U.S. patent application number 10/316315 was filed with the patent office on 2003-05-01 for process for making highly dispersible polymeric reinforcing fibers.
This patent application is currently assigned to W.R. GRACE & CO.-CONN.. Invention is credited to Berke, Neal S., Macklin, Michael B., Ranganathan, Anandakumar, Rieder, Klaus Alexander.
Application Number | 20030082376 10/316315 |
Document ID | / |
Family ID | 25289942 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030082376 |
Kind Code |
A1 |
Rieder, Klaus Alexander ; et
al. |
May 1, 2003 |
Process for making highly dispersible polymeric reinforcing
fibers
Abstract
Synthetic polymer reinforcing fibers provide dispersability and
strength in matrix materials such as concrete, masonry, shotcrete,
and asphalt. The individual fiber bodies, substantially free of
stress fractures and substantially non-fibrillatable, have
generally quadrilateral cross-sectional profiles along their
elongated lengths.
Inventors: |
Rieder, Klaus Alexander;
(Beverly, MA) ; Berke, Neal S.; (Chelmsford,
MA) ; Macklin, Michael B.; (Westford, MA) ;
Ranganathan, Anandakumar; (Waltham, MA) |
Correspondence
Address: |
Craig K. Leon, Esq.
W. R. Grace & Co.-Conn.
Patent Department
62 Whittemore Avenue
Cambridge
MA
02140-1692
US
|
Assignee: |
W.R. GRACE & CO.-CONN.
Columbia
MD
|
Family ID: |
25289942 |
Appl. No.: |
10/316315 |
Filed: |
December 11, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10316315 |
Dec 11, 2002 |
|
|
|
09843427 |
Apr 25, 2001 |
|
|
|
Current U.S.
Class: |
428/364 ;
428/294.7; 428/297.4; 428/373; 428/394 |
Current CPC
Class: |
D01D 5/253 20130101;
Y10T 428/249932 20150401; Y10T 428/2922 20150115; Y10T 428/2913
20150115; C04B 16/06 20130101; Y10T 428/2929 20150115; Y10T
428/2978 20150115; E04C 5/073 20130101; Y10T 428/2973 20150115;
C08J 5/04 20130101; Y10T 428/2964 20150115; Y10T 428/2967 20150115;
Y10T 428/2904 20150115; Y10T 428/24994 20150401 |
Class at
Publication: |
428/364 ;
428/394; 428/373; 428/297.4; 428/294.7 |
International
Class: |
D02G 003/00 |
Claims
It is claimed:
1. Fibers for reinforcing matrix materials, comprising: a plurality
of individual fiber bodies having an elongated length defined
between two opposing ends and comprising at least one synthetic
polymer, said individual fiber bodies being substantially free of
stress fractures and substantially non-fibrillatable when
mechanically agitated within the matrix material to be reinforced,
said fiber bodies having a generally quadrilateral cross-sectional
profile along said elongated length, thereby having width,
thickness, and length dimensions, wherein the average width is at
least 1.0 mm; wherein the average width is no more than 5.0 mm;
wherein the average thickness is at least 0.1 mm; wherein the
average thickness is no more than 0.3 mm; wherein the average
length is at least 20 mm; and wherein the average length is no more
than 100 mm.
2. The fibers of claim 1 wherein said average width is no less than
1.3 mm; said average width is no greater than 2.5 mm; said average
thickness is no less than 0.15 mm; said average thickness is no
greater than 0.25 mm; said average length is no less than 30 mm;
and said average length is no greater than 60 mm.
3. The fibers of claim 1 wherein, in said plurality of individual
fiber bodies, said individual fiber bodies are separated from each
other.
4. The fibers of claim 1 wherein, in said plurality of individual
fiber bodies, said individual fiber bodies are partially separated
from each other but are completely separable when mechanically
agitated within the matrix material.
5. The fibers of claim 1 wherein, in said plurality of individual
fiber bodies, said at least one synthetic polymer is selected from
the group consisting of polyethylene, polypropylene,
polyoxymethylene, poly(vinylidine fluoride), poly(methyl pentene),
poly(ethylene-chlorotrif- luoroethylene), poly(vinyl fluoride),
poly(ethylene oxide), poly(ethylene terephthalate), poly(butylene
terephthalate), polyamide, polybutene, and thermotropic liquid
crystal polymers.
6. The fibers of claim 1 wherein said fiber bodies comprise
polypropylene in an amount no less than 75% by weight and said
fiber bodies comprise polypropylene in an amount up to 100%.
7. The fibers of claim 6 wherein said fiber bodies comprise a blend
of at least two polymers or a co-polymer comprising at least two of
said polymers.
8. The fibers of claim 7 wherein said fiber bodies comprise
polypropylene and polyethylene.
9. The fibers of claim 1 wherein said fiber bodies have a Young's
modulus of elasticity of no less than 3 Giga Pascals and wherein
said fiber bodies have a Young's modulus of elasticity no more than
20 Giga Pascals.
10. The fibers of claim 1 wherein said fiber bodies have a tensile
strength of no less than 350 Mega Pascals and wherein said fiber
bodies have a tensile strength of no more than 1200 Mega
Pascals.
11. The fibers of claim 1 wherein said fiber bodies have a minimum
load carrying capacity in tension mode of no less than 40 Newtons
per fiber body and said fiber bodies have a minimum load carrying
capacity in tension mode of no more than 900 Newtons per fiber
body.
12. The fibers of claim 1 wherein said fiber bodies have a width to
thickness ratio of no less than 4 and wherein said fiber bodies
have a width to thickness ratio of no more than 50.
13. The fibers of claim 1 wherein said fiber bodies have a width to
thickness ratio of no less than 5 and wherein said fiber bodies
have a width to thickness ratio of no more than 20.
14. The fibers of claim 1 wherein said fiber bodies have an average
bendability "B" of no less than 20 mN.sup.-1*m.sup.-2 and wherein
said fiber bodies have an average bendability "B" of no more than
1300 mN.sup.-1*m.sup.-2, said bendability "B" of said fibers being
determined in accordance with the formula,
B=1/(3.multidot.E.multidot.I), wherein the moment of inertia "I"
for a generally rectangular cross-section is computed in accordance
with the formula, I.sub.rectangle=1/12.multidot.w.-
multidot.t.sup.3, wherein "w" is the average width and "t" is the
average thickness of the generally rectangular cross-section.
15. The fibers of claim 1 wherein said fiber bodies have an average
bendability "B" of no less than 25 mN.sup.-1*m.sup.-2 and wherein
said fiber bodies have an average bendability "B" of no more than
500 mN.sup.-1*m.sup.-2, said bendability "B" of said fibers being
determined in accordance with the formula,
B=1/(3.multidot.E.multidot.I), wherein the moment of inertia "I"
for a generally rectangular cross-section is computed in accordance
with the formula, I.sub.retangle=1/12.multidot.w.m-
ultidot.t.sup.3, wherein "w" is the average width and "t" is the
average thickness of the generally rectangular cross-section.
16. The fibers of claim 1 wherein said fiber bodies have an average
surface square area "S.sub.A" to volume "V" ratio of no less than
7.0 and wherein said fiber bodies have an average S.sub.A to V
ratio of no more than 22.1.
17. The fibers of claim 16 wherein said fiber bodies have an
average surface square area "S.sub.A" to volume "V" ratio of no
less than 10 and wherein said fiber bodies have an average S.sub.A
to V ratio of no more than 15.
18. The fibers of claim 1 further comprising a second plurality of
individual fiber bodies, wherein said second plurality differs in
terms of fiber composition, dimensions, a physical characteristic,
or combination thereof.
19. The fibers of claim 1 being coated, bundled, packaged,
packeted, coated, adhered, or contained together.
