efficient strengthening technique to increase the flexural resistance of existing rc slabs

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Efficient Strengthening Technique to Increase the FlexuralResistance of Existing RC Slabs

Everaldo Bonaldo1; Joaquim António Oliveira de Barros2; and Paulo B. Lourenço3

Abstract: Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity ofreinforced concrete �RC� members. Near-surface mounted �NSM� is one of the most promising strengthening techniques, based on the useof carbon fiber-reinforced polymer �CFRP� laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slitsopened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is anadequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increaseof the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of theslab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSMtechnique. A new thin layer of concrete bonded to the existing concrete at the compressed region is suitable to overcome this limitation.Volumetric contraction due to shrinkage and thermal effects can induce uncontrolled cracking in the concrete of this thin layer. Addingsteel fibers to concrete �steel fiber-reinforced concrete �SFRC��, the postcracking residual stress can be increased in order to prevent theformation of uncontrolled crack patterns. In the present work, the combined strengthening strategy, a SFRC overlay and NSM CFRPlaminates, was applied to significantly increase the flexural resistance of existing RC slabs. Experimental results of four-point bendingtests, carried out in unstrengthened and strengthened concrete slab strips, are presented and analyzed.

DOI: 10.1061/�ASCE�1090-0268�2008�12:2�149�

CE Database subject headings: Concrete, reinforced; Concrete slabs; Flexural strength; Experimentation; Composite materials;Fiber reinforced materials; Fiber reinforced polymers; Laminates; Composite structures.

Introduction and Background

The near-surface mounted �NSM� strengthening technique hasbeen used in the recent years, with notable effectiveness, to in-crease the flexural strength �FIB 2001; Carolin 2003; Barros andFortes 2005; Kotynia 2005; Jung et al. 2005; Liu et al. 2006� andshear resistance �De Lorenzis and Nanni 2001; Barros and Dias2006; Barros et al. 2006� of reinforced concrete elements. TheNSM technique involves the embedment of carbon fiber-reinforced polymer �CFRP� bars—of circular, square, or rectan-gular cross section—into grooves opened on the concrete surface.The CFRP bars are bonded to concrete using an epoxy-basedadhesive.

When compared to the externally bonded reinforcement �EBR�technique, the NSM technique assures a higher anchoring capac-ity to the FRP reinforcing material. As a consequence, a hightensile stress can be applied to the CFRP, as long as the member

1Ph.D. Student, Dept. of Civil Engineering, Univ. of Minho, Azurém,4800-058 Guimarães, Portugal �corresponding author�. E-mail: [email protected]

2Associate Professor, Dept. of Civil Engineering, Univ. of Minho,Azurém, 4800-058 Guimarães, Portugal. E-mail: [email protected]

3Full Professor, Dept. of Civil Engineering, Univ. of Minho, Azurém,4800-058 Guimarães, Portugal. E-mail: [email protected]

Note. Discussion open until September 1, 2008. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on August 24, 2006; approved on March 26, 2007. Thispaper is part of the Journal of Composites for Construction, Vol. 12, No.
2, April 1, 2008. ©ASCE, ISSN 1090-0268/2008/2-149–159/$25.00.
JOURNAL OF COMP

J. Compos. Constr. 20

load carrying capacity is not limited by a premature failure mode�Blaschko and Zilch 1999; FIB 2001; Carolin 2003�.

For reinforced concrete �RC� slabs of low or medium concretestrength, the increment of the flexural resistance that NSM canprovide might be limited by the maximum allowable compressivestrain in the extreme compressed concrete fiber. This drawbackcan be overcome by adding a concrete layer in the compressionzone of the existent slab �Walser and Steiner 1997; Barros andSena-Cruz 2001�.

To attain the desired structural performance �e.g., full compos-ite action�, the new concrete overlay and the existent concreteslab should behave monolithically. A sound bond between thenew layer and the existing concrete slab can be guaranteed if aproper epoxy compound is used �Bonaldo et al. 2005a,b�.

Since a thin concrete layer is enough for the aforementionedpurpose, the restrained shrinkage and the temperature variationcan induce uncontrolled cracking in the concrete of this layer.Adding discrete fibers to concrete, such as steel fibers, the crackwidths resulting from drying restrained shrinkage can be substan-tially reduced. Moreover, the postcracking residual tensilestrength of cement based materials can be increased by fiber ad-dition �Barros et al. 2005�, to prevent the formation of uncon-trolled crack patterns. The benefits of fiber reinforcement can beextended to the concrete compression behavior, since concretecrushing can be delayed or even avoided for compressive strainlevels attained in RC practical applications.

In the present paper, a strengthening strategy to increase theload carrying capacity of RC slabs was experimentally investi-gated. This strategy consists of applying CFRP laminates on thetensile bottom slab surface, according to the NSM technique, and
bonding, with appropriate epoxy-based adhesive, a steel fiber RC
OSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2008 / 149

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�SFRC� layer in the compression top slab surface �see Fig. 1�. Toassess the effectiveness of the NSM strengthening technique toincrease the flexural resistance of slab strips failing in bending,four point bending tests were carried out.

