Note: Descriptions are shown in the official language in which they were submitted.
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FIBER REINFORCED REBAR N
The present invention relates a method for manufacture of fiber
reinforced reinforcing bar or "rebar".
The term "rebar" as used herein is intended to include bars and rods
which are hollow, that is tubing. The outside surface is preferably but not
necessarily of circular cross section. The rods can be of any length including
elements which are relatively short so that they are sometimes referred to as
"bolts".
BACKGROUND OF THE INVENTION
The use of fiber reinforced plastics (FRP) rods in construction, marine,
mining and others has been increasing for years. This is because FRP has many
benefits, such as non-(chemical or saltwater) corroding, non-metallic (or non-
magnetic) and non-conductive, about twice to three times tensile strength and
1/4
weight of steel reinforcing rod, a co-efficient of thermal expansion more
compatible
with concrete or rock than steel rod. Most of the bars are often produced by
pultrusion process and have a linear or uniform profile. Conventional
pultrusion
process involves drawing a bundle of reinforcing material (e.g., fibers or
fiber
filaments) from a source thereof, wetting the fibers and impregnating them
(preferably with a thermo-settable polymer resin) by passing the reinforcing
material
through a resin bath in an open tank, pulling the resin-wetted and impregnated
bundle through a shaping die to align the fiber bundle and to manipulate it
into the
proper cross sectional configuration, and curing the resin in a mold while
maintaining
tension on the filaments. Because the fibers progress completely through the
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pultrusion process without being cut or chopped, the resulting products
generally
have exceptionally high tensile strength in the longitudinal direction (i.e.,
in the
direction the fiber filaments are pulled). Exemplary pultrusion techniques are
described in U.S. Patent No. 3,793,108 to Goldsworthy; 4,394,338 to Fuwa;
4,445,957 to Harvey; and 5,174,844 to Tong.
FRP uniform profile or linear rods offer several advantages in many
industrial applications. The rods are corrosion resistant, and have high
tensile
strength and weight reduction. In the past, threaded steel rods or bolts had
been
widely used in engineering practice. However, long-term observations in Sweden
of
steel bolts grouted with mortar have shown that the quality of the grouting
material
was insufficient in 50% of the objects and more bolts have suffered from
severe
corrosion (see reference Hans K.Helfrich). In contrast with the steel bolts,
the FRP
bolts are corrosion resistant and can be simultaneously used in the temporary
support and the final lining, and the construction costs of single lining
tunnels with
FRP rock bolts are 33% to 50% lower than of tunnels with traditional in-site
concrete
(see reference Amberg Ingenieurburo AG, Zurich). This FRP rock bolting system
is
durable and as a part of the final lining supports a structure during its
whole life
span. Furthermore, due to their seawater corrosion resistance, the FRP bolts
and
anchors are also proven as good solutions in waterfront (e.g., on-shore or off-
shore
seawalls) to reinforce the concrete structures. In general the fibreglass
rod/bolt'is
already an important niche, and will be a more important product to the mining
and
construction industries. The critical needs of these industries are for
structural
reinforcements that provide long-term reliability that is of cost-effective.
The savings
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in repair and maintenance to these industries will be significant, as the
composite
rebar will last almost indefinitely.
The mining industry requires composite rods for mining shafts or tunnel
roof bolts. These rods are usually carried by hand and installed overhead in
mining
tunnel, so there is a benefit that the fibreglass rod is 1/4 the weight and
twice the
strength of steel rebar which are widely used currently. Fibreglass rod also
does not
damage the mining equipment. In construction industries, such as bridges,
roads,
seawall and building structures, reinforcements of the steel rebar have been
widely
used and the most of steel rebars have been corroded after a few years of
service
life. Typically, the structures with the steel rebars are often torn down
after a period
of time. Therefore the use of the corrosion resistant composite rebars have
been
increased for construction industries in recent years.
Non-uniform profile or non linear threaded rods are also required in
many industrial applications. For example, threaded FRP rods and associated
nuts
have been used as rock bolting system in mining industries (e.g., for tunnel
roof
bolts), as threaded reinforcing rebar structures in construction industries
(e.g., in
bridge construction), as well as seawall bolting system in marine structures.
The structures of the threaded composite rods from existing
manufacturing technology consist of two styles:
(1) Pultruded rod with machined threads in outside surface, and
(2) Pultruded rod has a core of fiber rovings with plastic materials
molded outside the core to form threads.
In style (1), the problem of machining composite rebar surface aftetit
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is fully cured is that the fibers in a depth of surface are cut into segments.
