AU688031B2

AU688031B2 - Metal fiber with optimized geometry for reinforcing cement-based materials

Info

Publication number: AU688031B2
Application number: AU22517/95A
Authority: AU
Inventors: Nemkumar Banthia; Madhavaro Krishnadev
Original assignee: Universite Laval
Current assignee: Universite Laval
Priority date: 1994-08-31
Filing date: 1995-04-21
Publication date: 1998-03-05
Anticipated expiration: 2015-04-21
Also published as: DE69517668D1; KR960706001A; DE69517668T2; DK0725871T3; AU2251795A; KR100353732B1; MX192955B; WO1996006995A1; EP0725871B1; MX9601504A; ES2151059T3; ATE194198T1; EP0725871A1; CA2131212C

Abstract

A metal fiber (10) for reinforcing cement-based materials comprises an elongated, substantially straight central portion (12) and sinusoid shaped end portions (14, 14'). The sinusoid at each end portion (14, 14') has an optimum amplitude Ao,opt defined by: Ao,opt = [k1( sigma c)<k>2] [ sigma u< alpha > epsilon f< beta >] [Af/Pf], where k1 = 2.025 x 10<-2>, sigma c = compressive strength of the cement-based material in MPa, k2 = 3.19 x 10<-1>, sigma u = ultimate tensile strength of the metal in MPa, alpha = 6.60 x 10<-1>, epsilon f = ductility of the metal in percent, and beta = 3.20 x 10<-1>, Af = cross-sectional area of the fiber in mm<2>, and Pf = perimeter of the fiber in mm. The sinusoid further has a wavelength Ls defined by: Ls = (Lf - Lm)/2, where Lf = length of the fiber, Lm = length of the central portion, and wherein 0.5 Lf < Lm < 0.75 Lf. Since the optimum amplitude is defined as a function of the ultimate tensile strength and ductility of the fiber material as well as of the compressive strength of the matrix material, it is possible to tailor the fiber geometry according to the properties of the fiber and matrix materials chosen, and ultimately to the composite toughness desired in an actual structure.

Description

WO 96106995 PCTICA95100225 1 "METAL FIBER WITH OPTIMIZED GEOMETRY FOR REINFORCING CEMENT-BASED MATERIALS" TECHNICAL FIELD The present invention pertains to improvements in the field of fiber reinforced cement-based materials.

More particularly, the invention relates to a metal fiber having an optimized geometry for reinforcing cement-based materials.

BACKGROUND ART All cement-based materials are weak in tension. In addition, these materials have a very low strain capacity which places them in a brittle category with other brittle materials such as glass and ceramics. It is well known that concrete and other portland cementbased materials may be reinforced with short, randomly distributed fibers of steel to improve upon their mechanical properties. It is also known that for any improvement in the tensile strength, fiber volume fraction has to exceed a certain critical value.

Beyond matrix cracking, fibers form stress transfer bridges and hold matrix cracks together such that a further crack opening or propagation causes the fibers to undergo pull-out from the matrix. Pull-out processes being energy intensive, steel fiber reinforced concrete exhibits a stable load-deflection behavior in the region beyond matrix-cracking which places these materials in a category of pseudo-plastic or tough materials such as steel and polymers. Thus, wile a plain unreinforced matrix fails in a brittle a~ II WO 96/06995 PCT/CA95100225 2 manner at the occurrence of cracking stresses, the ductile fibers in fiber reinforced concrete continue to carry stresses beyond matrix cracking which helps maintaining structural integrity and cohesiveness in the material. Further, if properly designed, fibers undergo pull-out processes and the frictional work needed for pull-out leads to a significantly improved energy absorption capability. Therefore, fiber reinforced concrete exhibits better performance not only under static and quasi-statically applied loads but also under fatigue, impact and impulsive loadings.

