Mechanism Analysis of Helical and Twisted Reinforcement Mechanism Analysis of Helical and Twisted Reinforcement

Mechanism Analysis of Helical and Twisted Reinforcement

  • 期刊名字:武汉理工大学学报(材料科学版)(英文版)
  • 文件大小:406kb
  • 论文作者:ZHANG Junlin,ZHOU Bo
  • 作者单位:School of Materials Science and Engineering
  • 更新时间:2020-11-22
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论文简介

24Vol.23 No.1 ZHANG Junlin et al: Mechanism Analysis ofHelical and Twisted .DOI 1007/5115-06 1024-yMechanism Analysis ofHelical and Twisted ReinforcementZHANG Junlin, ZHOU Bo(School of Materials Science and Engineering, Tongji University, Shanghai 200092, China)Abstract: Although helical and twisted reinforcement has been used to reinforce concrete for morethan two decades, its rationale still remains unclear. With a brief review of current rescarches on the helicaland twisted reinforcement properties, this paper describes some new phenomenon of the helical and twistedreinforcement in concrete and other matrix by experimental studies, and then discusses on mechanism ofhelical effect of strengthening . This paper also discusses the mechanism of accessional helical effect ofstrengthening and its significance in industrial practice. Extensive tests indicate that twisting is the mosteffective way to improve reinforcement mechanical properties. The main results are: (1) They can greatlyenhance bond anchorage in base material. In some pull-out tests, the pull-out resistance increases withreinforcement slip within the specimens, which results not only in a higher pull-out load but also a larger slip upto 70%-80% of reinfrcement embedded length. (2) Concrete renforced by twisted bars demonstrates certainductility at failure. (3) The bond strength depends on the pitch space directly. (4) The twisted effect on materialstrengthening is from a three-dimensional interlocking force which is formed from material untwisting whenthey were pulled out from base specimens.Key words: twisted reinforcing materials; helical and twisted ffect; structural ductility1 Introduction2 ExperimentalAt the end of the seventies of the last century,2.1 Previous work: helieal effect ofcold-rolled and twisted bars were developed in China.straightening studied by nsSoon afterwards, many kinds of similar reinforcementThe phenomenon of helical-straightening and itsmaterials were developed, such as cold-drawing anduses are very common in daily life. As everyone knows,spiral bars, helical ribbed bars, cold drawn and helicalthe surface of the nail is smooth, but when rolled andbars, twisted fiber, etel". The same characteristics oftwisted along their longitudinal axis, the nails form ribsthese materials are their helical surfaces. Therefore,along their surfaces that improve their gripping (pull-outthey can be called in general as helical and twistedresistance) to a similar extent that a screw grips betterreinforcement. The previous industrial application and than nails in practice.structural experiments presented that the reinforcementstrength and its bond anchorage with concrete are bothFig.1 Figure of cold-olled and twisted barsenhanced by the twisting of the reinforcement, whichin turm greatly increased the bearing capability of theHowever, the mechanism of twisted bar isreinforce concrete components. But, the mechanismdifferent from that of a conventional screw. A lot ofof twisted reinforcement with concrete has remained different properties and outstanding performance fromunclear. By studying the results from the experiments,the twisted reinforcement with concrete have beenthis paper discusses some outstanding performance studied. In the past ten years, mechanical propertiesfrom the twisted reinforcement in comparison toon thousands cold-rolled and twisted bars (theiruntwisted reinforcement. Based on the discussion, this cross-section are rectangle as shown in Fig. 1), bondpaper develops the mechanism of twisted effect ofanchorage properties on hundreds specimens andstrengthening.stnuctural properties of several tens concrete structuralspecimens had bpen , inveetioaterd,2.2 H中国煤化工forcing bar(Received: March 23.2006; Accepted: Oet.10,2007)ZHANG Junlin(张钧林): Prof; Ph D Supervisor; E-mail:ItYHC N M H Grking stress ofzilongj@163.comtwisted reinforcement can reach 95% of their limitFunded by the National Natural Science Foundatin of China No.strength from our experiment. As a comparison, the50578119)working stress of deformed bars can barely reachJournal of Wuhan Universiy of Technology-Maler. Sci. Ed. Feb. 20082556% of their limit strength. Table 1 shows the testTable 309 shows the cracking results on testedresults from a group of reinforced concrete beams beams reinforced with different bars before failure. Itwith concrete strength 24-29 MPa and reinforced with is observed that the beams reinforced with cold-rolleddiferent reinforcing bars23]. Beside the higher working and twisted bars have more cracks distributed withinstress level, twisted bar reinforced concrete beams whole pure flexure section of the beam, which is verydemonstrated some properties only the prestressed beneficial to the structure.concrete structure can reach, such as higher ductilityThe enlargement of crack distribution rangewith the width of the maximum