12
Flexural strengthening of reinforced concrete slab–column connection using CFRP sheets Hazem A. El-Enein a , Hossein Azimi b,, Khaled Sennah b , Faouzi Ghrib a a Civil and Environmental Engineering Department, University of Windsor, Windsor, ON, Canada b Civil Engineering Department, Ryerson University, 350 Victoria, Toronto, ON, Canada highlights 6 specimens were tested, 3 control specimens and 3 CFRP strengthened counterparts. The proposed strengthening scheme was proven to be effective and have sufficient anchorage. Effect of column eccentricity on concentric, eccentric and edge columns was studied. Alteration in ductility due to the application of CFRP sheets and column eccentricity was studied. Code-based predicted capacities were not conservative for CFRP-reinforced eccentric columns. article info Article history: Received 16 April 2013 Received in revised form 19 January 2014 Accepted 24 January 2014 Available online 24 February 2014 Keywords: CFRP sheets Strengthening Reinforced concrete Slab–column connection Flat-plate system Experiments Ultimate load Ductility abstract The objective of this paper is to investigate experimentally the effectiveness of application of carbon fiber reinforced polymer (CFRP) sheets as a strengthening technique of a reinforced concrete (RC) slab–column connection in one-way flat-plate system to enhance its flexural strength. The experimental study was particularly conducted to examine whether there is enough anchorage with the use of CFRP wrapping to the discontinuous longitudinal CFRP sheets at column stub. Series of tests were conducted on six flat slab–column connection specimens organized in two groups of three specimens each. The first group included three control specimens with central, eccentric, and edge columns, respectively. The second group was geometrically identical to the first group, though with CFRP sheets installed on the tension side of the slab to increase flexural capacity at the negative moment region. The specimens with eccentric and edge columns are those having geometrical eccentricity and whose results are compared with those obtained for specimens with central column to study the effect of column eccentricity. The experimental work included fabrication of specimens and testing them under increasing monotonic gravity loads up to failure. Experimental results demonstrated that the flexural ultimate load carrying capacity increased by 33%, 37% and 67% for the tested specimens with central, eccentric, and edge column, respectively, when strengthened using CFRP sheets. The cross-sectional analysis was also undertaken to compare the exper- imental results with those obtained from Canadian Standards for FRP design for buildings. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The connection between reinforced concrete (RC) flat-slab and column is a critical as it is exposed to large flexural stresses. There may be a need to improve the flexural load carrying capacity of these flat slab structures especially when they are exposed to sever environmental conditions that cause corrosion of the steel rein- forcement (e.g. parking garages). There are few traditional tech- niques to restore the strength and stiffness of existing concrete flat-slabs, among them: steel plate bonding, section enlargement, and external post-tensioning. However, most of these techniques are either capital-demanding or are still hampered by steel corro- sion or deterioration of the bond at steel–concrete interface caused by the corrosion of steel [1–3]. Application of fibre reinforced polymer (FRP) composites is an alternative to traditional strengthening methods offering several advantages over traditional strengthening techniques. These advantages include high tensile strength-to-weight ratio, relatively easy installation, and noncorrosive nature. Among the FRP types, carbon fiber reinforced polymers (CFRP) sheets are commonly used in external strengthening of structural members that are known to http://dx.doi.org/10.1016/j.conbuildmat.2014.01.077 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +1 4163994974. E-mail addresses: [email protected], [email protected] (H. Azimi). Construction and Building Materials 57 (2014) 126–137 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Flexural Strengthening of Reinforced Concrete Slab–Column Connection

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Page 1: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

Construction and Building Materials 57 (2014) 126–137

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Flexural strengthening of reinforced concrete slab–column connectionusing CFRP sheets

http://dx.doi.org/10.1016/j.conbuildmat.2014.01.0770950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 4163994974.E-mail addresses: [email protected], [email protected] (H. Azimi).

Hazem A. El-Enein a, Hossein Azimi b,⇑, Khaled Sennah b, Faouzi Ghrib a

a Civil and Environmental Engineering Department, University of Windsor, Windsor, ON, Canadab Civil Engineering Department, Ryerson University, 350 Victoria, Toronto, ON, Canada

h i g h l i g h t s

� 6 specimens were tested, 3 control specimens and 3 CFRP strengthened counterparts.� The proposed strengthening scheme was proven to be effective and have sufficient anchorage.� Effect of column eccentricity on concentric, eccentric and edge columns was studied.� Alteration in ductility due to the application of CFRP sheets and column eccentricity was studied.� Code-based predicted capacities were not conservative for CFRP-reinforced eccentric columns.

a r t i c l e i n f o

Article history:Received 16 April 2013Received in revised form 19 January 2014Accepted 24 January 2014Available online 24 February 2014

Keywords:CFRP sheetsStrengtheningReinforced concreteSlab–column connectionFlat-plate systemExperimentsUltimate loadDuctility

a b s t r a c t

The objective of this paper is to investigate experimentally the effectiveness of application of carbon fiberreinforced polymer (CFRP) sheets as a strengthening technique of a reinforced concrete (RC) slab–columnconnection in one-way flat-plate system to enhance its flexural strength. The experimental study wasparticularly conducted to examine whether there is enough anchorage with the use of CFRP wrappingto the discontinuous longitudinal CFRP sheets at column stub. Series of tests were conducted on six flatslab–column connection specimens organized in two groups of three specimens each. The first groupincluded three control specimens with central, eccentric, and edge columns, respectively. The secondgroup was geometrically identical to the first group, though with CFRP sheets installed on the tensionside of the slab to increase flexural capacity at the negative moment region. The specimens with eccentricand edge columns are those having geometrical eccentricity and whose results are compared with thoseobtained for specimens with central column to study the effect of column eccentricity. The experimentalwork included fabrication of specimens and testing them under increasing monotonic gravity loads up tofailure. Experimental results demonstrated that the flexural ultimate load carrying capacity increased by33%, 37% and 67% for the tested specimens with central, eccentric, and edge column, respectively, whenstrengthened using CFRP sheets. The cross-sectional analysis was also undertaken to compare the exper-imental results with those obtained from Canadian Standards for FRP design for buildings.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The connection between reinforced concrete (RC) flat-slab andcolumn is a critical as it is exposed to large flexural stresses. Theremay be a need to improve the flexural load carrying capacity ofthese flat slab structures especially when they are exposed to severenvironmental conditions that cause corrosion of the steel rein-forcement (e.g. parking garages). There are few traditional tech-niques to restore the strength and stiffness of existing concrete

flat-slabs, among them: steel plate bonding, section enlargement,and external post-tensioning. However, most of these techniquesare either capital-demanding or are still hampered by steel corro-sion or deterioration of the bond at steel–concrete interface causedby the corrosion of steel [1–3].

