Effects of Bar-Placement Conditions on Steel-Concrete Bond

Materials and Structures (2006) 39:211–220

DOI 10.1617/s11527-005-9030-7

Effects of bar-placement conditions on steel-concrete bondTayfun A. Soylev · Raoul Francois

Received: 24 February 2005 / Accepted: 12 May 2005C© RILEM 2006

Abstract Vertical sections exhibit a variety of factors

that affect the bond between the embedded steel bars

and the concrete, like casting position, concrete char-

acteristics, and compaction procedure. The results ob-

tained in this project indicate that bond strength de-

creases if the concrete depth below the horizontal bar

increases, primarily because of the water-stop effect un-

derneath the bar, and of the settlement of fresh concrete.

This effect increases for larger values of water content

and water-cement ratio (w/c), which normally cause an

increase in slump, and a decrease in stability of concrete

(resistance to bleeding, settlement, and segregation).

However, very good bond properties were achieved

by using self-compacting concrete (SCC) tested ver-

sus standard concrete mixes, is a demonstration that in

the case of SCC higher slump values do not mean lower

bond properties.

Resume Sur la hauteur d’une section de beton, il y ade nombreux facteurs qui peuvent affecter l’adherenceentre les armatures et le beton. Cet article etudie plusparticulierement quelques parametres qui sont: la posi-tion des barres lors du coulage, les caracteristiques dubeton et la technique de mise en place. Les resultatsindiquent qu’il y a une chute de l’adherence avec

Tayfun A. SoylevIstanbul Kultur Universitesi, Turkey

Raoul FrancoisLMDC, INSA Toulouse, France

l’accroissement de hauteur de beton sous les armatureshorizontales, qui est du principalement a la formationde vides sous l’armature a cause des phenomenes deressuage et de tassement du beton frais. La meilleuretenue de l’adherence sur la hauteur de beton estobtenue dans le cas des betons auto-placant (BAP) parrapport aux betons classiques. Cette meilleure stabilitedes BAP montre qu’un plus grand affaissement au conene conduit pas necessairement a reduire l’adherenceacier-beton.

1. Introduction

The settlement and bleeding of fresh concrete favour

the formation of a void under the horizontal bars and

reduce steel-concrete bond. This “top-bar effect”, is

caused by several factors including the properties of

the concrete, the position and the shape of the bars, the

methods of concrete placement, and the type of form-

work [1–18]. Some design codes introduce the top-bar

effect by applying a reduction factor that increases the

anchorage length whenever the concrete depth under

the horizontal reinforcement exceeds a certain limit

[19, 20]. However, even if the lower bond is balanced

by the greater anchorage length, the corrosion of the

reinforcing steel is not ruled out. In addition to the for-

mation of a void underneath the bars (because of the

settling of fresh concrete), bleeding may change the

physicochemical properties of the concrete, by creating

212 Materials and Structures (2006) 39:211–220

small voids at the steel-concrete interface and provok-

ing both corrosion and bond-strength reductions [21].

This bond reduction is also worsened by the slump,

which increases above the maximum required by con-

crete settlement. According to Powers, above a mini-

mum of 17–18 mm, any further slump has to do with

the bleeding capacity, which in turn depends on the

excess water [22]. In the end, the “top-bar effect” is

thoroughly related to the slump.

One way to reduce the top-bar effect is to put less

water in the mix, by using water reducers and/or plas-

ticizers. For concretes of the same slump, high-range

water reducers (HRWR) or superplasticizers decrease

bleeding significantly. For the same water-cement ra-

tio, concretes with HRWR bleed more than the control

concrete [12] and exhibit a greater top-bar effect [6–

9]. At the same time, a viscosity-enhancing admixture

(VMA) can significantly reduce or eliminate bleeding

by increasing the viscosity of the paste. The combina-

tion of VMA and HRWR leads to a highly flowable

concrete with high stability (SCC) [14–18, 23–26].