crack greater than 1.5 improves the structural internal stress distributionmm, or the deflection reached one fiftieth span length and stress concentration. The increase of the crackat failure as shown in Fig.2Hl.quantity has postponed the appearance of the width ofAs shown in Table 2, after cold-working, thethe largest crack, which enhances the ductility of theelastic modulus of both helical and twisted bars and structure.deformed bars have reduced slightly within the values2.4 Unique bond anchorage properties oftheproposed by design code, which is conformed to be the twisted barsbasic principle of material science.To find the mechanism which resulted in aboveThe phenomenon that the helical and twisted mentioned phenomenon, extensive bond anchorageprocessing on reinforcing bar surface unexpectedly tests have been carried out in Tongji Universityenhanced the working stress to 95% of their limit to evaluate the twisted bars and compare theirintensity and doubled the cofficient of weight bearing performance to other kinds of reinforcing bars. Somestrength can be called as the acessional strengthening typical results are shown in Fig. 3.effect .The tested value of bond strength of cold-rolled2.3 Dense and slender eracks appeared on and twisted bars is improved as much as expected.test specimensOnly 1.5 times of that of plain bars were reached byTable 1 Flexural test results of reinforeed concrete beamType ofbarsWorkingLimRatio of working stressCoefficientnt ofbearingType of failurestress/MPa strength/MPato limit strengthcapability examineddeflection reachedcold colled twisted61565095%1.6-2.0one fieth span lengthdeflection reached onedeformed33057556%1.6fiftieth span lengthhot rolled Plain21037057%.3Table2 Comparison of mechanical properties of reinforced barsTensile strength(N/mm2)Extension Elastic modulusTypeCode rferedratio/%E,(N/mm')Standard valueDesigned valuePlain bars(HPR235, Q235)≥3758。≥23"2.1x10'(GB 50010-202)"Helical and twisted bars(type I)≥5803608≥4.51.9x 10'(JGJ 95-95)"Deformed bars(LL550)≥550δw≥81.9x10'(JGJ 115-97)“Table 3 The comparison of cracks occurred in beams reinforced by different barsThe configuration of barsThe condition of cracksBarsReinforcementPitch/cmLocation onAverageDistributionQuantityMaximumspecimensdistance/mmrange/mmwidth/mm2φ'10 cold-rolledfront side中国煤化工90.6014.40.06and twisted barsback sideTYCNMHG1317.25:2中'6.5 plain bars0.10526Vol.23 No.I ZHANG Junlin et al: Mechanism Analysis ofHelical and Tyisted ..40r66}30Col-rolled and\Etwisted barsDeformed bars昌2Plain bars .10,4)30405060Slip/mmsio/m(a) Plain bars(b) Deformed bars (c) Col-rolled andFig.2 Load-deflection comparison of the testedbeams with different barsFig.3 Pull-slip curves of the tested barstwisted deformed bars.Some conclusions can be withdrawn from the testObviously, there must be some other mechanisms results: (1) twisted bars have its special mechanicalmore than the bond strength in improving concretefeatures when working with concrete together; (2)structure properties. This kind of comprehensive result twisted bars can not be treated in the same way as theis helical effect of strengthening .other bars in design and construction; (3) the helicalFig. 3091 compares the pull-out load versus slipsurface figure has not only improved the bond strengthresponse of plain, deformed and cold-rolled and twistedbut also introduced a new functionality which changesbars. The curve is drawn by the x-y recorder during the the structural failure mode or process and thereforetests by authors, namely pull out-slip (P-s) curve. Curveimproves the performance of the structure.(C) from helical and twisted bar with a second peakreflects the essential difference to other bars.3 Results and DiscussionIn Fig.3, (OA) range represents adhesion stage;(A-B-C) range represents friction and mechanical3.1 Discussion on mechanism of nccessionalbond anchorage stage; and (C-D-E) is helical surface helical effeet of strengtheningresisting stage. p-s curve of cold-rolled and twisted3.1.1 Analysis of mechanical bond of twisted barsbars displays a big difference from the others: after theExtensive pull-out tests conducted by authorsfirst peak, the curve descends a lttle bit, and then riseson the reinforced concrete specimens with differentto the second peak (point D) which is even higher thantwisted shaped bars indicated that twisting is the mostthe first one. Afier the second peak, the curve descends effective way to improve reinforcement bond strength.slowly again (point E).The second peak appeared in p-s curve duringpull-out test indicates that the key feature of twistedrocted zotbars is that when the twisted bar is pulled out from aconcrete matrix, its resistance increases with its slipincrease. This feature results in a relatively higher pull-out resistance up to very large slips which is aboutFig.4 Occlusion condition case and mechanical analysis70%-80% of embedded length. The more it slips, theAs we know, the reinforcement bond strengthhigher its resistance is formed. The pul-out resistanceis composed by adhesion, friction, mechanical andof the twisted bars increases the average bond strength interlock actions. A smooth reinforcing bar has a weekwhich in turn leads to a direct increase of the