Note: Descriptions are shown in the official language in which they were submitted.
CA 02881271 2015-02-05
1
LIGHT WEIGHT COMPOSITE ARMOR WITH STRUCTURAL STRENGTH
FIELD AND BACKGROUND OF THE INVENTION
The present disclosure relates generally to passive ballistic armor for
military and civilian
use, and in particular, to lightweight composite armor structure with very
strong
structural strength based on a multi-material composite for absorbing and
limiting the
transfer of impact energy from most types and sizes of kinetic energy (KE)
threats and
chemical energy (CE) threats.
The present disclosure relates especially to ballistic protection for use in
military vehicles
of different types and other moving and stationary military platforms, or for
use in
permanent or temporary protection of different types in buildings or other
fixed or
mobile installations. The ballistic protection according to the present
disclosure can be
employed as protection against different types and sizes of low-velocity or
high-velocity
armor ballistic threats, such as armor piercing projectiles or threats based
on chemical
energy.
It is an object of the present disclosure to provide a composite armor that
will prevent
the penetration of projectiles in a structure while also providing structural
support to the
same structure.
Desired armor protection levels can usually be obtained if weight is not a
consideration.
For over a century, metals have been the material of choice in providing load-
bearing
capabilities and ballistic protection for military platforms. Especially
military vehicles
have traditionally been manufactured from high strength armor plate steel.
Development of modern fighting technology, directed generally towards
decreasing of
mass of vehicles construction, creates simultaneously the necessity of
increasing the
penetration resistance of protective layers to all types of impact threats.
Vehicles
designed for land conflict are often lightly or inadequately protected from
heavy machine
gun (HMG) projectiles, high velocity kinetic energy-based anti-tank long rod
penetrators
= (LRP), asymmetric threats such as fragments from Improvised Explosive
Devices (IED's),
Explosively Formed Projectiles (EFP's) and chemical energy threats (CE), which
are all
encountered with rising frequency by troops on operations.
The present disclosure provides an integral composite armor material for
absorbing and
dissipating kinetic energy from high-velocity armor piercing projectiles,
IED's, EFP's as
well as chemical energy threats such as RPG's. It is an improvement to
existing ceramic-
.
CA 02881271 2015-02-05
2
based armor.
The present disclosure improves upon existing composite armor designs through
the use
of Fiber Reinforced Cementitious Composite (FRCC), which is a composite
material with a
cementitious matrix and discontinuous reinforcement (short fibers), which are
made of
either inorganic or organic material, mostly steel fibers, polyvinyl alcohol
(PVA) fibers and
polyethylene (PE) fibers (FIG. 1).
More specifically, it relates to encapsulation and pre-stress of armor-grade
materials,
such as ceramics, with FRCC, providing armor with highly enhanced ballistic
efficiency,
physical durability, structural strength and environmental resistance.
The present disclosure results in superior ballistic characteristics of an
armor system and
can be ballistically qualified in different kind of shapes, sizes and
thicknesses, and can be
custom produced to meet any demand for ballistic protection. The overall shape
of the
panel will be determined by end user requirements. Often panels of the present
disclosure will be generally flat and with generally uniform thickness. For
more
specialized end user requirements, a panel can be shaped in almost any form of
curvature and varied thickness through the panel. The overall dimensions of
the panels
of the present disclosure will be determined by end user requirements, such as
the
impact conditions which they are required to resist, and the size and/or area
of the
object which the panel or an assembly of the panels is required to protect.
These
individual panels can then be laid into multiple-panel arrays to obtain broad
area
coverage of a contoured structure.
The transition between the different degrees of protection (light and heavier
protection)
can be implemented quickly and efficiently, and upgrading/downgrading of
existing
protection can be performed quickly without particularly sophisticated
equipment.
The result is a lightweight, composite hybrid structure for ballistic
protection particularly
suited to tactical ground vehicles. One advantage of the present disclosure is
an increase
in the ballistic penetration resistance of especially ceramic-based armor with
a
simultaneous decrease in the armor system weight. It also provides additional
strong
structural reinforcement.
Further areas of applicability will become apparent from the description
provided herein.
It should be understood that the description and specific examples are
intended for
purposes of illustration only and are not intended to limit the scope of the
present
diSclosure.
CA 02881271 2015-02-05
3
SUMMARY OF THE PRESENT INVENTION
Ballistic armor is well known in the art as is herein detailed and described
together with
explanation why the prior art could be improved, or in some essential features
is
different from the present disclosure.
To defeat lethal projectiles, laminate systems composed of a hard frontal
plate
supported by a metallic or polymer composite backing are employed. In this
system, the
high hardness face plate breaks (shatters, erodes, blunts) the projectile
defocusing the
kinetic energy to allow the backing to effectively catch the residuals. In
addition, the
backing serves as a breadboard for attachment to the vehicle. This is employed
in armor
systems including dual hard steel, titanium-aluminum laminates and ceramic-
composites.
Facial ceramics are most effective in this type of application, but have
limited fracture
toughness and damage tolerance; thus the ceramic is parasitic to the vehicle
structure.
There are two prevalent hard passive armor technologies in general use. The
first and
most traditional approach makes use of metals. The second approach uses
ceramics.
Each material has certain advantages and limitations. Metals are more ductile
and are
generally superior at withstanding multiple hits. However, they have a large
weight
penalty and are not as efficient at stopping armor-piercing threats. Ceramics
are
extraordinarily hard, strong in compression, light weight and brittle, making
them
efficient at eroding and shattering armor-piercing threats, but not as
effective at
withstanding multiple hits.
Light-weight metallic and ceramic armor designs are known. For example, metals
such as
titanium and aluminum alloys can replace traditional steel to cut weight.
Ceramics, such
as aluminum oxide, silicon carbide, and boron carbide, are used in combination
with a
supporting backing plate to achieve even lighter armor. When ceramics are
employed in
laminate constructions and are backed with high tensile strength, high-
toughness
"momentum trap" composites such as Kevlar or Spectra fibers, mass-efficient
armor
systems can be designed. The mass efficiency of such ceramic composite armor
systems
is generally two times (or more) higher than that associated with high
hardness steel or
similar high strength metallic armor plate.
State-of-the-art military armor systems for different platform protection
frequently make
use of lightweight, very high compressive strength ceramics such as silicon
carbide (SiC),
boron carbide (B4C) or alumina as the so-called "strike face" of an armor
laminate
CA 02881271 2015-02-05
4
package. The purpose of the strike face material, as often employed in high
performance
ceramic composite armor systems, is to blunt and defeat incoming metallic
projectiles by
overmatching the compressive properties of the incoming projectile during the
early
(compressive shock) portions of the impact event. High modulus, high strength
ceramics
can have four to five times the dynamic compressive strength of projectile
materials such
as steel, tungsten or tungsten carbide. Thus, it is possible to shock the
incoming
projectile to the extent that compressive fracture is initiated. This
decreases the ability of
the projectile to defeat the armor system.
Armor-grade ceramics can be classified in three distinct material categories,
a) Oxide
ceramics such as alumina, zirconia, silica, aluminum silicate, magnesia,
aluminum titanate
and other metal oxide based materials, b) Non-oxide ceramics such as carbides,
borides,
nitrides and silicides, c) Composite ceramics, such as particulate reinforced
ceramics,
fiber-reinforced ceramics, ceramic-metal ceramics, nano-ceramics and
combinations of
oxide ceramics and non-oxide ceramics. Grinding of ceramics is part of the
process of
making ceramics suitable for high performance ceramic applications, such as
armor
applications and where tight tolerances are necessary.
An important ceramic material today for ballistic protection of military
vehicles and ships
are Alumina (A1203). Owing to its excellent price-efficiency ratio, alumina is
the
preeminent ceramic armor material for vehicular applications. When an
extremely low
weight is required (e.g. for personal protection or for helicopters), silicon
and boron
carbide materials are often used. Other ceramic materials may also be
considered for the
purpose of ballistic protection, such as Silicon nitride (SN), Titanium boride
(TiB2),
Aluminium nitride (AIN), SIALON (Silicon aluminium oxynitride), Fiber-
reinforced ceramic
(e.g. C-SiC), and Ceramic-metal composite materials (CMC). However, ceramic
armor is
not without serious engineering and practical shortcomings. High hardness,
high elastic
modulus ceramic materials such as SiC and B4C are very brittle and have poor
durability
and resistance to dropping or even rough handling under field conditions.
Furthermore,
the low toughness of high performance ceramics implies that essentially all
armor-grade
ceramics have poor multiple hit capabilities. Once a monolithic ceramic part
is impacted,
the subsequent impact response of the armor is seriously compromised.
Multiple hits are a serious problem with ceramic-based armors. Armor-grade
ceramics
are extremely hard, brittle materials, and after one impact of sufficient
energy, the
previously monolithic ceramic will fracture extensively, leaving many smaller
pieces and a
CA 02881271 2015-02-05
reduced ability to protect against subsequent hits in the same vicinity.
