Enhancement of flexural behaviour of CFRP-strengthened reinforced concrete beams usingengineered cementitious composites transition layer

Hamdy M. Afefy*, Nesreen Kassem1 and Mohamed Hussein2

Structural Engineering Department, Faculty of Engineering, Tanta University, Tanta, Egypt

(Received 22 November 2013; final version received 7 March 2014; accepted 31 March 2014)

This paper aimed to develop and evaluate an efficient strengthening method for reinforced concrete beams, based onengineered cementitious composites (ECC) to be applied as a transition layer prior to the application of the carbon fibre-reinforced polymer (CFRP) strengthening sheet. The role of the proposed transition layer is to control the cracking ofconcrete and detain or even avoid premature de-bonding of the strengthening CFRP sheets. As the ability of the transitionlayer to exhibit a strain hardening behaviour is mainly dependent on the used fibre volumetric ratio, three ECC mixes withthree different polypropylene fibre volumetric ratios were used (fibre volumetric ratio of 0.5%, 1% and 1.5%). Theexperimental results showed that while the used CFRP strengthening sheet can increase the ultimate load by about 28.8%compared with the control un-strengthened beam, this increase can reach about 48.5% by applying the same CFRP sheet tothe proposed ECC transition layer that contains a fibre volumetric ratio of 1.5%. Moreover, this layer integrated with themention ratio of the fibre content enabled the CFRP sheet to be in a complete contact with the strengthened beam withoutany de-bonding up the rupture of the CFRP sheet at failure.

Keywords: crack width; cracks spacing; carbon fibre-reinforced polymers; ductility; engineered cementitious composites;reinforced concrete beams; strengthening

1. Introduction

The progressively rapid deterioration of infrastructure is

becoming a prime problem for both researchers and

engineers in construction industry. Engineers are con-

fronted with the continuous challenge of developing new

techniques by researchers in order to repair, replace or

rehabilitate the existing structures. Deterioration in all

types of reinforced concrete (RC) structures is further

provoked by load- induced stresses greater than the design

stress, excessive concentrations of chlorides in construc-

tion materials, by high humidity and temperatures, and by

marine environments. The construction industry is

urgently in need of non-corrosive materials as alternatives

to steel reinforcement or, at least, a proper protection for

the main steel and its concrete tension cover. In order to

remedy this problem, various solutions ranging from

replacement of the structure to strengthening with a variety

of techniques have been proposed.

In addition, many methods to counter the threat of

corrosion in steel reinforcement such as epoxy coatings,

cathodic protection, increased concrete cover thickness

and polymer concrete have been proposed, but none of

these measures has provided a long-term solution. In the

current research, the tension zone around the main tension

steel of the test beams was replaced by an engineered

cementitious compositess (ECC) material and then the

strengthening carbon fibre-reinforced polymer (CFRP)

sheets were bonded to its bottom surface. That was done

for the aim of enhancing the strengthened beams cracking

behaviour resulting in improved bond characteristics,

which in turn increases the strengthening efficiency along

with providing protection for the main tension steel,

simultaneously.

Fibre-reinforced polymer (FRP) materials as a new

construction strengthening materials have been developed

and used for recent, current and potential applications that

cover both new and existing structures. FRP composites

have become more popular in recent years due to the

reduction in their cost, combined with newer under-

standing of the versatility and benefits of the material

properties. Among different types of FRP materials, a

CFRP is used extensively in the structural engineering

field. The externally bonded reinforcement (EBR)

technique has been more generally applied due to its

simple installation procedure. Design guidelines and

specifications have also been established well for this

system (ACI 440.2R-08). In particular, their practical

implementations for strengthening by epoxy bonding are

numerous (Benzarti, Freddi, & Fremond, 2011; Cromwell,

Harries, & Shahrooz, 2011; Jumaat, Rahman, & Rahman,

q 2014 Taylor & Francis

*Corresponding author. Email: hamdyafefy@hotmail.com

Structure and Infrastructure Engineering, 2014

http://dx.doi.org/10.1080/15732479.2014.930497

2011; Ombres, 2010; Rasheed, Harrison, Peterman, &

Alkhrdaji, 2010).

Although external strengthening of RC beams with

epoxy-bonded CFRP has been established as an effective

tool for increasing flexural and/or shear strength, the

method still suffers from some drawbacks. Many of these

drawbacks are attributed to the characteristics of currently

available commercial CFRP-strengthening systems. Even

though CFRPs have high strength, they are very brittle.

When loaded in tension, FRPs exhibit a linear stress–

strain behaviour up to failure, without exhibiting a yield

plateau or any indication of an impending failure. As FRPs

behave differently than steel, they consequently suffer

from a significant loss in beam ductility, particularity

when CFRPs are used for flexural strengthening of RC

beams (Arduini & Nanni, 1997; Bencardino, Spadea, &

Swamy 2002; Lee, Pan, & Ma, 2004; Saadatmanesh &

Ehsani, 1991; Spadea, Swamy, & Bencardino, 2001;

Toutanji, Zhao, & Zhang, 2006).

