MXPA00000766A - Concrete reinforcing fiber - Google Patents

Abstract

An improved reinforcing fiber for concrete is formed with two types of anchors positioned adjacent to each axial end of the fiber. A drag anchor which frictionally resists being pulled from the concrete without fiber breakage and a dead anchor between the drag anchor and adjacent axial end of the fiber, the dead end engages the concrete to develop stresses at a weakened point in the fiber formed between the drag anchor and its adjacent dead anchor to break the fiber or deform the dead anchor before maximum tensile strength of the fiber is reached so that the dead anchor functions to maximize the load carrying capacity while at the same time protecting against fiber rupture and the drag anchor continues to function after release of the weak point of the fiber.

Description

FIBER OF RS_f8g_ CONCRETE Z0 DESCRIPTION OF THE INVENTION The present invention relates to a reinforcing fiber particularly suitable for reinforcing concrete. Concrete is considered a brittle material due to its low tensile strength and fatigue and thus requires reinforcement eg a reinforcing steel rod as reinforcement to provide a structural concrete generally known as reinforced concrete. Another way or method to reinforce concrete is to form a composite that incorporates short fibers such as steel fibers, which typically have a length of approximately 25mm (1 inch). By dispersing these fibers through the concrete, the fracture strength of the concrete can be increased several times in such a way that the amount of energy consumed before the break is significantly greater. A form of concrete where the reinforcing fiber is especially attractive is in a concrete known as Shotcrete which is a form of concrete that has dispersed in it a plurality of fibers that are sprayed together with the cement, water and aggregates to produce a Shotcrete with fiber reinforcement when cement sets in situ. Approximately 50% of the total global steel fiber demand is consumed by Shotcrete.

One of the main problems with the steel fibers used in the Shotcrete is known as "rebound" which occurs when the shotcrete mixture of cement and fiber aggregate is sprayed or projected onto its position and a high proportion of the fibers do not it is embedded in the resulting concrete, and in this way they are wasted. For example, with commercially available fibers, which have a generally approximate diameter of 0.5mm (some flat fibers are also used) and a length of about 25mm. as much as 75% of the steel fiber can bounce and not be present in situ in the final concrete. It is recognized that the reinforcing fiber that is pulled out of the concrete matrix in the fissures is the main mechanism that allows the concrete reinforced with steel fibers (SFRC) to be more ductile than the non-reinforced concrete. In this way, all the commercial reinforcement fibers currently available on the market, are deformed at the ends along their length, to improve the fixation of the fiber with the concrete matrix and generate a greater pulling resistance. The current state of technology in the design of fibers can be divided into two large groups with respect to their fixing mechanisms, mainly a "fixed fastener" and a "drag holder".

