber reinforced concrete (SFRC) can exhibit superior performances of high strength and toughness due to the enhancing effect of discontinuous steel fibers randomly distributed in concrete matrix. For better understanding the size effect on the fracture properties of SFRC structural elements, we developed a three-dimensional (3D) two-phase mesoscale model considering the random distribution and orientation of steel fibers in concrete, to investigate the quasi-static flexural responses of the notched SFRC beams with different beam depths, i.e., 30 mm, 60 mm, 90 mm, 120 mm and 150 mm. A new coupling method was employed to describe the interfacial relationship between steel fibers and concrete matrix in the present simulations. The fracture behaviors of SFRC beam were numerically studied in terms of the load versus cracking mouth opening displacement curve, the evolution of fracture process zone, the nominal flexural strength and the fracture failure patterns. Through the analysis and comparison between the present numerical results and the available test data, it was found that there is negligible size effect on the fracture patterns of SFRC beam. The size effect on the nominal flexural strength of SFRC beam can be well characterized and analyzed using the Bažant’s size effect law. Further, the numerical results of SFRC beams showed a favorable agreement with the corresponding test results, demonstrating the great feasibility and potential of the developed mesoscale modelling approach in simulating and investigating the mechanical behaviors of SFRC.

New system for vibrating and orientation of steel fibers in masonry mortars

The incorporation of reinforcing fibers during the kneading process has become an efficient method to improve the flexural strength of building binder materials. In this work, a new system for the vibration and orientation of metallic fibers has been developed. This allows the fibers to be positioned perpendicular to the direction of the flexural stress. This vibrating system for construction mortars in a fresh state improves the performance of mortars made with recycled aggregate, increasing their mechanical resistance and their application within the building sector. Testswere carried out with mortars made with two types of recycled aggregates and two different percentages of reinforcing fibers. The results show thatby applying the vibrating system implemented, it is possible to increase the mechanical resistance of mortars made with recycled aggregate by more than 15%. • A new vibrating device to orientate fibers was developed. • A comparison between concrete and ceramic aggregate mortars was performed. • It was possible to orient more than 50% of the steel fibers content in all the samples made. • The orientation of fibers transversely to the load increases the flexural strength of mortars made with recycled aggregate.

Flexural Behavior of Steel Fiber Reinforced Concrete Beams: Probabilistic Numerical Modeling and Sensitivity Analysis

One of the principle issues concerning the practical application of steel fiber reinforced concrete (SFRC) is the uncertainty related to its structural behavior, primarily caused by the partially random distribution and orientation of steel fibers in SFRC structural elements. This paper aims to provide a better understanding of how the variance of material properties of the SFRC affects the flexural behavior of SFRC beams. First, a distributed plasticity fiber finite element model of beam flexural behavior is proposed and validated. Then, probability distributions of selected material properties are defined based on existing probabilistic models and experimental results from the literature. Finally, a variance-based sensitivity analysis is performed using Sobol’ indices to identify uncertainties in material properties that contribute most to the uncertainties related to three characteristic points of a beam’s flexural behavior: first crack, yield, and collapse point. Sensitivity analysis is performed by surrogating the numerical model using polynomial chaos expansion. The variance in residual tensile strength is identified as the main contributor to the variance in the flexural behavior of an SFRC beam used in the case study.

Numerical Modeling for Flexural Behavior of UHPC Beams Reinforced with Steel and Sand-Coated CFRP Bars

Many properties of advanced materials need to be studied, including ultra high performance concrete (UHPC) as well as the Carbon Polymer Reinforcing Fiber (CFRP) rods of sand coated surfaces. The combined action of (UHPC) with (CFRP) is of constructive importance for researchers to know the behavior of the structural members in different loading stages. Eight samples were prepared with dimensions of 2.4 meters in length, 0.25 meters in depth, and 150 mm in width. The compressive strength of 110 MPa was selected after preparing many trail mixes. A computer model by ANSYS 11 program was made for the representation of all inputs, including steel fibers. All Parts of beams, testing accessories, and loads were simulated with adequate elements. The study examined the effect of the orientation of steel fibers on the flexural capacity in terms of load-deflection response. It was concluded that the distribution of the steel fibers plays a fundamental role in the behavior of the models, and thus the optimal distribution was found to be orthogonal distribution to the longitudinal direction of UHPC beam in equal ratios.

Monitoring steel fibre orientation in self-compacting cementitious composite slabs during pouring with dynamic X-ray radiography

This paper presents a new technique based on dynamic X-ray radiography that can be used to assess fibre orientation during pouring of steel fibre reinforced cementitious composites. Synthetic examples were used to assess the suitability and robustness of the technique, which was shown to provide reliable measurements of fibre orientation even when the signal-to-noise ratio is relatively high. A study was then carried out on the effect of formwork aspect ratio, time/duration of pouring, and rebar placement on the fibre orientation while pouring self-compacting cementitious composite slabs. Results demonstrated the ability of the technique in monitoring the movements of fibres while pouring, and the strong effect of the flow in inducing preferential fibre alignment within the slabs. Fibre orientation was found to vary progressively over time and could take about half of the duration of pouring to fully stabilise.