Over the years, many studies focusing on the

behaviour of CFRP-strengthened beams have been

conducted to better understand their behaviour under

different loading conditions, aiming also to develop the

best technique for the application of CFRP fabric sheets

and strips. Ehsan, Sobuz, and Sutan (2011) verified that the

attachment of CFRP laminates with edge strip plates had

substantially influenced the performance of CFRP-

strengthened beams. In addition, the important practical

issues that were encountered in strengthening of beams

with different types and different thicknesses of FRP were

addressed (Murali & Pannirselvam, 2011). Hence, a

simple method of applying FRP for strengthening the

beam with different FRP types with different thicknesses

was proposed. In addition to the practical experience for

choosing the strengthening configuration, design guide-

lines for FRP RC structures were stipulated (Pilakoutas,

Neocleous, Guadagnini, & Matthys, 2011)

ECC is a special category of the new generation of

high-performance fibre-reinforced cementitious compo-

sites with microstructures tailored according to micro-

mechanics theory, featuring high ductility and durability

characteristics (Dhawale & Joshi 2013; Li, 1993, 2012; Li

& Li, 2011a, 2011b). ECC’s high tensile ductility,

deformation compatibility with existing concrete and

self-controlled micro-crack width lead to their superior

durability under various mechanical and environmental

loading conditions such as fatigue, freezing and thawing,

chloride exposure and drying shrinkage (Li & Li, 2008,

2011a, 2011b; Sahmaran, Li, & Li, 2007). Experience of

the use of the ECC material in a bridge deck patch repair

case showed that high tensile strain capacity is one of the

most important properties of a durable repair material to

resist typical failures in repaired systems and that such

high tensile strain capacity can be achieved economically

in ECC materials (Li, 2004).

ECC is a mortar-based composite that is reinforced

with short random fibres, such as polyethylene (PE) or

polyvinyl alcohol (PVA). The use of PVA fibres has better

tensile crack-bridging properties while the use of PE fibres

shows a better compressive behaviour (Fischer & Li,

2007). ECC is a micromechanically designed material that

uses a micromechanical model to tailor-make the required

properties. By using this micromechanical tool, the fibre

amount used in the ECC is typically less than 2% by

volume. The moderately low fibre content has also made

shotcreting ECC viable. Furthermore, the most expensive

component of the composite fibres is minimised resulting

in ECC that is more acceptable to the highly cost-sensitive

construction industry (Dhawale & Joshi, 2013). Therefore,

ECC is designed to resist large tensile and shear forces but

at the same time has compatible properties, such as

compressive strength and thermal expansion, to ordinary

concrete based on Portland cement (Li, 2002).

Since the introduction of this material two decades ago,

ECC has undergone major evolution in both materials

development and the range of emerging applications. A

variety of experiments have been performed to assess the

performance of ECC at the structural level (Fischer & Li,

2003a, 2003b; Fukuyama, Matsuzaki, Nakano, & Sato

1999; Li & Fischer, 2002) for both seismic and non-seismic

structural applications. These experiments provide new

insights into how the material properties enhance the

structural performance of the structure. Under tensile

loading, in contrast to normal concrete where a single

unstable crack develops into a failure plane, a characteristic

of ECC is the development of multiple stable micro-cracks,

bridged by fibres (Spagnoli, 2009). As amicro-crack forms,

the fibres within the matrix bridge the crack, preventing it

from propagating into a failure plane. The multiple micro-

cracking behaviour of ECC is dependent on the fibre crack-

bridging law and on the degree of heterogeneity in the

material. Crack initiation sites are typically at material

flaws, which in the majority of cases are bubbles of

entrapped air (voids). Crack initiation behaviour is

therefore influenced by the size and spatial distribution of

voids in the material (Wang & Li, 2004). Final failure

occurs when one of the multiple cracks forms a fracture

plane.

Many researchers have confirmed that applying CFRP

laminates to RC beams can increase both the ultimate load

carrying capacity and the stiffness of the beams. Further

observations proved that the application also delays the

cracking moment and mitigates the development of cracks

(FIB, 2001). Most of the investigations carried out on RC

members strengthened in flexure have indicated that EBR

technique cannot fully utilise the strength of the FRP

composites. Brittle failure due to premature FRP de-

bonding and concrete cover separation has been the most

common type of failure (Teng, Chen, Smith, & Lam

2001). CFRP composites have very desirable high

H.M. Afefy et al.2

strength, but cannot be utilised properly if there is a poor

FRP–concrete bond. Typically, failure of a FRP–concrete

bond occurs within the concrete just a few millimetres

below the surface (Chen & Teng, 2001; Pan & Leung

2007; Wu, Hu, Wu, & Zheng, 2011; Yuan, Teng, Seracino,

Wu, & Yao, 2004). The de-bonding mechanism results in

the loss of the composite action between the concrete and

FRP laminates and initiates when high interfacial shear

and normal stresses exceed the strength of concrete.