This energy absorption attribute of fiber reinforced concrete is often termed "toughness" Concrete is a strain-softening, micro-cracking material. In steel fiber reinforced cement-based composites, fiber bridging action sets in even prior to the occurrence of the perceived matrix macro-cracking.

The critical fiber volume fraction or the magnitude of strength improvement at a certain fiber volume fraction, therefore, depends upon the geometry of the fiber. Also dependent upon the geometry is the pull-out resistance of an individual fiber from the cementitious matrix around it, which in turn, governs the shape of the load-deflection plot beyond matrix cracking and the achievable improvement in composite toughness.

An improvement in the strength of the composite at a certain fiber volume fraction or, in other words, a reduction in the required critical fiber volume fraction, is possible by excessively deforming the fiber. However, this may lead to too good a fiber anchorage with the matrix and causes a brittle mode of fracture in the post-matrix cracking region. Toughness II r WO 96106995 PCT/CA95/00225 3 reductions in the case of excessively deformed fibers, therefore, can be significant. The other possible way is to increase the number of fibers in the composite by reducing the size of the fibers. This solution is known to cause extreme difficulties in terms of concrete mixing and workability, and uniform fiber dispersion often becomes impossible as the fibers tend to clump together giving a highly non-uniform distribution.

In US Patent No 4,585,487 which proposes a concrete-reinforcing fiber having uniform wave shaped corrugations distributed over its entire length, the sole fiber performance characteristics considered for optimization is the fiber pull-out performance. The same also applies in respect of Canadian Patent Nos.

926,146 and 1,023,395 which disclose concretereinforcing fibers having a straight central portion with shaped ends. Some fibers have ends which are formed thicker; others have ends which are hooked. All these characteristics are intended to improve anchoring of the fiber in the concrete.

For fibers that are used as a reinforcement distributed randomly in a moldable concrete matrix, the property of interest is the overall composite toughness. The composite toughness, although dependent on the pull-out resistance of fibers, cannot quantitatively be derived from the results of an ideal fiber pull-out test where the fiber is aligned with respect to the load direction, since in a real composite, once the brittle cementitious matrix cracks, the fibers are not only embedded to various depths on both sides of the matrix but also inclined at various angles with respect to the loading direction. Further, I_--s WO 96/06995 PCT/CA95/00225 4 fibers pulling out as a bundle have a very different performance as compared to a single fiber owing primarily to fiber-fiber interaction. Also, in a real composite, the contribution from the matrix is not entirely absent while fibers are pulling out (as assumed in an ideal pull-out test) due to aggregate interlocking, discontinuous cracking and crack bands.

Thus, the idealistic single fiber pull-out test with the fiber aligned with respect to the loading direction is not a realistic representation of what is happening in a real composite. So far, no attempt has been made to rationally optimize the fiber geometry with respect to the properties of the matrix material, i.e.

concrete, and the fiber material, i.e. steel or other metal.

DISCLOSURE OF INVENTION It is therefore an object of the present invention to relate the fiber geometry to the properties of both the matrix and fiber naterials, with a view to optimizing the overall composite toughness.

It is another object of the invention to provide a metal fiber with an optimized geometry for reinforcing cement-based materials such that the fiber fully utilizes matrix anchoring without fracturing in the pre-matrix macro-cracking region and pulls out at the maximum pull-out resistance in the post-matrix macrocracking region giving the highest possible toughness.

In accordance with the present invention, there is thus provided a metal fiber for reinforcing cementbased materials, which comprises an elongated, WO 96/06995 PCT/CA95/00225 5 substantially straight central portion and sinusoid shaped end portions. The sinusoid at each end portion has an optimum amplitude Ao,opt defined by: Ao,opt [k 1 2 [a uI [A/Pf] (1) where k i 2.025 x 10-2, a0 compressive strength of the cement-based material in MPa, k 2 3.19 x 10-1, au ultimate tensile strength of the metal in MPa, a 6.60 x 10-1, 8f ductility of the metal in percent, and 3.20 x 10-1, Af cross-sectional area of the fiber in mm 2 and Pf perimeter of the fiber in mm.

