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Title Influence of Existence of Construction Joints on Fatigue Behavior of Reinforced Concrete Beams

Author(s) Yokomichi, Hideo

Citation Memoirs of the Faculty of Engineering, Hokkaido University, 13(Suppl), 75-82

Issue Date 1972-05

Doc URL http://hdl.handle.net/2115/37902

Type bulletin (article)

File Information 13Suppl_75-82.pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

75

Japan-UAugustSeattle

,S. Science23--27, 1971, U･S･Ae

Seminar

INFllUIENCE

OF

OF EXISMENCE ON IEiAM:GUIE REIN[FiORCED

OF CONSMRUCTION JOINTS BEHAVIORCONCRETE BEAJM[S 1 By Hideo YOKOMICHX

Synopsis

Results of fatigue tests on T-bearns having a span of 3m,a reinforcing deformed bar of 22mm diameter with a speeial typeof coupler joint and two construction joints of concrete werediseussed and compared with those of monolithic beams. It wascertified that construction joints of concrete had not,any effecton fatigue behavior Qf reinforced concrete beam.

Keywords: £atigue, crack±ng, flexure, beam, reinforcedconcrete, deformed bar, precast concrete, conerete joint,

le In±roduction

One of the most important probMems to be solved in designand construction of reinforced conerete beams assembled of precastelements is the influenee of existence of jo±nts on fatiguebehavior of beams eoncerning flexural strength and crackingbehavior under repeated loads. This problem contains two items;the one eoncerns the eonstruet±on joints of eonerete and theother the joints of reinforcing bars. The present paper deals with a discussion on the former itemon the basis of the resul.ts of both tests previously reportedand lately condueted by the author's team.

2. Fatigue Behavior ofMonolith±e Reinforced Concrete Beams

About ZOO reinforced eoncrete beams having T--shape crosssections of 28 x 30cm, a span of 300 em and reinforcing deformedbars were tested under repeated loads with less than two millionpvLlsations at a rate of 250 eycles per minute (1)(2)(5). Theresults shows that, rkn case of under--reinforced beams, the failureof beams is always eaused by fatigue fracture of reinforcementsand that the inerease of eraek openings and deflections hardlyoecunes by dynamic Zoads, when steel stresses are under O.8 timesyielding point of the reinforeing steel, even when the initialeraek openings were O.5mm. Mg.l shows a typical test result(l).

Z. ProfessorHokkaido

of Civi: Engineer±ngsUniversity, Japan.

Faeu]mty of Engineering,

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Fig.1 Result of fatigue test on monolith±c

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A more precise investigation on reinforced eoncrete beams(4).shows that a slight increase of crack widths is obsevved underrepeated loads (Fig.2). This may be understood as folZows: A erack width is generally expressed by

,,.=･-((i.i,}o --- ifli£)L

where 6ke == steel stress in the cra' eked seetion, (SE.= meam stress of surrounding concrete in a given section, ew.= mean value of 6;an over the length Z, pt = ratio of cross-sect±onal area of the bar against that of concrete, L= craek spacing. Sinee CiZ.is the stress which is transmitted by the bond fromsteel to conerete, the increase of i" may be caused by gradualrelaxation of bond stresses which does not necessarily resuZt ina detrimental fa±:ure of the bond, because the static Mexuraltest after fat±gue test shows that the bond stresses are restoredat higher Zoads than that in the fatigue test as shown in the-f1gu]'e.e

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5o FatigueHaving

Behavior ofConstruct±on

Re±nforeed Conerete BeamJoints of Concrete

Tests on the influence of existence of eonstruction joints ofeoncrete on fatigue behav±or of reinforeed concrete beams werecarried out by the author's team as follows:

]!LATERXA]LS AND MEST PROeEDURE

Mhe ordinary Portlaxid cernent was used. Seashore sand of2.17 £±neness modu Lus and river gravel of Z5mm maximum size wereused.