There are several sources of information which shows that confining the
ceramics results
in an increase in penetration resistance One relatively obvious and popular
method to
overcome the disintegration of ceramic armor is to encapsulate ceramic armor
with a
5 layer of surrounding metal.
In the laboratory, ceramics show much higher performance when their boundaries
are
heavily encapsulated. The problem is to devise methods to realize some or all
of this
encapsulation effect so it can be reduced to practical application in real
armor systems. If
the ceramic tile is not encapsulated, the fractured pieces can move away
easily, and
residual protection is lost.
State-of-the-art integral armor designs work by assembling arrays of armor-
grade
ceramic tiles/spheres/pellets within an encapsulation of polymer composite
plating or
metallic frames. Such an armor system will erode and shatter (armor-piercing)
projectiles. Different designs are in current use over a range of
applications. Substantial
development efforts are ongoing with this type of armor, as it is known that
its full
capabilities are not being utilized.
There are several deficiencies with the encapsulation of ceramic material in
the prior art.
Because of the properties of the proposed metals, conventional casting
processes cannot
be readily and effectively utilized to encapsulate ceramic material. For
example, the very
high solidification shrinkage of metals precludes this process as the
encapsulating metal
exerts undue stresses on the ceramic material, and can result in the
fracturing of the
ceramic. Encapsulation of ceramic armor can also be performed by a number of
other
means, such as shrink-fitting ceramic tiles or bricks into metallic
containers, or by other
bonding methods involving the use of welded, bolted, brazed or adhesively
bonded
metallic containers. In the past, such layers have for instances been formed
on or around
ceramic material or tiles by techniques such as powder metallurgical-forming,
diffusion
bonding, and vacuum casting of liquid metal layers.
Accordingly, a need exists for an armor component formed of an encapsulated
ceramic
material that has improved penetration resistance, and for an inexpensive
method for
forming an armor component from a ceramic material that has improved
penetration
resistance.
Snedeker, et al. used a hybrid metal/ceramic approach in U.S. Pat. No. 5, 686,
689.
Ceramic tiles were placed into individual cells of a metallic frame consisting
of a backing
CA 02881271 2015-02-05
6
plate and thin surrounding walls. A metallic cover was then welded over each
cell,
encasing the ceramic tiles.
More expensive encapsulation processes, such as, powder metallurgy techniques
are
used as disclosed in U.S. Pat. No. 4, 987, 033, which shows methods for
metallic
encapsulation of ceramic material with powdered metal layers that are cold
isostatically
pressed, vacuum sintered and then hot isostatically pressed to final density.
These methods have severe shape limitations, involve the use of relatively
costly cold
isostatic press tooling, requires a complicated and costly multiple step
processing
sequence, and still requires complicated and costly post-machining to produce
a metallic
en,capsulating layer with consistent areal density. U.S. Pat. Nos. 3, 616, 115
and 7, 069,
836 respectively, shows methods for metallic encapsulation of ceramic armor
based on
vacuum hot pressing and/or diffusion bonding of ceramic tiles and metallic
stiffening
layers into machined arrays of lattice-type metallic frameworks. While capable
of
producing well-bonded and geometrically-consistent metallic encapsulation
layers, these
methods are also costly and very limited with regard to their shape-forming
capability
and the related ability to be transitioned to large-scale manufacturing, as
they require
expensive restraint tooling and die sets that essentially limit vacuum hot
press or die
pressing-based diffusion bonding to flat plate geometries.
Modifications of conventional liquid metal casting processes have been used as
in U.S.
Pat. No. 7, 157, 158. These methods, while capable of providing for
encapsulation of
different ceramic materials as well as complex shapes, require complex and
costly molds,
and the casting process itself presents many challenges since most metals of
interest for
encapsulation (Al, Mg, Ti etc.) shrink anywhere from about 3 to 12% upon
solidification.
The high coefficient of thermal expansion relative to armor-grade ceramics
such as
silicon carbide, boron carbide or alumina frequently leads to liquid metal
casting-based
encapsulation results generating very high stresses around the ceramic
material, which
can easily result in the fracturing of the ceramic being encapsulated. This
situation would
also be worsened for more complex ceramic armor tile geometries.
U.S. Pat. No. 5, 361, 678 issued to Roopchand reveals coated ceramic bodies in
composite armor where the ceramic bodies are embedded in a metal matrix.
French patent No. 2526535 issued to Pequignot reveals ceramic elements
embedded into
a metallic plate and thermally stressed.
U.S. Pat. No. 6, 532, 857 issued to Shih reveals a ceramic array armor
confined with shock
CA 02881271 2015-02-05
7
isolated ceramic tiles with rubber between the tiles and over the top of them.
Polysulfide
is used as an encapsulation component.
U.S. Pat. No. 7, 117, 780 issued to Cohen reveals composite armor plate using
a layer of
pellets held by elastic material.
The present disclosure is in the field of improved and lighter armor materials
for military
and civilian purposes.
"Smaller and lighter" is today's paradigm for future combat vehicles. Passive
armor is,
and will be for many years to come, the last line of defense for vehicular
survivability.
Future fighting vehicles will require lighter and more efficient armor
materials for greater
survivability, mobility and transportability.
It is increasingly difficult to defend against the destructive forces of
projectiles being
produced and developed for the penetration and destruction of current armor
materials.
These projectiles are also here referred to as ballistic threats or simply
"threats". Ballistic
threats vary in size and type.
To meet these threats, exceptional mechanical and physical properties are
required in
armor materials. Toughened metal alloys, fiber reinforced plastics and high
technology
ceramic materials are some of the major armor materials used today. Other
materials
used in armor materials are glass, glass-ceramic and sintered refractory
material. Many
modern armors are multicomponent systems, using several plates of different
materials
bonded together in order to exploit the individual material properties to best
advantage.
Ceramics like alumina, silicon carbide, boron carbide, beryllium oxide etc.,
represent one
of the most important developments in armor materials. Their very high
compressive
strength and hardness, coupled with relatively low density, provide excellent
armor
performance against a wide range of high energy ballistic threats.
Ceramic armor materials are lighter than conventional metallic solutions,
including
titanium, and are two to three times harder. Use of ceramics lowers the weight
of a
passive armor system while improving the ability to defeat ballistic threats.
Ceramic materials have seen increasing use in ballistic applications where a
combination
of high compressive strength and low density are important. The very high
compressive
strength of ceramic materials offers the potential for more efficient
destruction of
penetrators than more conventional monolithic metals. However, the extreme
localized
loading of the ceramic during a ballistic impact often generates early failure
and
CA 02881271 2015-02-05
8
comminution of these brittle materials, and subsequent considerable loss of
ballistic
efficiency.
Ceramic materials are hard and brittle. The high hardness contributes to
flatten the nose
part of the incoming projectiles, which increases the forces to stop the
projectiles.
Regardless of the ceramic material used, ceramic armor is damaged on impact,
and this
damage propagation affects the subsequent ceramic performance. This damage is
caused
by the activation of preexisting defects by the shear and tensile forces that
are generated
on impact on the ceramic.
The brittle properties of ceramics are not good for sustained defeating of
projectiles,
however, the damage zone forms due to this helps to distribute the impact
force over a
larger area. Another effect of brittleness of ceramic material is the long
cracks usually
expanding from the point of hit due to bending. The long cracks and resulting
small
pieces of ceramic material are harmful for the defeat of projectiles, because
not much
constraint exist in-plane to keep the material in the damage zone and to
contribute
resistance forces.
Ceramics are usually employed in an armor system where backing and surround
plates
are utilized in an attempt to increase efficiency of the comminuted ceramic.
Such
attempts at backing or surrounding the ceramic with different materials have
been met
with mixed success, partly because the underlying principals who influence
successful
system design are not clearly understood. Engineering a better backing and/or
surrounding that can enhance the performance of the ceramic, would lower its
weight
and space burden in a structural armor application.
Different factors (backing plate stiffness, ceramic compressive strength,
ceramic/-
encapsulation impedance mismatch, etc.) have been shown to be important
contributors
to'the overall efficiency of the ceramic/encapsulation system. The resulting
behavior of
the ceramic is a complicated combination of the integrated responses of the
damaged
and undamaged regions. Since ceramics is rarely, if ever, used as a stand-
alone armor,
the mechanical response of the entire system determines the degree to which
the
damage is generated and how well the damage is encapsulated.
Ceramics are currently the subject of intensive research and improvement. The
field
applications of ceramic armors, however, have been limited by especially the
high
material cost. Incorporation of ceramics in hybrid armor systems can result in
significant
weight reductions, and are an excellent prospect for next-generation energy
absorbing
CA 02881271 2015-02-05
9
systems for ballistic protection. The present disclosure relates to the
encapsulation and
pre-stress of armor-grade materials, such as ceramics, with fiber reinforced
cementitous
composites (FRCC). The characteristics of FRCC can be utilized to exert a
highly beneficial
confinement and compressive stress on a ceramic material. Such beneficial
confinement
and compressive stress makes the encapsulated material to defeat the
projectiles much
more effectively by delaying the formation of cracks in the ceramics.
