Apparatus and process for producing fibers for concrete and the like

Abstract

A fiber for reinforcing concrete and the like, made of a polymer material, preferably polypropylene, and provided, along at least one longitudinal section, with a plurality of first recesses on a first side of the section, and with a plurality of second recesses on a second side, opposite the first side, of the section, said first and second recesses being staggered from each other, the minimum distance between a first recess and a second recess arranged in series with each other being equal to or greater than 90% of the maximum height of the fiber when measured at a cross-section without recesses.

Description

[0001] "Apparatus and process for producing fibers for concrete and the like"

[0002]

[0001] FIELD OF THE INVENTION

[0003]

[0002] The present invention relates to the field of the production of polymer fibers and specifically to an apparatus and process for producing reinforcing fibers for concrete and the like.

[0004]

[0003] Specifically, the present invention relates to manufacturing fibers preferably based on polyolefin such as homo- or co-polymer of polypropylene, which can provide excellent mixing with cement and cementitious mixtures such as concrete. The fibers object of the invention are so-called structural macro-fibers, that is, fibers with a diameter greater than 0.2 mm, preferably at least 0.3 mm.

[0005]

[0004] KNOWN PRIOR ART

[0006]

[0005] As known, concrete is a cement-based composite that is significantly brittle and has poor tensile and bending strength compared to, for example, metal materials.

[0007]

[0006] As a matter of fact, concrete undergoes shrinkage adaptations during the fluid / plastic phase and subsequent drying. Shrinkage is basically due to two combined actions: autogenous shrinkage due to the hydration reaction of cement and drying shrinkage, also called hygrometric shrinkage due to the evaporation of excess hydration water contained in fresh concrete. These two different actions are called "plastic shrinkage".

[0008]

[0007] Therefore, in use, concrete is subject to fissures (cracks) and breakages which decrease its mechanical properties, reduce or jeopardize the strength of the manufactured article, reduce or jeopardize the “durability" of the work and can cause water seepages. If water seeps in, and outside temperatures drop, the water could freeze and fracture the concrete. Building structures made of concrete (both residential and functional, such as roads and bridges) can therefore be subject to structural failure, with obvious undesirable consequences.

[0009]

[0008] In addition, concrete may have micro-defects formed during its preparation, which can form subsequent cracks upon application of an external load.

[0010]

[0009] For example, the use of metal reinforcing elements is known to improve the mechanical properties of concrete. For example, in the construction of pavements, it is known and commonly used to have a mesh formed by iron bars arranged and tied to form a mesh. The concrete is poured over the mesh, which as a result is embedded in the concrete in the manufactured finished product. It is also known to reinforce concrete by dispersing fibrous, typically discontinuous, materials into it, in place of or in addition to the wire mesh. The function performed by the fibers added to the concrete mix is to counteract the internal stresses which are triggered in a manufactured article during hardening or curing and which create the so-called cracking phenomenon. The anti-cracking action of concrete fibers is well known. In some cases, not only the fibers added to the concrete mixture have the ability to prevent and stop the cracking phenomenon, but also have such robustness that they can take on more significant loads of a structural nature.

[0011]

[0010] A fiber particularly suitable to be added to concrete has historically been asbestos fiber. However, subsequent research has demonstrated the harmfulness of asbestos fibers to humans, particularly its carcinogenic properties, so that reinforcing concrete with asbestos is no longer a viable solution.

[0012] [Oi l] Alternative solutions led to the use of different fibers, including metal fibers or synthetic fibers.

[0013]

[0012] Metal fibers generally have high cost and are subject to degradation due to oxidation in open environment (particularly in the presence of water or moisture). Moreover, in the event that concrete is used to pave portions of roads or anyway portions intended for traveling vehicle, metal fibers brought to the surface by the continued use of the pavement can damage the tires of traveling vehicles.

[0014]

[0013] The synthetic fibers used are, for example, polyolefins, polyacrylonitrile, polyamides and homo- and co-polymers of polyester. Among polyolefins, a material particularly suitable for making synthetic reinforcing fibers for cement, both anticracking and structural, is homo- and co-polymer of polypropylene (PP), preferably virgin PP, by virtue of its chemical-physical and mechanical properties.

