Bond of tension bars in underwater concrete: effect of bar diameter and cover

1 23

Materials and Structures ISSN 1359-5997 Mater StructDOI 10.1617/s11527-014-0414-4

Bond of tension bars in underwaterconcrete: effect of bar diameter and cover

Camille A. Issa & Joseph J. Assaad

1 23

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ORIGINAL ARTICLE

Bond of tension bars in underwater concrete: effect of bardiameter and cover

Camille A. Issa • Joseph J. Assaad

Received: 14 October 2013 / Accepted: 28 August 2014

� RILEM 2014

Abstract This research project is undertaken to

assess the effect of washout loss on the drop in bond

properties of reinforcing steel bars embedded in

underwater concrete (UWC). Special emphasis was

placed to evaluate bond using the same concrete

mixtures that were subjected to washout. Testing was

realized using the beam-end specimen method, and

parameters evaluated included level of washout loss,

bar diameter, and concrete cover. Test results showed

that bond between steel and UWC is affected by the

level of washout loss, which in turn is directly

influenced by the mixture composition. Similarly to

bond in concrete cast and consolidated above water,

the ultimate UWC bond strength increases for smaller

bar diameters and higher confinement reflected by

increased concrete covers. UWC mixtures with

increased washout losses exhibited higher drops in

compressive, tensile, and bond strengths, as compared

to concrete cast above water. Two boxes depending on

the level of washout loss, i.e. from 4.2 to 6.9 % and

from 8.8 to 10.8 %, have been proposed to predict the

extent of bond decrease in UWC mixtures.

Keywords Underwater concrete �Washout loss �Bond properties � Concrete cover � Bar diameter

1 Introduction

Bond of steel reinforcement to concrete cast and

consolidated above water has been studied extensively

over the last decades and large amount of experimental

and analytical data are published in literature [1–3].

For deformed bars, ACI 408 committee reported that

force transfer from the reinforcement to surrounding

concrete occurs by chemical adhesion between both

materials, frictional forces arising from the roughness

of interface, and mechanical bearing of the steel ribs

against the concrete surface [1]. Numerous parameters

were found to affect the bond behavior including bar

properties (i.e., yielding strength, cover, size, position

in the cast element, geometry, relative rib area, epoxy

coating, and others) and concrete properties (i.e.,

tensile strength (ft), compressive strength (f’c), den-

sity, presence of mineral admixtures, workability,

method of consolidation, and others). The develop-

ment (or embedment) length required to ensure

adequate transfer of stresses can be calculated using

equations specified in various Building Codes [4–6].

For instance, Eq. 12.1 in ACI 318 Building Code

reports that the development length for tension bars is

inversely proportional to the square root of f’c,

multiplied by additional factors to account for special

C. A. Issa

Civil Engineering Department, Lebanese American

University (LAU), P.O. Box 36, Byblos, Lebanon

J. J. Assaad (&)

Holderchem Building Chemicals,

P.O. Box 40206, Baabda, Lebanon

e-mail: [email protected]; [email protected]

Materials and Structures

DOI 10.1617/s11527-014-0414-4

Author's personal copy

design considerations [4]. For example, for bars coated

with epoxy, ACI 318 recommends multiplying the

development length by a factor of 1.5 for bars with a

cover less than 3 db or clear spacing between bars less

than 6 db (where db is the bar diameter), and a factor of

1.2 for other cases [4]. In the AASHTO bridge

specification, these factors are 1.5 and 1.15, respec-

tively [5].

The current knowledge for bond behavior and use

of existing equations and factors to evaluate develop-

ment and splice lengths become unwarranted when

casting takes place underwater. In fact, the in situ ft

and f’c for underwater concrete (UWC) is highly

dependent on the mixture proportions and casting

conditions, thus making difficult a realistic estimation

of the contribution of concrete strength to the bond

against steel bars. Typical residual f’c values reported

in literature varied from 80 to 90 % for UWC cast

using the tremie/hydrovalve technique [7], 50–70 %

for self-consolidated UWC depending on turbulence

of water and location of extracted cores for strength

testing [8], and as low as 40 % for concrete having a

slump of 230 mm made without anti-washout admix-

ture (AWA) [9]. The drop in UWC strength can be

attributed to a combination of factors such as washout

loss of cementitious particles together with alteration

in concrete homogeneity due to aggregate segregation.

Additionally, water infiltration within the freshly cast

concrete during placement increases the specified

water-to-cement ratio (w/c), thus reducing ft and f’c.

Several testing methods are available in literature to

evaluate washout loss of UWC. For example, Sakata

proposed measuring the turbidity of water using

standard light transmittance apparatus following the

drop of 0.5-kg of fresh concrete into a vessel

containing around 5 l of water [10]. By calibration

using known dispersions of cement in water, the

amount of washout occurring as a result of concrete

falling through water can be determined. Another test

developed by Hughes [7] consisted on evaluating the

segregation susceptibility of coarse aggregate of fresh

concrete when placed under water. This later test

describes the scattering of concrete after having

dropped over a cone from two hoppers, once in air

and another time through water. The US Army Corps

of Engineers proposed using a more practical proce-

dure (CRD C61 test method) for assessing washout

loss of UWC [11]. This test consists on placing

approximately 2 kg of fresh concrete in a wire-mesh

basket and allowing it to free fall vertically in a 2-m

high column of water. The sample is then hauled

slowly to the surface at a constant speed of 0.5 m/s and

weighed to determine washout loss. Staynes and

Corbett reported that CRD C61 test results relate well

to practical underwater casting conditions that corre-

spond to the free fall of concrete from a pump delivery

hose through 1–2 meters of water [12].

2 Context and objectives of this project

Limited studies were undertaken to assess the effect of

washout loss and aggregate segregation on the drop in

UWC bond strength. Also, no design provisions or

correction factors have been proposed to enable

adequate estimation of development length for bars

in tension during underwater works. The lack of

research is primarily due to the difficulty of obtaining a

representative concrete sample capable of reflecting

actual UWC properties that result during underwater

casting. For instance, the common approach to eval-

uate f’c of UWC consists on dropping fresh concrete

into molds placed in water tanks. However, this

approach presents the inconvenience of not reflecting

actual conditions or accurately estimating the degree

of washout loss that the fresh UWC sample has

undergone during the dropping process in water [13,

14]. On the other hand, it is not accurate to correlate

washout loss to compressive or bond strength deter-

mined from cores extracted from existing underwater

structures, given the direct effect of the casting method

on washout loss [8, 15].

