•
ACI
MATERIALS
JOURNAL
TECHNICAL
PAPER
Title no. 88-M26
Bond
of
Epoxy-Coated
Reinforcement:
Bar
Parameters
by Can Chul Choi, Hossain Hadje-Ghaffari, David Darwin, and Steven
L.
McCabe
The
effects
of
coating thickness, deformation pattern, and bar size on
the reduction
in
bond strength between reinforcing bars and concrete
caused by epoxy coating are described. Tests include beam-end and
splice specimens containing
No.5,
6,
8, and
11
bars with average
coating thicknesses ranging
from
3 to
17
mils (0.08 to 0.43 mm).
Three deformation patterns
are
evaluated.
All
bars
are
bottom~cast.
Beam-end specimens have covers
of
two bar diameters, while splice
specimens have covers that depend on bar size and are
less
than 2 bar
diameters.
The results are compared with the splice tests that
were
used to
es-
tablish the epoxy-coated bar provisions
in
the 1989 A
CI
Building
Code and
1989
AASHTO
Bridge Specifications. Epoxy coatings
are
found to reduce bond strength significantly, but the extent
of
the
re-
duction
is
less than that used to select the development length modi-
fication
factors
in the
ACI
Building Code
and
AASHTO
Bridge
Specifications. Coating thickness has little effect on the amount
of
bond strength reduction
for
No. 6 bars and larger. However, the
thicker the coating, the greater the reduction
in
bond strength
for
No.
5 bars. In general, the reduction
in
bondstrength caused by
an
epoxy
coating increases with bar size. The magnitude
of
the reduction
de-
pends on the deformation pattern; bars with relatively larger rib-
bearing areas with respect to the bar cross section
are
affected
less
by
the coating than bars with smaller bearing
areas.
This
is
the first
in
a
series
of
papers concerning
bond
of
epoxy-coated reinforcement.
Subsequent papers will address the effects
of
concrete cover, bar po-
sition, concrete strength, and transverse reinforcement.
Keywords: bond (concrete to reinforcement); coatings; deformed reinforce-
ment; epoxy resins: lap connections; pullout tests; reinforcing steels; splicing;
structural engineering.
Epoxy-coated
reinforcing steel has been
in
general
use
for
about
15
years. Its
application
to
reduce
the
cOirosion
of
reinforcing
steel is increasing each year.
While epoxy coating protects the steel, it also reduces
the
bond
between the steel
and
concrete. The reduction
in
bond
strength has been demonstrated in two princi-
pal studies.
Using beam-end specimens containing transverse re-
inforcement,
Johnston
and
Zia
1
observed a
15
percent
reduction in
bond
strength with the use
of
epoxy-coated
bars. Using splices
without
transverse reinforcement,
Treece
and
Jirsa
2
reported
an
average reduction
of
34
percent.
Largely
based
on
the
recommendations
of
Treece
and
Jirsa,
ACI
Committee· 318
3
adopted modi-
fication factors
to
increase the development length for
ACI
Materials Journal I March-April
1991
epoxy-coated bars. The factor is 1.5 (a 50 percent in-
crease) for bars with cover less
than
3
bar
diameters
or
with clear spacing between bars less
than
6
bar
diame-
ters.
It
is
1.2 for all
other
conditions.
AASHT04
has
adopted factors
of
1.5
and
1.15 based
on
the same cri-
teria. The new
ACI
and
AASHTO
provisions include
no recognition
of
the effect
of
confining reinforcement
on
the
strength
reduction
obtained
with epoxy
coat-
ings.
The 1.5 modification factor
is
based
on
only
12
speci-
mens with epoxy-coated reinforcement
and
9 specimens
with
uncoated
reinforcement.
A single
deformation
pattern
was evaluated,
and
no
specimens were repli-
cated. Considering the high variability typical
of
bond
tests, it
is
not
clear
that
these few tests provide a relia-
ble picture
of
the effect
of
epoxy coating.
This
is
the first in a series
of
papers
that
describe a
large-scale study
to
determine
the
effect
of
epoxy coat-
ing
on
bond
strength. This
paper
addresses the effects
of
parameters
associated
with
the
bars
themselves:
coating thickness, deformation pattern,
and
bar.size.