Experimental Program

Specimen and Test Configuration

To assess the effectiveness of the hybrid strengthening techniquefor the increase of the flexural load carrying capacity of RC slabs,the slab strips represented in Fig. 2 were used, including un-

Fig. 1. Hybrid strengthening technique for concrete slabs

Fig. 2. Slab strips: �a� test configuration; �b� specimens cross-sectional dimensions �all dimensions are in mm�

150 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARC


strengthened �or reference� slabs and strengthened slabs �with andwithout concrete overlay�. The test setup and the cross-sectionaldimensions of the tested slab strips are also illustrated in Fig. 2.

The number of CFRP laminates applied in each RC slab wasdesigned to increase the service load of the reference slabs by50%, which was assumed equal to the load producing a midspandeflection of � /250=1,800 mm /250=7.2 mm, where � is the slabspan. The arrangement of the CFRP laminates and the dispositionof the SFRC overlay are indicated in Fig. 2�b�.

The width of the slit where the CFRP is installed usuallyranges from 4.0 to 5.0 mm for the NSM strengthening with CFRPlaminates. To assess the influence of the thickness of the adhesivelayers that bond the CFRP laminate to concrete and, consequently,the influence of the adhesive deformability on the NSM strength-ening performance, three different widths for the slits were con-sidered. NSM slits having widths of about 4.0, 7.5, and 14.0 mmwere selected for the hybrid slab strips, whereas a width of about4.0 mm was taken for the slabs only strengthened with NSM.

The experimental program, therefore, consists of three series:SLi−—reference slabs; SLiS—NSM strengthened slabs; andSLiSO—NSM and SFRC strengthened slabs. In the designationof the series, i represents the number of the slab. A concrete oflow-to-moderate strength was selected for the reference slabs,aiming at reproducing the characteristics of typical existing rein-forced concrete structures. A concrete overlay thickness of ap-proximately 30 mm was applied for the SFRC strengthened slabs.

Measuring Devices

Fig. 3 depicts the positioning of the sensors for data acquisition.To measure the deflection of the slab strip, five linear voltagedifferential transducers �LVDT #1, LVDT #2, LVDT #3, LVDT#4, and LVDT #5� were supported in a suspension yoke �see Fig.3�a��. LVDT #3, placed at the slab strip midspan, was also used tocontrol the test at 20 �m /s up to the deflection of 49 mm. Afterthis deflection, the actuator internal LVDT was used to control thetest at 25 �m /s displacement rate up to the failure of the slab.The total applied force �F� was measured using a load cell�±200 kN and accuracy of 0.5%� placed between the loading steelframe and the actuator of 600 kN load capacity.

To measure the strains in the steel reinforcements, three elec-trical resistance strain gauges �SG1, SG2, and SG3� were installedon the internal steel reinforcements, according to the arrangementindicated in Fig. 3�c�. Five strain gauges were installed on oneCFRP laminate �SG4, SG5, SG6, SG7, and SG8� for evaluatingthe strain variation along the laminate �see Figs. 3�b and d��. Twostrain gauges �SG9 and SG10� were also bonded on the top con-crete surface to determine the maximum concrete compressivestrain �Fig. 3�e��.

Material Properties

Tables 1 and 2 include values, obtained from experimental tests,for the main properties of the materials used in the present work�Bonaldo et al. 2006�. Two concrete batches were prepared for thesame composition, with small variation in the mechanical proper-ties �1.5% in terms of compressive strength and 11% in terms offlexural tensile strength�. In Tables 1 and 2 the compressivestrength and the static modulus of elasticity in compression weredetermined according to NP-EN 12 390-3 �NP-EN 2002� andRILEM �1994�. The flexural behavior of the concretes was char-acterized by three-point bending tests with notched beams accord-
ing to RILEM �1985� for ordinary concrete, and following the
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recommendations of RILEM �2002� for the SFRC overlay. Thevalue of axial tensile strength �fctm�, included in Table 1, wasderived from the flexural tensile strength �fctm,fl�, obtained fromtests on notched beams, according to the CEB-FIP �1993�. Tocharacterize the steel bars, uniaxial tensile tests were conductedaccording to the standard procedures found in NP-EN 10 002-1�NP-EN 1990�. Unidirectional pultruded CFRP laminates, spe-cially designed to be bonded into narrow slits in concrete struc-tures, were used in this study. The uniaxial tensile tests on CFRPcoupons were carried out according to the ISO 527-5 �ISO 1997�recommendations. For the laminates and overlay epoxy adhe-sives, uniaxial tensile tests were performed complying with theprocedures outlined by ISO 527-2 �ISO 1993�. Further detailsrelated to the properties of the materials can be found in Bonaldo

Fig. 3. Arrangement of displacement transducers and strain gauges:�a� displacement transducers; �b� position of strain gauges at CFRPlaminate—side view; �c� layout of strain gauges at steel bars; �d�layout of strain gauges at CFRP laminate—bottom view; and �e�strain gauges at concrete slab top surface—top view �all dimensionsare in mm�

et al. �2006�. All symbols are defined in the “Notation.”

