ACI STRUCTURAL JOURNAL TECHNICAL PAPER

Title no. 85-826

Investigation of "Top Bar'' Effects in Beams

by Paul R. Jeanty, Denis Mitchell,· and M. Saeed Mirza

An experimental study was performed on full-scale beam specimens bars placed horizontally at the top of the formwork
to study the "top bar" effect on the responses. The behavior of (top-cast). In addition, Soretz, 8 Leonhardt and
beams containing top-cast bars is compared with companion beams
Walther, 9 and Ferguson and Thompson 10 performed
containing bottom-cast bars. A top bar factor for these tests is de-
rived and the results are compared with current ACI predictions and beam tests comparing the behavior of bottom-cast with
with the suggested provisions made by ACI Committee 408. In addi- top-cast specimens.
tion, the influence of transverse reinforcement is investigated. In the case of a vertically oriented reinforcing bar, it
was concluded that the settlement of the concrete re-
Keywords: anchorage (structural); beams (supports); bond (concrete to rein- sulted in better consolidation of the concrete above the
forcement); position (location); reinforced concrete; reinforcing steels; struc- deformations than below the bar deformations. The
tural design.
bond resistance is therefore more favorable when the
bar is pulled against the direction of casting rather than
The influence of the casting position of reinforcing in the casting direction. The lower bond strength of
bars on bond characteristics has been recognized since top-cast compared to bottom-cast horizontal bars is at-
1913. 1 The top bar factor was introduced in the 1951 tributed to the greater settlement of concrete immedi-
ACI Building Code for top bars defined as horizontal ately below the top-cast bar and to a 10 to 20 percent 11
bars so placed that more than 12 in. (305 mm) of con- lower tensile strength of the concrete at the top of the
crete is cast in the member below the bars. This factor casting. The pullout tests enabled the following classi-
and the definition of a top bar have remained essen- fication for casting position and loading direction of
tially the same in the ACI Building Code for more than bars in order of decreasing bond strength:
30 years. I. Vertically oriented bar loaded in direction oppo-
site to casting direction;
RESEARCH SIGNIFICANCE 2. Horizontally oriented bottom-cast bar;
There is a need for more research in this important 3. Horizontally oriented top-cast bar; and
area to provide a better understanding of the effects of 4. Vertically oriented bar loaded in direction of cast-
casting position on the bond performance. The effects ing.
of water-cement ratio, the vertical position of bars in From their pullout tests, Ferguson, Breen, and
the casting height, the orientation of the bars, the pres- Thompson 7 observed that top-cast bars slipped at the
ence of superplasticizers, and the use of large-diameter unloaded end at relatively low loads and then contin-
bars need to be investigated. This paper presents the ued to accept more load, whereas bottom-cast bars did
results of a series of experiments on full-scale rein- not slip at the unloaded end until almost at their maxi-
forced concrete beams to examine the top bar effects on mum load.
the responses. Recognition of this phenomenon was first intro-
duced into the ACI Building Code" in 1951 in the form
HISTORICAL DEVELOPMENT OF TOP BAR of allowable bond stresses at working loads based on
FACTOR the tests carried out by Clark.' The allowable bond
The effects of casting position on the bond charac- stress for a top-cast bar was 0.7 times the allowable
teristics were reported by Abrams; 1 Davis; 2 Clark; 3 stress for a bottom-cast bar. Top bars were defined as
Menzel; 4 Dutron; 5 Rehm; 6 and Ferguson, Breen, and
Thompson. 7 These researchers performed experiments
Received May 6, 1987, and reviewed under Institute publicationpolicies.
on pullout specimens that included reinforcing bars Copyright :<:: 1988, American Concrete Institute. All rights reserved, mcludmg
placed vertically in the formwork, bars placed horizon- the making of copies unless permission is obtained from the copynght propn-
etors. Pertinent discussion will be published in the March-Apnl 1989 ACI
tally at the bottom of the formwork (bottom-cast), and Structural Journal if received by Nov. I, 1988.