Fixed fasteners are generally produced by deforming the fibers with a hook or cone adjacent to each of their ends, under tension, in an aligned fiber (ie, under axial tension) the fastener is generally designed to fail (eg, pull ) at a maximum strength below the strength of the steel. However, these fixed fasteners, after failure, have a significantly reduced capacity to resist pull movement. Drag fasteners are generally formed by expanding the fiber adjacent to its end so that during pulling, the enlargement generates friction with the matrix as the fiber is dragged out of the concrete. This type of fiber generally develops a lower maximum pull strength compared to the fixed fastener but its effect tends to last for a greater pulling displacement and therefore a higher pull energy is consumed at the end of the pulling process. Various types of fastening mechanisms are shown for example in the North American Patent NO. 4,883,713 issued November 28, 1989 to Destree et al, which shows a reinforcing fiber with an expanded head at each axial end of the fiber and U.S. Patent No. 5,215,830 issued June 1, 1993 to Cinti which shows a reinforcing fiber of metal wire with a straight central portion and holding parts misplaced at opposite ends. Canadian Patent No. 2,094,543 published November 9, 1993 whose inventor is Nemegeer discloses a fiber with hook ends. U.S. Patent No. 5, 443, 918 issued on August 22, 1995 to Banthia et al, describes a metal fiber to reinforce a cement-based metal which incorporates deformed end portions in a specific manner made in accordance with the properties of matrix and fiber to obtain the desired composite strength in the resulting composite. U.S. Patent No. 5,451,471 issued September 19, 1995 to Over et al discloses a reinforcing fiber deformed near both ends thereof over a selected distance such that the selected amount of the undistorted portion of the fiber is found. between the deformities. The fibers are also provided with a large number of notches that extend at an angle along the longitudinal axis of the fiber and increase the pulling strength of the fiber when used as reinforcement in the concrete matrix. It is an object of the present invention to provide an improved reinforcing fiber for covering concrete, more particularly, it is an object of the present invention to provide an improved fiber geometry for reinforcing concrete composites formed by casting or shotcreting methods. Broadly, the present invention relates to a reinforcing concrete fiber comprising fiber means defining an adjacent pull fastener but separate from each axial end of the fiber, means for forming a fixed fastener between each means forming the pull fastener and its adjacent axial end of the fiber and fixed fastener release means that reduce the load borne by the fixed fastener when the applied load for the fiber develops a tension in the release means that exceeds a selected maximum. Preferably, the fixed fastener releasing means comprises means defining a stress concentration point in the fiber between each fixed fastener and its adjacent pull fastener. Preferably the weak point is built to fail under stresses when the fiber is subjected to a load below a maximum load bearing capacity of the fiber between the weak points of stress concentration to release the fixed fastener when the fiber that is found between the weak points of voltage concentration is under a load less than the maximum load.