Application of fibre reinforced polymer (FRP) composites is analternative to traditional strengthening methods offering severaladvantages over traditional strengthening techniques. Theseadvantages include high tensile strength-to-weight ratio, relativelyeasy installation, and noncorrosive nature. Among the FRP types,carbon fiber reinforced polymers (CFRP) sheets are commonly usedin external strengthening of structural members that are known to

Page 2: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137 127

be practically immune to chemical attacks and have high strengthand relatively high modulus. Studies have shown that FRPs in-crease the strength of flexural members significantly. The behav-iour of the beam–column connection strengthened by CFRP wassignificantly investigated by Antonopoulos and Triantafillou [4]and Bousselham [5]. However, few authors dealt with strengthen-ing of slab–column connection in flat-plate system RC structuresusing FRP sheets.

Michaluk et al. [6] studied flexural and shear behaviour of one-way slabs reinforced with FRP reinforcing bars. Successful CFRPsheet application on a troubled post-tensioned concrete slab of aparking garage in Atlanta, USA, was investigated by Vatovec et al.[7], where proof load testing demonstrated successful performanceof the CFRP repair of spans with delaminated Gunite beams.Robertson and Johnson [8] presented the results of repaired RCslab–column connections with epoxy injection subjected to cycliclateral loads and gravity loads. Mosallam and Mosalam [9] studiedthe application of CFRP laminates in 2640 � 2640 � 76.2 mm RCtwo-way slabs, showing significant upgrade in the structuralcapacity in the order of 500% in unreinforced and 200% in rein-forced specimens.

A strengthening technique using CFRP sheets was investigatedby Binici and Bayrak [10] to increase punching shear capacity inRC flat-plates depicting about 50% augmentation in the capacity.Punching shear behaviour of CFRP-strengthened RC two-way slabswas also studied by El-Salakawy et al. [11] at the slab–column con-nection of an edge column having un-balanced moment. The effectof various CFRP strip configurations on the punching shearcapacity of two-way slabs was investigated by Sharaf et al. [12].The effect of externally bonded FRP composites in the stiffnessenhancement of one-way slabs and the corresponding deflectioncontrol capabilities were studied by Ospina [13].

The structural behaviour of a cracked RC one-way slab rehabil-itated with CFRP sheets was studied by Thanoon et al. [14], demon-

(a)

(c)

Fig. 1. (a) Example of the studied system in a parking garage; (b) example of the studiedcolumn location.

strating the enhancement in ultimate capacity while reducing theductility. Polies et al. [15] and Halabi et al. [16] investigated theefficiency of the application of CFRP sheets as rehabilitation andstrengthening technique of a flat-plate slab–column connectionsubjected to monotonic gravity loading and unbalanced moment.Results showed that the rehabilitation technique was able to re-store and enhance the ultimate load capacity and stiffness of allcracked specimens. However, the ductility index reduced afterrehabilitation and the mode of failure changed [15].

Despite the considerable amount of research on the flexuralstrengthening of beams and slabs with CFRP strips, the effect of col-umn eccentricity on the CFRP flexural strengthening requires moreexperimental research. This paper presents the experimental inves-tigation of three types of slab–column connections of one-way RCflat slabs strengthened by CFRP strips. The slab–column connectionis also referred to as the negative moment region of the slab wherethe top of the slab is in tension and the bottom is under compres-sion. One type of these one-way flat slabs [17] is shown inFig. 1(a) and (b). This type of flat slab considers the flat slab actingin one direction normal to the main shallow beams connecting col-umns in the other direction. At columns, the negative moment onthe shallow beams is much more than that in the slab short direc-tion, thus one-way action is dominant. Depending on the locationof columns, three slab–column types are shown in Fig. 1(c).Type 1 represents the internal columns, which are usually centeredin the shallow beam. The other two types illustrate side columnsdue to a column with eccentricity (Type 2) or an edge column(Type 3). Types 1 and 3 are common types that are always partsof the slab system shown in Fig. 1(a). Type 2 may be used insteadof Type 3 in some applications where the slab is extended few mil-limeters outside the column edge to accommodate continuousexterior wall or cladding. For instance, if the perimeter masonryor stud wall insulation needs to be continuous along buildingperimeter, the column should be cast close to the slab edge with

(b)

system in a corner column [17]; (c) various types of the system depending on the

Page 3: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

128 H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137

an eccentricity equal to or greater than the wall thickness. Theinfluence of CFRP flexural strengthening for such cases is studiedin this paper.

2. Proposed flexural strengthening scheme

The proposed strengthening scheme is shown in Fig. 2 for aninterior column of a flat slab system or a system with shallowbeams. Fig. 2 shows that the CFRP sheets are attached to the topsurface of the concrete at slab–column location (i.e. negative mo-ment region) where the top surface of concrete is in tension andbottom surface is under compression. Since this system will betested in one-way slab or beam system, Fig. 2 shows the strength-ening scheme in one direction only. However, this proposedscheme can be applied to the perpendicular direction too in atwo-way slab system.