The
benefit of high tensile strength of the fibers are lost when they are cut into
short
lengths. The strength of the threads now rely on the shear strength of the
cured
resin which is much less than that of the fibers. Thus, the rebar could not be
used
under tension since the threads of the rebar will shear away from the core.
The
rebar uses a specially designed nut that compresses against the rebar to give
it
holding strength when a load is placed on the rebar. The nut threaded onto the
rebar has just enough resistance to take up any slack between the nut and the
thread surface. Therefore the nut is used without pre-tension.
In style (2), the rebar has a core of fiber glass rovings and a plastics
molded threads surface. This rebar is only capable of withstanding a small
amount
of longitudinal loads. This is because the threads formed by the molded
plastics
lack the fiber glass reinforcements for having the longitudinal strength.
Other rebars,
such as those shown in a brochure by Marshall fndustries Composites Inc C-BAR
1996, are a combination of a fiber-reinforced polyester core and a urethane-
modified
vinyl ester outer skin, which do not include the thread features in rebar
surface.
There is therefore a need in mining, construction and other industries
for composite rod and nut fastening system that the rod and nut have a ful.ly
threaded feature without the disadvantages of the style (1) and (2) described
in the
paragraph above.
SUMMARY OF THE 1NVENTION
lt is one object of the present invention to provide a novel reinforcing
bar formed from fiber reinforced resin.
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According to a first aspect of the invention there is provided a
reinforcing bar comprising:
a series of inner rovings of reinforcing fibers arranged longitudinal to
the bar;
5 a first helical wrapping or wrappings of at least one roving wrapped
around the inner rovings in a first direction of wrapping;
a second helical wrapping or wrappings of at least one roving wrapped
around the inner rovings in a second opposed direction of wrapping;
a thermoset resin permeated through both the inner rovings and
through the wrappings to form a structure integrated by the permeated
thermoset
resin;
the bar having an outer surface portion which extends along at least
most of the length of the bar;
at the outer surface portion, the inner rovings having parts thereof
between the first and second wrapping or wrappings exposed and bulged
outwardly
by tension applied by the wrapping or wrappings during curing;
the bulged parts defining components of the outer surface portion of
the bar which are thus rough and exposed for engaging a material to be
reinforced
so as to transfer longitudinal loads between the material to be reinforced and
the
inner rovings.
Preferably the resin is exposed on the outside surfaces of the inner
rovings and the wrapped rovings.
Preferably the outside surface portion is free from bonded exterior
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roughening elements attached onto the outside surface of the resin.
Preferably, at the outer surface portion, the resin is cured while the
inner and wrapped rovings are free from external pressure such that the shape
of
the outer surface is defined solely by the shape of the inner and wrapped
rovings as
the resin is cured.
Preferably the bar includes at least one additional outer surface portion
where the resin and the inner rovings and the wrapped rovings are compressed
to
form a polygonal cross section for engaging a correspondingly shaped chuck by
which the bar can be rotated about a longitudinal axis of the bar.
According to a second aspect of the invention there is provided a
reinforcing bar comprising:
a series of inner rovings of reinforcing fibers arranged longitudinal to
the bar;
a first helical wrapping or wrappings of at least one roving wrapped
around the inner rovings in a first direction of wrapping;
a second helical wrapping or wrappings of at least one roving wrapped
around the inner rovings in a second opposed direction of wrapping;
a resin permeated through both the inner rovings and through the
wrappings to form a structure integrated by the permeated resin;
the bar having an outer surface portion which extends along at least
most of the length of the bar;
at the outer surface portion, the inner rovings having parts thereof
between the first and second wrapping or wrappings exposed and bulged
outwardly
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by tension applied by the wrapping or wrappings during curing;
the bulged parts defining components of the outer surface portion of
the bar which are thus rough and exposed for engaging a material to be
reinforced
so as to transfer longitudinal loads between the material to be reinforced and
the
inner rovings;
wherein, at the outer surface portion, the resin is cured while the inner
and wrapped rovings are free from external pressure such that the shape of the
outer surface is defined solely by the shape of the inner and wrapped rovings
as the
resin is cured.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side elevational view of a portion of a reinforcing bar
according to the present invention.
Figure 2 is a cross sectional view along the lines 2-2 of Figure 1.
Figure 3 is a cross sectional view similar to that of Figure 2 on an
enlarged scale.
Figure 4 is a cross sectional view along the lines 4-4 of Figure 1.