It
also considers the effect
of
embedment length
on
the
relative strength
of
coated
and
uncoated bars
to
estab-
lish the suitability
of
the specimen configurations used
in
the
study.
The
overall
study
also considers
the
ef-
fects
of
concrete cover,
bar
position, concrete strength,
and
transverse reinforcement. These topics will be cov-
ered in subsequent papers. The full details
of
this por-
tion
of
the study are presented in Reference 5.
RESEARCH SIGNIFICANCE
Epoxy-coated· reinforcing
bars
are
used
in
concrete
structures where corrosion protection
is
a principal de-
sign
requirement.
The
bars
exhibit
a
lower
bond
strength
to
concrete
than
uncoated
bars.
Considering
the
increasing
application
of
epoxy-coated reinforce-
ment,
the
conservatism
of
current
design provisions,
ACI
Materials Journal,
V.
88,
No.2,
March-April
1991.
Received Apr. 30, 1990, and reviewed under Institute publication policies.
Copyright
© 1991, American Concrete Institute. All rights reserved, including
the making
of
copies unless permission
is
obtained from the copyright propri-
etors. Pertinent discussion will be published in the January-February
1992
ACI
Materials Journal
if
received by Oct.
1,
1991.
207
A
CI
member Oan Chul Choi
is
a graduate research assistant and PhD candi-
date
in
civil engineering at the University
of
Kansas. He obtained his
BS
and
MS
from Seoul National University. He has served
as
a structural designer at
Hyundai Construction Co., Seoul, and taught at
Ulsan
University
in
Korea. His
research interests include the experimental study
of
bond
and finite element
analysis
of
reinforced concrete.
A
CI
member Hossain Hadje-Ghaffari
is
a graduate research assistant and PhD
candidate
in
civil engineering at the University
of
Kansas. He holds a BS
in
ar-
chitectural studies
from
the University
of
Nebraska at Lincoln and
an
MS
in
civil engineering
from
the University
of
Kansas.
p
IITIIITJ
Test Setup
p
ITIIITJI
David Darwin, FACI,
is
Deane E. Ackers Professor
oj
Civil Engineering and
Director
of
the Structural Engineering and Materials Laboratory at the Univer-
sity
of
Kansas. He
is
a member
of
the
ACI
Board
of
Direction and Technical
Activities Committee and is Past President
of
the Kansas Chapter
of
ACI.
He
is
also a member and past-chairman
of
ACI
Committee 224, Cracking. He
is
a
member
of
ACI
Committees
408,
Bond and Development
of
Reinforcement;
446,
Fracture Mechanics; joint
ACI-ASCE
Committees 445, Shear and
Tor-
sion; 447, Finite Element Analysis
of
Reinforced Concrete Structures; and the
Concrete Materials Research Council.
o
2C
s
As
Cast
1.25in.
208
(0)
25.4 mm)
Bar
L
L'
Is
No.
of
s
b
d
h
Cs
Cb
No. (ft.)
(ft.)
(in.)
splices
(in.)
(in.)
(in.) (in.) (in.) (in.)
5
4
4 12
3
6
15.75
14.69
16
2
1
5
4
4 12
2
6
10.5
14.69
16
2
1
6 4 4 12
2
7
11
14.63
16
2
1
8
4 4
16
2
7
12
14
16
2 1.5
11
4.5
6
24
2
6
13.65
13.30
16
2 2
in. (229 mm) wide by
24
in. (610 mm) long.
For
No.
11
bars, the width was increased
to
10
in. (254 mm). Spec-
imen depth was adjusted
to
provide
15
in.
of
concrete
above the bar and 2
bar
diameters
of
cover below the
bar (all bars discussed in this paper were bottom-cast).
Two polyvinyl chloride (PVC) pipes were used as
bond breakers to limit the bonded length
of
the test bar
and prevent a cone-type failure
on
the front face. The
bonded lengths
of
the test bars were selected
to
insure
that
the
bars
did
not
yield
before
bond
failure oc-
curred.