The CFRP strengthening scheme consists of four longitudinalCFRP sheets, namely: A, B, C and D as shown in Fig. 2. Sheets Dand C are continuous over the negative moment region and alongthe sides of the column, while sheets A and B are discontinuousat column location. In order to provide sufficient anchorage forsheets A and B at the column location, they are bent upward andglued to the column width over a 200-mm length. Then, the bentportions of these sheets are anchored using CFRP wrapping sheet,noted as sheet E in Fig. 2, with a width of 200 mm. The capacityof the proposed anchorage details for the discontinuous sheets atthe column location is as yet unavailable in the literature andcan only be evaluated experimentally. The lengths of sheetsA to D depend on the length of negative moment region and thepotential for premature peeling off the CFRP sheets from concretesurface at their ends. More details of the proposed flexuralstrengthening scheme together with associated dimensions willbe mentioned later in this paper.

3. Experimental program

3.1. Test specimens

In this paper, the three types of slab–column connections were studied corre-sponding to three column locations of centric, eccentric, and edge as shown inFig. 1(c). The slab dimensions were kept 2.0 � 1.0 m with the aspect ratio of 2 whichis usually taken as the limit for one-way slab behaviour. Strengthening schemes

(a) Proposed strengthening scheme

Fig. 2. General view of the prop

Table 1Specimen definition and concrete characteristics.

Group Specimen no. Column location Strengthening using CFRP

S-l Central NoI S-2 Eccentric No

S-3 Edge No

S-4 Central YesII S-5 Eccentric Yes

S-6 Edge Yes

with CFRP strips were applied on the tension side (top of the slab at the negativemoment region), then the specimens were inverted and tested under gravity loadto apply bending moments at the slab–column location, where the tension sidewas at the bottom of the specimens. This was done to simplify the testing procedureby applying a downward jacking load on top of the column stub while the corners ofthe slab rest over point supports.

The experimental program included testing up-to-failure six specimens in twogroups as shown in Table 1. The first group consisted of three specimens with con-crete slab–column connection system without CFRP strengthening (control speci-mens), while the second group was identical to the first group but withstrengthening using externally-bonded CFRP sheets. Each tested specimen wascomposed of 2000 � 1000 � 150 mm slab, with 200 � 200 mm column stubextending 750 mm above the slab and 400 mm below the slab. The dimension ofthe slabs and the column stub were kept unchanged for all specimens, howeverthe location of the column stub was variable along the short direction of the spec-imens as shown in Fig. 3. These column locations were identified herein as centralcolumn for specimen S-1, eccentric column for specimen S-2 and edge column forspecimen S-3. Specimens S-4, S-5, and S-6 were geometrically identical to speci-mens S-1, S-2 and S-3, respectively, except that the former specimens werestrengthened in the tension side of the slab and around the column stub using CFRPsheets. Fig. 3(a)–(c) shows the dimensions of test specimens in plan-view. Cross-sectional view showing the slab thickness and steel reinforcement is depicted inFig. 3(d).

Steel reinforcement in the slab consisted of two meshes of M10 (i.e. 11.3 mmdiameter and 100 mm2 cross-sectional area) steel bars near the top and bottom sur-faces of the slabs at equal spacing of 100 mm and clear concrete cover of 25 mm.Four 10 M steel bars, one at each corner of the cross-section, were used to reinforcethe column stub. Fig. 3(d) shows details of steel reinforcement with a view of thesteel reinforcement and wooden formwork for specimen S-1 shown in Fig. 3(e).

After concrete hardening, specimens S-4, S-5 and S-6 were strengthened usingCFRP sheets. The CFRP sheets were applied as recommended by the manufacturerand were installed in the tension side of the slab per the sequence shown inFig. 4(a)–(c). It can be observed that CFRP strips that intersect with the column werebent 90� and glued to the column side over a length of 150 mm. Then, those bentstrips were wrapped with a 200 mm width CFRP strip for better anchorage. Itshould be noted that the end of the CFRP sheets in the slab short direction in spec-imens S-5 and S-6 were bent vertically and glued to the vertical side of the slab, asshown in Fig. 4(b) and (c) to provide better anchorage at those locations. Fig. 4(d)views an example of strengthened specimen S-5.

3.2. Material properties

High-early strength concrete, with a specified compressive strength of 35 MPaafter seven days was used. The concrete was ordered in two batches. The first batchwas used in casting specimens S-1, S-2 and S-4, while the second batch was used tocast specimens S-3, S-5 and S-6. The results of testing concrete cylinders in the formof compressive and splitting strengths at the time of testing are shown in Table 1.Steel reinforcing bars, which were used for reinforcing the six slabs and all their col-umn stubs, were 10 M bars with specified yield strength of 400 MPa and modulus ofelasticity of 200 GPa.

(b) Detailing of strengthening scheme

osed strengthening scheme.

Concrete compressive strength (MPa) Concrete splitting strength (MPa)

40.1 3.940.1 3.939.2 3.7

40.1 3.939.2 3.739.2 3.7

Page 4: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

(a) (b) (c)

(d)

(e)

- All dimensions are in mm

Fig. 3. (a), (b), and (c) Geometry of specimens in plan view; (d) steel reinforcement used in all specimens; (e) view of the steel mesh and formwork for specimen S-1.

(a) (b)

(c) (d)

- All dimensions are in mm

Fig. 4. Details of CFRP strengthening (a) specimen S-4, (b) S-5, (c) and S-6; (d) view of specimen S-5 after CFRP application.

H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137 129

The CFRP reinforcement material was Tyfo SCH-41S Composite Fibre System[18] and was applied longitudinally and transversely on the tension side of the slab.This system consisted of Tyfo S Epoxy and Tyfo SVH-41S reinforcing fabrics. The

Tyfo SCH-41S is a unidirectional carbon fabric with aramid cross fibres. It has beenstitched, with the carbon material oriented in 0� direction, and aramid fibres at 90�.The CFRP system properties are summarized in Table 2.