DETAILED DESCRIPTION OF THE INVENTION
In Figure 1 is shown a reinforcing bar generally indicated at 10 which
has a first section 11 extending along most of the length of the bar together
with a
second section 12 which extends a part of the length of the bar. The bar is
generally
formed in continuous construction so that the first and second sections are
repeated
alternately. The fength of the second section generally will comprise only a
short
portion relative to the length of the main section 1 so that for example the
main
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section may be 12 feet long and the second section only 6" long.
The reinforcing bar is formed solely from a resin material 14 which is
permeated through to sections of reinforcing fibers including longitudinal
reinforcing
fibers 15 and wrapping reinforcing fiber 16, 17.
The longitudinal reinforcing fibers 15 constitute the main volume of the
structure so that typically the fiber content may be constituted as
longitudinal fibers
90 to 97% and wrapping fibers 3 to 10 %, where the resin content can be of the
order of 20 to 30 % by weight.
The structure in the area of the portion 11 is formed without any
compression of any of the fibers by a pultrusion process. Thus neither the
inner
core formed by the longitudinal fibers 15 nor the outer wrapping 16 and 17
pass
through a die structure so that they are free to take up their positions as
determined
by the tensions in the material when formed.
The resin may be a two part resin which sets without heat but more
preferably is a thermosetting resin which is heated by any one of a number of
available heating techniques such as microwave heating, forced air heating,
infra-
red heating, RF-heating, or induction heating where at least one metal fiber
is
included in the structure to absorb the electromagnetic energy. Thus the heat
is
applied to the structure to effect curing of the resin without contact by the
heating
device on the structure. In this way the fibers in the first section 11 are
free to take
up their position depending upon their tension and they take up a position
within the
resin so that the resin extends both through the longitudinal fibers and the
wrapping
fibers.
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In order to obtain this situation where the resin 14 extends outwardly to
the outer surface 18 and permeates through all of the fibers, the longitudinal
fibers
and the wrapping fibers are both preferably wetted preferably using a bath or
dipping
process so that the fibers are fully enveloped with the resin prior to entry
into the
forming system generally described above and shown in more detail in the above
US patent of the present inventor, the disclosure which can be referred to for
further
details.
The wetting of the fibers ensures that the resin permeates through the
whole structure of the outside surface 18.
The absence of any compression by the provision of any form of die
through which the core of longitudinal fibers passes ensures that the wrapping
fibers
16 and 17 apply pressure onto those parts of the longitudinal fibers which are
contacted by the wrapping fibers squeezing those longitudinal fibers inwardly
and
causing bulging of the longitudinal fibers in the sections 19. Thus between
each
wrapped strip of fibers there is a portion of the longitudinal fibers which is
squeezed
and bulged outwardly so that it projects to a position which is preferably
slightly
proud of the outside surface of the wrapping fibers.
The wrapping fibers are of course spaced in the longitudinal direction
by a helical wrapping action so that the width of the wrapping fibers is less
than the
width of the bulged intermediate sections 19.
Typically the wrapping fibers in each direction can be spaced of the
order of 1 to 3 to the inch. However a wider or lesser spacing may be used
provided
the longitudinal fiber are properly controlled and provided there is enough
space to
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ensure bulging between the wraps.
The wrapping fibers may be wrapped as a single roving in a single
start wrapping process or as multiple rovings applied in a multi-start
wrapping
process. In such a multi start process the number of rovings side by side may
be in
5 the range 3 to 10. The number of rovings or the thickness of the roving at
the
wrapping position may vary depending on the diameter of the core.
The wrapping action occurs in both directions so that the wrapping
fibers overlap one another as they cross as shown for example at 20. In this
way
the bulged sections are generally diamond shape in front elevation and are
10 squeezed at the top and bottom by the wrapping action of the wrapping
fibers. Thus
the bulging sections 19 are individual and separated by the wrapping fibers
and yet
the longitudinal fibers are properly contained and held into the structure by
the
wrapping at top and bottom of the bulging sections.
The provision of the wrapping or wrappings symmetrically in both
directions tends to contain and locate the inner longitudinal rovings and
maintain
them in the longitudinal direction even when tension is applied. Thus the full
strength of the longitudinal fibers in the longitudinal direction is
maintained and is not
reduced or compromised by any tendency of the longitudinal fibers to twist.
Any
such twisting of the longitudinal fibers can significantly reduce strength by
applying
loads sequentially to different fibers leading to sequential failure. In
addition the
wrappings in opposite directions accommodate torque applied to the rod in both
directions.
