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csi = one-half of the bar clear spacing; and imum displacement capacity of the beam end at failure for
db = diameter of bar. higher strength concretes.
c max
= 0.1 + 0.9 1.25 (3.20)
c min 3.9 Anchorage of Epoxy-Coated
Bars in Tension
where cmax = maximum (cb, cs)
The object of this phase of NCHRP Project 12-60 was to
Ktr = (0.52trtdAtr/sn)f c1/2 (3.21)
evaluate the bond strength of epoxy-coated bar lap splices in
where concrete with strengths up to 15 ksi. An extensive literature
tr = 9.6 Rr + 0.28 1.72; review of test data was supplemented with 12 additional tests
Rr = relative rib area of the reinforcement; of top cast epoxy-coated bar splices. The variables considered
td = 0.78db + 0.22; in the experimental program included bar size (#6 and #11),
Atr = area of each stirrup or tie crossing the potential plane concrete strength (12 to 17 ksi), and the amount of transverse
of splitting adjacent to the reinforcement being de- reinforcement over the splice length.
veloped, spliced, or anchored;
n = number of bars being developed or spliced; and
3.9.1 Literature Review
s = spacing of transverse reinforcement.
Epoxy-coated bars have been used as an economical
method of protection against deterioration of reinforced con-
3.8.9 Summary and Conclusions
crete structures associated with corrosion of steel reinforce-
On the basis of the analysis of results from the tests of six ment. Treece and Jirsa (1989) tested 21 beams in 9 series. The
beam specimens with lap-spliced uncoated bars embedded in variables were bar size (#6 and #11), concrete strength (4, 8,
higher strength concretes conducted as part of NCHRP and 12 ksi), casting position, and coating thickness (5 and 12
Project 12-60 and the evaluation of an extensive database of mils). The splice lengths were selected so that the bars would
test results compiled by ACI Committee 408, the following fail in bond before reaching yield, and no transverse rein-
conclusions can be drawn: forcement was provided in the splice region. Test results
showed that epoxy-coated bars with an average coating thick-
· The ratios of test maximum stress on the top spliced bars to ness above 5 mils developed 67 percent of the bond strength
the stress calculated from the design equation in the 318 of black bars.
Code (ACI 2005) ranged from 1.13 to 1.59. A similar ratio DeVries, Moehle, and Hester (1991) reported the test re-
of test maximum stress to stress calculated from the sults of 36 beams. The variables were casting position, bar size
AASHTO specifications ranged from 1.48 to 2.14. Thus, the (#6 and #9), and the presence of an antibleeding agent in the
procedure in the 318 Code (ACI 2005) and the AASHTO concrete. The range of concrete strengths was 8 to 15 ksi. Test
specifications for top bar uncoated splice and development results indicated that the ratio of bond strength of epoxy-
length in tension can be extended to normal-weight con- coated bars to black bars was 0.84. Based on the test results,
crete with uniaxial cylinder strength up to 16 ksi. De Vries and Moehle indicated that the effects of casting po-
· The design equation in the 318 Code (ACI 2005) and the sition and epoxy coating were not cumulative and that the
design equation in the AASHTO LRFD Bridge Design Spec- modification for top cast epoxy-coated bars relative to bot-
ifications, Equations 3.11 and 3.15, respectively, overesti- tom cast epoxy-coated bars was not needed. Also, the results
mated the bar stress in several of the bottom cast specimens showed that the presence of an antibleeding agent in the con-
in the ACI 408 Committee Database, especially for speci- crete did not significantly alter the bond stress of the splice for
mens with concrete compressive strength higher than 10 either top cast or bottom cast bars.
ksi. However, the calculated result proposed by ACI 408 Choi et al. (1991) reported on the tests of 15 beams. The
Committee, Equation 3.18, resulted in fewer cases where variables were bar size (#5, #6, #8, and #11), average coating
the ratio of test to calculated stress was less than 1.0. It also thickness (3 to 17 mils), and deformation patterns (three pat-
resulted in more conservative estimates of the bond terns designated S, C, and N). The concrete strength was
strength defined by the stress of spliced bars embedded in around 6 ksi. Test results indicated that the ratio of the bond
higher strength concrete beams. strength of epoxy-coated bar splices to that of black bar
· Based on the maximum bar stress and beam end displace- splices varied from 0.71 to 0.94 with an average value of 0.82.