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under the standard curing. The reinforcing bars used were oftwisted deformed ones as shown in Fig.5, 22mm in diameter and ofnominal yield point of 40kg/mm2.

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The dimensions of the test beams are shown in Fig.4. Mhe loadswere applied to the beam at third points. All the beams had aFY-coupier joint for reinforcing bars at midspan. The EY-coupleris a special type of screw-threaded coupler (Fig.5) which wasdeveloped by the author's team and experimentaUy tested to securethe transmission of the entire strength of the bar to be joined(5).

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Fig.5 Fy-coupler

A total of 8 beams were tested, arnong which 5 bearns had twoeonstruction joints of eonerete as shown ±n F±g.4 and the othershad not any concrete joint.

Bearn tests were carried out by Lohsenhausen type fatiguemachine at rate of 250 cycZes per minute. In the fatigue testthe ratio of minimum Zoad to maximun Zoad was set to O.3. Themaximum range of loads was set to a magnitude which caused initialcrackings of O.2mm or more maximum width.

TEST RESULTS AIND DISCUSSIONS

A view of typieal state of crackings of beam whieh had con-struetion joints of concrete and the variation of crack widthsunder repeated loads were as shown in Fig.6 and those of beamwithout construction eoncrete joint were as shown typicaUy inFig.7. These figures show that the line of construction joint did notalways coineide with one of cracking lines, and. that the faUureof beam took place through one of eracking lines of larger widthwhich did not coincide with any of the joint lines, and moreoverthat the increase of crack openings was hardly observed irre-speetive of existence of construetion joints of eoncrete. The faUu]re of all the beams was caused by fatigue fractureof reinforcing bars, this being the same as in ease of monolithicbeams previously mentioned.

The test re6'ul-ts were plotted all together as shown in Fig.8showing relation between maximum loads in steel stresses 61 andnumber of cycles at fatiguLe failune of beam. It can be seen fromthis figutre that all the plotted points fe!l approximately on asame curve irrespeetive of existence of concrete joints. Thisshows also that the existence of construction joints of concretehad seareely affected fatigue behavior of reinforced concret beams.

CONCLUSIONS

Zn case of reinforced conerete beam where a beam consists ofprecast elements having deformed bars as reinforcement, the ele-ments are joined by connecting bars by meams of secrew-threadedcouplers which are statically strong enough to secure the trans--misBion of the entire strength of the bar to be joined, and eachgap between two adjacent elements is filled up by pouring conereteof the same quality as that of precast concrete. the existence ofeonstr"uction joints of concrete has seareely affected fatigue be-havior of beam concerning flexural strength and cracking underrepated loads with Less than two million pulsations.

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on tatigue behavior of reinforced eonerete beam

' ' References (J: im Japanese)

H.Yokom:chi:･ On ･New DeveZopment of Reinforeed Concrete andPRe System, (J), Hokkaido Branch of Japan Soeiety of CiviZEngineers, Feb, Z964gysgge;g¥g･,gdnglgjksa,.il{.ls.zaNg' yhsg2glfi,,,Og.¥g??ffyg Rggg¥zgr,,

onv= Engeneering Research Xnstttute, Hokkaido DevelopmentBureau, No.37, Oct. Z965.'Y.Kaknta: EMndamental :nvestigation on Cracking and DeformationBehavior of Reinforced eoncrete Beams, (J), D±ssertat±on,Yokomiehi Laboratory, FacutLty of Eng., Hokkaido University,Dee. I967.H.Yokom±chi: Prestressed Rein£orced Concrete system, Preli-ntnary Pub:ication of 8th Congress of ZABSE (New York) MhemeZVbeZ968eH.Yokomiehi and Y.liLuLjita: Tests on FY--coupXer Joints o£Deformed Bars, (J), Yokomichi Laboratory, FacuLty of Eng.,Hokkaido Univ., May :965

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