Engineering Structures 27 (20

a

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even older; thus reinforcement consists of smooth rebars, since only in the 1970s did early applications of deformed rebars appear. Technical

literature on mechanical performances of anchored smooth rebars is non-comprehensive, mainly from the deformation standpoint, despitethe relevance of this aspect to the response of critical regions, i.e. beam to column joints and column bases. In the present paper a series ofexperimental tests on smooth rebars are presented; they are aimed at describing in detail the forceslip relation for the bond mechanism forstraight rebars and for anchoring end details, i.e. circular hooks with a 180 opening angle. 2005 Elsevier Ltd. All rights reserved.Keywords: Old type r.c. constructions; Seismic assessment; Smooth reinforcement; Anchorages; Bond

1. Introduction

The first step in upgrading strategies for addressingexisting reinforced concrete (r.c.) structures is the assess-ment of seismic performances of materials and structuralsystems. In fact, many existing constructions in seismicareas have been designed only for gravity loads or accord-ing to outdated seismic rules, resulting in low availableductility and lack of a strength hierarchy. The measure ofglobal ductility for framed structures is the interstorey driftratio, that for reinforced concrete frames is dependent upondifferent contributions like the beam plastic rotation, thecolumn flexural behaviour and the beam to column jointregion deformation [1]. The latter is generally divided intotwo components related to shear deformation of the panelzone and to fixed-end rotation that is predominant in under-designed structures and depends on the bond properties of

deals with the smooth reinforcement widely used up to the1970s in a very large number of existing constructions;they exhibit poor bond performances resulting in mandatoryanchoring end details able to ensure the required level ofinteraction. Thus, the behaviour of anchored smooth rebarsis a key issue in the development of reliable procedures forthe evaluation of available bearing and/or displacement ca-pacities of buildings. In fact, advanced structural analysesare able to take account of actual element dimensions,critical details, laboratory and field test data on the concreteand reinforcement, as shown in [3]. In the following,the results of an experimental evaluation on the behaviourof straight smooth rebars and 180 circular hooks are dis-cussed; some interesting features of the structural responseunder service loads and in the large post-yielding field arepointed out.Experimental behaviour ofold type reinforced

Giovanni Fabbrocinoa,, Gerardo MaDepartment S.A.V.A., University of Molise

bDepartment of Structural Analysis and Design, UniversityReceived 1 December 2004; received in rev

Abstract

Modelling of existing reinforced concrete (r.c.) frames designed wupgrading and seismic assessment. In many European countries a verreinforcement and anchoring devices [2]. The present paper

Corresponding author. Tel.: +39 0874 404779; fax: +39 0874 404855.E-mail address: giovanni.fabbrocino@unimol.it (G. Fabbrocino).

0141-0296/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2005.05.00205) 15751585www.elsevier.com/locate/engstruct

nchored smooth rebars inoncrete buildings

. Verderameb, Gaetano Manfredib

ia De Sanctis 86100 Campobasso, ItalyNaples Federico II, Via Claudio, 21 80125 Napoli, Italy

d form 2 May 2005; accepted 3 May 2005

ut specific seismic rules is a key problem for maintenance, structuralarge percentage of reinforced concrete buildings are 40 years old, or1.1. Literature review

Early experimental studies [4,5] were aimed at evaluatingthe effectiveness of end details on smooth reinforcement

rin

le i

experimental works on smooth rebars carried out by [9,10]offered a series of dstudies in the field ofand experimental dataconcrete structures ardevelopment of a relirebars placed in criticcan be found in the frthe performances ofsafe design provisionused as a reference buif a large post-yieldin

This is the case arebars that today areelements [12].

del and the idealisedonents identified foren the smooth rebarsnd the anchoring end

of the pull-out forceof the anchorage; a

eraction mechanismsurrounding concreten has a number ofof the anchored rebarues reliable [14] andpost-yielding field.sults of an experi-ata used as references in many laterbonding. However, available technicalon smooth rebars as reinforcement fore not fully satisfactory as regards theable numerical modelling of anchoredal regions. In fact, the majority of dataamework of studies aimed at assessingdeformed reinforcements and definings [14]; accordingly, smooth rebars aret are not fully investigated, particularlyg phase is considered.lso for more recent research on plaincommonly used for precast concrete

Fig. 1 reports the hooked rebar moconstitutive relationship of the companalysing the problem: the bond betweand the concrete in the straight rebars, adetail response.