The current research is undertaken in the aim of

enhancing the bond characteristics between the externally

bonded CFRP (EB-CFRP) strengthening materials and the

substrate concrete beams by replacing the concrete tension

zone around the main tension steel by an ECC transition

layer with different fibre contents. The polypropylene (PP)

fibres were used in the ECC material mix with three

different volume ratios in order to accomplish the

optimum ratio of fibre that can guarantee the complete

composite action between the CFRP sheets and the

substrate beams up to failure. That was done in order to be

able to utilise the overall strength of the CFRP sheets. At

this stage of research, the contribution of the ECC layer in

enhancing the ultimate strength was not of prime

importance. On the other hand, enforcing the CFRP

sheet to use its full tensile capacity was the most important

issue. In addition, this layer will provide a good protection

for the strengthened beams steel reinforcement against any

future attack that leads to corrosion.

2. Experimental work programme

2.1 Test beams

For the purposes of this study, six beams were fabricated

and cast then tested up to failure. The beams were divided

into two groups: the first group contained three beams

representing the reference beams, and the second group

contained three beams representing the CFRP-strength-

ened beams where three different fibre ratios were

implemented in the ECC transition layer. All beams had

the same concrete dimensions, i.e. the total length of the

beams was 2200mm while the centre-to-centre span was

2000mm. The beams cross section was 150mm width £300mm total depth. The main steel of the beams was two

high tensile steel bars of 12mm diameter representing a

reinforcement ratio of 0.56% that guarantee the tension-

controlled failure according to the current ECP 203 (2007)

and ACI 318-11 (2011) codes. The secondary steel was

two high tensile steel bars of 10mm diameter, while the

stirrups were mild steel bars of 8mm diameter and spaced

every 100mm all over the entire span of the beams.

Two beams of group 1 were completely cast with

concrete without any ECC transition layer, which

represented the control beam (B-C) and the reference

CFRP-strengthened beam (B-C-CF). The remaining

beam of group 1, beam B-C-ECC, was cast with 50mm

depth £ 150mm width ECC layer where the main

tension steel was embedded into the ECC transition layer

as illustrated in Figure 1. The second group, group 2,

represented the CFRP-strengthened beams. The three

beams had the same configuration as that of the reference

beam (B-C-ECC) as shown in Figure 1 but the used fibre

content ratio was the control parameter where it had three

ratios namely: 0.5%, 1% and 1.5% for the three beams. It

is worth mentioning that the fibre content of the reference

beam, B-C-ECC, was 1%.

All test beams were cast at the same time vertically in

wooden forms upside down where the tension sides were

at the top. Both beam B-C and B-C-CF were cast

completely with concrete, while the remaining four

beams were cast partially leaving the top 50mm depth

without concrete. Two days after casting, the standard

cubes, the concrete prism of 150mm 3 150mm 3 700 -

mm and the sides of the specimens B-C and B-C-CF were

stripped from the moulds and covered by plastic sheets,

while the sides of partially cast beams were left in

position to act as shuttering when ECC transition layers

were poured. The upper surface of all specimens were

cured by water until the 10th day and then allowed air-

drying. After about 4 weeks, the partially cast beams

were prepared for ECC pouring where the top surfaces

were roughened using chisel to remove slurry cement

from external surfaces of coarse aggregates. Before

pouring the ECC layers, the contact surfaces of the beams

were recleaned with brush and high-pressure air to ensure

a clean bonding surface, and then they were adequately

damped (Li, 2004). Just before casting the ECC layers, a

bonding agent (Addibond 65, provided by CMB

Company, Tanta, Egypt) was applied to the concrete

surfaces as illustrated in Figure 2. The upper surface of

the ECC layers were cured by water for 7 days and then

allowed air-drying until the testing day.

Four beams (B-C-CF, BS-0.5, BS-1 and BS-1.5) were

strengthened using CFRPs, in accordance with ACI 440

(2008) recommendations (ACI 440.2R-08) as shown in

Figure 1. Because the adopted strengthened beams are

considered as bond-critical application, surface prep-

aration requirements should be based on the intended

application of the CFRP system. The concrete surface

was prepared with abrasive techniques. The common

feature of the four strengthened beams was that all beams

were strengthened typically using CFRP sheets of

dimensions 150mm width, 0.13 mm thickness and

1800mm length extended at the tension side of the

beams. In addition, two 100-mm width CFRP anchorage

U-shaped sheets were used at both ends of the CFRP

sheets and ran up to 250mm depth at both sides in order

to prevent the premature peeling of the sheets at both

ends (Bilal, Amal, Hage, & Harajli, 2005; Sarah, James,

& Oguzhan, 2008).