ACI Structural Journal I May-June 1988 251

 j. Presence ot transverse remtorcement crossmg the
Paul R. Jeanty is an engineer with the Commission des Sen·ices Electriques of
the City of Montreal, Quebec, Canada. plane of potential splitting
The experimental results are compared with the cur-
Denis Mitchell, FACJ, is a professor in the Department of Civil En!(ineerinR
and Applied Mechanics at McGill University in Montreal. He is currently a
rent ACI Building Code (ACI 318-83) provisions 15 and
member of ACI-ASCE Commillee 445, Shear and Torsion, and ACI Commit- the suggested provisions 16 •1' of ACI Committee 408. By
tees 408, Bond and Development of Reinforcemem; and E 901, Scholarships; comparing the responses of top-cast specimens and
and is a member of the Canadian Concrete Code Commillee.
companion bottom-cast specimens, a suitable top bar
M. Saeed Mirza, FACI, is a professor in the Department of Civil Engineering factor is suggested.
and Applied Mechanics at McGill University. He is currently a member of ACI
Commillees 444, Models of Concrete Structures, and 435, Deflection of Con-
crete Structures.
DESCRIPTION OF TEST SPECIMENS
The cross-sectional dimensions of all the test speci-
mens were 9 in. (229 mm) wide by 18 in. (457 mm)
deep. The specimens were tested over a simple span
length of 10 ft. (3048 mm) with a central load, as
embedment l embedment I shown in Fig. I. The beams contained the following
length length
reinforcment:
10' -a· (3048mm)
I. A No. 8 reinforcing bar (25-mm diameter) with a
(a) Beam in Testing Position
clear concrete cover of 1.5 in. (38 mm) was placed with
equal embedment lengths on both sides of the beam
centerline. The yield stress f. was 59.5 ksi (410 MPa).
2. Two No. 6 (19-mm diameter) reinforcing bars
were provided in the corner of the stirrups running the
open stirrup full length of the beam. The yield stress was 58.5 ksi
(403 MPa).
2-#6
1-#8 test bar 3. Two No. 3 (9.5-mm diameter) reinforcing bars
were provided in the corners of the stirrups. The yield
BOTTOM CAST TEST BAR TOP CAST TEST BAR stress was 60.3 ksi (416 MPa).
4. Either open or closed No. 3 stirrups were placed at
(b) Casting Positions and Cross-Section Details
8-in. (203-mm) spacing throughout the beam length.
The beams were cast in pairs, one with the reinforc-
Fig. 1- Test set-up and positions of casting (1 in. ing placed in the formwork to provide a bottom-cast
25.4 mm)
test bar and the other with an inverted reinforcing cage
to provide a top-cast test bar [see Fig. !(b)]. All of the
"horizontal bars so placed that more than 12 in. of specimens were tested, as shown in Fig. !(a), with the
concrete is cast in the member below the bar." The main tension steel on the bottom to enable a compari-
1963 ACI Building Cadell introduced ultimate strength son of the performance of top versus bottom-cast bars.
design and used an ultimate bond stress expression with The embedment lengths were varied from 30 to 48 in.
the same top bar bond stress reduction factor as the (762 to 1219 mm) to determine the embedment length
1951 ACI Code. necessary for yielding of the main tension reinforce-
The 1971 ACI Building Code 1" replaced the tradi- ment for both the top-cast and the bottom-cast speci-
tional bond stress calculation with expressions for de- mens. Since bond failure was initiated by a longitudi-
velopment length. In this code and the 1983 ACI nal splitting crack in the concrete cover over the test
Building Code, 15 the top bar effect is accounted for by bar, some of the stirrups were closed [see the dotted
multiplying the development length by a factor of 1.4, line in Fig. !(b)] to study the effects of transverse rein-
which corresponds to the top bar bond stress reduction forcement crossing the plane of splitting. Table I sum-
factor of 0. 7 in the 1951 ACI Building Code. Since the marizes the important test specimen details. The odd-
1951 ACI Code, the top bar definition and the top bar numbered specimens contain top-cast bars, while the
factor have not been changed. Further research is even numbered specimens contain bottom-cast test
needed to determine the influence of casting position on bars.
the bond performance in modern concrete construc- The concrete consisted of normal portland cement,
tion. natural sand, and crushed limestone for the coarse ag-
gregate having a maximum size of % in. (19 mm). A
OBJECTIVES OF EXPERIMENTAL PROGRAM water-cement ratio of 0.54 was used, and a water-re-
The objectives of this investigation are to test full-size ducing admixture was included in the mix. The com-
beam specimens to study the effects on the responses of ponents of the concrete mix are given in Table 2. The
the following parameters, varied systematically, one at compressive strength for each specimen was deter-
a time: mined from six standard 6 x 12 in. (!52 x 305 mm) test
1. Top-cast versus bottom-cast bars; cylinders tested on the day after the beam was tested.
2. Embedment length of the test bars; and The average compressive strength for each specimen is
252 ACI Structural Journal I May-June 1988
 DEFLECTION, mm
Table 1 - Test specimen details
0 10 15 20
Top cast Bottom cast
Embedment I
I
f', f',
813
length, psi psi Stirrup so
in. (mm) Specimen (MPa) Specimen (MPa) type
30 Bl 4040 82 3890 Open
(762) (27.9) (26.8)
200
36 83 4070 84 4240 Open 40
(914) (28.1) (29.2)
40 85 4030 86 4070 Open
(1016) (27.8) (28.1)
44 4000 88 Open 150
87 4000
30
(1118) (27.6) (27.6) .,
c.
48 813 4560 - - Open :;: ~
(1219) (31.4) ci ci
<( <(
0 0
30 89 4560 810 4080 Closed ...J
Specimen
Embedment
100 ...J
(762) (31.4) (28.1) 20 Length, In (mm)

