Research Article An Experimental Study on Concrete Flat...

Research ArticleAn Experimental Study on Concrete Flat Slabs Prestressed withCarbon Fibre Reinforced Polymer Sheets

Yin Shen1 Shaohui Lu2 and Fangyuan Li1

1Department of Bridge Engineering College of Civil Engineering Tongji University 1239 Siping Road Shanghai 200092 China2Liuzhou OVMMachinery Co Ltd Longquan Road Liuzhou Guangxi 545005 China

Correspondence should be addressed to Fangyuan Li fylitongjieducn

Received 30 June 2015 Revised 29 September 2015 Accepted 5 October 2015

Academic Editor Stefano Sorace

Copyright copy 2015 Yin Shen et alThis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Carbon fibre reinforced polymer (CFRP) is currently used to reinforce buildings in civil engineering in the common forms of sheetswhile the utilization efficiency of a CFRP materials greatly decreased when the CFRP material is directly bonded to the structurebecause of the lack of the effect of the exertion of a prestress A paper spool-inspired anchoring method is proposed to overcomethe shearing problem in the anchoring system through the friction between layers Anchoring and jack-up tensioning devices forCFRP sheets are also designed and produced A prestress is successfully applied to single andmultiple CFRP sheets (80 tensioningstrength is achieved) thus verifying the tensioning effect of the prestress Based on these results prestressed concrete flat slabs weredesigned with pretensioned CFRP sheetsThe correspondingmechanical properties of the concrete flat slabs are tested to verify thefeasibility of using CFRP sheets to apply a prestress The results show that the uniformity of the fibre stress during the tensioningof the CFRP sheet is the key to the success of the application of the prestress

1 Introduction

Fibre reinforced polymers (FRPs) have many advantagessuch as high strength low creep moderate elastic modulusand good corrosion resistanceTherefore they can be used toreplace the steel bars and prestressed steel bars in concretestructures and steel plates externally bonded to concretefor reinforcement Since the 1960s through research anddevelopment and engineering pilot projects FRPs have beenused in the construction reconstruction and reinforcementof bridges and buildings in the United States (US) CanadaEurope and Japan [1]

FRPs are composed of high-performance fibres andbinders Based on the type of fibres used common FRPs arecategorized as glass fibre reinforced polymers carbon fibrereinforced polymers (CFRPs) and aramid fibre reinforcedpolymers CFRPs are high-tech products that have excellentproperties such as high specific strength high specific mod-uli high temperature resistance high corrosion resistancehigh fatigue resistance high creep resistance high electricalconductivities high thermal conductivities and low thermalexpansion coefficients CFRPs can be used both as structural

materials to bear loads and as functionalmaterials to performtheir functional roles Therefore CFRPs have been develop-ing rapidly in recent years and have been widely used inmany fields such as aeronautics astronautics the automobileindustry environmental engineering chemical engineeringenergy traffic building construction electronics and sportsequipment CFRPs are praised as the new material of the 21stcentury [2]

CFRP tendons are the FRP material used in concretestructures with the best prospects because CFRP tendonsare regarded as the replacement for steel bars in steelreinforced structures Currently CFRP sheets and CFRPplates are used for structural reinforcement more commonlythan other materials [3 4] First CFRP sheets and CFRPplates are relatively easy to produce Second repairing andreinforcing the existing structures worldwide represent alarge guaranteed demand for CFRP sheets and CFRP plateswhichwill also help tomaintain relatively lowprices forCFRPsheets and CFRP plates for the foreseeable future Henceresearchers from several countries have conducted studieson CFRP sheets and plates which have contributed to thecontinuous improvement and perfection of the production

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2015 Article ID 792320 11 pageshttpdxdoiorg1011552015792320

2 Advances in Materials Science and Engineering

processes [3 5] According to the literature 95 of theCFRP sheets and plates are used to strengthen and reinforceexisting structures worldwide while CFRPs are directly usedas structural materials only in trial experimental studiesTherefore based on their price advantage the utilization ofthe economy and excellent properties of fibre sheets or platesas well as the development of new applications of fibre sheetsor plates have high potential economic value

The steel plate bonding and reinforcement techniqueis commonly used to strengthen reinforced concrete (RC)structures However steel plates are heavy and difficult totransport in addition the cutting and moulding of steelplates are also relatively complicatedThese shortcomings canbe avoided by using fibre sheets or fibre plates which arelightweight easy to machine and have a very high uniaxialtensile strength [6ndash9] Since the 2000rsquos First Symposium onFibre Reinforced Plastic Concrete Structures the research onthe application of FRPs in civil engineering and constructionin China has reached an unprecedented scale Based onthe requirements of the China Association for EngineeringConstruction Standardization the National Industrial Build-ing Diagnosis and Retrofit Engineering Technology ResearchCentre of China (the Central Research Institute of Buildingand Construction the Ministry of Metallurgical Industry)the Sichuan Institute of Building Research and other relevantunits have jointly compiled the Technical Specification forStrengthening Concrete Structures with Carbon Fibre Rein-forced Polymer Laminate (China Engineering ConstructionStandardization 1462003) [10] A new revised version of thistechnical specification is currently being prepared