The sinusoid further has a wavelength L s defined by: Ls (Lf L m /2 (2) where Lf length of the fiber, Lm length of the fiber central portion, and wherein 0.5 Lf Lm 0.75 Lf.

As it is apparent from equation both the ultimate. te'sile strength and the ductility of the fiber material as well as the compressive strength of the cement-based material are important factors in defining the optimum amplitude. The equation also takes i IIIP"--- ai WO 96/06995 PCT/CA95/00225 -6 into account the cross-sectional area and perimeter of the fiber. It is therefore possible to tailor the fiber geometry according to the properties of the fiber and matrix materials chosen, and ultimately to the composite toughness desired in an actual structure.

Where use is made of a cement-based material having a compressive strength ac ranging from about to about 60 MPa, the value of kl(ac)k 2 in equation (1) then ranges from about 6 x 10-2 to about 7.5 x 10-2. A preferred valve of k,(ac)k 2 which provides an optimum amplitude Ao, opt in the concrete compressive strength range of 30-60 MPa is about 7 x 10-2.

The fiber according to the invention preferably has an end angle 0 less than 200, the angle 0 being defined by 0 tan- 1 4(A 0 (3)

L,

The angle 0 preferably ranges from about 12° to about 150. Such a small end angle 0 prevents the fibers from undergoing balling so that there is no problem with mixing.

The fibers of the invention which have sinusoids only at the end portions as opposed to those that have sinusoids along their entire length, such as in the case of US Patent No 4,585,487, provide better reinforcing. At a crack where fibers form stresstransfer bridges and are subjected to pull-out forces, those with deformations over the entire length transmit the entire pull-out force immediately back to the matrix through anchorage. In the case of fibers WO 96/06995 PCT/CA95100225 7 deformed only at the extremities, the stresses are slowly transferred from the crack face to the interior of the matrix with the major transfer of forces taking place only at the extremities. Such a gradual transfer of stresses averts a possible crushing and splitting of the matrix at the crack face which is commonly observed in fibers deformed all along the length. It is due to the matrix crushing and splitting that fibers unfavorably affect each others ability to reinforce when in a group and the overall toughness of the composite is severely reduced. Since the optimum amplitude of the sinusoid shaped end portions of the fibers according to the invention is defined as a function of the ultimate tensile strength and ductility of the fiber material as well as of the compressive strength of the matrix material, such amplitude is generally less than 5% of the fiber length. The low fiber amplitude leads to a more gradual transfer of stresses back to the matrix and hence less crushing and splitting of the matrix around the fibers.

A particularly preferred metal fiber according to the invention has a uniform rectangular cross-section with a thickness of about 0.4 mm and a width of about 0.8 mm, a length Lf of about 50 mm and a length Lm of about 25 mm. The wavelenth L s of the sinusoid at each end portion of the fiber is about 12.5 mm.

Fiber reinforced concrete incorporating the fibers of the invention can be used in slabs on grade, shotcrete, architectural concrete, precast products, offshore structures, structures in seismic regions, thin and thick repairs, crash barriers, footings, hydraulic structures and many other applications.

WO 96/06995 PCT/CA95/00225 8 BRIEF DESCRIPTION OF DRAWINGS Further features and advantages of the invention will become more readily apparent from the following description of preferred embodiments, reference being made to the accompanying drawings in which: Figure 1 is a side elevational view of a steel fiber according to the intention; Figure 2 is a load deflection plot in which the toughness of concrete reinforced with the fiber illustrated in Fig. 1 is compared with that of concrete reinforced with conventional fibers; and Figure 3 is a graph showing the relationship between post-crack strength and beam mid-span deflection expressed as a fraction of the span for the same fibers.