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Strengthening Procedures

When the concrete slabs attained, approximately, 28 days of age,the slits were opened in the strengthened slabs. The slits werefilled with the epoxy adhesive using a spatula, and the CFRPlaminates were then introduced into the slits. To ensure goodadhesion between the new concrete overlay and the old concrete,the top surface of the last one was sandblasted to remove thecement laitance layer.

The SFRC overlay was bonded to the concrete slab sand-blasted surface, using the epoxy adhesive with the properties in-dicated in Table 2. The bond product was spread over thesubstrate top surface with a spatula, and then the SFRC overlaywas poured. A mini slipform was used to consolidate both the thinSFRC overlay and SFRC beam specimens. The mini slipformintends to simulate the real conditions of compaction of a thinSFRC overlay �Bonaldo et al. 2006�. The manufacturers specifi-cations and ACI guidelines ACI 503.2-92 �ACI 1992b�, ACI503.5R-97 �ACI 1997�, and ACI 503.6R-92 �ACI 1992a� werefollowed to bond the fresh SFRC overlay to the hardened con-crete.

Results and Analysis

Load-Displacement Response

The load-midspan deflection curves of the tested slabs are pre-sented in Fig. 4. The relationship between the displacements re-corded in the other LVDTs and load are included elsewhere�Bonaldo et al. 2006�. It can clearly be noticed in Fig. 4 that theexperimental load-displacement curves of the unstrengthened andstrengthened slabs have a typical trilinear diagram, which corre-spond to the following behavioral phases: the uncracked elastic;crack propagation with steel bars in elastic stage; and steel rein-forcement postyielding stage. The unstrengthened control slabsbehaved in a perfectly plastic manner in the postyielding phase,whereas the strengthened slabs exhibited continuous hardeninguntil maximum load.

In the SL3S, SL4S, SL5SO, and SL6SO slabs the width of theslits where the CFRP laminates were installed was of about

Table 1. Characteristics of Plain Concrete

Mix Slab

Property

fcm

�MPa�fctm,fl

�MPa�fctm

a

�MPa�Ec

�GPa�

SL1b

B1 SL4Sc 25.97 2.91 1.39 27.69

SL6SOd �2.16%� �20.87%� �20.64%� �6.80%�

SL7SOd

SL2b

B2 SL3Sc 26.35 3.26 1.56 26.61

SL5SPd �3.36%� �26.19%� �25.75%� �6.13%�

SL8SOd

Note: �Value� Coefficient of variation �COV�= �standard deviation /average��100.aDerived from CEB-FIP �1990�.bControl slabs.cNSM strengthened slabs.dNSM and SFRC strengthened slabs.

4.0 mm. In the SL7SO and SL8SO slabs the width of the NSM


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slits was about 14.0 and 7.4 mm, respectively. By examining theload-deflection curves in Fig. 4, it can be noticed that the width ofthe NSM slits had no influence on the deformational behavior ofthe slabs.

Table 2. Summary of Characteristics of Steel Reinforcement, CFRP and

Steel reinforcement CFRP laminatea Lam

�s=8 mm tf =1.41 mm

Es=200.32 GPa wf =9.37 mm E

�sy=465.75 MPa Ef =156.10 GPa �au

�sy=0.25% �fu=2,879.13 MPa �

�su=557.12 MPa �fu=1.85%

aPultruded CFRP laminate provided by the S&P Company.

Fig. 4. Load-deflection at midspan of all tested slabs

Table 3. Summary of Results in Terms of Loads and Deflections

Strengthening Slab ID

Cracking

Load�kN�

Deflection�mm�

Reference SL1 2.57 0.69

SL2 3.50 0.96

CFRP laminate strengthening SL3S 3.78 1.03

SL4S 3.77 0.87

CFRP+SFRC overlay strengthening SL5SO 7.61 0.73

SL6SO 6.98 0.70

SL7SO 6.79 0.77

SL8SO 6.96 0.68

Note: �Value� Coefficient of variation �COV�= �standard deviation /averagaDeflection registered at ultimate load in strengthened slabs; The deflectio0.3% in the strain gauges on top of slab.bWith respect to the reference slabs.cWith respect to the CFRP laminate strengthened slabs.d
NA=not applicable.