The objectives of the present disclosure are as follows:
= Enhancement of hydrostatic encapsulation of a material made of armor-
grade
materials, such as ceramic, to increase dwell time for a projectile on the
front
face of the encapsulated material thus promoting mushrooming and defeat of
anti-armor projectiles of any types and sizes.
= Enhancement of multiple hit penetration resistances for individual tiles,
tile
arrays or more complex shapes in which the encapsulated material is divided
into.
= Enhancement of the durability and damage tolerance against physical abuse
and
routine handling for the inner brittle material by providing a robust
container for
individual tiles, tile arrays or more complex shapes.
= Providing a composite armor that will prevent the penetration of
projectiles in a
structure while also providing structural support to the same structure.
The present disclosure is a major improvement to current ceramic-based
integral armor,
which results in superior ballistic performance and survivability, multi-hit
capability
including reduced damaged area, lower areal density, more flexible design as
well as
strong structural strength.
It is to be understood that the following detailed description present
embodiments of
the present disclosure and are intended to provide an overview or framework
for
understanding the nature and character of the present disclosure as it is
claimed. The
aceompanying drawings are included to provide a further understanding of the
present
disclosure and are incorporated into and constitute a part of this
specification. The
drawings illustrate different embodiments of the present disclosure, and
together with
the description, serve to explain the principles and operations of the present
disclosure
but not to limit the present disclosure to these descriptions only.
The main idea of the present disclosure is accomplished by forming fiber
reinforced
CA 02881271 2015-02-05
cementitious material around the perimeter of an encapsulated armor-grade
material,
especially ceramic, to encapsulate and pre-stress this encapsulated material
to increase
dwell time and avoiding the inner core from lateral expansion when impacted by
a
ballistic threat, such as a projectile or a fragment.
5 Fiber reinforced cementitious composite (FRCC) is a very universal
term for all
cementitious materials that are reinforced by any kind of fibers. FRCC is
concrete
containing fibrous material which increases its structural integrity. It
contains short
discrete fibers that are uniformly distributed and randomly oriented. Fibers
are man-
made, such as steel, titanium, glass, carbon, polymers or synthetic. The
character of FRCC
10 changes with varying concretes, fiber materials, geometries,
distribution, orientation and
densities.
Many different so called high performance materials have been developed. The
considered materials in this disclosure to encapsulate ceramic materials are
all
cementitious composites reinforced by randomly oriented short discrete fibers.
Adding
fibers enhances the compressive, tensile and shear strengths, flexural
toughness,
durability and resistance to impact and penetration as well as resistance to
plastic
shrinkage cracking.
Concrete is widely used in structural engineering with its high compressive
strength, low
cost and abundant raw material. But normal strength concrete has some
shortcomings,
for example, shrinkage and cracking, low tensile and flexural strength, poor
toughness,
high brittleness, low shock resistance and so on, that restrict its
applications. To
overcome these deficiencies, additional materials are added to improve the
performance
of normal strength concrete.
Cementitious matrices such as concrete have low tensile strength and low
strain capacity
and therefore fails in a brittle manner. As a result, the mechanical behavior
of the
concrete is critically influenced by crack propagation. Concrete can exhibit
failure
through cracks which are developed due to brittleness. A more ductile material
can be
achieved by the use of fibers in the concrete.
The use of fibers in concrete to improve pre- and postcracking behavior has
gained
popularity, and several different fiber types and materials have been
successfully used in
concrete to improve its mechanical and physical properties.
Reinforcing fibers will stretch more than concrete under loading. Therefore,
the
composite system of fiber reinforced concrete is assumed to work as if it were
CA 02881271 2015-02-05
11
=
unreinforced until it reaches its "first crack strength." It is from this
point that the fiber
reinforcing takes over and holds the concrete together. For fibers
reinforcing, the
maximum load carrying capacity is controlled by fibers pulling out of the
composite.
Some fibers are more "slippery" than others when used as reinforcing and will
affect the
toughness of the concrete product in which they are placed.
Many catastrophic failures of reinforced concrete structures subjected to
impact are
associated with the brittleness of concrete material in tension. Although a
compressive
stress wave is generated on the loading side of the structure by impact, it
reflects as a
tensile stress wave after hitting a free boundary on the back side of the
structural
element. In addition, the tensile strength of concrete is lower (by about an
order of
magnitude) than its compressive strength. Therefore, concrete tensile
properties
generally govern concrete failure under impact.
Concrete is considered a brittle material, primarily because of its low
tensile strain
capacity and poor fracture toughness. Concrete can be modified to perform in a
more
ductile form by the addition of randomly distributed discrete fibers in the
concrete
matrix
Ductile concrete would be highly desirable to suppress the brittle failure
modes and
enhance the efficiency and performance of current design approaches. The most
effective means of imparting ductility into concrete is by means of fiber
reinforcement.
An extremely ductile fiber reinforced brittle matrix composite is of great
value to
protective structures that may be subjected to dynamic and/or impact loading.
Compared to normal strength concrete and fiber reinforced concrete, a ductile
concrete
based composite has significant improved tensile strain capacity with strain
hardening
behavior several hundred times higher than that of normal strength concrete
and fiber
reinforced concrete, even when subjected to impact loading (FIG. 2).
While the fracture toughness of concrete is significantly improved by fiber
reinforcement, most fiber reinforced concrete still shows quasi-brittle post-
peak tension-
softening behavior under tensile load where the load decreases with the
increase of
crack opening. The tensile strain capacity therefore remains low, about the
same as that
of normal concrete. Significant efforts have been made to convert this quasi-
brittle
behavior of fiber reinforced concrete to ductile strain hardening behavior
resembling
ductile metal. In most instances, the approach is to increase the volume
fraction of fiber
as much as possible. As the fiber content exceeds a certain value, normally 4-
10%
CA 02881271 2015-02-05
12
depending on fiber type and interfacial properties, the conventional fiber
reinforced
concrete may exhibit moderate strain hardening behavior.
The typical stress-elongation response of FRCC indicate two properties of
interest, the
stress at cracking and the maximum post-cracking stress (FIG. 3). While the
cracking
strength of the composite is primarily influenced by the strength of the
matrix, the post-
cracking strength is solely dependent on the fiber reinforcing parameters and
the bond at
the fiber-matrix interface. Thus, improving the post-cracking strength is the
key to the
success of the composite.
Special types of concrete are those with out-of-the-ordinary properties or
those
produced by unusual techniques. Concrete is by definition a composite material
consisting essentially of a binding medium and aggregate particles, and it can
take many
forms.
The first fiber reinforced cementitious material that was developed in the
early 1960s is
steel fiber reinforced concrete (SFRC). SFRCs exhibit ductile behavior
compared to the
brittle matrix, but their flexural and tensile strengths are not very high,
and especially the
compressive strengths of these materials do not practically change with the
fiber volume
frktion. Although it shows certain improvements compared to normal concrete,
it is not
considered to be a high performance material. SFRC consists of a normal
strength
concrete matrix containing fine and coarse aggregates, which is reinforced by
relatively
long straight steel fibers. The primary improvements of traditional SFRC
compared to
normal strength concrete are a higher toughness and energy absorption
capacity, greater
spalling and delaminating resistance, improved durability characteristics
through better
crack control, as well as a higher ductility on a material level. Compared to
normal
strength concrete the tensile strength of SFRC is not improved significantly
and SFRC still
exhibits "quasi-brittle" failure (no strain hardening). The reasons for this
are that low
amounts of fibers are used, that the used fibers are relatively long and that
the matrix
contains too much course aggregate. However, compared to unreinforced concrete
the
failure is more ductile exhibiting a smother softening behavior.
Originally fiber reinforced materials, straight steel fibers at relatively low
volume
contents were used to improve the mechanical properties of traditional
concrete. The
addition of larger volume contents of fibers was mainly prevented by
workability
problems. In order to improve workability and restrict "balling" of fibers,
the quantity of
cement was increased and the amount of coarse aggregate reduced. Further
CA 02881271 2015-02-05
13
improvement of workability could be achieved by the introduction of high-range
water-
reducing admixtures. Hereby the possible volume content of fibers could be
increased.
Together with the use of improved cementitious matrices with fewer coarse
aggregate
and carefully adjusted properties, this finally leads to the high performance
fiber
reinforced materials we know today.
FRCC includes the entire class of fiber reinforced cementitious composites
(FIG. 4), and
comprises fiber reinforced concrete (FRC), fiber reinforced mortar (FRM), high
performance fiber reinforced composite (HPFRCC) and ductile fiber reinforced
cementitious composite (DFRCC). DFRCC is a broader class of materials than
HPFRCC.