Page 5: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

130 H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137

3.3. Steel and CFRP reinforcement ratios

As stated before, 10 M steel bars were used in all directions as tension and com-pression steel in all specimens. The steel reinforcement used corresponds to 1.02%steel reinforcement ratio (qs) which falls in the range of 0.5–1.2% commonly usedfor flat slabs [19]. This reinforcement ratio was chosen so that the flexural failureof test specimens starts with yielding of tension steel followed by concrete crushingat the compression side of the slab. In order to understand the expected behaviourof the test specimens, Fig. 5 was drawn. Assuming complete one-way slab action oftest specimens, three distinct areas are recognized in Fig. 5 depending on the steeland CFRP reinforcement ratios defined as qs = As/bd and qfrp = Afrp/bd, respectively[20], where b = 1000 mm is the width of the section, d = 108.05 mm is the depthof tension steel reinforcement, As is the tension steel area and Afrp is the area of CFRPsheets. Area (1) in Fig. 5 is due to compression failure in concrete before steel yield-ing or rupture in CFRP, also referred as over-reinforced section. Area (2) correspondsto yielding in steel with no rupture in CFRP which is the preferred behaviour.Finally, rupture in CFRP occurs in Area (3) with yielding in steel bars, leading to asudden drop in the slab flexural capacity. These three areas are divided by twostraight lines as depicted in Fig. 5, whose function is given by:

½/sðFy þ e0sEsÞ�qs þ ½/frpefrpEfrp�qfrp ¼ /ca1b1f 0cb2dc ð1Þ

where /s, /frp, and /c are resistance factors for steel bars, CFRP sheets, and concretewith values equal to 0.85, 0.65, and 0.65, respectively, as specified by CSA S806-12[21]. Other parameters in Fig. 5 are: steel yield strength Fy = 400 MPa; steel tensilemodulus Es = 200 GPa; CFRP tensile modulus Efrp = 65.1 GPa; concrete compressivestrength f 0c as reported in Table 1; equivalent concrete stress block factorsa1 = 0.85–0.0015f 0c and b1 = 0.97–0.0025 f 0c ; and c is the depth of the neutral axis.In Eq. (1), it is assumed that the areas of tension and compression steel reinforce-ment are equal (As = A0s) which is the case of the current study.

For the line of the balanced section (i.e. es = ey when concrete crushes, the linebetween Areas (1) and (2) in Fig. 5), the followings should be substituted in Eq. (1):

c ¼ ecu

ecu þ eyd; e0s ¼

d0 � cc

ecu 6 ey; efrp ¼h� c

cecu ð2Þ

where concrete ultimate strain ecu = 0.0035; steel yield strain ey = 0.002; depth ofcompression reinforcement d0 = 41.95 mm, and h is the slab thickness. Similarly,for the line representing balance condition of the CFRP rupture (i.e. line betweenAreas (2) and (3) in Fig. 4), the followings should be substituted in Eq. (1):

c ¼ ecu

ecu þ efrpuh; e0s ¼

d0 � cc

ecu 6 ey; efrp ¼ efrpu ð3Þ

Table 2Properties of CFRP composite laminate.

Description: Primary carbon fibre, 0�; aramid fibre 90�Ultimate tensile strain 1.21%Ultimate tensile strength 786 MPaTensile modulus 65.1 GPaLaminate thickness 1 mm

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8 9 10 11 12

CFR

P re

info

rcem

ent r

atio

( Afr

p/bd)

, %

Steel reinforcement ratio (ρs=As/bd), %

S-1, S-2 and S-3S-4S-5S-6

ρs, min

Balanced section:εs=εy

εfrp=εfrpu

Area (1): Compression failure: εs<εy

Area (2):Concrete crushing-Steel yielding: εs>εy; εfrp<εfrpu

Area (3): FRP rupture: εfrp>εfrpu

f 'c=39.2 MPaf 'c=40.1 MPa

Fig. 5. Flexural failure modes of CFRP strengthened one-way slab.

where efrpu is the CFRP ultimate strain equals to 1.21% as shown in Table 2. The statescorresponding to the steel and CFRP reinforcement ratios used for the test specimensare shown in Fig. 5. The associated point to control specimens S-1 to S-3 withqs = 1.02% is located on the horizontal axis showing the preferred under-reinforcedbehaviour. The CFRP reinforcement ratios qfrp were 0.53%, 0.43%, and 0.33% appliedfor specimens S-4, S-5, and S-6, respectively, such that all the three CFRP-strength-ened specimens be located on the preferred Area (2) which is outside of the CFRPrupture Area (3) as shown in Fig. 5.

3.4. Test setup, instrumentation, and test procedure

As stated earlier, CFRP sheets were bonded to the tension side of the slab at col-umn location which is the top surface of the slab in a building. However, the spec-imens were inverted in the test setup to simplify the testing procedure as shown inFig. 6. In this case, the specimens rest over 4 supports at the slab corners, whilejacking load was applied on top of the column and a 450-kN load cell was usedto record the applied load. Fig. 6 shows a schematic diagram of the test setup.The instrumentations used for these tests include steel strain gauges located onthe bottom layer steel reinforcement (tension steel), concrete strain gauges to mea-sure concrete strains on top concrete fiber in the compression side of the slab, andlinear variable differential transformers (LVDTs) to measure slab deflection. Steeland concrete strain gauges were located in long and short directions to study thebehaviour of the slab in both directions to evaluate the significance of one-way ac-tion of the slab. Eleven steel strain gauges were installed in the long direction ofeach test specimen to record the transverse strain distribution. Fig. 7 shows loca-tions of these sensors in the three slab–column types considered in this study.

The following steps were considered in each test: (i) the specimen was accu-rately placed over the four corner supports; (ii) LVDTs were placed in their markedpositions to measure deflections; (iii) steel strain gauges, concrete strain gauges,LVDTs, and load cell were connected to the data acquisition system; and (iv) thejacking load was applied monotonically in increments and crack initiation and crackpropagation were marked. The specimen was considered failed when it could notabsorb more jacking load. Fig. 8(a)–(f) shows views of specimens during testingor after failure.