The cracking, yield, and maximum loads, and the strength in-creasing ratio are given in Table 3. In an experimental test, thecracking load is normally reported as a load corresponding to theoccurrence of a crack for the first time. However, the crackingload is considered here as the load at which a significant changein the slope of the total load-deflection relationship is observed.The yield load is herein defined as the load which leads to a strainin the steel reinforcement that is equal to the yield strain mea-sured from tension tests in coupons. A significant change in theslope of the total load-deflection curve is also observed in thevicinity of yield load. In Table 3, the strength increasing ratio wascalculated using the following expressions:—CFRP strengthened slabs:

FuCFRP − Fu

REF

FuREF �1�

and—CFRP & SFRC strengthened slabs:

FuCFRP&SFRC − Fu

REF

FuREF �2�

hesive, and SFRC and Its Adhesive

dhesive SFRC overlay Overlay adhesive

fcm=43.23 MPa

Ec=30.08 GPa

GPa Ea=3.62 GPa

feqm,2=4.31 MPa

3 MPa �au=26.56 MPa

feqm,3=3.47 MPa

8% �au=1.07%

fRm,1=4.45 MPa

fRm,4=2.24 MPa

ielding Ultimate Averageultimate

load�kN�

Strength increaseratio

Deflection�mm�

Load�kN�

Deflectiona

�mm�

16.76 14.34 102.52 �56.59� 14.73 NAd

17.29 15.12 102.74 �71.41�

18.00 35.60 69.63 �41.96� 36.64 +1.49

17.99 37.67 79.48 �35.64�

12.20 65.13 56.54 �46.93� 66.37 +3.51b

11.65 65.46 64.11 �43.95�

12.12 66.85 57.84 �47.23� �2.02%� +0.81c

11.84 68.04 56.47 �45.52�

00.

ackets is the deflection corresponding to a concrete compressive strain of

Its Ad

inate a

a=7.47

=33.0

au=0.4

Y

Load�kN�

10.56

12.10

17.93

19.15

31.55

30.08

30.57

30.64

e��1

n in br

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FuCFRP&SFRC − Fu

CFRP

FuCFRP �3�

where FuCFRP, Fu

REF, and FuCFRP&SFRC=average maximum load of

CFRP strengthened slabs, reference slabs, and CFRP and SFRCoverlay strengthened slabs, respectively.

The monitored strains at the midspan, in the concrete top sur-face, steel bars, and CFRP laminates are listed in Table 4. Fromthe analysis of the strain values of Table 4, it can be concludedthat, at the onset of the slabs’ failure, the maximum strain in theCFRP ranged from 1.1 to 1.34%, which is 60–73% of the CFRPlaminate ultimate strain �see Table 2�.

As expected, the concrete maximum compressive strain of theslabs only strengthened with CFRP laminates has exceeded0.35%, while in the hybrid strengthened slabs the concrete maxi-mum compressive strain was below this value.

Analytical Prediction of Ultimate Loads and FailureModes

Table 5 includes the theoretical calculations of the load capacity,the experimental loads at failure, the ultimate deflections, and the

Table 4. Summary of Monitored Strains

Strengthening Slab

Con

Reference SL1

SL2

CFRP laminate strengthening SL3S

SL4S

CFRP+SFRC overlay strengthening SL5SO

SL6SO

SL7SO

SL8SO

Note: �Value� Corresponding load in brackets.aMaximum value of SG9 and SG10.bMaximum value of SG1, SG2 and SG3.cMaximum value recorded in SG7 and SG8.dThe load in brackets is the maximum load.eNA=not applicable.fNE=not evaluated.

Table 5. Theoretical Calculations Compared with Loads at Failure, Ultim

Strengthening Slab

Calculatedload�kN�

Reference SL1 13.84

SL2 14.01

CFRP laminate strengthening SL3S 37.58

SL4S 37.58

CFRP+SFRC overlay strengthening SL5SO 66.90

SL6SO 66.54

SL7SO 66.36

SL8SO 68.26aDeflection registered at ultimate load in strengthened slabs.bSlab specimen with severe crushing of concrete followed by diagonal shc
Flexo-shear failure mode is considered here, since steel yielding and shear fail
JOURNAL OF COMP


failure modes of the tested slabs. Fig. 5 illustrates the internalstrain and stress distribution for the CFRP strengthened crosssection under flexure at the ultimate limit state, for the theoreticalcalculations of the resistant bending moment of a NSM strength-ened RC cross section. In the calculations, the materials pro-perties from Tables 1 and 2 were assumed, together with therecommended values �=0.85 and �1=0.72, according to ACI318-05, section 10.2.7.3 �ACI 2005�. The same strain and stressdistribution presented in Fig. 5 can be also used for the CFRP andSFRC strengthened cross sections, since the depth of the equiva-lent rectangular stress block remains in the SFRC layer.

Reference SL1 and SL2 slabs failed in flexure, i.e., by yieldingof internal reinforcement, with extensive cracking in the tensionflange, followed by concrete crushing at the top surface.

The SL3S and SL4S slabs, only strengthened with laminates,have also failed in flexure: yielding of the internal steel reinforce-ment followed by concrete crushing at the top surface. However,specimen SL3S failed in shear after significant deflection, by theformation of an intermediate shear crack mechanism.