Recently, Assaad and Issa adopted a new approach

to assess the effect of washout loss and aggregate

segregation on the drop in UWC bond properties [16].

The approach consisted of subjecting a series of UWC

samples taken from the same batch to washout loss as

per the CRD C61 test method, and then after securing

enough materials, the samples are vigorously mixed

together in a clean container for subsequent use in

bond testing. For simplicity reasons, the direct pullout

bond test was used as it required the least amount of

UWC materials (around 5-kg of washed samples were

enough to fill the specimen measuring 150-mm in

diameter and 120-mm in height). Steel bars having

12-mm diameter whether coated or not with epoxy

were tested. The authors reported that bond strength is

directly dependent on the level of washout loss which,



in its turn, is affected by the mixture composition and

casting conditions [16]. For example, the residual

bond strength dropped from 90 to 65 % for mixtures

possessing CRD C61 washout loss values ranging

from 7.5 to 15 %, respectively, and aggregate segre-

gation varying from 70 to 95 kg/m3, respectively. The

hydrostatic water pressure at the casting location and

interfacial concrete/water velocity are crucial param-

eters that can magnify the drop in UWC strength [17].

Although the previous studies enabled good under-

standing of UWC bond properties, it is important to

note that the tests done using the direct pullout method

cannot be conclusive mainly because of the dissimi-

larity between the pullout test conditions and actual

loading situations encountered in service, especially in

flexural members [1, 18]. In fact, as the bar is loaded in

tension during a direct bond test, the concrete is

restrained in compression against the loading frame.

This is quite unrealistic compared to real engineering

situations which makes difficult the extrapolation of

such data to real-life design. Therefore, the main

objective of this paper is to evaluate bond properties

between UWC and steel bars using tests realized as per

the general guidelines of ASTM A944 for beam-end

specimen [19]. Unlike the direct pullout test, the

beam-end specimen is capable of duplicating the stress

state occurring in structural members where both

concrete and steel are placed in tension. Washout loss

of UWC was measured using an adapted version of the

CRD C61 test capable of securing greater amounts of

materials for testing. A total of 30 mixtures sampled in

dry or washed conditions were tested to quantify the

effect of washout on the drop in UWC bond properties

for different bar diameters and concrete covers.

Regression models enabling the prediction of bond

strength are established with respect to the level of

washout loss, bar diameter, concrete cover, f’c, and ft

values.

3 Experimental program

3.1 Materials

Portland cement conforming to ASTM C150 Type I

was used in this study. Its Blaine surface area and

specific gravity were 345 m2/kg and 3.14, respec-

tively. A polycarboxylate-based high-range water-

reducing (HRWR) admixture and liquid cellulosic-

based AWA with specific gravities of 1.1 and 1.12,

respectively, and solid contents of 40 and 30 %,

respectively, were employed. The gradations of

crushed limestone aggregate having nominal maxi-

mum particle size of 20 mm and siliceous sand were

within ASTM C33 specifications [20]. The coarse

aggregate and sand had fineness moduli of 6.4 and 2.5,

respectively, and bulk specific gravities of 2.72 and

2.65, respectively.

Commercially available deformed bars complying

to ASTM A615 [21] specification, Grade 60, were

used to evaluate bond properties. Four types of bars

including No. 13, 16, 19, and 25 with nominal

diameter (db) of 12.7, 15.9, 19.1, and 25.4 mm,

respectively, were tested. Their bond indexes (or,

relative rib area ratio) were equal to 0.048, 0.069,

0.088, and 0.106, respectively. The bond index was

calculated as (de2–di2)/4 db S, where de is the external

bar diameter (top of rib), di is core diameter (bottom of

rib), and S is longitudinal spacing of the ribs [22]. The

bars had upper yield and tensile strengths equal to

420 ± 6 MPa and 625 ± 10 MPa, respectively, when

tested as per ASTM E8 [23].

3.2 Mixture proportioning

As summarized in Table 1, two series of UWC

mixtures proportioned to achieve different compres-

sive strengths in dry condition (f’c(dry)) of 32 ± 2 and

57 ± 3 MPa were tested; the corresponding cement

content increased from 350 to 425 kg/m3, respec-

tively, and w/c decreased from 0.56 to 0.47, respec-

tively. In each series, different combinations of

admixtures commonly used for proportioning flow-

able to highly flowable UWC for repair or new

construction were used. The AWA was added at

0.5 % of cement mass, while the HRWR was adjusted

to secure a slump of 220 mm or slump flow of

450 mm. Also, a UWC mixture having 220 mm

slump made without AWA was tested. Additional

discussion regarding the mixture composition and

optimization of washout loss of UWC can be seen in

other publications [17, 24]. The sand-to-total aggre-

gate ratio was fixed at 0.46 for all tested concrete. A

gluconate-based set-retarder was added at a dosage of

0.35 % of cement mass to minimize slump or slump

flow loss during testing. It is to be noted that the

mixture codification used throughout the paper refers



to cement content-w/c-Consistency (S22: slump of

220 mm; SF45: slump flow of 450 mm)-AWA.

3.3 Mixing and characterization of fresh

properties

All mixtures were prepared in an open-pan mixer of

100-Liters capacity. The mixing sequence consisted of

homogenizing the sand, aggregate, and around 50 %

of mixing water before introducing the cement. After

one minute of mixing, the other 40 % of water was

added, followed by the HRWR, and then the AWA

diluted in the remaining 10 % of water. The concrete

was mixed for two additional minutes.

Following the end of mixing, the workability, air

content, and washout mass loss were evaluated. The

slump, slump flow, and air content were determined as

per ASTM C143 [25], C1611 [26], and C231 [27],

respectively. Values of air content were found equal to

2.3 ± 0.4 % for all tested mixtures. Filling ability of

flowable UWC having 450 mm slump flow was

evaluated using the L-box test, and found to vary

from 38 to 55 % [17]. All characterization of fresh

properties was realized under laboratory conditions

where ambient temperature and relative humidity was

21 ± 3 �C and 60 ± 8 %, except for washout testing

as per the adapted CRD C61 test where outdoor

temperature varied from 14 to 20 �C.

3.4 Evaluation of washout mass loss

Washout of UWC was determined using two different

methods. The first was according to the standard CRD

C61 test which consists on placing a fresh concrete

sample in a wire-mesh basket and allowing it to free

fall vertically in a 1.7-m high column of water [11].

