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Composites: Part B 38 (2007) 712–719

Punching shear behavior of concrete flat plate slab reinforcedwith carbon fiber reinforced polymer rods

Ran Li a, Young Sang Cho b,*, Sumei Zhang a

a Harbin Institute of Technology, Department of Civil Engineering, Harbin 150001, PR Chinab Hanyang University, Department of Architectural Engineering, Kyunggi-Do 425-791, Republic of Korea

Received 10 May 2006; accepted 28 July 2006Available online 27 December 2006

Abstract

Flat plate slab system is widely adopted by engineers as it provides many advantages . The system can reduce the height of the build-ing, provide more flexible spatial planning due to no beams present, and further reduce the material cost. However, the main problem inpractice is the brittle failure of flat plate slab under punching shear. In this paper, the punching shear behavior has been studied and anexperimental work using carbon fiber reinforced polymer (CFRP) rods as shear reinforcement has been conducted in flat plate slabsystem.

This exploratory research is to study the behavior of the flat plate slab with CFRP-rods reinforced in punching shear zone under con-stant gravity load and lateral displacements in a reversed cyclic manner. Three specimens of interior column-slab connection specimenswere tested including one standard specimen without any shear reinforcement, the second one reinforced with CFRP-rods and the thirdone reinforced with stud rails as the reference to the second one. The slabs were 3000 mm long · 2800 mm wide · 150 mm deep, and weresimply supported at four corners. Punching shear failure occurred for the standard specimens at a lateral drift-ratio, lateral drift dividedby the length of vertical member, of approximately 5%. The specimen reinforced by CFRP-rods had significant flexural yielding andsustained deformations up to a drift ratio of approximately 9% without significant losses of strength, and punching shear was notobserved in this specimen. The displacements increased up to 1.79 times larger than that of the standard specimen. And this specimenshowed 42% superior ductile performance than the standard specimen and even the same capability with the stud-rail reinforced spec-imen. The results of the experiment indicate that CFRP-rods using in the flat slab has a better foreground.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon–carbon composites (CCCs); A. Plates; B. Strength; D. Mechanical testing; Punching shear

1. Introduction

The flat slab system has been adopted and widely usedfor many structures that were recently constructed suchas large-scale supermarket, store and underground garage,bridge decks, etc. Also, flat plate slab system is frequentlyused in high rise buildings with mix use purposes or inbuildings with flexible spatial arrangements during their lifespan. Flat plate slab [1] is a very common and competitivestructural system, in which columns directly support floor

1359-8368/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2006.07.017

* Corresponding author.E-mail addresses: liran.hit@gmail.com (R. Li), ycho@hanyang.ac.kr

(Y.S. Cho), smzhang@hit.edu.cn (S. Zhang).

slabs without beams. The system has following additionaladvantages. First, construction period and cost can bereduced by efficient formwork compared with other kindsof structural systems. Secondly, this system can minimizethe ceiling height which can reduce a total building heightand further reduce the corresponding material cost. Mean-while beams, column capitals and drop panels take upspace which makes a flexible design difficult. On the otherhand, the main problem in practice is often to provide theresistance against a punching shear failure at the slab-col-umn connections. The punching shear capacity of theslab-column connection is affected by the size of column,the depth of slab and compressive strength of concrete,etc. The punching shear capacity can be increased by using

R. Li et al. / Composites: Part B 38 (2007) 712–719 713

a larger column diameter, a larger effective depth, moreflexural reinforcement, higher concrete compressivestrength or by additional shear reinforcement. Amongthese methods, the most effective way to enhance theslab-column connection is to use the shear reinforcementat the punching shear zone [2].

In this paper, a new type of shear reinforcement calledCFRP rod is studied and an experimental work was carriedout. Three interior slab-column connection specimens weretested consisting of one standard specimen without anyshear reinforcement, one specimen reinforced with CFRProd, and one specimen reinforced with stud-rail for com-parison. With respect to applied loading, lateral displace-ment was applied using a horizontal hydraulic actuatorof 250 kN capacity for the application of cyclic loadingat the top of each specimen’s column.

2. Research significance

Punching shear design requirements are included inmany design codes worldwide, especially under gravityand lateral loads [3,5]. There were numerous damagesand collapses under gravity loads and seismic loads suchas the Sampong department store in Korea, 1995. Theproblem of the punching shear strength of reinforced con-crete slabs, subjected to both vertical and lateral loads, hasreceived great attention over several decades because of itsimportance in flat plate slab systems. Recently there wereadvances in research about FRP rods such as the mechan-ical performance of pultruded composite rods with embed-ded fiber-optic sensors [6], durability of FRP rods forconcrete structures [8] and analytical and experimentalstudy on bonded-in CFRP bars in glulam timber [7] etc.