Another way to reduce bleeding is through the use of

fine materials, such as silica fume, which can greatly

reduce bleeding by absorbing the excess water [12].

Last but not least, the size of cement grains and fine

aggregates (sand) and the use of air-entraining agents

are other important factors that affect bleeding [12].

While mechanical vibration has been shown to re-

duce the voids caused by the pouring of concrete, the

bleeding capacity is not markedly affected [12]. How-

ever, it has been established for a number of years

that internal vibrations can provide improved concrete-

steel bond when compared with hand rodding (for low-

slump concrete) [1]. In different studies, the vibrations

have been found more beneficial for higher slump con-

crete since in this case more compaction is needed to

overcome the extra settlement induced by high slump

values [6, 7]. On the other hand, the adverse effect

of vibrating concrete longer than necessary were con-

firmed, although these effects are small in comparison

with those of re-vibration [1]. This result is in contra-

diction with the findings of another research study on

the effects of the re-vibration, which indicated a sig-

nificant increase in bond strength for top-cast bar and

a less significant decrease in bond strength for bottom-

cast bars with a slump increase [8].

Self-compacting concrete achieves good

compaction without vibration due to its special

mix design [14–18, 23–26] that is based on a number

of factors aimed to increase both the fluidity and the

stability: reduced coarse-aggregate content to limit the

contact among aggregate particles, lower water-binder

ratio (w/b) which increases the viscosity of the paste

(to prevent the coarse particles from colliding), and

need to use a superplasticizer to increase workability

[26]. This combination is often supplemented by a

viscosity-enhancing admixture (VEA) to maintain the

moderate viscosity necessary to ensure the stability of

highly fluid concretes.

There are many studies on the top-bar effect, but

further research is needed to fully investigate the ben-

efits of SCC. In the present research project, the bond

qualities of SCC and conventional concrete are com-

pared. To this end, smooth bars are used because they

are more sensitive to interface quality and the better

bond qualities of SCC are more evident.

2. Experimental program

A plywood formwork was used for casting five deep

panels. The ordinary mixes were compacted by inter-

nal vibration. The concrete was cast in several lifts, and

each lift was vibrated. Vibrators were inserted rapidly,

held in place for a few seconds and then removed slowly

as described in references [6–8]. Each panel had 13 re-

inforcing bars that were placed horizontally and rigidly

fixed to the formwork. The distances from the bottom

of the panels to the centre of the reinforcing bars varied

from 0.1 m to 1.9 m. The centre-to-centre spacing be-

tween adjacent bars was 150 mm. Smooth round steel

bars (φ = 10 mm) were used.

The panels were removed from the formwork after 6

days and were cured at 100 % relative humidity for 28

days. Then, each panel was cut in 13 prismatic speci-

mens, with a single steel bar at the centre of each prism

(Fig. 1).

2.1. Concrete mixture proportions

Five different mix designs were adopted for the con-

crete (Table 1): C20, C40, SCC40, C50, and SCC50.

The mixes were named with respect to their com-

pressive strength (20, 40 and 50 MPa). C stands for vi-

brated concrete and SCC stands for self-compacting

concrete.

Materials and Structures (2006) 39:211–220 213

Table 1 Mix design

Materials (kg/m3) C20 C40 SCC40 C50 SCC50

CPA CEM I 52.5 PM 304 365.5 430 450

ES CP2

CPA CEM I 52.5 310

CP2

Silica fume 30

Total water 229 195 186 167 186.9

Sand 3.15 R 490

Sand 0/4 C 330

Sand 0/4 R 728

Sand 0/5 R 990 736 780

Coarse aggregate 4/10 C 460

Coarse aggregate 4/10 R 992

Coarse aggregate 10/14 C 360

Coarse aggregate 5/15 R 862 1117 1020

Limestone filler 140

Viscocrete 2100 3.84

(plasticizer)

Viscocrete 3010 4.05 7.68

(VMA - HRWR)

Plastiment HP 1.8

(plasticizer)

Sikatell 200 (VMA) 1.28

Glenium 27 8.6

(superplasticizer)

Water/cementitious 0.75 0.53 0.60 0.39 0.39

materials ratio

Fig. 1 Concrete panel measuring 2000 mm in height and sam-ples obtained by sawing: the 150 mm part was used for pullout test and the other part to study steel-concrete interface atvıdeomicroscope.