6
Standard bonded lengths
of
3
Y2
in. (89 mm)
for
No.5
bars,
4Y2
in. (114 mm) for
No.6
bars, 8 in.
(203
mm) for No. 8 bars, and 9 in. (229 mm) for No.
11
bars were used. The corresponding lengths
of
bond-
breaking
PVC
pipe
at
the
front
of
the
bars
(lead
lengths) were
2%,
2%,
3%, and 1
Y2
in. (60, 70, 95, and
38
mm), respectively. Additional specimens were tested
to help evaluate the effect
of
epoxy coating as a func-
tion
of
lead length and bonded length. The results for
270 beam-end specimens are summarized in this paper.
The
splice specimens (Fig. 2) consisted
of
simply
supported beams, similar
to
those tested by Treece and
Jirsa.
2
Splice lengths were
12
in.
(305
mm) for
No.5
and 6 bars,
16
in. (406 mm) for
No.8
bars, and
24
in.
(610 mm) for No.
11
bars. Each specimen contained
two
or
three splices in the constant moment region.
Three splices were used for the No. 5 bars.
An
addi-
tional beam with two splices
of
uncoated No. 5 bars
was used
to
evaluate the usefulness
of
double splice
specimens for later tests. The strengths
of
the double
and triple splice specimens were nearly proportional to
ACI Materials Journal I March-April
1991
Fig.
2-Sp/ice
specimens
(1
in.
Support bar
Steel conduit
2 in.
(b)
24
in.
Lead Bonded
length length in.
Test bar
Plywood
form
side
h:
15
in. +
bar
diameter
+
cover
b: 9 in.
for
No.5, No.6 and No.8
bars
10
in.
for
No.11 bars
Fig.
I-(a)
Beam-end specimen dimensions;
(b)
test bar
installation
(1
in. = 25.4 mm)
and the limited
data
upon which those provisions are
based, an improved understanding
of
bond behavior
is
warranted. The goal
is
to improve economy and con-
structibility while maintaining an adequate margin
of
safety.
r4i~
I
15
in.
I
c
======:_:=
=+:~==========:::::J
'"-
J_......I~_2
db
EXPERIMENTAL PROGRAM
Test
specimens
Two types
of
test specimens were tested: beam-end
specimens (Fig.
1)
and splice specimens (Fig. 2). Beam-
end specimens containing
No.5,
6, and 8 bars were 9
A
CI
member Steven L. McCabe
is
an
assistant professor
of
civil engineering at
the University
of
Kansas.
He
is active in research involving reinforced con-
crete, structural analysis/design
for
dynamic loading, and finite element tech-
niques. McCabe
is
a member
of
A
CI
Committees
439,
Steel Reinforcement, and
446,
Fracture Mechanics.
He
also serves on the Board
of
Direction
of
the Kan-
sas Chapter
of
A CI.
Table 1 - Average test bar data
Yield
Bearing Bearing
Bar
Deformation
strength,
Deformation Deformation
Deformation
area
area
size
pattern
ksi spacing, in.
gap, in.
angle, deg
per in.*
ratio in.-
I
*
5 S 70.6 0.423
0.159
"90
0.113 0.361
5
C 72.3 0.413 0.140
60 0.143 0.471
5
N
68.4 0.379 0.158
70
0.166
0.545
6
S
63.8 0.502 0.154 90
0.139
0.320
6
C
70.9 0.467 0.122
60 0.188 0.420
6
N 64.2 0.462
0.151
70
0.201 0.448
8
S
67.0 0.674
0.176 90 0.202 0.256
8
C
t
0.656 0.195 60 0.241
0.305
8
N
63.8
0.602 0.160 70
0.250 0.316
11
S
64.6
0.945 0.217
90
0.313 0.202
11
C 63.1 0.840
0.196 60 0.302 0.196
11
N
64.3 0.914 0.195 70 0.287 0.185
*Bearing area based on closely spaced mesurements
of
ribs; bar areas based on nominal dimensions.
tYield strength
is
greater than 70.0 ksi.
1 in.