Preferably each fixed fastener has a load bearing capacity when in situ in particular below each drag holder. Preferably, each pull fastener is formed by a pair of side flanges projecting to the sides projecting one in each pair of opposite sides of the fiber in the first distance. Preferably, the pair of lateral flanges extending sideways is formed by a deformity in the fiber locally reducing its thickness without producing areas of significant stress concentrations to reduce the axial tensile existence of the fiber. Preferably, the means defining the fixed fasteners are formed by a deformity in the fiber by reducing its thickness to provide a second pair of side flanges projecting to the sides projecting laterally from the fiber a second distance greater than the first distance. Preferably, the first and second flanges are placed in substantially parallel planes. Preferably, the means defining the weak points are in a tension concentration area formed in the fiber adjacent to where the fixed fastener connects to the fiber, on one side of the fixed fastener adjacent to its adjacent pull fastener.

Preferably, the fiber has a ratio of fiber length in the square root of the diameter of the fiber of less than 30mm @ l / 2. Preferably, the fiber has a fiber length of between 20 and 35mm and a diameter of between 0.6 and 1mm. BRIEF DESCRIPTION OF THE DRAWINGS Characteristics, objects and additional advantages will be obvious from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which: Figure 1 is a diagram of fiber rebound as percentage of mass bounced against the fiber length on the square root of the diameter of the fiber in millimeters; Figure 2 is a side view of a preferred embodiment of an end of a fiber constructed with the present invention; Figure 3 is a plan view looking towards the direction of the arrow 3 in Figure 2, Figure 4 is a diagram of the pulling displacement against the nominal tension in the steel for a commercially available fiber having only one fixed fastener, a commercially available fiber having only one pull fastener and for a fiber having a combination of fixed and pull fasteners constructed with the preferred embodiment of the present invention; Figure 5 is a diagram of fiber length versus fracture energy of the shotcrete for four different fiber lengths constructed in accordance with the present invention; Figure 6 is a diagram of fiber diameter versus fracture energy of the shotcrete for three fibers with different diameter of the present invention; Figure 7 is a load versus displacement diagram in a flexural strength test (ASTM C1018) compared to the shotcrete made using the two different types of commercial fibers used in the tests shown in Figure 4 with the fiber-made shotcrete constructed in accordance with the present invention (average of at least 4 tests). Before describing the preferred embodiment of the invention, it should be noted that in the test performed, the material used in all the fibers is steel conventionally used in the manufacture of reinforcing fibers, in this way, the description should be read based on that the fibers are made of steel or a material with equivalent mechanical properties. If a different, but adequate material is to be used, the size and configuration will have to be modified according to the physical characteristics of the material from which the fibers are made. Obviously, the ductility of a fiber material can provide that certain materials, in fact many materials, are not suitable for use, ie, materials that are too ductile or too brittle, which will not be suitable. As indicated above, the amount of fiber rebound seriously increases the strength of reinforced concrete product in that if the fiber bounces and is not retained within the concrete it can not function to improve strength. A series of experiments were conducted using steel fibers in circular cross section having diameters and lengths as follows: diameters 0.5, 0.61, 0.65, 0.76 and lmm and lengths of 3, 12.5, 19, 24.5 and 40mm. The fibers of each diameter were made at each length. Shotcrete was produced using the dry mix technique and the fiber rebound was evaluated and the in situ fiber content was determined. The results obtained are shown in Figure 1. Applicants have found that there is a substantially linear relationship between the fiber rebound Rf and an aspect ratio provided by the fiber length divided by the square root of the diameter of the fiber, i.e. : Rf = function. lf / .phi. @ l / 2 where Rf = the fiber bounce If = fiber length .phi. = fiber diameter It will be obvious that a reduction in rebound Rf significantly increases the amount of fiber retained in the concrete produced to the extent that if a fiber rebound is reduced from the 75% figure that characterizes the fibers currently in the 50% market, the fiber content in situ is doubled for the final shotcrete produced. As can be seen in Figure 1, if the fiber rebound is below about 70%, which is less than that of conventional fibers, the fiber length ratio of the square root of the diameter of the fiber will be below about 30mm @ ^ _ (for steel). Figures 2 and 3 show the half (one end) of a preferred fiber constructed in accordance with the present invention, ie having a preferred fiber geometry. The other half is essentially the same since each fiber is symmetrical on opposite sides of its average length. As shown, the fiber 10 has a diameter d and has a length of fixed lf, which in the illustrated arrangement is intended by the dimension lf / 2 since only half the length of the fiber is shown. The other half of the fiber is essentially the same as that shown in Figures 2 and 3.