2.2. Materials

The cement used for SCC 40 was a French Portland

cement (CPA-CEMI 52.5 CP2). For the other mixes,

a French Portland cement (CPA-CEMI 52.5 PM ES

Table 2 Physical properties of cement

CPA CEMI 52.5 PM ES CPA CEMI 52.5

CP2 CP2

Density 3.15 3.15

blaine (cm2/g) 3700 4400

CP2) with a low-C3A content (sulfate-resistant) was

used. The properties of the cements are given in Ta-

ble 2. The silica fume used had a specific surface of

15 m2/g. The particle size of the limestone filler was

≤0.1 mm.

Aggregates from three sources were used:� natural siliceous limestone sand and gravel for the

vibrated concretes (0 to 5 mm & 5 to 15 mm� natural siliceous limestone sand and gravel for the

SCC (0/3, 0/4, 4/10)� crushed aggregates for the SCC (0/4, 4/10, 10/14)

Various types of admixtures were used, mostly by

SIKA (Plastiment HP = high-range plasticizer; Vis-

cocrete 2100 = plasticizer and water reducer; Vis-

cocrete 3010 SCC plasticizer for SCC and viscosity-

enhancer; Sikatell 200 VP = cohesiveness enhancer;

Glenium 27 by MBT = superplasticizer).

All mixes were prepared in 110-litre batches. In each

batch, 80 litres were intended for the reinforced panel

and 30 litres for concrete testing.

2.3. Test program

The slump, slump flow, bulk density and entrapped-air

content by volume of fresh concrete were measured.

Both the compressive and the tensile strength (splitting

test) at 28 days were measured by cylindrical specimens

(L = 220 mm; φ = 110 mm, Tables 3 and 4).

Prior to concrete casting, the extremities of each

bar were fitted up with plastic pipes (100 mm) to

limit the bond between the bar and the concrete to

Table 3 Tests on fresh concrete

C20 C40 SCC40 C50 SCC50

Slump (cm) 15.8 7.6 23

Slump flow (cm) 63 60

Entrapped air (%) 2 1.4 2.2 1.4 1.8

Density 2.38 2.43 2.33 2.48 2.41


Table 4 Strength at 28 days (MPa)

C20 C40 SCC40 C50 SCC50

Compressive strength 27.4 45.8 43.9 55.4 57.1

(MPa)

Tensile strength (MPa) 3.0 3.8 3.4 4.4 5.1

(Brazilian test)

50 mm. The pull-out load versus slip was recorded.

The average ultimate bond strength (τmax) was calcu-

lated by Eq. (1) which has the following, well-known

expression

τmax = Pmax

πdl(1)

where Pmax, d and l correspond to the ultimate pull-

out load, the bar diameter and the anchored length,

respectively.

The steel-concrete interface was analysed by means

of a video-microscope at a magnification of 25 and

175 times. The areas of defects were quantified in

terms of steel length in the radial direction not

bonded to concrete at the lower part of the bar

(Fig. 2).

Fig. 2 Steel-concrete interface observed at videomicroscopewith an amplification of 25 times.

3. Results and discussion

3.1. Effect of bar position

3.1.1. Concrete below the bar

The scattering of τmax with the thickness of the concrete

layer underneath the bar is plotted in Fig. 3. It appears

clearly that the larger the cover underneath the bar, the

lower the bond strength. The decrease appears to be

more important for conventional concrete mixes.