The latter can be given in terms(and/or stress) and slippage at the endglobal representation of localised intinvolving the hooked rebar and the scan be obtained. Such an idealisatioadvantages, since it makes the analysisusing consolidated numerical techniqgives stable solutions even in the large

The present paper reports the re1576 G. Fabbrocino et al. / Enginee

Fig. 1. Hooked rebar model and its ro

pull-out. In the same period, the research by Saliger [6]was very interesting due to the large number and varyingkinds of tests; these were aimed at evaluating the strengthof anchorages, without any consideration of performancein terms of deformation. Straight and hooked rebars (180opening angle) were tested; different rebar diameters,curvature radii and transverse rebar arrangements wereconsidered. Results on pull-out tests agreed with Bachstests on beams [4], which exhibited an increased flexuralstrength due to end anchorage and demonstrated that theshape of the anchorage and transverse reinforcement couldgive beneficial effects. Later, research by Mylrea [7] tookinto consideration both the strength and the deformation ofhooked smooth rebars with a 180 opening angle and triedto outline the influence of the end hook radius and transversereinforcement. Results showed a slight influence of thehook radius on the deformation and indicated the role oftransverse reinforcement in the type of failure. In particular,plain specimens showed a non-ductile behaviour by concretefailure, compared to rebar failure induced by appropriatetransverse rebar detailing. Approaching the 1950s, earlytypes of deformed rebars were investigated and comparedas regards the forceslip response to straight and anchoredsmooth rebars [8]. More than forty specimens were tested,varying the hook radius, surface type (smooth or ribbed),development length and opening angle. During the 1960sg Structures 27 (2005) 15751585

n the deformation of critical regions.

In summary, a more comprehensive approach is requiredin order to develop reliable numerical procedures and giveconsistent predictions of the r.c. construction response underseismic actions or of the residual bearing capacity of existingbuildings [11,13].

2. Research objective

A review of experimental analyses aimed at theevaluation of smooth rebars and related anchoring enddetails reveals a lack of data in the large post-yielding fieldresponse.

A number of studies on the subject were carried out, butthis was many years ago, so the limitations of the testingequipment due to the technology available at that time leadto results being non-comprehensive. In fact, it has beenfound that results rarely refer to strain levels beyond yieldingand that the use of equipment under force control preventsthe reporting of descending branches that could be relevantfor modern applications.

From a behavioural perspective, the response of hookedsmooth rebars results from the interaction between twodistinct components: the straight rebar portion, where thebehaviour is basically related to the bond interaction; andthe anchoring device, made of a circular hook, where thespecific rebar shape activates local interaction mechanismsthat involve large volumes of concrete.mental programme aimed at calibrating proper constitutive

truG. Fabbrocino et al. / Engineering S

relationships for anchoring devices; specific tests for char-acterising the two components of the anchored smooth re-inforcement (straight rebar and hooked end) are discussed.These tests represent the experimental background of re-liable models of reinforced concrete joints where smoothrebars are used [15].

3. Test programme

The experimental programme described in this paperconsists of 20 tests with distinct aims:

evaluation of smooth rebar bond properties with threebeam tests and three pull-out tests;

evaluation of the response of hooked anchorages bothin service and at ultimate load with fourteen pull-outtests.

All the tests are carried out on both straight and hooked12 mm rebars. The selection of steel rebars was based onthe mechanical and surface properties of materials used inthe decade 19601970 [16]. The hooked rebar geometrywas defined after a comprehensive review of Italian andinternational design codes and manuals used as reference inthe reference period [17].

In particular, the hook geometry can be described byreferring to two dimensionless parameters: the ratio betweenthe inner diameter of the hook and the rebar diameter, equalto 5, and the ratio between the straight end length and therebar diameter, generally equal to 3.