CFRPs are brittle materials therefore it is necessary toconsider the requirements of structural strain during thedesign processThe carbon fibres used in structures generallyhave a strain of 12ndash15 which is smaller than the strainin a steel strand (6) thus such carbon fibres completelysatisfy the basic requirements of structural deformation andcracking and signs of structural damage or overloadingcan be observed Similar to concrete CFRP can also failfrom creep rupture under sustained high-loading conditionsWhen the sustained load exceeds 75ndash80 of the ultimatetensile strength the life of the CFRP will be affected [11ndash13]Therefore the long-term load on a steel tendon is limitedto 50ndash60 of the tensile strength of the tendon understatic loading conditions to reduce the permissible value ofthe prestress and the applicable sustained load Howeverthis limitation is not an obstacle during the design processbecause when the stress on CFRP tendon reaches 60 ofthe tensile strength of the CFRP tendon under static loadingconditions or lower the CFRP tendon still maintains its totaltensile capacity and the reserved 40 of the tensile strengthcan then be used for bending deformation and fatigue loadafter the initial stress occurs

Even though high tensile strength and moderate elasticmodulus CFRP tendons are suitable for prestress CFRPtendons are still disadvantageous because they are hetero-geneous and brittle The heterogeneity may cause a CFRPtendon to easily fail when used to apply a prestress toan anchor To date no anchoring system has been devel-oped that can ensure that CFRP tendons reach the optimal

level of stress transfer [11 14ndash17] Therefore developingan anchoring system that can ensure that CFRP tendonsreach the optimal top stress transfer that is developingthe anchorages required when using CFRPs as prestressingtendon material has become one of the focuses of theresearch using CFRPs as prestressing tendons [18 19] There-fore combining the mature CFRP sheet material techniqueand the prestressing technique provides new applicationmodes and ideas for CFRP materials and the prestressingtechnique which is the research objective and goal of thisstudy

2 Material and Methods

21 Method for Applying a Prestress to a CFRP Sheet If therotation of a paper spool is hindered when the paper on thespool is pulled the gap between the layers will be compressedand the paper will be elongated during the process until thegap between layers is completely compressed When pullingthe paper the pressure on the paper will gradually decreaseif the layers of the paper are maintained in parallel Thebalancing force to the pulling force is from both the tensionon the topmost layer of the paper and the friction betweenthe topmost layer of the paper and the layer of the paperimmediately below the topmost layer of the paper This waythe direct shearing on fibres that occur when using otheranchoring methods such as the anchoring method that usesclips can be avoided and the brittle failure of fibres can beavoided

Inspired by these paper spools in this study CFRP sheetis clipped onto a cylindrical drum and the CFRP sheetis then wound using the friction between layers (119879) tocreate tension in the CFRP sheet during the process whilethe corresponding length of the CFRP sheet continuouslydecreases The anchoring force of the CFRP sheet materialis the sum of the friction between the rotating drum and theCFRP sheet (119873) and the self-resistance (1198791015840) that is1198791015840+sum120591 =119879 (Figure 1) The two layers of the CFRP sheet are the sameand there is adequate contact area between the two layersTherefore compared to traditional anchoring methods suchas the anchoring method that uses clips the shearing forceof the anchorage generated using this method is relativelydispersed and does not result in the failure of the fibres byshear forces

Figure 1 summarizes the initial idea of this study Exper-imentation is required to verify the feasibility of this ideaThe authors of this study and the researchers at the OVMResearch Centre on Prestress at Tongji University jointlydesigned the experimental frameWhen designing the exper-imental frame we considered the effect of the deformationof the steel drum used for winding the CFRP sheet on theprestress loss of the CFRP sheet and the jack-up constructiontime required to reach the curing age needed for the requiredstrength of the concrete as well as how to overcome theeccentricity of the CFRP sheet relative to the rotating drumduring the winding process Tension is increased usingjacks on both sides The jack-up tension is checked usingthe pressure sensor located at the front of each jack Toperform the experiments a rotating drum device is required

Advances in Materials Science and Engineering 3

T

The reversed torque

The roller shell

(a) Layout of roller with sheets

T

The inner layer

The outer layer

(b) Location of sheets

120591

T998400

N

T

(c) Action between different lay-ers

Figure 1 Diagram of the principle of the anchor tension

The tension must also be measured to control the tension ofthe CFRP

22 Material The Tonggu CFRP sheets (model numbersCFC2-2 and CFC20-3) manufactured by Shanghai YichangIndustry Corporation were used as the experimental CFRPsheets The detailed parameters of the CFRP sheets aresummarized in Table 1

