Interwoven steel fibers with slip hardening behavior in cement-based matrices

Abstract

Title of the invention: Interwoven steel fibers with shear hardening behavior in cementitious matrixes\nInterwoven steel fibers are a product for use in fiber concrete with a certain volume percentage. The use of steel fibers with shear hardening behavior allows for the achievement of a fiber concrete with strain hardening behavior. Therefore, the main goal of a fiber with shear hardening behavior is to be easy to manufacture and easy to access. For this purpose, the twisting of two straight high-strength steel wires together with different twisting cycles per unit length was used. According to the results, the slip hardening behavior for the steel fibers twisted together was formed at different twistings. In such a way that, based on the defined parameters, the maximum pull-out force increases by 15 to 25 percent of the embedded length of the fibers. And a slip hardening behavior is formed up to this amount of slip, or the force-slip diagram of some samples shows a slip hardening behavior up to 50 to 70 percent of their insertion length. And the energy absorption capacity of these proposed steel fibers has increased by 50 to 90 percent compared to the hooked steel fibers available in the market.

Description

Description of the invention Title of the invention (as stated in the declaration) Interwoven steel fibers with slip hardening behavior in cement-based matrices Technical background of the relevant invention The claimed invention is in the field of civil engineering and relates to types of steel fibers used in the manufacture of fiber concrete. Technical problem and stating the objectives of the invention Fiber-reinforced concretes are classified into either strain-softening or strain-hardening behavior based on their response to tensile loading. Strain-hardening behavior is accompanied by the formation of multiple cracks under bending and after the formation of the first crack, and achieving this predictable and non-negligible tensile capacity in the post-cracking stages is one of the characteristics and goals of fiber-reinforced concrete. One of the methods that can provide the possibility of achieving strain-hardening behavior in the composite is the use of fibers with slip-hardening behavior. In conventional steel fibers, the main goal is either to increase the tensile strength of fiber-reinforced concrete, or access to fibers with existing slip-hardening behavior (complex fibers) is very difficult or its cost is very high (PVA fibers). Therefore, the main goal of this design is to achieve a fiber with slip-hardening behavior that increases the possibility of achieving a fiber concrete with strain-hardening behavior while simultaneously increasing its tensile strength, and that is very easy to manufacture and access.Its bond strength and ductility properties can be improved compared to a fiber with a different geometry and similar diameter. A description of the state of the prior art and the history of developments related to the claimed invention. In the early days of steel fiber reinforced concrete, the fibers were mostly undeformed (straight). More recent research shows that mechanical anchoring in deformed fibers effectively improves pullout resistance. Mechanical anchoring in steel fibers is achieved by deformation at the ends of the fibers, such as hook-end fibers, or deformation along the length of the fibers, such as corrugated fibers or twisted fibers. Hooked steel fibers utilize the plastic energy of steel deformation. However, only a small portion of the fiber length is used to increase the pullout resistance, which is due to the formation of two plastic hinges at the end hook. This plastic hinge formation leads to a slip-hardening response up to a specific slip. Twisted fibers are made of high-strength steel. The design of these fibers is such that when pulled out of the matrix, with increasing slip, its pull-out resistance increases and they maintain this resistance up to 70 to 90 percent of the buried length. Creating mechanical restraint in these fibers is achieved by twisting them along the length, and their shape becomes a screw or drill. And they have a triangular and square cross-section. For this reason, the cost of manufacturing them is high and access to these fibers is difficult. Twisted steel fibers are a new type of mechanical anchor for which no patent has been filed yet. Only one paper has examined its pull-out behavior in detail, and no slip hardening behavior has been observed. Therefore, by making changes in the way the twisted fibers are made and changing the straight fibers used, it became possible to achieve new fibers with slip-hardening behavior. Providing a solution to an existing technical problem along with an accurate, sufficient, and integrated description of the invention To achieve a fiber with slip hardening behavior and easy access, the twisting of two steel wires together was used as a new mechanical anchor. The characteristics and behavior of the interface between the fibers and the cementitious base matrix affect the mechanical properties of steel fiber reinforced concrete. For this purpose, many studies analyze the pullout behavior of a single fiber. The pullout test studies the behavior of the fiber-concrete interface. Therefore, a pullout test was conducted to investigate the behavior of the twisted steel fibers. To twist two steel wires together, a DC motor with a speed of 10 rpm and a steel disk with two holes (for connecting the straight fibers to the motor) were prepared. The ends of the straight fibers were hooked to a force gauge with a capacity of 150 Newton while being twisted together (Figure 1). To make the twisted steel fibers, two steel wires with diameters of 0.387 mm (S1) and 0.29 mm (S2) were selected, and the equivalent diameters of the twisted steel fibers after twisting were 0.537 mm and 0.41 mm, respectively. The length of each twisted steel fiber was taken as 40 mm. Figure (2) shows the twisted steel fibers with different twists. The geometric and physical properties of the steel wire and steel fibers used for the pull-out test are given in Table 1. Figure (3) shows pull-out test specimens made with interwoven steel fibers. Table 1: Steel fiber specifications Density [g / cm3] Aspect ratio [] End-Type Type of fiber Fiber Class ≥2700 200 7.85 - - 0.387 No hook Steel wire S1 ≥2780 200 7.85 - - 0.29 No hook Steel wire S2 200 7.85 74.48 40 0.537 No hook Twin-twisted steel fiber D1 200 7.85 97.56 40 0.41 No hook Twin-twisted steel fiber D2 ≥2700 200 7.85 92 35 0.387 Yes Hook Hooked-End D3 ≥2400 200 7.85 70 35 0.5 Yes Hook Hooked-End D4 آزمایش انجام شده: For each diameter of the twisted fibers, three different pitch types (2 turns, 3 turns, and 4 turns) were adopted, and a total of 6 models of twisted steel fibers were subjected to pull-out tests (each twist of the fibers is considered equivalent to 360 degrees of rotation of a separate wire in a spiral). To investigate the possibility of using twisted steel fibers with different twists per unit length and compare their performance with other fiber types, the most common steel fibers