81 30 (762)
36 Bll 4080 812 4560 Closed
(914) (28.1) (31.4) 83 36 (914)

85 40 (1016)
10
so
Table 2 - Concrete mix components for one 87 44 (1118)

cubic yard 813 48 (1219)

Component Weight
Fine aggregate: sand 1610 lb 0
Coarse aggregate: Y.-in. stone 510lb 0.30 0.60 0.90
'h-in. stone 840lb
DEFLECTION, in.
V.-in. stone 340lb
Type I portland cement 500 lb
Water 270 lb Fig. 2-Load-deflection responses of top-cast beams
Water-reducing admixture 35 oz having different embedment lengths
I lb. ~ 4.448 N.
DEFLECTION, mm

given in Table 1. The average measured slump was 4Y2 10 15 20

in. (114 mm) for all of the test specimens.

The No. 8 (25-mm diameter) test bar was instru- so

mented with electrical-resistance strain gages at a

spacing of 4 in. (102 mm), while the No. 6 (19-mm di-
ameter) bars were instrumented with strain gages at a 200
40
spacing that varied from 6 to 12 in. ( 152 to 305 mm).
Strain gages near the compression face of each beam
were used to measure the concrete strains.
150
30
.,
EXPERIMENTAL RESULTS c.
:;: ~
Effect of embedment length and position of ci
<(
ci
<(
casting on the responses 0
...J
100
0
...J

The measured load versus central deflection re- 20

Embedment
Specimen
sponses for the beams with top-cast test bars (B I, B3, Length, In (mm)

B5, B7, and Bl3) and for the beams with bottom-cast 82 30 (762)

test bars (B2, B4, B6, and B8) are shown in Fig. 2 and 84 36 (914)
50
3, respectively. All of these specimens contained open 10
86 40 (1016)
stirrups. The test bars in these beams had embedment 88 44 (1118)
lengths varying from 30 to 48 in. (762 to 1219 mm). If
the responses of the companion top-cast and bottom-
cast specimens having the same embedment lengths are 0.30 0.60 0.90
0

compared, it is evident that the bottom-cast beams ex-

DEFLECTION, in.
hibited better overall behavior than the top-cast beams.
The dashed lines shown in Fig. 2 and 3 are load-deflec- Fig. 3-Load-deflection responses of bottom-cast
tion predictions using the computed ultimate flexural beams having different embedment lengths
strengths and the calculated average effective moments
of inertia of the beams for different load levels, assum- Fig. 4 and 5 compare the variations in the measured
ing perfect bonding between the concrete and the rein- strains in the No.8 bar for top-cast specimens (Bl, B3,
forcing bars. B5, and B7) with those measured in the bottom-cast
ACI Structural.Journal I May-June 1988 253
 Beam 1Centerline
2500 2500