[0015]

[0014] The effect of the fibers on the cement is also related to the interaction between the surface of the polyolefin fibers and the cement. The fibers have the function of anchoring the concrete when micro-cracks begin to form in the cement matrix in order to reduce the formation of larger cracks. The mechanism of crack stopping in fiber- loaded concrete is affected by the amount of fibers per unit volume of the mixture, the mechanical properties of the fibers, e.g., their toughness, and the ability of the fibers to anchor themselves to the concrete matrix.

[0016]

[0015] A problem with the use of synthetic fibers as an admixture for concrete and cementitious mixtures is due to the difficulty of achieving good and uniform mixing of the fibers within the cement paste, i.e., mixture. In particular, there are limits to the weight amount of polyolefin fibers that can be added to a cement matrix, over which the mixture is inhomogeneous with aggregation of fibers with each other.

[0017]

[0016] In particular, a problem with synthetic fibers is that they are not compatible with cement paste, meaning that the fibers tend to slip relative to the paste or concrete matrix, thereby jeopardizing their ability to retain the cement and hinder crack formation.

[0018]

[0017] It is also well known to make recesses on the side surface of the fiber to increase the bond of the concrete to the fiber, that is, the anchorage of the fiber to the cement matrix. However, these recesses create weakening areas in the fibers, resulting in reduced toughness thereof; in order to use indented fibers as reinforcement in concrete, the amount of fibers per unit volume to be added to the mix must be increased.

[0019]

[0018] It is therefore object of the present invention to provide synthetic fibers, particularly polypropylene fibers, adapted to be used as reinforcement in concrete, and in particular having good cement matrix retention properties together with good mechanical properties.

[0020]

[0019] SUMMARY OF THE INVENTION

[0021]

[0020] These and other purposes are achieved by the present invention according to one or more of the appended claims.

[0022]

[0021] Fibers according to claim 1, and a respective production method according to claim 8 are, in particular, object of the present invention. A process for making concrete with a fiber according to the invention is further described in claim 10.

[0023]

[0022] Specifically, according to an aspect of the present invention, for the reinforcement of concrete and the like, made of polymeric material, preferably polypropylene, provided along at least one longitudinal section with a plurality of first recesses on a first side of the section, and a plurality of second recesses on a second side, opposite to the first side, of the section. The first and second recesses are staggered from each other, the minimum distance DI between a first recess and a second recess arranged in series with each other is equal to or greater than 90% of the maximum height D of the fiber section, preferably equal to or greater than 100% of the maximum height of the fiber section. In a preferred embodiment, said minimum distance is within a range such that 0.90D < DI < 1.10D, more preferably 0.95D < DI < 1.O5D; or, in other words, the D / Dl ratio is between 0.90 and 1.10, preferably 0.95 and 1.05.

[0024]

[0023] Specifically, the maximum height of the fiber can be calculated as follows. In the longitudinal section of the fiber, all the segments perpendicular to the fiber axis which connect the fiber axis to the first side of the section, and all segments perpendicular to the fiber axis which connect the fiber axis to the second side of the section, have to be considered. Among these, the segment with the longest length has to be selected. Then, the length of that segment has to be multiplied by two. The value obtained represents the maximum fiber height. Therefore, in a fiber having circular section, this operation consists in selecting the radius of the fiber having the greatest length in the section, multiplying its length by two, so as to derive the value of the maximum diameter of the fiber.

[0025]

[0024] The maximum height is then measured in a region of the fiber section without recesses.

[0026]

[0025] The minimum distance is the length of the shortest line segment connecting the first and second recesses.

[0027]

[0026] In particular, as the applicant has been able to observe, this value is particularly critical for fiber performance. The present invention ensures that the distance between opposite recesses is always similar to or greater than the maximum height of the fiber section. This allows the fiber to preserve sufficient toughness to effectively perform the designated task. The effect of having a value of the minimum distance between two recesses according to the invention (i.e., a value equal to or greater than 90%, preferably equal to or greater than 95%, more preferably equal to or greater than 100% but not greater than 110%, preferably not greater than 105%, of the maximum height of the fiber section) is that at this position, where there is the minimum distance between two recesses, the area of a fiber section measured in a plane which extends between said two recesses and is inclined with respect to the longitudinal axis of the fiber, along said minimum distance, is equal to or greater than 90%, preferably between 95% and 105%, more preferably about 100% of the area of a fiber crosssection measured at the maximum fiber height.