This paper focuses on the punching shear behavior ofconcrete flat plate slab structure reinforced with CFRProd under lateral cyclic load.

This paper will discuss about the effect of punchingshear capacity that were provided by the CFRP rods. Thisresearch also concerns how to improve structural safety,

a b

Fig. 1. Detail of floor framing plan (a) building model of column-slab conn

convenience of construction, saving of structural material,construction cost. In this study, specimens representing theinterior slab column connection from a 32-storey building,were used as shown in Fig. 1.

The paper pays attention to use the CFRP rods as theshear reinforcement based on the experimental results.Punching shear capacity with CFRP shear reinforcementis also presented [4].

3. Test program

3.1. Material and shear reinforcements

A 28-day compressive strength test of reinforced con-crete was conducted and the result was about 34.3 MPa.Steel bars were used as the flexural reinforcement in slaband column. The steel reinforcement bars included HD-19, HD-13, and HD-10 with the size of 19, 13, and10 mm in diameter, respectively.

The nominal yield strength of the flexural reinforcementbar is 440 MPa. The nominal yield strength of shear studsused in the third specimen was 525 MPa. The standard ten-sile test for the CFRP rod was also conducted to determinethe tensile properties of this kind of material. Table 1shows the detailed information of the reinforcement bar.The test results of CFRP rod material properties are shownin Table 2.

3.2. Test specimens

Three test specimens were tested. Among the three spec-imens, one standard specimen did not contain any shearreinforcement within it. The other two were the specimenswith shear reinforcement: the first specimen with CFRProds and the second specimen with stud rails.

Each specimen consisted of a 3000 · 2800 · 150 mm slaband a 150 · 150 · 1310 mm column at center with the 1/2scale of actual size. All the test specimens containedthe same slab flexural steel reinforcement and column

ection and (b) floor framing plan of base floor in a 32 storey building.

Table 1Information of steel bar and stud used in the specimen

Specimen Steel yield strength Reinforcement in slab Reinforcement in column

Flexible, fy

(MPa)Stud, fyr

(MPa)Top Bottom Longitudinal

reinforcement barStirrup

Reinforcedratio (%)

Steel grade Reinforcedratio (%)

Steel grade Reinforcedratio (%)

Steel grade Reinforcedratio (%)

Steel grade

SP-1 440 – 0.59 HD-13 0.21 HD-10 2.52 HD-19 0.48 HD-10SP-2 440 – 0.59 HD-13 0.21 HD-10 2.52 HD-19 0.48 HD-10SP-3 440 525 0.59 HD-13 0.21 HD-10 2.52 HD-19 0.48 HD-10

Table 2Property of CFRP rods

Description Value (CFRP)Ultimate strength 1819.86 MPaUltimate strain 1790 leYoung’s modulus 112083.58 MPa

Fig. 2. Top reinforcement bar arrangement.

Fig. 4. Column-slab connection A–A section.

Fig. 5. Column-slab connection B–B section.

714 R. Li et al. / Composites: Part B 38 (2007) 712–719

reinforcement. Figs. 2 and 3 show the arrangement of thetop and bottom layers of reinforcement respectively [1].And Figs. 4 and 5 show the reinforcements including hoopsinside the column. The bottom re-bars are spaced at240 mm on center each way, and the top bars are spaced

Fig. 3. Bottom reinforcement bar arrangement.

at 110–200 mm on center. In column, hoops were placed150 mm on center. Clearance of the concrete cover was15 mm.

The other two were basically the same with the standardone in terms of reinforcement and material propertiesexcept additional punching shear reinforcement. Shearreinforcement of the second specimen was made of CFRProds with a size of 9 mm in diameter.

Fig. 6 shows the specific configuration of the CFRProds. The third specimen was reinforced with stud railsand the placement is shown in Fig. 7.

3.3. Test procedure

The experimental set-up is shown in Fig. 8. The speci-men was simply supported at four corners of the slab by

Fig. 6. (a) Placement of CFRP rods and (b) diagram of the CFRP rod shear reinforcement.

Fig. 7. Configuration of studs rail.