Yielding
2000

"'0
1500
"'0
X
1500
l:·jl\;~
I ·~~
"""[] 84
o I o
•
,o \
\
/6_6,....6-o\ o
)<
z 0
<
a: o-o I o~\ \\,
,!f:/,
f-
rJ)

I -· \ \ ,

1f~
1000 1000

i \\\
500 500
0 i \ 0 \0

0~-L~~~~~~~~-L~~~_u~~~ 0
H4" Typical 48
(102 mm)
Typical
(102 mrn)

POSITION OF GAGES
POSITION OF GAGES

Fig. 4-Measured tensile strains in No. 8 bar at failure Fig. 5-Measured tensile strains in No. 8 bar at failure
for specimens Bl, B3, B5, and B7 for specimens B2, B4, B6, and B8

specimens (B2, B4, B6, and B8). As expected, larger sponse in Fig. 2, Beam B 13 displayed an extremely
strains are developed in the No. 8 bar as the develop- ductile response similar to that for bottom-cast Beam
ment length is increased. It is clear from these two fig- B6 having a test bar embedment length of 40 in. (1016
ures that the top-cast bars required an embedment mm).
length of 44 in. ( 1118 mm), while the bottom-cast bars Photographs of the side faces and tension faces of
required only 36 in. (914 mm) to yield the reinforce- Beams B3 and B4 are shown in Fig. 8. For these beams,
ment. and in general for all the beams tested, the bottom-cast
The measured load versus central deflection re- specimens displayed better cracking response (i.e., a
sponses for Specimen B3 (top-cast) and B4 (bottom- larger number of more evenly spaced smaller cracks)
cast) are shown in Fig. 6. The various stages of behav- than the companion top-cast specimens due to the
ior, such as cracking at the bar cut-offs, the initiation larger tensile strength of the bottom-cast concrete. In
of longitudinal splitting cracks, and the appearance of addition, the top-cast specimens exhibited more severe
major shear cracks close to maximum load, are shown longitudinal splitting cracks than the bottom-cast spec-
in this figure. It is interesting to note that the top-cast imens (see tension faces of Beams 83 and B4 in Fig. 6).
specimen displayed flexural cracking at slightly lower From this series of tests on companion beams with
loads than the bottom-cast specimen, indicating that top-cast and bottom-cast No. 8 (25-mm diameter) bars
the tensile strength of the concrete at the top of the and an average concrete strength of 4170 psi (28.8
casting is lower than that at the bottom of the casting. MPa), it can be seen that a development length of 36
The effects of increasing the embedment length to 44 in. (914 mm) is required for a bottom-cast bar, while a
in. (1118 mm) for Beam B7 (top-cast) and Beam B8 length of 44 in. (1118 mm) is required for a top-cast
(bottom-cast) are apparent from Fig. 7. The test bar in bar. This implies a top bar factor of 44/36 = 1.22 for
the top-cast specimen just reached yield at maximum these tests. It is noted that the top-cast test bar had 15.5
load (see Fig. 4). Beam B8 displayed yielding of the test in. (394 mm) of concrete cast below it (see Fig. I).
bar along a length of about 28 in. (711 mm), as can be
seen from Fig. 5. Effect of transverse reinforcement and position
Only one beam, top-cast Specimen Bl3, had an of casting on the responses
embedment length of 48 in. (1219 mm). Due to the sig- Fig. 9 compares the responses of bottom-cast Beam
nificant yielding displayed in Beams B6 and B8 having B2, without transverse reinforcement crossing the plane
embedment lengths of 40 and 44 in. (1016 and 1118 of splitting, and bottom-cast Beam BIO, containing No.
mm), respectively, it was unnecessary to test a compan- 3 closed stirrups at 8-in. (203-mm) spacing crossing the
ion bottom-cast specimen with a longer embedment plane of splitting. The significant difference in the re-
length. As can be seen from the load-deflection re- sponses of these two beams in terms of their overall be-
254 ACI Structural Journal I May-June 1988
 DEFLECTION, mm