20. The fibers of claim 19 wherein said individual fiber bodies are
partially connected together as a scored sheet which is operative
to separate into said individual fiber bodies when said sheet is
introduced into, and mechanically agitated, in a hydratable
cementitious composition.
21. The fibers of claim 1, wherein said matrix materials are
hydratable cementitious compositions.
22. Fibers for reinforcing matrix materials, comprising: a
plurality of individual fiber bodies having an elongated length
defined between two opposing ends and comprising at least one
synthetic polymer, said individual fiber bodies being substantially
free of stress fractures and substantially non-fibrillatable when
mechanically agitated within the matrix material to be reinforced,
said fiber bodies having a generally quadrilateral cross-sectional
profile along said elongated length, thereby having width,
thickness, and length dimensions wherein the average width is no
less than 1.0 mm; wherein the average width is no more than 5.0 mm;
wherein the average thickness is no less than 0.1 mm; wherein the
average thickness is no more than 0.3 mm; wherein the average
length is no less than 20 mm; wherein the average length is no more
than 100 mm; wherein the average fiber width to thickness ratio is
no less than 5; wherein the average fiber width to thickness ratio
is no more than 50; wherein said fiber bodies have a Young's
modulus of elasticity no less than 3 Giga Pascals; wherein said
fiber bodies have a Young's modulus of elasticity no more than 20
Giga Pascals; wherein said fiber bodies have a tensile strength no
less than 350 Mega Pascals; wherein said fiber bodies have a
tensile strength of no more than 1200 Mega Pascals; wherein said
fiber bodies have a minimum load carrying capacity in tension mode
no less than 40 Newtons per fiber body; wherein said fiber bodies
have a minimum load carrying capacity in tension mode no greater
than 900 Newstons per fiber body; wherein said fiber bodies have an
average square area to volume ratio no less than 7.0; wherein said
fiber bodies have an average square area to volume ratio no more
than 22.1; wherein said fiber bodies have an average bendability
"B" no less than 25 mN.sup.-1*m.sup.-2; and wherein said fiber
bodies have an average bendability "B" no more than 500
mN.sup.-1*m.sup.-2; said bendability "B" of said fibers being
determined in accordance with the formula,
B=1/(3.multidot.E.multidot.I), wherein the moment of inertia "I"
for a generally rectangular cross-section is computed in accordance
with the formula, I.sub.retangle=1/12.multidot.w.multidot.t.sup.3,
wherein "w" is the average width and "t" is the average thickness
of the generally rectangular cross-section.
23. A matrix composition comprising said fibers of claim 1 and a
matrix material selected from the group consisting of adhesives,
asphalt, composite materials, plastics, elastomers, and hydratable
cementitious materials.
24. The matrix composition of claim 23 wherein said matrix material
is a hydratable cementitious composition and said fibers comprise
polypropylene.
25. The matrix composition of claim 23 wherein said fibers are
present in the matrix composition in the amount no less than 0.05%
by volume and wherein said fibers are present in the matrix
composition in an amount no greater than 10% by volume.
26. Method for modifying a matrix material, comprising introducing
into a matrix material the fibers of claim 1.
27. Process for manufacturing fibers, comprising: melt extruding at
least one synthetic polymeric material through a sheet dye; cooling
the extruded sheet to below ambient temperature; cutting the
extruded sheet to form separate fibers to achieve a generally
quadrilateral cross-sectional provide and resultant average width
and thickness dimensions, wherein the average width is at least 1.0
mm, wherein the average width is no more than 5.0 mm, wherein the
average thickness is at least 0.1 mm, and wherein the average
thickness is no more than 0.3 mm; stretching said fibers
longitudinally by a factor no less than 10 and no greater than 20;
and cutting said fibers to provide average fiber length no less
than 20 and no greater than 100 mm.
28. The process of claim 27 wherein said stretching said fiber
longitudinally precedes said cutting to provide average fiber
lengths of no less than 20 and no greater than 100 mm.
29. The process of claim 27 wherein said cutting to provide average
fiber lengths of no less than 20 and no greater than 100 mm
precedes said stretching.
30. The process of claim 27 wherein said cooling comprises taking
up said melt extruded polymeric material on a chill roll.
31. The process of claim 27 wherein said cooling comprises passing
said melt extruded polymeric material through a water bath.
32. The fibers of claim 1 wherein said individual fiber bodies have
a variability of thickness or width along the individual fiber body
length of no less than 2.5 percent deviation from average thickness
or width as the case may be, and wherein said individual fiber
bodies have a variability of thickness or width along the
individual fiber body length of no greater than 25 percent
deviation from the average thickness or width as the case may
be.
33. The fibers of claim 1 wherein said individual fiber bodies
comprise at least two synthetic polymers, one of said at least two
synthetic polymers comprising an alkaline soluble polymer disposed
on the outward fiber surface thereby being operative to dissolve
when said fiber bodies are mixed into the alkaline environment of a
wet concrete mix.
34. The fibers of claim 1 wherein said individual fiber bodies are
coated with an alkaline soluble polymer operative to dissolve when
said fiber bodies are mixed into the alkaline environment of a wet
concrete mix.
35. The fibers of claim 1 wherein said plurality of fibers are
contained in packaging with an admixture.
36. The fibers of claim 35 wherein said admixture is selected from
the group consisting of a superplastizicer, water reducer, air
entrainer, air detrainer, corrosion inhibitor, set accelerator, set
retarder, shrinkage reducing admixture, fly ash, silica fume,
pigments, or a mixture thereof.
Description
FIELD OF THE INVENTION
[0001] The invention relates to fibers for reinforcing matrix
materials, and more particularly to a plurality of synthetic
polymer fibers having excellent dispersibility and reinforcibility
properties in hydratable cementitious compositions. Individual
fiber bodies are elongated and highly bendable, with generally
quadrilateral cross-sectional profiles, thereby minimizing fiber
balling and maximizing fiber bond.
BACKGROUND OF THE INVENTION
[0002] Although fibers of the present invention are suitable for
reinforcing various matrix materials, such as adhesives, asphalts,
composites, plastics, rubbers, etc., and structures made from
these, the fibers that will be described herein are especially
suited for reinforcing hydratable cementitious compositions, such
as ready-mix concrete, precast concrete, masonry concrete (mortar),
shotcrete, bituminous concrete, gypsum compositions, gypsum- and/or
Portland cement-based fireproofing compositions, and others.
[0003] A major purpose of the fibers of the present invention is to
reinforce concrete, e.g., ready-mix, shotcrete, etc., and
structures made from these. Such matrix materials pose numerous
challenges for those who design reinforcing fibers.
[0004] Concrete is made using a hydratable cement binder, a fine
aggregate (e.g., sand), and a coarse aggregate (e.g., small stones,
gravel). A mortar is made using cement binder and fine aggregate.
Concretes and mortars are hence brittle materials. If a mortar or
concrete structure is subjected to stresses that exceed its maximum
tensile strength, then cracks can be initiated and propagated
therein. The ability of the cementitious structure to resist crack
initiation and crack propagation can be understood with reference
to the "strength" and "fracture toughness" of the material.
[0005] "Strength" relates to the ability of a cement or concrete
structure to resist crack initiation. In other words, strength is
proportional to the maximum load sustainable by the structure
without cracking and is a measure of the minimum load or stress
(e.g., the "critical stress intensity factor") required to initiate
cracking in that structure.
[0006] On the other hand, "fracture toughness" relates to the
specific "fracture energy" of a cement or concrete structure. This
concept refers to the ability of the structure to resist
propagation--or widening--of an existing crack in the structure.
This toughness property is proportional to the energy required to
propagate or widen the crack (or cracks). This property can be
determined by simultaneously measuring the load required to deform
or "deflect" a fiber-reinforced concrete (FRC) beam specimen at an
opened crack and the amount or extent of deflection. The fracture
toughness is therefore determined by dividing the area under a load
deflection curve (generated from plotting the load against
deflection of the FRC specimen) by its cross-sectional area.