In the combined strengthening case, for the other fours speci-

ompressiveaina

%�Steel strainb

�%�CFRP laminate strainc

�%�

14.05�d 0.84 �14.00� NAe

�14.56� 1.99 �14.89� NAe

�35.49� NEf 1.11 �35.58�

�34.39� 2.08 �25.07� 1.25 �36.91�

�65.13� 2.09 �39.20� 1.34 �65.13�

�65.24� 0.75 �44.84� 1.22 �65.46�

�60.61� 2.15 �41.87� 1.31 �66.85�

�67.62� 1.21 �45.64� 1.34 �68.04�

eflection, and Mode of Failure

xperimentalload�kN�

Experimental/calculated

ratio

Ultimatedeflectiona

�mm�Type offailure

14.34 1.04 102.52 Flexure

15.12 1.08 102.74 Flexure

35.60 0.95 69.63 Flexureb

37.67 1.00 79.48 Flexure

65.13 0.97 56.54 Flexo-shearc

65.46 0.98 64.11 Flexo-shear

66.85 1.01 57.84 Flexo-shear

68.04 1.00 56.47 Flexo-shear

ack formation.

crete cstr�

0.36 �

0.31

0.47

0.51

0.34

0.34

0.28

0.35

ate D

E

ear cr

ure was observed after significant deflection.


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mens with laminates and SFRC overlay, all slab strips failed inshear, by the formation of an intermediate shear crack mechanismwith extensive cracking in the tension flange.

The failure modes of the slabs strengthened only with lami-nates and strengthened with laminates and SFRC overlay are il-lustrated in Fig. 6.

In all the tested slabs strengthened by the NSM technique, onthe contrary to what has been commonly verified experimentallyfor the extensively studied EBR strengthening technique �Grace2001, Smith and Teng 2002; Teng et al. 2003; Mosallam andMosalam 2003; Ashour et al. 2004; Wu and Hemdan 2005�, nopremature CFRP-debonding occurred.

Early concrete cover delamination and plate-end failure, due todirect transverse tensile stress at the concrete cover-internal rein-forcement interface and adhesive-concrete interface, respectively,are other common failure modes usually present in RC membersstrengthened with externally bonded FRP reinforcement �Ahmedet al. 2001; Smith and Teng 2002�. Although this type of collapsehas also been reported as a failure mechanism for the NSMstrengthening technique �De Lorenzis and Nanni 2002; Rizkallaet al. 2003; El-Hacha and Rizkalla 2004; Hassan and Rizkalla2004; Barros and Fortes 2005; Kotynia 2005�, such as occurrencewas not observed in the present tests, even in the slab specimenshaving a concrete of low strength class �C20/25�, a relative smalldistance between the NSM slits �about 70 mm�, and a high lon-gitudinal equivalent reinforcement ratio, �s,eq ��s,eq=1.08 and

Fig. 5. Internal strain and stress distribution fo

Fig. 6. Failure mode: �a� slab strengthened with laminates—slab SL



0.71%, for slabs strengthened with NSM and slabs strengthenedwith NSM and SFRC overlay, respectively�. Here, the equivalentreinforcement ratio is defined as

�s,eq =As

b · ds+

Ef

Es·

Af

b · df�4�

where b=width of the slab section; As, Es, and ds=cross-sectionalarea, Young’s modulus, and effective depth of the internal steelreinforcement; and Af, Ef, and df =cross-sectional area, Young’smodulus, and effective depth of the laminates. However, moreresearch about delamination is needed, taking into account vari-ables such as the distance between the slits, the termination pointsof the NSM strengthening, the concrete strength, the percentageand arrangement of internal steel reinforcement, the effective re-inforcement ratio, and the ratio of CFRP strengthening with re-spect to the internal steel reinforcement �Af /As�.

Bond Stress between CFRP Laminates and Concrete

Using the strains recorded in the strain gauges installed in thelaminates, the average CFRP-concrete bond stresses developedalong the CFRP laminates were evaluated. The average bondstress �bm

RL� in a CFRP laminate, in between the strain gaugesposition, SGL �left� and SGR �right�, was determined according tothe following equation �refer to Fig. 7�

gthened section under flexure at ultimate stage

� slab strengthened with laminates and SFRC overlay—slab SL8SO

r stren

3S; �b

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bmRL =

Ef · Af · �RL

2 · wf · LRL �MPa� �5�

where LRL=distance between two consecutive strain gauges;�RL=axial strain difference between the strain gauge at right andleft sections; and wf =width of the CFRP laminate, respectively.In this approach, constant bond stress between the strain gaugesSGL and SGR is assumed. The bm

RL variation during the loadingprocess of the SL4S and SL6SO slabs is shown in Fig. 8, which istypical of the bm

RL variation observed in the remaining slabsstrengthened with the NSM technique �Bonaldo et al. 2006�.