Fig. 3 (a) Ultimate bond strength as a function of concrete coverunderneath the bar (five mixes). (b) – Ultimate bond strength asa function of concrete cover underneath the bar (five mixes).


3.1.2. Casting position

The effect of the casting position is particularly evident

when top-cast and bottom-cast bars are compared. The

ratio of top-to-bottom τmax values (bond efficiency ra-

tio) is plotted in Fig. 4.

The largest decrease in bond efficiency occurs when

the concrete cover underneath the bar is 40 cm for C20,

C40, and SCC40 (54 to 64 percent decrease with respect

to bottom-cast bar). In spite of the scattering of the

results, it appears that τmax decreases slightly for large

values of the concrete cover underneath the bar. The

decrease in the bond strength varies between 77 and 32

Fig. 4 (a) Bond efficiency as a function of concrete cover un-derneath the bar (five mixes).(b) Bond efficiency as a function ofconcrete cover underneath the bar (five mixes).

percent with respect to the bottom bar for these three

concrete mixes, except for C20 in the case above 160

cm.

τmax decreases less than in the other mixes for con-

crete depth ≤40 cm (28 percent). For larger values of

the concrete depth (up to 160 cm) τmax decreases by

21–47 percent.

The unexpected increase of the bond strength above

the bottom-cast bar for C50 may be due to the free fall of

the fresh concrete [15], which is more likely to cause a

segregation because of the high amount of HRWR used

without any incorporation of fine material or VEA [6–

9, 14–18, 23–27]. This trend is the same up to 85 cm.

Above this depth the bond strength decreases are lower

than that of the cast-bottom bar (13 to 58 percent).

3.2. Effect of concrete properties

3.2.1. Slump

There are many studies on the combined effect of the

slump and of the concrete cover underneath the bar for

reducing bond strength [1–10, 15–18]. However two

conventional concrete mixes, C20 and C40 have similar

trends in the terms of bond efficiency as a function of

the cover (in spite of the marked difference between the

slump values of the mixes, 160 mm in the former case

and 80 mm in the latter case). For SCC40, the variation

of bond strength is similar to those of C20 and C40,

but the loss of bond strength is slightly lower, in spite

of its higher slump. The last two concrete mixes C50

and SCC50, in spite of the higher slump with respect to

conventional concrete, exhibit lower bond decrements.

The anchorage length should be increased by multi-

plying the development length of the top-bars by an am-

plification factor to maintain a constant bond strength

at any height in the fresh concrete. This factor (called

casting position factor) is the inverse of the previously

defined bond efficiency ratio and has been shown to in-

crease with the concrete bottom cover and/or with the

slump [4].

In Fig. 5 the casting position factor is plotted as a

function of concrete bottom cover. The linear function

of the C40 has the highest slope and the C50 the lowest.

The slope of the linear function of C20 is very close to

that of C40 if the two top bars with a complete loss of

bond are not considered.

ACI Code [19] and Eurocode [20] require to in-

crease of the development lengths by 30 and 40 %,


Fig. 5 Plots of the casting position factor as a function of con-crete cover underneath the bar (five mixes).

respectively, for reinforcing bars whenever the bottom

cover is larger than 300 and 250 mm respectively. Code

requirements are indicated on the Fig. 5.

The large slopes of C20 and C40 can be expected

because a smooth bar has no ribs on its underside to

maintain contact with the settled concrete and only a

slight amount of settlement has a large effect in re-

ducing the surface area of the bar in contact with the

concrete [2].

3.2.2. Bleeding

The voids under horizontal bars increased with the con-

crete bottom cover. In many previous studies, these

voids were supposed to be caused by bleeding and set-

tlement [1–18,21,27]. This assumption is based on two

criteria as confirmed by the results of the present re-

search: (1) Since the depth of concrete available for

settlement and bleeding increases, the voids increase;

(2) Void increase is correlated to the water increase and

w/c, which are the main factors determining the set-

tlement and bleeding in a conventional concrete. The

defect factor, which is the ratio of the length not bonded

to the bar perimeter, is plotted in Fig. 6.