3.1. Material properties

The smooth rebars used in the context of the present workare hot rolled and classified as Feb22k [16]; in particular,tensile tests carried out on 12 mm reinforcements haveshown a mean yielding stress s,y = 320 N/mm2, initialhardening under strain sh = 3%, ultimate stress s,u =440 N/mm2 and ultimate strain s,u = 23%. Stressstrainplots are reported in Fig. 2(a), where the significant ductilitycan be recognised together with a large strain hardening ratio(1.375).

The concrete has been prepared according to typicalmixing rules of the 1960s [18] and tests on cubes 150 mmwide were used to define the mean concrete strength.

Table 1 reports the concrete mix design data for bothbeam test and pull-out specimens that have been preparedin two distinct phases, characterised by different strengthdevelopments probably due to the different humidity ofthe coarse aggregates. Specimens and cubes for strengthevaluation have been cast together and cured in the sameopen air environmental conditions for 28 days beforetesting.

The beam test specimens exhibited a mean cubiccompressive strength of 34.20 MPa; Table 2. The pull-outtest specimens exhibited a mean cubic compressive strength

of 29.34 MPa; Fig. 2(b).ctures 27 (2005) 15751585 1577

Fig. 2. Steel stressstrain plot (a); concrete compressive strength of pull-outspecimens (b).

Table 1Concrete mix design

Component

Water/cement ratio 0.45Aggregate size (04 mm) (kN/m3) 10.14Aggregate size (410 mm) (kN/m3) 3.13Aggregate size (1020 mm) (kN/m3) 5.16

Table 2Compressive strength of beam test specimens

Cube Cubic strength Cylindrical strength(MPa) (MPa) (MPa) (MPa)1 33.60 26.902 34.00 27.203 33.60 34.20 26.90 27.304 35.70 (2.49) 28.60 (2.54)5 33.60 26.80

6 34.70 27.70

n1578 G. Fabbrocino et al. / Engineeri

Fig. 3. Beam test set-up (a); specimen type (b).

3.2. Test set-up

The beam test has been carried out according to the set-up described in Fig. 3. Specimens were composed of twoconcrete blocks connected by a reinforcing rebar. The loadtransfer is slightly different with respect to standard beamtests [19], since a steel hinged beam, Fig. 3(a), is used toapply the load on the concrete using shear studs. The loadis transferred on each side of the cylindrical hinge locatedon the symmetry axis and the axial load T on the rebarcan easily be evaluated using the equation of equilibriumbetween the moment due to external force and the resistanceone due to tensile stresses in the rebar. The embedmentlength, Lb, is assumed equal to 10; in order to avoidany interaction with surrounding concrete, plastic pipes areused. The embedded length Lb is used to evaluate the bondstress, which is calculated assuming a constant distributionof the bond stress along the rebar. The load on the steelbeam is applied using a mechanical actuator in displacementcontrol; a load cell, inductive transducers and strain gaugesare used to measure the load, slippage and strain of the rebarsrespectively; transducers give the slippage at the loaded andthe unloaded ends of rebar Fig. 3(b).

The set-up for the pull-out tests is shown in Fig. 4;specimens were made of cubes, 300 mm wide, thatincorporate the rebar to be tested. A plastic pipe is usedto avoid interaction between the rebar and the surroundingconcrete except in the embedded zone, 10 long. The testingequipment is completed by a bolted steel envelope thatrestrains the concrete block by means of threaded rebars onthe lateral surfaces, as shown in Fig. 4. It is worth noting that

special care has been devoted to avoiding any tensile forceg Structures 27 (2005) 15751585

Fig. 4. Test arrangement for pull-out type tests on straight bars.

Fig. 5. Test arrangement for pull-out type tests on hooked smooth bars; Fulltype specimen.

in the threaded rebars and keeping the concrete unconfined,without lateral compressive stresses.

Measurements of the tensile force F , of the rebar strainand of the slips between the loaded and the unloaded endsand the concrete are taken; tests are carried out underdisplacement control, so that descending branches can befully detected.

The second phase of the programme consists of modifiedpull-out tests on hook anchorages. The main parametersinvestigated are: the concrete cover; the cast direction; theposition of the circular branch with respect to the topsurface of the specimen. In fact, two types of specimenshave been designed in order to modify the hook concretecover in compliance with the reinforcement detailing in basecolumn/internal beam to column joints and external jointsrespectively.