10 15 20
•
B3
50

-r
'
\

"\
/
200 .)
40 B3

(a) Beam B3- "Top-Cast"

150
30
"'
c.
:;: ~
ci ci
< ~Flexural Cracks at Cut-Off <
0 0
--' 100 --'
20

10 50

Embedment Length • 36in (914mm)

(b) Beam B4- "Bottom-Cast"

0.30 0.60 0.90 Fig. 8-Effect of casting position on the crack patterns
DEFLECTION, in.
on side faces and tension faces of Beams B3 and B4

DEFLECTION, mm

Fig. 6-Effect of casting position of the reinforcement 10 15 20

on the load-deflection responses of Beams B3 and B4 810 (closed stirrup)
50

DEFLECTION, mm
200
10 15 20
40
88 (bottom-cast)
50
87 (top-cast)

150
30
200
40 "'
c.
:;: z
ci
"'ci
< <
0 0
--' 100 --'
20
150
30
.,
c.
:;: ~
ci ci
< < 10
50
0 0
--' 100 --'
20

Embedment Length • 301n (762mm)

0.30 0.60 0.90

10 50
DEFLECTION, in.

Embedment Length • 44in ( 1118mm)

Fig. 9-Effect of transverse reinforcement on the load-
0 deflection response for bottom-cast beams
0.30 0.60 0.90

DEFLECTION, in.
havior (i.e., cracking, stiffness, maximum strength, and
maximum deformations) is clearly evident in Fig. 9.
Fig. 7-Effect of casting position of the reinforcement Both beams have embedment lengths of 30 in. (762
on the load-deflection responses of Beams B7 and B8 mm). The improved response of Beam BIO is due to the
ACI Structural Journal I May-June 1988 255
 DEFLECTION, mm
Beam! Centerline
10 15 20
2500

"'~
2000
)/\ y-- Yielding
50

40
B 11 (closed stirrup)

200

~ 1500 83 (open stirrup)

<(
a: 150
f-
rJ) 30

"'c.
:>: ~
1000 o" o"
<( <(
~ Flexural Cracks at Cut-off
0 0
--' 100 --'
20

50
10

Embedment Length • 36in (914mm)

4" Typical
(102 mm)

0.30 0.60 0.90

POSITION OF GAGES
DEFLECTION, in.

Fig. 10-Measured tensile strains in No. 8 bar at fail-

ure for specimens B2 and BJO Fig. 11-Effect of transverse reinforcement on the
load-deflection response for top-cast beams
presence of transverse reinforcement crossing the plane
of splitting. As can be seen from Fig. 10, significantly and from the ACI Committee 408 expressions 16 with the
larger strains were attained in the No. 8 bar of Speci- development length required to produce yielding of the
men B I 0. Specimen B 10 experienced significant yield- test bar. In the calculations, the average concrete
ing, whereas Specimen B2 did not reach yield, indicat- strength of 4170 psi (28.8 MPa) was used. For the cases
ing a bond failure. with no transverse reinforcement crossing the plane of
The responses of top-cast Beams B3 (without closed splitting, the ACI predictions are unconservative (by 19
stirrups) and Bll (with closed No. 3 stirrups at 8-in. percent for the bottom-cast bar and 7 percent for the
[203-mm] spacing) are compared in Fig. II. Both of top-cast bar), while the predictions using the recom-
these beams had embedment lengths of 36 in. (914 mendations of Committee 408 are conservative (by 16
mm). Once again, the improved response of the mem- percent for the bottom-cast bar and by 23 percent for
ber with the transverse reinforcement crossing the plane the top-cast bar).
of splitting (Beam B 11) is noted. For those cases with transverse reinforcement cross-
From the results of tests on specimens with closed ing the plane of splitting, the ACI method does not di-
stirrups (B9, BIO, B11, and Bl2), it can be seen that a rectly account for the presence of this reinforcement,
development length of 30 in. (762 mm) is required for while the ACI Committee 408 provisions account for
the bottom-cast No. 8 test bar while a length of 36 in. the size, yield strength, and spacing of the transverse
(914 mm) is needed for the top-cast No. 8 test bar. reinforcement. Once again, the ACI Committee 408
It is important to emphasize that the provision of predictions are conservative for both top-cast and bot-
transverse reinforcement crossing the plane of splitting tom-cast bars, and this approach provides a good pre-
reduces the development length required from 36 to 30 diction of the beneficial effects of the transverse rein-
in. (1016 to 762 mm) for a bottom-cast bar and from forcement.
44 to 36 in. (1219 to 914 mm) for a top-cast bar. These
results show that this amount of transverse reinforce- CONCLUSIONS
ment reduces the required development length by about This paper presents experimental data on the effects
20 percent. These results also imply a top bar factor of of top- versus bottom-cast bars on the response of full-
approximately 1.20. scale beams. An examination of the test results has led
to the following conclusions.
Comparison of development length provisions I. Beams with bottom-cast bars showed improved
with test results behavior in terms of the cracking, stiffness, strength,
Table 3 compares the development lengths computed and deformation response over the companion top-cast
from the ACI Building Code (ACI 318-83) expressions 15 beams.