MODES FOR CARRYING OUT THE INVENTION As shown in Fig. 1, the steel fiber illustrated which is generally designated by reference numeral comprises an elongated, substantially straight central portion 12 with sinusoid shaped end portions 14 and 14'. The sinusoid at each end portion is defined by Y Ao sin (4) Lz where the coordinate system is as illustrated in Fig. 1 and A o is the amplitude of the sinusoid. Also illustrated in Fig. 1 are the length Lf of the fiber I li i I, 1- 111 WO 96/06995 PCT/CA95/00225 9 the length Lm of the central portion 12 and the length Ls of the end portions 14,14', as well as the end angle 0. The length Lf of the fiber 10 may vary from about 25 to about 60 mm. As explained herein, the fiber geometry is optimized by giving tt. he sinusoid an optimum amplitude Ao,opt as defined in equation For example, the optimum following three steels with properties are given in Table 1, Af/Pf 1.33 x 10-1 mm: amplitudes for the different mechanical where a, 40 MPa and TABLE 1 Steel Type and Properties (bulk) Optimum Amplitude, Ao,opt Steel A: type C1018 1030 MPa; Ef 0.60%) 0.7 mm Steel B: Martensite Steel (a u 1550 MPa; Ef 1.2 mm Steel C: HSLA* Steel (a 1350 MPa; ef 1.5 mm High Strength Low Aluminum In the embodiment illustrated in Fig. 1, the fiber has a uniform rectangular cross-section. Such a fiber may also have a circular cross-section.

I,

WO 96/06995 PCT/CA95/00225 10 Fibers with optimized geometry at a dosage rate of kg/m 3 were used in reinforcing concrete matrices having an unreinforced compressive strength of 40 MPa.

Beams made from the fiber-reinforced concrete were tested in third point flexure, along with their unreinforced companions. The beam displacements were measured using a yoke around the specimen such that the spurious component of the load point displacement due to the settlement of supports was automatically eliminated. The resulting load deflections plots are set forth in Fig. 2, where the toughness of concrete reinforced with the fibers of the invention (Fl) is compared with that of concrete reinforced with conventional fibers (F2 to F5). The conventional fibers investigated for comparative purpose were the following: L' TABLE 2 Fiber Geometry Cross- Length Size Tensile Weight Number Design- Section (mm) (mm) Stzength per kg ation Shape (MPA) F2 Hooked- Circular 60 0.8 1115 0.263 3800 end diam.

F3 Twin-cone Cicular 62 1.0 1198 0.403 2480 di al.

F4 Crimped C.*rcular 60 1.0 1037 0.420 2380 di am.

Crimped Crescent 52 2.3 x 1050 0.393 2540 1 0.55 WO 96/06995 PCTCA)5/00225 12 The plots were analyzed according to conventional techniques (ASTM C1018; JSCE SF-4) as well as to the PCS technique described by Trottier, "Toughness of Steel Reinforced Cement-Based Composites", Ph.D.

Thesis, Laval University, 1993, with a view to determining the toughness parameters. The results are given in Table 3 and plotted in Figure 3: WO 96/06995 PCT/CA95/00225 -13 TABLE 3 Post Crack Strength Plain Concrete Concrete with Fl at beam displacement (ac 40 MPa; Fibers (ac 43 MPa; of span/rn, PCSM Ec 39 GPa) Ec 39 GPa)

PCS

300 0 0 6.3-6.5 MPa

PCS

1500 0 6.0-6.5 MPa

PCS

60 0 0 5.8-6.0 MPa

PCS

400 0 5.5-5.8 MPa

PCS

300 0 5.0-5.3 MPa

PCS

200 0 4.0-4.8 MPa Modulus of Rupture (MOR) 5.19 MPa 5.5-5.9 MPa Toughness Indices (ASTM-C1018) 1.0 4.7-5.0 1.0 9.0-9.5 120 1.0 17.2-20.0 130 1.0 22.0-23.0 160 1.0 45.0-50.0 JSCE (SF-4) Factor 5.2-5.8 MPa WO 96/06995 PCTr/CA95/0225 14 In Table 3, Ec is the elastic modulus of concrete as per ASTM C-469. The JSCE SF-4 technique takes the total area (elastic and plastic) under the curve up to a deflection of span/150 and converts into an equivalent post-crack strength.