Fig. 8 shows that, as expected, up to concrete cracking thestrain in the CFRP laminates is very low. In general, a low bondstress profile is also developed at laminate-epoxy interfaces, at theserviceability limit deflection, since the average bond stress com-puted at this deflection level �7.2 mm at midspan� was below1.0 MPa. It can also be noticed that the bond stress was below6.5 MPa at the ultimate load. When the longitudinal bars startedyielding, an abrupt increase of bond stress occurred between SG6and SG7 since the maximum bending moment occurs at the slabcross section where SG7 is located and the laminates need tosupport the load increment applied to the slab. In general, a shearcrack was formed between SG5 and SG6. After the formation ofthis crack, the bond stress between these two SG increasedabruptly, followed by a significant decrease of the bond stressbetween SG6 and SG7. These two opposite tendencies of thebond stress in these two regions remained up to the failure of theslab. However, the maximum bm

RL was considerably lower than theaverage bond stress �bm=12 MPa� obtained in pullout-bendingtests carried out to assess the bond stress-slip relationship for theNSM technique �Sena-Cruz and Barros 2004a,b; Sena-Cruz2004�. This indicates that the bond length of the laminate, at eachside of the shear failure crack formed in the tested slabs, was

Fig. 7. Average bond stress �bmRL� inbetween two consecutive strain

gauges installed to CFRP laminate

sufficiently high to preclude considerable slip.

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Crack Spacing Analysis

The distinct crack feature of reference and strengthened slabs,shown in Fig. 9, clearly reveals the significant improvement in thecrack behavior provided by the used strengthening strategies.

The expression from the FIB �2001� was considered herein tocalculate the theoretical average crack spacing for the tested slabstrips. The average crack spacing, experimentally measured afterthe slabs have been tested, was determined as schematically de-scribed in Fig. 10 �Bonaldo et al. 2006�.

According to FIB �2001�, assuming stabilized cracking, themean crack spacing, srm, taking into account the effect of both theinternal reinforcement and EBR, can be calculated as

srm =2 · fctm · Ac,eff

sm · us·

Es · As

Es · As + �b · Ef · Af�6�

where �b=fm·Es ·�s ·uf /sm·Ef ·4 ·Af; Ac,eff=effective area ofconcrete in tension, the area of concrete surrounding the tension

Fig. 8. Bond stress variation for slab strips: �a� SL4S; �b� SL6SO

reinforcement; us=bond perimeter of the internal steel reinforce-


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ment; sm=1.8· fctm=average bond stress of the steel �CEB-FIP1990�; fm=average bond stress of the CFRP strengthening: fm

=1.25· fctm for EBR �FIB 2001�, and fm=1.85· fctm is used in thepresent work for NSM. The coefficient of 1.85 for calculating thevalue of fm was determined through an inverse analysis of bondstress between SG7 and SG6 when the intermediate shear crackwas formed �see the section “Bond Stress between CFRP Lami-nates and Concrete,” above�; and uf =bond perimeter of the CFRPstrengthening for EBR �FIB 2001�. Here, in the NSM context, uf

is considered as the perimeter length of the cross section of thefailure plane, which is assumed to be 1.0 mm offset from theNSM laminate �CIDAR 2006�: uf =nf · ��tf +2�+2· �wf +1��; nf

=number of NSM CFRP laminates; �s=mean bar diameter of theinternal steel reinforcement; Es=modulus of elasticity of the steelreinforcement; wf =width of the CFRP laminate; tf =thickness ofthe CFRP laminate; and Ef =modulus of elasticity of the CFRPlaminate.

The results indicated in Table 6 show that the enhancementsintroduced in the FIB formulation �FIB 2001� have allowed agood prediction of the crack spacing measured experimentally.

Ductility Analysis

In strengthening of RC structures with EBR it is notable that theincrease in strength and stiffness is sometimes attained at theexpense of a loss in ductility, or loss of capacity of the structuresto deflect inelastically while sustaining a significant percentage of

Fig. 9. Bottom view of typical final crack pattern �a� referenceslabs—slab SL1; �b� NSM strengthened slabs—slab SL3S; �c� and�d� NSM and SFRC strengthened slabs—slab SL6SO and slabSL8SO, respectively

Fig. 10. Determination of average crack spacing �srm� in tested slabstrips



its maximum load. Ductility is an important property for safedesign of strengthening of any structural element. As NSMstrengthening with CFRP is a relative recent strengthening strat-egy, understanding the effect of this technique on the ductility ofa RC member is crucial. A method, based on the ductility indexcommonly used, is considered herein to analyze the ductility ofthe slabs.

Ductility of RC members has generally been measured by pa-rameters designed as bending ductility indexes. In the presentwork, the ductility indexes are expressed as the ratio between thedeflection, or curvature, at an ultimate condition �u and �u, re-spectively� and the deflection, or curvature, at the yield load �y

and �y, respectively�, as follows

� =u

y�7�

and

�� =�u

�y�8�

The deflection at ultimate condition is the one recorded whena compressive strain of 0.3% was attained in the slab top surface,since it is the average of the strain at concrete compressivestrength �about 0.25%� and the strain corresponding to concretecrushing �0.35%� according to CEB-FIP �1990�. Table 7 lists thevalues of the ductility indexes for each of the slabs, as well as, theaverage values of these indexes for the unstrengthened andstrengthened slabs. By examining Table 7, a decrease of about42% in the average values of � index for the NSM strengthenedslabs is noticed, if � of the reference slabs is used for compari-son purposes. As previously mentioned, the deformational capa-bility of the slabs strengthened using NSM was limited byconcrete crushing in the extreme compressive fibers. Furthermore,no significant change in the ductility index of the slabs strength-ened using NSM and SFRC overlay, with respect to theunstrengthened control slabs, was observed. In the slabs strength-ened with the hybrid system, the increase in the energy absorptioncapacity at the compression softening phase, provided by the fiberreinforcement mechanisms, avoided the SFRC overlay to crush.However, the deformability of these slabs was limited by theshear failure.