[0028]

[0027] Therefore, the recesses allow the fiber to stay in place in the concrete, while the distance between first and second recesses prevents excessive weakening of the fiber, which is therefore less subject to breakages within the concrete. This property is respected by all pairs of first and second recesses arranged in series.

[0029]

[0028] The Applicant could verify that even an excessive distance value can reduce the performance of the end product, i.e., the concrete. Indeed, although a greater value of the distance helps to strengthen the fiber, it reduces the number of recesses per unit length and therefore reduces the fiber's ability to be constrained to the concrete matrix.

[0029] Therefore, the minimum distance between two recesses is preferably between 90% and 110% of the value of the maximum fiber height, more preferably between 95% and 105% of the maximum fiber height.

[0030]

[0030] According to a possible aspect, the longitudinal distance between a first recess and a second recess arranged in series is greater than 20% of the maximum height of the fiber, preferably greater than 30% of the maximum height of the fiber, more preferably greater than 50% of the maximum height of the fiber.

[0031]

[0031] This distance is measured along the fiber axis. The end and beginning of a recess are preferably found where the fiber reaches 99% of the maximum height value.

[0032] According to a possible aspect, the depth of the recesses is between 14 microns and 3 mm deep.

[0032]

[0033] Typically, the first and second recesses have same depth. In general, first and second recesses have identical size and shape with each other and are arranged at the same pitch. In preferred embodiments, a second recess is interposed between two first recesses, at half the distance between two first recesses.

[0033]

[0034] According to an aspect, there is at least one longitudinal section without recesses. The recesses do not extend along the entire circumference of the fiber section.

[0035] The fiber can have various sections, but in preferred embodiments it has circular section not considered at the recesses. The fiber preferably has maximum height of the fiber section between 0.25 and 0.75 mm, more preferably about 0.5 mm.

[0036] According to a possible aspect, the length of the fiber is between 10 and 100 mm, preferably between 30 and 70 mm. Preferably, the "aspect ratio" of the fibers, defined as the ratio of fiber length to diameter, i.e. maximum fiber height, is between 30 and 500, more preferably between 100 and 500.

[0034]

[0037] According to a possible aspect, the fiber is surface treated with a plasma treatment.

[0035]

[0038] An aspect of the invention also relates to a process for making a polymer fiber for reinforcing concrete and the like, comprising the steps of: a) Extruding a polymer filament; b) Forming, preferably by pressworking or calendering, a plurality of recesses on two opposite sides of at least one longitudinal section, said first and second recesses being staggered from each other, the minimum distance between a first recess and a second recess arranged in series with each other being equal to or greater than 90% of the maximum fiber height, preferably equal to or greater than 100% of the maximum fiber height; c) Cutting the polymer filament to make a plurality of fibers.

[0036]

[0039] Preferably, as discussed, the process also provides the step of surface treating the filament or fibers (i.e., before or after cutting) by plasma treatment.

[0037]

[0040] An aspect of the invention also relates to a process for making concrete, comprising the step of mixing a fiber according to one or more of the previous aspects, in an amount between 2 and 8 kg of fiber per cubic meter of concrete, that is, per cubic meter of end product.

[0038]

[0041] In preferred embodiments, this amount is between 2.5 and 6 kg / m3, and preferably between 3 and 5 kg / m3.

[0039]

[0042] BRIEF DESCRIPTION OF THE FIGURES

[0040]

[0043] The invention and exemplary and non-limiting embodiments are now discussed with reference to the attached figures, in which:

[0041] Figure 1 is a schematic cross sectional view of a fiber according to an embodiment of the invention;

[0042] Figure 2 is a schematic sectional view on the plane Pl of a portion of the fiber of figure 1;

[0043] Figure 3 is a schematic view, not to scale, of a possible apparatus for forming the recesses in a fiber according to the invention;

[0044] Figure 4 is a schematic diagram of how fibers work to control crack formation.