Examples of modern high performance materials are engineered cementitious
composites (ECC), sometimes also called "Bendable Concrete", hybrid fiber
concretes
(HFC), multi-scale fiber reinforced cementitious composites (MSFRCC), compact
reinforced composites (CRC) and reactive powder concretes (RPC), non-high
performance
fiber reinforced concrete and steel fiber reinforced concrete (SFRC).
Considering the mechanical properties of FRCC, these composites can be
categorized into
two classes: quasi-brittle and pseudo strain-hardening. Conventional FRCC fall
into the
first category whereas HPFRCC fall into the latter.
Quasi-brittle materials, such as normal strength concrete and conventional
FRCC, usually
fail due to the formation of a single macro-crack, whereas pseudo strain-
hardening
cementitious materials such as HPFRCC undergo multiple cracking. For
conventional
FRCC, the typical upper limit for fiber volume fraction is 3%. For such
relatively low fiber
content, the fibers mainly enhance the crack arresting ability, post cracking
ductility,
fatigue and impact resistance. The stress at first crack, maximum stress and
the
corresponding strain are not significantly improved compared to normal
strength
concrete.
HPFRCC show a large improvement in both strength and toughness compared with
the
normal strength matrix. The main feature of these materials is the optimum
combination
of strength and toughness which approaches the structural properties of steel.
HPFRCC is a generic term encompassing many different materials ranging from
those that
employ ultra-compact matrices and those that do not. However, the common point
of all
HPFRCC materials is their hardening tensile behavior that helps control
cracking to a
much better extent than usual FRCC.
The criterion which separates a HPFRCC from a traditional FRCC, such as SFRC,
is its
CA 02881271 2015-02-05
14
response in tension. If the material is strain hardening in the inelastic
regime it is
considered to be a HPFRCC. The fundamental difference between composites that
show
strain hardening and those who don't is that the ductility of the latter is
only effective on
a material level and does not affect the overall structural ductility. Or in
other words the
material does react in a ductile manner, but because it is softening in
tension, the plastic
deformations are restricted to a small area, resulting in damage localization
and failure in
a small zone with large crack openings. On a structural level this kind of
ductility has little
influence since the failure occurs locally and the rest of the structure
remains elastic.
Strain hardening fiber reinforced cementitious composites on the other hand
retard
localization and lead to multiple cracking and structural ductility even
without the
addition of structural reinforcement bars. For this reason they are called
high
performance fiber reinforced cementitious composites.
Because several specific formulas are included in the HPFRCC class, their
physical
compositions vary considerably. However, most HPFRCCs include at least the
following
ingredients: fine aggregates, a superplasticizer, polymeric or metallic
fibers, cement and
water. Thus the principal difference between HPFRCC and typical concrete
composition
lies in HPFRCCs lack of coarse aggregates. Typically, a fine aggregate such as
silica sand is
used in HPFRCCs.
Ultra high performance fiber reinforced concrete (UHPFRC) is a relatively new
cementitious material, which has been developed to give significantly higher
material
performance than normal strength concrete, FRC or ECC (FIG. 5). UHPFRCC
denotes a
subclass of FRCCs that encompasses a number of ultra high strength concretes
that are
reinforced with steel fibers.
Many UHPFRCCs are also HPFRCCs and therefore exhibit strain-hardening and
multiple
cracking in direct tension. UHPFRCC are a sub group of HPFRCC combining the
ductility of
strain hardening cementitious composites with the high compressive strength of
DSP
concrete. UHPFRCC is furthermore distinguished between other FRCCs as a
material
exhibiting strain hardening in tension, whereas other FRCCs may exhibit a
hardening
behavior in bending, but are characterized by strain softening in tension.
UHPFRC have
good potential for absorbing energy through flexure. Studies of this material
under
dynamic loading have shown an increase in the ultimate strength with
increasing strain
rate.
UHPFRC can be mixed and cast like normal strength concrete with no special
facilities or
CA 02881271 2015-02-05
handling. "Ultra high performance" refers principally to improved mechanical
strength,
fractural toughness and durability. A mix is designed to combine high cement
content
with a very low water/cement ratio. The selection of fine aggregates achieves
maximisation of the particle packing density and minimises any localised non
5 homogeneity. Post-set heat treatment at 90 degrees Celsius can be applied
to further
improve the microstructure. This process results in a very high compressive
strength
concrete, typically between 150-200 MPa. The addition of a high dosage of high
tensile
steel fibers, normally 13 mm in length and 0.2 mm in diameter, results in a
high flexural
tensile strength, typically between 25-50 MPa. This material also has a very
high capacity
10 to absorb damage, with fracture energy in the range 20,000-40,000 .1/m2.
Fibers are incorporated in UHPFRC in order to enhance the fracture properties
of the
composite material. The additional role of fibers in UHPFRC, in comparison to
the role of
fibers in ordinary and in high strength fiber reinforced concrete, is to
provide sufficient
ductility of the material in tension without a decrease in stress. This is
achieved by
15 choosing the appropriate type and quantity of fibers. In recommendations
for UHPFRC,
minimal fiber strength is limited to 2000 MPa. Fibers used in UHPFRC are
typically short,
smooth and straight, while hooked fibers are more often used in high-strength
or
ordinary concretes. Required fiber geometry can be estimated based on the
relationship
between pullout force and fiber-breaking force. The resulting high-strength
and energy
absorbing properties of UHPFRC are far superior to those of normal concrete.
DFRCC is a broader class of materials than HPFRCC. HPFRCC is an FRCC that
shows
multiple cracking and strain hardening in tension, therefore in bending as
well. On the
other hand, DFRCC encompasses a group of FRCCs that exhibit multiple cracking
in
bending only, in addition to HPFRCCs. Multiple cracking leads to improvement
in
properties such as ductility, toughness, fracture energy, strain hardening,
strain capacity
and deformation capacity under tension, compression and bending. The advantage
of
DFRCCs is the increased toughness under tensile stress condition. Among a
variety of
DFRCCs, some DFRCCs achieve pure tension toughness and ductility that are
comparable
to those of metallic materials, while others show increased toughness only
under flexural
tension.
ECCs make up a particular type of HPFRCC. HPFRCC also includes Slurry-
infiltrated Fibrous
Concrete (SIFCON) and Slurry-Infiltrated Mat Concrete (SIMCON).
ECC are ultra ductile fiber reinforced cennentitious composite materials. ECC
essentially
CA 02881271 2015-02-05
16
consists of two components: fibers and a cementitious matrix. Using a micro-
mechanical
approach, fiber and matrix properties are adjusted in order to obtain the
desired
macroscopic material behavior. The most characteristic material property of
ECC is its
extremely ductile, strain-hardening-like behavior in the inelastic tensile
regime. This
behavior has been termed "pseudo-strain hardening" referring to the post-yield
strain
hardening usually exhibited by metals. Although the macroscopic performance of
ECC in
the inelastic regime is very similar to the strain hardening observed for
metals, the
responsible micromechanical mechanism is a completely different one, hence the
term
"pseudo". In metals strain hardening is a consequence of a change in the
molecular
structure, whereas in ECC it is produced by multiple cracking and bridging of
the cracks
by the incorporated fibers.
ECC is a special type of HPFRCC which has been microstructurally tailored
based on
micromechanics. The most obvious beneficial mechanical property of ECC is its
extremely
ductile response in tension. Experiments show, that tensile strains up to 6%
can be
reached before softening sets in. This means that structural elements made
from ECC are
able to bear very large imposed deformations proved durability characteristics
through
better crack control, as well as a higher ductility on a material level which
are of interest
for impact loading. Microstructure optimization allows ECC to be made with
fiber content
less than 2-3%. ECC deforms pseudo uniformly due to dense and fine multiple
cracks
(FIG. 6); therefore it shows deformation capacity comparable and compatible to
that of
steel. While conventional reinforced concrete members suffer from steel
yielding at
localized cracks, reinforced ECC members attain deformation compatibility and
utilize the
deformation capacity of steel to greater extent.
Depending on the matrix constitution, the fiber geometry and the bond
properties
between fibers and matrix, a certain minimal volume content of fibers is
necessary to
achieve the typical pseudo strain hardening behavior of ECC. If the fiber
content is too
low, quasi-brittle failure as it is exhibited by traditional fiber reinforced
concrete will
occur. In this case damage localization and softening will set in immediately
after
formation of the first crack. Therefore the fiber content is usually chosen
just above the
critical volume fraction which allows for the pseudo-strain hardening and
ductile
behavior, but without using excessive amounts of fibers. In general synthetic
fibers such
as UHMWPE (ultra high molecular weight polyethylene) or PVA (polyvinyl
alcohol) fibers
are used (FIG. 7). The difference between these two fiber types is, that
UHMWPE fibers
CA 02881271 2015-02-05
17
show little chemical bond with the cement matrix whereas the chemical bond
strength
between PVA fibers and the cement matrix is considerable. Therefore untreated
PVA
fibers are not very adequate for pullout behavior and preliminary surface
treatment of
the PVA fibers might be necessary. The used fiber volume contents are usually
somewhere between 0.5% and 4%. Typically the synthetic fibers in ECC have a
higher
aspect ratio (fiber length to fiber diameter) than steel fibers used in steel
fiber reinforced
concrete. Their interfacial bond strength on the other hand is usually lower.