[0007] In the cement and concrete arts, fibers have been designed
to increase the strength and fracture toughness in reinforcing
materials. Numerous fiber materials have been used for these
purposes, such as steel, synthetic polymers (e.g., polyolefins),
carbon, nylon, aramid, and glass. The use of steel fibers for
reinforcing concrete structures remains popular due to the inherent
strength of the metal. However, one of the concerns in steel fiber
product design is to increase fiber "pull out" resistance because
this increases the ability of the fiber to defeat crack
propagation. In this connection, U.S. Pat. No. 3,953,953 of Marsden
disclosed fibers having "J"-shaped ends for resisting pull-out from
concrete. However, stiff fibers having physical deformities may
cause entanglement problems that render the fibers difficult to
handle and to disperse uniformly within a wet concrete mix. More
recent designs, involving the use of "crimped" or "wave-like"
polymer fibers, may have similar complications, depending on the
stiffness of the fiber material employed.
[0008] Polyolefin materials, such as polypropylene and
polyethylene, have been used for reinforcing concrete and offer an
economic advantage due to relative lower cost of the material.
However, these polyolefinic materials, being hydrophobic in nature,
resist the aqueous environment of wet concrete. Moreover, the
higher amount of polyolefin fibers needed in concrete to
approximate the strength and fracture toughness of steel
fiber-reinforced concrete often leads to fiber clumping or
"balling" and increased mixing time at the job site. This tendency
to form fiber balls means that the desired fiber dosage is not
achieved. The correct concentration of fibers is often not attained
because the fiber balls are removed (when seen at the concrete
surface) by workers intent on achieving a finished concrete
surface. It is sometimes the case that locations within the
cementitious structure are devoid of the reinforcing fibers
entirely. The desired homogeneous fiber dispersion, consequently,
is not obtained.
[0009] Methods for facilitating dispersion of fibers in concrete
are known. For example, U.S. Pat. No. 4,961,790 of Smith et al.
disclosed the use of a water-soluble bag for introducing a large
number of fibers into a wet mix. U.S. Pat. No. 5,224,774 of Valle
et al. disclosed the use of non-water-soluble packaging that
mechanically disintegrated upon mixing to avoid clumping and to
achieve uniform dispersal of fibers within the concrete mix.
[0010] The dispersal of reinforcing fibers could also be achieved
through packaging of smaller discrete amounts of fibers. For
example, U.S. Pat. No. 5,807,458 of Sanders disclosed fibers that
were bundled using a circumferential perimeter wrap. According to
this patent, the continuity of the peripheral wrapping could be
disrupted by agitation within the wet concrete mix, and the fibers
could be released and dispersed in the mix.
[0011] On the other hand, World Patent Application No. WO 00/49211
of Leon (published Aug. 24, 2000) disclosed fibers "packeted"
together but separable when mixed in concrete. A plurality of
fibers were separably-bound together, such as by tape adhered to
cut ends of the fibers, thereby forming a "packet." Within a wet
cementitious mix, the packets could be made to break and/or
dissolve apart to permit separation and dispersal of individual
fibers within the mix.
[0012] The dispersal of reinforcing fibers may also be achieved by
altering fibers during mixing. For example, U.S. Pat. No. 5,993,537
of Trottier et al. disclosed fibers that progressively fibrillated
upon agitation of the wet concrete mix. The fibers presented a "low
initial surface area" to facilitate introducing fibers into the wet
mix, and, upon agitation and under the grinding effect of
aggregates in the mix, underwent "fibrillation," which is the
separation of the initial low-surface-area fibrous material into
smaller, individual fibrils. It was believed that homogeneous fiber
distribution, at higher addition rates, could thereby be
attained.
[0013] A novel approach was taught in U.S. Pat. No. 6,197,423 of
Rieder et al., which disclosed mechanically-flattened fibers. For
improved keying within concrete, fibers were flattened between
opposed rollers to attain variable width and/or thickness
dimensions and stress-fractures perceivable through microscope as
discontinuities and irregular and random displacements of polymer
on the surface of the individual fibers. This microscopic stress
fracturing was believed to improve bonding between cement and
fibers, and, because the stress-fractures were noncontinuous in
nature, the fibers were softened to the point at which
fiber-to-fiber entanglement (and hence fiber balling) was
minimizied or avoided. The mechanical-flattening method of Rieder
et al. was different from the method disclosed in U.S. Pat. No.
5,298,071 of Vondran, wherein fibers were interground with cement
clinker and retained cement particles embedded into the
surface.
[0014] In this vein, the nature of the fiber surface has also been
a frequent topic of research in fiber dispersion and bonding in
concrete. For example, U.S. Pat. No. 5,753,368 of Hansen disclosed
a list of wetting agents such as emulsifiers, detergents, and
surfactants to render fiber surfaces more hydrophilic and thus more
susceptible to mixing in wet concrete. On the other hand, U.S. Pat.
No. 5,753,368 of Berke et al. taught that the bonding between
concrete and fibers could be enhanced by employing particular
glycol ether coatings instead of conventional wetting agents that
tended to introduce unwanted air at the fiber/concrete
interface.
[0015] Of course, as mentioned in U.S. Pat. Nos. 5,298,071 and
6,197,423 as discussed above, physical deformation of the fiber
surface was also believed to improve the fiber-concrete bond. U.S.
Pat. No. 4,297,414 of Matsumoto, as another example, taught the use
of protrusions and ridges to enhance bond strength. Other surface
treatments, such as the use of embossing wheels to impose patterns
on the fiber, were also used for improving fiber-concrete bond.
Fiber designers have even bent fibers into sinusoidal wave shapes
to increase the ability of fibers to resist being pulled out from
concrete. However, the present inventors realized that increased
structural deformations in the fiber structure may actually enhance
opportunities for unwanted fiber balling to occur.
[0016] Against this background, the present inventors see a need
for novel polymeric synthetic reinforcing fibers having ease of
dispersibility in concrete so as to avoid fiber balling and to
achieve intended fiber dosage rates, while at the same time to
provide strength and fracture toughness in matrix materials and
particularly brittle materials such as concrete, mortar, shotcrete,
gypsum fireproofing, and the like.
SUMMARY OF THE INVENTION
[0017] In surmounting the disadvantages of the prior art, the
present invention provides highly dispersible reinforcing polymer
fibers, matrix materials reinforced by the fibers, and methods for
obtaining these. Exemplary fibers of the invention provide ease of
dispersibility into, as well as strength and fracture toughness
when dispersed within, matrix materials, particularly brittle ones
such as concrete, mortar, gypsum or Portland cement-based
fireproofing, shotcrete, and the like.
[0018] These qualities are achieved by employing a plurality of
individual fiber bodies having an elongated length defined between
two opposing ends, the bodies having a generally quadrilateral
cross-sectional shape along the elongated length of the fiber body.
The individual fibers thereby have a width, thickness, and length
dimensions wherein average width is 1.0-5.0 mm and more preferably
1.3-2.5 mm, average thickness is 0.1-0.3 mm and more preferably
0.15-0.25 mm., and average length is 20-100 mm and more preferable
30-60 mm. In preferred embodiments, average fiber width should
exceed average fiber thickness by at least 4 times (i.e., a ratio
of at least 4:1) but preferably average width should not exceed
average thickness by a factor exceeding 50 times (50:1). More
preferably, the width to thickness ratio of the fibers is from 5 to
20 (5:1 to 20:1).
[0019] While individual fiber bodies of the invention may
optionally be introduced into and dispersed within the matrix
material as a plurality of separate pieces or separable pieces (ie.
fibers in a scored or fibrillatable sheet, or contained within a
dissolvable or disintegratable packaging, wrapping, packeting, or
coating) the fibers can be introduced directly into a hydratable
cementitious composition and mixed with relative ease to achieve a
homogeneous dispersal therein. Individual fiber bodies themselves,
however, should not be substantially fibrillatable (i.e. further
reducible into smaller fiber units) after being subjected to
mechanical agitation in the matrix composition to the extent
necessary to achieve substantially uniform dispersal of the fibers
therein.