4. Experimental results and discussion

4.1. General behaviour and the effect of column eccentricity

4.1.1. Control specimensResults showed that all of the unstrengthened control speci-

mens (S-1, S-2, and S-3) failed in flexure by concrete crushing atthe top surface after yielding of internal steel reinforcements.However, observations for the first crack and the ultimate loadwere different. At a jacking load of 125, 90, and 67 kN, the first vis-ible tension crack was observed on specimens S-1, S-2, and S-3,respectively. This crack appeared along the short direction of thetension side (bottom surface of the specimen) at column location.With increase in the applied load, more tension cracks were ob-served away from the column location. These bottom cracks pene-trated through the slab thickness towards the compression side(top surface of the specimen) with increase in the applied load.Crack propagations on the vertical side of the free edge of the slabwere also observed. With increase in the applied load, slab deflec-tion and strains were observed increasing and concrete cracks onthe vertical side of the slabs continued to widen and penetrate intothe slab thickness till concrete crushed at the top surface of thespecimens S-1, S-2, and S-3 at column location at jacking loads of164, 151, and 113 kN, respectively.

In order to compare the results of specimens, the normalizedultimate load should be used to account for the effect of differentcompressive strength utilized in these specimens as shown in Ta-ble 1. Since the failure in specimens was all due to flexure, the nor-malized ultimate load is obtained by [22,23]:

Pnorm ¼Pu

a1b1f 0cð4Þ

where Pu is the ultimate experimental jacking load. With the com-parison of the values calculated for Pnorm as tabulated in Table 3, thenormalized ultimate loads of specimens S-2 with column eccentric-ity and S-3 with edge column are 9% and 31%, respectively, less than

Page 6: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

Fig. 6. Schematic diagram of the test setup.

H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137 131

that for specimen S-1 with central column. A significant reductionin the cracking and ultimate load was observed as a result of chang-ing the column location to be closer to the edge of the slab. A non-uniform flexural stress distribution across slab cross-section wasobserved for all specimens as the column shift away from the col-umn, as expected. To illustrate this finding, the flexural stress distri-

CL: Concrete strain gauge in longitudinal direction; CSSL: Steel strain gauge in longitudinal direction; SS

- All dimensions are in mm

(b)

(a)

(c)

Fig. 7. Location of internal and external instruments: (a) steel strain gauges, (b)

bution was obtained using elastic linear finite-element analysisusing SAP2000 software [24] incorporating shell elements to modelthe slabs of the tested specimens. Such stress distribution is shownin Fig. 9 in the form of normalized flexural stress. It should be notedthat the slab flexural stresses were first obtained from SAP2000software at column location and then divided by the maximumflexural stress (smax) to obtain a normalized stress distribution tofacilitate the comparison between results. Fig. 9 clearly shows theeffect of column eccentricity on the flexural stress distributionwhich confirms the experimental findings of the difference betweenjacking loads at the initiation of visible cracks of 125, 90, and 67kNobtained for specimens S-1, S-2, and S-3, respectively. Also, resultsshow that steel bars at column location would yield first beforethose away from column location. Moreover, concrete crushingwould start first at column location and extend away from the col-umn to the sides of the slab.

4.1.2. CFRP-strengthened specimensThe strengthened specimens S-4, S-5 and S-6 showed similar

mode of failure (i.e. flexure failure) as that for control specimensbut with a delay in the appearance of visible cracks as a result ofthe presence of CFRP sheets. Due to the presence of CFRP sheets,it was difficult to observe the crack initiation in the CFRP-coveredareas. However, the first visible cracks at the free side of the slabswere noticed at 154, 125, and 105 kN for specimens S-4, S-5 andS-6, respectively. While, the ultimate jacking loads were recorded

: Concrete strain gauge in short direction: Steel strain gauge in short direction ;LV: LVDT

concrete strain gauges, (c) location of LVDTs to measure vertical deflection.

Page 7: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

(c) Specimen S-3 during test

(b) Specimen S-2 during test

(a) Specimen S-1 after failure (d) Specimen S-4 after failure

(e) Specimen S-5 after failure

(f) Specimen S-6 after failure

Fig. 8. Views of all specimens during test and after failure.

132 H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137

as 221, 208, and 189 kN for specimens S-4, S-5 and S-6, respec-tively. These loads are summarized in Table 3.

Similar to control specimens, the ultimate jacking load of theCFRP-strengthened specimens decreased as the column eccentric-ity increased. However, the rate of change in ultimate load carryingcapacity was observed to be less in slabs strengthened with CFRPsheets. Since different concrete strengths were used in CFRP-

Table 3Summary of theoretical and experimental results.

Specimen Pcrackinga (kN) Py

b (kN) Puc (kN)

Exp.d Theory Exp. CSA (2012)

S-l 24 2 118.8 125.5 139.6S-2 23.3 118.8 112 139.6S-3 20.1 118.6 86 138.8

S-4 33.4 177.3 180 251.3S-5 29.4 166.1 171 229.4S-6 23.7 155.0 138 208.5

a Cracking load.b Steel yielding load.c Ultimate load.d Value obtained from experiment.e Normalized ultimate load = Pu/(a1 b1 f 0c).f Deflection at steel yielding load.g Deflection at ultimate load.

strengthened specimens, the normalized ultimate loads calculatedby Eq. (4) should be used when comparing the specimens. Com-pared to specimen S-4 with central column, the reduction in Pnorm

as shown in Table 3 was about 4% and 13% for specimens S-5 witheccentric column and S-6 with edge column, respectively. None ofthe specimens experienced CFRP rupture except specimen S-5. Itwas observed in specimen S-5 that at the ultimate stage of loading

Penorm Py/Pu Deflection (mm) Ductility

Exp. Exp. Exp. (%) Dyf Du

g l

165.6 6.01 76 14.4 28.2 2.0151.1 5.49 74 13.3 35.2 2.6112.7 4.17 76 17.4 52.0 3.0

220.8 8.02 82 14.8 23.2 1.6207.7 7.68 82 15.4 23.3 1.5188.6 6.97 73 15.7 29.7 1.9

Page 8: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137 133

prior to concrete crushing, debonding followed by the CFRP rup-ture occurred at the longitudinal CFRP strip with 150 mm widthon the short side of the column denoted as sheet C in Fig. 4(b). Aview of CFRP rupture at this location is shown in Fig. 10. Thismay be attributed to the fact that this location exhibited the high-est flexural stresses across the slab cross-section, leading peelingoff the CFRP sheet followed by concrete crushing at the compres-sion side of the slab.

