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    Inelastic cyclic behaviourof as-received and pre-corroded

    S500s tempcore steelreinforcement

    Ch. Alk. Apostolopoulos and C.A. RodopoulosDepartment of Mechanical Engineering and Aeronautics,

    University of Patras, Patras, Greece

    Abstract

    Purpose Seismic loading can induce significant deformations to steel reinforcement. The recentapproach suggested by Eurocode 8 indicates that steel reinforcement shall sustain repeated loading

    well within its elastic region, excluding by definition seismic loading. This paper aims to examine thebehaviour of S500s steel reinforcement at strain ranges representing strains corresponding tosmall/medium earthquakes while significant attention has been paid to cases where the reinforcementhas been corroded as this is most representative to aged buildings. The work concludes that thecomplex behaviour of steel reinforcement under low cycle fatigue conditions can be successfully treatedvia the use of the viscous stress. The latter is found to be independent to corrosion exposure while itholds the merits of ductility exhaustion on which most degradation models are based.

    Design/methodology/approach This paper establishes a relationship between the cumulativeeffect of low cycle fatigue and that of the viscous stress.

    Findings The work identifies that the viscous stress follows an exponential growth behaviourwhich terminates at a plateau. The plateau value is found to be independent to corrosion exposure andstrain rate and hence providing a strong potential for being a characteristic indicator of the behaviourof steel reinforcement under realistic inelastic loading.

    Research limitations/implications The study is limited to S500s grade steel. Further study ondifferent steel grades is necessary to increase the potential of viscous stress.

    Originality/value The significance of this paper is the introduction of viscous stress in an areawhere traditional approaches of cumulative damage are based on a large number of empiricalparameters and assumptions.

    KeywordsMechanical behaviour of materials, Steel, Stress (materials), Reinforcement, Viscosity

    Paper typeResearch paper

    IntroductionSince the 1970s, there is a growing research interest towards the fatigue response ofhigh-yield steel reinforcement (from now on called rebars; Tilly, 1979; Highway

    Research Board, 1962; ACI Committee 215, 1974). Rebars represent the basicstrengthening element of steel reinforced concrete, RC, structures and are responsible forcarrying, distributing and controlling loads and displacements.

    Cyclic loading in the form of constant amplitude or spectra is mainly due to thegrowing use of RC in bridges, towers, offshore structures and buildings founded inearthquake zones (ACI Committee 215, 1974; Chang and Mander, 1994). Despite the factthat in the last 20-30 years rebar production has doubled to 100 million t/year, while itconstantly retains the 10 per cent of steel production, the cyclic behaviour of rebars has

    The current issue and full text archive of this journal is available at

    www.emeraldinsight.com/1757-9864.htm

    IJSI1,1

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    International Journal of Structural

    Integrity

    Vol. 1 No. 1, 2010

    pp. 52-62

    q Emerald Group Publishing Limited

    1757-9864

    DOI 10.1108/17579861011023793

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    received little attention and is mainly concentrated in the region of elastic loading (S/Ncurves; Hellenic Regulation for the Technology of Steel in Reinforced Concrete, 2008)regardless of the design requirements under seismic response conditions where rebarsare expected to deform into inelastic range and dissipate energy via a stable hysteretic

    behaviour. This is because severe ground motion can result into cracking of concrete,leaving the rebars governing the response of the RC section (Filipou et al., 1983).

    Although, the cumulative nature of fatigue damage is generally recognised as animplicit factor controlling total life, current design practise and codes fail to recognisethe effect of non-peak inelastic cycles (Chai, 2005). This is particularly hazardous in thecase of long-duration ground motion or in the case of multiple seismic events with preor after shocks combined with the main shock. Accelerograms taken from Mexico Cityindicate that ground motion events can have a random behaviour both in terms ofmagnitude and duration (Ordaz and Singh, 1993). In the same work, ground motionsranging from 55-215 centimeter per square second and with duration range between 2.9up to 32 second have been reported. The above two factors represent a critical input interms of identifying the magnitude of seismic energy contributing to damage inrelation to the cyclic plastic energy capacity of the structure (Chai et al., 1998).

    The potential of rebars being the self-governing and most critical constituent phaseto the response of RC structures is further reinforced in cases where the volumetricexpansion of corrosion products has caused spalling of concrete cover and interfacialbond strength degradation (Chang et al., 1999; Maslehuddin et al., 1983; Fang et al.,2004). Measurements taken from corroded reinforcement in buildings still in public useand in locations of significant seismic propensity have shown that mass loss could beas much as 18 per cent (Apostolopoulos, 2007). In Apostolopoulos et al. (2006), it wasalso shown that corrosion of rebars degrades their tensile mechanical strength andductility properties to levels below the minimum required values set by the HellenicStandard ELOT 971 (1994).