The fibers of the inventions even at a low dosage of 40 kg/m 3 lead to strengthening in the system as evident from the increase in the load carrying capacity over the plain, unreinforced matrix. Also, after the matrix cracking, the composite is capable of carrying approximately the same level of stresses as when at matrix cracking and as such very high toughness is derived. The composite behaves almost in an elastoplastic manner.

A minor increase (about in the compressive strength of concrete due to fiber addition indicates that an adequate fiber dispersion and mix compaction were achieved.

As it is also apparent from Figs 2 and 3, the fiber with optimized geometry according to the invention behaves superior tio existing commercial fibers and provides higher flexural toughness. It is believed that the fiber geometry fully utilizes the potential of steel and that of the cement matrix to produce an optimized composite.

Claims

1. A metal fiber for reinforcing cement-based materials, which comprises an elongated, substantially straight central portion and sinusoid shaped end portions, the sinusoid of each end portion having an optimum amplitude Ao,opt defined by: Aoopt [k,(or(ck 2 [E efP] where ki 2.025 x 10-2, ac compressive strength of the cement-based material in MPa, k, 3.19 x 10-1, oY ultimate tensile strength of the metal in MPa, a 6.60 x 10- 1 E ductility of the metal in percent, and

3.20 x 10-1, Af cross-sectional area of the fiber in mm 2 and Pf perimeter of the fiber in mm, said sinusoid further having a wavelength L s defined by: s (Lf Lm)/ 2 where Lf length of the fiber, L m length of the central portion, and wherein 0.5 Lf Lm 0.75 Lf. i 16 2. Fiber according to claim 1, wherein the length Lf of the fiber ranges from about 25 to about 60 mm. 3. Fiber according to claim 1, wherein said central portion and said end portions have a uniform rectangular cross-section.

4. Fiber according to ciaim 3, wherein said central portion and said end portions have a thickness of about 0.4 mm and a width of about 0.8 mm, and wherein the length Lf of the fiber is about 50 mm and the length La of the central portion is about 25 mm.

5. Fiber according to claim 1, wherein said central 15 portion and said end portions have a uniform circular cross-section. o

6. Fiber according to claim 1, wherein the cement- based material has a compressive strength CT ranging from about 30 to about 60 Mpa and wherein k 1 (Gc)k 2 ranges from about 6 x 10 2 to about 7.5 x 10-2. *o

7. Fiber according to claim 6, wherein k 1 2 is about 7 x 10 2 S

8. Fiber according to claim 7, wherein the cross- sectional area Af and the perimeter Pf of the fiber are such that Af/Pf 1.33 x 10-1 mm.

9. Fiber according to claim 8, wherein said metal is steel. Fiber according to claim 9, wherein said steel is of type C1018 having an ultimate tensile strength cu of about 1030 Mpa and a ductility Ef of about 0.60%, and wherein said sinusoid has an optimum amplitude Ao,opt of about 0.7 mm. H:\KarenKeep\22517-95-UNVERSIT-LAVAL.1 17/12/97 L L-II 17

11. Fiber according to claim 9, wherein said steel is a martensite steel having an ultimate tensile strength ou of about 1550 Mpa and a ductility Ef of about 1% and wherein said sinusoid has an optimum amplitude Ao,ot of about 1.2 mm.

12. Fiber according to claim 9, wherein said steel is a high strength low aluminum steel having an ultimate tensile strength Cu of about 1350 Mpa and a ductility Ef of about and wherein said sinusoid has an optimum amplitude Ao,opt of about 1.5 mm.