[0045]

[0044] EMBODIMENTS OF THE INVENTION

[0046]

[0045] Referring at first to Fig. 4, the leftmost fiber in the figure is a fiber which is anchored to the concrete matrix that broke under the stress of crack formation and did not stop crack propagation. The fiber immediately to the right is a fiber that has not broken but, not being anchored to the matrix, has slipped out of its initial position, again failing to control propagation of the crack. The rightmost fiber is a fiber that is sufficiently anchored and tough to stay in place without breaking, thus reducing or stopping crack propagation.

[0047]

[0046] A fiber 1 for reinforcing concrete and the like is made of polymer material. Preferred materials are homo- and co-polymer of polypropylene. The fibers 1 are typically made by extruding polymer filaments, and are then cut to the desired length. Preferred lengths are between 10 and 100 mm, more preferably between 30 and 70 mm. Preferred lengths are about 50 mm.

[0048]

[0047] The fiber 1, along at least one longitudinal section, comprises a plurality of first recesses 11, 12, 13 and second recesses 21, 22, 23 on opposite sides LI, L2 of the section of the fiber 1.

[0049]

[0048] The first recesses 11, 12, 13 and the second recesses 21, 22, 23 are arranged in a staggered manner. Thus, there is no cross section, i.e., a section perpendicular to the fiber axis, that simultaneously crosses a first recess and a second recess.

[0050]

[0049] All possible line segments connecting a first recess and the second recess of a pair formed by a first recess and a second recess arranged in series with each other, are now considered.

[0051]

[0050] With reference to the figures, the first recess 12 and the second recess 22, for example, are considered.

[0052]

[0051] The length of the segment having the shortest length among all these segments is defined herein as the minimum distance DI between the first and the second segments.

[0053]

[0052] This minimum distance DI is equal to or greater than 90% of the length of the maximum height D of the fiber section, measured at a fiber cross-section without recesses 11, 12, 13, 21, 22, 23.

[0054]

[0053] As discussed, the value of the maximum height D can be calculated as the maximum distance between the axis A of the fiber, and either side LI, L2 multiplied by two. In the case of fiber with a circular section, this value corresponds to the fiber diameter (before the recesses are formed). The value of the maximum height D is preferably between 0.25 and 0.75 mm, more preferably between 0.3 or 0.4 and 0.6 mm. The maximum height in preferred embodiments is about 0.5 mm. Typically, these fibers have linear densities of about 1200-1800 den, preferably about 1600 den.

[0055]

[0054] Preferably, this minimum distance is equal to or greater than the maximum height D. In other words DI > 0.9D, preferably DI > D.

[0055] As discussed, the minimum distance DI also has a maximum value, so that 0.90D < DI < 1.10D, more preferably 0.95D < DI < 1.05D.

[0056]

[0056] This property applies to all pairs which comprise a first and a second recess arranged in series. For example, in Figure 2, in addition to the pair first recess 12 - second recess 22, it also applies to the pairs (denoted only by the numeral references) 11 - 21, 21 - 12, 22 - 13, 13 - 23.

[0057]

[0057] Preferably, for each pair of first and second recesses arranged in series, the longitudinal distance D2 between the recesses is greater than 20% of the maximum height D of the fiber section, preferably greater than 30% of the maximum height of the fiber, more preferably greater than 50% of the maximum height of the fiber section.

[0058] The longitudinal distance, excluding manufacturing tolerances, is substantially constant for each pair of first and second recesses arranged in series of the fiber 1.

[0058]

[0059] The end of a recess is found at the section S, S' where the height of the longitudinal section of the fiber (that is, the distance between the two sides LI and L2 of the longitudinal section of the fiber) reaches 99% of the value of the maximum height D.

[0059]

[0060] The depth P of the recesses, measured radially to the axis A of the fiber, is preferably between 14 microns and 3 mm. In any case, preferred values are less than 30% of the maximum height D, more preferably less than 20% of the maximum height D.

[0060]

[0061] The recesses are arranged distal to the axis A of the fiber 1. The distances DI and D2 are therefore typically measured on the longitudinal section where the recesses have maximum depth. It should be noted that the planes on which each recess reaches its maximum depth may be slightly offset due to manufacturing tolerances.

[0061]

[0062] The distances DI and D2 are then measurable on any plane passing through the axis A of the fiber 1 in which both recesses to be considered reach at least 90% of their maximum depth. In general, values DI and D2 can be found on all longitudinal fiber sections crossing both the first and second recesses 11, 12, 13, 21, 22, 23.