Therefore, three test set-ups have been considered:

Full type specimens, shown in Fig. 5, that consist of aconcrete cube 300 mm wide and the rebar centred in thecross section; this leads to a significant concrete coverthat can be representative of the above-mentioned basecolumn or the internal beam to column condition.

End type specimens, shown in Fig. 6, that arerepresentative of the typical location of rebars in externalbeam to column joint regions; in this case, the concretecover is 22 mm, and the concrete block has a 180 mmthick by 300 mm wide cross section.

Full-H type specimens; these are characterised by thesame geometry as Full type specimens, but the cast

direction is perpendicular to the rebar and the location of

StruG. Fabbrocino et al. / Engineering

Fig. 6. Test arrangement for pull-out type tests on hooked smooth bars; Endtype specimen.

the circular branch with respect to the block top surfaceis changed.

In all cases, the load is applied to the free end of therebar and the reaction force is imposed by an external steelbox restrained to the concrete block using bolts embeddedin the concrete. This specific set-up has been chosen inorder to avoid compressive stresses on the top surface of theconcrete and to fit the real conditions of rebars under tensionin cracked sections. The bolts used as shear connectorsare the only reinforcement present in the concrete blocksand are characterised by zero pre-tension to avoid lateralconfinement of concrete, similarly to the bond tests.

The main aspect of the test set-up is the directmeasurement of the slip at the end section of the anchorage;in fact, interaction of the straight branch is prevented usinga plastic pipe, as shown in Figs. 5 and 6, and the slippageat the end of the circular branch is measured using a highperformance draw-wire displacement sensor. Preliminaryvalidation of measurements taken by draw-wire transducershas been performed in order to avoid incorrect data; the axialtensile force generated by the transducer and the very lowflexural stiffness of the wires used for measurements of theslippage ensured the reliability of the system.

In addition, an extensometer has also been usedthroughout the load process in order to evaluate thestressstrain relationship of each rebar tested. The tests havebeen carried out using a uniaxial testing system able to applythe load under displacement control and measuring the slipof the anchorage inside the concrete block.

4. Experimental results

4.1. Bond test

Due to the nature of the reinforcement and the geometryof the specimen, splitting phenomena did not occur, soconcrete blocks were not damaged macroscopically duringthe tests. Measurements of slippage at loaded and unloadedends demonstrated that the differences between them arenegligible, so the constitutive relationships have been plotted

depending on the unloaded end slip.ctures 27 (2005) 15751585 1579

Fig. 7. Beam test results: (a) steel stressslip plot; (b) bond stressslip plot.

Figs. 7(a) and 8(a) show both beam test and pull-out testresults; evaluation of the bond stress b has been carriedout, depending on the bonded length Lb = 10 and on thetensile reinforcement stress s as follows:

b = s As Lb (1)

where As and are the area and the rebar perimeterrespectively.

Figs. 7(b) and 8(b) show clearly the different phases ofthe interaction phenomenon: adhesion with negligible slips,interlocking with increasing slip up to a peak value and thena friction based residual stress.

The mean value of the bond stress is about 1.42 MPa,corresponding to a slip of about 0.04 mm for beam test typespecimens, and 1.96 MPa, corresponding to a slip of about0.14 mm for pull-out test type specimens; see Table 3 fordetails.

In the same plot the theoretical bond stresssliprelationship suggested by Model Code 90 (MC90) [20] is

also presented; it is worth noting that peak bond stress values

ing1580 G. Fabbrocino et al. / Engineer

Fig. 8. Pull-out test results: (a) steel stressslip plot; (b) bond stressslipplot.

Table 3Results for pull-out and beam test specimens

Specimen Pull-out test Beam testSlip b max Slip b max(mm) (MPa) (mm) (MPa)

1 0.14 2.30 0.05 2.162 0.14 1.90 0.03 1.063 0.15 1.67 0.05 1.05Mean value 0.14 1.96 0.04 1.42(COV) (4.12) (16.26) (28.86) (44.92)

are higher than the MC90 maximum stress, but the lattermatches well with the residual experimental stress.