available on the market, which are hooked steel fibers, were selected with diameters of 0.5 mm (D3) and 0.387 mm (D4). To understand the pull-out behavior of twisted steel fibers from a concrete matrix, fibers with a number of twists per 10 mm (2 turns, 3 turns, and 4 turns), three buried lengths (10 mm (E10), 15 mm (E15), and 20 mm (E20)), three matrix strengths (47 MPa (C1), 80 MPa (C2), 110 MPa (C3)), and two equivalent diameters of twisted fibers (0.537 mm (D1), 0.41 mm (D2)) were investigated under static loading conditions (loading rate 1 mm / min). Test results: In all the pull-out diagrams of the fibers twisted together from the matrix, except in cases where the fibers are broken, due to the participation of the mechanical restraint of the fibers, with increasing slip, the pull-out force increases and a slip hardening behavior is formed. In such a way that based on the defined parameters, the maximum pull-out force increases up to 15 to 25 percent of the embedded length of the fibers and a slip hardening behavior is formed up to this amount of slip, or the force-slip diagram of some samples shows a slip hardening behavior up to 50 to 70 percent of their embedded length, which was observed in samples made in concrete with a compressive strength of 47 MPa. In map number (4), map number (5) and map number (6), some pull-out diagrams of the steel fibers twisted together are shown. By comparing the force-slip behavior of the hooked fibers and the interwoven steel fibers as shown in Figure 6, a significant improvement in the hooks is observed due to the interwoven fibers twisting together. In the hooked fibers, the formation of a plastic hinge leads to a slip-hardening response up to a certain slip. However, the pullout mechanism of the interwoven steel fibers is based on the torsional moment resistance of the fibers, which is distributed throughout its embedded length, which makes it so that even if the pullout force and maximum bond strength of the hooked fibers and the interwoven fibers are equal, the slip rate of the interwoven steel fibers before the fiber-matrix bond failure is much higher than that of the hooked fibers. As a result of this difference, a significant increase in the energy absorption capacity during the pullout of the interwoven steel fibers is observed. In such a way that the pulling energy of the twisted fibers was between 50 and 90 percent higher than that of the hooked steel fibers.For this reason, the equivalent bond strength of the interwoven fibers is also increased, which is closely related to the strain capacity and also the formation of multiple cracks in FRC with strain hardening behavior. And the ductility of fiber reinforced concrete with interwoven fibers can be significantly increased. Explanation of shapes, maps and diagrams Figure 1) shows a view of the configuration made to bend two straight steel wires together. (1) DC motor with a speed of 10 rpm (2) A steel disk with a diameter of 40 mm (3) Holes made on the steel disk with a diameter of 1 mm on the right and left sides of the steel disk for connecting the steel wires to the motor (4) Steel wires (5) Clamp to hold two steel wires (6) Force gauge Figure 2) Schematic of twisted steel fibers after cutting to a length of 40 mm. (1) Twisted steel fibers with 2 twists per 10 mm (2) Twisted steel fibers with 3 twists per 10 mm (3) Twisted steel fibers with 3 twists per 10 mm Map No. 3) Dimensions and sizes of concrete specimens for tensile testing of twisted steel fibers Map No. 4) Force-displacement diagram of twisted steel fibers with 2 turns of twist per 10 mm of fiber length 2 turns of twist in 10 mm of fiber length = TT2 Buried length of 20 mm in concrete specimen = E20 Concrete with a compressive strength of 47 MPa = C1 Concrete with a compressive strength of 79 MPa = C2 Concrete with a compressive strength of 109 MPa = C3 Equivalent diameter of twisted fibers = 0.537 mm D1 Map No. 5) Force-displacement diagram of twisted steel fibers with 3 turns of twist per 10 mm of fiber length 3 turns of twist in 10 mm of fiber length = TT3 Buried length of 20 mm in concrete specimen = E20 Concrete with a compressive strength of 47 MPa = C1 Concrete with a compressive strength of 79 MPa = C2 Concrete with a compressive strength of 109 MPa = C3 Equivalent diameter of twisted fibers = 0.537 mm D1 Map No. 6) Force-displacement diagram of twisted steel fibers with 4 turns of twist per 10 mm of fiber length 4 turns of twist in 10 mm of fiber length = TT4 Buried length of 20 mm in concrete specimen = E20 Concrete with a compressive strength of 47 MPa = C1 Concrete with a compressive strength of 79 MPa = C2 Concrete with a compressive strength of 109 MPa = C3 Equivalent diameter of twisted fibers = 0.537 mm D1 A clear and precise statement of the advantages of the claimed invention over prior inventions. The advantages of this invention: 1. Simplicity of the method of manufacturing braided steel fibers with available facilities compared to other steel fibers 2. Reduction of balling effect in fiber concrete due to straight fiber geometry 3. Lower manufacturing cost of twisted steel fibers due to their circular cross-section compared to triangular and square cross-section fibers with slip hardening behavior. 4. Achieving slip hardening behavior up to 50 to 70 embedded lengths in twisted fibers with 4 complete twists per unit length 5. Achieving high elongation energy in fibers with 2 turns of bending and 3 complete turns of twisting per unit length and forming a sliding softening behavior with a very low slope after the sliding hardening behavior compared to conventional fibers available on the market. 6. Achieving slip hardening behavior up to slip values ​​of 50 to 70 percent of the fiber length in normal strength concrete. Description of at least one implementation method for implementing the invention This invention is used to make steel fiber reinforced concrete. Explicit mention of the industrial application of the invention Brief description of the invention Title of the invention: Interwoven steel fibers with slip hardening behavior in cement-based matrices Interwoven steel fibers are a product for use in fiber concrete with a certain volume percentage. The use of steel fibers with slip hardening behavior allows for the achievement of a fiber concrete with strain hardening behavior. Therefore, the main goal of a fiber with slip hardening behavior is to be easy to manufacture and accessible. For this purpose, the twisting of two straight high-strength steel wires together with different twisting cycles per unit length was used. According to the results, slip hardening behavior was formed for interwoven steel fibers at different twists. In such a way that, based on the defined parameters, the maximum pull-out force increases by 15 to 25 percent of the embedded length of the fibers. And a slip hardening behavior is formed up to this slip value, or the force-slip diagram of some samples shows a slip hardening behavior up to 50 to 70 percent of their embedded length. And the energy absorption capacity of these proposed steel fibers has increased by 50 to 90 percent compared to the hooked steel fibers available on the market.