Structure and Infrastructure Engineering 3

2.2 Material properties

The used concrete was made from ordinary Portland

cement (type I), natural sand and a mixture of crushed pink

limestone types I and II, as the coarse aggregate of

maximum size of 14mm, with a mixture proportion as

shown in Table 1 in order to obtain concrete with a target

strength of 25MPa. The actual compressive strength of the

used concrete was determined at the testing day as the

average of three standard cubes of 150mm side length as

reported in Table 3.

ECC layer was made with ingredients typically found

in concrete, including ordinary Portland cement, sand in

addition to silica fume and super-plasticiser based on poly-

carboxylic ether. The mix proportions of the ECC used as

strengthening materials are listed in Table 1. Water-to-

binder ratio (W/B) was 0.26, while 10% of the design

cement content was replaced by silica fume. High strength

PP fibre was selectively chosen, and its volume in mix was

varied to be 0.5%, 1% and 1.5%. The diameter and length

of the PP fibres were 0.025 and 12mm, respectively. The

tensile strength of the used PP fibres was 626MPa. PP

fibre can decline the brittleness of structure effectively by

improving the continuity and uniformity of concrete and

reduce the cracking obviously during plastic shrinkage.

The target compressive strength of the used ECC was

determined to be 60MPa, while the actual strength for

2200 mm

300

150

2 10

2 12

8@ 100 mm

750 mm 750 mm500 mm

2000 mm

Beam B-C

2000 mm

300

150

2 10

2 12

8@ 100 mm 50

50 mm thickness ECC layer (1% fiber content)

2000 mm

300

150

2 10

2 12

8@ 100 mm

EB-CFRP sheet (150 mm width x 0.13 mm thicness x 1800 length)

CFRP anchorage sheets

100 mm1600 mm100 mm

250

Beam B-C-ECC

Beam B-C-CF

300

150

2 10

2 12

8@ 100 mm 50

50 mm thickness ECC layer (0.5%, 1% and 1.5% fiber content)

Beams BS-0.5, BS-1, BS-1.5

EB-CFRP sheet (150 mm width x 0.13 mm thicness x 1800 length)

2000 mm

100 mm1600 mm100 mm

250

Figure 1. Concrete dimensions and reinforcement detailing for all tested beams.

H.M. Afefy et al.4

each beam was determined based on the compressive test

results of three standard cubes of 150mm side length as

reported in Table 3. The tensile behaviour of the used ECC

was determined by uniaxial tensile test of nine dumbbell-

shaped specimens, three specimens for each fibre content

percentage (tested cross section: 10 £ 30 mm). Figure 3

shows a sample of the stress–strain relationship measured

from the uniaxial tensile tests based on average sense for

each fibre content value. On the other hand, the concrete

tensile strength of the tested beams was determined by

splitting tensile test for three cylinders 150mm diameter

and 300mm height. The average value was approximately

2.49MPa.

In order to determine the mechanical properties of the

used 10 and 12mm diameter deformed high tensile steel

bars, tensile tests were performed on three specimens. The

mean value of tensile yield strength, ultimate strength and

Young’s modulus were 410MPa, 631MPa and 205GPa,

respectively, for 10mm bar and 396MPa, 571MPa and

200GPa, respectively, for 12mm bar. For the 8-mm mild

steel bars thatwere used as stirrups, themeanvalue of tensile

yield strength, ultimate strength andYoung’s modulus were

250MPa, 364MPa and 205GPa, respectively. Regarding

the used CFRP sheets, carbon fibre fabric SikaWrap

230C/45 and epoxy-based impregnating resin Sikadur-330

were implemented (the materials were provided by Sika-

Egypt Company, Cairo, Egypt). Table 2 presents the

mechanical properties for CFRP fabric sheet along with the

epoxy resin as provided by the manufacturer.

2.3 Test set-up and instrumentation

One bay of three-dimensional steel frame as presented in

Figure 4 was equipped to carry out the tests. A 100-mm

gauge length linear variable differential transducer

(LVDT) was used in order to measure the vertical

deflection at mid-span point of the test beams, while 10-

mm strain gauges were used in order to measure the

developed strains in the internal reinforcement at the

tension side along with the CFRP strain for the CFRP-

strengthened beams at the mid-span section. In addition,

a Pi-gauge of 100mm gauge length was used in order to

measure the deformation at the concrete tension and

compression sides. Hence, the concrete compressive strain

can be obtained. The beam was loaded by means of four

points loading test. In several steps, the beam was loaded

up to failure. The load on the beam was measured by a load

cell of 600 kN capacity. A laser level was used to ensure

the coincidence of the axes of the beam, load cell and the

loading beam before testing.

After each loading step, the vertical mid-span

deflection, the Pi-gauge readings, the strains in the

longitudinal steel and the CFRP sheets were recorded. The

loading rate for all beams ranged from 0.05 to 0.07 kN/s.