4.2. Hook test

Pull-out tests on circular hooks in the three differentarrangements (Full, End, Full-H) are reported in the presentsection. The reinforcement has a diameter of 12 mm, incompliance with the previously discussed bond tests. It is

worth noting that Full and End type specimens allow forStructures 27 (2005) 15751585

analysing the influence of the concrete cover on the hookbehaviour, while Full-H tests indicate the role of the concretecasting direction.

Five Full type specimens, described in Fig. 9(c), arefirst examined. In particular, Fig. 9(a) reports the measuredstressstrain relationships of the rebars, Fig. 9(b) representsthe stressslip relationship at the hook end and finallyFig. 9(d) gives the strain versus hook slip relationship plot.

Analysis of experimental results indicates a strongly non-linear behaviour of the anchoring device even for low stresslevels in the rebar; in more detail, an initial high devicestiffness is observed, since zero slips are measured for stresslevels lower than 50 MPa. The slippage of the circular hooktaken at the yielding stress (strain) exhibits a considerablevariability ranging from 0.80 mm to an upper bound of2.55 mm.

The behaviour of the hook in the post-yielding phaseof the rebar is characterised by an interesting phenomenonthat can be recognised with reference to Fig. 9(d). In fact,slippage at the yielding stress remains basically constantover the whole plastic plateau of the rebars and increasesonly when strain hardening starts; this circumstance is alsoconfirmed by a stressslip relationship that seems to becontinuous and does not show a sudden increase at theyielding stress.

This phenomenon is common to all the tests, confirmingthat yielding spreading does not occur along the circularbranch and the interaction is basically governed by amechanical interlock, which leads to the concentration ofthe normal stress at the end of the hook without yieldingpenetration along the curved branch. On the other hand,evaluation of the stressstrain relationship of rebars showthat the plastic deformation takes place over the wholeunbonded straight branch of the rebar.

The strain hardening phase, as already mentioned, is thencharacterised by an increase of the slippage at the hook endactivated by the load increase, even if a progressive reductionof the stiffness is observed up to rebar failure. The slippagemeasured at rebar failure ranges between 2.66 and 7.86 mm.

In Fig. 10(a), a Full type specimen during the testis reported on; Fig. 10(c) indicates the tensile failure ofthe rebar. Both pictures clearly show the devices used tomeasure the rebar strain and the anchoring device slippage.The specimen cross section is then reported in Fig. 10(b) forafter a pull-out test; it is easy to recognise that the unloadedend of the hook is basically affected by slippage withoutvisible concrete damage.

The plots of Fig. 11 referring to the End test series showthat the shape of the curve is similar to the previous ones,but the results are less scattered in terms of the hook slipat yielding; in fact measured values range from 1.62 to2.00 mm. The stress/strain curves of the rebar, however,indicate that a sudden loss of load occurs in the large post-yielding field; this loss is significant since it can reach even60% of the maximum load. This phenomenon is due to a

splitting type of failure that occurs in the concrete cover,

Str

trigas s

Nevstre

f

nFig. 9. Summary of experimental results of pull-out tests; Full type specimens.

Fig. 10. Full type specimen. Set-up (a), final state of the anchorage after the pull-out test (b), anchorage failure (c).

gering a relevant increase of the slip at the hook end,hown clearly by the rebar strainhook slip relation.ertheless, the anchorage is still able to bear tensile

cracking load can be recovered and a progressive pull-out othe rebar develops.

On this subject, it is worth noting that the solutioG. Fabbrocino et al. / Engineeringsses in the cracked state also, so in many cases the pre-uctures 27 (2005) 15751585 1581adopted to restrain the concrete block in End type specimens

in

did nlaterlatiomen

Adiffecrackat ththatby ahooktypesuddbranslipsFig.charwith

Fafter

st;lariceon

ontheng

ofonofilyastedto

in,rt,s);Fig. 11. Summary of experimental results of pull-out tests; End type specimens.

ot affect the response of the hook due to the absence ofal confinement and to the tolerances used for bolt instal-n (parallel to hooks), resulting in free relative displace-ts between the concrete and surrounding steel envelope.