3 Experiments and Results

There are two types of experiments that were completedTo verify the feasibility of the proposed method tensioningtests on single and multiple CFRP sheets were conductedConcrete flats with or without CFRP sheet prestressed werecompleted to verify the effectiveness

31 Experiment to Verify the Method toGet the Prestressing with CFRP Sheet

311 Single CFRP Sheet The trial winding test results showthat the number of the winding layers has a relatively largeimpact on the tension in the CFRP sheet In the initialstage of the winding process there is friction between anytwo adjacent layers however when the CFRP sheet wassubjected to an external force only the outermost 2-3 layersare able to effectively provide friction An excessive numberof winding layers are detrimental to stress uniformity becausean increase in the length error between two points results inan inconsistency between the extended lengths In additionan increase in the number of winding layers results in anincrease in the winding thickness which in turn rendersthe tensile deformation difficult to control Therefore it isconcluded that the optimal number of winding layers is threeto get the uniform stress This conclusion also provides acompelling basis for saving CFRP sheet materials

Figure 2(a) shows thewinding process inwhich theCFRPsheet is wound onto the rotating drum on one side Figures2(b) and 2(c) show themanual pretension process performedbefore the CFRP sheet is unwound onto the rotating drumon the other side and the preparation before the final jack-uptension process respectively Figure 2(d) shows the process ofclamping the CFRP sheet onto the rotating drumon the otherside Figures 2(e) and 2(f) show the bidirectional windingprocess and the check-up process before the jack-up process

Following the tensioning scheme an experiment is per-formed on one CFRP sheet using the fabricated tensiondevice Figure 3 shows the experimental effect During theexperiment it was difficult to apply tension uniformly to allthe fibres in the sheet material Figures 3(a) and 3(b) showthe results of the tensile failure of longitudinal fibres causedby nonuniform tension on one side and uniform tension onthe other The data in the figure show that nonuniform stressresults in an excessive stress on local fibres which results inthe gradual failure of fibres at the cross section By adjustingthe loading speeds of the jacks on both sides the horizontalposition and the initial winding state of the CFRP sheetmaterial during multiple runs a near-ideal stress conditionis achieved (Figure 4)

Based on the parameters of the material provided bythe manufacturer and considering the prestressing effectthe minimum jack-up tension is determined The minimumcontrol stress is set at 05 times that of the tensile strengthConsidering the safety of the experiment the maximumcontrol value of the tensile stress is set at 08 times that ofthe tensile stress Based on the determined minimum jack-up tension minimum control stress and maximum controlvalue of the tensile stress the corresponding jack-up controlforces are estimated (Table 2)

The tensioning control stress is measured thrice whiletest-running the jack-up tensioning frame The results ofthe tests are 2027MPa 2684Mpa and 2521MPa whichcorrespond to 057 076 and 071 times that of the tensilestrength respectively Because the effect of the uniformityof the CFRP sheet is not considered the tensioning controlstress does not reach the expected value (08119891

119910119888) in any of

the three tests and tensioning stops when the failure of localfibres occurs in all of the tests Through constant adjust-ments these possible unfavourable conditions are eliminatedAfterwards the ultimate tensioning stress is tested thriceTheresults show that the ultimate tensioning stress does not reachthe tensile strength provided by the manufacturer in any ofthe tests the local tensile failure of the CFRP sheet occursinstead The results of the three tests show that the ultimatetensioning stress was 080 089 and 087 times that of thetensile strength These results have practical significance forthe selection of the design value of the tensile strength of theCFRP sheet in application


Table 1 Specifications and parameters of the CFRP sheets used in the experiments

Productthickness

Carbon fibretype Tensile strength Tensile elastic modulus Elongation at fracture Weight per unit area Sheet width

012mm T-700-12K ge3500MPa 220GPa 18 300 gm2 100mm017mm T-700-12K ge3500MPa 220GPa 18 200 gm2 100mm

Table 2 List of the control parameters for tensioning

Elastic modulus of the CFRP sheet 119864 = 220GPaTensile strength 119891

119910119888= 3550MPa

Cross section 119860119891= 012mm times 100mm = 12mm2

Minimum control tensioning stress 119891min = 05119891119910119888= 05 times 3550 = 1775MPa

Minimum control tensioning strain 119891min119864 = 1775(220 times 101198903) = 0008068Minimum control tension 119865min = 119891min times 119860119891 = 1775 times 12 = 21300N = 213 kNMaximum control tensioning stress 119891max = 08119891

119910119888= 08 times 3550 = 2840MPa

Maximum control tensioning strain 119891max119864 = 2840(220 times 101198903) = 0012909Maximum control tension 119865max = 119891max times 119860119891 = 2840 times 12 = 3408 kN

(a) Winding onto the rotating drum on one side (b) Applying tension before unwinding

(c) Adjusting the position of the other rotating drum (d) Fixing the CFRP sheet using a clip

(e) Bidirectional winding before jack-up tensioning (f) Check-up on the fixation before the jack-up process

