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Tensile Behavior of Cement‐Based Composites with Random Discontinuous Steel Fibers
Journal of the American Ceramic Society
Abstract
In this paper, the tensile properties of cement-based composites containing random discontinuous steel fibers are reported. Direct tensile tests were performed to study the effects of fiber length (hence fiber aspect ratio), interfacial bonding, and processing conditions on composite properties. Composite tensile strength and ductility are highlighted and discussed.
... Hence, the research and development of new high-strength, high-toughness roadside filling materials has become an inevitable trend. With the aim of enhancing the toughness and durability of concrete, endeavors have been undertaken to augment concrete through the incorporation of unconventional materials, including fibers [13][14][15], rubber particles [16][17][18], carbon nanotubes [19][20][21][22], and ceramics [23][24][25]. Several studies have demonstrated that the addition of steel fibers can substantially enhance the strength and toughness of the concrete matrix [26][27][28][29][30][31]. ...
Against the background of the prevailing green development paradigm, numerous coal mines have embraced the adoption of gob-side entry retaining mining technology. The most commonly employed form of gob-side entry retaining involves building an artificial wall along the edge of the goaf behind the working face to maintain the roadway. The pivotal challenge in gob-side entry retaining lies in the roadside support. Currently, commonplace concrete serves as the predominant material for the roadside filling body. Nevertheless, traditional concrete exhibits drawbacks, including inadequate tensile strength and poor toughness, leading to wall cracks or even collapses in the retaining wall. Steel fiber, a frequently employed reinforcement and toughening agent in concrete, has found widespread application in the construction sector and other fields. However, its use as a roadside filling material in underground coal mines remains infrequent. Therefore, in this paper, the flow and mechanical properties of steel fiber concrete were tested and analyzed, and field industrial tests were conducted. Results of indoor experiments show that steel fibers reduce the slump of concrete. The addition of steel fibers shifted the pore compacting stage, linear elasticity stage, and destabilization stage forward and improved the post-peak bearing capacity. The addition of steel fibers makes the concrete compressive and tensile strength show a “first increase and then decrease” trend; both peaked at 1.5%, and the increase in tensile strength is more pronounced. Steel fibers enhance the strength of compressive strength of concrete at an early age, weaker at a late age, and tensile strength inversely. The addition of steel fiber can change the concrete matrix from tensile damage to shear damage, and the toughness index shows the trend of “first increase and then decrease”, and reaches the peak value when the dosage is 1.5%. Industrial test results show that steel fiber concrete as a roadside filling body can reduce the surrounding rock surface displacement and bolt (cable) force.
... The results of the direct tensile test conducted on the grout showed that the tensile strength ranged from 0.45 to 3.2 MPa [28,29,43]. Therefore, considering an anchor diameter of 10 cm that is used in typical construction sites, the grout tensile failure load is expected to be 3-25 kN. ...
As ground anchors are widely used to stabilize various structures, load distributive compression anchor (LDCA) is gaining popularity owing to their high load-bearing capacity and ease of removing strands. Unlike conventional anchors, an LDCA consists of multiple anchor bodies, thus allowing the distribution of the load among them. In addition, interference effects that differentiate the behavior of LDCA from that of single compression anchor are induced. These effects must be considered when designing an LDCA; however, the lack of research on its supporting mechanism has led to LCDA design being based on conventional criteria and experience. Furthermore, existing studies have not focused on the behavior of LDCA in soil, which is more vulnerable than in rock. This study proposes a physical model test that evaluates the load transfer mechanism of an LDCA. A series of pull-out tests is performed on an LDCA installed in residual soil to investigate the behavior of the LDCA under various conditions, by varying parameters such as the total anchor length, number and spacing of anchor bodies, and loading conditions. The test results show that the movement of the upper anchor body causes a tensile load on the grout and additional grout–soil shear stress on the lower anchor body, which reduces the ultimate bearing capacity of the LDCA. Moreover, the test results reveal that the interference effect between anchor bodies increases as the spacing decreases, thus resulting in a greater reduction in the ultimate bearing capacity. The findings of this study are anticipated to develop the basis for the design and application of LDCA.