A high
aspect ratio is essential in minimizing the fiber volume content necessary for
the pseudo-
strain hardening behavior. The cementitious matrix in ECC consists of a
Portland cement
paste or mortar. The addition of fly ash and microsilica is possible and often
used (FIG. 8).
The function of the synthetic fibers in ECC is to lead to steady state
cracking and multiple
cracking of the composite. In turn these two mechanisms are responsible for
the pseudo-
strain hardening behavior and large strain capacity of ECC. In order to
guarantee multiple
cracking, the bridging stress that can be transmitted by the fibers has to be
larger than
the first cracking strength of the intact matrix. This condition is referred
to as the
strength criterion for multiple cracking.
In general no or little aggregate with a relatively small grain size is used
for the
cementitious matrix of ECC. The main reason for this is to keep the fracture
toughness of
the matrix low. Adding large amounts of aggregate would result in longer
fracture path
distances and by consequence a higher fracture toughness. Having a low
fracture
toughness is a further condition to obtain multiple cracking with moderate
fiber volumes.
SIFCON, SIMCON: SIFCON is produced by infiltrating slurry into pre-placed
steel fibers in a
formwork, and due to the pre-placement of fibers, its fiber volume fraction
can amount
to 20% at maximum. The confining effect of numerous fibers yields high
compressive
strength reaching over 200 MPa, and the strong fiber bridging leads to tensile
strain
hardening behavior in some SIFCONs. The fracture energies of SIFCON's are
about 1350
times that of normal strength concrete. SIMCON uses pre-placed fiber mat
instead of
steel fibers.
RPC's represent a new generation of concretes which utilizes reactive powder,
and it is
designed with optimal packing theory. The cube strengths are between 200 and
800
MPa, the tensile strengths are between 25 and 150 MPa, and the unit weight is
of 2500 -
30.00 kg/m3. The fracture energy of these materials can reach up to 40000
J/m2, as
compared to 100 to 150 1/m2 for ordinary concretes. The fracture energies of
RPCs are
CA 02881271 2015-02-05
18
about 300 times that of normal strength concrete. The RPC microstructure has a
more
compact particle arrangement and is enhanced by the presence of the strongest
cementitous hydrates as compared to HPC. RPCs are produced by using very fine
sand,
cement, silica fume, superplasticizer and short cut steel fibers. Their very
low porosity
gives them important durability and transport properties and makes them
potentially
suitable materials for storage of industrial wastes. These features are
achieved by 1)
precise gradation of the particles in the mixture to yield a matrix with
optimum density,
2) reducing the maximum size of the particles for the homogeneity of the
concrete, 3)
reducing the amount of water in the concrete, 4) extensive use of the
pozzolanic
properties of highly refined silica fume, 5) optimum composition of all
components, 6)
the use of short cut steel fibers for ductility, 7) hardening under pressure
and increased
temperature, in order to reach very high strengths.
Ductal is a range of ultra high performance concrete (UHPFRC) with very high
compressive strength and non-brittle tensile behavior offering compressive
strength of
160 to 240 MPa and tensile strength of over 10 MPa and with true ductile
behavior.
Ductal is an inorganic composite material based on the concept of RPC. The
properties
are characterized by high strength, high durability and high flowability.
Ductal is a cement
based composite reinforced with steel fibers under the concept of high
strength and high
toughness. W/C ratio is in the region of 0.2. The sand used has a fine
grading, with the
largest grains not exceeding around 600 um in diameter. The addition of silica
fume and
optimized use of admixtures are both absolutely essential. Last but not least,
the
concrete is reinforced with metal fibers, which have also been optimized for
several
criteria, involving optimizing not only the behavior of the individual fibers,
but also their
interactions within the matrix. A content of 2% by volume of 13-15 mm long
fibers with
diameters of around 0.2 mm emerges as a good compromise. Calculating the mean
spacing of these fibers in the matrix gives a result of around 1.6 mm. which
is perfectly
compatible with the sand grading used.
UHSCs or Ultra High Strength Concretes are concretes with a very densely
packed matrix,
which causes them to withstand high compressive loads. They usually contain
large
amounts of fly-ash and silica fume and can reach compressive strengths between
120
MPa and 250 MPa.
In FRCCs, fibers can be effective in arresting cracks at both macro and micro
levels. Most
of the strain hardening FRCC is limited to single fiber type.
CA 02881271 2015-02-05
19
Mono fiber composites containing high stiffness fibers normally show high
ultimate
strength, low strain capacity and small crack width properties, while those
containing low
stiffness fibers show low ultimate strength, high strain capacity and large
crack width
properties.
Recently hybrid fiber reinforced cementitious composites (HFRCC) exhibiting
strain
hardening behavior is also developed. In hybrid fiber composites, two or more
different
types of fibers are suitably combined to exploit their unique properties. The
use of
optimized combinations of two or more types of fibers in the same concrete
mixture can
produce a composite with better engineering properties than that of individual
fibers. A
hybrid composite, with proper volume ratio of high and low stiffness fibers,
show
simultaneous improvement in ultimate strength, strain capacity and crack width
properties.
The hybridization of fibers in FRCC can be done in different ways, such as by
combining
different lengths, diameters, modulus and tensile strengths of fibers. Large
macro fibers
bridge the big cracks and provide toughness while small micro fibers enhance
the
response prior or just after the cracking. Micro fibers also improve the pull
out response
of macro fibers, thus produce composites with high strength and toughness.
Over the last three decades, high compressive strength concretes and high
tensile
ductility concretes have emerged as two distinct classes of concrete.
Materials at the
frontiers of both of these classes include very high strength concrete (VHSC)
with
compressive strength around 200 MPa, and engineered cementitious composites
(ECC)
with tensile ductility in the range of 3%-6%. The development of these two
concretes was
based on two different design philosophies that targeted two different
structural
pe.rformances. VHSC and similar high strength concretes (RPC, Ductal, MDF and
DSP)
were designed to achieve size efficiency in structural members for very large
structures
and to provide additional strength safety margin for strategically critical
and protective
structures. ECC and similar high performance fiber reinforced cementitious
composites
(HPFRCCs) were developed to ensure ductility of structural elements and
massive energy
absorption in the face of extreme load displacement events such as
earthquakes.
However, the decoupled development of VHSC and ECC resulted in mutual
exclusion of
each other's desirable properties. VHSC is an order of magnitude less ductile
than ECC,
whereas the compressive strength of ECC is three to four times less than VHSC.
A
combination of high strength and high ductility in one concrete material is
highly
CA 02881271 2015-02-05
desirable.
Recently, there have been a few notable investigations on combining high
compressive
strength and high tensile ductility in one concrete with limited success. Some
mechanical
test results of ultra high performance - strain hardening cementitious
composites (UHP-
5 SHCC) shows average compressive strength of 96 MPa and tensile ductility
of 3.3% at 14
days after casting. The development of another such material, ultra high
performance -
fiber reinforced composite (UHP-FRC) shows compressive strength of about 200
MPa and
tensile ductility of 0.6%. Although both of these materials attempt to combine
tensile
ductility and compressive strength in one concrete, UHP-SHCC has a compressive
10 strength that is only about half that of VHSC, and UHP-FRC has tensile
ductility which is at
least five times smaller than ECC.
Newly development of a new composite material, high strength - high ductility
concrete
(HSHDC), shows both the desirable properties of high compressive strength
(similar to
VHSC) and high tensile ductility (similar to ECC) and are integrated into a
single
15 composite material. This results in higher specific energy absorption
(or composite
toughness) in HSHDC as compared to any other material in the class of high
performance
cementitious composites. The micromechanics-based principles that guide the
design of
=ECC, combined with a VHSC matrix, led to the development of HSHDC.
Due to controlled cracking enabled by micromechanical tailoring of the
composite
20 material, HSHDC exhibits high tensile ductility in spite of a high
strength brittle matrix.
Specific energy (or composite toughness) of HSHDC, is calculated as the area
under the
stress-strain curve before attaining ultimate stress capacity. A comparison of
other
composite properties of HSHDC with other similar high performance concrete
materials
is shown in FIG. 9. It can here be observed, that the specific energy of HSHDC
is the
largest among all the materials presented, which is a result of the
combination of high
tensile strength and high tensile ductility. Such material behavior leads to
high energy
absorption, which is critical for structures to withstand extreme loading
conditions.
Fibers used in FRCCs
The use of short fibers in concrete to improve pre- and post cracking behavior
has gained
popularity. The mechanical properties of fiber reinforced concrete depend on
the type
= and the content of the added fibers. Several different fiber types and
materials have
been successfully used in concrete to improve its mechanical and physical
properties. In
=
CA 02881271 2015-02-05
21
often used fiber reinforced concrete, steel and synthetic fibers are mainly
used, although
a great variety of fibers made of other materials exists. This is related to
the strength and
stiffness that is required of the desired fiber contribution.