An automatic data logger unit had been used in order to

record and store data during the test for load cell, steel

strain gauges, CFRP strain gauges, Pi-gauges and LVDT.

3. Results and discussion

The test results of the reference group included the control

beam, the beam provided with an ECC transition layer

only and the CFRP-strengthened beams in addition to the

strengthened group included CFRP-strengthened beams

where the strengthening CFRP sheets were bonded to ECC

Table 1. Mix proportions of RC substrate beams and the ECC materials having different fibre contents for one cubic meter (kg/m3).

Concrete mix W/Ba Cement Sand Crushed pink limestone Water Silica fume Super-plasticiser PP fibre (12mm)

RC 0.45 300 650 1300 126 – – –ECC-0.5% 0.26 1100 660 – 315 110 25 6ECC-1% 0.26 1100 660 – 315 110 25 12ECC-1.5% 0.26 1100 660 – 315 110 25 18

aW/B is the water/binder ratio, B ¼ cement þ silica fume.

Figure 2. Preparing the contact surfaces just before casting theECC layers.

Structure and Infrastructure Engineering 5

transition layers having different fibre content as presented

in Table 3. Generally, the proposed strengthening

technique proved to be efficient both in the reconstruction

of the tensile zone around the tension reinforcement of RC

beams and in improvement of the CFRP-strengthened

beams performance as a whole. Further discussion is

presented in the sequence regarding mode of failure, load–

deflection response, cracking characteristics of the ECC

transition layer, ultimate capacity and ductility.

3.1 Failure mode

The normal flexural failure is usually observed in under-

reinforced unstrengthened beams and is characterised by

extensive yielding of the internal steel reinforcement in

tension, followed by crushing of concrete in the

compression zone. This failure occurred in the control

beam, B-C, as shown in Figure 5(a). As for beam B-C-

ECC, more flexural cracks were developed where hair

cracks were developed in the ECC transition layer and

stopped to propagate along the RC beam. Then the cracks

widened significantly after the yielding of the steel

reinforcement, and finally the concrete crushed in

compression side at the mid-span section as illustrated in

Figure 5(b).

The beam B-C-CF exhibited different kinds of failure

mode. The CFRP de-bonding process initiated at a major

flexural crack near the mid-span section and then

propagated towards the end where the CFRP anchorages

located. When this occurred, the CFRP sheet was ruptured,

as shown in Figure 5(c), accompanied by peeling-off the

concrete cover. The de-bonding of the CFRP sheet

occurred due to high local interfacial stress near wide

cracks. The widening of the crack was the direct driving

force of propagation, and the relative vertical displace-

ment between the two faces of the crack produced the

peeling stress on the interface. This peeling stress resulted

in the peeling-off failure because the cracked concrete

Table 2. Mechanical properties of the CFRP sheets along withthe epoxy resin.

Criteria CFRP sheets Epoxy

Tensile strength (MPa) 3500 30Modulus of elasticity (GPa) 230 21.40Failure strain (%) 1.50 4.80Thickness (mm) 0.13 –

Figure 3. Stress–strain relationships for uniaxial tensile tests ofECC material with different fibre content.

750 mm 500mm 750mm

Pedestal

h =300 mm

Load cell

LVDT

Pedestal

Pi-gauge

Loading beam

The beam of the main testing frame

Figure 4. Test set-up.

H.M. Afefy et al.6

cover was unable to transmit this force. No failure of the

end anchors was observed while the rupture of the CFRP

sheet was occurred near the mid-span of the beam.

Figure 6 depicts the failure modes of the CFRP-

strengthened beams of group 2. It can be seen that changing

the fibre content of the ECC transition layer has a great

influence on the bond efficiency between the CFRP sheet

and the ECC transition layer. For a small fibre content of

beam BS-0.5, a complete separation between the CFRP

sheet and the beam was observed just after the rupture of

CFRP sheet as shown in Figure 6(a). This can be attributed

to the poor cracking control provided by the relatively small

amount of fibre content used in the ECC layer. Increasing

the fibre content to 1% as beam BS-1 showed enhanced

bond effect where localised de-bonding was developed

around the mid-span of the beam as shown in Figure 6(b).

The mode of failure for beam BS-1.5 (fibre

content ¼ 1.5%) showed outstanding result of increased

bond between the CFRP sheet and the ECC transition layer

where no separation between theCFRP-strengthening sheet

and the beam was observed even after the complete failure

of the beam as illustrated in Figure 6(c).