[0020] Exemplary individual fiber bodies of the invention are also
substantially free of internal and external stress fractures, such
as might be created by clinker grinding or mechanical flattening.
The general intent of the present inventors is to maintain
integrity of the individual fiber bodies, not only in terms of
structural fiber integrity, but also integrity and uniformity of
total surface area and bendability characteristic from one batch to
the next.
[0021] A generally quadrilateral cross-sectional profile provides a
higher surface area to volume ratio (S.sub.a/V) compared to round
or oval monofilaments comprising similar material and having a
diameter of comparable dimension. The present inventors believe
that a quadrilateral cross-sectional shape provides a better
flexibility-to-volume ratio in comparison with round or elliptical
cross-sectional shapes, and, more significantly, this improved
flexibility characteristic translates into better "bendability"
control. The individual fiber bodies of the invention will tend to
bend predominantly in a bow shape with comparatively less minimal
twisting and fiber-to-fiber entanglement, thereby facilitating
dispersal. In contrast, for a given material modulus and
cross-sectional area, the prior art fibers having circular or
elliptical cross section with major axis/minor axis ratios of less
than 3 will have greater resistance to bending, thereby having a
greater tendency for fiber balling when compared to fibers of
generally quadrilateral (e.g., rectangular) cross-section.
[0022] The present inventors further believe that a generally
quadrilateral cross-section will provide excellent fiber surface
area and handability characteristics when compared, for example, to
round or elliptical fibers. In this connection, preferred fibers of
the invention have a "bendability" in the range of 20 (very stiff)
to 1300 (very bendable) milli Newton.sup.-1*meter.sup.-2
(mN.sup.-1m.sup.-2), and more preferably in the range of 25 to 500
milli Newton.sup.-1*meter.sup.-2. As used herein, the term
"bendability" means and refers to the resistance of an individual
fiber body to flexing movement (ie. to force that is perpendicular
to the longitudinal axis of the fiber) as measured by applying a
load to one end of the fiber and measuring its relative movement
with respect to the opposite fiber end that has been secured, such
as within a mechanical clamp or vice, to prevent movement. Thus, a
fiber can be called more bendable if it requires less force to bend
it to a certain degree. The bending flexibility of a fiber is a
function of its length, shape, the size of its cross-section, and
its modulus of elasticity. Accordingly, the bendability "B" of the
fiber is expressed in terms of milli Newton.sup.-1*meter.sup.-2
(mN.sup.-1m.sup.-2) and is calculated using the following formula 1
B = 1 3 E I
[0023] wherein "E" represents the Young's modulus of elasticity
(Giga Pascal) of the fiber; and "I" represents the moment of
inertia (mm.sup.4) of the individual fiber body. A fiber having a
lower bendability "B" will of course be less flexible than a fiber
having a higher bendability "B." The moment of inertia "I"
describes the property of matter to resist any change in movement
or rotation. For a cross-sectional profile having a generally
quadrilateral (or approximately rectangular) shape, the moment of
inertia can be calculated using the formula
I.sub.rectangle=1/12.multidot.w.multidot.t.sup.3
[0024] wherein "w" represents the average width of the rectangle
and "t" represents the average thickness of the rectangle.
[0025] In further exemplary embodiments, the "bendability" of
fibers can be further improved if the thickness and/or the width of
the fibers are varied along the length of the fibers, for example
from 2.5-25 percent maximum deviation from the average thickness or
width value. This small variation of the thickness and/or the width
of the fiber also improves the bond between the reinforcing matrix
and the fiber.
[0026] The inventors realized, in view of the above equation for
"bendability" "B" of fibers having generally quadrilateral
cross-sections, that an increase in the fiber modulus of elasticity
"E" will result in a corresponding decrease in bendability and,
consequently, make fiber dispersibility more difficult. The
inventors then realized that to maintain the same level of
bendability, the moment of inertia "I" must be decreased, and this
could be achieved, for example, by reducing the thickness of the
fibers while maintaining the cross-sectional area of the
fibers.
[0027] In further embodiments of the invention, preferred
individual fiber bodies have the following properties when measured
in the longitudinal dimension (end to end) along the axis of the
fiber body: a Young's modulus of elasticity of 3-20 Giga Pascals
and more preferable 5-15 Giga Pascals, a tensile strength of
350-1200 Mega Pascals and more preferable 400-900 Mega Pascals, and
a minimum load carrying capacity in tension mode of 40-900 Newtons
more preferable 100-300 Newtons.
[0028] A particularly preferred method for manufacturing the fibers
is to melt-extrude the polymeric material (e.g., polypropylene as a
continuous sheet); to decrease the temperature of this extruded
sheet melt below ambient temperature (e.g., below 25.degree. C.);
to cut or slit the sheet (after cooling) into separate or separable
individual fiber bodies having generally quadrilateral
cross-sections to stretch the individual fibers by at least a
factor of 10-20 and more preferably between 12-16, thereby to
achieve an average width of 1.0-5.0 mm and more preferably 1.3-2.5
mm and an average thickness of 0.1-0.3 mm and more preferably
0.15-0.25 mm; and to cut the fibers to obtain individual fiber
bodies having an average fiber length of 20-100 mm and more
preferably between 30-60 mm. Further exemplary processes are
described hereinafter.
[0029] The present invention is also directed to matrix materials,
such as concrete, mortar, shotcrete, asphalt, and other materials
containing the above-described fibers, as well as to methods for
modifying matrix materials by incorporating the fibers into the
matrix materials.
[0030] Further advantages and features of the invention are further
described in detail hereinafter.
BRIEF DESCRIPTION OF DRAWING
[0031] An appreciation of the advantages and benefits of the
invention may be more readily comprehended by considering the
following written description of preferred embodiments in
conjunction with the accompanying drawings, wherein
[0032] FIGS. 1-3 are microphotographic enlargements of the
cross-sections of PRIOR ART reinforcing fibers;
[0033] FIGS. 4 and 5 are microphotographic enlargements of the
generally quadrilateral cross-sectional profile of exemplary fibers
of the present invention;
[0034] FIG. 6 is microphotographic enlargement (at 25.times.
magnification) of the surface of an exemplary individual fiber body
of the present invention before mixing in a concrete mixture (which
would contain fine and coarse aggregates), and FIG. 7 shows the
fiber after mixing;
[0035] FIG. 8 is microphotographic enlargement (at 200.times.
magnification) of the surface of an exemplary individual fiber body
of the present invention before mixing in a concrete mixture (which
would contain fine and coarse aggregates), and FIG. 9 shows the
fiber after mixing;
[0036] FIG. 10 is microphotographic enlargement (at 900.times.
magnification) of the surface of an exemplary individual fiber body
of the present invention before mixing in a concrete mixture (which
would contain fine and coarse aggregates), and FIG. 11 shows the
fiber after mixing;
[0037] FIG. 12 is a microphotographic enlargement (at 900.times.
magnification) of a PRIOR ART fiber mechanically flattened in
accordance with U.S. Pat. No. 6,197,423;
[0038] FIG. 13 is a graphic representation of tensile load versus
strain behavior of different fibers;
[0039] FIG. 14 is a graphic representation of tensile stress versus
strain behavior of different fibers;
[0040] FIG. 15 is a photographic of a wedge-splitting device for
testing load on cementitious matrix materials containing
reinforcing polymer fibers; and
[0041] FIG. 16 is a graphic representation of stress vs. crack
mouth opening displacement behavior of different fibers.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0042] The present inventors believe that the reinforcing polymer
fibers of the present invention may be used in a variety of
compositions and materials and structures made from these. The term
"matrix materials" therefore is intended to include a broad range
of materials that can be reinforced by the fibers. These include
adhesives, asphalt, composite materials (e.g., resins), plastics,
elastomers such as rubber, etc., and structures made therefrom.