= 25.4 mm; 1 ksi = 6.89 MPa, bar sizes:
No.5
=
16
mm,
No.6
=
19
mm,
No.8
=
25
mm, No.
11
=
35
mm.
the number
of
splices. Based
on
this admittedly limited
evidence, double splice beams were used for
No.6,
8,
and
11
bars. Cover was 1 in.
(25
mm) for
No.5
and 6
bars, 1
Y2
in.
(38
mm) for
No.8
bars, and 2 in.
(51
mm)
for No.
11
bars. The clear spacing between splices was
equal
to
4 in. (102 mm) and side cover was equal to 2
in.
(51
mm) for all beams. Additional dimensions and
data are included in Fig. 2. The spliced bars were all
bottom-cast, in contrast to the Treece/Jirsa specimens,
which primarily used top-cast bars. The results for
15
splice specimens are reported in this paper.
Materials
Reinforcing
steel-ASTM
A
615
7
Grade 60,
No.5,
6,
8,
and
11
bars were used. Bars with three deformation
patterns, designated S, C, and N, were tested (Fig. 3).
Bars
of
each size and deformation pattern were from
the same heat
of
steel. Yield strengths and deformation
properties are shown in Table
1.
Epoxy
coatings
were
applied
in
accordance
with
ASTM A 775
8
and
ranged in thickness from 3
to
17
mils (0.08 to 0.43 mm) as measured by a pulloff-type
thickness gage.
8
Readings were taken at 6 points around
the circumference
of
the
bar
between each set
of
ribs
within the bonded length. Average readings within the
bonded lengths are reported. A wide range
in
coating
thickness, beyond the ASTM A 775 limits
[5
to
12
mils
(0.13
to
0.30 mm)], was used to evaluate the effects
of
coating thickness
on
bond
strength.
Concrete-Nonair-entrained concrete with Type I
portland cement
and
% in. (19 mm) nominal maximum
size coarse aggregate was used. Water-cement ratios
from 0.41
to
0.55 were used
to
obtain
concrete with
nominal strengths
of
5000
or
6000 psi (34 to
41
MPa).
Concrete
of
6000 psi
(41
MPa) was used for the major-
ity
of
the
specimens. Mix
proportions
and
concrete
properties are listed in Appendix A
* and Reference
5.
Concrete strengths are listed in Tables 2 and
3.
*The appendixes are available in xerographic.or similar form from ACI
headquarters, where they will be kept permanently on file, at a charge equal to
the cost
of
reproduction plus handling at time
of
request.
ACI
Materials Journal I March-April
1991
Fig.
3-Reinforcing bar deformation patterns
Test procedure
The beam-end specimens were tested using apparatus
developed by Donahey and Darwin
9
and modified by
Brettmann, Darwin,
and
Donahey.6
No.5
and 6 bars
were loaded
at
approximately 3.0 (13.3 kN) kips per
min. No. 8 and
11
bars were tested at about 6.0 kips
(26.7 kN) per min.
Splice specimens were inverted
and
tested as illus-
trated
in Fig. 2. Splices were located within the con-
stant moment region. Crack locations and widths were
recorded during the progress
of
the tests, which lasted
20
to
25
min.
Results and observations
Beam-end
specimens-Test
variables
and
ultimate
bond
forces
of
the
individual
bars
in
the
beam-end
specimens are listed in Appendix B and Reference 5.
Fig. 4 illustrates load-versus-unloaded end slip curves
for No. 5 bars. A splitting-type bond failure occurred
in all tests. Uncoated bars obtained a higher strength
than
bars
with a
nominal
5 mil (0.13 mm)
coating,
which in
turn
had
a greater
bond
strength
than
bars
with a
12
mil (0.30 mm) coating. The initial slope
of
the
load-slip curve decreases as the coating thickness in-
creases. As will be discussed later, only
No.5
bars ex-
hibited a marked sensitivity
to
coating thickness.