post-adhesive and frictional bond strength resisting its pullcracking strength of the composite, the specimen out fom the concrete. When it slips out, it results in atoughness which results in ductility destruction and thelimited damage to the surrounding concrete. If the barenergy absorption capacity as shown from the total area is replaced by a twisted bar, not only the mechanicalbetween curve and abscissa.anchorage (grip) can be greatly increased, but alsoAnother important phenomenon observed in the cause an untwist torsion effect while the bar is beentesting is that when the twisted bars were pulled outpulling out from the conerete (Fio 41 is generated. Thisfrom the concrete specimens, a torsion effect appeared untwist中国煤化工:chanism on the .on the specimens. The same effect was also observedreinforeIYHCNMHGdescribedasanby authors in the pull-out tests with smooth and helical additional nterlock mecnanism.twisted fiber steel and organic glass, and was reportedIn other words, when the twisted bar is pulled outby other researcherslol.from the concrete, the pull and untwists effect forms aJourral of Wuhan University of Technology-Mater. Sci. Ed. Feb. 200827three-dimensional mechanics (Fig, 4) on the reinforcing curve. By matbematical analysis, it is easy to find thatbar. There, untwisting greatly consumespart of the the volume of concrete cylinder is increased with thepull-out force, and increases the resistance to the pull increase of the difference between a and b. That is toout strength which is the reinforcement bond strength.say, if the rib space is kept unchanged, the level of theGuan Pinwul"l and other researchers have second peak on p-s curve is determined by the crossobserved the untwisting during their pull-out tests section shape of the helical and twisted bars. The largerbefore, and called it as anti- symmetry trigonometrythe difference value between the long side b and thefunction. By spltting the failed specimen, they found short side a of the cross section, the higher the secondthat near loading side of the bar, there are concrete peak and bond anchorage ductility.powders and remarkable slipping marks along the first 3.1.3 The examination and analysis of the pitch spacehalf of the bar, which indicates that it is this anti-Another essential parameter on helical and twistedsymmetry trigonometry function that keeps the bond reinforcement is the space between the pitches, or instrength higher.other words, is the number of ribs or twists per unitR Gustavson!2l and Antoine E Naaman!3l also length.explored this mechanism. R Gustavson explained thisIn the course of pull out test, the bars have thephenomenon by an inconsistency in bar: if the surface tendency of moving at the direction of pulling, andis not perfectly constant, the bar does not ft perfectly in exert the pressure to the front side concrete of the pitch.the concrete channel during increased slippage which At the mean time, the concrete prevents the movementis the case for plain bars. R Gustavson called it the of bars and, in tumn, exerted the counteracting pressurelack-of-fit efect'"." Antoine E Naaman described itas a to the bar surface. Because there is an angle betweenrepetitive stick-slip mechanism and it was generatedhelical ribs and axis, the pressure may be decomposedby the untwisting wave that travels along the embedded into a force component in the opposite direction oflength of the fber when it was pulled out.pulling. Thus, the bond strength between helical and3.1.2 Analysis of the cross section shapetwisted bars and concrete is enbanced. Therefore, ifA concrete specimen reinforced with a twisted the pitch spacing is increased, or in other word, thebar under pull out test can be divided into three layers number of ribs or twists is reduced, the opposite forcefrom inside as shown in Fig.4. The most important componeht is also reduced, then, the bond strength islayer affecting the pull- out strength which forms the reduced. That is, the longer the pitch space, the lowersecond peak on the p-s curve, is the layer of concretethe bond strength is.surrounding the twisted barl4l.The equations which describes the relationshipWith a rectangle cross section, the vertical between the ribs space and the first peak value or theprojection of a twisted reinforcement is a rotundity second peak can be presented ashaving a diameter of (a2+b)', where, a and b representr-=-0.015l+5.89(R-=-0.19)(1)the width and the length of the rectangle respectively.