3.2 Cracking, yielding and ultimate loads

The cracks first appeared in RC beams once the tensile

capacity of the concrete was exhausted. In the current

research, there are two different materials having

dissimilar tensile capacity where the tensile capacity of

the ECC layer is about two times that of the substrate

concrete beam. As a consequence, for all the beams having

the ECC transition layers, cracks began to appear at

substrate beam, and soon later they propagated into the

ECC transition layer as illustrated in Table 3. With further

increases in acting load, cracks propagated in the tension

side penetrating both substrate beam and the ECC

transition layer. However, more cracks were noticed in

the ECC layer compared with those appeared in the

substrate beam where more hair cracks were developed in

the ECC layer and stopped to propagate to the substrate

beam. The recorded cracking loads for both ECC

transition layers along with the substrate beams for all

beams are presented in Table 3. It can be noted that

providing either an ECC transition layer or EB-CFRP

sheets delayed the appearance of cracks in substrate beams

by about 48% and 54%, respectively, for beams B-C-ECC

and B-C-CF compared with that of control beam, B-C.

Furthermore, it can be observed that the combined effect

of both ECC transition layer and the EB-CFRP sheets

showed more delay of crack appearance as exhibited by

beams BS-0.5, BS-1 and BS-1.5. In addition, increasing

the fibre content yielded more delay in crack appearance.

Regarding the yielding load based on the slope of the

load–deflection relationship for the test beams, using an

ECC transition layer delayed the yielding load by about

15%, while the application of CFRP sheet on a virgin RC

beam delayed the yielding load by about 29% compared

Table 3. Failure characteristics of all beams.

Concrete cubestrength (MPa)

Crackingload (kN) Maximum

steel strain,micro-strain

MaximumCFRP strain,micro-strainGroup no. Beam RC ECC RC ECC Py (kN) Pu (kN) Dy (mm) Du (mm)

Group 1 B-C 24.8 – 24.3 – 84.1 97.2 7.7 34.1 3436 –B-C-ECC 25.3 56.3 36.1 40.4 96.3 103.6 7.6 23.2 3806 –B-C-CF 25.3 – 37.4 – 108.8 125.2 7.4 20.3 3037 5370

Group 2 BS-0.5 26.2 57.4 38.4 45.5 109.3 131.9 7.9 27.1 3920 6480BS-1 26.2 57.4 39.5 49.3 110.3 137.5 7.7 28.2 5353 9610BS-1.5 26.2 57.4 41.3 53.3 107.7 140.5 5.4 31.6 7367 11,789

Py ¼ total vertical load at yielding, Pu ¼ ultimate failure load, Dy ¼ mid-span deflection at yielding, Du ¼ mid-span deflection at complete failure.

B-C-ECC beam B-C-CF beam Control beam, B-C

(a) (b) (c)

Figure 5. Failure modes of all beams of group 1.

Structure and Infrastructure Engineering 7

with that of the control beam, B-C. On the other hand, the

combination of the CFRP sheet and the ECC transition

layer showed approximately the same yielding load for

beams BS-0.5, BS-1 and BS-1.5 compared with that of the

beam B-C-CF. However, the corresponding deflections

decrease with increasing the fibre content ratio.

With respect to the ultimate capacity, providing an

ECC transition layer can increase the ultimate load-

carrying capacity of the beam by about 6.5% (compare the

ultimate capacities of beams B-C-ECC and B-C), while

applying one ply of the EB-CFRP sheet yielded an

increase of about 28.8% in the ultimate capacity (compare

the ultimate capacities of beams B-C-CF and B-C). The

algebraic sum of both increases is 35.3%, while the

manifested increase when applying both of them

simultaneously, as beam BS-1, is about 41%, which is

higher than the algebraic sum of individual increases. That

can be attributed to the enhanced composite action

attained by bonding the CFRP-strengthening sheet to a

transition layer with enhanced cracking behaviour.

Increasing the fibre content to 1.5% resulted in increased

capacity by about 44.5% as exhibited by beam BS-1 when

compared with that of beam B-C. That can be attributed to

the developed new mode of failure due to the enhanced

composite action between the ECC transition layer and the

EB-CFRP sheet.

3.3 Load–deflection response

Figure 7 depicts the load–deflection relationship for all

beams. Each beam exhibited linear behaviour up to the

cracking load, beyond which a rapid change in the slope of

the load–deflection curve was observed. By further

increasing the loads, the yielding of the internal steel

reinforcement occurred in the control beam B-C and then a

steady plateau in the load–deflection curve was observed

soon after that till the complete failure of the control beam.

Beam B-C-ECC showed a similar behaviour as that of the

control beam except that its post-cracking stiffness is

higher than that of the beam B-C. Providing one ply of

CFRP sheet resulted in decreased vertical deflection and

increased instantaneous stiffness at the same vertical load

as those of both B-C and B-C-ECC beams. In addition, the

exhibited load–deflection plateau beyond the yielding

load was shorter than that of the control beam where a

sudden drop in the vertical load was observed due to the

sudden rupture of the CFRP sheet.

When considering the CFRP-strengthened beams of

group 2, the manifested load–deflection plateaus after

yielding were varied according to fibre content provided in

the ECC transition layer. That can be attributed to the

different post-peak behaviour due to changing the fibre

volumetric ratio. Figure 7 shows that, for all CFRP-

strengthened beams (BS-0.5, BS-1 and BS-1.5), both

strength and stiffness were enhanced significantly com-

pared with those of the reference beams (B-C and B-C-CF).