209
Table 2 - Beam splices
Average Concrete
Bar stress
Bar Deformation Splice coating strength,
No.
of
Widest
for crack Ultimate Ultimate
Group
no. pattern length, in.
thickness, mils
psi
cracks crack, mils comparison, ksi moment, k-in. stress, ksi C/U*
SP1
5 N
12
0.0 5360 7
9
" 40.9
521
58.7
5
t
N
12
0.0
8
7 42.1
813
61.2
5
t
N
12
9.5
6
7 42.1
609 45.5
0.74
SP2
6 S
12
0.0 6010
6
7 36.7
543
43.2
6
S
12
8.3
3 9
36.7
511
40.6 0.94
6
C
12
0.0
5
5 36.7 610 48.7
6
C
12
8.8
6 5
36.7
466 36.9
0.76
SP3
8
S
16
0.0
5980
6
7
25.9
854
40.1
8
S
16
9.4
4 5
25.9
768
35.9
0.90
8 N
16
0.0
5
9 25.9
858
40.3
8 N
16
9.5 7
7
25.9
737
34.4 0.85
SP4
11
S
24
0.0
5850
5
7 24.0
1459 37.6
11
S 24
9.3
5
9
24.0
1053
26.6
0.71
11
C
24
0.0
7 7 24.0
1372 35.2
11
C
24
10.3 6
10
24.0
1128
28.6
0.81
Mean =
0.82
*C/U
= ratio
of
bond strengths
of
coated to uncoated bars.
tThese beams contained 3 splices.
1 in. = 25.4 mm; 1 mil = 0.001 in. = 0.025 mm; 1
psi=
6.89 kPa; 1 ksi = 6.89 MPa; 1 k-in. = 0.113 N-m.
Table 3(a) - Summary
of
beam-end tests for specimens with standard configuration
Uncoated
Coated
Concrete No.
of
bars No.
of
bars
Bar Deformation
Group strength,
uncoated
bond coated
bond
C/ut
U/U,t
C/U,t
size pattern
no.
psi
bars
force,lb
bars
force, lb
group
all all
5
S
9 5650
3
14,154
6
11,753
0.83
1.01
0.84
5 S
21
5990
3
14,598
6 12,005
0.82
104
0.86
Average
=
0.85
5
C
0.93
5
C
0.93
Average
=
0.91
1.02 0.93
5
N
11
5970
3
12,964
3 11,998
0.93 0.92 0.86
5
N
12
5940
3
14,003
3
12,425
0.89
1.00
0.89
5
N
13
5840
3
13,107
3 11,977
0.91 0.93 0.85
Average
=
13,358
12,133
0.91 0.95 0.87
Average
of
all
No.5
bars§
=
14,021
12,342
0.88 1.00 0.88
*Numerator and denominator based on group average.
tNumerator based on group average. Denominator based on average for three deformation patterns for each bar size; each deformation pattern weighted equally.
§Each
deformation pattern weighted equally.
1 psi = 6.89 kPa; 1
Ib
= 4.45 N.
0.40
0.50
- Uncoated
- - - Coated·
0.30
...
,
,
,
,
,
,
,
0.20
Total Deflection (in.)
0.100.00
01.....---'-_-L.--..L_----1-_.L...----'-_....L..----L_~_.L...--_.L_---.J
-0.10
20000
40000
10000
30000
-0
o
o
..J
20000
15000
-::0
.:::::;,
-0
10000
0
0
..J
5000
OL..L..4...L..L.&.I-L..L...J...J....L....L...L..J.-l...I.-I--L-L-JL....L...J-L....L...L.................L..J....L..I-L..L...J...J....L....L...L..J.-l...I.-I--L-L-JL....L../...JL....L...J-L..J....L.I
-0.001
0.000
0.001
0.002 0.003
0.004 0.005
0.006 0.007
0.008 0.009
0.010
Unloaded
End
Slip
(in.)
Fig.
4-Load-slip
curves forS-pattern
No.5
bars
(1
lb
= 4.45 N; 1
in.
= 25.4 mm)
Fig.
5-Load-deflection curves for S-pattern
No.8
bar
splice specimens
(1
lb
= 4.45 N; 1
in.
= 25.4 mm)
Splice
specimens-The
load-deflection curves (ap-
plied load-versus-center line deflection minus average
load-point deflection) for the splice specimens (Fig.