... Although its tensile strength (5.62 MPa) is slightly lower than that of R-0 (6.66 MPa), it still meets the needs of most engineering applications. In previous studies on hybrid Strain Hardening Cementitious Composite (SHCC) [62][63][64], a similar trend has been observed. By combining polypropylene (PP) fibers and PE fibers and studying the effects of fiber properties and proportions, researchers have found that the addition of low-cost and low-performance PP fibers can improve specific mechanical properties of hybrid SHCC, making it suitable for various engineering applications. ...
... This powerful tool has been developed for the pullout behavior of different fibers; in fact, it is widely recognized as the design guideline for achieving high strain capacity alone [2]. The micromechanical models have assisted civil material engineers in producing various SHCCs by using many fibers with different mechanical and interfacial properties, such as polyethylene (PE), polyvinyl alcohol (PVA -as the most useful fiber in this technology by now), polypropylene (PP), steel [3,4] and even natural fibers [5] in combination with an optimized cementitious matrix. Nowadays, SHCC is a family of materials with a vast range of ultimate tensile strengths and strain capacity; multiple cracks under tensile load have been developed and modified, depending on the demands of a particular structure [6]. ...
The use of Load Distribution Compression Anchors (LDCAs), known for their high bearing capacity and removable strands after construction, has grown recently. Unlike conventional anchors, LDCAs have multiple anchor bodies along their length, presenting unique pull-out behaviors that remain insufficiently understood. This study aimed to investigate the failure mechanisms of LDCAs through physical model tests conducted in residual soil with a 90% relative density, adjusting variables such as anchor length, the number of anchor bodies, and the spacing between these bodies. To assess the effect of the surrounding soil on these mechanisms, the results of the experiment were compared with those from experiments conducted at a 70% relative density. Findings revealed that anchor body-grout tensile failure occurred during the initial loading stage. The tensile failure was determined by the grout strength itself rather than the effect of the surrounding soil. After anchor body-grout tensile failure, each anchor body behaves independently. However, when the spacing between the anchor bodies was small, local grout-ground interface failure occurred at both 70% and 90% relative density conditions, resulting in the overlapping of grout axial loads. Lastly, it was confirmed that the ultimate bearing capacity of LDCAs (multiple anchor bodies) can be less than that of compression-type anchors (single anchor body). The shear field developed by each anchor body in LDCAs overlapped and reduced the grout-ground shear strength. The reduction in ultimate bearing capacity was more pronounced in the dense soil condition and narrow anchor body spacing condition.
The present research is conducted to optimize and introduce the sustainable and cost-effective hybrid ECC mixtures. The availability, expensive nature, and high carbon footprints of standard ECC constituents: silica sand (SS) and polyvinyl alcohol fiber (PVA) restricted its wide application. The designed hybrid ECC were prepared with three different profiles of fibers: PVA, PET and MSE. This study also investigates the usability of stone processing waste (SPW) in ECC. To analyse the effect of SPW on the robustness of hybrid ECC, six different ECC mixtures were prepared having SPW up to 100% as a substitute for SS. To check the role of various constituents in designed matrices compressive response, tensile performance, flexural characteristics, air permeability, electrical resistivity, sorptivity, environmental, economic performance, effect of chemical immersion were firmly recorded. Evaluated viability of hybrid ECC mixes depicted that strength, durability, strain, and ecological parameters improved appreciably. Thus, it was revealed that the incorporation SPW improved the robustness behaviour of designed ECC mixes. Economic index of developed mixes revealed that the cost of hybrid ECC mixes lowered by 43.3% as compared to fully PVA and SS containing mixes. In addition, developed hybrid ECC mixes improved sustainability by reducing carbon footprints which is beneficial for eco-friendly environment.
This paper evaluates the efficiency of polypropylene fibers in improving the tensile and flexural characteristics of fiber-reinforced alkali-activated slag composites with strain-hardening behavior. Several different composites were produced and the mechanical properties of the composites were investigated during compressive and flexural tests. Furthermore, the mid-span curvature of the beams was measured by applying the digital image correlation technique to the flexural tests, and the curvature-deflection diagrams of the composites were analyzed and demarcated by structural mechanics. Finally, an analytical model considering strain-hardening behavior was proposed by fitting the calculated moment-curvature diagrams to the experimental results to recognize the tensile behavior of composites more precisely than the existing indirect methods. The results showed that an ultra-high tensile strain capacity was obtained for composites cured under laboratory conditions. Also, it was found that the fiber interfacial bond with the matrix was improved by using slag with lower Al2O3 content. Moreover, weakening the interfacial transition zone between the fine aggregate and the paste, the heat-treatment curing reduced the fracture energy of the composites which is favored to make strain-hardening cementitious composites.