ment at peak load in all the specimens, the use of stirrups They reported that all splice specimens exhibited extensive
in the amount of Ktr from 0.37 to 0.67 in the splice region longitudinal and transverse cracking in the region of the
resulted in increases in both maximum bar stress and max- splices at failure. The salient conclusion was that differences
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in coating thickness have little effect on the amount of the strengths vary from 4 to 10 ksi. The relationship between the
bond strength reduction for #6 bars and larger with coating bond efficiency (the ratio of test stress to stress calculated using
thicknesses between 5 and 12 mils. 318 Code [ACI 2005]) of the spliced bars and the square root of
Hamad and Jirsa (1993) reported on an experimental concrete compressive strength in this literature review is shown
study in which 12 beams were tested. The main variables were in Figure 3.62. Note that the upper limit on the fc of 100 psi
bar size, bar spacing, and the amount of transverse reinforce- was removed in this calculation. Generally, the calculated stress
ment in the splice region. The concrete strength was around was conservative in the range of higher strength concretes.
4 ksi. Failure of all beams was governed by splitting of the
concrete cover in the splice region. Test results indicated that
3.9.2 U.S. Design Specifications
the presence of transverse reinforcement in the splice region
increased the deformation capacity of the beams and im- 3.9.2.1 318 Code (ACI 2005)
proved anchorage strength of epoxy-coated bar splices rela-
318 Equation 12-1 (ACI 2005) for estimating tension splice
tive to black bar splices more than 10 percent.
and development length requirements, Equation 3.12 in this
Cleary and Ramirez (1993) reported on an experimental
report, contains several factors. One of these is t, the tradi-
study in which 23 beam splice tests were subjected to repeated
tional reinforcement location factor of 1.3 to reflect the
loadings and then tested to failure to compare the service and
adverse effects of the top reinforcement casting position.
ultimate load behavior of beams with coated and uncoated
Parameter e is the specific coating factor to deal with epoxy-
reinforcement. The range of concrete strengths was 4 to 7 ksi.
coated bars. It is 1.5 with cover less than 3db or clear spacing
They reported that the differences in crack widths, deflec-
less than 6db, and it is 1.2 for all other cases. These factors are
tions, and reinforcement stresses in beams with coated and
consistent with the ratio of bond strength of coated bars to
uncoated reinforcement were reduced with repeated loading.
bond strength of uncoated bars reported in the literature of
The ratio of the average bond stress at failure for a beam
1/1.5 = 0.67 and 1/1.2 = 0.82. However, the product of t and
containing epoxy-coated bars to its companion specimen
e need not be taken greater than 1.7. All other factors are the
containing uncoated reinforcement ranged from 0.82 to 0.96,
same as for uncoated bars. In addition, as for uncoated bars,
with an average of 0.88.
when calculating anchorage length requirements for tension
Hester et al. (1993) tested 65 beam and slab splice specimens
lap splices, these should be as required for Class A or B splice
containing #6 and #8 bars. The average coating thickness
but not less than 12 in., where
ranged from 6 to 11 mils, and concrete strength ranged from 5
to 6.5 ksi. The Hester et al. study concluded that transverse re- Class A splice..................1.0 ld
inforcement improved the strength of splices containing both Class B splice..................1.3 ld
coated and uncoated bars, and the percentage increase in
strength was approximately the same for both coated and un-
3.9.2.2 2004 AASHTO Specifications (Section 5.11
coated bars with an equal amount of transverse reinforcement.