The sample was then slowly retrieved to the surface

and weighed to determine washout loss. Cumulative

washout loss after 3 drops in water is reported and

referred to as W3. Around 2-kg are normally used

when testing washout loss as per the CRD C61 test

method.

In order to secure enough washed materials for

subsequent filling in the beam-end specimen mold, an

adapted version of the CRD C61 test was developed.

The principle is quite similar to the standard test,

however, a larger wire-mesh basket (i.e., 300 mm in

diameter) was used to allow testing up to 8 kg of fresh

concrete. The circular perforations of the basket as

well as the distance between adjacent perforations

remained the same as in the original CRD C61 test

(i.e., 3 and 5 mm, respectively). The differences

between the standard and adapted CRD C61 test

methods are listed in Table 2. After 15 s at the bottom

of the test column, the basket was retrieved at constant

speed of 0.5 m/s, and weighed to determine washout

mass loss referred to as W. Washout tests were

conducted using a large water container measuring

2.2 m in height and 1.8 m in diameter (Fig. 1).

Different UWC mixtures were batched and tested

for washout to validate the procedure and select the

appropriate number of drops in water to be used in this

testing program. The mixtures possessed different

cohesiveness levels secured by adjusting the HRWR to

yield various consistency levels and/or adding various

AWA concentrations to modify mixture viscosity. As

can be seen in Fig. 2, W determined after 1 drop tends

to under-estimate W3, mostly due to increased con-

crete volume (8 kg vs. 2 kg) that reduces the extent of

relative washout upon contact with water. Conversely,

the increase in the number of drops to three led to

higher W values with considerable decrease in the

Table 1 Mixture composition of tested UWC

f’c(dry) = 32 ± 2 MPa f’c(dry) = 57 ± 3 MPa

Cement content = 350 kg/m3

and w/c = 0.56

Cement content = 425 kg/m3

and w/c = 0.47

Water, kg/m3 196 200

Fine aggregate, kg/m3 820 790

Coarse aggregate, kg/m3 970 930

Combination of admixtures used HRWR adjusted to yield slump of 220 mm

AWA = 0.5 % of c.m.; HRWR adjusted for 220 mm slump

AWA = 0.5 % of c.m.; HRWR adjusted for 450 mm slump flow



correlation coefficient (R2) to 0.73. Hence, W deter-

mined after two drops in water was selected through-

out this project when using the adapted CRD C61 test,

as almost similar washout values than the intrinsic W3

were obtained with acceptable R2 of 0.93.

Generally speaking, W and W3 determined using

the adapted or standard CRD C61 tests are similarly

affected by the mixture composition. For example,

lower values are obtained for UWC prepared with

combinations of higher cement content and lower w/c.

Also, the addition of AWA resulted in lower washout,

for given consistency level. This can be attributed to

the mode of function of such additives which binds

part of the mixing water, increases cohesiveness, and

leads to lower washout upon concrete contact with

surrounding water [24]. The increase in the level of

consistency (i.e., from slump of 220 mm to slump flow

of 450 mm) led to higher washout. Additional discus-

sion regarding the effect of mixture composition on

washout variations can be seen in other references [17,

24].

3.5 Determination of UWC bond properties

3.5.1 Securing materials for bond testing

As already noted, the bond stresses of UWC to

embedded steel were determined using the same

concrete samples that were subjected to washout.

The procedure used for securing enough materials is as

follows:

Two operators holding two wire-mesh baskets

containing approximately 8-kg of fresh concrete each

stood around the 1.8-m diameter water container

(Fig. 1). The baskets were dropped twice in water,

retrieved to determine W, and then moved to a clean

container which was covered by wet burlap. Subse-

quently, other fresh samples were taken from the same

concrete batch, subjected to similar washout loss

testing, and again stored in the same container. This

process was repeated a third time to obtain a total mass

Table 2 Differences between standard and adapted CRD C61

test methods

Standard

CRD

C61 test

Adapted CRD

C61 used for

testing larger

concrete

volumes

Height of water tube 1.7 m 2.2 m

Diameter of water tube 200 mm 1.8 m

Diameter of the basket 130 mm 300 mm

Nominal diameter of the

basket’s perforations

3 mm 3 mm

Distance between

adjacent perforations

5 mm 5 mm

Sample used for testing Around 2 kg Around 8 kg

Number of drops in water 3 drops 2 drops

Washout referred to as: W3 W

Fig. 1 Photo of the container used for measuring washout of

larger UWC volumes as per the adapted CRD C61 test

R² = 0.96

y = 0.92 xR² = 0.93

y = 1.14 xR² = 0.73

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14

Was

ho

ut

loss

as

per

ad

apte

d C

RD

C

61 t

est

(W),

%

Intrinsic CRD C61 washout loss (W3), %

W after 1 drop

W after 2 drops

W after 3 drops

y = 0.801 x

Fig. 2 Relationships between intrinsic W3 and washout

determined using the adapted CRD C61 test



of around 45 kg (taking into consideration the material

lost due to washout), which was then rigorously mixed

for filling the beam-end specimen mold. The entire

time needed to complete the three cycles of washout

did not exceed 20 min, whereby slump and slump flow

loss thresholds were limited to less than 30 mm. It is to

be noted that the washout values obtained from both

operators between the first and last cycle of testing

were found to vary by ± 0.55 % for UWC mixtures

having a total washout loss ranging from 4 to 6 %.

Such variation was less than ± 0.9 % for mixtures

having total washout varying from 7 to 11 %.

3.5.2 Bond strength of UWC to steel bars

Given the difficulty to secure large volumes of washed

UWC materials, bond properties were determined

using adapted beam-end specimens following the

general guidelines of ASTM A944 [19]. The speci-

mens measured 220 mm in width, 250 mm in length,

and either 250 or 300 mm in height depending on

concrete cover. The front and side views of tested

specimen are shown in Fig. 3a, b, respectively. The

test bar enters the beam-end specimen at the loaded

end, extends into the specimen along a short un-

bonded length, extends further along a bonded length,

and has an additional un-bonded length before termi-

nating within a hollow steel conduit to provide access

to the free end for measuring slip. The bar ribs were

randomly oriented in the test set-up. The specimen is

positioned in a test rig so that the bar can be pulled

slowly from the concrete. When the bar is pulled

during testing, the specimen is restrained from trans-

lation through a compression reaction plate, and

restrained from rotation through a tie-down, thus

approximating boundary conditions of simply sup-

ported beams [1, 18].