Unlike continuous fiber composites, the external loads are not directly
applied to the
fibers in FRCC. The load applied to matrix materials is transferred to the
fibers via fiber
ends and the surfaces of fibers. As a consequence, the properties of FRCC
greatly depend
on fiber length and the diameter (i.e., fiber aspect ratio) of the fibers.
Further, several
factors such as fiber orientation, volume fraction, fiber spacing, fiber
packing
arrangement, and curing parameters also significantly influence the properties
of FRCC.
Using micro and macro fibers of different mechanical, geometrical and physical
properties reduces the brittleness of cementitious materials. Adding short
needle-like
fibers to cementitious matrices enhances their mechanical properties,
particularly their
toughness, ductility and energy absorbing capacity under impact. The fibers
can and
should be engineered to achieve optimal properties in terms of shape, size and
mechanical properties, as well as compatibility with a given matrix. All
fibers are made of
either inorganic or organic material. The inorganic category includes
materials such as
metals, minerals, ceramic, carbon and glass. The list of organic fiber
materials, on the
other hand, seems to be limited only by the creativity of nature and the
chemical
industry. Nature still holds the record for the strongest fibers, which are
spun by spiders.
Other natural fibers include cellulose, silk and cotton. Manmade organic
fibers include
nylon, polypropylene, polyvinyl alcohol (PVA), polyethylene and aramid, among
many
others.
Materials used in fiber reinforcing can include acrylic, asbestos, cotton,
glass, nylon,
polyester, polyethylene, polypropylene, rayon, rockwool and steel. Of these,
acid
resistive glass and steel fibers have received the most attention. Plastic
fibers have
shown to be of little value in reinforcing concrete until only recently.
Natural fibers are
subject to alkali attack and are also determined to have little value. The
premium fibers
are graphite reinforced plastic fibers, which are nearly as strong as steel,
lighter-weight
and corrosion-proof. Some experiments have had promising early results with
carbon
nanotubes. Fiber (steel or "plastic" fibers) reinforced concrete is less
expensive than
hand-tied rebar, while still increasing the tensile strength many times.
Short fibers used in concrete can be characterized in different ways. First,
according to
the fiber material: natural organic (such as cellulose, sisal, jute, bamboo,
etc.); natural
CA 02881271 2015-02-05
22
mineral (such as asbestos, rockwool, etc.); man-made (such as steel, titanium,
glass,
carbon, polymers or synthetic, etc). Second, according to their
physical/chemical
properties: density, surface roughness, chemical stability, non-reactivity
with the cement
matrix, fire resistance or flammability, etc. Third, according to their
mechanical
properties: tensile strength, elastic modulus, stiffness, ductility,
elongation to failure,
surface adhesion property, etc.
Short fibers are mainly characterized by the material and its mechanical
properties and
by their geometry. Once a fiber type has been selected, an infinite
combination of
geometric properties related to its cross sectional shape, length, diameter or
equivalent
diameter and surface deformation can be selected. The cross section of the
fiber can be
flat, circular, rectangular, diamond, square, triangular or any other
significant polygonal
shape.
The effectiveness of fibers on the mechanical properties of brittle matrix
varies with the
geometrical, mechanical and physical properties of fibers. Fibers with surface
roughness
and large specific surface area develop good bond with the matrix due to, as a
results,
micro-cracking mechanism before the occurrence of peak load is arrested in the
presence
of fibers, and fibered concrete exhibits high value of peak load compared to
normal
strength concrete without fibers.
The properties of concrete matrix and of the fibers greatly influence the
character and
performance of FRCC. The properties of fibers which are of interest include
fiber
stiffness, bond between fiber and concrete matrix, fiber concentration, fiber
geometry,
fiber orientation, fiber distribution and fiber aspect ratio.
In order to be effective in concrete matrices, fibers must have the following
properties:
1) a tensile strength significantly higher than that of the concrete (two to
three orders of
magnitude); 2) a bond strength with the concrete matrix preferably of the same
order as
or.higher than the tensile strength of matrix; and 3) unless self-stressing is
used through
fiber reinforcement, an elastic modulus in tension significantly higher than
that of the
concrete matrix. The Poisson's ratio and the coefficient of thermal expansion
should
preferably be of the same order for both the fiber and the matrix.
In relation to the elastic modulus, fibers are divided into two types, those
where the
elastic modulus of fibers is less than the elastic modulus of the matrix: i.e.
cellulose fiber,
polypropylene fiber, polyacrylonitrile fiber, etc.; and those where the
elastic modulus of
fibers is greater than the elastic modulus of the matrix: i.e. asbestos
fibers, glass fiber,
CA 02881271 2015-02-05
23
steel fiber, carbon fiber, aramid fiber, etc.
To' develop bond with matrix, specific surface area and surface conditions of
the fiber
play an important role. Increasing the average bond strength leads to a direct
increase in
the post cracking strength of the composite and other important properties as
well, such
as toughness and energy absorption capacity. The different bond components are
adhesion, friction, mechanical and interlock. In some fibers the surface is
etched or
plasma treated to improve bond at the microscopic level.
To develop better bond between the fiber and the matrix, the fiber can be
modified
along its length by roughening its surface or by inducing mechanical
deformations. Thus
fibers can be smooth, indented, deformed, crimped, coiled, twisted, with end
hooks,
paddles, buttons, or other anchorage.
When micro-cracks are developed, the stress in fiber increases gradually with
the
increase of crack openings, and a stage of either pulling out from the matrix
before the
stress in fiber exceeds its tensile strength capacity will happen or a stage
of breakage of
fiber will happen if the fiber are not pulled out from the matrix before the
stress in fiber
exceeds its tensile strength capacity. In order to enable the transfer of
force with a small
crack opening and sustain tensile force without breaking, a high modulus of
elasticity and
high strengths are required.
To transfer the stress across the crack edges (bridging action of fibers),
length of fiber
compatible with maximum aggregate size is important. Interfacial transition
zone
between the aggregate and the cement paste is the weakest phase in the
concrete. In
order to bridge this zone and to get highest effect of fibers, length of the
fiber and the
diameter of the aggregate must be coherent with each other. To develop an
efficient
bridging action, fiber must be embedded into the matrix on both ends beyond
the
aggregate particles. For that, fiber length must be at least greater than 2
times maximum
aggregate size. Also to get better efficiency, fiber length should be 2 to 3
times the
maximum size of the coarse aggregate.
Ideally the amount of fibers and aspect ratio should be as large as possible
to maximize
the improvements in the mechanical properties.
The length and diameter of synthetic fibers vary greatly. Single filament
fibers can be as
little as 10 micrometers in diameter such as for Kevlar or carbon fibers, and
as large as
0.8 mm such as with some polypropylene or poly-vinyl-alcohol (PVA) fibers.
Generally in
concrete applications, the aspect ratio, that is, the ratio of length over
diameter or
CA 02881271 2015-02-05
24
equivalent diameter, of very fine fibers exceeds 100 while that of courser
fibers is less
than 100. Most synthetic fibers (glass, carbon, kevlar) are round in cross
section; flat
synthetic fibers cut from plastic sheets and fibrillated are suitable when
very low volume
content is used.
Most common steel fibers are round in cross section, have a diameter ranging
from 0.4
to 0.8 mm, and a length ranging from 25 to 60 mm. Their aspect ratio is
generally less
than 100, with a common range from 40 to 80.
Different types of steel fibers have been developed (FIG. 10). They differ in
size, shape
and surface structure. Such fibers have different mechanical properties such
as tensile
strength, grade of mechanical anchorage and capability of stress distribution
and
absorption). Hence they have different influence on concrete properties. Some
other
types of closed-loop steel fibers such as ring, annulus, or clip type fibers
have also been
used and shown to significantly enhance the toughness of concrete in
compression.
SFRC with the ring-type steel fibers (RSFRC), fails by more energy consuming
mechanisms
other than fiber pullout, whereby significant improvements in flexural
toughness is
obtained as compared to that of SFRC with conventional straight steel fibers.
Fiber¨
matrix interfacial bond strength is provided by a combination of adhesion,
friction and
mechanical interlocking. While the mechanical performance of traditional
straight steel
fibers relies on the fiber-matrix interfacial bond strength, ring-type steel
fibers are mainly
designed to mobilize fiber yielding rather than fiber pullout. Three different
types of
flexural failure mechanisms of RSFRC are involved: fiber rupture after
yielding and cone-
type concrete fracture and separation between ring-type steel fibers and
concrete
matrix. Toughness indices of RSFRC are affected by fiber contents, ring
diameter and
fiber diameter.
Due to the formulation of the mechanics of the composite, the fiber content in
cement
matrices is specified by volume fraction of the total composite. Because of
fiber materials
of different densities, the same volume fraction of fibers of different
materials leads to
different weight fractions of fibers. Fibers are purchased by weight, but
mechanical
properties of composites are based on volume fraction, not weight fraction of
fibers.