[0043] Preferred matrix materials of the invention include
hydratable cementitious compositions such as ready-mix concrete,
precast concrete, masonry mortar and concrete, shotcrete,
bituminous concrete, gypsum-based compositions (such as
compositions for wallboard), gypsum- and/or Portland cement-based
fireproofing compositions (for boards and spray-application),
water-proofing membranes and coatings, and other hydratable
cementitious compositions, whether in dry or wet mix form.
[0044] A primary emphasis is placed upon the reinforcement of
structural concrete (e.g., ready-mix concrete, shotcrete), however,
because concrete (whether poured, cast, or sprayed) is an extremely
brittle material that presents challenges in terms of providing
reinforcing fibers that (1) can be successfully introduced into and
mixed in this matrix material and (2) can provide crack-bridging
bonding strength in the resultant fiber reinforced concrete
structure.
[0045] Prior to a detailed discussion of the various aforementioned
drawings and further exemplary embodiments of the invention, a
brief discussion of definitions will be helpful to facilitate a
deeper understanding of advantages and benefits of the invention.
As the fibers of the invention are envisioned for use in the paste
portion of a hydratable wet "cement" or "concrete" (terms which may
sometimes be used interchangeably herein), it is helpful to discuss
preliminarily the definitions of "cement" and "concrete."
[0046] The terms "paste," "mortar," and "concrete" are terms of
art: pastes are mixtures composed of a hydratable cementitious
binder (usually, but not exclusively, Portland cement, masonry
cement, or mortar cement, and may also include limestone, hydrated
lime, fly ash, blast furnace slag, pozzolans, and silica flume or
other materials commonly included in such cements) and water;
mortars are pastes additionally including fine aggregate (e.g.,
sand); and concretes are mortars additionally including coarse
aggregate (e.g., gravel, stones). "Cementitious" compositions of
the invention thus refer and include all of the foregoing. For
example, a cementitious composition may be formed by mixing
required amounts of certain materials, e.g., hydratable
cementitious binder, water, and fine and/or coarse aggregate, as
may be desired, with fibers as described herein.
[0047] Synthetic polymer fibers of the invention comprise at least
one polymer selected from the group consisting of polyethylene
(including high density polyethylene, low density polyethylene, and
ultra high molecular weight polyethylene), polypropylene,
polyoxymethylene, poly(vinylidine fluoride), poly(methyl pentene),
poly(ethylene-chlorotrif- luoroethylene), poly(vinyl fluoride),
poly(ethylene oxide), poly(ethylene terephthalate), poly(butylene
terephthalate), polyamide, polybutene, and thermotropic liquid
crystal polymers. A preferred synthetic polymer is polypropylene.
Exemplary individual fiber bodies of the invention may comprise
100% polypropylene, or, as another example, they may comprise
predominantly polypropylene (e.g., at least 70-99%) with the
remainder comprising another polymer (such as high density
polyethylene, low density polyethylene) or optional fillers,
processing aids, and/or wetting agents, such as are conventionally
used in the manufacture of polymer fibers.
[0048] The molecular weight of the polymer or polymers should be
chosen so that the polymer is melt processable. For polypropylene
and polyethylene, for example, the average molecular weight can be
5,000 to 499,000 and is more preferably between 100,000 to 300,000.
Different grades of polyethylene may be used, including ones
containing branches and comonomers such as butene, hexene, and
octene, and further including the so-called "metallocene"
polyethylene materials. If polypropylene polymer is used, it is
preferred that no more than about 30 weight percent polymerized
comonomer units or blended resins be present in order to maintain
smooth process operation, with up to about 10% being preferred.
Propylene homopolymer resins are most preferred, with
general-purpose resins in the nominal melt flow range of about 1 to
about 40 grams/10 minutes (ASTM D2497 1995). Preferred resins also
have weight average molecular weight to number average molecular
ratios of about 2:1 to about 7:1.
[0049] FIG. 1 is a cross-sectional view, originally taken at about
100.times. magnification, of a PRIOR ART polypropylene fiber with
an elliptical cross-section having dimensions of 0.96 mm in width
and 0.63 mm in thickness. The width is close to thickness, and the
fiber can twist almost equally well in all directions about its
longitudinal axis.
[0050] FIG. 2 is a cross-sectional view, originally taken at about
100.times. magnification, of a PRIOR ART elliptical (or
oval)-shaped fiber made from polyvinylacetate having 0.78 mm width
and 0.42 mm thickness.
[0051] FIG. 3 is a cross-sectional view, originally taken at about
1 00.times. magnification, of a PRIOR ART fibrillatable fiber
commercially available under the tradename GRACE.RTM. Structural
Fibers. This fiber is designed to fibrillate or break into smaller
fibrils when mixed in concrete. The cross-sectional profile
resembles a tri-lobed peanut.
[0052] FIG. 4 is a cross-sectional view, originally taken at about
100.times. magnification, of an exemplary individual fiber body of
the present invention. The generally quadrilateral cross-sectional
profile is evident, in that four sides can be discerned, although
the small right side is not completely straight. The quadrilateral
shape could more accurately be characterized as trapezoidal in
nature, because the longer pair of sides (which define the width)
are generally parallel to each other, while the two smaller sides
are somewhat angled with respect to the longer sides and to each
other. The inventors believe that when such individual fiber bodies
are slit from a larger sheet using cutting blades, the angle or
attitude of the blades can define whether the smaller sides will
have an angle such as in a trapezoid (wherein the two smaller sides
will have different angles), parallelogram (wherein the two smaller
sides, in addition to the two longer sides, will be parallel to
each other), or rectangle (opposing sides are equal, and the angles
are all about 90 degrees).
[0053] The term "quadrilateral" or "generally quadrilateral" as
used herein shall mean and refer to a cross-sectional profile that
has four sides, at least two of which are generally parallel to
each other and define the width dimension of the fiber. The two
shorter sides or faces (which therefore define the thickness aspect
of the fiber) may or may not be parallel to each other. The two
shorter sides or faces may not even be straight but could assume,
for example, a concave or convex shape if the fibers were extruded
as separate bodies rather than being cut from a sheet.
[0054] FIG. 5 is a partial cross-sectional view, originally taken
at about 200.times. magnification, of an exemplary individual fiber
body of the present invention, having 0.19 mm measured thickness.
In this enlarged microphotograph, the small side is generally
perpendicular to the two longer sides (which are 0.19 mm apart),
but there is a slight imperfection at the corners. While sharper
corners are preferred, because they are believed by the present
inventors to decrease fiber-to-fiber entanglement, some rounding or
imperfections due to the manufacturing process are to be
expected.
[0055] FIG. 6 is a view, originally at about 25.times.
magnification, of the outer surface of an exemplary individual
fiber body of the present invention. Exemplary fibers are
substantially non-fibrillatable when mixed and substantially
uniformly dispersed in concrete. Accordingly, there are
substantially no stress-fractures or discontinuities to be seen in
the relatively smooth polymer surface of the fiber, although some
surface streaking and imperfections due to the extrusion process
and/or slitting process will be seen under magnification. The
present inventors believe that introducing into concrete individual
fiber bodies that are not mechanically flattened (to the point of
having micro-stress-fractures over the entire surface) and that are
not fibrillatable (reducible into still smaller fibrils when
subjected to mechanical agitation in concrete) will lead to more
uniform dispersing and reinforcing characteristics, due to uniform
fiber surface area to fiber volume ratios and structural integrity
from fiber to fiber. Moreover, the surface of the fibers of the
invention, upon being subjected to mechanical agitation within the
aggregate-containing concrete, will attain a desirable surface
roughness that will facilitate bonding of fibers within the
concrete matrix when the concrete is solidified.