Development and Splices of Reinforcement)
A maximum development length modification factor of 1.35
was proposed for design with epoxy-coated reinforcement. In 1989, on the basis of several test programs that showed
Grundhoffer et al. (1998) reported on a series of 94 in- that the bond strength of epoxy-coated bars is reduced
verted half-beam specimens. The variables were bar size (#6,
Choi et al. (1991) Hamad and Jirsa (1993) Treece and Jirsa (1989)
#8, and #11), bar surface (epoxy and uncoated), concrete
DeVries, Moehle, and Hester (1991) Grundhoffer et al. (1998)
strength (6, 10, 12, and 14 ksi), and the addition of micro-
silica to concrete. A comprehensive review of the effect of 3.0
Bond Efficiency (Test / ACI-05)
epoxy-coating on bond strength was conducted using the 2.5
results of this study and 151 test results from seven other 2.0
research studies. They concluded that ACI's 1989 318 Code
1.5
was more conservative than the 1995 318 Code for all the test
results based on the comparison between experimental re- 1.0
sults and the values of design bond strength calculated using 0.5
ACI's 1989 and 1995 318 Code equations. 0.0
The review of past work shows that only two specimens of 0 50 100 150
Treece and Jirsa (1989), eight specimens of DeVries, Moehle, Concrete Strength (f'
c)
and Hester (1991) and some specimens of two groups out Figure 3.62. Bond efficiency of the spliced
of eight groups in Grundhoffer et al. (1998) used concrete bars with concrete strength relationship
strengths greater than 10 ksi. Other researchers' concrete (1 psi-6.89 kPa).
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because coating prevents adhesion between the bar and the In summary, although the factors are the same in both the
concrete, two factors--1.5 and 1.2 (function of the amount ACI Code and AASHTO LRFD Bridge Design Specifications
of concrete cover or bar spacing)--were introduced in the with respect to development and splice length of tension of
318 Code provisions for development length of bars in ten- epoxy-coated reinforcement, the same differences observed
sion. No factors were stated for similar bars in compression in the case of uncoated bars for the calculation of tension
or epoxy-coated bars terminated by means of standard hooks development length remain.
anchored to resist tension. Similar factors are currently em-
ployed in the AASHTO LRFD Bridge Design Specifications. 3.9.3 Experimental Program
The rest of the approach is the same as for uncoated bars. No
3.9.3.1 Test Specimens
factors were stated for similar bars in compression or epoxy-
coated bars terminated by means of standard hooks anchored The experimental program covers the testing of 12 beam
to resist tension. splice specimens reinforced with epoxy-coated bars. The spec-
The tension development length, ld , shall not be less than imen dimensions and variables are shown in Table 3.38. The
the product of the basic tension development length, ldb (see test variables are bar size, concrete cover, concrete strength,
Equation 3.15), and the modification factor specified in Arti- and transverse reinforcements in the splice region in higher
cle 5.11.2.1.2 (1.5 for epoxy-coated bars with cover less than strength concretes. The cover value given in Column 3 is both
3db or with clear spacing between bars less than 6d and 1.2 for top and side clear cover to the bar being developed or spliced.
epoxy-coated bars not covered above). The tension develop- Details of typical specimen are shown in Figure 3.63. In Spec-
ment length shall not be less than 12.0 in., except for lap imens II-15 through II-18, transverse reinforcement was used
splices specified in Article 5.11.5.3.1 (Class A splice . . . 1.0 ld, in the splice region to confine the concrete as shown in Figure
Class B splice . . . 1.3 ld, Class C splice . . . 1.7 ld). When the 3.64(b). In the splice region, the transverse reinforcement
changes that eliminated many concerns regarding develop- consisted of #3 @8 in. for Specimens II-15 and II-17 and #4 @8
ment length of tension lap splices due to closely spaced bars in. for Specimens II-16 and II-18, respectively.
were introduced in the 1989 version of the 318 Code, Class C The splice length shown in Column 7 of Table 3.38 was
splices were eliminated. selected to provide a direct link with previous tests in order to
Table 3.38. Specimen dimensions and variables.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Specimen Bar Cover Beam Effective Number Splice 318-05 Compressive
Size (in.) Size Depth of Length Cal. Strength
(B x H) (in.) Spliced (in.) Stress ( f c , ksi)
(in.) Bars (ksi)
II-7 #6 0.75 9 x 18 16.875 3 16 34.82 12.4
II-8 #11 1.50 18 x 18 15.750 3 36 33.34 12.3
II-9 #6 2.25 18 x 18 15.375 3 16 66.67 13.6
II-10 #11 4.50 24 x 18 12.750 2 36 63.83 13.6
II-11 #6 0.75 9 x 18 16.875 3 16 40.78 16.8
II-12 #11 1.50 18 x 18 15.750 3 36 39.05 16.8
II-13 #6 2.25 18 x 18 15.375 3 16 73.50 16.6
II-14 #11 4.50 24 x 18 12.750 2 36 70.38 16.6
II-15* #6 0.75 9 x 18 16.875 3 16 54.51 17.2
II-16* #11 1.50 18 x 18 15.750 3 36 51.76 17.2
II-17* #6 2.25 18 x 18 15.375 3 16 72.93 16.4
II-18* #11 4.50 24 x 18 12.750 2 36 69.83 16.4
1 in. = 25.4 mm; 1 ksi = 6.89 MPa (* denotes specimens with transverse reinforcement in the splice
region). B = specimen width. H = specimen height.