[0062]

[0063] The recesses preferably do not extend over the entire outer surface of the fiber 1.

[0063]

[0064] Typically, therefore, there is at least one longitudinal section of the fiber 1 without recesses.

[0064]

[0065] For example, in the embodiment shown in the figures, a longitudinal cross section along the plane Pl has recesses, whereas a longitudinal section of the fiber 1 along the plane P2 has no recesses.

[0065]

[0066] It should be noted that the longitudinal sections discussed herein are longitudinal sections that comprise the axis A of the fiber 1 (when the fiber 1 is arranged in a straight line).

[0066]

[0067] Recesses can be made by different processes.

[0067]

[0068] Preferred solutions include the use of shaped calenders 100, as schematically shown in Figure 3, or the use of molding presses.

[0068]

[0069] According to a preferred aspect, the fibers are surface treated by electronic plasma treatment. This allows the fiber to be made hydrophilic, so that its efficiency within the cement paste is increased.

[0069]

[0070] These treatments are known in the art and are not discussed in detail herein. Typically, the plasma treatment process comprises spraying an excited gas onto the surface of the filaments / fibers. Excited gas is the result of the removal of charged particles after it has been electrically excited by a plasma jet.

[0070]

[0071] As discussed, a preferred use of the fiber is in making concrete.

[0071]

[0072] In that case, the fiber is added to the cement paste adapted to form concrete, typically in an amount of between 2 kg and 8 kg of fiber per cubic meter of concrete.

[0072]

[0073] A preferred example of fiber making is described below.

[0073]

[0074] Example 1

[0074] A polypropylene filament, having circular section with linear density of 1600 den and maximum height D of 0.5 mm, is cut into fibers having length of 50 mm.

[0075] Before cutting, 51 notches are made on opposite sides of the filament every 50 mm. In other words, the notches, that is to say the recesses, are opposite and staggered from each other, with 51 recesses every 50 mm of fdament.

[0076] The notches are made so that the minimum distance DI is substantially equal to the maximum height D. Specifically, if manufacturing tolerances were zero, the minimum distance DI would be 0.503 mm.

[0077] The longitudinal distance D2 between pairs of first and second recesses is 0.3 mm, i.e. 60% of the maximum height D. The maximum depth Pl of the recesses is 0.08 mm, i.e. 16% of the maximum height D.

[0078] The fibers have toughness of 9 g / den and elongation at break of 8%.

[0079]

[0075] Example 2 A polypropylene fiber according to Example 1 was used to prepare fiber-reinforced concrete (FRC), which was tested to measure flexural strength, according to EN 14651 + Al, EN 14845-1 and EN 14845-2 standards.

[0080] In particular, concrete contains 3 kg of fiber per cubic meter of concrete.

[0081] According to the standard, six concrete specimens measuring 150x150x600 mm were prepared, cured in water and subsequently tested, resulting in a load of 1.77 MPa applied at a crack mouth opening displacement (CMOD) of 0.5 mm, and a load of 1.42 MPa, at an opening (CMOD) of 3.5 mm. The six specimens also exhibited very similar behavior to each other, highlighting the repeatability of this result. The results of this test are summarized in the following table.

[0082] Table 1

[0083] It should be noted that, according to EN 14889-2, this type of concrete is suitable for meeting certification requirements as it exceeds the limit values of 1.5 MPa and 1.0 MPa for 0.5 mm CMOD and 3.5 mm CMOD, respectively.

[0084]

[0076] The Applicant was also able to verify that fiber-loaded concrete specimens, in which the value of the minimum distance DI is more than 1.1 times the value of the height D or less than 0.9 times the value of the height D, do not meet these minimum stress requirements, if the concrete is reinforced with 3 Kg / m3of fiber.

[0085]

[0077] Indeed, fibers made of the same material but with distance DI less than 0.9 times the height D, are subject to breakages, whereas fibers with distance DI greater than 1.1 times the height D are not sufficiently constrained to the concrete (i.e. cement paste).

[0086]

[0078] In order to obtain suitable concrete specimens with these fibers, which are not in accordance with the invention, the fiber density within the concrete must be increased, typically to values greater than 4.5 kg / m3, and in some cases equal to or greater than 6 kg / m3. Moreover, the Applicant found that the standard deviation of the values of these concrete specimens, which are not in accordance with the invention, is significantly higher than that of a fiber according to the invention, thus exhibiting greater unpredictability of concrete behavior.