[0056] FIG. 7 is a view at 25.times. magnification of the fiber of
FIG. 6 after it has been mixed in concrete for five minutes at
twenty-five rpm in a twin shaft mixer (and removed for purposes of
illustration herein). Although the fiber surface remains
substantially free of micro-stress fractures (e.g., cracks), it
will experience a roughening or increased opacity due to the effect
of the aggregate in the concrete mix. At 200.times. magnification,
as shown in FIG. 8, the surface of the fiber, before being
introduced into concrete, is substantially free of deformities, the
only features being perceived at this level of magnification are
slight streaking and imperfections due to the extrusion method used
for making the sheet from which the individual fibers are cut.
After being substantially uniformly dispersed in a concrete mix,
the fiber, as shown at the same 200.times. magnification in FIG. 9,
does not demonstrate substantial stress-fracturing or fibrillation.
However, a desirable surface roughening is discernible when viewed
at this magnification level. Also, because the polymeric material
of the fibers of the present invention will be highly oriented, it
is not unusual that at higher magnifications there will be evident
some small strands sticking out from the fiber body, but this can
be attributed to having molecular pieces separate from each other,
or otherwise to imperfections or scraping and does not constitute
substantial fibrillation wherein the fiber body splits into smaller
fibril units.
[0057] The polymer fiber surfaces of FIGS. 10-12 were all
photographed at about 900.times. magnification and evidence major
differences between exemplary fibers of the present invention
(FIGS. 10, 11) and a mechanically-flattened PRIOR ART fiber (as
shown in FIG. 12). FIGS. 10 and 11 show the fiber surface,
respectively, before and after being mixed in wet concrete using a
twin shaft mixer (having counter-rotating blades) to attain
substantially uniform dispersion of fibers in the concrete. The
extrusion streaking, which is seen in FIG. 10, is desirably
roughened as shown in FIG. 11, but without substantial
stress-fracturing or subsurface discontinuities. Even after being
mixed in the concrete (which contains sand and coarse aggregate
such as crushed stone or gravel), the surface of the fiber of the
present invention (FIG. 11) does not develop a micro-stress
fractured morphology (e.g., sinewed discontinuities) as seen in the
mechanically-flattened PRIOR ART fiber (FIG. 12), but nevertheless
is able to provide a desirably roughened surface and overall
integrity as well as to provide desirable bendability
characteristics for achieving dispersion of a plurality of
individual fiber bodies within the concrete matrix.
[0058] As used herein and above, the terms "plurality" of
"individual fiber bodies" refer to situations wherein a number of
fibers that are identical in terms of material content, physical
dimensions, and physical properties are introduced into the matrix
material. Exemplary fiber bodies of the invention are substantially
free of surface stress fractures and substantially
non-fibrillatable when mechanically agitated within the matrix
material to be reinforced, and they have a generally quadrilateral
cross-sectional profile along said elongated length, wherein
average width is 1.0-5.0 mm. and more preferably 1.3-2.5 mm,
average thickness is 0.1-0.3 mm and more preferably 0.15-0.25 mm.,
and average length is 20-100 mmn. In preferred embodiments, average
fiber width should exceed average fiber thickness by at least 4:1
but by no more than 50:1, and more preferably the width to
thickness ratio (for fibers having average length of 20-100 mm) is
5-20 (5:1 to 20:1).
[0059] In further exemplary embodiments of the invention, a first
plurality of individual fibers can be mixed with a second plurality
of individual fiber bodies (i.e. comprising different materials,
different physical dimensions, and/or different physical properties
in comparison with the first plurality of fibers) to modify the
matrix composition. The use of additional pluralities of fibers,
having different properties, is known in the art. Hybrid blends of
fibers is disclosed, for example, in U.S. Pat. No. 6,071,613 of
Rieder and Berke, and this use of hybrid blending may be used in
association with the fibers of the present invention as well. For
example, a first plurality of fibers may comprise polymeric
material having geometry, dimensions, minimum load carrying
capacity, and bendability as taught by the present invention,
whereas a second plurality of fibers may comprise another material
such as steel, glass, carbon, or composite material. As another
example, a first plurality of fibers may have a particular
bendability characteristic and/or physical dimension (in terms of
average width, thickness, or length), while a second plurality of
fibers may comprise identical or similar-polymer materials and
employ a different bendability characteristic and/or physical
dimension(s).
[0060] Exemplary pluralities of fibers as contemplated by the
present invention may be provided in a form whereby they are
packaged or connected together (such as by using a bag, peripheral
wrap, a coating, adhesive, or such as by partial cutting or scoring
of a polymer precursor sheet, etc.). However, as previously
discussed above, "individual fiber bodies" of the invention are
defined as being themselves separated from other fiber bodies or as
being separable from other fibers when mixed into the concrete.
Thus, exemplary fibers of the invention can be said to comprise a
plurality of individual fiber bodies wherein the individual fiber
bodies are separated from each other or wherein individual fiber
bodies are connected or partially connected to each other but
capable of becoming separated after being introduced into and mixed
within the matrix composition (to the point of substantially
uniform dispersion).
[0061] The present inventors believe that the bendability of
individual polymer fibers can be controlled more precisely, in
part, by using the generally quadrilateral cross-sectional profile.
The present inventors sought to avoid too much flexibility whereby
fibers became wrapped around other fibers (or around themselves)
such that fiber balling arises. They also sought to avoid extreme
rigidity, which is often associated with strength, because this too
can lead to undesirable fiber "balling." Flexibility that is too
high (such as in wet human hair) can be just as troublesome as
stiffness (such as in the "pick-up-sticks" game played by children)
because self-entanglement can arise in either case. A high degree
of fiber balling or entanglement means that substantially uniform
dispersion has not been attained in the matrix material; and this,
in turn, means that the fiber dosage will be inadequate and the
material properties of the fiber reinforced material will be
subject to significant variation.
[0062] The present inventors believe that for best dispersion
properties, bendability needs to be sufficiently high to minimize
stress transfer among the other fibers. In order to achieve this,
the inventors believed that alterations in the shape and size of
the fiber and elastic modulus of fibers were worth consideration.
For example, a lower elastic modulus will increase the bendability
of the fiber, if the shape and size of its cross-section remain
constant. On the other hand, inventors also believe it is necessary
to consider the elastic modulus of the matrix material to be
reinforced. For polypropylene fibers, the elastic modulus is in the
range of 2-10 Giga Pascals; and for a matrix material such as
concrete (when hardened) the elastic modulus is in the range of 20
to 30 Giga Pascals, depending on the mix design used. The present
inventors believe that to improve the properties of the matrix
material (hardened concrete) especially at small crack openings or
deflections, the elastic modulus of the fiber should preferably be
at least as high as the elastic modulus of the matrix material
(hardened concrete). As mentioned above, an increase in elastic
modulus usually means a decrease in bendability, which has a
negative impact on dispersion properties of the plurality of
fibers. Thus, in order to keep the bendability high, the present
inventors have chosen to modify the both the shape and
cross-sectional area of the individual fiber bodies. Fracture tests
of concrete specimens containing the fibers have indicated that a
minimum load-carrying capacity under tension (and not minimum
tensile stress) of fibers is needed for transferring significant
stresses across a cracked section of concrete. This also helps to
keep the number of fibers per unit volume of concrete down, and
this lowered dosage requirement has a positive effect in terms of
improving workability of the fresh fiber reinforced concrete. It is
a well-known fact that micro-fibers (having diameters of 20-60
micrometers) which are added to concrete for plastic shrinkage
cracking control (rather than structural reinforcement, for
example) can not be added in large volumes due to the high number
of fibers per unit weight (e.g., high surface area). Typical dosage
rates for these fibers range from 0.3 kg/m.sup.3 to 1.8 kg/m.sup.3
(0.033 vol. % to 0.2 vol. %). Fibers added at these low dosage
rates do not have a significant effect on the hardened properties
of concrete. Fibers that are supposed to have an effect on the
hardened properties of concrete need to be added in larger volumes
due to the significant higher stresses needed to be transferred
across cracked concrete sections.