-=-0.0151+10.74(R=-0.63)(2)While the twisted is been pulling out, a cylinderwhere, R is a relevance cofficient.composed of concrete and twisted bar is formed inEquation (1) and (2) are got by the linearthe close contact region between helical surface andregression of test results respectively, as shown in Fig.concrete. The volume of concrete in this cylinder 5 and 6. The concrete strength used in these tests is 30affects the slip resistance of the twisted bar, which MPa. All tested specimens were reinforced with thefinally affects the level of the second peak on p-s same twisted bars having a cross section of 10 4 mm3之中国煤化工57090110 130150170MYHCNMHG-1301soThe pitch space/mmFig.5 Relationship between the pitch and ribFig.6 Relationship between the pitch and ribspace and the first peak valuespace and the second peak value28Vol.23 No.1 ZHANG Junlin et al: Mechanism Analysis of Helical and Twisted一processed by plain reinforcement in 8 mm diameter.higher the second peak and the better bond anchorageComparing the test results in Fig.5 to Fig. 6, we ductility.find the pitch space has more significant effect on thed) The pich space, or the number of ris per umnitsecond peak than that on the first peak. The cause is length along a twisted bar resulted from the degreethat when the first peak is reached, the bar slip is still of twisting is another essential parameter of helicalvery small and the untwisting is not conspicuous yet. and twisted reinforcement, which affects the bondThat is to say, although the pitch space reduction willperformance directly. By deducting the pitch space,increase both first and second peaks, only the second both first and second peak values are increased. But thepeak will be increased significantly.pitch space has less efect on the first peak.3.2 The prospect: the significance of studye) Helical and twisted bar is just one kind of theon accessional helical and twisted effeet of helical and twisted materials. With further studiesstrengtheningconducted in this accessional helical and twisted effectWith further development in high strengthof strengthening area, the engineers wish for a kindconcrete, the higher working stress of reinforcing bars of material with high tensile strength, strain hardeningwithin the concrete is expected, which results in thebehavior, ductile and toughness is not a dream in theexpectation of higher bond strength. In this case, a near future.peculiar mode of anchorage provided by helical andReferencestwisted shape on reinforcement has a great attraction.The helical and twist technology discussed and[1] F K Liang. The Current Situation and Development Prospectf Cold-rolled and Twisted Bars Reinforced Structure[C].illustrated above can be applied to metallic or non-ofcAnthology on Applied Technique of Cold- rlled and Twistedmetallic materials, continuous reinforcing bars orBars, Beijing, 1999discontinuous fibers, frrous or non-ferrous metals, [2] 儿Zhang Experimental Study on Stnctural Perfomame ofCold-rolled and Twisted Bars Reinforced Concrete Slab [C].even anchored bars for post-tensioned in prestressedAnthology on Applied Technique of Cold-rolled and Twistedconcrete.Bars,Beijing, 19993] JL Zhang Deign and Engineering ofCldrolled and TwistedBars Reinforced Concrete Structure[M]. Shanghai: Tongji4 ConclusionsUniversity Press,2000[4] Ministry of Construction of the People s Republic ofChina. Hol-rolled Low Carbon Steel Wire Rods[S]. GB/Ta) Working stresses of twisted bars can reach 95%701-1997,1997of their limit strength. Bond strength demonstrates the[5] Ministry of Construction of the People s Republic ofChina. Code for Design of Concrere[Srructures[S].GBcharacteristic of ductility. The cracks on the cracked50010-2002002beams are thin and dense. The beams deform greatly [6] Ministry of Construction of the People s Republice of China.Technical Specification for Concrelep Structure Wih Cold-before failure. The helical and twisted fbers have theroller and Twisted Bars[S] JGJ 115-971997same performance as what helical and twisted bars[7] Ministry of Construction of the People s Republic of China.Specification for Concrete Structure With Cold-roller RibbedSieel Wires and Bars[S]. JGJ 95-95,1995b) The test on the pull-out load versus slip[8] J L Zhang. Study and Discussion on the Anchorage Propertyresponse with different shaped twisted bars indicatedof Reinforced Concrete[J]. Concrete and Cement Products,1998, (4):30-35that twisting is the most effective way to improve the9] J L Zhang. Experimental Study on Helical Effect of themechanical component of reinforcement bond. TheSurface of Col-olled and Twisted Bars[J]. Concrete andbond strength resists a great portion of the pullout loads[10] Y L xU, L X LiU. Experiment and Research in the BondCement Products, 1993,(6):51-54with a very large bar slip which is about 70%-80% ofAbility of Cold-rolled Twisted Bars[J]. Architectureits embedded length. When pulled from the concrete,Technology, 1998,(8):538-540helical and twisted material bears pull and untwist[11] P W Guan. Experimental Study on Mechanical Propertyand Bond Property of Cold-rolled and Twisted Bars[D].which results in a three-dimensional mechanics definedZhengzhou: Zhengzhou University, 1997as an interlock mechanism. The untwisting consumes[12] R Gustavson. Experimental Studies of the Bond Responseof Three-wire Strands and Some Influencing Parameters[J].the pull-out load and increases the energy consumption.Materials and Structures.2004.(31:96-106c) With a constant noumber of ribs per unit length中国煤化工Fibers with Optialalong a twisted bar, the second peak on p-s curve islent Composites[J].affected by the cross section shape of the helical and[14]JoYHC N M H Gy, 20.1)241-251cnang,」L Clnang. DUIu AlCUIage of Spirilly Twistedtwisted bars. The larger the difference value betweenBar with Different Crossection[J]. Journal of BuildingMaterials, 2005, (3): 256-260its long side b and the short side a of the section, the

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