13. Fiber according to claim 1, wherein said end 15 portions each have an end angle 8 below 200, the angle 0 below 200, the angle 0 being defined by: 4(Ao.o,) 0 tan- Ls 20 14. Fiber according to claim 13, wherein said angle 9 ranges from about 120 to about a a~ *0 a a a. jgsRA ,1 -0LIJ) TO -O~I Cul l{:\Karen\Keep\22S17-95-UNIVERSIT-LAvAL1 11/12/97 18 A metal fiber reinforced cement-based material, which comprises a cement-based material in admixture with metal fibers, said metal fibers each having an elongated, substantially straight central portion and sinusoid shaped end portions, the sinusoid of each end portion having an optimum amplitude A o ,opt defined by: Ao,opt.= [kl(CJ)k 2 [ou EXP] [A/Pf] where k i 2.025 x 10-2, *e a f•o 19 oC compressive strength of the cement-based material in MPa, k2 3.19 x 10-1, au ultimate tensile strength of the metal in MPa, a 6.60 x 10-1, 6f ductility of the metal in percent, and 3 3.20 x 10-1, Af cross-sectional area of the fiber in mm 2 and Pf perimeter of the fiber in mm, said sinusoid further having a wavelength L s defined by: Ls (Lf Lm)/2 where Lf length of the fiber, Lm length of the central portion, and wherein 0.5 Lf Lm 0.75 Lf.

16. A metal fiber reinforced cement-based material as claimed in claim 15, wherein the length Lf of the fibers range from about 25 to about 60 mm.

17. A metal fiber reinforced cement-based material as claimed in claim 15, wherein said central portion and said end portions have a uniform rectangular cross-section. S

18. A metal fiber reinforced cement-based material as claimed in claim 17, wherein said central portion and said end portions have a thickness of about 0.4 mm and a width of about 0.8 mm, and wherein the length Lf of the fibers is A about 50 mm and the length Lm of the central portion is about mm. 20

19. A metal fiber reinforced cement-based material as claimed in claim 15, wherein said central portion and said end portions have a uniform circular cross-section. A metal fiber reinforced cement-based material as claimed in claim 15, wherein the cement-based material has a compressive strength o~ ranging from about 30 to about 60 MPa and wherein k (oc) k 2 ranges from about 6 x 10 2 to about 7.5 x 10-2

21. A metal fiber reinforced cement-based material as claimed in claim 20, wherein k,(oG) 2 is about 7 x 10-2.

22. A metal fiber reinforced cement-based material as claimed in claim 21, wherein the cross-sectional area Af and che perimeter Pf of the fibers are such that Af/Pf 1.33 x 10-1 mm.

23. A metal fiber reinforced cement-based material as claimed in claim 22, wherein said metal is steel.

24. A metal fiber reinforced cement-based material as claimed in claim 23, wherein said steel is of type C1018 having an ultimate tensile strength ou of about 1030 MPa and .a ductility er of about 0.60%, and wherein said sinusoid has an optimum amplitude, Ao,opt of about 0.7 mm. A metal fiber reinforced cement-based material as claimed in claim 23, wherein said steel is of martensite steel having an ultimate tensile strength o u of about /T 0 21 1550 MPa and a ductility E, of about and wherein said sinusoid has an optimum amplitude, Ao,opt ,l about 1.2 mm.

26. A metal fiber reinforced cement-based material as claimed in claim 23, wherein said steel is a high strength low aluminum steel having an ultimate tensile strength ao of about 1350 MPa and a ductility 8, of about and wherein said sinusoid has an optimum amplitude, Aoopt of about mm.

27. A metal fiber reinforced cement-based material as claimed in claim 15, wherein said end portions each have an end angle 8 below 20°, the angle 0 being defined by: tan-1 4( L,

28. A metal fiber re-nforced cement-based material as a a claimed in claim 27, wherein said angle 0 ranges from about 120 to aDout Dated this 19th day of November 1997 UNIVERSITE LAVAL By Its Patent Attorneys: GRIFFITH HACK Fellows Institute of Patent S Attorneys of Australia. I