All steel bars were properly cleaned with a wire

brush prior to use. They were embedded inside the

specimens at fixed lengths (Le) of 90 mm, resulting in

db/Le ratios equal to 0.141, 0.177, 0.212, and 0.282 for

No. 13, 16, 19, and 25 bars, respectively. It is to be

noted that the rib orientation parameter was not

considered in this study, as the bars were randomly

embedded inside the specimen. Two concrete covers

(Cc) of 60 and 100 mm were tested in this project,

resulting in various Cc/db varying from 2.4 to

relatively high ratios of 7.9 which may be particularly

specified for underwater applications. Tolerances for

bonded lengths, concrete covers, and overall specimen

dimensions were ± 2, 3, and 5 mm, respectively. Two

stirrups placed on each side were provided for shear

resistance, but were oriented parallel to the ‘‘pull’’

direction to avoid confining the test bar along its

bonded length. The closed stirrups were made of No.

10 plain bars (ASTM A615) with db of 9.5 mm.

The concrete samples were placed in two consec-

utive lifts in the beam-end specimen molds, and

Fig. 3 Front and side views of the adapted beam-end specimen test



internally vibrated using a 150-Hz frequency vibrator.

Care was taken in the insertion of the vibrator to avoid

as much as possible the formation of air bubbles

around the steel bars. Also, bond of reference mixtures

sampled in dry conditions (i.e. without immersion in

water) was determined. The UWC and reference

specimens were demolded after 24 h, covered with

plastic bags, and allowed to cure at 23 �C for 28 days.

Before testing, the specimen was shimmed and aligned

so that the test bar was parallel to the loading frame.

The tensile load was gradually applied at a rate of

25 ± 4 kN per minute until bond failure occurred. The

rebar’s relative slips to concrete were monitored from

measurements of two LVDTs placed at the loaded and

free bar surfaces.

3.6 Determination of f’c and ft of UWC mixtures

A procedure similar to the one described earlier was

adopted to secure materials used for determining the

residual compressive and tensile strengths of UWC.

However, only one cycle was enough to secure enough

washout concrete for filling some 100 9 200 mm

steel cylinders. Note that strength of reference mix-

tures sampled in dry conditions was also determined.

All UWC and reference mixtures were placed in two

lifts in the molds and consolidated by rodding, as per

ASTM C31 Standard Practice [28]. All samples

(whether sampled in dry or wet conditions) were

demolded after 24 h, cured in water under similar

conditions, and tested at 28 days for compressive and

tensile strengths as per ASTM C39 [29] and C496 [30]

Test Methods, respectively.

4 Test results and discussion

Table 3 summarizes the properties of reference con-

crete mixtures sampled in dry conditions including

tensile strength (ft(dry)), compressive strength (f’c(dry)),

ultimate bond strength (su(dry)) representing the max-

imum peak load, normalized bond stress which is the

ratio of su(dry) to the square root of f’c(dry), and mode of

failure (MF). The bond stresses were calculated as the

ratios of measured pullout loads divided by the

corresponding rebar’s embedded area (3.14 9 db 9

bonded length (Le) of 90-mm), assuming a uniform

load distribution along the embedded length [31].

Also, the various Cc/db ratios are indicated in Table 3. Ta

ble

3C

om

pre

ssiv

e,te

nsi

le,

and

bo

nd

pro

per

ties

of

test

edco

ncr

ete

sam

ple

din

dry

con

dit

ion

s(i

.e.,

wit

ho

ut

imm

ersi

on

inw

ater

)

f t(d

ry),

MP

a

f’c(d

ry),

MP

a

Bar

No.

Concr

ete

cover

,C

c=

60

mm

Concr

ete

cover

,C

c=

100

mm

Cc/

db

s u(d

ry),

MP

a(k

N)

s u(d

ry)7

Hf’

c(d

ry)

MF

(Sli

p,m

m)

Cc/

db

s u(d

ry),

MP

a(k

N)

s u(d

ry)7

Hf’

c(d

ry)

MF

(Sli

p,m

m)

350-0

.56-S

22

2.6

333.1

13

4.7

212.6

(45.2

)2.1

9P

(0.6

4)

7.8

712.8

(45.9

)2.2

2P

(0.7

2)

16

--

--

6.2

912.5

(56.3

)2.1

8P

(0.8

)

19

3.1

48.4

5(4

5.6

)1.4

7S

(0.6

)5.2

39.2

(49.6

)1.6

S(0

.51)

350-0

.56-S

22-A

WA

2.5

832.4

13

4.7

214.2

(50.9

)2.4

9P

(0.4

5)

7.8

716

(57.5

)2.8

2P

(0.5

8)

25

--

--

3.9

48.1

(58.1

)1.4

2S

(0.5

2)

350-0

.56-S

F45-A

WA

2.5

532.2

16

3.7

710.7

(48.1

)1.8

9P

(0.7

7)

--

--

425-0

.47-S

22

4.6

257.3

13

4.7

214.8

(53.1

)1.9

6P

(0.8

3)

7.8

717.5

(62.8

)2.3

1P

(0.7

1)

16

3.7

712.5

(56.1

)1.6

5P

(0.5

)6.2

815.1

(67.7

)1.9

9P

(1.0

5)

19

--

--

5.2

312.1

(65.3

)1.6

P/S

(0.4

4)

425-0

.47-S

22-A

WA

4.8

158.2

25

2.3

66.4

(45.9

)0.8

4S

(0.3

1)

--

--

Mix

code

refe

rsto

cem

ent

conte

nt-

w/c

-Consi

sten

cy(S

22:

slum

p=

220

mm

;S

F45:

slum

pfl

ow

=450

mm

)-A

WA

MF

refe

rsto

mode

of

fail

ure

(PP

ull

out;

SS

pli

ttin

g;

P/S

Pull

out

wit

hm

inim

alsp

litt

ing)

The

ult

imat

epull

out

forc

e(i

nkN

)an

dco

rres

pondin

gsl

ip(i

nm

m)

atfa

ilure

are

giv

enfo

rin

dic

atio

npurp

ose

s



The washout loss (W) of UWC mixtures tested as

per the adapted CRD C61 test along with the

resulting ft, f’c, su, normalized bond stress, and MF

are summarized in Table 4 (the intrinsic W3 values

are given for indication purposes). The residual Dft,

Df’c, and Dsu values are calculated as [1—(strength

in dry conditions–strength after washout)/strength in

dry conditions] 9 100. Such indices will be used in

this paper to quantify the relative decrease in UWC

strength resulting from washout loss. It is to be

noted that for the 425-0.47-S22 mixture having a Cc

of 100 mm, bond failure occurred slightly after

yielding of the No. 13 bar for specimens sampled in

dry conditions or after washout loss. For all other

tests, bond failure occurred before yielding of test

bars.