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Specimens with transverse reinforcement in the splice region:
#4@8" with #11 Bars (#3@8" with #6 Bars)
18" (9")
Ls = 36" (16" )
54" 48" 54"
Specimens with #11 Bars:
#4@4.5" Splice Region #4@4.5"
Specimens with #6 Bars: #3@8"
Figure 3.63. Typical beam-splice specimen reinforced with
epoxy-coated bars (1 in. 25.4 mm).
extend the specifications to higher strength concretes for for purposes of estimating the required shear reinforcement
epoxy-coated bars and to permit a more straightforward to resist the maximum shear associated with reaching the mo-
cover effect evaluation among specimens. The splice lengths ment capacity of the section at the support. In the overhang
have been selected to get a yielding stress in the basic speci- region, the spacing of shear reinforcement was #3 @8 in. on
mens with 3db concrete cover (II-9 and II-10) as shown in centers and #4 @4.5 in. on centers for specimens with #6 bars
Column 8 of Table 3.38. Using Equation 3.11 with appropri- and specimens with #11 bars, respectively. Figure 3.64 depicts
ate modification factors, including the epoxy-coated bar the construction of the specimens.
factor, and with a splice class factor of 1.0, it was possible to
calculate stress and force in the bar for various anchorage
3.9.3.2 Test Setup and Loading Protocol
conditions. To determine the calculated stress, fs, ld is replaced
by the splice length provided, 16 and 36 in. Note that all the The test setup is shown in Figure 3.65(a). In all speci-
specimens were cast with more than 12 in. below the splice. mens, the distance between the loading points and the sup-
As shown in Table 3.38, all the bars in specimens with 3db port was 48 in., and the distance between supports was also
concrete cover had a calculated stress greater than 60 ksi. 48 in. To investigate the characteristics of spliced beams,
The specimens were reinforced in the overhang region to the applied loads, resulting deflections at each beam end
prevent premature shear failures outside of the test region. and midspan, and strains developed in longitudinal bars
For safety against shear failure, a stress of 1.25 times the yield and stirrups were monitored using load cells, LVDTs
strength of the longitudinal bar was assumed in the overhang attached to an external reference frame, and electrical
(a) Specimens II-7 & II-8 (b) Specimens II-17 & II-18
Figure 3.64. Construction of beam-splice specimen with epoxy-coated bars.
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(a) Loading & Supporting (b) Measuring by LVDT
#6 Bar Specimens (Ls = 16", G1=19", G2=13")
G2=15" G1=39" (Plan for gage location and bars)
Ls = 36"
6" 48" 48" 48" 6"
(Elevation for Supporting)
(c) Location of gages and support (Specimen II-8)
Figure 3.65. Test setup for beam-splice specimens reinforced with epoxy-coated bars (1 in. 25.4 mm).
resistance strain gages affixed to the bars as shown in Fig- uniaxial compressive stress by age are shown in Figure 3.66(b).
ure 3.65 (b) and (c). As shown, the strength of Mix II continued to increase after 28
days and achieved a strength of 17 ksi at 56 days.