Fabrication of ultra-high-performance concrete (UHPC) is costly, especially when commercial materials are used. Additionally, in contrast to conventional concrete, numerical procedures to simulate the behaviour of ultra-high-performance fibre-reinforced concrete (UHPFRC) are very limited. To contribute to the foregoing issues in this field, local materials were used in the fabrication process, while accounting for environmental issues and costs. Micro steel fibres (L: 13 mm, d: 0.16 mm, and ft: 2600 MPa; L: length, d: diameter, ft: tensile strength) were used in 2% volumetric ratios. Compression and indirect tests were carried out on cylindrical and prismatic beams according to international standards. To further enrich the research and contribute to the limited simulation data on UHPFRC, and better comprehension of the parameters, numerical analyses were performed using the ATENA software. Finally, nonlinear regression analyses were employed to capture the deflection-flexural response of the beams. The results were promising, indicating cost-effective fabrication using local materials that met the minimum requirements of UHFRC in terms of compressive strength. Furthermore, inverse analysis proved to be an easy and efficient method for capturing the flexural response of UHPFRC beams.
This dissertation presents the investigation results on fiber/cement bond property modification in relation to interfacial microstructure. The fiber/matrix interfacial bond properties generally refer to the elastic bond between fiber and matrix, fracture energy of the fiber/matrix interface, interfacial friction, or the characteristics of the fiber/cement interfacial microstructure. Improving fiber/matrix bond properties is a key to optimize the composite performance. The focus of this study is on the evaluation of the effects of transition zone densification and fiber surface modification on bond enhancement, the age effect on bond property, and the mechanism of interfacial debonding. The results provide insights into various bond enhancing mechanisms and guidelines for bond property tailoring. The issue of different debond modes, strength-governed or fracture-governed, is first examined. The debond mode is a material property and has to be identified by material testing. A concept of effective bond strength able to replace the fracture-based bond property, the interfacial fracture toughness, conditionally, is proposed. This theoretical concept allows the evaluation of the debond mode for any given fiber/matrix system. The experimental study carried out for steel and brass fibers with different diameters indicate that these systems exhibit a strength based debond mode. The nature of the transition zone between fiber and cement matrix suggests various bond tailoring mechanisms, including transition zone densification and fiber surface modification. The investigation of transition zone densification in this study suggests that this mechanism would be effective in bond improvement only for the material systems with high fiber/matrix adhesion. One example has been brass fiber/cement matrix, as reported in this dissertation. For material systems with low fiber/matrix adhesion, such as most synthetic fibers, fiber surface modification is necessary. The bond properties of plasma treated polymeric fibers are examined in this study. It is found that significant improvement on the bond property using plasma treatment to modify the surface condition of synthetic fibers can be achieved. Different bond failure types, adhesive and cohesive, are identified and both types of bond failure have important implication on fiber debonding and interfacial bond property. One of the important characteristics of cement-based materials--aging effect, is also studied in this work. The time-dependency on the interfacial bond and the formation of transition zone microstructure of two polymeric fiber systems are examined. It is found that the formation of fiber contact layer in the transition zone is mostly responsible for the bond strength development. The characteristics of time-dependency of interfacial bond, however, is expected to be different in the material systems with cohesive type of bond failure. The early development of bond strength is in favor of the strain-hardening behavior of a composite in early age.
Concrete beams containing both conventional steel reinforcing bars and different volumes of fibrillated polypropylene fibers were tested in an instrumented, drop-weight impact machine. Impact strength and fracture energy were determined. The presence of the fibers led to a considerable increase in fracture energy; the peak bending loads were not affected. The increase in fracture energy was much greater than the sum of the effects of polypropylene fibers and reinforcing bars considered separately.