    In this work, the authors attempt to compare and explain the inelastic cyclic responseof as-received and pre-corroded S500s grade steel rebar and identify the mechanisms offailure. Herein, it is also necessary to note that terminologically inelastic cyclic loading isalso referred as low cycle fatigue or extremely low cycle fatigue.

    Experimental procedure and resultsTempcore S500s ribbed rebar of nominal diameter 12 millimeter was received from aGreek steel manufacturer. S500s rebars have extensively been used in Greece duringthe period 1987-2006. The chemical composition is shown in Table I. The material hasa minimum yield stress of Rp 500 MPa, minimum fracture strength ofRm 550 MPa and minimum elongation at failure of 12 per cent.

    The tempcoring process consists of three stages. During the first stage, hotaustenite at,1,0008C (surface temperature) is quenched with water down to 2808C. Atthe end of this process the rebars have an austenite core surrounded by a layer

    C P S N Mn Fe

    As-received 0.21 0.018 0.030 0.009 0.96 RemainingELOT 1421 (ELOT 971, 1994) ,0.24 ,0.055 ,0.055 ,0.014 Remaining

    Table I.Chemical composition of

    as-received S500s againstnational standards

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    composed of a mixture of austenite and martensite. The duration of this first stagemainly controls the thickness of the martensite layer. The second stage involves aircooling. At the beginning of this stage the temperature of the core is around 900 8Cwhich by induction reheats the surface to temperatures close to 7808C. This results into

    self-tempering of martensite. The third stage involves final cooling on a bed. Thisdelivers a quasi-isothermal transformation of the remaining austenite leading to amixture of two phase ferrite-pearlite (polygonal ferrite and pearlite) and bainite. Fromthe above, it is also understood that the percentage of martensite also depends on thediameter to be produced while the final product (rebar) should be considered as acomposite material. A typical cross section of the rebar shown in Figure 1.

    Prior to testing, cross sections of the rebars were examined under scanning electronmicroscope (Philips XL40) after successive polishing to a surface roughness ofRa , 0.8 micrometer. The analysis revealed extensive porosity close to the surface(martensite layer; Figure 2). 3D surface profilometry revealed that the porosity canreached depths larger or equal to 55-72 micrometer as shown in Figure 3. The numberis only indicative since the actual tip of the pore is unknown. Energy dispersion X-rayanalysis was performed at different locations including the pores. Results from thepores indicate the presence of O and Cu. Typical results are shown in Figure 4.

    Figure 1.Optical view of the phasesconsisting S500s tempcoreafter Nital etching

    Two phase ferrite-pearlite

    Martensite

    Bainite

    Figure 2.Extensive porosity foundin the martensite layer

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    The amount of oxygen found is high enough as to be primarily attributed to the largeamounts of oxygen used during the hearth operation (EUROFER, 2007). Coppercontent should be attributed to the quality of the scrap metal used.

    Strain control cyclic tests were performed in fully reversed loading without the useof anti-buckling device. The latter was due to the fact that under realistic seismicconditions, rebars are prone to buckling between stir-ups. The phenomenon hasreceived increasing scientific attention in the last years indicating that thepost-buckling hysteretic behaviour should primarily control the potential of rebarsto dissipate seismic energy (Chai et al., 1998). Testing of as-received material wasperformed in a 250KN MTS servo-hydraulic rig at a frequency of 1 hertz and at threedifferent (D1ranges, namely: 2, 5 and 8 per cent. The strain amplitude was chosen inorder to replicate typical seismic response (Franchiet al., 1996; Pipa and Vercesi, 1996;

    Pipaet al., 1994). The above ranges should be considered as mediocre since values ashigh as D1 28 per cent have been reported (Sheng and Gong, 1997).

    In addition, rebars were exposed in salt-spray environment according to ASTMB117. This accelerated corrosion tests was selected as being closer to the case where astructural member of a building has experienced failure of its concrete cover while itslocality is close to sea front. The salt solution was prepared by dissolving five parts bymass of sodium chloride (NaCl) into 95 parts of distilled water. The pH of the salt spraysolution was such that when dissolved at 358C the solution was in the pH range of6.5-7.2. The duration of exposure was 45 and 90 days. After exposure and prior totesting the specimens were cleaned from the corrosion products. During all tests thehysteretic loop was recorded. Typical results are shown in Figure 5.

    In general the material experiences a continuous softening degradation mechanism

    up to failure. Further analysis is provided in the next section. The total number ofcycles to failure is shown in Table II.