256 ACI Structural Journal I May-June 1988

 2. For this test series of beams, both with and with- Table 3 - Comparison of required development
out transverse reinforcement crossing the plane of lengths computed by ACI method and
splitting, the top bar factor was found to be about 1.22. Committee 408 method with test results
3. The presence of transverse reinforcement across Required development length,
the plane of potential splitting can reduce significantly in. (mm)
r----
Committee
the required development length for both bottom-cast Casting Transverse ACl 408
and top-cast bars. This reduction was 20 percent for position reinforcement method'' method''' Test
this test series. Bottom None 29 42 36
4. The provisions suggested by Committee 408 pro- (737) (1067) (914)
vide more accurate predictions of development lengths Top None 41 54 44
(1041) (1372) (1118)
required than the provisions of the ACI Building Code.
In comparing these two methods it must be noted that Bottom No. 3 at 8 in. 29 33 30
(9.5 mm (737) (838) (762)
a <I> factor of 0.8 was used in making the predictions us- diameter at
ing the Committee 408 expression. 203 mm)
The aim of this experimental program is to provide Top No. 3 at 8 in. 41 43 36
(9.5 mm (1041) (1092) (914)
additional data on the behavior of beams with top-cast diameter at
and bottom-cast bars. Further research is needed to de- 203 mm)
velop more appropriate treatment of this important Note: Average f.' taken as 4170 psi (28.8 MPa). Capacity reduction factor
of 0.8 was used in Committee 408 expression for dewlopment length.
phenomenon in codes.

9. Leonhardt, Fritz, and Walther, Rene, "Schubversuche an Plat-

tenbalken mit unterschiedlicher Schubbewehrung," Bulletin No. 156,
Deutscher Auschuss fiir Stahlbeton, Berlin, 1963, 84 pp.
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5. Dutron, R., "Adherence des Armatures au Bewn," Revue des troit, 1971, 78 pp.
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Concrete," C&CA Library Translation No. 134, Cement and Con- troit, 1983, Ill pp.
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7. Ferguson, Phil M.; Breen, John E.; and Thompson, J. Neils, Standard Hook Provisions for Deformed Bars in Tension," (ACl
"Pull-Out Tests on High Strength Reinforcing Bars," ACI JouRNAL, 408.1 R-79), American Concrete Institute, Detroit, 1979, 3 pp.
Proceedings V. 62, No.8, Aug. 1965, pp. 933-950. 17. Jirsa, James 0.; Lutz, LeRoy A.; and Gergely, Peter, "Ra-
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et sur le Beton Arme," Annates, I.T.B.T.P. (Paris), Series Beton visions for Deformed Bars in Tension," Concrete International: De-
Arme No. 64, Nov. 1961, pp. 1199-1218. sign & Construction, V. I, No.7, July 1979, pp. 47-61.

ACl Structural Journal I May-June 1988 257

v shear force acting on beam section 4. MacGregor, J. G., "Challenges and Changes in the Design of
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I. Ritter, W., "Die Bauweise Hennebique," Schweizerische Bauz- Stress," ACI JoURNAL, Proceedings V. 81, No. 3, May-June 1984,
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268 ACI Structural Journal I May-June 1988