Several mixtures were tested three times to evaluate

the validity of the testing procedure developed to

determine strength properties of UWC using the same

samples that were used for washout measurement.

Acceptable reproducibility of ft, f’c and su values was

obtained throughout testing as the coefficient of

variation (C.O.V.) was less than 11.4, 6.5 to 14.2 %,

respectively.

4.1 Modes of failure

Under similar conditions, the modes of failure that

resulted during UWC testing (i.e., pullout vs. splitting)

were quite similar to those observed when using

concrete cast above water [1, 32]. For instance, failure

was by pullout mode for specimens tested using bars

No. 13 and 16, whereby the bar pulled out slowly as

the load dropped steadily (Fig. 4, left). Nevertheless,

minimal splitting was noticed along the top surface of

the test bar No. 16 for the 350-0.56-SF45-AWA UWC

mixture sampled after washout and having a Cc of

60 mm, as shown in Fig. 4 (middle).

Splitting failure occurred when tests are realized

with bars No. 25 and some of those prepared with bars

No. 19. In fact, at approximately � to � of the load

capacity, three radial cracks appeared on the front

loaded-end face, which extended on the two side and

top faces, as shown in Fig. 4 (right) for the 350-0.56-

S22-AWA mixture. The radial cracks widened as the

bar is being pulled out from the specimen. Note that a

pullout failure accompanied with minimal splitting

took place for bar No. 19 in the 425-0.47-S22 mixture

having a Cc of 100 mm.

4.2 Pullout force versus displacement responses

Typical variations of the pullout force versus dis-

placement (slip of the bar at free end) curves

determined on UWC and reference mixtures with Cc

of 100 mm and bars No. 13 and 25 are plotted in

Fig. 5. For given bar diameter, UWC subjected to a

certain washout loss exhibited initially similar trends

of pullout force versus displacement responses, as

compared to the reference mixture sampled in dry

conditions. Thus, all curves yielded initially a stiff

response that varied up to around 10 or 15 % of the

maximum force, beyond which the bars free end

started to slip noticeably. In this region, the bond

resistance consists mainly of chemical adhesion and

friction at the interface between the bar and concrete

matrix [1, 3, 33].

Following the formation of internal cracks and

initiation of debonding, the stiffness of the ascending

curves gradually softens until reaching a maximum

pullout force value. The ultimate force and corre-

sponding slip were, however, directly dependent by

the type of concrete, i.e., whether sampled in dry

conditions or after washout loss. Hence, the maximum

force decreased from 45.2 kN for the 350-0.56-S22

reference concrete to 37.3 kN when the UWC was

sampled after 8.8 % washout loss. As can be seen in

Tables 3 and 4, UWC mixtures sampled after being

subjected to some washout are characterized by lower

compressive and tensile strengths than those sampled

in dry conditions. For instance, ft decreased from 2.63

to 2.25 MPa and f’c from 33.1 to 26.8 MPa for the

350-0.56-S22 mixture. This can reduce the concrete

confining stresses against the steel bar, thus decreasing

bond resistance and leading to internal cracking at

lower bond stresses.

As shown in Fig. 5, the decrease in maximum

pullout force due to washout is coupled with shifting

of the bars free end displacement towards lower

values. Hence, the slip at failure for No. 25 bars

decreased from 0.51 to 0.38 mm for the 350-0.56-S22-

AWA mixtures sampled in dry or washed conditions,

respectively. Practically, this indicates that the pullout

deformation and ability to redistribute load tend to

decrease for concrete mixtures (such as UWC sub-

jected to some washout) characterized by lower matrix

strength and toughness [34]. The debonding continues

in the post-peak region for both UWC and reference

mixtures until the entire length is totally debonded,



Ta

ble

4W

ash

ou

t,co

mp

ress

ive,

ten

sile

,an

db

on

dp

rop

erti

eso

bta

ined

usi

ng

UW

Cm

ixtu

res

sam

ple

dfo

llo

win

gte

stin

gu

sin

gth

ead

apte

dC

RD

C6

1te

st

W,

%

f t,

MP

a

Df t

,

%

f’c,

MP

a

Df’

c,

%

Bar

No

.

Co

ncr

ete

cov

er,

Cc

=6

0m

mC

on

cret

eco

ver

,C

c=

10

0m

m

s u,

MP

a

(kN

)

s u7

Hf’

c

Ds u

,

%

MF

(Sli

p,m

m)

s u,

MP

a

(kN

)

s u7

Hf’

c

Ds u

,

%

MF

(Sli

p,m

m)

35

0-0

.56

-S2

2(W

3=

9.2

%)

8.8

2.2

58

5.6

26

.88

0.9

13

9.2

(33

)1

.78

73

P(0

.66

)1

0.4

(37

.3)

2.0

18

1.1

P(0

.61

)

16

--

--

9.0

2(4

0.5

)1

.74

59

.9P

(0.6

6)

19

5.2

(28

.1)

16

1.5

S(0

.28

)6

.6(3

5.6

)1

.27

71

.7S

(0.3

5)

35

0-0

.56

-S2

2-A

WA

(W3

=7

.1%

)6

.92

.32

89

.92

8.4

87

.61

31

2.3

(44

.1)

2.3

18

6.6

P(0

.41

)1

4.4

(51

.7)

2.7

89

.8P

(0.3

9)

25

--

--

6.3

(45

.2)

1.1

87

7.8

S(0

.38

)

35

0-0

.56

-SF

45

-AW

A(W

3=

11

.2%

)1

0.8

2.3

90

.22

4.5

76

.11

66

.95

(31

.2)

1.4

71

.9P

/S(0

.67

)-

--

-

42

5-0

.47

-S2

2(W

3=

5.8

%)

5.6

4.5

29

7.8

54

.39

4.8

13

13

.1(4

7)

1.7

88

8.5

P(0

.7)