Fig. 10. Rupture in CFRP at failure of specimen S-5.

4.2. Load–deflection behaviour and ductility

Deflections were measured by LVDTs at certain locations alongthe slab as shown in Fig. 7(c). Fig. 11 presents the load–deflectionrelationships for all tested specimens, at a point in the slab alongthe slab longitudinal axis passing through the column centroidand 400 mm away from it, namely LV1 and LV4, respectively. Itcan be generally observed that at a certain load level, slab deflec-tion decreased as a result of the presence of the CFRP sheets. Thismay be attributed to the contribution of the CFRP sheets in increas-ing slab flexural stiffness.

The formation of the initial cracks in the slab usually occurs in aload level (Pcracking) much less than the ultimate load due to the lowsplitting strength of the concrete compared to the concrete com-pressive strength. While the first flexural crack was observed visu-ally during testing, it can be obtained from the load–deflectioncurves where there is a sudden change in the slope of the initialportion of the curve as a result of reduction in slab flexural stiffnessfor cracked section. The cracking loads were then obtained fromthe load–deflection curves and tabulated in Table 3. It can be ob-served that these cracking loads are much less than those obtainedfrom visual inspection during testing. In addition to the crackingload, the steel-yielding load (Py) can also be obtained from load–deflection curves where the slope of the curves starts to changeonce again in a load level much greater than the cracking load.The yielding load, Py, was then obtained and summarized in Table 3.The corresponding deflection is denoted by Dy in Table 3 as ob-tained for LV1. In addition, the ultimate load and correspondingdeflection denoted by Pu and Du, respectively, are reported inTable 3.

One clear observation in Fig. 11 is the pre-yielding stiffness ofspecimens which is the slope of the load–deflection curve afterthe cracking load (Pcracking) and before the yielding load (Py). Forboth CFRP-strengthened and control specimens, the pre-yieldingstiffnesses of specimens with central column (i.e. S-1 and S-4)are almost identical to those with eccentric column (i.e. S-2 andS-5). However, the specimens with edge column demonstrated

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000

Nor

mal

ized

stre

ss (s

/sm

ax,c)

Location across slab (mm)

Central column (Specimen S-1)Eccentric column (Specimen S-2)Edge column (Specimen S-3)

Fig. 9. Elastic transverse stress distribution at face of column.

lower pre-yielding stiffness, which is much more significant inthe case of control specimens (i.e. specimen S-3 compared to S-1and S-2, and also specimen S-6 compared to S-4 and S-5). Thismay be attributed to the non-uniform flexural stress distributionin the transverse direction as mentioned earlier. At any given loadbeing Pcracking or Py, considering the non-uniform stress distribu-tion, those sections close to the column fully contributed in carry-ing load while the farther sections from the column did partiallycontribute. Consequently, Dy obtained for specimen S-3 was17.4 mm which was much more than 14.4 and 13.3 mm obtainedfor specimens S-1 and S-2, respectively, since the pre-yielding stiff-ness of specimen S-3 is much smaller than those for S-1 and S-2.However, this finding was less significant in CFRP-strengthenedspecimens since the presence of CFRP sheets would alleviate theeffect of non-uniformity of flexural stresses after concrete crackingas mentioned earlier. For specimen S-6, Dy was obtained as15.7 mm compared to 14.8 and 15.4 mm obtained for specimensS-4 and S-5.

Table 3 also includes the ductility for each specimen, which isdefined as ultimate deflection divided by the yield deflection(l = Du/Dy) [25]. As it can be observed in Table 3, ductility in-creased with the increase in column eccentricity in control speci-mens. Ductility was obtained as 2.0, 2.6, and 3.0 for specimensS-1, S-2, and S-3, respectively. When specimens started to yield,those sections close to column stub yielded first and the rest ofthe slab cross-section gradually experienced yielding due to non-uniformity in the transverse stress distribution. Therefore, speci-mens S-2 and S-3 showed more softening behaviour after yieldingdeflection (Dy) as shown in Fig. 11 which is the main reason formore ductility observed in those specimens compared to S-1. How-ever, the effect of column eccentricity on the observed ductilitywas less significant in CFRP-strengthened specimens. The ductilityobtained for specimen S-6 was 1.9 compared to 1.6 and 1.5 for S-4and S-5, respectively. This may be attributed to the effect of CFRPsheets in limiting the tendency of the section to undergo relativelywide cracks after steel yielding which eventually reduces the ulti-mate deflection, Du.

4.3. Effect of CFRP strengthening

Application of CFRP strips to strengthen the control specimenssignificantly increased the ultimate load capacity, Pu, and yieldingloads, Py. Summary of CFRP effect on each type of the slabs is de-picted in Table 4. The cracking load, Pcracking, at which the concretestarted to crack at the tension side was mainly affected by the sec-tion modulus defined as S = I/y where I is the transformed momentof inertia and y is the distance of the extreme tensile fibre of con-crete from neutral axis. The application of CFRP sheets would in-crease I and decrease y, that subsequently increase the sectionmodulus. The amount of CFRP sheets used in specimen S-4 was

Page 9: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

0

50

100

150

200

250

0 10 20 30 40 50 60

App

lied

load

(kN

)

Deflection (mm)

S-1S-2S-3

0

50

100

150

200

250

0 10 20 30 40 50 60

App

lied

load

(kN

)

Deflection (mm)

S-4S-5S-6

0

50

100

150

200

250

0 10 20 30 40 50 60

App

lied

load

(kN

)

Deflection (mm)

S-1S-2S-3

0

50

100

150

200

250

0 10 20 30 40 50 60

App

lied

load

(kN

)

Deflection (mm)

S-4S-5S-6

(d) With CFRP, LVDT No. 4 (LV 4)(c) No CFRP, LVDT No. 4 (LV 4)

(a) No CFRP, LVDT No. 1 (LV 1) (b) With CFRP, LVDT No. 1 (LV 1)

Fig. 11. Load–deflection relationships for all specimens (note: LVDT locations are shown in Fig. 7c).