Figure 2 Jack-up tensioning process


(a) Tensile failure from nonuniform tension (b) Tensile failure from uniform tension

Figure 3 Different jack-up failure modes of the CFRP sheet

(a) After jack-up tensioning (the jacks are removed) (b) Ideal tension effect

Figure 4 Ideal tension state of the CFRP sheet

312 Applying Tension to Three CFRP Sheets Simultane-ously In practical applications to simplify the constructionprocedure or to reduce the required equipment there arevery few occasions in which a single CFRP sheet is usedCompared to a single CFRP sheet it is more difficult to applytension to three CFRP sheets simultaneously because it isnecessary to maintain uniform tension on all three CFRPsheets and to ensure the uniformity of the tension on each ofthe three CFRP sheets Based on the experience gained fromthe tensioning of a singleCFRP sheet and through continuousattempts the final tensioning test was completed Figure 5shows the jack-up end and the anchorage end during thetensioning process

To ensure that the prestress reaches the expected valuein the concrete flat slab test 017mm thick CFRP sheetsare used in the experiment Through trial and error usingthe method described above the final maximum jack-upforce is 134 kN The mean tensioning stress on each sheetis 2627MPa which is approximately 074 times that of thetensile strength whereas the tensioning stress on the sheet is09 times that of the tensile strength when a single sheet isused which warrants attention in practical application

32 Mechanical Properties of the Concrete Flat SlabPrestressed with CFRP Sheets

321 Experimental Scheme and Design To verify the feasi-bility of the proposed method and the practical effect of theapplication of prestress the actual situation inwhich concrete

slab structures are prestressed with CFRP sheets is testedTo date there have been no related experimental studies ondirectly prestressing concrete structures with CFRP sheetsTherefore the experimental study on prestressing concreteflat slabs with CFRP sheets is an exploratory study Acomparative experiment is also conducted that is resultsfrom an ordinary RC flat slab CFRP sheet prestressed RCflat slab and a CFRP sheet prestressed concrete flat slab arecompared

The slab is 2100mm long 600mm wide and 80mmhigh with 1800mm calculation span The design live load(119902119896) is 15 kNm2 C50 concrete is used (119891cm = 26MPa

and 119891119910= 210MPa) The concrete has a deadweight (120574) of

23 kNm3The reinforcement design for the simply supportedplate is performed according to the Design Specification forReinforcement Concrete Structures In addition 12060161206011095(41206016 and 312060110) were selected and a total of 1281206016120were used as the distribution steel bars Figure 6 shows thereinforcement drawing

Three 100mm wide CFRP sheets were placed withinthe range of 119887 = 600mm and each CFRP sheet had atensile strength (119891

119910119888) of 3550MPa Lacking a proper design

reference the CFRP sheets are designed to be tensionedbased on 50 of the tensile strength It is assumed that theCFRP sheet reaches 70 of its tensile strength at failure Theflexural design for the simply supported plate is performedagain according to the Design Specification for ReinforcementConcrete Structures The stresses on the upper and lower


(a) Winding of CFRP sheets at the jack-up end (b) Winding of the CFRP sheets at the anchorage end

Figure 5 Photographs of the jack-up tensioning of multiple CFRP sheets

6020 80

15 105 105 152040

600

1515

Distribution steel bars 1206016120Force-bearing bars 12060110190Force-bearing bars 1206016190

16 times 120

6times95

Figure 6 Construction drawing of the ordinary RC slab

edges of the cross section of the concrete are calculated to be053MPa and minus545MPa respectively

Considering the disadvantages of cracks caused by fullprestressing for example excessively fast development andpoor structural ductility some ordinary steel bars are addedto the fully CFRP prestressed concrete slab to study the effectof ordinary steel bars on the CFRP prestressed structureConsidering the positions of steel bars limited by the actuallayout of the CFRP sheets 41206016190 are only added to thegaps among the CFRP sheets based on the CFRP sheetprestressed flat slab that is one steel bar is placed in each ofthe gaps among the three CFRP sheets (Figure 7)

322 Construction of the Prestressed Concrete Flat SlabsA flat slab is poured at the Structure Laboratory in theDepartment of Bridge Engineering at Tongji University Thefollowing steps are used in the construction

(1) The 20mm thick bottom of the concrete is poured(Figure 8(a)) vibrated (Figure 8(b)) and levelled(Figure 8(c))

(2) The anchoring device is jacked up and tension isapplied to the CFRP sheets

(3) The 60mm thick top layer of the concrete is pouredand levelled

The jack-up tensioning is controlled based on the pressuremeasured at the end of the jack The tension design control

force is 119873 = 065119891119910119888119860119875= 118 times 10

5N however whenstabilized the readings indicate that the jack-up forces on thetwo sides are 55 kN and 53 kN After the concrete is pouredthe readings show that the jack-up forces further decrease to39 kN and 42 kN The calculation shows that only 446 ofthe tensile strength of the CFRP sheets is reached