[0063] Ideally, the present inventors believe that fibers, used in
a concrete structure that is cracked, provide a balance between
anchoring in concrete and pull-out from concrete. In other words,
about half of the fibers spanning across the crack should operate
to pull out of the concrete while the other half of the fibers
spanning the crack should break entirely, at the point at which the
concrete structure becomes pulled completely apart at the crack.
Thus, exemplary fibers of the present invention are designed with
particular physical dimensions that combine dispersibility with
toughness for the purpose at hand.
[0064] An exemplary process for manufacturing fibers of the
invention comprises: melt extruding a synthetic polymeric material
(e.g., polypropylene, polypropylene-polyethylene blend) through a
dye to form a sheet; cooling the extruded polymer sheet (such as by
using a chill take-up roll, passing the sheet through a cooling
bath, and/or using a cooling fan); cutting the sheet to provide
separate individual fibers (such as by pulling the sheet through
metal blades or rotary knives), whereby a generally quadrilateral
cross-sectional profile is obtained (preferably having the average
width and thickness dimensions as described in greater detail
above); stretching the polymer in the longitudinal direction of the
fibers by a factor of at least 10 to 20 and more preferably by a
factor of 12-16. After the stretching and cutting steps, the
individual fibers can be cut to form individual bodies having
average 20-100 mm lengths. Thus, exemplary individual fiber bodies
of the invention will have elongated bodies, comprising one or more
synthetic polymers, having an orientation (stretch ratio) in the
direction of the length of the fiber bodies (a longitudinal
orientation) of at least 10-20 and more preferably 12-16.
[0065] A further exemplary method for making the fibers with
generally quadrilateral cross-sections comprises extruding the
polymer or polymeric material through a four-cornered, star-shaped
die orifice, stretching the extruded fibers by a factor of 10-20
(and more preferably by a factor. of 12-16), and cutting the
stretched fibers to 20-100 mm lengths. In still further exemplary
embodiments, fibers having round or elliptical shapes may be
extruded, and, while still at a high temperature, be introduced
between rollers (which optionally be heated) to flatten the fibers
into a generally quadrilateral shape (although in this case the
smaller faces of the fibers may have a slightly arched or convex
shape).
[0066] In addition to the fiber body embodiments mentioned above,
still further exemplary fiber embodiments are possible. For
example, individual fiber bodies may have a variability of
thickness and/or width along individual fiber body length of at
least 2.5 percent deviation (and more preferably at least 5.0
percent deviation) and preferably no more than 25 percent deviation
from the average (thickness and/or width). For example, it may be
possible during cutting of the polymer sheet that the blades can be
moved back and forth so that the width of the fibers can be varied
within the 20-100 mm length of the individual fiber bodies.
[0067] In further exemplary embodiments, individual fiber bodies
may comprise at least two synthetic polymers, one of said at least
two synthetic polymers comprising an alkaline soluble polymer
disposed on the outward fiber surface thereby being operative to
dissolve when said fiber bodies are mixed into the alkaline
environment of a wet concrete mix. Alternatively, individual fiber
bodies may be coated with an alkaline soluble polymer. When
dissolved in the alkaline environment of a wet concrete mix, the
outer surface of the fiber could be increased for improved keying
with the concrete when hardened. An alkaline soluble (high pH)
polymer material suitable for use in the present invention could
comprise, for example, polymers of unsaturated carboxylic
acids.
[0068] Exemplary fibers of the invention may also be packaged with
one or more admixtures as may be known in the concrete art.
Exemplary admixtures include superplastizicers, water reducers, air
entrainers, air detrainers, corrosion inhibitors, set accelerators,
set retarders, shrinkage reducing admixtures, fly ash, silica fume,
pigments, or a mixture thereof. The one or more admixtures may be
selected, for example, from U.S. Pat. 5,203,692 of Valle et al.,
incorporated by reference herein. The fibers may also be coated
with wetting agents or other coating materials as may be known to
those of ordinary skill in the concrete industry.
[0069] Further features and advantages of the exemplary fibers,
matrix compositions, and processes of the invention may be
illustrated by reference to the following examples.
EXAMPLE 1
Prior Art
[0070] Prior art fibers having an elliptical shaped cross section
were tested in terms of bendability and dispersibility in a
concrete mix. These elliptical fibers were 50 mm long, 1.14 mm
wide, 0.44 mm. thick, and had a Young's modulus of elasticity of 4
Giga Pascal. The "bendability" formula discussed above may be
employed, wherein bendability "B" was computed as
B=1/(3.multidot.E.multidot.I), and the moment of inertia (I) for
ellipses is calculated by the formula,
I.sub.elipse=Pi/64.multidot.a.multidot.b.sup.3, where "a" is half
the width of the elliptical fiber (major axis of the ellipse, i.e.,
widest dimension through the center) and "b" is half the thickness
of the elliptical fiber (minor axis of the ellipse, i.e. thinnest
dimension through the center point of the ellipse). The bending
deflection "B" was computed to be 17.5 mN.sup.-1*m.sup.-2. This
fiber is considered a "stiff" fiber. 30 minutes were required for
introducing 100 pounds of these elliptical fibers into 8 cubic
yards of concrete. The concrete resided in the drum of a ready-mix
truck and was rotated at 15 revolutions per minute (rpm). Excessive
fiber balling was observed. The elliptical fibers did not disperse
in this concrete.
EXAMPLE 2
[0071] In contrast to the prior art elliptical fibers of Example 1,
fibers having a generally quadrilateral cross-section were used.
These quadrilateral fibers had the following average dimensions: 50
mm long, 1.35 mm wide, and 0.2 mm thickness, with a Young's modulus
of elasticity of 9 Giga Pascal. The bendability "B" of these fibers
was computed in accordance with the formula,
B=1/(3.multidot.E.multidot.I), wherein the moment of inertia "I"
for rectangular cross-section was computed in accordance with the
formula, I.sub.rectangle=1/12.multidot.w.multidot.t.s- up.3,
wherein "w" is the average width and "t" is the average thickness
of the rectangle. Using the equation, the bendability "B" was
computed as 41.2 mN.sup.-1*m.sup.-2. This fiber is considered
flexible. When 100 pounds of these fibers were introduced into 8
cubic yards of concrete, located in a ready-mix truck drum and
rotated at the same rate as in Example 1, a homogeneous fiber
distribution was achieved in just 5 minutes. No fiber balling was
observed.
EXAMPLE 3
[0072] The mechanical properties of the fibers themselves have a
huge impact on the behavior of the fibers in concrete, if there is
sufficient bond between the fiber and the brittle concrete matrix.
If the fibers have not bonded well to the matrix (e.g. fiber
pull-out is the major fiber failure mechanism observed when the
fiber reinforced concrete is broken apart), then the fiber
properties will have minimal impact on the behavior of the
composite material. As mentioned earlier, due to the fiber geometry
and dimensional ranges inventively selected by the present
inventors, sufficient bond adhesion between the matrix material
(when hardened) and the fibers can be achieved to obtain, ideally,
half fiber failure (breakage) and half fiber pull-out. Therefore,
fiber properties such as elastic modulus of elasticity, tensile
strength, and minimum load carrying capacity were selected so as to
maintain as closely as possible the ideal 50:50 balance between
fiber pull-out failure and fiber failure. The optimum mechanical
properties of the fibers will highly depend on the strength of the
matrix: a higher strength matrix will require a fiber with a higher
elastic modulus, higher tensile strength, and higher minimum load
carrying capacity.