ASTM A615 Grade 60 reinforcing bars were used for both
3.9.3.3 Materials
longitudinal and transverse reinforcement. The yield
Table 3.39 shows the design concrete mixes. The water-to- strength, calculated by a 0.2-percent offset from tensile tests
cement ratio was 0.32 for the 10-ksi Mix I and 0.20 for the of samples of the reinforcing bars, was 70.3 ksi and 74 ksi for
14-ksi Mix II. A sample of the uniaxial stress versus strain re- the #6 and #11 bars, respectively. The average thickness of
lationship for the concrete is shown in Figure 3.66(a). The epoxy coating was 12.5 mils and 11.5 mils for the #6 and #11
average modulus of rupture was 566 psi and 834 psi at 28 days bars, respectively. The relative rib area was 0.091 and 0.135
for Mix I and Mix II, respectively. Also, typical data for for the #6 and #11 bars, respectively. The measured tensile
Table 3.39. Concrete mix (per cubic yard).
Contents Mix I: 10 ksi Mix II: 14 ksi
Cement (lb) 780 900
Silica fume (lb) 50 200
Water (lb) 265 220
1,600 1,800
Coarse aggregate (lb)
(3/8" pea gravel) (1/2" crushed limestone)
Fine aggregate (lb) 1,240 1,000
High-range water reducer (oz) 190 520
Normal-range water reducer (oz) 35 38
3 3
1 lb = 0.454 kg; 1 oz = 28.35 gr; 1 yd = 0.765 m ; 1 ksi = 6.89 MPa
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Concrete Stress vs. Strain Relationship
20
Compressive Strength, ksi
15
10
5
10 ksi Mix
14 ksi Mix
0
0.0000 0.0005 0.0010 0.0015 0.0020
Strain, in/in
(a) Concrete Stress vs. Strain Relationship
20
18
Compressive Strength (ksi)
16
14
12
10
8
6
4 10 ksi
2 14 ksi
0
0 20 40 60 80 100
Age (days)
(b) Concrete Strength vs. Age Relationship
100 100
90 90
80 80
70 70
Stress (ksi)
Stress (ksi)
60 60
50 50
40 40
30 30
20 20
10 10
0 0
0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000
Strain (x10^-6) Strain (x10^-6)
(c) # 6 (#19M) Bars (d) #11 (#35M) Bars
Figure 3.66. Material properties for beam-splice specimens reinforced with
epoxy-coated bars (1 in. 25.4 mm; 1 ksi 6.89 MPa).
stress versus strain curves for #6 and #11 bars are shown in With the increase of beam end loads, a shear crack appeared
Figure 3.66(c) and (d). in the overhang region. Near the peak load, splitting hori-
zontal cracks appeared along the longitudinal bars in the
3.9.4 Experiment Results splice region. Finally, the deformations pushed the concrete
away from the bar by wedge action. Typical failure crack pat-
3.9.4.1 Cracking Pattern and Mode of Failure
terns are shown in Figure 3.67. All the specimens failed in
In nearly all tests, the cracking sequence was similar. First, splitting mode after yielding of the spliced bars in the con-
a flexural crack appeared in the constant moment region. stant moment region.
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(a) Specimen II-7 (b) Specimen II-8
(c) Specimen II-9 (d) Specimen II-10
(e) Specimen II-17 (f) Specimen II-18
Figure 3.67. Typical failure crack pattern for beam-splice specimens reinforced with epoxy-coated bars.
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3.9.4.2 Load versus End Displacement 3.9.4.3 Summary of Test Results
Characteristics
The test results are summarized in Table 3.40 and findings
The applied load versus deflection at the tip of the over- from these results are presented on the basis of three main
hang response for Specimens II-7 to II-10 and II-17 and II-18 parameters.
is shown in Figure 3.68. Load was calculated by averaging the
two values from the actuators, and deflection was obtained Concrete Cover (db). Comparison of Specimens II-7 and
averaging displacements at both ends of the beam. II-9 (#6 bars) and comparison of Specimens II-11 and II-13
II-7 II-8
30 80
70
25
60
Load (kips)
Load (kips)
20 50
15 40
30
10
20
5 10
0 0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Displ. (in) Displ. (in)
(a) Specimen II-7 (b) Specimen II-8
II-9 II-10
35 80
70
30
60
25
Load (kips)
Load (kips)
50
20
40
15
30
10 20
5 10
0 0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Displ. (in) Displ. (in)
(c) Specimen II-9 (d) Specimen II-10
II-17 II-18
35 80
30 70
25 60
Load (kips)
Load (kips)
20 50
40
15
30
10
20
5
10
0 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Displ. (in) Displ. (in)
(e) Specimen II-17 (f) Specimen II-18
Figure 3.68. Applied load versus deflection at the tip of the overhang response (Specimens II-7
through II-10 and II-17 through II-18).
