FIELD OF THE INVENTION
The present invention relates to couplers used for joining reinforcing rebars. The
invention further relates to a process of joining rebars during constructional
phases.
BACKGROUND OF THE INVENTION
The existing couplers are generally manufactured from medium carbon low alloy
steel, hot rolled plane round bars, forged bars or from tubes of different
diameters. The known manufacturing process of the couplers is illustrated in the
flow chart of figure 1.
There are a number of patent references such as US Patent No. 4143986,
US4469465, US 4666326 that discuss about the above mentioned known
processes. The major drawbacks with the existing processes are as follows:
a. The outer surface of the couplers are either plane or without any sufficient
protrusions/ribs. This leads to lesser bond strength with the concrete
material in the coupled zone.
b. In BIS Standard 1786:2008 for "High Strength Deformed Steel Bars and
wires for Concrete Reinforcement Specification", the carbon content of the
rebars used in construction have been restricted to 0.25 max and 0.30
max for Fe 500/500D and Fe 600 grades, respectively. The existing
couplers have carbon generally in the range 0.4-0.45%. When such
couplers are fitted in the rebars having an altogether different chemistry,

there is a greater chance of galvanic cell formation leading to accelerated
corrosion and possibility of failure of the structural component at junction
points or developing structural deficiencies with the passage of time.
c. Further, the Ultimate tensile strength of the existing coupler material e.g.
(EN8D/EN9) used is low (about 670-690 N/mm2) which seems to be
inadequate for joining high strength rebars (having UTS>740).
In light of the above mentioned limitations, there is need of developing
couplers which have Ultimate tensile strength to join high strength rebars
and also have better adhesion/bonding with concrete material.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved coupler for
joining reinforcement bars, which reduces galvanic corrosion in the rebar-coupler
joint portions.
Another object of the invention is to propose an improved coupler for joining
reinforcement bars, which increases bond strength of the couplers with concrete
material.
A further object of the invention is to propose an improved coupler for joining
reinforcement bars, which has increased tensile strength to allow joining of high
strength rebars (UTS>740 MPa).
A still further object of the invention to propose a process to produce an
improved coupler for joining high strength rebars.

SUMMARY OF THE INVENTION
Accordingly in one aspect of the invention, there is provided an improved coupler
for joining high strength reinforcement bars. The couplers of the invention has
similar chemistry to that of the joinable rebars hence, the possibility of galvanic
corrosion at the joints is eliminated. Further the couplers of the present invention
are rolled to form special groove/protrusion on the outer diameter so that there
is an improvement in the bond strength. This hot rolled material is heat treated
to impart high strength to this invented couplers and is thus become capable to
join high strength rebars (UTS>740 MPa).
In a second aspect of the invention, there is provided a process to produce an
improved coupler for joining high strength rebars.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Shows a process chart of manufacturing prior art coupler.
Figure 2 Shows vertical and cross sectional views of the coupler of the
invention.
Figure 3 Shows the coupler and the rebar of the invention, in disassembled
Condition.
Figure 4 Shows the coupler and the rebar of the invention in assembled
condition.
Figure 5 Shows a cross sectional view of the coupler and rebar assembly of
the invention.

Figure 6(a) Shows a process flow chart for producing an improved coupler of
the invention.
Figure 6(b) Shows an alternative process flow chart for producing an improved
coupler of the invention.
Figure 7 Shows a comparative results in respective of corrosiveness data
between the inventive coupler and prior art coupler collected
respectively after 6 days and 9 days.
Detailed Description of the invention
Couplers as per the invention have the following chemistry:
Carbon 0.40% (max)
Sulphur 0.40% (max)
Phosphorous 0.40% (max)
Manganese 0.50-1.50%
Vanadium 0.065% (max)
The above chemistry is substantially similar to the rebar chemistry and provide
the following mechanical properties :
a) Yield Strength - 400 MPa (min.)
b) Ultimate Tensile Strength - 500 MPa (min.)
c) Percentage Elongation - 10% (min.)
As shown in figure 2, U is the length of the inventive Coupler, L2 is the diameter,
Dl is the major diameter and PI is the pitch of the thread.

Each coupler is a single unit. The couplers are threaded from inside and profiled
from outside. The outer protrusion help to form a firm grip with the concrete
material whereas the inner threads form a grip with the threads of the rebars to
be joined together. The outer diameter and the length of the coupler is decided
based on the rebars' diameters to be joined. Figures 2,3,4,5 illustrate the
construction of the proposed coupler.
As shown in figure 6(a), and 6(b), there is provided a process to produce an
improved coupler for joining high strength rebars. The couplers manufactured by
the alternative processes of the invention possess desired properties. The
coupler produced by the processes and chemistry as specified hereinabove were
tested for various mechanical properties such as Yield strength (YS) and Ultimate
tensile strength (UTS). Two different sizes of the couplers corresponding to 20
and 25mm mother rebar were tested. Table 1 stipulates the mechanical
properties of coupled splices of the coupler of the present invention.

Table 1: Mechanical Properties of coupled splices as per the current invention.
The table above shows that all mechanical splice successfully satisfy the
requirement of Indian standard for reinforcement - IS 1786.

Further, the couplers of the present invention were tested vis-a-vis conventional
couplers (EN8D couplers) for corrosion resistance. The corrosion resistance of
the current rebar coupler was tested in Carbonated and chlorinated simulated
pore solution (condition leading to severe corrosion in concrete). Figure 7 shows
a comparison in respect of corrosiveness of the couplers of the present invention
vis-a-vis the conventional couplers, wherein the data were collected after 6 days
and 9 days. The figure shows that the invented coupler perform much better
than the conventional (EN8D) coupler in harsh corrosive environment.
Further, the inventive couplers have better splice strength than the conventional
couplers. The coupler of the present invention provides better adhesion/bonding
to the concrete due to the profiled protrusion provided on the outside diameter
of the coupler.

WE CLAIM :
1. An improved coupler for joining high strength reinforcement bars;
comprising an elongated component having a threaded portion in the
inside diameter, and protrusions/ribs in the outside diameter produced by
rolling, and wherein the mother rebars are cold upsetted and having
thread in the outer diameter, and wherein the mother rebars are joined
along the longitudinal axis.
2. A process to produce an improved coupler for joining high strength
rebars, comprising the steps of :-
producing a billet in a known steel making process and casting thereof;
reheating the billet in a known furnace upto a soaking zone;
rolling the reheated billet in a rolling mill upto a desired finish diameter
with a rib profile;
quenching the rolled rod under adjusted temperature after a distance of
20-25 meters, the adjusted temperature being between 540°C to 660°C;
cooling the rod in a cooling bed and cutting the rod to required length;
and
machining of the quenched and tempered rod by cutting, drilling, and
tapping or threading to produce the coupler.
3. The process as claimed in claim 2, wherein the rolled rod is soft quenched
in a water box and wherein the core heat is used to temper the quenched
surface of the rod.
4. The process as claimed in claim 3, wherein the finish rolling temperature
is between 700°C -1000°C.

5. The process as claimed in claim 2 or claim 3, wherein the billets are
reheated to a temperature between 1000°C to 1250°C.

ABSTRACT

The invention relates to an improved coupler for joining reinforced bar. The
couplers of the invention possess almost the similar chemistry as that of
reinforced bars to be joined and are profiled from outside as well. This improved
coupler exhibits better corrosion resistance (especially at coupler-rebar joint)
than conventional coupler and have better bond strength with concrete. The
invention further discloses alternative processes for producing improved coupler
for joining rebars.