review of experimental tests indicates that threerent responses of the anchoring device in the post-ing phase occurred. The first behaviour is characterised

e crack formation by a sudden loss of bearing capacitycan be estimated as 40% of the peak load and thengradual reloading phase affected by large slips of theup to rebar failure (specimens 1 and 2). The second

of behaviour exhibits, similarly to the previous one, aen loss of load at crack formation, but the reloadingch is not able to trigger the rebar failure due to largeof the hook, so anchorage failure can be recognised in

12(c) (specimen 4). The last type of behaviour is thenacterised by a Full type specimen stressslip response,out any clear loss of load (specimens 3, 5).ig. 12 reports a number of pictures taken during and

shows the final state of a specimen after the pull-out tean estimation of the slip at the unloaded end of the circuhook can be made with reference to the measuring devplaced upon the cracked surface. Furthermore, a comparisbetween Figs. 12(b) and 10(b) indicates a large extensiof the concrete damage in End type specimens due tolocal stresses between the steel rebar and the surroundimedium.

The last set of plots, in Fig. 13, show the responsehooks depending on the casting direction and on the positiof the circular branch with respect to the top surfacethe specimens (Full-H type). The results can be easdivided into two groups, showing the influence of the lparameter on the response of the hooks. In fact, hooks placdownwards are stiffer both at yielding and at collapse duerebar failure.

Table 4 reports slips measured at the yielding stress/strasy , and the one corresponding to the strain hardening stassh , for each group of tests (Full and End type specimen1582 G. Fabbrocino et al. / Engineerthe End type specimen tests. In particular, Fig. 12(b)g Structures 27 (2005) 15751585mean values of the above parameters are also given.

Str

Itchartypearou

ngoferFig. 12. End type specimen. Set-up (a), final state of the anchorage after the pull-out test (b), anchorage failure of specimen 4 (c).

Fig. 13. Summary of experimental results of pull-out tests; Full-H type specimens.

is worth noting that basically End type specimens areacterised by a larger deformability compared with Fullones; the increase of deformation can be estimated as

of concrete, since it occurs in the advanced strain hardeniphase; conversely it can be related to a different levelconfinement of the hook depending on the concrete covG. Fabbrocino et al. / Engineeringnd 20%. This result is not necessarily related to crackinguctures 27 (2005) 15751585 1583thickness.

in1584 G. Fabbrocino et al. / Engineer

Table 4Results of pull-out test: Full and End type specimens

Type Full End

Specimen sy ssh su sy ssh su(mm) (mm) (mm) (mm) (mm) (mm)1 1.58 1.58 4.26 1.96 2.10 2 2.55 2.55 7.86 2.00 2.03 3 1.45 1.45 4.45 1.62 1.71 4 0.96 1.07 3.40 1.86 1.91 5 0.80 0.97 2.66 1.62 1.71 Mean value 1.47 1.52 4.53 1.81 1.89 (COV) (46.73) (41.27) (44.07) (10.08) (9.50)

However, the different evolutions of phenomena inthe strain hardening phase prevents a comprehensivecomparison between slips at failure su , so Table 4 reportsonly the values measured for Full type specimens.

Table 5 provides an estimation of the influence of thecast direction on the response of the anchoring devices. Infact, slips measured at the yielding stress/strain, sy , andthose corresponding to the strain hardening start, ssh , andto rebar failure, su , are reported with reference to Full-H type specimens; mean values of the above parametersare also given. It is easy to recognise that up specimensexhibit a larger deformability compared with down ones;the increase can be estimated as 80% in the case of yieldingslip and about 20% in the case of ultimate slip.

Table 5Results of pull-out tests: Full-H type specimens

Type Full-H (down) Full-H (up)Specimen sy ssh su sy ssh su(mm) (mm) (mm) (mm) (mm) (mm)1 0.72 0.73 2.80 1.83 1.99 4.162 1.27 1.36 3.75 1.83 1.93 4.25Mean value 1.00 1.05 3.28 1.83 1.96 4.21

A combined review of results given in Tables 4 and 5demonstrates that Full-H down specimens, representativeof beam top smooth reinforcement, are characterisedby a stressslip response that is stiffer that the Full-Hup configuration, representative of beam bottom smoothreinforcement, and of the Full one, which is columnreinforcement.