16

.1(5

7.8

)2

.18

92

P(0

.74

)

16

10

(45

)1

.36

80

.3P

(0.4

2)

13

.2(5

9.3

)1

.79

85

.5P

(0.6

)

19

--

--

11

(59

.4)

1.4

99

0.9

S(0

.33

)

42

5-0

.47

-S2

2-A

WA

(W3

=4

.4%

)4

.24

.69

5.6

56

.49

6.9

25

5.6

(40

.2)

0.7

58

7.5

S(0

.24

)-

--

-

No

tes:

Th

eW

3v

alu

esd

eter

min

edfr

om

the

stan

dar

dC

RD

C6

1te

star

eg

iven

for

ind

icat

ion

pu

rpo

ses

on

ly

Df t

,%

=[1

-(f

t(dry

)-f t

)/f t

(dry

)]9

10

0

Df’

c,

%=

[1-

(f’ c

(dry

)-f’

c)/

f’c(d

ry)]

91

00

Ds u

,%

=[1

-(s

u(d

ry)-

s u)/s u

(dry

)]9

10

0

Th

eu

ltim

ate

pu

llo

ut

forc

e(i

nk

N)

and

corr

esp

on

din

gsl

ip(i

nm

m)

are

giv

enfo

rin

dic

atio

np

urp

ose

s



causing the steel bars to slide out dynamically from the

specimens.

4.3 Parameters affecting normalized bond stress

of UWC

4.3.1 Mixture composition

The effect of mixture composition on the normalized

bond stress is plotted in Fig. 6 for various UWC

mixtures tested using different steel bars placed at a

cover of 100 mm. Under given testing conditions, the

addition of AWA led to considerable improvements in

the normalized bond stress. For example, such improve-

ment was from 2.01 to 2.7 when adding AWA to the

350-0.56-S22 mixture tested using bars No. 13 (Fig. 6).

This can be related to reduction in the bleed water which

may strengthen the cement paste in the transition zone

adjacent to the reinforcing bars [1, 15, 16].

The coupled effect of higher cement content and

lower w/c is shown to increase the normalized bond

stress for UWC mixtures sampled after washout

(Fig. 6). For example, such increase was from 1.27

to 1.49 when the cement content increased from 350 to

425 kg/m3 and w/c decreased from 0.56 to 0.47,

respectively, for mixtures tested using bars No. 19.

This can normally be due to improved particle packing

and bearing strength capacity of the concrete in front

of the bar ribs which lead to an increase in bond

stresses [1].

4.3.2 Washout loss

The effect of washout is evaluated by plotting the

relationship between washout loss and the difference

between normalized bond stress determined on con-

crete sampled in dry condition minus the one deter-

mined on UWC sampled following immersion in

water (Fig. 7). Although the scatter in data mostly due

to different concrete covers and bar diameters, it is

evident that such difference is always positive, thus

confirming that bond stress of concrete to embedded

steel decreases when casting is realized under water.

Fig. 4 Typical pullout failure for bars No. 13 (left), pullout with minimal splitting for 350-0.56-SF45-AWA, bar No. 16, Cc of 60 mm

(middle), and splitting failure for 350-0.56-S22-AWA, bar No. 25, Cc of 100 mm (right)

0

10

20

30

40

50

60

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Pu

llou

t fo

rce,

kN

Slip at free end, mm

1

2

3

4

Fig. 5 Typical responses of pullout force versus displacement

for UWC and reference mixtures

2.01

1.74

1.27

2.7

2.18

1.79

1.49

1

1.4

1.8

2.2

2.6

3

No

rmal

ized

bo

nd

str

ess

350-0.56-S22

350-0.56-S22-AWA

425-0.47-S22

Fig. 6 Effect of mixture composition on normalized bond

stress of UWC



On the other hand, it can be noted that the extent of

reduction in the normalized bond stress tends to

increase for UWC mixtures exhibiting higher washout

losses. In fact, such mixtures are characterized by a

decrease in the cementitious phase coupled with a

potential increase in the specified w/c, which can both

decrease the contribution of concrete strength to the

bond against steel [16].

It is important to note that the effect of mixture

composition and washout loss on the normalized bond

stress of UWC determined using the beam-end

specimen is quite similar to that determined using

the direct pullout bond test [16]. In other words, the

normalized stress increases for stable mixtures exhib-

iting reduced levels of washout upon casting in water.

Although the bond at failure is evaluated differently

using both methods, this suggests that the approach

that consists on subjecting UWC to washout and then

testing bond strength is appropriate to assess the drop

in normalized stress due to washout.

4.3.3 Bar diameter (db)

Typical variations of the normalized bond stresses

with respect to db for tested UWC are plotted in Fig. 8

(the effect of concrete cover can also be seen in this

figure). For given concrete cover and mixture compo-

sition, a reduction in normalized bond stress is noticed

with the increase in db. For example, the normalized

bond stress dropped from 2.01 to 1.27 for the

350-0.56-S22 mixture when db increased from 12.7

to 19.1 mm, respectively, for a cover of 100 mm. The

lowest normalized stress of 0.75 was registered for the

425-0.47-S22-AWA mixture tested using the largest

db of 25.4 mm (Table 4). This indicates that larger bar

diameters require longer development or splice length

to achieve proper transfer of stresses when designing

steel reinforcement for UWC.

The influence of db on bond behavior between steel

and concrete cast above water has been studied in

literature [1, 22]. For instance, Metelli and Plizzari

reported a reduction in bond strength by about 25 %

and secant bond stiffness by 70 % when db increased

from 12 to 50 mm, tested using the Rilem/CEB/FIP

pullout method [22]. The rebars were embedded in

cubic specimen for a length equal to 5 db and a cover of

4.5 db. In the case of this study, it is interesting to note

that the extent of bond decrease due to db is relatively

more pronounced when testing UWC mixtures sub-

jected to washout, as compared to concrete cast above

water. For example, su decreased by 43 % (i.e., from

9.2 to 5.2 MPa) when db increased from 12.7 to

19.1 mm (Cc/db equal to 4.72 and 3.14, respectively)

for the 350-0.56-S22 mixture sampled after washout.

Such decrease was equal to 33 % (i.e., from 12.6 to

8.45 MPa) when the same mixture was cast above

water under similar conditions. This highlights the

importance of using small-diameter bars when casting

is expected to occur under water.