Technologies for creating directional orientation of fiber reinforcement in concrete

Information on the analysis of methods for creating directional orientation of steel fiber rodding of reinforced concrete structures is presented. The design of a mobile experimental setup for the manufacture of fiber-concrete and fiber-reinforced concrete linear elements with a directional orientation of steel fibers by mechanical means has been developed. The test results of the pilot plant are presented. The required dimensions of the bunker for the fiber-reinforced concrete mixture, the type of the leveling element and the method of increasing the fiber-reinforced concrete mobility during casing using a pilot unit have been determined. The creation of a directed orientation of fiber threads in fiber-reinforced concrete makes it possible to make the most of this material‘s advantages. Despite a sufficient number of theoretically possible ways to create a directional orientation of fibers in a concrete mixture, not all of them can be applied in real construction conditions. Also, a number of methods introduce additional difficulties into the concreting process, which can adversely affect the construction processes duration. The use of a mechanical method for creating a directional orientation of steel fiber reinforcement in concrete made it possible to obtain the desired result on a mixture already containing fiber threads, without the need for their separate laying. The optimal design of the mechanical element leveling and the features of the process of leveling steel fibers in concrete have been established.

Design, preparation and mechanical properties of full-field aligned steel fiber reinforced cementitious composite

In this study, a full-field aligned steel fiber reinforced cementitious composites (FASFRC) prism specimen was designed, prepared and tested. The special feature of the FASFRC is the orientation of steel fibers in the specimen is consistent with the principal stress when apply flexural loading to the specimen. In order to optimise the design of the FASFRC prism, the orientation of the major principal stress of the prism subjected to flexural load was determined by finite element analysis, which is the expected orientation of steel fibers in a true specimen. Then, in the preparation of the FASFRC prisms, an external magnetic field with magnetic line consisting with the direction of the principal stress was used to control the direction of steel fibers to be consistent with the designed orientation when the mixtures were compacted. Finally, the flexural strength, toughness and residual strength of FASFRC, conventional steel fiber reinforced cementitious composite (SFRC) and aligned steel fiber reinforced cementitious composite (ASFRC) prism specimens were tested by four-point bending tests. Through the tests, it is found that the mechanical properties of the FASFRC specimens were significantly higher than other specimens mainly due to the enhancement of the reinforcement of steel fibers.

Flexural behavior of HPFRCC: Enhancing post-crack strength and toughness by magnetic alignment of the reinforcement

This paper presents an investigation on the effect of magnetic orientation of steel fibers on the physical–mechanical and electrical properties of a High Performance Fiber Reinforced Cement Composite. A simplified magnetic circuit was proposed to align the reinforcement within the cementitious matrix. X-ray Computer Tomography was employed to qualitatively access the fiber distribution in hardened specimens. Physical and electrical properties were characterized by ultrasound propagation velocity and electrical resistivity, while mechanical performance was evaluated by flexural and compressive tests. Scanning electron microscopy was used to study the damage processes in the fiber–matrix interfaces. Results showed that the magnetic circuit designed in this study was able to generate a preferential orientation of the steel fibers, increasing the ultrasound propagation velocity and causing anisotropy in the electrical properties. Furthermore, the modulus of rupture and toughness of the composite were significantly enhanced, leading to a feasible reduction in the critical fiber volume for bending when fibers are aligned.

Experimental study on flexural behavior of damaged reinforced concrete (RC) beam strengthened by toughness-improved ultra-high performance concrete (UHPC) layer

The flexural test was conducted to investigate crack resistance, loading bearing capacity, deformation characteristics and failure modes of damaged reinforced concrete (RC) beams strengthened by reinforced ultra-high performance concrete (UHPC) layer (Abbreviated as UC). Also, their mechanical properties were compared with those of the unstrengthened RC beam (Abbreviated as I–C). Additionally, the effects of pre-damage degrees in the RC beams and three strategies for the improvement of toughness of the reinforced UHPC layer on the cracking and flexural performance of UC were discussed. The results showed that UC acted monolithically under flexure without interfacial debonding before typically flexural failure. Compared with I–C, cracking and ultimate loads of UC increased by 1.57–3.32 times and 1.72–2.21 times, respectively. The reinforced UHPC layer effectively suppressed the cracking of the RC beam, making crack width in the RC beam propagate slowly with the load. Moreover, the severer the pre-damage degree of the RC beam, the smaller improvement of the flexural performance of UC. The addition of steel wire mesh, orientation of steel fibers and moderate-temperature steam curing further improved the cracking and flexural performance of UC with the most evident improvement in the addition of steel wire mesh. Finally, the theoretical model and formula were proposed for the calculation of load bearing capacity of UC with consideration of the influence of the pre-damage degrees in the RC beams, and the experimental results verified that the theoretical formula can accurately predict the load bearing capacity of UC.