5)
indicate little difference .in the t
..
esponse
of
the mem-
bers, with the principal exception that epoxy-coated bar
210
specimens consistently failed at a lower load than un-
coated bar specimens.
Crack widths were measured within a region span-
ning
12
in.
(305
mm)
on
either side
of
the splice. The
number
of
cracks and maximum crack widths are sum-
ACI Materials Journal I March-April
1991
0.73
0.83
0.82
0.90
0.89
0.80
0.98
1.06
0.96
1.00
0.74
0.83
0.84
0.90
35,584
31,303
38,827
35,238
41,409
42,365
45,461
43,078
C 0.94
C 0.83
N 0.99
N 0.96
N 5070 - 5270 0.77
N 5260 - 5290 0.79
C 5070 - 5270 0.74
C 5260 - 5290 0.87
Average
=
Average =
Average =
Average
of
all
No.8
bars§
=
8
C
2
5700 1
47,184
3
37,976
0.80
1.10 0.88
8 C
5 5920
3 36,504
9
34,784
0.95
0.85
0.81
8
C
6 5870 2
45,880
2 35,600
0.78 1.07 0.83
8
N 4
6130 3 46,104
3
37,208
0.81 1.07 0.86
8 N
6
5870
2
43,304 2 41,296
0.95
1.01
0.96
8
N
15
6000
3 43,464
0 0 0.00
1.01
0.00
8 N
18
4790 - 5430
3 48,256
3
38,800
0.80
1.12 0.90
Average
= 0.97
Average
of
all
bars!l
= 0.85
Average
of
all No.
11
bars§
= 0.83
Average
= 0.90
Average
= 0.78
Average
= 0.80
Average
of
all
No.6
bars§
0.89
*Numerator and denominator based on group average.
tNumerator based on group average. Denominator based on average for three deformation patterns for each bar size; each deformation pattern weighted equally.
§Each
deformation pattern weighted equally.
1 psi
= 6.89 kPa; 1 lb = 4.45 N.
Uncoated
Coated
bars bars
Concrete
No.
of
bond No.
of
bond
Bar
Deformation
Group
strength,
uncoated force,
coated force,
C/U*
U/U,t
C/U,t
size pattern
no.
psi
bars
lb bars
lb
group
all all
8
S
3 6090
3
41,384
9
29,472
0.71
0.96 0.68
8
S
6
5870 2
45,104
2 34,512
0.77
1.05 0.80
8
S
15
6000 2
42,680
6 31,600
0.74
0.99 0.73
8
S
18
4790 - 5430
3
41,312
3
34,064
0.82
0.96
0.79
Uncoated
Coated
bars bars
Concrete
No.
of
bond No.
of
bond
Bar
Deformation
Group strength,
uncoated force,
coated
~
force,
C/U*
U/U,t
C/U,t
size pattern
no. psi
bars lb bars lb
group
all all
6
S
14
5800
3
19,363
6
15,498
0.80
1.00
0.80
6
S
17
5850
3
18,720
6
15,525
0.83 0.97
0.81
Uncoated
Coated
bars bars
Concrete No.
of
bond
No.
of
bond
Bar
Deformation
Group strength,
uncoated
force,
coated
force,
C/U*
U/U,t
C/U,t
size
pattern
no.
psi
bars lb bars
lb
group
all
all
11
S
19
5070 - 5270 3
39,033
3
33,138 0.85 0.94
0.80
11
S
20
5260 - 5290 3
41,994
3
41,580
0.99
1.01
1.00
Table 3(c) - Summary
of
beam-end tests for specimens with standard configuration
*Numerator and denominator based
on
group average.
tNumerator based on group average. Denominator based on average for three deformation patterns for each bar size; each deformation pat-
tern weighted equally. .
§Each
deformation pattern weighted equally.
!lEach
bar
size weighted equally. .
1 psi
= 6.89 kPa; 1 lb = 4.45 N.
ACI Materials Journal I March-April
1991
211
Table 3(b) - Summary
of
beam-end tests for specimens with standard configuration
Table 3(d) - Summary
of
beam-end tests for specimens with standard configuration