The fiber is provided with a drag holder 12 having a length Id and a width wd measured at the maximum width of the drag holder 12. The drag holder 12 in the illustrated arrangement is a deformity in the fiber arrangement to reduce the thickness to td by deforming the fiber with a die or the like having a radius rg which causes the width of the fiber to increase in the area Thickness reduced to width wd, that is, the width wd in the drag holder will be greater than the diameter d of the fiber. While it is preferred to use a radius with a radius rg, ie a circular shape, this is not essential, however care should be taken in deforming the fiber and not forming areas or areas of high voltage under load in the fiber, which It can cause the fiber to break prematurely. Adjacent to the axial end 14 of Figure 10, there is a connecting section 16 having a length measured in the axial direction of the fiber indicated in it (it is small in relation to Id and in some cases it may be zero (0) ) and adjacent and preferably extending from the free end 14 of the fiber 10 to the section 16 is a fixed fastener 18 having a length I measured in the axial direction of the fiber a thickness t which is significantly less than the thickness td of the fiber. 12 fastener - s «5 * j_: drag, and a width w significantly greater than the width wd of the drag section 12. A concentration of voltage or weak point 20 causes a concentration of tension and ensures the breaking of the fiber at the point of concentration of tension under load conditions higher than normal. This stress concentration point is preferably formed by a lowering neck section 22 where the configuration of the fiber is significantly altered to be included in the fixed fastener 18, i.e. the cross-section of the fiber is significantly flattened and wider (to form the fixed fastener which will normally have approximately the same cross-sectional area as the non-deformed fiber) over a short length ln formed in the array illustrated by a slice having a radius n to define a voltage concentration or weak point 20 which provides the breaking point through which the fiber is intended to break during use when the fiber is subjected to a sufficiently high load to develop a tension at the stress concentration point 20 above the breaking point. This break occurs to provide ineffectiveness for the fixed fastener and thereby decrease fiber tension levels. For the fiber to break at the proper load it is required that the fixed fastener 18 provide sufficient drag strength outside the concrete to generate a fiber tension greater than can be accommodated by the weak point 20, that is, the tension at 20 becomes so high that the fiber breaks at the area 20. In this way, the overall thickness t and width w in fact generate the clamping power of the fixed fastener 18 on the fiber 10 as illustrated , must develop sufficient flexure or bond with the concrete so that a pulling force required to generate the tension in the tension concentration pin 20 is high enough to break the fiber at the weak point 20 and the fiber can be applied axially between traction 12 and fasteners 18 fixed. In some chaos, the flanges or side protrusions 19 and 20 of the fastener 18 fixed on opposite sides of the fiber tend to form loops or bend, which reduces the sliding resistance of the fixed fastener 18 and causes the fixed fastener 18 to be less effective to withstand a high load so that the ability to withstand a maximum load in these cases is reduced by the formation of loops of the fixed fastener 18 to reduce the load on the fiber. In this way, the object of the invention to ensure that the fixed fastener is released to reduce the tension in the fiber can be obtained in at least two ways mainly by designing the fiber to break at a point 20 of concentration of tension between the fixed 18 and the drag clamps 12 and / or causing the fixed fastener 18 itself to be deformed and released. The geometry of the fixed fastener 18 which allows it to be released by the deformation of the fixed fastener at a peak load before breaking at the weak point 20 (if a weak point 20 is provided) and in any case to reduce the stress on the fiber, for the design shown in Figures 2 and 3, it depends mainly on the thickness t of the fixed fastener 18. Since, as indicated above, the tension concentration or weak point 20 may not be a main factor that causes the release of the fixed fastener, it is preferred to include said point in the design of the fiber since it can be designed with more accuracy to ensure the release of tension in the fiber under the appropriate loading conditions. The load bearing capacity of the fiber between the weak points 20 of stress concentration is not exceeded when the fiber breaks at weak points 20 of stress concentration. The drag holder 12 functions in essentially the same manner as the conventional drag holder in a conventional reinforcing fiber. Nevertheless, the maximum tensile force or the axial force applied to the fiber 10 to be able to allow the drag holder to be dragged through the concrete is less than the maximum force required to break the fiber 10. The additional incremental forces that are supported by the fixed fastener 18 under peak conditions cause stresses at the weak point 20 to break the fiber at the weak point 20 or the tensions at the fixed fastener to deform the fixed fastener 18 and cause it to be released. In this way, the fixed fastener 18 functions to reinforce the concrete in one case until the break occurs in 20 or in the second case until the fixed fastener is deformed. In any case, as shown in Figure 4, the energy that can be absorbed by the fiber is substantially greater than that which can be absorbed using conventional reinforcing fibers with conventional fastener structures. This system allows the application of a higher total draft load without the risk of breaking the fibers since the fixed fastener is released before the stress on the rest of the fiber, including the drag clamp, exceeds its rupture modulus. Generally, the drag holder 12 will be designed to support at least 80% of the peak load and preferably 90% or more so that the incremental load supported by the fixed fastener is small and the carrying capacity of the fiber is not dramatically reduced when the fixed fastener is released.