[0087]

[0079] Therefore, the particular geometry of the present solution allows quality concrete, in particular adequate according to EN 14889-2, to be obtained even when the amount of fiber within concrete is relatively small, that is, in the range of 2.5 kg / m3to 4.0 kg / m3. In other words, the technical effect of said fiber geometry according to the invention is to decrease the amount of fibers per unit volume required to obtain concretes that meet the requirements of EN 14889-2 standard.

[0088]

[0080] A lower amount of fiber results in reduced cost, better mixing in the preparation of the mix and also better workability of the concrete during casting, with the same mechanical properties of the concrete.

[0089]

[0081] In particular, it has been verified that thanks to the fibers of the present invention, the use of wire mesh can be avoided in applications such as pavements; this results in further significant economic benefits due to the elimination of labor costs resulting from the construction of the mesh at the site of the concrete casting.

Claims

CLAIMS1. Fiber (1) for reinforcing concrete and the like, made of a polymer material, preferably polypropylene, and provided, along at least one longitudinal section, with a plurality of first recesses (11, 12, 13) on a first side (LI) of the longitudinal section, and with a plurality of second recesses (21, 22, 23) on a second side, opposite the first side (LI), of the longitudinal section, said first and second recesses (11, 12, 13, 21, 21, 22) being staggered from each other, the minimum distance (DI) between a first recess (12) and a second recess (22) arranged in series with each other being equal to or greater than 90% of the maximum height (D) of the fiber (1) when measured at a cross-section without recesses, preferably equal to or greater than 100% of the maximum height (D) of the fiber (1), more preferably being between 90% and 110% of said maximum height (D) of the fiber (1).

2. Fiber (1) according to claim 1, wherein said minimum distance (DI) is between 95% and 105% of said maximum height (D) of the fiber (1).

3. Fiber (1) according to claim 1 or 2, wherein the longitudinal distance (D2) between a first recess (13) and a second recess (22) arranged in series is greater than 20% of the maximum height (D) of the fiber, preferably greater than 30% of the maximum height (D) of the fiber, more preferably greater than 50% of the maximum height (D) of the fiber.

4. Fiber (1) according to claim 1, 2 or 3, wherein the depth (P) of said first and second recesses (11, 12, 13, 21, 22, 23) is between 14 microns and 3 mm.

5. Fiber (1) according to one of the preceding claims, wherein there is at least one longitudinal section without recesses.

6. Fiber (1) according to one of the preceding claims, whose maximum height (D) is between 0.25 and 0.75 mm.

7. Fiber (1) according to one of the preceding claims, whose length is between 10 and 100 mm, preferably between 30 and 70 mm.

8. Fiber (1) according to one of the preceding claims, surface-treated with a plasma treatment.

9. Process for making a polymer fiber (1) for reinforcing concrete and the like, comprising the steps of:- extruding a polymer filament;- forming a plurality of first and second recesses (11, 12, 13, 21, 22, 23) on twoopposite sides (LI, L2) of at least one longitudinal section of the fiber (1), said first and second recesses (11, 12, 13, 21, 22, 23) being staggered from each other, the minimum distance (DI) between a first recess (12) and a second recess (22) arranged in series with each other being 90% or more of the maximum height (D) of the filament when measured at a cross section without recesses, preferably 100% or more of the maximum height (D) of the filament, more preferably between 90% and 110% of said maximum height (D),- cutting the polymer filament to make a plurality of fibers.

10. Process (1) according to claim 9, wherein said minimum distance (DI) is preferably between 95% and 105% of said maximum height (D) of the fiber (1).

11. Process according to claim 9 or 10, comprising at least one of the steps of surface treating the fibers or filament by plasma treatment and the step of forming said recesses by pressworking or calendering.

12. Process for making concrete, comprising the step of mixing a fiber according to one of claims 1 to 7 with a cement paste, in an amount preferably between 2 and 8 kg of fiber per cubic meter of concrete.

13. Process according to claim 12, wherein said amount is between 2.5 and 5 Kg of fiber per cubic meter of concrete.

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