When pouring the top layer of the concrete the CFRPsheets must be protected from damage caused by the tensionDuring the pouring process a spacing of 100mm betweentwo CFRP sheets is shown to be unfavourable to the vibrationof the concrete that is the concrete vibrator can onlymove ina 100mmwide gap which affects the construction speed andis detrimental to the construction qualityThat is the vibratorcould easily destroy the fibres in the CFRP sheets during thevibration process Figures 8(c) and 8(d) show the situation inwhich the surface of the concrete slab is treated as well as thefinal curing condition of the concrete slab respectively

323 Experimental Testing and Results An evenly dis-tributed load is applied to the flat slab However consideringthe convenience of the experimental operation and theconditions set in the laboratory the common loading modefor bending-resistant components is used when loading theone-way slab in practice The three-dividing-point loadingmethod (Figure 9) is used to compare the flexural perfor-mance of the ordinary RC slab and the CFRP prestressed RCslab Figure 10 shows the actual loading process


2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

Distribution steel bars 1206016120CFRP sheet Logitudinal steel bars 1206016190

Figure 7 Construction layout of the CFRP sheet prestressed concrete slab (unit mm)

(a) Pouring concrete (b) Vibrating concrete

(c) Levelling concrete (d) Curing concrete

Figure 8 Construction process of the concrete slab

Manual loading with a jack

Pressure sensorShared load beam

Displacement meterA B

Figure 9 Schematic of the three-dividing-point loading process

To obtain the mechanical properties of the concreteslab during the loading process concrete strain gauges areattached to the roof and the bottom of the concrete slab at the

midspan section and at the locations of the force-bearing steelbars Figure 11 shows the corresponding measuring pointsThe concrete strain gauges on the CFRP prestressed flat slabcorrespond to the locations of the longitudinal steel bars inthe ordinary concrete slab

Figure 12 shows a comparison between the midspandeflection curves of the RC slab and the CFRP prestressedslab The data in Figure 12 show that the cracking load ofthe RC slab is insignificant whereas the cracking load ofthe CFRP slab is approximately 15 kN and the deflectionincreases rapidly after the slab is cracked Figure 13 shows thecorresponding strain curves of concrete and steel bars at themidspan cross section

In terms of the ultimate bearing capacity the designedCFRPprestressed slab does not reach the expected load value


Figure 10 Photograph of the loading process

Steel bar strain gauge Concrete strain gauge

1020

Center symmetry line

balsCR

balsCRP

PRFC

Steel bar strain gauge

Con

cret

e str

ain

gaug

e

Figure 11 Layout of the measuring points on the RC slab and CFRP PRC slab Note A total of seven steel bar strain gauges and a total of sixconcrete strain gauges (three gauges on the bottom of the slab and three gauges on the top of the slab)

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18

Point A of CFRP slabPoint B of CFRP slabMiddle section of CFRP slab

Displacement (cm)

Load

(kN

)

Point A of RC slabPoint B of RC slabMiddle section of RC slab

Figure 12Deflection-load curves for theRC slab and theCFRP slab

primarily because of the prestress loss and the maximumdevelopment of cracks Because the effective prestress of theCFRP prestressed slab was relatively low the cracking loadof the CFRP prestressed slab is nearly the same as that forthe RC slab However because ordinary steel bars inhibitand disperse the development of cracks the ultimate bearing

capacity of ordinary steel bars is greater than the prestress ofthe CFRP prestressed slab

4 Analysis

Although 09 times tensile strength can be reached for asingle CFRP sheet the verified tests proved that the meantensioning strength is only about 075 times that of thetensile strength So about 07 times tensile strength is morepossible for practice Further experimentation is required todetermine whether a higher tensioning stress can be reachedwhen multiple sheets are used

Several methods that can be used to improve the unifor-mity of the tension on three sheets are summarized below

(1) Ensure that the winding lengths of the three CFRPsheets are the same

(2) Improve the accuracy of the initial cutting length(3) Complete the fixing and winding of the three sheets

at the same time(4) Use a winding method with guide rails to allow one

rotating drum to rotate along the guide rails in theparallel way as the other rotating drum to ensure thatthe two rotating drums are parallel

The experiment on the CFRP prestressed slab that con-tained no steel bars was not performed therefore becausethere is a larger contact area between the CFRP sheet andthe concrete further studies are necessary to understand


0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)

(a) Strain curve of the ordinary steel bars in the RC slab

0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)

(b) Strain curve of the steel bars in the CFRP slab

0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e

Load (kN)Bottom1Bottom2Bottom3

Top1Top2Top3

(c) Strain curve of the concrete in the RC slab

0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)

Bottom1Bottom2Bottom3

Top1Top2

(d) Strain curve of the concrete in the CFRP slab

Figure 13 Strain curves of the concrete and the steel bars at the midspan cross section

the effect of the CFRP sheet on inhibiting and dispersingcracks A possible reason for the rapid dispersion of cracksis the followingThe stress on the CFRP sheet is nonuniformresulting in the premature failure of the CFRP fibres whichin turn results in rapid structural damage