Although the classic Griffith-Orowan one-parameter model cannot be used to describe the crack growth in fiber-reinforced cementitious materials, elastic fracture mechanics is valid. The characteristic of fiber-reinforced materials is the development of stable crack growth because of the resistance due to bridging fibers. The main difficulty in modelling this crack growth is that it is non-linear because the crack closure forces depend upon the crack opening which is in turn dependent partly on the closure stresses. Exact solutions for crack growth are therefore difficult. After reviewing past attempts at satisfactory practical solutions a simple approximate method is described. In this method, the exact stress displacement relationship is linearized and it is assumed that the crack flanks in the fiber bridging zone remain straight. With the assumption that equilibrium crack growth occurs when the effective stress intensity factor at the tip of the continuous matrix crack reaches a critical value KIc, it is only necessary to use iteration to find the length of the fully developed fiber bridging zone that produces the critical crack mouth opening.
Slurry-infiltrated fiber concrete (SIFCON) composites differ from conventional fiber reinforced concrete in two aspects: they contain a much larger volume fraction of fibers and they use a matrix consisting of very fine particles. They could be made simultaneously to exhibit outstanding strength and ductility properties. This research deals with the tensile stress-strain properties of SIFCON and comprises an experimental and an analytical program. Parameters investigated include the matrix composition and the fiber type. It is shown that SIFCON composites can exhibit tensile strength up to 4 ksi (28 MPa) at peak strains ranging from 1 to 2 percent. A model is proposed to predict the ascending branch of the stress-strain curve of SIFCON from its compressive strength and its fiber-reinforcing parameters.
Micromechanical model constructed on the basis of fracture mechanics and fiber bridging provides a means of selection of fiber, matrix and interface property combinations for which short random fiber reinforced cementitious composites will undergo pseudo strain-hardening when loaded beyond the elastic limit. This paper reviews the basic elements of the micromechanical model, and demonstrate with experimental results how pseudo strain-hardening has been achieved. Emphasis will be placed on the fact that fiber volume fraction is but one adjustable parameter in tailoring the material composition for pseudo strain-hardening. The role of other parameters, such as fiber aspect ratio, matrix toughness, and interfacial bond strength in controlling pseudo strain-hardening, are discussed.
Apart from imparting increased fracture toughness, one of the useful purposes of reinforcing brittle matrices with fibers is to create enhanced composite strain capacity. This paper reviews the conditions underwhich such a composite will exhibit the pseudo strain-hardening phenomenon. The presentation is given in a unified manner for both continuous aligned and discontinuous random fiber composites. It is demonstrated that pseudo strain hardening can be practically designed for both gills of composites by proper tailoring of material structures. 18 refs., 8 figs., 2 tabs.
Flexural tests were carried out on cement-based composites containing, individually or in combination, continuous polypropylene nets, chopped glass strands and continuous glass rovings. It is shown that the combined-fibre-reinforced composites, especially the ones with glass rovings, result in higher limit of proportionality and post-cracking stiffness compared with the individually reinforced composites. A minor increase in the volume fraction of glass rovings can significantly improve the load-carrying capacity of the combined-fibre-reinforced composites.
This paper analyzes the pseudostrain-hardening phenomenon of brittle matrix composites reinforced with discontinuous flexible and randomly distributed fibers, based on a cohesive crack-mechanics approach. The first crack strength and strain are derived in terms of fiber, matrix, and interface micromechanical properties. Conditions for steady-state cracking and multiple cracking are found to depend on two nondimensionalized parameters that embody all relevant material micromechanical parameters. The results are therefore quite general and applicable to a variety of composite-material systems. Phrased in terms of a failure-mechanism map, various uniaxial load-deformation behaviors for discontinuous fiber composites can be predicted. The influence of a snubbing effect due to local fiber/matrix interaction for randomly oriented crack-bridging fibers on the composite properties is also studied.
Toughening of cement-based materials when reinforced with relatively high fiber volume is studied in the paper. Toughening mechanisms of composites have been experimentally studied by quantitative image analysis, laser holographic interferometry, and acoustic emission. It is found that damage localization in cement-based materials can be greatly reduced by the use of fibers. Based on fracture mechanism observed, an R-curve approach has been proposed to predict toughening of matrices due to fiber reinforcement. Theoretical predictions show good agreement with experimental results.