Defect development corresponds to three zones of

the sedimentation due to concrete bleeding, which is

described in Reference [12]. Those zones make it pos-

sible to directly compare the internal bleeding mecha-

nism to void formation and bond strength. Up to 25–40

cm is the compression zone, where the concrete is com-

pacted by its own weight. Above this zone, there exists

a constant zone of defect and above 160 cm, void for-

Fig. 6 (a) Plots of the defect factor as a function of concretecover underneath the bar (five mixes). (b) Plots of the defectfactor as a function of concrete cover underneath the bar (fivemixes).

mation is more important, corresponding to the zone of

clear water. In this last zone there is a marked accumu-

lation of the bleed water. This zone was very significant

C20, which displayed a complete loss of bond. The de-

fects occurred above 40 cm for C20 and above 25 cm

for C40 and SCC40. For the strength class of 50 MPa,

there were no defects at the interface.

In Fig. 7, bond efficiency ratio and the defect fac-tor appear to be roughly linearly related. In the middle

part of the concrete panel, there is a good correlation

between bond strength and void formation below the

horizontal bar. But near the bottom and top of the con-

crete, there is no proportionality. Near the bottom, be-

cause of the higher bulk density of the concrete, the

bond efficiency factor decreases without a change in

the defect factor. The upward movement of bleed water


Fig. 7 Bond efficiency as a function of defect factor.

reduces the concrete quality in the top part of the con-

crete [12, 15–18, 24, 25, 27]. This deterioration may

have an effect on the loss of bond as the bond strength

is proportional to the strength of the surrounding con-

crete.

In Fig. 8 the capillary porosity of the five mixes con-

sidered is plotted as a function of the bottom concrete

cover. Above 25 cm, the porosity is mostly indepen-

dent of the bottom cover regardless of the concrete mix

except the C20, which exhibits a slight increase due to

its higher w/c. In the bottom part of the panels (10 cm),

there is a denser zone of concrete due to the compaction

caused by the self-weight.

In spite of the tendency to a slight decrease with the

bottom concrete cover, the compressive-strength values

exhibit a marked scatter (Fig. 9).

This scatter is mostly due to the perpendicular coring

of specimens with respect to the direction of casting

[15,16]. The results found in the literature confirm this

pattern of scatter accompanied by a slight tendency to

decrease with the bottom concrete cover [15, 16, 24].

According to Khayat [25], the compressive strength

tends to decrease with the bottom concrete cover at

large strength values and becomes irrelevant above 80

MPa.

However, the decrease in pull-out resistance that oc-

curred (Fig. 3) at the interface (before 25 cm to 40 cm

concrete depth for C40, SCC40, C20 and along the

whole concrete section for C50 and SCC50) must be

due to concrete quality combined with interface quality.

3.2.3. Compressive strength class

The bond strength increases with the compressive

strength (Fig. 4). The average bond strength of 13 bars

for each concrete panel was 3.77 MPa for C20, 5.87

Fig. 8 (a) Porosity as a function of concrete cover underneaththe bar (five mixes). (b) Porosity as a function of concrete coverunderneath the bar (five mixes).

MPa and 5.4 MPa for C40 and SCC40 respectively,

and 8.77 MPa and 11.28 MPa for C50 and SCC50 re-

spectively. The differences between C50 and SCC50

may be due to the high amount of HRWR used in C50.

The bond strengths were normalized with reference

to the concrete strength of 27.4 MPa, (which is the

minimum compressive strength in this project, MIX

C20) assuming for the bond strength a square-root de-

pendency on the compressive strength. Fig. 10 shows

the normalized τmax as a function of the thickness of

the bottom concrete cover. The average bond strength

values were still in the same ascending order for C20,

SCC40, C40, C50, SCC50 as 3.77 MPa, 4.54 MPa,

4.27 MPa, 7.94 MPa, 6,07 MPa, respectively. This data

emphasizes the role that the steel-concrete interface has

on bond strength development.