The bulging sections 19 are thus presented on the outside surface 18
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for engagement with material within which the bar is embedded. Thus if the
material
to be reinforced is concrete, the concrete sets around the reinforcing bar and
engages the bulging sections 19. Longitudinal loads from the concrete to the
reinforcing bar are therefore transferred to the bulging sections 19 and not
only to
the wrapping section 16 and 17. The wrapping sections because of their angle
to
the longitudinal direction have less ability to accommodate longitudinal
tension than
do the longitudinal fibers which are longitudinal and continuous. Thus
transferring
the loads in the longitudinal direction to the bulged sections 19 ensures that
the
loads are transferred into the longitudinal fibers and avoid transference to
elements
which can be moved longitudinally or stripped from the outside surface 18. The
bulge sections 19 cannot of course move longitudinally since they are part of
longitudinal fibers.
Yet the outside surface thus can be free from additional bonded
projecting elements such as grit or sand which is commonly applied to the
outside
surface of such reinforcing bars.
The fact that the resin is permeated throughout both the longitudinal
fibers and the wrapping fibers to the outside surface 18 ensures that the
wrapping
fibers are bonded effectively into the structure.
The second section 12 is formed periodically along the bar as it is
formed by clamping the portion of the bar within a clamping die. The clamping
die
may move with the structure as it moves forwardly or the movement could be
halted
while the clamping action occurs and the curing occurs in the clamped
position.
Generally the formation of the clamped section occurs before the remainder of
the
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bar moves into the heating section to complete the curing action. The clamping
die
has an inside surface which is shaped to a polygonal shape such as square and
squeezes both the wrapping fibers and the longitudinal fibers to form them
into the
required outer shape 22 as shown in Figure 4. The clamping action squeezes the
fibers together and may reduce the cross sectional area due to squeezing of
the
resin from the structure. The longitudinal fibers extend through the clamp
section
and also the wrapping fibers extend through the clamp section as shown in
Figure 4.
Thus the wrapping fibers in both directions of wrap are clamped into the
structure at
the polygonal second section 12.
As an alternative to the polygonal shape, any other non-circular shape
may be used such as a compressed flat shape.
As a further alternative the rough rebar may be formed with a hole
through the fibers to provide a connection for an anchor.
The second section 12 is thus shaped so that the bar can be grasped
by a chuck or other clamping element so that the bar can be rotated around its
axis
during insulation of the bar in particular circumstances. The wrapping of the
fibers
16 and 17 ensures that rotation at the second section 12 is transmitted into
torque
throughout the length of the bar by those wrapped section 16 and 17.
In one example of use of an arrangement of this type, the bar can be
inserted into a drilled hole in rock in a mining situation and the drilled
hole filled with
a suitable resin. The stirring action in the resin caused by the rotation of
the bar
grasping the second section 12 and rotating the first section 11 causes the
resin to
be spread through the hole around the periphery in an effective stirring
action
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caused by the bulged sections 19. Thus the bar can be bonded into place within
the
drilled hole to act as reinforcement for mining structures at for example the
roof area
of a mine.
In another alternative use of reinforcing bars of this type, a drill tip can
be attached at one section 12 and the bar grasped at another section 12
allowing
the bar to be rotated with the drill tip causing a drilling action driving the
bar directly
into a drilled hole while the bar causes the drilling of the hole. The bar can
then
remain in place and the drill tip selected be of a sufficiently disposable
type so that it
can be discarded within the hole.
Again the direct connection between the polygonal section 12 and the
main portion of the bar caused by the presence of the wrapping fibers 16 and
17
within the resin allows the transfer of loads between the polygonal section
and the
main section 11.
The arrangement described herein has been found to be significantly
advantageous in that it provides an improved embedment strength which is a
factpr
used in calculating parameters for reinforcing bars in concrete. Thus the
shape of
the outer surface (wrappings in both directions, bulging of the longitudinal
strands)
provides a higher degree of attachment with the adhering material (concrete or
epoxy resin). This higher mechanical bond translates into a high embedment
strength.
The arrangement described herein has been found to be significantly
advantageous in that it provides an improved control of crack width.
Measurement
of crack width is another factor used in calculating parameters for
reinforcing bars in
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concrete with the intention of maintaining a low crack width factor. When
designing
for crack control reinforcement, the nature of this product and its high
embedment
strength will allow for a smaller bond dependant co-efficient to be used (for
example,
sand coated bars use 0.8, and a smooth pultruded bar would be higher). A lower
bond dependant co-efficient translates into smaller crack widths, or less
reinforcement required for the same crack width.