The experimental results indicate that there is no dif-ference in the use of the structure other than the ordinarysteel bars being replaced by the CFRP sheets however thecracking load remains the same or even increases and theultimate bearing capacity changes The CFRP prestressedslab is more advantageous based on the comparison betweenthe routine service stages of the two slabs Structurally theCFRP sheets have a smaller deadweight and the crackingload of the CFRP sheets remains the same or even increasesAdditionally the durability of the CFRP sheets is morefavourable to the service life of the structure Further cost

comparisons must be based on the specific engineeringapplication

5 Conclusions

(1) Prestress was applied using a CFRP sheet materialthat had been tensioned using a winding processTheshearing of an anchoring device that uses clips wasavoided by using the prestressing method thereforethe possibility of the shear failure of fibres wasreduced The CFRP sheet material was prestressedusing the fabricated jack-up tensioning device

(2) When applying uniform tension the tensioningstrength was ensured to reach 3160MPa which was90 of the tensile strength of the material otherwise


the tensioning strength was only 2027MPa whichwas 60 of the tensile strength of the materialIt was more difficult to uniformly tension multipleCFRP sheets simultaneously than to do so for onesingle CFRP sheet When multiple CFRP sheets weretensioned simultaneously the mean tensioning stresson each CFRP sheet was only 2627MPa which wasapproximately 74 of the tensile strength of thematerial

(3) The uniformity of the stress on the fibres in theCFRP sheet could be improved through adjustingthe initial cutting length and the jack-up speeds onboth sides and improving the precision of the parallelwinding Because of the nonuniformity of the jack-up tensioning method there is a design limit for thetensioning control stress of the CFRP sheet

(4) Themonitoring data obtained from the sensor duringthe concrete pouring process show that the stabilityof the jack-up frame significantly impacted the stresson the CFRP sheet Additionally the monitoring datashow that the stress loss of the CFRP sheet wasrelatively significant Further studies are necessary tounderstand the cause for this stress loss

(5) Based on the development of cracks in the slabpartially prestressed with CFRP sheets it was con-cluded that the prestressing effect was still effective Itcould be seen from the distribution and developmentpatterns of cracks in the RC slab and the CFRP sheetprestressed slab that the CFRP prestressed slab mightstill need ordinary steel bars to inhibit and dispersethe development of cracks and avoid the excessiveconcentration of cracks and the occurrence of largecracks to avoid the occurrence of brittle failure

(6) The relatively low ultimate bearing capacity of theCFRP sheet prestressed slab might be related tothe nonuniform stress on the CFRP sheets and theprestress loss that occurred at a later stage Furtherstudies are needed to search for a method that can beused to apply a more effective prestress

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] S Pimenta and S T Pinho ldquoRecycling carbon fibre reinforcedpolymers for structural applications technology review andmarket outlookrdquoWaste Management vol 31 no 2 pp 378ndash3922011

[2] L C Hollaway ldquoA review of the present and future utilisationof FRP composites in the civil infrastructure with referenceto their important in-service propertiesrdquo Construction andBuilding Materials vol 24 no 12 pp 2419ndash2445 2010

[3] J Jayaprakash E Pournasiri K K Choong C G Tan andF DersquoNan ldquoExternal CFRP repairing of pretested beams rein-forced using prestress rebarsrdquo Journal of Reinforced Plastics andComposites vol 30 no 20 pp 1753ndash1768 2011

[4] NWahab T Topper and K A Soudki ldquoModelling experimen-tal bond fatigue failures of concrete beams strengthened withNSM CFRP rodsrdquo Composites Part B Engineering vol 70 pp113ndash121 2015

[5] M Rezazadeh I Costa and J Barros ldquoInfluence of prestresslevel on NSM CFRP laminates for the flexural strengthening ofRC beamsrdquo Composite Structures vol 116 no 1 pp 489ndash5002014

[6] P Nabipay andD Svecova ldquoShear behavior of CFRP prestressedconcrete T-beamsrdquo Journal of Composites for Construction vol18 no 2 Article ID 04013049 2014

[7] M M Onal ldquoStrengthening reinforced concrete beams withCFRP and GFRPrdquo Advances in Materials Science and Engineer-ing vol 2014 Article ID 967964 8 pages 2014

[8] H D Yapa and J M Lees ldquoRectangular reinforced concretebeams strengthened with CFRP strapsrdquo Journal of Compositesfor Construction vol 18 no 1 2014

[9] S M Mosavi and A S Nik ldquoStrengthening of steel-concretecomposite girders using carbon fibre reinforced polymer(CFRP) platesrdquo Sadhana Academy Proceedings in EngineeringSciences vol 40 no 1 pp 249ndash261 2015

[10] National Industrial Building Diagnosis and Retrofit Engineer-ing Technology Research Centre of China Technical Specifi-cation for Strengthening Concrete Structures with Carbon FibreReinforced Polymer Laminate China Planning Press BeijingChina 2007