R. Li et al. / Composites: Part B 38 (2007) 712–719 715

four transducer struts and constant gravity shear ratio (Vg/Vc) of 0.3 was controlled by load blocks and hydraulic jackof 500 kN capacity. The load was applied in a cyclicreversed displacement manner on the top surface of the col-umn using a 250 kN MTS loading system [9]. 40 kN of con-stant uniform gravity load was applied using four bundlesof steel blocks. The locations of the load blocks were deter-mined by the result of finite element analysis which showedthat moment shear ratio was similar to that of prototypebuilding. On the cross sides of the slab, four small steelbeams were bolted on the girders to restrict the rotation

Fig. 8. (a) Diagram of experimental set-up an

of slabs. On the two ends of column, the steel boxes werebolted to the column so that actuator and hydraulic jackcan be fixed on the concrete column. According to ACIT1.1R01, the cyclic load should be applied step by step insequence in order to mask undesirable brittle failure modesthat might occur in the inelastic response range during aloading process. The actual loading steps are shown inFig. 9. Total 67 cycles were carried out with an incrementalmanner until the failure of specimens. Actually, loadingprocedure was divided into two parts because of the strokelimitation of the actuator. Cyclic load reversals were onlypossible up to 6% drift ratio (phase I); beyond this, loadingcontinued in one direction only (phase II).

3.4. Test results

3.4.1. Cracks and failure pattern

In early steps, namely in the elastic step, the deformationof specimens steadily increased in accordance with theincrease of the lateral displacements applied on the topend of column. Lateral drifts and the applied load devel-oped a linear increase relationship and it showed no evi-dent changing in appearance. During application of thegravity load, slab cracking occurred on the top surface ofslab adjacent to the column. As lateral displacements wereapplied in an incremental manner, these cracks extended to

d (b) photo of actual experimental set-up.

-150

-100

-50

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Cycles

Dis

plac

emen

t (m

m)

Phase I Phase II

Fig. 9. Loading procedure.

716 R. Li et al. / Composites: Part B 38 (2007) 712–719

the edge of the slab while additional flexural and torsionalcracking occurred. Gradually, the cracks on the surface ofthe slab expanded to all regions and the main cracksappeared clearly around the column. The sound of thespalling from the inside of the slab could be heard duringthe test. When the displacement was added to 94 mm, thespecimen reached the ultimate load 50.5 kN. After thiscycle, the load, that the standard specimen could endure,decreased sharply and several cycles later, the specimenfailed in shear failure without any obvious warning. Thefinal crack pattern of a typical slab after 8% lateral driftis shown in Fig. 10a. The standard specimen without slabshear reinforcement failed due to punching shear at 5% lat-eral drift ratio.

The specimen with CFRP rods vertically placed as shearreinforcement was tested with the same method. This spec-imen reached a 57.71 kN ultimate strength when the actu-ator went forward to 120 mm. Before that cycle thisspecimen behaved a similar property with the standardone.

After entering into plasticity step, the growing trend ofthe loading resistance appeared a slowly rising rate. Whenthe load achieved 40% of the ultimate load, the fine cracksexpanded and the main cracks evidently enlarged adjacentto the column. When the load reached 70% of ultimate

Fig. 10. Failure pattern of the specimen (a) SP-1 (contr

load, cracks spread all the surface of the slab and cracksappeared on the heel post. The dehiscence sounds aggra-vated along with the increasing of the lateral displace-ments. This slab-column connection with slab shearreinforcement of CFRP rods did not suffer from punchingshear failure even though they were cycled to nearly 10%lateral drift ratio.

This third specimen showed nearly the same perfor-mance with SP-2, but differences existed as seen in the hys-teretic response curve and backbone curve shown in Figs.11c and 12, respectively. The two reinforced specimens dis-played similar lateral load and drift ratio but those speci-mens reached the ultimate load from the backbone curvein different phase. This is proved by the crack propagatingmodes in Fig. 10, which shows the specimen with studs onthe rail experience the punching shear failure at the lastperiod of loading procedure.

The cracks of bottom surface of the slab were mainlyconcentrated on the intersection of the slab and the col-umn. All the specimens show the same crack modes atthe bottom of slab.

3.4.2. Analysis of load–displacement

As the result of the experiment, yield load (Py), ultimateload (Pu), yield displacement (dy), ultimate displacement

ol), (b) SP-2 (CFRP-rods) and (c) SP-3 (stud rails).