Finally, comparison of experimental data obtainedfrom modified beam tests and pull-out tests indicatesthe uncertainties of beam test data related to indirectmeasurement of slippage with respect to the alternativesolution adopted for the pull-out tests; as a result, theimproved reliability of the pull-out tests allows for the

assessment of the actual response of hooked anchors.g Structures 27 (2005) 15751585

5. Conclusions

Seismic assessment of old type r.c. constructions is ofinterest for structural engineers; however, knowledge ofbasic interaction phenomena involving smooth rebars is notcomprehensive, especially due to the lack of data rangingfrom yielding to the large strain hardening field. Thiscircumstance represents a limitation on the use of advancednon-linear analysis procedures.

The present paper gives an experimental contribution inthis research area. Beam tests and pull-out tests allowedfor describing in detail the forceslip relation of the bondmechanism for straight rebars and that of anchoring enddetails, i.e. circular hooks with a 180 opening angle. Theresults indicate some particular aspects of the behaviourunder monotonic loading. The slippage due to anchoringdevices is relevant and cannot be neglected, especially inthe large post-yielding field; mechanisms governing thestressslip response of hooks allow a reduced yieldingspreading in the anchoring device, so at yielding, the hookslip does not show a plastic plateau and increases onlywhen strain hardening starts. The concrete cover plays arole in the large post-yielding field, since splitting typefailures have been observed in End specimens that fitthe rebar embedment in the external beam to columnregions. The casting direction seems to have an influenceon the behaviour, together with the relative position ofthe hook with respect to the top surface of the concretespecimen.

References

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[4] Bach C. Deutcher Ausschus fur Eisenbeton. Hefts 9 and 10.1911.[5] Abrams DA. Test of bond between concrete and steel. Bulletin no. 71.

Urbana: Engineering Experiment Station, University of Illinois; 1913.p. 238.

[6] Saliger R. Schubwiderstand und Verbund in Eisenbeton-balken. 1913.[7] Mylrea TD. The carrying capacity of semi-circular hooks. ACI

Journal, Proceedings 1928;24:24072.[8] Fishburn CC. Strength and slip under load of bent-bar anchorage and

straight embedment in haydite concrete. ACI Journal, Proceedings1947;44(4):289308.

[9] Rehm G. ber die grundlagen des verbundes zwischen stahl un beton.DafStb, H. 138. Berlin: W. Ernst u. Sohn; 1961.

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[11] Kankam LK. Relationship of bond stress, steel stress and slip inreinforcing concrete. Journal of Structural Engineering 1997;7985.

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[13] Fabbrocino G, Verderame GM, Manfredi G, Cosenza E. Experimentalresponse and behavioural modelling of anchored smooth bars inexisting rc frames. In: International conference Bond in concrete from research to standards. 2002.

[14] Eligehausen R, Popov EP, Bertero VV. Local bondstress relation-ships of deformed bars under generalized excitations. UCB/EERC 83,23. 1983.

[15] Fabbrocino G, Verderame G, Manfredi G, Cosenza E. Structuralmodels of critical regions in old-type r.c. frames with smooth rebars.Engineering Structures 2004;26:213748.

[16] Verderame GM, Stella A, Cosenza E. Mechanical propertiesof reinforcement used for r.c. constructions in 1960s. In: 10th

national conference LIngegneria Sismica in Italia. 2001 [inItalian].

[17] Fabbrocino G, Verderame G, Manfredi G, Cosenza E. Experimentalbehaviour of smooth bars anchorages in existing r.c. buildings. In: fib2002 Osaka congress. 2002. Paper W-463.

[18] Verderame GM, Manfredi G, Frunzio G. Mechanical properties ofconcrete used for r.c. constructions in 1960s. In: 10th nationalconference LIngegneria Sismica in Italia. 2001 [in Italian].

[19] RILEM technical recommendations for the testing and use ofconstruction materials. RC5: Bond test for reinforcing steel. 1. Beamtest, 1982. UK: E & FN Spon; 1994.

[20] CEB Bulletin No. 213/214. CEB-FIP model code 90. 1993.

Experimental behaviour of anchored smooth rebars in old type reinforced concrete buildingsIntroductionLiterature review

Research objectiveTest programmeMaterial propertiesTest set-up

Experimental resultsBond testHook test

ConclusionsReferences