Typically a 1% volume fraction of steel fibers in normal-weight concrete
amounts to
about 80 kg/m3 of concrete; however, a 1% volume fraction of polypropylene
fibers
amounts to about only 9.2 kg/ m3.
CA 02881271 2015-02-05
A lightweight composite armor is disclosed wherein one or successive layers of
discrete
armor-grade objects, such as monolithic ceramic blocks, are encapsulated
within a fiber
reinforced cementitious composite (FRCC). The FRCC is used to (1) encapsulate
the
armor-grade material, (2) pre-stress the encapsulated armor-grade material.
5 Studies shows that better confining of armor-grade ceramic results in
an increase in
penetration resistance, and that ceramic yields much higher performance when
their
boundaries are heavily encapsulated, because if the ceramic material is not
encapsulated, the fractured pieces can move away easily, and residual
protection is lost.
The type of encapsulation can influence the ballistic efficiency of the
ceramic based
10 armor, and that "dwell" type defeat of penetrators can be achieved on
the ceramic front
surfaces. Two key parameters here are suppression of cracked tile expansion
and putting
the ceramic in an initial state of high compressive stress to delay or stop it
from going
into a state of tensile stress during impact. Tensile stresses are the cause
of the
premature failure in ceramic components, since in general ceramic have higher
strength
15 in compression than in tension.
The advantage of such compressive stresses on ceramic component is two-fold.
First, the
ceramic material will have a higher tensile strength and will be more
effective in
defeating the projectile, as the projectile will spend more time (and more
energy) before
it causes the ceramic component to develop cracks and failure. This can allow
the
20 disclosed composite structure to defeat projectiles with minor damage
to the ceramic
component, and therefore will allow the structure to take multiple hits. The
ceramic
component can be preserved due to the relative high elastic strain limit of
cementitious
composites. Even though the effectiveness of the system will be reduced (in
the case of
formation of cracks in ceramic), the remaining compressive stresses will
maintain some
25 effectiveness of the ceramic for subsequent hits, and at the minimum
will keep the un-
cracked portion of ceramic in place to defeat the projectile and dissipate its
energy.
Yiwang Bao etal (materials letters December 2002) reported substantial
enhancement in
projectile penetration resistance in encapsulated and pre-stressed ceramic
material, and
experimental results by Holmquist and Johnson (EDP Sciences 2003) also shows
that pre-
stressed ceramics does improve performance. Other sources shows that three
dimensional stressing of ceramic provides a higher enhancement in the
penetration
resistance than a two dimensional stressing. There is a clear need to improve
the impact
and penetration resistance, ballistic efficiency and structural integrity of
ceramic armor
CA 02881271 2015-02-05
26
employed on a widespread basis in many types of armor systems. Over about the
past
twenty years, it has been discovered that the ballistic performance of ceramic
armor is
critically dependent on the specific design attributes and geometrical
configuration of
the entire armor system. In particular, it has been observed that enhanced
destruction
and fragmentation of an incoming projectile can be obtained by increasing the
so-called
"dwell" time of the projectile on the front face of the ceramic armor during
the very early
stages (the first 5-10 microseconds) of the ballistic impact event.
"Dwell," the duration of projectile erosion without target penetration, is an
indicator of
the ceramic's ballistic efficiency. Strategies to prolonging projectile dwell
on ceramics
include retarding damage and retaining dynamic toughness in the damaged state.
Cdramic failure caused by excessive structural bending from ballistic loading
is another
possible limiting factor to improving ballistic performance. Improving ceramic
armor can
be obtained with improved structural support for the ceramics. Desirable
attributes of a
backing material include shock mitigation and high stiffness to resist
bending.
In general, the longer the dwell time on the front face of the ceramic armor,
the more
completely the projectile can be attenuated and fragmented. Enhanced dwell
time on
the front face of the ceramic armor leads to a phenomenon that is called
interface
defeat, wherein the projectile face mushrooms radially outward without
significant
penetration in the thickness direction; this increases the projectile frontal
area and thus
decreases its subsequent ability to penetrate the (ceramic) armor (FIG. 12).
If the
interface defeat is not sufficient, there will be an initial dwell and
subsequent
penetration into the armor (FIG. 11).
The phenomenon of dwell is used to particular advantage in medium or heavy
ceramic
armor systems that are intended to defeat larger caliber high kinetic energy
projectiles
(12.7mm HMG and above). It has been found that physical encapsulation of
ceramics
such as B4C, SiC or TiB2 increases the dwell time and delays the lateral and
axial spreading
of the comminuted zone ahead of the projectile, thus increasing the ballistic
efficiency of
the ceramic.
There are several advantages to using FRCC as a surrounding material for
ceramic
material. One is the relative high "yield" strength of FRCC that can be
utilized to constrain
the ceramic material very effectively and impede the material's
disintegration. When the
armor package takes a hit, the ceramic material will tend to fracture and
dimensionally
expand due to opening cracks. In this situation, the surrounding FRCC will be
forced to
CA 02881271 2015-02-05
27
stretch out and the material's resistance to yielding will be an important
factor in
impeding the disintegration of the ceramic material. This constrain of the
ceramic
material when hit by an incoming projectile results in:
= Constraints of material to prevent material "flee" from the impact zone
= Improved hardness of
ceramic to flatten the tip of projectile at the initial
stage of impact
= Transfer of impact force to surrounding and supporting materials
= Small damage zone
= Other aspects to defeat projectile by involving more materials in the
impact
zone
Terminology
Brittle A material is called brittle if it loose its tensile strength
immediately after first
cracking under uniaxial tension and is no longer able to resist any stress.
Cementitious materials The binding component of fiber reinforced cementitious
composites (FRCC). These are: cement, mortar or concrete.
Composite materials (or composites for short) Engineered materials made from
two or
more constituent materials with significantly different physical or chemical
properties
which remain separate and distinct on a macroscopic level within the finished
structure.
Concrete A construction material composed of cement (commonly Portland cement)
as
well as other materials such as fly ash and slag cement, aggregate (generally
a coarse
aggregate such as gravel, limestone, or granite, plus a fine aggregate such as
sand),
water, and chemical admixtures. It is known as normal concrete, but also
called normal
weight concrete or normal strength concrete.
DFRCC Stands for a subclass of FRCCs called ductile fiber reinforced
cementitious
composites which are hardening under flexural conditions (deflection-
hardening) but not
strain-hardening in direct tension (see also HPFRCC). The expression ductile
emphasizes
the fact that these composites exhibit multiple cracking which can be
considered to be a
form of ductility.
Ductile A ductile material is a material that does not fail immediately under
uniaxial
tension after reaching its first cracking strength. It first enters a strain-
hardening phase,
which is then followed by a crack opening phase and localization of failure.
CA 02881271 2015-02-05
28
FRCC Stands for fiber reinforced cementitious composites and describes
cementitious
materials that are reinforced by randomly oriented short fibers.
Glass An amorphous (non-crystalline) solid material. Glasses are typically
brittle and
optically transparent. In science, however, the term glass is usually defined
in a much
wider sense, including every solid that possesses a non-crystalline (i.e.,
amorphous)
structure and that exhibits a glass transition when heated towards the liquid
state. In this
wider sense, glasses can be made of quite different classes of materials:
metallic alloys,
ionic melts, aqueous solutions, molecular liquids, and polymers. Polymer
glasses (acrylic
glass, polycarbonate, polyethylene terephthalate) are a lighter alternative to
traditional
silica glasses.
Glass-ceramic Materials that share many properties with both non-crystalline
glass and
crystalline ceramics. They are formed as glass, and then partially
crystallized by heat
treatment.
HPFRCC Stands for high performance fiber reinforced cementitious composite. It
delimits
a subclass of FRCCs which are strain-hardening in direct tension. It is also
called pseudo
strain-hardening or quasi strain-hardening. Strain-hardening refers to a true
material
property and should not be confounded with hardening due to a redistribution
of
internal stresses such as within the cross-section of a beam (referred to as
deflection-
hardening).
Localized Crack A localized crack is a crack at which the damage accumulates
and where
deformations start concentrating. It should be characterized by the crack
opening
displacement rather than a strain since the latter is gauge-dependent.
Multiple Cracking Means that a FRCC is capable of arresting the further
opening of cracks
by fiber bridging action and by consequence new cracks tend to form in the
close vicinity.
This is a fundamental property of HPFRCCs.
Cluasi-Brittle The expression quasi-brittle describes a material that starts
softening di-
rectly after first cracking under uniaxial tension. However, quasi-brittle
materials are still
capable of transferring some reduced amount of stress which gradually
decreases with
increasing crack opening.
Strain hardening / pseudo strain hardening Strain hardening describes a
phenomenon
that, under uniaxial tension, transmitted tensile stress increases
successively even after
first cracking, with continued tensile straining. The term "pseudo strain
hardening" is
sometimes used instead, since the strain hardening mechanism of DFRCC is
different
CA 02881271 2015-02-05
29
from that of metallic materials. During strain hardening/pseudo strain
hardening, the
stress-strain curve is uniquely defined, and is a true material property.