[11] A Mostafa and A G Razaqpur ldquoCFRP anchor for preventingpremature debonding of externally bonded FRP laminates fromconcreterdquo Journal of Composites for Construction vol 17 no 5pp 641ndash650 2013

[12] A A B Mostafa and A G Razaqpur ldquoA new CFRP anchorfor preventing separation of externally bonded laminates fromconcreterdquo Journal of Reinforced Plastics and Composites vol 32no 24 pp 1895ndash1906 2013

[13] S Ghasemi A A Maghsoudi H A Bengar and H RRonagh ldquoFlexural strengthening of continuous unbonded post-tensioned concrete beams with end-anchored CFRP laminatesrdquoStructural Engineering and Mechanics vol 53 no 6 pp 1083ndash1104 2015

[14] Y J KimM F Green andRGWight ldquoEffect of prestress levelsin prestressed CFRP laminates for strengthening prestressedconcrete beams a numerical parametric studyrdquoPCI Journal vol55 no 2 pp 96ndash108 2010

[15] Y Kim K Quinn W M Ghannoum and J O Jirsa ldquoStrength-ening of reinforced concrete T-beams using anchored CFRPmaterialsrdquo ACI Structural Journal vol 111 no 5 pp 1027ndash10362014

[16] J Michels E Martinelli C Czaderski and M MotavallildquoPrestressed CFRP strips with gradient anchorage for structuralconcrete retrofitting experiments and numerical modelingrdquoPolymers vol 6 no 1 pp 114ndash131 2014

[17] M Q Zhu H Pan F J Wei and Q Liu ldquoAn experimentalstudy and analysis of full process of external prestressing CFRPconcrete box girderrdquo Applied Mechanics and Materials vol 351-352 pp 1008ndash1013 2013


[18] G Sakar and H M Tanarslan ldquoPrestressed CFRP fabricsfor flexural strengthening of concrete beams with an easyprestressing techniquerdquoMechanics of Composite Materials vol50 no 4 pp 537ndash542 2014

[19] W Sun and W M Ghannoum ldquoModeling of anchored CFRPstrips bonded to concreterdquoConstruction and BuildingMaterialsvol 85 pp 144ndash156 2015

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


processes [3 5] According to the literature 95 of theCFRP sheets and plates are used to strengthen and reinforceexisting structures worldwide while CFRPs are directly usedas structural materials only in trial experimental studiesTherefore based on their price advantage the utilization ofthe economy and excellent properties of fibre sheets or platesas well as the development of new applications of fibre sheetsor plates have high potential economic value

The steel plate bonding and reinforcement techniqueis commonly used to strengthen reinforced concrete (RC)structures However steel plates are heavy and difficult totransport in addition the cutting and moulding of steelplates are also relatively complicatedThese shortcomings canbe avoided by using fibre sheets or fibre plates which arelightweight easy to machine and have a very high uniaxialtensile strength [6ndash9] Since the 2000rsquos First Symposium onFibre Reinforced Plastic Concrete Structures the research onthe application of FRPs in civil engineering and constructionin China has reached an unprecedented scale Based onthe requirements of the China Association for EngineeringConstruction Standardization the National Industrial Build-ing Diagnosis and Retrofit Engineering Technology ResearchCentre of China (the Central Research Institute of Buildingand Construction the Ministry of Metallurgical Industry)the Sichuan Institute of Building Research and other relevantunits have jointly compiled the Technical Specification forStrengthening Concrete Structures with Carbon Fibre Rein-forced Polymer Laminate (China Engineering ConstructionStandardization 1462003) [10] A new revised version of thistechnical specification is currently being prepared

CFRPs are brittle materials therefore it is necessary toconsider the requirements of structural strain during thedesign processThe carbon fibres used in structures generallyhave a strain of 12ndash15 which is smaller than the strainin a steel strand (6) thus such carbon fibres completelysatisfy the basic requirements of structural deformation andcracking and signs of structural damage or overloadingcan be observed Similar to concrete CFRP can also failfrom creep rupture under sustained high-loading conditionsWhen the sustained load exceeds 75ndash80 of the ultimatetensile strength the life of the CFRP will be affected [11ndash13]Therefore the long-term load on a steel tendon is limitedto 50ndash60 of the tensile strength of the tendon understatic loading conditions to reduce the permissible value ofthe prestress and the applicable sustained load Howeverthis limitation is not an obstacle during the design processbecause when the stress on CFRP tendon reaches 60 ofthe tensile strength of the CFRP tendon under static loadingconditions or lower the CFRP tendon still maintains its totaltensile capacity and the reserved 40 of the tensile strengthcan then be used for bending deformation and fatigue loadafter the initial stress occurs