Improvement effect of fiber alignment on resistance to elevated temperature of ultra-high performance concrete

This paper investigated mechanical properties of ultra-high performance concrete (UHPC) after exposure to elevated temperatures ranging from 200 °C to 800 °C. Two casting methods were applied to prepare UHPC specimens containing randomly distributed or aligned steel fibers. X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were utilized to evaluate the microstructure degradation of UHPC with high temperatures. Experimental results indicated that compressive strength, flexural strength and toughness were significantly reduced by 83.4%, 82.5% and 84.6% for UHPC specimens with randomly distributed steel fibers after exposure to 800 °C. The alignment of steel fibers can positively alleviate the degradation of mechanical properties of UHPC, inducing an increase of 33.8%, 72.4%, 43.9% and 58.8% for the residual compressive strength, flexural strength, toughness and deflection at post-cracking peak point, respectively. According to microstructure measurements, cement hydration products were badly decomposed by high temperature exposures with the occurrence of coarsening pore structure in cement paste and worsening the interface between cement paste and steel fibers, subsequently causing the mechanical performance degradation of UHPC specimens. Although the tensile strength of fibers was notably reduced by high temperatures, the fibers still performed bridging effects on cracks in UHPC. This is the main reason that the fiber alignment can significantly improve the residue mechanical properties of UHPC after exposure to high temperatures. Therefore, the high temperature resistance of UHPC can be improved by optimizing the orientation of steel fibers.

Do polymer fibres affect the distribution of steel fibres in hybrid fibre reinforced concretes?

This study investigates the effects of the hybrid use of different types of fibres in terms of their orientations, volume and hybridisation properties on the mechanical and fracture characteristics of self-compacting fibre reinforced concrete (FRC). In the experiments, three different types of macro fibres were used in two different proportions (1 or 2 vol%) to produce monotype or hybrid FRC with the same matrix phase and flowability. The fibre types used include (1) typical steel fibre, which is commonly used in many applications, (2) high-density polyethylene (HDPE) fibre with superior tensile strength, and (3) nylon fibre with characteristic diminishing effects on early and late age shrinkage. The fibre orientation was examined by image analysis and its relationship with post-cracking behaviour, which was determined by three-point flexural tests, was quantitatively evaluated. The study also evaluates and compares the orientations of different types of fibres in both monotype and hybrid FRC and reviews the interaction of fibre orientation. The results indicate that the highest flexural strength was obtained by using the nylon-steel FRC, while the HDPE-steel hybrid FRC series provided the highest fracture energy. The post-cracking residual strengths were found to have increased with increasing effective fibre reinforcement ratio, which is dependent largely on the orientations of fibres. The results of the image analysis indicate that the presence of HDPE or nylon fibres affects the orientation of steel fibres in hybrid FRC and these results were found to be in agreement with the mechanical and fracture properties of concretes. The test results demonstrate how the interactions of different types of fibres in terms of their orientations influence the flexural behaviour of hybrid FRC.

Inductive method for the orientation of steel fibers in recycled mortars

Mortars elaborated with recycled aggregates raise two main difficulties: lower mechanical strengths and higher shrinkage compared to traditional mortars. For this reason, fibers were used to improve the properties. Steel fibers are the most employed for their ferromagnetic qualities, what helps to orient them influenced by magnetic field. This paper argues how recycled mortars with steel fiber oriented under the influence of 500 mT field reach flexural strengths 10% higher than those with non-oriented fibers. Thus, mechanical strengths are closer to traditional mortars. Inclusion of fibers in mortar is suitable for obtaining shrinkage values similar to those obtained in traditional mortars.

Three‐dimensional modeling of the distribution and orientation of steel fibers during the flow of self‐compacting concrete

AbstractSteel fibers may be added to self‐compacting concrete (SCC) to improve its mechanical performance; however, such improvement is subject to a number of conditions, an important one of which is the distribution and orientation of the fibers. The objective of this study is to propose a new method capable of predicting the variation of the distribution and orientation of steel fibers immersed in the SCC during flow. The flow of the steel fiber reinforced SCC is simulated by a computational fluid dynamics software, in which the volume of fluid method is used to track the free surface of concrete. On the basis of the velocity field of concrete obtained by the software, the motion of steel fibers is calculated by solving the rigid body dynamics equation. To validate the proposed method, a casting process of a concrete beam with steel fibers is simulated. The effectiveness of the numerical simulation is confirmed by a comparison with the experimental results. Meanwhile, the variation of fiber orientation during the flow of SCC is discussed by analyzing the fiber orientation factor.