Strain softening Strain softening describes a phenomenon that, under uniaxial
tension,
transmitted tensile stress decreases upon first cracking or after strain
hardening.
Structure A structure generally relates to the way elements are organized in
relation to
each other and relative to the whole they are in. This can be physical,
spatial or
systematically.
Tension toughness, compression toughness, flexure toughness Toughness
describes
energy absorption which is given by the area below stress-strain curve or load-
displacement curve either in tension, compression, or flexure. In practice,
toughness is
calculated based on the area up to a prescribed strain or displacement.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings and photos, which are incorporated in and form a
part of
this specification, illustrates the technical background and embodiments of
the present
disclosure, and together with the description, serve to explain by way of
example only,
the principles of the present disclosure:
FIG 1 Illustrates the different compositions between normal strength concrete
and UHPC;
FIG 2 A photo of a high strength - high ductility concrete (HSHDC) plate under
bending;
FIG 3 Illustrates characteristics of cementitious materials: Definition of A:
brittle, B: quasi
brittle, and C: ductile behavior as well as strain softening and strain
hardening under
uniaxial tensile loading;
FIG 4 Illustrates some different high performance fiber reinforced
cementitious materials
and their classification;
FIG 5 Illustrates flexural performances of beam specimens made of different
fiber
reinforced cementitious materials;
FIG 6 Illustrates multiple cracking pattern of PVA-ECC under uniaxial tension;
FIG 7 Photos of polyethylene (PE) fibers and polyvinyl alcohol (PVA) fibers
used in ECC;
FIG 8 Illustrates the composition of a typical ECC formulation and a concrete
formulation,
showing weight percent;
FIG 9 Illustrates and compares mechanical properties of HSHDC with some other
fiber
reinforced cementitious materials;
CA 02881271 2015-02-05
FIG 10 A photo of typical profiles of steel fibers commonly used in some fiber
reinforced
concretes;
FIG 11 A photo of the sequence of three flash X-ray radiographs showing the
initial dwell
of a penetrator and subsequent penetration into thick ceramic target;
5 FIG 12 A photo of the sequence of three flash X-ray radiographs
showing complete dwell
of a penetrator on a thick ceramic target;
FIG 13 Photo collage of different forms and sizes of armor-grade ceramics,
which can be
used in the present disclosure;
FIG 14 A schematic cross-sectional perspective view of a first exemplary
embodiment of
10 the construction of a lightweight composite armor according to the
present disclosure;
FIG 15 A schematic cross-sectional perspective view of a second exemplary
embodiment
of the construction of a lightweight composite armor according to the present
disclosure;
FIG 16 A schematic cross-sectional perspective view of a third exemplary
embodiment of
the construction of a lightweight composite armor according to the present
disclosure;
15 FIG 17 A schematic cross-sectional perspective view of a fourth
exemplary embodiment
of.the construction of a lightweight composite armor according to the present
disclosure;
FIG 18 A schematic cross-sectional perspective view of a fifth exemplary
embodiment of
the construction of a lightweight composite armor according to the present
disclosure;
FIG 19 A schematic cross-sectional perspective front view of a sixth exemplary
20 embodiment of the construction of a lightweight composite armor
according to the
present disclosure;
FIG 20 A schematic cross-sectional perspective back view of a sixth exemplary
embodiment of the construction of a lightweight composite armor according to
the
present disclosure;
DETAILED DESCRIPTION OF THE INVENTION
With reference now to the figures of certain preferred embodiments in detail,
it is
stressed that the particulars shown are by way of example and for purposes of
illustrative
discussion of the preferred embodiments of the present disclosure only, and
are
presented in the cause of providing what is believed to be the most useful and
readily
understood description of the principles and conceptual aspects of the present
disclosure. In this regard, no attempt is made to show structural details of
the present
CA 02881271 2015-02-05
31
disclosure in more detail than is necessary for a fundamental understanding of
the
present disclosure. Only a few examples of how the composite armor structure
can be
configured in different shapes, sizes and thicknesses, and in different
configurations, and
how the encapsulated armor-grade material can be configured and arranged in
different
spacious patterns are shown here.
The description taken with the drawings makes it apparent to those skilled in
the art how
the several forms of the present disclosure can be embodied in practice.
Referring to FIG 14, a cross-sectional perspective view of an embodiment of a
ballistic
structure showing an encapsulant (1) confining an armor-grade material (2) for
absorbing
and limiting the transfer of impact energy from a ballistic threat, such as a
kinetic energy
projectile.
The encapsulant is fabricated from a fiber reinforced cementitious composite
which
preferably has a greater tensile strength than the tensile strength of the
encapsulated
material.
The encapsulated material is preferably comprised of armor-grade ceramic
material. But
it must be understood that the principles of the present disclosure are
applicable to any
armor-grade materials such as glass, glass-ceramics, sintered refractory
material, other
armor-grade materials having high hardness or mixtures thereof.
The encapsulating structural layer (1) is configured to encapsulate the armor-
grade
material (2). In one embodiment, the encapsulating structural layer (1) pre-
stresses the
encapsulated material (2). Without pre-stress, at least simple mechanical
contact or
binding is needed.
Referring to FIG 15, in another embodiment of the present disclosure, the
encapsulating
layer of fiber reinforced cementitious composite and the encapsulated armor-
grade
material can be layered in a laminated structure, where the alternating layers
of armor-
grcle material and fiber reinforced cementitious composite are composed as
shown.
Referring to FIG 16, in another embodiment of the present disclosure, the
encapsulated
armor-grade material are formed as spheres and are all held and fully encased
with an
encapsulating layer of fiber reinforced cementitious composite. The gaps
between
adjacent armor-grade spheres are made to be small enough for avoiding the
creation of a
weak point and stopping an anticipated projectile between the spheres.
Referring to FIG 17, in another embodiment of the present disclosure, the
layer of fiber
reinforced cementitious composite (1) encapsulates multiple tiles of armor-
grade
CA 02881271 2015-02-05
32
material (2); in this example four tiles are used.
Referring to FIG 18, in another embodiment of the present disclosure, two
separate
layers of fiber reinforced cementitious composite (2) and (3) encapsulates
tiles of armor-
grade material (4). Fastening elements (1) are used to obtain that the
encapsulation
layers of fiber reinforced cementitious composite provides pre-stress to the
encapsulated
armor-grade material.
Referring to FIG 19, in another embodiment of the present disclosure, two
separate
layers of fiber reinforced cementitious composite (1) and (2) and an
additional layer of
fiber reinforced cementitious composite (3) encapsulates armor-grade material.
Fastening elements (4) are used to obtain that the encapsulation layers of
fiber
reinforced cementitious composite (1) and (2) provides pre-stress to the
encapsulated
armor-grade material. The additional layer of fiber reinforced cementitious
composite (3)
are configured to provide pre-stress to the encapsulated armor-grade material.
Referring to FIG 20, is the same embodiment of the present disclosure as
referred to in
figure 19, but is here shown in a cross-sectional perspective back view.
In any of the above embodiments of the present disclosure shown in FIG 14 to
FIG 20 the
thickness of the encapsulating layer can be varied as well as the dimensions
of the
encapsulated material can be varied.
Often panels of the present disclosure will be generally flat and with
generally uniform
thickness. For the purpose of constructing the panel, the front face is that
which will face
the direction from which the ballistic impact is expected, and the other is
the back face.
Likewise, the overall dimensions and the overall shape of the panels of the
present
disclosure will be determined by end user requirements, such as the impact
conditions
which they are required to resist, and the size and/or area of the object
which the panel
or an assembly of the panels is required to protect. For more specialized end
user
requirements, a panel can be shaped in mostly any form of curvature. Whatever
its
overall shape, the fact that it is a panel implies that its thickness will be
smaller than its
other dimensions, e.g. its length and width, and it will have two faces
separated by its
thickness.
The shapes shown in in FIG 14 to FIG 20 are by way of example only. Other
polygonal
shapes can be used, such as cylinders and special shaped pellets. In addition,
the shape
of the tiles shown in FIG 14 and FIG 15 need not be a regular geometric shape.
The tile
can have any shape needed for a particular application, such as triangles,
squares,
CA 02881271 2015-02-05
33
rectangles, hexagons or combinations of polygons thereof, which nest to give
complete
coverage in one layer. In another configuration, polygon shaped tiles or
combinations
thereof are to be used in a first layer, and any gaps in the first layer are
protected by a
second layer to obtain complete coverage. It is most often desired to achieve
complete
coverage in one layer. Often used tile shapes used for this are square and
hexagonal.
While the present disclosure has been described with reference to certain
preferred
embodiments, numerous changes, alterations and modifications to the described
embodiments are possible without departing from the spirit and scope of the
present
disclosure, as defined in the appended claims and equivalents thereof.
15
25
=