[0073] All the mechanical tests performed on the fiber itself have
to be done in direct tension (i.e., longitudinal direction), which
is also the mode the fibers fail when embedded in hardened
concrete. (Commercially available machines for such testing are
available from known sources such as Instron or Material Testing
Systems). For these mechanical tests, a fiber filament, usually 100
mm long, is fixed on both ends with special fiber yarn grips that
do not allow the fiber to slip. The fiber is slightly pre-stretched
(less than 2 Newton of load is measured). A load cell measures the
tensile load while the fiber is being pulled apart at a constant
rate. Typical rates of loading range from 25 mm/min. to 60 mm/nmin.
The strain is measured using an extensometer, which is clamped onto
the sample. Strain is defined as the length change divided by the
initial length (also called gauge length) multiplied by 100 and is
recorded in terms of percentage. The initial gauge for the
measurements was set to 50 mm.
[0074] FIG. 13 shows various load versus strain curves of fibers
with different cross sectional areas. Fibers with number 1 are
thinner than fibers with the number 2. The letters "A", "B", "C"
are related to the width of the fibers: "A"is the fiber with the
smallest width, while "C" is the fiber with the largest width.
Therefore, the fiber with the smallest cross sectional area is
fiber "1A", while the fiber with the largest cross section is fiber
"2C".
[0075] These curves provided in this example show that a fiber with
a small cross sectional area has a much lower minimum load carrying
capacity than a fiber with a larger cross sectional area.
Individual fiber bodies should have a minimum load carrying
capacity such that a plurality of the fibers will cumulatively
provide a total load-carrying capacity exceeding the tensile stress
at which the concrete matrix material failed (i.e. the typical
stress at failure for the concrete matrix is somewhere in the range
of 2 to 5 Mega Pascals). The inventors believe that a minimum load
carrying capacity (in tension) of the fiber is necessary in order
to transfer stresses effectively as well as keeping the number of
individual fibers down. By keeping the fiber numbers down, the
workability of the fresh concrete can be maintained.
EXAMPLE 4
[0076] FIG. 14 shows the tensile stress versus strain curves of the
fibers decribed in the previous example. "Stress" is defined as the
load divided by the cross sectional area of the fiber. The slope of
the initial part of the ascending curve is directly proportional to
the modulus of elasticity of the fiber material. As mentioned
earlier, the modulus of elasticity of the fiber should preferably
be as close as possible to the modulus of elasticity of the matrix
material, so as to transfer tensile loads across cracks in the
matrix immediately after they have been initiated. On the other
hand, a higher elastic modulus decreases bendability (ie. increases
stiffness) of the fibers; the inventors discovered that this
diminishes the dispersibility of fibers in wet concrete. To
minimize the adverse effect of a high elastic modulus on the
bendability of the fiber, the inventors selected a generally
quadrilateral cross-sectional profile and selected a thinner and
wider fiber.
[0077] The stress-versus-strain curves shown in FIG. 14 indicate
that the elastic moduli and tensile strengths of the different
fiber samples are approximately the same (up to around 7% strain).
However, as shown in FIG. 16, the use of different cross-sectional
dimensions had a profound effect on the performance of the
different fiber samples in the concrete.
EXAMPLE 5
[0078] The effect of different geometries of the fibers, as well as
different minimum load carrying capacities on the mechanical
properties of fiber reinforced concrete, can be studied using
fracture tests. The basic principle of a fracture test performed on
a given material is to subject a specimen (in this case the fiber
reinforced concrete) to a load that initiates cracking in a
controlled manner, while measuring the applied load and the
deformation and eventual crack opening of the specimen. A suitable
test for concrete is the Wedge Splitting Test, which is based on a
modified Compact-Tension specimen geometry. The test set-up is
described in the Austrian Patent AT 390,328 B (1986) as well as in
the Austrian Patent AT 396,997 B (1996).
[0079] FIG. 15 depicts a typical uniaxial wedge splitting test
device that can be used for measuring load on concrete materials. A
notched cube-shaped concrete specimen resting on a linear support
(which is much like a dull knife blade is split) with load
transmission equipment situated in a rectangular groove extending
vertically down into the top of the sample concrete specimen. The
load transmission equipment consists of a slim wedge (a) and two
load transmission pieces (b) with integrated needle bearings. The
crack mouth opening displacement (CMOD) is measured by two
electronic displacement transducers (Linear Variable Differential
Transducer or "LVDT" gauges) located on opposing sides of the
crack. Both LVDTs (d) are mounted in a relatively simple way on a
CMOD measurement device (c) that is attached to the specimen with
screw bolts.
[0080] The crack initiates at the bottom of the starter notch and
propagates in a stable manner from the starter notch on top of the
concrete sample to the linear support below the sample. To obtain a
load-versus-displacement curve, the two crack mouth opening
displacement sensors, CMOD1 and CMOD2, and the applied load
(downward through the wedge), are recorded simultaneously.
[0081] To maintain an approximately constant rate of crack opening,
the test is performed with a rigid testing machine at a constant
cross-head speed of 0.5 mm/min. to 1.0 mm/min. depending on the
wedge angle. The applied machine load, F.sub.M, the vertical
displacement, .delta..sub.V, and the crack mouth opening
displacement, CMOD, are recorded simultaneously at least every
second. The fracture energy, G.sub.F, a measure of the energy
required to widen a crack, is determined from a load-displacement
curve by using the formula 2 G F = 1 B W 0 C MOD max F H ( CMOD ) (
CMOD ) with CMOD = 1 2 ( CMOD1 + CMOD2 )
[0082] where "B" is the ligament height, "W" is the ligament width
(B times W is the crack surface area), and "F.sub.H" is the
horizontal splitting load which may be calculated using the
following equation, 3 F H = F M + m w 9.81 2 tan ( / 2 )
[0083] wherein "F.sub.M" is the applied machine load, "m.sub.w" is
the mass of the splitting wedge, and ".alpha." is the wedge
angle.
[0084] As a measure for the energy for crack initiation, the
critical energy release rate "G.sub.Ic" is calculated (plane stress
assumed): 4 G Ic = K Ic 2 E with K Ic = k F H , max
[0085] where "K.sub.Ic" is the critical stress intensity factor,
which is proportional to the maximum splitting load "F.sub.H, max".
The constant k depends on the specimen geometry and can be
calculated by a finite element program.
[0086] The stress factor "K.sub.I" is defined as following:
K.sub.I=k.multidot.F.sub.H
[0087] where "F.sub.H" is the horizontal load measured during the
fracture of the specimen. The stress factor is independent of the
specimen size, which can be used to compare the behavior of
different specimens and materials.
[0088] The effect of the fiber on the mechanical properties of the
composite material can be seen after a crack is initiated. FIG. 16
shows the stress-versus-crack opening behavior of different fiber
geometries and fiber materials. The larger the area under the
curve, the more energy the composite material can absorb while it
is being broken apart. This phenomenon is also called `toughening`
of a material. The higher the `toughness` of a material with a
certain fiber dosage (volume %), the higher is the resistance to
crack propagation of the material. If a certain fiber achieves
similar toughness at a lower dosage, as compared to other fibers,
then such a fiber will be considered to be a more effective
reinforcing fiber.
[0089] FIG. 16 shows that flat, substantially non-fibrillatable
fibers of the present invention are much more effective when
compared to the performance of fibrillatable fibers of similar
dimensions (when initially introduced into the concrete) and
similar dosage. FIG. 16 also demonstrates that the performance of a
flat PVA fiber (used at 25% higher dosage rate) with respect to
resisting propagation at small crack openings is slightly better
than that of other fibers. However, at larger crack openings, the
exemplary flat fibers of the present invention clearly outperformed
the flat PVA fiber in resisting higher deformations.
[0090] The present invention is not to be limited by the foregoing
examples which are provided for illustrative purposes only.
* * * * *