Fig. 9 (a) Compressive strength as a function of concrete coverunderneath the bar (five mixes). (b) Compressive strength as afunction of concrete cover underneath the bar (five mixes).

3.3. Effect of compaction

There was no significant difference between the devel-

opment of bond strength for C20, C40, and SCC40, in

spite of the marked differences in w/c and slump [4].

This result may be due to the vibration, which tends to

decrease the accumulation of bleed water beneath the

horizontal bars [1, 6–8]. In previous studies, the vibra-

tion has been found more beneficial for higher-slump

concrete since higher-slump concrete need more com-

paction to overcome the extra settlement that occurs

with high slump [6, 7]. However, the upward move-

ment of this bleed water causes higher amount of

accumulated bleed water in the top part of the vertical

concrete (external bleed water). Consequently, there is

Fig. 10 (a) Ultimate normalized bond strength as a function ofconcrete cover underneath the bar (five mixes). (b) Ultimate nor-malized bond strength as a function of concrete cover underneaththe bar (five mixes).

a complete loss of bond for C20, which has a higher w/b.

SCC40 performed similarly to the vibrated C40, despite

its higher w/b ratio. Lower values of w/b were accom-

panied by a lower bleeding and, C50 and SCC exhibited

more stability with increasing concrete depth and they

performed similarly. However C50 was prone to segre-

gation during the free fall of the fresh concrete towards

the bottom. Another effect was the marked segregation

between the paste and the aggregate in the top part of

the vertical concrete, which could be directly observed.

4. Conclusions

Bond strength decreases as a function of the concrete

depth underneath horizontal bars. The decrease is less

marked for high-grade concretes. The highest decrease


was measured at depth values equal to or larger than 25

cm, that is the limit depth of concrete above which

a correction for the anchorage length is required in

some design codes [19, 20]. The bond strength de-

crease is strongly dependent on the stability of the con-

crete. However, the fluider the concrete, the lower the

its stability, but SCC maintain its stability in spite of

the higher slump. These results support the findings of

previous work [1–11, 15–18], and lead to the following

conclusions:

Void formation underneath the horizontal bars and

bond strength are strongly related to the thickness of the

concrete layer underneath the bars. The quantification

of the void formation with a video-microscope seems to

be an appropriate way to measure the effect of internal

bleeding.

The capillary porosity of concrete increases as a

function of concrete depth underneath the bars, but

above 25 cm the increase is insignificant. The compres-

sive strength exhibits a higher variability as a function

of concrete depth but with a slight tendency to decrease.

Concrete-strength development as a function of the dis-

tance from the bottom face seems to have a minor effect

on the decrease of the pull-out resistance

The number of control points used in the present

research is much higher than in previous work [1–11,

15–18]. As a result, a higher precision was reached

in identifying some zones along the bottom concrete

cover. As a first approximation, bond strength decreases

linearly with the thickness of the concrete cover under-

neath the bar. However, a three-stepped function may

give a better description of the dependency of τmax on

the three zones with different bleeding characteristics:

The highest bond values occur up to 25–40 cm. There-

after, a very significant drop occurs for cover thick-

ness (thickness values >25–40 cm) and then more the

behaviour becomes more uniform in spite of a small

decrease up to 160 cm. Finally an abrupt decrease can

be seen above 160 cm. This behaviour matches exactly

the void formation.

5. Future study

The top-bar effect can be overcome to a significant

degree by the gripping action of ribbed bars [1–3]. It

would be useful to test and compare the performance

of several types of bars, bonded to SCC, with different

rib shapes and sizes. Finally, further testing of smooth

bars may be instrumental in studying the effects of re-

inforcement corrosion on bond, since bond strength in

smooth bars is very sensitive to nay change in interface

properties.

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Effects of Bar-Placement Conditions on Steel-Concrete Bond

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