-150 -100 -50 0 50 100 150 200 250

-60

-40

-20

0

20

40

60

Late

ral l

oad(

kN)

Displacement (mm)

-150 -100 -50 0 50 100 150 200 250

-60

-40

-20

0

20

40

60

Late

ral L

oads

Displacements-150 -100 -50 0 50 100 150 200 250

-60

-40

-20

0

20

40

60

Late

ral L

oads

(kN

)

Displacements (mm)

a b c

Fig. 11. Hysteretic response of specimens (a) SP-1 (control), (b) SP-2 (CFRP-rods) and (c) SP-3 (stud rails).

-150 -100 -50 0 50 100 150 200 250

-70

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-50

-40

-30

-20

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0

10

20

30

40

50

60

70

Late

ral L

oads

(kN

)

Displacements (mm)

SP-1

SP-2

SP-3

Phase IIPhase I

Fig. 12. Backbone curve.

R. Li et al. / Composites: Part B 38 (2007) 712–719 717

(du), strength ratio (Pu/Py), ductility (du/dy) were calculatedand shown in Table 3. In this study, the yield displacementis defined using the Park method presented in FEMA 356.The ultimate load is the maximum load point where thespecimen fails. The ultimate displacement is the displace-ment point where the ultimate load occurs. The yield loadis defined as the load point of 75% of the ultimate load. Theyield displacement is the intersection point between thehorizontal line passing ultimate load and the secant linepassing the point of 75% of ultimate load. From the hyster-etic response of the three specimens, a better performanceof ductility and shear capacity using CFRP in flat plate

Table 3Experimental results

Specimen Py

(kN)Pu

(kN)dy

(mm)du

(mm)

P u

P y

du

dy

Drift ratio(%)

SP-1 37.88 50.50 71.89 103.45 1.33 1.44 4.93SP-2 43.28 57.71 99.02 192.5 1.33 1.94 9.14SP-3 43.63 58.18 110.0 200.95 1.33 1.83 9.57

slab were found. The standard specimen showed a suddenfailure and the other two specimens showed a ductile pat-tern which could also be seen in Fig. 11. Especially, theCFRP rod specimen exhibited a similar behavior with thespecimen reinforced with stud rail. Fig. 12 shows the back-bone curve of the three specimens. The curve of the stan-dard specimen falls down sharply when the lateraldisplacement just reached approximately 120 mm. In theopposite, the specimen reinforced with CFRP rods showeda better lateral load resistance and had a long descendingbranch. The displacements increased up to 1.86 times thanthat of the standard specimen. The CFRP rod reinforcedspecimen showed 50% superior ductile performance thanthe standard specimen, almost the close capability withthe stud rail reinforced specimen, and a better crack prop-agation mode compared to specimen with stud rail.

In comparison with the specimen using CFRP rods, thestuds rail reinforced specimen went until Phase II. Obvi-ously, the standard specimen showed a sudden brittle fail-ure. The reinforced ones gave a better ductile performanceuntil Phase II as they gradually crushed. In comparison tothe specimen with stud rails, specimen using CFRP rods inthis experiment shows more ductile and the crack propaga-tion in specimen No. 2 seems to be a better mode shown inFig. 13, and this initial result needs further comprehensiveresearch to generalize.

In comparison of the backbone curves of the two rein-forced specimens, it showed that one reinforced by CFRProds achieved the maximum load earlier than the stud railreinforced one, but both of them reached the maximum lat-eral displacements due to the limitation of actuator. Figs.14 and 15 show the ultimate and yield load anddisplacements.

The ultimate load and ultimate displacement of SP-2showed a relative increase compared with SP-1. In addi-tion, the nearly equivalent ultimate load and displacementwith SP-3 also revealed that the SP-2 hold a ductile per-formance. The calculated ductility of the three specimensis given in Fig. 13 since the ultimate displacement, du, usedthe maximum displacement measured at the top of col-umn although this is limited by the actuator. This ductility

SP-1 SP-2 SP-320

30

40

50

60

Load

(kN

)

Pu Py

Fig. 14. Ultimate and yield load of each specimen.

SP-1 SP-2 SP-30.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0D

uctil

ity

Fig. 13. Ductility comparison of each specimen.

SP-1 SP-2 SP-3

40

80

120

160

200

240

Dis

plac

emen

ts (

mm

)

δuδy

Fig. 15. Ultimate and yield displacement curve of each specimen.