Figure 4 shows the effectiveness of the present invention in improving the energy absorption that can be obtained from the individual fibers having the fastener of the present invention relative to the individual commercially available fibers with fasteners. The commercial fiber having only one pull fastener (curve 1 in Figure 4) provides a relatively gradual increase in tension as the displacement (pull) increases to about 1.5mm. When a fiber with only one fixed fastener was tested (curve 2 in Figure 4) the maximum or peak voltage that can be applied was significantly higher, approximately 900 MPa. (tensile strength of the steel used in all cases is 1100 MPa, but the displacement that can be tolerated is less than about X in. In both cases, the nominal fiber tension rapidly decreases (even more so for the fixed fastener). that for the drag holder) since the displacement increases beyond the point of peak tension The fiber having the combination of the fixed and drag fasteners 18 and 12 of the present invention, (curve 3, Figure 4) shows a very significant increase in the voltage that can be tolerated, that is, the rated voltage for the fiber reaches above 1000 MPa while accommodating a displacement of approximately 2 * mm and then the voltage that can be allowed decreases but it is not reduced to that of the commercial towing fastener by itself until a very substantial amount of hauling has been carried out, that is, in the order of about 7mm. breaks or the fixed fastener 18 is deformed to release the fixed fastener the peak tension is obtained which occurs before the breaking strength of the fiber is reached, thereby preventing the fiber breakage load from being applied to the fiber. It became obvious from fiber 4, that the energy absorbed using the present invention from the combination of fixed and pull fasteners (curve 3) is able to significantly absorb more energy than either of the two fasteners of the prior art. (curves 1 or 2) (the energy absorbed is measured by the area under their respective curves). Thus, it is obvious that significant improvements in the amount of pull energy that can be absorbed can be obtained using the present invention. EXAMPLE To optimize the present invention, fibers were made from a wire of fixed diameter with a diameter of 0.89 mm formed with lengths of 12.5, 19, 25.4 and 40mm and all were tested at a rate of 60 kg / m. 3 in shotcrete to determine its cumulative fracture energy under a flex load of an ASTM C 1018 standard test on beam specimens 100. times .100. times .350 mm. (area under the load of bending against the displacement curve at a displacement of 2 mm). The results obtained are shown in Figure 5 where it is obvious that the fiber length of between 20 to 40 mm, preferably about 25 mm, was found to be optimal. After flexing an optimal length of 25.4 mm, the diameter fibers of 0.61, 0.76 and 0.89 were tested. The results of these tests are shown in Figure 6, where it is clearly indicated that a fiber diameter of approximately 0.75mm (0.74 to 0.8mm) was optimal. Based on these dimensions, mainly, a length if = 25.4mm and a diameter d = 0.76mm, the dimensions of the fiber illustrated in Figures 2 and 3 were optimized. In this arrangement, the diameter rg of the notch forming the drag section 12 was 10.7 mm, the thickness td was approximately 0.46 times the diameter d, the width wd was 1.45 times the diameter d. Based on the dimensions rg and td the length Id can be derived. The length I of the fixed hook section was set to 1.4 the diameter of the fiber and the thickness t was 0.23 times the diameter d which produces a width w of 2.36 times the diameter. The dimension was 0.2 mm and In and the radius rn for this example were equal to and less than 0.5mm. In other words, in one of the preferred embodiments of the present invention for the shotcrete, it uses a fiber diameter of 0.76mm, a thickness td of 0.35mm, a width wd of 1.1mm, a thickness t of 0.18mm and a width w of 1.79 mm. EXAMPLE 2 The fibers as described in the previous example were produced in a sufficient amount and tested in a shotcrete application and were compared using a standard ASTM C1018 test with 100. times .100. times .350mm. 5 specimens under the flexure test with commercial fibers were used for the same application. The results of these tests are shown in Figure 7 where curve A is a diagram of the results obtained using the present invention, and curve B was obtained using fibers sold under the trademark Dramix by Bekaert and curve C using a fiber FE sold by Novocon. It is obvious that the present invention is capable of accommodating more load bearing capacity and therefore consuming more fracture energy (the area contained by the curves in Figure 7) than either of the other two commercial products.

The above description has been directed mainly to Shotcrete applications since they are more complicated because the rebound of the fiber plays an important role, however the present invention can also be used with cast concrete. The fibers that are used in cast concrete can, for example, have a significantly longer length than those for Shotcrete, in fact the length can be approximately doubled. Having described the invention, the modifications will be obvious to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims (20)