Claims

Claim: Claim 1) What is claimed is a new geometry for the types of steel fibers available on the market, which is made by twisting two high-strength steel wires along the length of the fibers, and its length after twisting was considered to be 40 mm, to be used to prepare fiber concrete with a certain volume percentage, and the name of this new steel fiber was called twisted steel fiber. Claim 2) Dependent on claim (1), the geometry of these new steel fibers is such that deformation continues along the entire length of the fibers, and the mechanism of their extraction from the concrete due to tensile force is based on the torsional moment resistance. Claim 3) Dependent on claim (1), the fiber twisting mechanism has caused the formation of slip hardening behavior. Claim 4) Dependent on claim (2), the fiber pull-out mechanism is associated with the fibers twisting apart along their entire length, which increases the energy absorption capacity of the fibers after tensile cracking of the concrete. Claim 5) Dependent on claim (2), the mechanical attachment of the fibers to the concrete can be increased by increasing the number of twists from 2 to 3 and 4 per 10 mm of its length. Claim 6) Dependent on claim (3), it is also possible to achieve the slip hardening behavior of twisted fibers in normal strength concrete. Claim 7) Dependent on claim (1), due to the straight shape of the fibers, the possibility of fibers balling up inside the concrete is reduced when making fiber concrete. Claim 8) Dependent on claim (3), the volume percentage required for making fiber concrete with strain hardening behavior is reduced.

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