718 R. Li et al. / Composites: Part B 38 (2007) 712–719

index can be taken to evaluate the capacity of specimenunder lateral load. Under the cyclic load test in this study,higher ductility index, ultimate displacement divided byyield displacement, indicates the higher margin betweenthe ultimate load and the yield load. The specimen rein-forced by CFRP rods showed an excellent ductility in thisstudy.

4. Conclusions

This paper presents the experiment of three specimens ofcolumn-slab connection under constant gravity load and

variant lateral displacement in a reversed cyclic manner.The material selection, specimen design, fabrication, andexperimental procedures are based on American standardssuch as ACI code, FEMA, etc. Careful data record andobservations were taken during the experiment, and theresults were presented accordingly by using figures, tablesas well as photographs.

1. Based on experimental observations, it was noted thatthe specimens’ failure accompanied by main crackingaround the face of column and slight cracks appears inthe part of heel post. Spalling of concrete was noticedon the surface of slab, starting gradually during thewhole procedure and hearing of the cracking soundand increase gradually until the specimen reached theultimate load. Several strong crushing sounds wereheard during ultimate load. Punching shear failure wasnoticed in the standard specimen while the other twodid not failed as punching shear failure and showed duc-tile behavior with nearly two times drift ratio.

2. Based on the results of the experimental program, it wasfound that the CFRP rods reinforced specimen showeda better load–displacement curves that climb to peakload in phase I followed by unloading, without punch-ing shear failure. It seems that in elastic stage of con-crete, CFRP rods take efficient roles in slab comparedto steel stud rails due to the elastic modulus of CFRProds is smaller than that of steel studs. The initial stiff-ness of the three specimens showed small difference witheach other, and than decrease with the increase of thedisplacement applied on the column. Slight damage withfine cracks was found in the heel post just before itreaches the ultimate load.

3. In phase II, the load was applied only in one direction asthat the failure cracks appeared only in one side. Ulti-mate and yield load curves showed that the increasingtendency drive to approximately the same stage but onthe other hand the ultimate and yield displacement curveperforms differently. The yield displacement increasesslightly in sequence but the ultimate displacementincrease sharply from SP-2, the CFRP rods reinforcedspecimen. Neither the ultimate and yield load nor theultimate and yield displacement between SP-2 and SP-3 represents minute distinctiveness.

4. Punching shear failure occurred for the standard speci-mens at a lateral drift-ratio of approximately 5%. TheCFRP rods reinforced specimens had significant flexuralyielding and sustained deformations up to a drift ratioof approximately 10% without significant losses ofstrength, and punching shear was not observed in thisspecimen. The displacements increased up to 1.86 timesthan that of the standard specimen. The CFRP rod rein-forced specimen showed 50% superior ductile perfor-mance than the standard specimen, almost the closecapability with the stud rail reinforced specimen, and abetter crack propagation mode compared to specimenwith stud rail.

R. Li et al. / Composites: Part B 38 (2007) 712–719 719

5. The important ideas noted from this experimental resultare that the CFRP rods specimen behaves a promisingperformance. This indicates that FRP material using inthe flat slab has a reasonable potential to research further.

Acknowledgements

This research was supported by SRC/ERC program ofMOST (Grant#R11-2005-056-01004-0). This study wasalso supported by The Korea Science Foundation underthe joint research program with China Science Foundation(F01-2005-000-10226-0). This work was also supported by2003 Hanyang University international journal paper com-petition project (HY-2004).

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[4] EIGhandour AW, Pilakoutas K, Waldron P. Punching shear behaviorof fiber reinforced polymers reinforced concrete flat slabs: experimen-tal study. J Compos Constr ASCE 2003;7(3).

[5] Kang TH-K, Kee S-H, Han SW, Lee L-H, Wallace JW. Interior post-tensioned slab-column connections subjected to lateral cyclic loading.In: Proceedings of 8th national conference on earthquake engineering,San Francisco, USA; April 2006.

[6] Kalamkarov AL, Fitzerrald SB, et al. The mechanical performance ofpultruded composite rods with embedded fiber-optic sensors. ComposSci Technol 2000;60(8):1161–9.

[7] Lorenzis LD, Scialpi V. Analytical and experimental study on bonded-in CFRP bars in Glulam timber. Compos Part B–Eng 2005;36(4):279–89.

[8] Micelli F, Manni A. Durability of FRP rods for concrete structures.Constr Build Mater 2004;18(7):491–503.

[9] Robertson IN, Johnson G. Repair of slab-column connections usingepoxy and carbon fiber reinforced polymer. J Compos Constr ASCE2004;8(5).