According to the FIB �2001� recommendations, the minimumductility index, in terms of curvature, should be approximately1.7 and 2.6 for concrete types C35/45 or lower and concrete types

Table 6. Experimental and Analytical Crack Spacing

Strengthening Slab

Average crack spacing, srm

�mm�

Experimentala FIB �2001�

Reference SL1 80 �81� 93

SL2 82

CFRP laminatestrengthening

SL3S 45 �45� 42

SL4S 45

CFRP+SFRC overlaystrengthening

SL5SO 53 �52� 52

SL6SO 51

SL7SO 53

SL8SO 50aMeasures taken at the bottom of the slab lateral face �cf. Fig. 10�; �value�average value.

higher than C35/45, respectively. The ductility index should ex-

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ceed this minimum value to prevent the occurrence of suddenfailure in the strengthened flexural members. The minimum ac-ceptable ductility index ensures that the internal reinforcementexperiences plastic deformation in order to provide the desiredwarning prior to failure of the member.

The enhanced bond condition provides a much more ductilebehavior for the NSM strengthening technique compared to theEBR strengthening technique and the bond ductility when thecomposite material is applied into slits is comparable to the bondductility of embedded steel rebars.

Conclusions

An experimental program was conducted in order to: �1� investi-gate the NSM strengthening technique for reinforced concreteslabs; and �2� evaluate structural performance of a hybridstrengthening technique for RC slabs, composed of CFRP lami-nates on the tensile surface and SFRC overlay on the top com-pressive surface. For this purpose a total of eight slabs weretested. The following conclusions can be drawn from this study:1. The experimental program carried out demonstrated that the

hybrid strengthening technique has great potential applica-tion towards flexural strengthening of RC slabs, not only interms of increasing the slab ultimate load capacity, but alsoits stiffness;

2. Using for comparison purposes the RC slabs of 1.8 m spanlength, with a steel reinforcement ratio of 0.63%, it was veri-fied that a CFRP strengthening ratio of 0.25% �Af /As

�40% � increased the service load to about 55%. The slabsstrengthened by the NSM technique and SFRC presented anincrease of approximately 244% in the service load with re-spect to that of the reference slabs. Compared to the NSMtechnique, the hybrid strengthening strategy led to an in-crease of about 122% in the load at the deflection service-ability limit state;

3. The hybrid strengthening system also led to an increase ofabout 350% in the RC slab maximum load carrying capacitywith respect to that of the reference slabs, and an increase ofabout 80% in comparison to that of the slabs only strength-ened with the NSM technique. When compared to the refer-ence slabs, an increase of about 150% was obtained for theload carrying capacity when applying the NSM strengtheningtechnique;

4. The concrete cracking spacing measured at the bottom of the

Table 7. Ductility Ratios of Slab Specimens Tested

Strengthening Slaby

�mm�u

a

�mm�

D

�

Reference SL1 16.76 56.59

SL2 17.29 71.41

CFRP laminate strengthening SL3S 18.00 41.96

SL4S 17.99 35.64

CFRP+SFRC overlay strengthening SL5SO 12.20 46.93

SL6SO 11.65 43.95

SL7SO 12.12 47.23

SL8SO 11.84 45.52aThe ultimate load is taken as the load corresponding to a concrete compbThe curvature values were determined from the data recorded in the me

slabs’ lateral face indicates that the NSM technique provides

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a significant improvement in the crack behavior of RC slabs.In the results obtained, the average crack spacing �srm� of thelightly reinforced concrete slabs strengthened with NSM wasabout 45% lower the srm measured in the reference slabs. Areduction of about 36% in the srm was observed in the hybridstrengthened slabs;

5. For the stabilized cracking phase and taking into accountappropriate considerations for the NSM strengthening sys-tem, the average crack spacing of the strengthened slabs maybe achieved with acceptable accuracy using the analyticalexpression suggested by FIB �2001�;

6. When compared to the bond strength recorded in pullout-bending tests, a very low bond stress profile was observedthrough the CFRP laminate-epoxy adhesive interface alongthe laminates;

7. The ductility index for slabs strengthened with the NSM sys-tem and SFRC overlay, based on deflections measurements,was the same as the reference slabs. The slabs strengthenedonly with NSM laminates had a ductility index 42% lowerthan the reference slabs. This reduction in ductility is attrib-uted to the low resistance of the concrete, since early com-pressive failure was observed at the top surface of the slabs;

8. The NSM strengthening system has also provided a signifi-cant increase in the stiffness and deformability, consistentwith the high stress redistribution �closely spaced crack pat-tern� owing to the prominent composite action between theCFRP reinforcement and concrete. Since the hybrid strength-ening system led to a substantial increase in the flexural re-sistance, the shear capacity of the composite slabs limitedtheir deformability; however, the stiffness of the slabs in-creased significantly and the ductility level was maintained;and

9. The SFRC overlay bonding procedures can be consideredadequate, since no slip was observed, and the connectionbetween SFRC overlay and the RC slab was able to transferhorizontal shear stress, consequently forming a monolithicflexural cross section.