4.3.4 Concrete cover (Cc)

Regardless of the bar diameter, the increase in Cc from

60 to 100 mm led to increased normalized bond stress

of tested UWC (Fig. 8), mostly attributed to higher

concrete confinement surrounding the embedded steel

[1, 2]. Practically, this means that the increase in

concrete cover could be beneficial to limit the

y = 0.06 x - 0.14R² = 0.84

y = 0.058 x - 0.19R² = 0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

3 4.5 6 7.5 9 10.5 12

No

rmal

ized

bo

nd

str

ess

(dry

co

ncr

ete)

min

us

no

rmal

ized

bo

nd

st

ress

(U

WC

)

Washout loss as per adapted CRD C61, %

Cc = 60 mm

Cc = 100 mm

Linear (Cc = 60 mm)

Linear (Cc = 100 mm)

Fig. 7 Effect of washout loss on the drop in normalized bond

stress

Fig. 8 Effect of bar diameter (and concrete cover) on

normalized bond stress



detrimental effect of washout on the drop in UWC bond

strength. For example, as can be seen in Fig. 9, the use

of 100 mm Cc resulted in improved normalized bond

stress for the 425-0.47-S22 UWC mixture, despite the

5.6 % washout loss that occurred as compared to the

reference mixture sampled in dry conditions with Cc of

60 mm. Nevertheless, the 5.6 % washout may be

considered as a threshold value, beyond which further

increase in Cc may be required to compensate the

effect of washout loss on the drop in bond strength.

Hence, the normalized bond stress remained lower

with the use of 100 mm Cc for the 350-0.56-S22 UWC

mixture having a washout value of 8.8 %, as compared

to the reference mixture sampled in dry conditions

having a Cc of 60 mm (Fig. 9).

It is to be noted that the effect of db and Cc on

normalized bond stress for UWC is similar to that

resulting from concrete cast and consolidated above

water [1, 2]. This indicates that, once hardening

occurred, both UWC and concrete cast above water

behave similarly with respect to bond against steel.

Thus, the drop in bond for UWC is mainly related to

the washout that could take place during the under-

water casting process.

Although the limited number of data points,

acceptable correlations are plotted in Fig. 10 between

Cc/db and normalized bond stress for all tested UWC,

indicating that the increase in Cc/db can result in

higher normalized bond stresses. In other words,

concrete confinement plays an important role to

provide increased resistance to bond failure during

underwater casting (as washout loss can reduce such

bond stresses). Such results are in agreement with

those determined on concrete cast above water [1, 32].

The relationships between normalized bond stress

determined on concrete sampled in dry conditions and

Cc/db are as follows:

For Cc ¼ 60 mm; Normalized bond stress dryð Þ¼ 0:551 Cc=db� 0:358 R2 ¼ 0:88

For Cc ¼ 100 mm; Normalized bond stress dryð Þ¼ 0:284 Cc=db þ 0:218 R2 ¼ 0:83

4.4 Effect of washout loss on UWC properties

4.4.1 Residual UWC strengths

The relationships between washout loss determined

using the adapted CRD C61 method and UWC residual

strengths (Dft, Df’c, and Dsu) are plotted in Fig. 11.

Mixtures with increased washout losses exhibited

higher Dft and Df’c for the 5 tested data points. As

already mentioned, this is due to washout of cemen-

titious particles coupled with potential increase in w/c

due to water infiltration inside the specimens, which

can both decrease strength of UWC. The R2 of 0.97

obtained using the compression data indicates that

washout can be better related to compressive strength,

rather than tensile strength having a lower R2 of 0.61.

Regardless of the concrete cover or bar diameter, the

increase in washout is shown to reduce proportionally

the residual bond strength (Dsu) (Fig. 11). For exam-

ple, the increase in washout from 4.5 to 10.5 % resulted

in reduced Dsu from approximately 90–65 %, respec-

tively. As reported in literature for concrete cast and

consolidated above water, the ultimate bond strength is

directly dependent on the contribution of concrete

strength and quality of the interfacial transition zone

between the paste and embedded steel [34]. Hence,

UWC mixtures exhibiting relatively low washout

possess improved resistance against dilution and

reduced risks of water inclusion inside the specimen,

thus increasing the contribution of concrete strength to

the bond against steel bars. Conversely, UWC with

higher washout becomes more prone to a decrease in

the ultimate bond strength due to cementitious losses

coupled with an increase in the specified w/c.

4.4.2 Prediction of drop in bond strength for UWC

The relationship between Dsu and the drop in

normalized bond stress calculated as the difference

2.19

1.47

1.96

1.65

2.01

1.27

2.18

1.79

1.0

1.5

2.0

2.5

3.0N

orm

aliz

ed b

on

d s

tres

s

Mix in dry conditions, Cc = 60 mm

Washed UWC mix, Cc = 100 mm

Fig. 9 Effect of concrete cover on normalized bond stress for

UWC and reference mixtures



between that determined under dry or washed condi-

tions is illustrated in Fig. 12. As expected, the

difference between normalized bond stresses

decreases for UWC mixtures possessing higher Dsu

values, with R2 of 0.9. Thus, two boxes depending on

the level of washout loss are distinguished in Fig. 12 to

predict the drop in UWC bond strength, as follows:

a. Washout loss varying from 8.8 to 10.8 %: UWC

samples possessing such levels of washout are

characterized by relatively high losses of cemen-

titious particles and potential increase in w/c,

leading to dramatic drops in residual bond

strength. The corresponding Dsu values ranged

from approximately 60–82 % (Fig. 12). In other

words, to eliminate the detrimental effect of such

levels of washout on the drop in normalized bond

stresses, the ultimate bond strength of UWC

should be increased by 18 % (100–82) to 40 %

(100–60). As earlier discussed, this can be

achieved by different ways including higher Cc,

reduced db, increased development length, addi-

tion of AWA to reduce washout, and use of higher

strength concrete.

b. Washout loss varying from 4.2 to 6.9 %: Obvi-

ously, this category of mixtures is characterized

by improved concrete/water interface capable of

resisting loss of fines and water inclusion inside

the specimen, thus leading to Dsu ranging from 78

to 93 % (Fig. 12). Therefore, the bond should be

increased by 7 % (100–93) to 22 % (100–78) in

this category of mixtures to eliminate the detri-

mental effect of washout.