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Table 3.40. Summary of test results.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Speci- Max. Displ. 318-05 318-05* AASHTO Test (7)/(4) (7)/(5) (7)/(6)
men Load at Peak Cal. Cal. Cal. Stress Max.
(kips) Load Stress Stress (ksi) Stress
(in) (ksi) (ksi) (ksi)
II-7(db) 20.7 0.311 34.82 27.86 31.37 63.81 1.83 2.29 2.03
II-8(db) 61.5 0.423 33.34 33.34 37.55 65.50 1.96 1.96 1.74
II-9 29.0 0.687 66.67 48.94 31.75 78.39 1.18 1.60 2.47
(3db)
II-10 49.4 0.701 63.83 58.57 37.99 66.19 1.04 1.13 1.74
(3db)
II-11 21.0 0.315 40.78 32.63 31.37 65.50 1.61 2.01 2.09
(db)
II-12 64.5 0.395 39.05 39.05 37.55 65.00 1.66 1.66 1.73
(db)
II-13 32.1 1.161 73.50 53.96 31.75 83.45 1.14 1.55 2.63
(3db)
II-14 52.9 0.793 70.38 64.58 37.99 69.31 0.98 1.07 1.82
(3db)
II-15** 28.8 0.602 54.51 43.60 31.37 65.34 1.20 1.50 2.08
(db)
II-16** 92.0 0.662 51.76 51.76 37.55 65.96 1.27 1.27 1.76
(db)
II-17** 32.4 1.185 72.93 53.54 31.75 84.80 1.16 1.58 2.67
(3db)
II-18** 67.4 1.924 69.83 64.08 37.99 86.41 1.24 1.35 2.27
(3db)
1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 ksi = 6.89 MPa
* shows stress calculated by removing bar size factor and using one epoxy-coated bar factor of 1.5.
** shows specimens with transverse reinforcement in the splice region.
(#6 bars) show that increasing the concrete cover increased maximum stress or deflection at failure. In Specimens II-10
both maximum stress and deflection at failure. This result can and II-14 with larger cover (3db), increasing the concrete
also be seen in comparison of Specimens II-8 and II-10 (#11 strength resulted in increases in both maximum stress and
bars) and comparison of Specimens II-12 and II-14 (#11 deflection at failure.
bars). However, the increase in maximum stress for #11 bar
specimens was less than the increase in maximum stress for Effect of Minimum Amount of Transverse Reinforce-
#6 bar specimens. ment in Higher Strength Concretes. A comparison of
Specimens II-11 and II-15 (#6 bars) shows that the use of
Effect of Concrete Strength. For Specimens II-7 and II-11 transverse reinforcement over the splice region did not re-
(#6 bars) with small cover (equal to db), increasing the con- sult in an increase in the maximum stress but more than
crete strength led to an increase in maximum stress, but did doubled the deflection at failure when the small cover (db)
not significantly increase the maximum deflection at failure. was used. When the large cover (3db) was used, it resulted
When larger cover (3db) was used, increasing the concrete in increases to both maximum stress and deflection at fail-
strength increased both maximum stress and deflection at ure, as can be seen by comparing Specimens II-13 and II-17.
failure as can be seen by comparing Specimens II-9 and II-13. Comparison of Specimens II-12 and II-16 (#11 bars) and
For Specimens II-8 and II-12 (#11 bars) with small cover comparison of Specimens II-14 and II-18 (#11 bars) show
(db), an increase in concrete strength did not increase the that the use of transverse reinforcement over the splice
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region resulted in increases to both maximum stress and mens II-17 and II-18 (with larger covers and transverse rein-
deflection at failure. forcement) to the ratios for Specimens II-13 and II-14 (with
larger covers but no transverse reinforcement) is due to the
3.9.5 Comparison of Calculated Stress requirement that the ratio of (cb+ Ktr)/db in Equation 3.11
and Test Results should not be taken greater than 2.5.