Acknowledgments

The writers acknowledge the financial support of the PortugueseScience and Technology Foundation �FCT�, Ph.D. Grant No.SFRH/BD/11232/2002. Thanks also go to the companies “Com-panhia Geral de Cal e Cimento S.A. �SECIL�,” Sika S.A.,

atio

m�Average �

�mm/mm��y

b

�m−1��u

a

�m−1�

Ductile ratio��

�m−1 /m−1�Average ��

�m−1 /m−1�

3.75 0.044 0.080 1.81 2.33

0.047 0.133 2.86

2.16 0.058 0.212 3.64 4.00

0.054 0.234 4.36

3.84 0.039 0.161 4.17 3.97

0.039 0.150 3.88

0.040 0.152 3.81

0.040 0.163 4.03

strain of 0.3% in the strain gauges on top of slab.

devices �SG� situated at the cross section in the midspan of the slab.

uctile r�

mm/m

3.38

4.13

2.33

1.98

3.85

3.77

3.90

3.84

ressive

“Central do Pego,” “Pedreiras Bezerras,” Bekaert NV, “Degussa


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Construction Chemicals Portugal S.A.,” S&P Reinforcement,which generously have supplied cement; overlay bond product;fly ash; aggregates; steel fibers; superplasticizer and CFRP adhe-sive; and CFRP laminate, respectively.

Notation

The following symbols are used in this paper:Ac,eff effective area of concrete in tension �effective

area of concrete in tension�;Af total area of NSM laminate reinforcement;As total area of internal steel reinforcement;b width of slab cross section;

df effective depth of laminate reinforcement;ds effective depth of internal steel reinforcement;Ea modulus of elasticity of adhesive;Ec modulus of elasticity of concrete;Ef tensile modulus of elasticity of laminate;Es modulus of elasticity of the internal steel

reinforcement;F total applied force;

Fc resultant compressive action;Ff resultant tensile force in laminate

reinforcement;Fs resultant tensile force in internal steel

reinforcement;Fu

CFRP average maximum load of CFRP strengthenedslabs;

FuCFRP&SFRC average maximum load of CFRP and SFRC

overlay strengthened slabs;Fu

REF average maximum load of reference slabs;fcm mean concrete cylinder compressive strength;fctm mean concrete axial tensile strength;

fctm,fl mean concrete flexural tensile strength;feqm,2, feqm,3 mean equivalent flexural tensile strength

parameters;f f stress level in laminate reinforcement;

fRm,1, fRm,4 mean residual flexural tensile strengthparameters;

fs stress level in internal steel reinforcement;h overall thickness of slab cross section;

hSFRC SFRC overlay thickness;hSlab slab thickness;LRL distance between two consecutive strain

gauges;� slab span length;

M flexural strength of section;n number of crack spacing measurements taken

in constant moment zone;nf number of NSM laminates;

sr,i ith crack spacing measurement;srm average crack spacing;

tf laminate thickness;uf bond perimeter of CFRP strengthening �for

EBR�, or perimeter length of cross sectionof failure plane �for NSM�;

us bond perimeter of internal steel reinforcement;wf laminate width;

x distance from extreme compression fiber toneutral axis;

� multiplier on fcm to determine intensity ofequivalent rectangular stress distribution for
concrete;


�1 ratio of depth of equivalent rectangular stressblock to depth of neutral axis;

u deflection at ultimate condition;y deflection at yield load;

�RL difference in axial strain between strain gaugeat right and left sections;

�au strain corresponding to ultimate strength ofadhesive;

�c strain level in concrete;� f strain level in laminate reinforcement;

� fu strain corresponding to ultimate strength oflaminate reinforcement;

�L strain recorded in strain gauge at left section;�R strain recorded in strain gauge at right

section;�s strain level in internal steel reinforcement;

�sy strain corresponding to yield strength ofinternal steel reinforcement;

� deflection ductility index;�� curvature ductility index;�b bond parameter;

�s,eq longitudinal equivalent reinforcement ratio;�au adhesive tensile strength;�fu laminate tensile strength;�su ultimate tensile strength of internal steel

reinforcement;�sy yield strength of internal steel reinforcement;bm average bond stress �from pullout bending

test�;bm

RL average bond stress inbetween two consecutivestrain gauges;

fm average bond stress of CFRP strengthening;sm average bond stress of internal steel

reinforcement;�s nominal diameter of steel bars;�u curvature at ultimate condition; and�y curvature at the yield load.

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