134 H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137

more than that used in S-5, and the amount of CFRP sheets in S-5was more than that in S-6. Therefore, the increase in section mod-ulus was highest in specimen S-4 and was lowest in specimen S-6.This trend was observed in experimental results as the crackingload was affected by 38%, 26%, and 18% in specimens with central,eccentric, and edge column, respectively, with the application ofCFRP sheets as shown in Table 4.

After concrete cracking, CFRP sheets further engaged the sec-tions farther from the column stub by relaxing the non-uniformityof flexural stress distribution along slab cross-section. This aug-mented the contribution of CFRP strengthening in increasing yield-ing and ultimate loads when the column stub shifted from centerto the edge of specimen. As shown in Table 4, the yielding loadof specimens with central, eccentric and edge column increasedby 43%, 53% and 60% with the application of CFRP sheet strength-ening, respectively, while the ultimate load carrying capacity in-creased by 33%, 40%, and 67%, respectively. The increase in slabstrength and stiffness with the application of CFRP sheets was at-tained at the expense of reduction in ductility as shown in Table 4.This finding is in agreement with the findings of other researchersas the application of CFRP strengthening decreases ductility[15,16]. Although the order of strength gained is much more thanthe loss of ductility, the designer should consider loss of ductilitywhen selecting CFRP reinforcement ratio depending on the desiredapplication.

4.4. Strain measurements

Steel and concrete strains were measured by strain gauges in-stalled in the long and short directions as shown in Figs. 7(a) and(b). Steel strain gauges were installed only on the reinforcement lo-cated on the tension side of the slab, and concrete strain gaugeswere attached to the concrete compression side. Selective steel

and strain results are depicted in Fig. 12 representing the areasof high stress concentration. Fig. 12(a)–(c) shows a comparison be-tween jacking load–strain curves on the long direction for speci-mens with central, eccentric, and edge column, respectively.Fig. 12(d) provides a sample of strain readings in the short direc-tion of specimens with central column. It can generally be ob-served that at a given load, steel and concrete strains decreasedas a result of the presence of the CFRP sheets. This may be attrib-uted to the contribution of the CFRP sheets in increasing slabstrength and stiffness and relieving some tensile stresses in thesteel bars, as compared to those for the control specimens.

In addition, Fig. 12 shows that reinforcing steel bars yielded inthe long direction, irrespective of the presence of CFRP strengthen-ing, demonstrating an under-reinforced behaviour for which spec-imens were originally designed for. However on the shortdirection, there was no yielding observed in steel strains. Forexample, the maximum strain was recorded as bout 1100 micro-strain in Fig. 12(d) which is significantly smaller than the conven-tional value of 2000 microstrain used as yield strain of reinforcingsteel. In addition, comparison between Fig. 12(d) and (a) for steelstrains in the short and long directions, respectively, in the speci-mens with central column shows that the specimen behaviourwas one-way, as expected. Although the strain in the short direc-tion is given only for slabs with central column, similar findingswere observed for other specimens.

The transversal distribution of longitudinal steel strain is shownin Fig. 13(a)–(c). The horizontal axis represents the strain gaugenumbers shown in Fig. 7(a). Although strain readings are usuallyaffected by crack propagation and crack interception, the generaltrend could be understood from those figures which could be com-pared with those shown in Fig. 9 as obtained from finite-elementmodeling. Steel strain distributions are shown at 20%, 40%, 60%,and 80% of the ultimate load, Pu. As stated earlier, the steel strains

Page 10: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

Table 4Effect of CFRP strengthening.

Column location Strength effect DPcracking (%) DPy (%) DPu (%) Dl (%)

Central S-4/S-1 38 43 33 �20Eccentric S-5/S-2 26 53 40 �43Edge S-6/S-3 18 60 67 �37

0

50

100

150

200

250

-5000 0 5000 10000 15000

App

lied

load

(kN

)

Concrete and steel microstrain

No CFRP With CFRP

Concrete strain

Steel strain

0

50

100

150

200

250

-5000 0 5000 10000 15000

App

lied

load

(kN

)

Concrete and steel microstrain

No CFRP With CFRP

Concrete strain

Steel strain

0

50

100

150

200

250

-5000 0 5000 10000 15000

App

lied

load

(kN

)

Concrete and steel microstrain

No CFRP With CFRP

Concrete strain

Steel strain

0

50

100

150

200

250

-5000 0 5000 10000 15000

App

lied

load

(kN

)

Concrete and steel microstrain

No CFRP With CFRP

Concrete strain

Steel strain

(a) (b)

(c) (d)

Fig. 12. Concrete and steel strain distributions in the longitudinal direction of various column locations: (a) central, (b) eccentric, (c) edge; and (d) central in the shortdirection.

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5 6 7 8 9 10 11 12

Teni

sle

mic

rost

rain

Strain gauge number

20% Pu

40% Pu

60% Pu

80% PuNo CFRPWith CFRP

Column stub locatin

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5 6 7 8 9 10 11 12

Teni

sle

mic

rost

rain

Strain gauge number

20% Pu

40% Pu

60% Pu

80% Pu

No CFRPWith CFRP

Column stub locatin

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5 6 7 8 9 10 11 12

Teni

sle

mic

rost

rain

Strain gauge number

20% Pu

40% Pu

60% Pu

80% PuNo CFRPWith CFRP

Column stub locatin

(b)(a)

(c)

Fig. 13. Transverse distribution of longitudinal strain in steel bars on the tension side of the slab at column face of specimens with: (a) central column, (b) eccentric column,and (c) edge column (note: strain gauges locations are shown in Fig. 7a).

H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137 135

Page 11: Flexural Strengthening of Reinforced Concrete Slab–Column Connection

136 H.A. El-Enein et al. / Construction and Building Materials 57 (2014) 126–137

in the CFRP-strengthened members are generally less than thosefor control specimens which is the general trend depicted inFigs. 13(a)–(c) with few exceptions. One exception is the readingof strain gauges number 1–3 shown in Fig. 13(c), showing thatthe presence of CFRP sheets engaged more those regions that arefar from the location of applied load. This was previously men-tioned to be one of the reasons of significant increase of 67% inthe ultimate load of specimen S-6 with CFRP strengthening com-pared to that obtained for specimen S-3 without strengthening,as shown in Table 4.