In addition, the structural performance was enhanced with

increased fibre content from 0.5% to 1.5%. Among all

CFRP-strengthened beams, beam BS-1.5 showed an

enhanced performance from the viewpoints of stiffness,

ultimate capacity and the manifested deflection plateau.

Increasing the fibre content ratio for beams BS-1 and

BS-1.5 showed a remarkable increase in the tangential

stiffness after the first cracking of the beams compared

with that of the CFRP-strengthened beam without ECC

BS-1.5 beamBS-1 beamBS-0.5 beam

(a) (b) (c)

Figure 6. Failure modes of all beams of group 2.

Figure 7. Load–deflection relationship for the test beams.

H.M. Afefy et al.8

transition layer. After yielding of the main tension steel,

the combined behaviour of both the CFRP sheet and the

ECC transition layer controlled the behaviour. For beam

BS-0.5, approaching failure state, the CFRP sheet began to

de-bond that showed increased deflection with small

increased load. That was due to the strain hardening

behaviour of the ECC layer. With further loading, the

CFRP sheet was ruptured suddenly resulting in a step drop

in the resisting load.

For beam BS-1, a similar trend was noticed as

manifested by the beam BS-0.5 with higher stiffness and

resisting load. That can be attributed to the enhanced

bond characteristics between the EB-CFRP sheet and the

ECC transition layer which retained the de-bonding and

mitigated its propagation of the CFRP sheet compared

with that exhibited by the beam BS-0.5. Regarding beam

BS-1.5, a different mode of failure was noticed, while the

CFRP sheet was in complete bond to the ECC layer up to

failure, the ECC transition layer was failed resulting in

the separation between the RC substrate beam and the

ECC transition layer. That separation enforced the CFRP

sheet to work solely in an unbounded manner up to the

end of the test.

3.4 Crack width and crack spacing developed in theECC transition layer

Table 4 illustrates the measured major crack width along

with the average crack spacing developed in the ECC

transition layers along with those developed in the

concrete substrate of beam B-C-CF near the failure state.

It is well recognised that the de-bonding of the CFRP sheet

is highly dependent on the crack width and spacing,

because high local interfacial stress will be developed near

wide crack. The relative vertical displacement between the

two faces of the crack produces the peeling stress on the

interface. This peeling stress could result in the peeling-off

failure based on the ability of the cracked concrete cover to

transmit this force. This means that reducing the crack

width along with better crack distribution along the

interface of the CFRP sheets will result in better bond

characteristics along the CFRP interface up to failure.

Based on the test results of beam B-C-CF, the

relatively wide major crack (2mm) and the large crack

spacing (100mm) resulted in complete de-bonding in the

constant bending moment region as illustrated in Figure 5

(c). Increasing the fibre content of the ECC transition layer

from 0.5% to 1%, to 1.5% resulted in reduced crack width

along with reduced crack spacing that exhibited better

bond along the CFRP interface with the ECC transition

layer. As a result, the mode of failure had been changed

from de-bonding of the CFRP sheet as for beam BS-0.5 to

the rupture of the CFRP sheet without de-bonding as

manifested by beam BS-1.5.

Figure 8 displays the front view of both beams BS-1

and BS-1.5. It can be noticed that increasing the fibre

content ratio from 1% to 1.5% showed better crack

distribution where the number of cracks was doubled for

beam BS-1.5. On the other hand, separation between the

ECC transition layer and the substrate beam was happened

resulting in premature failure of the beam, BS-1.5. In other

words, the ultimate tensile capacity of the ECC transition

layer is a significant parameter and has to be increased in

order to show better enhancement in the ultimate capacity

of the beam due to the EB-CFRP sheet.

3.5 Ductility

Mufti, Newhook, & Tadros (1996) suggested that, in order

to determine the ductility of a RC beam strengthened with

Table 4. Cracking characteristics of the ECC transition layernear failure.

BeamMajor crackwidth (mm)

Average crackspacing (mm)

B-C-ECC 1 62B-C-CF 2a 100a

BS-0.5 0.42 55BS-1 0.39 38BS-1.5 0.35 19

a The crack width and spacing are for the substrate concrete beam.

Beam BS-1 Beam BS-1.5

Figure 8. Crack distribution along the ECC transition layer for beams BS-1 and BS-1.5.

Structure and Infrastructure Engineering 9

FRP, it is necessary to consider a deformability factor (DF)

and a strength factor (SF), and then combine these two

parameters to define an overall factor called performance

factor (PF). The DF can be determined by dividing the

deflection at ultimate limit state by the deflection at

serviceability limit state corresponding to a concrete

compressive strain of about 0.1%. SF is the ratio of the

ultimate load to the serviceability load.