It is to be noted that the two previously defined

washout boxes can also be identified in Fig. 13 when

plotting the relationship between Df’c and Dsu values

for all tested UWC. An acceptable R2 of 0.7 is

obtained. Thus, for mixtures having washout varying

from 8.8 to 10.8 %, Df’c varies from 75 to 83 %. In the

second category of UWC having 4.2 to 6.9 %

washout, Df’c ranges from 87 to 97 %.

4.5 Regression models for evaluating bond

strength in UWC

A series of regression models have been developed to

determine the ultimate bond strength of UWC, and

subsequently being able to assess the effect of given

variable on its magnitude. The models given in Eqs. 1

to 5 below are considered valid within the conditions

of the testing program, i.e. concrete having an f’c

varying from 25 to 58 MPa, ft from 2.25 to 4.8 MPa,

Cc/db from 2.36 to 7.87, db/Le from 0.141 to 0.282, db

from 12.7 to 25.4 mm, and W3 ranging from 0 %

(corresponding to reference concrete cast above

water) to 11 %. Note that the W3 values (Table 4)

were chosen in the regression models so as to use the

intrinsic washout losses determined using the stan-

dardized CRD C61 test method.

Equations 1 and 2 have been proposed as a function

of concrete tensile strength (ft). Acceptable accuracy

resulting in R2 of 0.71 and 0.76 is obtained when

comparing the predicted-to-measured values. The

signs of the estimates (±) indicate the positive or

negative effect of given variable on the ultimate bond

strength. For example, the increase in ft and Cc/db

leads to an increase (?) in bond. Conversely, the

increase in db/Le and W3 will reduce (-) the bond.

y = 0.537 x - 0.604R² = 0.87

y = 0.308 x - 0.149R² = 0.82

0.5

1.0

1.5

2.0

2.5

3.0

2 3 4 5 6 7 8 9

No

rmal

ized

bo

nd

str

ess

Cc / db ratio

Cc = 60 mm

Cc = 100 mm

Fig. 10 Relationships between normalized bond stress and Cc/

db ratios

y = -4.36 x + 111.7R² = 0.69

y = -3.61 x + 113.6R² = 0.97

y = -2 x + 106.01R² = 0.61

60

70

80

90

100

3 4.5 6 7.5 9 10.5 12

Res

idu

al s

tren

gth

s, %

Washout loss as per adapted CRD C61, %

Bond

Compression

Tensile

Linear (Bond)

Linear (Compression)

Linear (Tensile)

Fig. 11 Effect of washout loss on residual compressive, tensile,

and bond strengths of UWC



smax MPað Þ ¼ 0:078þ 0:68Cc

db

� 0:09W3

� �ft

R2 ¼ 0:71� �

ð1Þ

smax MPað Þ ¼ 2þ 0:45Cc

db

� 5:2db

Le

� 0:08W3

� �ft

R2 ¼ 0:76� �

ð2Þ

Higher R2 of 0.79 and 0.88 are obtained between

the predicted-to-measured values when using Eqs. 3

and 4, respectively. Those later models have been

proposed as a function of the square root of f’c,

suggesting that compressive strength can better be

used to predict the ultimate UWC bond strength.

smax MPað Þ ¼ 0:22þ 0:336Cc

db

� 0:052W3

� � ffiffiffiffif0c

q. . .

R2 ¼ 0:79� �

ð3Þ

smax MPað Þ¼ 1:64þ0:22Cc

db

�4:5db

Le

�0:046W3


q

R2¼0:88� �

ð4Þ

Given that Le remained fixed at 90 mm throughout

the testing program, Eq. 4 has been re-formulated into

Eq. 5 whereby the term db/Le is changed to db, as

follows:

smax MPað Þ¼ 2:6þ0:15Cc

db

�0:09db�0:044W3


q

R2¼0:91� �

ð5Þ

5 Summary and conclusions

This study aims at assessing the effect of washout loss

on the drop in UWC bond strength. Special emphasis

was placed to evaluate bond properties using the same

concrete mixtures that were subjected to washout as

per an adapted version of the CRD C61 test. Tests

were realized using the beam-end specimen method,

and parameters evaluated included level of washout

loss, bar diameter, and concrete cover.

For given bar size and concrete cover, test results

showed that bond between steel and UWC is affected

by the level of washout loss, which in turn is directly

influenced by the mixture composition. For example,

the increase in cement content, decrease in w/c, and

addition of AWA can reduce washout loss and risks of

increase in w/c, thus improving the contribution of

UWC strength to the bond against steel bars. Similarly

to bond behavior in concrete cast and consolidated

above water, ultimate bond strength of UWC increases

for smaller bar diameters and higher confinement

reflected by increased concrete covers. Practically,

this means that the increase in concrete cover could

limit the detrimental effect of washout on the drop in

UWC bond strength. UWC mixtures with increased

washout losses exhibited higher drops in compressive,

tensile, and bond strengths, as compared to the

concrete cast above water. Two boxes depending on

the level of washout loss, i.e. from 4.2 to 6.9 % and

from 8.8 to 10.8 %, have been proposed to predict the

y = -0.013 x + 1.3R² = 0.90

0.0

0.1

0.2

0.3

0.4

0.5

0.6

60 65 70 75 80 85 90 95 100

No

rmal

ized

bo

nd

str

ess

(dry

co

ncr

ete)

min

us

no

rmal

ized

b

on

d s

tres

s (U

WC

)

Residual bond strength, %

Washout = 4.2% to 6.9% => To increase UWC bond by 7% to 22%

Washout = 8.8% to10.8% => To increase UWC bond by 18% to 40%

Fig. 12 Effect of washout loss on residual bond strength and

normalized bond stress of UWC

y = 50.41 e0.0068x

R² = 0.70

70

75

80

85

90

95

100

60 65 70 75 80 85 90 95 100

Res

idu

al c

om

pre

ssiv

e st

ren

gth

, %

Residual bond strength, %

Washout = 8.8% to10.8%

Washout = 4.2% to 6.9%

Fig. 13 Effect of washout loss on residual compressive and

bond strengths of UWC



extent of bond decrease in UWC mixtures. Regression

models developed as a function of washout loss, bar

diameter, concrete cover, and compressive or tensile

strengths have been proposed to estimate the corre-

sponding ultimate bond strength.

Acknowledgments The authors would like to acknowledge

the support provided by CNRS-Lebanon and University

Research Council of the Lebanese American University

(LAU), Byblos, Lebanon. The authors also wish to thank the

experimental support provided by the Laboratory of the Civil

Engineering Department in LAU.

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