1. CLAIMS 1. A concrete reinforcement fiber comprising a fiber having a maximum load bearing capacity, means defining a tension fastener adjacent to but separated from each axial end of the fiber, means forming a fixed fastener between the ends of the fiber. means forming the drag holder and its adjacent axial end of the fiber and a fixed fastener release which reduces the load borne by the fixed fastener when an applied load of less than the maximum support capacity is applied to the fiber and whose Applied load develops a tension in the fixed fastener release that exceeds a preselected voltage. The concrete reinforcement fiber according to claim 1, characterized in that the fixed fastener release comprises means defining a weak point of stress concentration in the fiber between each fixed fastener and its adjacent fastener. 3. The concrete reinforcement fiber according to claim 2, characterized in that the weak point is constructed to fail under the stresses to release the fixed fastener when the fiber is under an applied load less than the maximum load bearing capacity of the fiber. The concrete reinforcement fiber according to claim 1, characterized in that each fixed fastener has a load bearing capacity when it is in situ in the concrete smaller than each drag holder. 5. The concrete reinforcement fiber according to claim 2, characterized in that the fixed fastener has a load bearing capacity when it is in situ in the concrete smaller than each drag fastener. The concrete reinforcement fiber according to claim 3, characterized in that each fixed fastener has a load bearing capacity when it is in situ in the concrete smaller than each drag holder. The concrete reinforcing fiber according to claim 2, characterized in that the means defining the weak point of stress concentration is an area of stress concentration formed in the fiber adjacent to where the fixed fastener is connected to the fiber to one side of the fixed fastener adjacent to this adjacent to the drive fastener. The concrete reinforcing fiber according to claim 3, characterized in that the means defining the weak point of stress concentration is an area of stress concentration formed in the fiber adjacent to where the fixed fastener is connected to the fiber on one side of the fixed fastener adjacent to it adjacent to the drive fastener. 9. The concrete reinforcement fiber according to claim 2, characterized in that the drag fastener is formed by a pair of side flanges projecting to the sides projecting one on each pair of opposite sides of the fiber at a first distance. The concrete reinforcement fiber according to claim 9, characterized in that the pair of side flanges extending sideways is formed by a deformity in the fiber locally reducing its thickness without producing areas of significant stress concentrations that reduce the axial tensile strength of the fiber. The concrete reinforcement fiber according to claim 9, characterized in that the means define a fixed fastener that is formed by a deformity in the fiber reducing its thickness to provide a pair of side flanges projecting laterally projecting laterally of fiber a second distance greater than the first distance. The concrete reinforcing fiber according to claim 11, characterized in that the first and second flanges are arranged in substantially parallel planes. 13. The concrete reinforcement fiber according to claim 3, characterized in that each fastener The dragging is formed by a pair of lateral flanges projecting to the sides projecting one in each pair of opposite sides of the fiber at a first distance. 14. The concrete reinforcement fiber according to claim 13, characterized in that the pair of side flanges extending sideways is formed by a deformity in the fiber locally reducing its thickness without introducing areas of concentration of significant tension that reduces the axial tensile strength of the fiber. 15. The concrete reinforcement fiber according to claim 14, characterized in that the means define a fixed fastener that is formed by a deformity in the fiber reducing its thickness to provide a 15 second pair of side flanges projecting to the sides projecting laterally from the fiber a second distance greater than the first distance. 16. The concrete reinforcement fiber according to claim 15, characterized in that the lashes The first and the second are placed in substantially parallel planes. 17. The concrete reinforcement fiber according to claim 1, characterized in that the fiber has a fiber length of between 20 and 35 mm and a diameter of 25 fiber between 0.6 and 1 mm. The concrete reinforcement fiber according to claim 1, characterized in that each drag holder is formed by a pair of lateral flanges projecting towards the sides projecting one envelope each pair of opposite sides of the fiber at a first distance. 19. The concrete reinforcement fiber according to claim 18, characterized in that the pair of lateral flanges extending to the sides is formed by a deformity in the fiber locally reducing its thickness without producing areas of significant concentrations that reduce the strength to the axial tension of the fiber. 20. The concrete reinforcement fiber according to claim 2, characterized in that the fiber has a fiber length of between 20 and 35mm and a fiber diameter of between 0.6 and 1mm. _ * ' "Su- SUMMARY An improved reinforcing fiber is formed for concrete with two types of fasteners placed adjacent to each axial end of the fiber: a drag holder that resists friction being pulled from the concrete without breaking the fiber and a fixed fastener between the drag clamp and the adjacent axial end of the fiber, the fixed end engages the concrete to develop stresses at a weakened point in the fiber formed between the drag clamp and its adjacent fixed clamp to break the fiber or deform the fixed clamp before the maximum tensile strength of the fiber is reached such that the fixed fastener works maximizing the load carrying capacity while at the same time protecting against breakage of the fiber and the continuous drive fastener running after the release of the weak point of the fiber.