Table 5 shows the DF, SF and PF for all tested beams.

It is empirically defined that the FRP-strengthened

members should present a PF larger than 4 to guarantee

an adequate ductile behaviour (Garcez, Meneghetti, &

Filho 2008). Table 5 also presents that the PF has been

increased when an ECC transition layer was implemented

solely (beam B-C-ECC) compared with that of the control

beam, B-C. On the other hand, the CFRP-strengthened

beam, B-C-CF, exhibited lower PF than that of the beam

B-C. In addition, its PF is lower than 4. When applied the

CFRP sheet on an ECC transition layer, the obtained PFs

are in-between those of beams B-C-ECC and B-C-CF.

Beam BS-1 manifested the highest PF among all

CFRP-strengthened beams. In addition, it showed

adequate ductility according to Garcez et al. (2008). The

beam BS-1.5 showed higher ductility compared with that

of the beam B-C-CF. However, it failed to exhibited

adequate ductility according to Garcez et al. (2008).

Despite that the beam BS-1.5 exhibited the highest SF

among all strengthened beam along with the control beam,

its DF showed the lowest one. That can be attributed to the

occurrence of local de-bonding between the ECC

transition layer and the substrate concrete and the

corresponding premature failure of of the ECC transition

layer.

4. Conclusions

Based on the studied hybrid strengthening techniques of

RC beams, loading scheme, the thickness of the ECC

transition layer, the configuration of the EB-CFRP sheet

and according to the used concrete dimensions and

reinforcement detailing, the following conclusions can be

drawn:

(1) Based on the results of the present experimental

programme, it can be concluded that the proposed

strengthening technique, even with the possibility

of further improvements, as any other technique,

proved to be efficient both in the reconstruction of

the tensile zone around the tension reinforcement

of RC beams and in the improvement of the entire

beam performance, particularly in a more efficient

exploration of the resistance properties of

strengthening with sheets of CFRP.

(2) The application of one ply of EB-CFRP sheet as a

strengthening material, solely, can enhance the

ultimate load-carrying capacity of the RC beams

on the expense of their ductility. However,

utilising an ECC transition layer in the hybrid

strengthening system showed enhanced ultimate

load-carrying capacity and ductility of the CFRP-

strengthened beams.

(3) Compared with the control unstrengthened beam,

utilising an ECC transition layer alone can

increase the ultimate load-carrying capacity of

the strengthened beam by about 6.5%, while

applying one ply EB-CFRP sheet yielded an

increase of about 28.8% in the ultimate capacity.

The algebraic sum of both increases is 35.3%,

while the manifested increase when applying the

EB-CFRP sheet on the beam having an ECC

transition layer, as beam BS-1, is about 41%,

which is higher than that the algebraic sum of

individual increases. That can be attributed to the

enhanced composite action attained by bonding

the CFRP strengthening sheet to a transition layer

with enhanced cracking behaviour. Also, increas-

ing the fibre content to 1.5% resulted in increased

capacity by about 44.5%.

(4) Increasing the fibre content in the ECC transition

layer results in decreased major crack width along

with better crack distribution leading to complete

Table 5. Ductility parameters for test beams.

Displacement (mm) Loading (kN)

Group no. Beam Ds Du DF Ps Pu SF PF

Group 1 B-C 6.1 31.1 5.10 67.3 97.2 1.45 7.40B-C-ECC 5.8 21.7 3.74 77.1 103.6 1.34 5.01B-C-CF 5.7 14.3 2.51 87.1 125.2 1.44 3.61

Group 2 BS-0.5 5.9 17.5 2.97 87.4 131.9 1.51 4.48BS-1 5.1 15.9 3.12 88.2 137.5 1.56 4.87BS-1.5 4.1 9.6 2.34 86.2 140.5 1.63 3.81

Ps ¼ total vertical load at service limit state, Pu ¼ ultimate failure load, Ds ¼ mid-span deflection at service loading, Du ¼ mid-span deflectioncorresponding to the ultimate load.

H.M. Afefy et al.10

bond between the EB-CFRP-strengthening sheet

and the ECC transition layer up to failure as

exhibited by beam BS-1.5. Thus, the whole tensile

capacity of the CFRP sheet can be transferred to

the strengthened beam. As a result, both ultimate

capacity and ductility can be significantly

improved.

(5) The exhibited mode of failure showed that the

tensile resistance of the ECC transition layer has to

be increased in order to show better enhancement

due to EB-CFRP-strengthening sheet. In sum, the

bond characteristics between the EB-CFRP and

the ECC transition layer are not the only parameter

that guarantees the enhanced global behaviour of

the studied beam. Hence, more parameters such as

the effect of the ingredients of the ECC layer have

to be studied such as the type of the fibre and

proportions of the ingredients of the mix as a

further research.

Notes

1. Email: nessreenkasem@yahoo.com2. Email: m_hussein_20@yahoo.com

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