The comparison of stress in the bar calculated using Equa-
tions 3.11 and 3.15 with appropriate modification factors and 3.9.6 Design Recommendation
test maximum stress is also shown in Table 3.40. The ratios of When the spliced bar stress was calculated using 318 Code
test maximum stress to ACI-calculated stress in the bar (ACI 2005), without a limitation on the square root of the
ranged from 0.98 to 1.96, as shown in Column 8. The ratios compressive concrete strength, only Specimen II-14 with 3db
of test maximum stress to AASHTO-calculated stress in the concrete cover had a ratio of test maximum stress to calculated
bar ranged from 1.73 to 2.67, as given in Column 10. It should stress of less than 1. However, the average ratio of test maxi-
be noted that the second part of Equation 3.15 controlled the mum stress to 318 Code (ACI 2005) calculated stress for the
basic development length. The findings from these compar- specimens with 3db cover was less than average of the same ratio
isons are discussed on the basis of three main parameters for the specimens with db cover. Therefore, it is possible to con-
studied: effect of concrete cover, effect of concrete strength, clude that the current cover contribution may be overestimated
and effect of minimum amount of transverse reinforcement in the case of higher strength concrete specimens with large cov-
in higher strength concretes. ers. Using only one coating factor of 1.5 may be the simplest way
to handle the possible overestimation of cover contribution.
Effect of Concrete Cover. In the specimens without trans-
Regarding bar size factor, no stress ratios less than 1 were
verse reinforcement, the ratio of test maximum stress to
obtained when the calculated stress included the 0.8 bar size fac-
ACI-calculated stress (see Column 8 in Table 3.40) in the
tor within the range of specimens covered in this study. The
specimens with 3db concrete cover was 0.98 to 1.18. The ratio
three specimens shown in Figure 3.62 with a ratio of test maxi-
of test maximum stress to ACI-calculated stress in the speci-
mum stress to calculated stress (defined in Figure 3.62 as "bond
mens with db concrete cover was 1.61 to 1.96, much higher
efficiency") of less than 1 were specimens reinforced with #11
than in the specimens with 3db concrete cover. This tendency
(#35M) bars. However, following the position of ACI Commit-
was consistent regardless of other parameters, such as con-
tee 408, the authors of this report also suggest not using the 0.8
crete strength and bar size.
bar size factor. Column 5 of Table 3.40 shows the calculated
Effect of Concrete Strength. In higher strength concrete stress without the bar size modification factor. Figure 3.69
specimens without transverse reinforcement, the average shows a comparison of bond efficiency (defined as the ratio of
ratio of test maximum stress to ACI-calculated stress for the
specimens with db and 3db concrete cover was near 1.64 and Purdue No Stirrups With Stirrups Purdue(S)
1.06, respectively (see Column 8 in Table 3.40). These obser-
vations point to the possibility that the current cover contri- 3.5
bution in the code may be overestimated in the case of higher
3.0
strength concrete specimens for larger covers.
Bond Efficiency (Test / ACI-05*)
2.5
Effect of Minimum Amount of Transverse Reinforce-
ment in Higher Strength Concretes. The ratios of test max- 2.0
imum stress to ACI-calculated stress for Specimens II-11 and
II-14 are 1.61 and 0.98, respectively; the ratios of test maxi- 1.5
mum stress to ACI-calculated stress for Specimens II-15 and
II-18 are 1.20 and 1.24, respectively. These data show that for 1.0
specimens with transverse reinforcement over the splice re-
0.5
gion the difference between ratios of test maximum stress to
ACI-calculated stress was smaller than the difference between 0.0
ratios for specimens without transverse reinforcement (see 600 2080 40 100 120 140
Column 8, Table 3.40). The lower ratios in Specimens II-15 Concrete Strength (f' c)
*Stress calculated by removing bar size factor and the epoxy-coated bar factor of 1.5
and II-16, with small covers, came from higher calculated
stress, considering the contribution factor of confining rein- Figure 3.69. Comparison of bond efficiency with
forcement. However, the similarity of the ratios for Speci- concrete strength.