5. Analysis of results

Since the failure occurred in all cases due to flexural failurerather than punching shear failure, the flexural resistance is onlycalculated herein. The conventional cross-sectional analysis isundertaken assuming full one-way slab action to calculate the ulti-mate moment capacity (Mu) of the slab at a section on the face ofthe column. Then, from the statics of the test setup, Pu was ob-tained as 2Mu/L where L was the clear distance of the supportsfrom the column in the long direction equal to 750 mm, as shownin Fig. 6.

Following the conventional approach specified by CSA S806-12 [20] for the one-way flexural capacity of the steel-reinforcedspecimens, the ultimate theoretical Pu was calculated and tabu-lated in Table 3. The small difference between Pu calculated forspecimen S-3 with the other two specimens S-1 and S-2 is dueto the slightly different concrete compressive strength of spec-imen S-3. For specimen S-1 with the centric column, the calcu-lated ultimate load was 139.6 kN compared to 165.6 kNobtained from experiment, with the experimental value 19%greater than the predicted value for specimen S-1. However,the experimental ultimate loads were 8% larger and 19% smal-ler than the theoretical ultimate loads for specimens S-2 and S-3, respectively. This percentage difference from one specimento another is mainly due to the uneven transverse distributionof flexural stresses at column location in the tested specimensthat is augmented with increase in column eccentricity. Itshould be noted that the theoretical Pu was calculated usingCSA S806-12 provisions considering uniform flexural stress dis-tribution in the transverse direction at column location whichcontradicts with the uneven stress distribution shown in Figs. 9and 13.

For CFRP-strengthened specimens, the one-way flexural resis-tance is calculated in accordance with CSA S806-12 [21] consider-ing the presence of CFRP sheets. As specified by CSA S806-12 [21],the maximum tensile strain in the CFRP sheets is taken as:

efrp;max ¼ 0:41

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffif 0c

nfrpEfrptfrp

s6 0:007 ð5Þ

where nfrp and frp are number and thickness of CFRP sheets. Inaccordance with the material properties mentioned earlier, efrp,max

is obtained as 0.007 for all CFRP-strengthened specimens, whichis 42% smaller than the ultimate tensile strain of CFRP sheets shownin Table 2. The calculated ultimate loads shown in Table 3 for spec-imens S-4, S-5, and S-6 are 251.3, 229.4, and 208.5 kN, respectively,which are 14%, 10%, and 10% larger than those obtained experimen-tally, respectively. It can be concluded that (i) the conventionalcross-sectional analysis overestimates the real flexural capacity ofCFRP-strengthened one-way slabs, (ii) when CFRP sheets are ap-plied, the effect of column eccentricity on the cross-sectional anal-ysis decreases significantly compared to steel-reinforcedspecimens. In addition to the ultimate load (Pu), the yielding load(Py) was calculated and summarized in Table 3. In the calculationof Py, the conventional approach was undertaken with linear stress

distribution in concrete under compression with no concrete tensileresistance. The maximum tensile strain in all CFRP-strengthenedspecimen at Py was calculated as 3.2 microstrain which is far lessthan efrp,max = 0.007.

The experimental and theoretical Py for centric (i.e. S-1 and S-4) and eccentric specimens (i.e. S-2 and S-5) are quite similarwhile they are different in case of specimens with edge column.In control specimen S-1, the theoretical Py is 6% smaller thanthat obtained from experiment, while it is 6% and 28% largerfor S-2 and S-3, respectively. In case of CFRP-strengthened spec-imens S-4 and S-5, this ratio is only 2% and 3% smaller thanthose obtained experimentally, while it is 11% greater for speci-men S-6. Hence, with the application of CFRP strengthening, sim-ilar to the trend of Pu, the experimental Py could be betterestimated by theoretical calculations particularly for specimenswith edge column where there is significant transverse stressnon-uniformity. This may be due to the effect of CFRP sheetswhich alleviates the non-uniform transverse stress distribution,leading to better cross-section engagement to increase slab flex-ural capacity.

6. Conclusions

The feasibility and effectiveness of using CFRP sheets for flex-ural strengthening of RC slab–column connections was investi-gated for flat slabs with drop beams in one direction. Thefollowing conclusions may be drawn from the experimentalstudy:

1. The proposed strengthening technique using CFRP sheets astension reinforcement applied on the region of slab–columnconnection was proven to be effective in increasing the flexuralstrength and stiffness of one-way slab–column connections.The proposed CFRP wrapping around column to the bent por-tions of the discontinuous CFRP sheets along the column widthis proved as effective anchorage details to maintain full flexuralcapacity of the strengthened slab.

2. The presence of CFRP sheets significantly reduces slab deflec-tion and increases both the yielding load and the ultimate loadcarrying capacity of the slab–column connection. The experi-mental ultimate load increased by 33%, 40%, and 67% for speci-mens with central, eccentric and edge columns, respectively,after strengthening. However, there was a reduction in speci-men ductility of about 20% for the specimen with central col-umn compared to about 40% in specimens with eccentric oredge column.

3. Experimental results demonstrated that column eccentricityconsiderably reduced the ultimate load and increased ductility.This is due to the uneven transverse distribution of flexuralstresses at column location. However, the presence of CFRPsheets alleviated the effect of column eccentricity.

4. The conventional cross-sectional analysis specified by CSAS806-12 overestimated the experimental flexural capacity ofCFRP-strengthened one-way slabs, leading to un-conservativedesign practices. The column eccentricity increased the differ-ence between the predicted values and experimental results.Based on experimental findings, a reduction factor may beapplied to the calculated flexural capacity using CSA S806-12to match experimental findings.

Acknowledgements

The in-kind contribution of Fyfe Co., LLC The Fibrwrap Com-pany, San Diego, CA, USA., and Watson Inc. of New York, USA., isgreatly appreciated.

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