Even though high tensile strength and moderate elasticmodulus CFRP tendons are suitable for prestress CFRPtendons are still disadvantageous because they are hetero-geneous and brittle The heterogeneity may cause a CFRPtendon to easily fail when used to apply a prestress toan anchor To date no anchoring system has been devel-oped that can ensure that CFRP tendons reach the optimal

level of stress transfer [11 14ndash17] Therefore developingan anchoring system that can ensure that CFRP tendonsreach the optimal top stress transfer that is developingthe anchorages required when using CFRPs as prestressingtendon material has become one of the focuses of theresearch using CFRPs as prestressing tendons [18 19] There-fore combining the mature CFRP sheet material techniqueand the prestressing technique provides new applicationmodes and ideas for CFRP materials and the prestressingtechnique which is the research objective and goal of thisstudy

2 Material and Methods

21 Method for Applying a Prestress to a CFRP Sheet If therotation of a paper spool is hindered when the paper on thespool is pulled the gap between the layers will be compressedand the paper will be elongated during the process until thegap between layers is completely compressed When pullingthe paper the pressure on the paper will gradually decreaseif the layers of the paper are maintained in parallel Thebalancing force to the pulling force is from both the tensionon the topmost layer of the paper and the friction betweenthe topmost layer of the paper and the layer of the paperimmediately below the topmost layer of the paper This waythe direct shearing on fibres that occur when using otheranchoring methods such as the anchoring method that usesclips can be avoided and the brittle failure of fibres can beavoided

Inspired by these paper spools in this study CFRP sheetis clipped onto a cylindrical drum and the CFRP sheetis then wound using the friction between layers (119879) tocreate tension in the CFRP sheet during the process whilethe corresponding length of the CFRP sheet continuouslydecreases The anchoring force of the CFRP sheet materialis the sum of the friction between the rotating drum and theCFRP sheet (119873) and the self-resistance (1198791015840) that is1198791015840+sum120591 =119879 (Figure 1) The two layers of the CFRP sheet are the sameand there is adequate contact area between the two layersTherefore compared to traditional anchoring methods suchas the anchoring method that uses clips the shearing forceof the anchorage generated using this method is relativelydispersed and does not result in the failure of the fibres byshear forces

Figure 1 summarizes the initial idea of this study Exper-imentation is required to verify the feasibility of this ideaThe authors of this study and the researchers at the OVMResearch Centre on Prestress at Tongji University jointlydesigned the experimental frameWhen designing the exper-imental frame we considered the effect of the deformationof the steel drum used for winding the CFRP sheet on theprestress loss of the CFRP sheet and the jack-up constructiontime required to reach the curing age needed for the requiredstrength of the concrete as well as how to overcome theeccentricity of the CFRP sheet relative to the rotating drumduring the winding process Tension is increased usingjacks on both sides The jack-up tension is checked usingthe pressure sensor located at the front of each jack Toperform the experiments a rotating drum device is required


T

The reversed torque

The roller shell


T

The inner layer

The outer layer


120591

T998400

N

T













119910119888) in any of




Productthickness





119910119888= 3550MPa




119910119888= 08 times 3550 = 2840MPa

























6020 80

15 105 105 152040

600

1515


16 times 120

6times95









force is 119873 = 065119891119910119888119860119875= 118 times 10





2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

















1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




T

The reversed torque

The roller shell


T

The inner layer

The outer layer


120591

T998400

N

T













119910119888) in any of




Productthickness





119910119888= 3550MPa




119910119888= 08 times 3550 = 2840MPa

























6020 80

15 105 105 152040

600

1515


16 times 120

6times95









force is 119873 = 065119891119910119888119860119875= 118 times 10





2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

















1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





Productthickness





119910119888= 3550MPa




119910119888= 08 times 3550 = 2840MPa

























6020 80

15 105 105 152040

600

1515


16 times 120

6times95









force is 119873 = 065119891119910119888119860119875= 118 times 10





2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

















1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






















6020 80

15 105 105 152040

600

1515


16 times 120

6times95









force is 119873 = 065119891119910119888119860119875= 118 times 10





2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

















1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




2040120

600

15

2060

120 120 120 120105

100

80

120120 120 120 120 120 120 120 120 120 105 15

100

100

5010

050

100

















1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






1020


balsCR

balsCRP

PRFC


Con

cret

e str

ain

gaug

e


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


Displacement (cm)

Load

(kN

)





4 Analysis









0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0 5 10 15 20 25 30 35 40 45 50 550

500

1000

1500

2000

2500St

rain

of s

teel

bar

Load (kN)


0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

Stra

in o

f ste

el b

ar

Load (kN)


0 5 10 15 20 25 30 35 40 45 50 55minus1200

minus1000

minus800

minus600

minus400

minus200

0

200

400

600

Stra

in o

f con

cret

e


Top1Top2Top3


0 5 10 15 20 25 30minus600

minus500

minus400

minus300

minus200

minus100

0

100

Stra

in o

f con

cret

e

Load (kN)


Top1Top2






5 Conclusions











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials











References




























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Research Article An Experimental Study on Concrete Flat...

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