    Herein, it is easily understood that at high strains, the effect of corrosion incontrolling cyclic loading resistance becomes independent to corrosion exposure.

    Analysis of results and discussionThe breakdown of a hysteretic loop in isotropic stress (R), backstress (X) and viscousstress (sv) is known to provide an efficient tool in understanding the mechanisms

    Figure 3.Indicative analysis of pore

    morphology

    z

    55

    50

    45

    40

    35

    30

    25

    20

    15

    5

    0

    60

    65

    5045

    40

    35

    30

    25

    20

    15

    10

    5

    0

    10

    (a) (b)

    y/mmx/mm

    0.016

    00.0040

    0.00400.0060

    0.00800.01

    0.0120.014

    0.0080 0.00200

    0.012

    Notes:(a) The selected pore; (b) the corresponding 3D morphology

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    Figure 4.

    Point A

    Point B

    (a)

    (b)

    (c)

    Notes:(a) Backscattered image of the cross section. Point A indicates the location for EDX

    analysis away from the pore (b) and Point B indicates the EDX analysis at the pore (c)

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    governing cyclic failure, especially in the case of large plastic strains (Rees, 1981;Feaugeas, 1999; Kuhlmann-Wilsdorf and Laird, 1979; Honycombe, 1968; Cottrell, 1953).The breakdown of a hysteretic loop is schematically shown in Figure 6. Plasticstraining in polycrystals develops dislocation walls and subgrain boundaries. Theabove create long-range internal stresses due to strain incompatibility and leads to

    easier reverse flow. This mechanism is associated to the backstress (also known asinternal). Additionally, the isotropic stress is the stress locally required for adislocation to move and is related to short range stresses like friction stress (crystallattice, solutes, etc.) and stresses from dislocation forests. Finally, the viscous stresscontrols the transition from viscous rupturing to brittle fracture (Honycombe, 1968). Ingeneral, the behaviour of the viscous stress indicates the creation and propagation ofmicro-cracks in a stable manner related to the potential external energy and theinstability (unsteady mode) in terms of critical crack coalescence. It is also imperative

    Figure 5.Typical behaviour of

    hysteretic loop

    S500s, = 8%, as-received

    Strain

    0.06 0.04 0.02 0.00 0.02 0.04 0.06

    Stress(M

    Pa)

    800

    600

    400

    200

    0

    200

    400

    600

    800

    Hysteretic loop Hysteretic loop

    (a) (b)

    S500s, = 5%, as-received

    Strain

    0.03 0.02 0.01 0.00 0.01 0.02 0.03

    Stress(M

    Pa)

    800

    600

    400

    200

    0

    200

    400

    600

    800

    S500s, = 8%, 45 days

    Strain

    0.06 0.04 0.02 0.00 0.02 0.04 0.06

    Stress(MPa

    )

    800

    600

    400

    200

    0

    200

    400

    600

    800

    Hysteretic loop Hysteretic loop

    (c) (d)

    S500s, = 5%, 45 days

    Strain

    0.03 0.02 0.01 0.00 0.01 0.02 0.03

    Stress(MPa

    )

    800

    600

    400

    200

    0

    200

    400

    600

    800

    Notes:(a)-(b) As-received at strain range of 8 and 5 per cent and (c)-(d) the corresponding behaviour after

    45 days of salt spray exposure; in every case, the loading sequence is compression tension

    Condition D1 2% D1 5% D1 8%

    As-received 529 28 1045 days of exposure 335 26 990 days of exposure 263 23 8

    Table II.Average number of

    cycles to failure (of threetests) of as-received and

    corroded S500s

    Inelastic cyclicbehaviour

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    to note that the viscous stress can more clearly preserve information regardingbuckling histories while components X and R have the tendency to filter them viasuccessive accumulation effects.

    The hysteretic breakdown components are given by:

    Xsmax se min

    2 1

    sv smax 2 se max 2

    Rse max 2 se min

    2 3

    A typical response of viscous stress for the whole range of the experimental matrix isshown in Figure 7.

    Generally speaking, in every case, the viscous stress shows a similar tendency toincrease, to saturate and finally to decrease. In the as-received condition it can be clearlyseen that the rate towards saturation increases with strain. In this case the effect ofmaterial to control buckling canbe seen at the viscous stress of the first cycle. The termcontrol is used in quotation marks since buckling is assumed to take place as a post-yieldcondition. Herein, at D1 2 per cent the viscous stress initiates at 49 MPa followed by65 and 85 MPa for D1 5 and 8 per cent, respectively. After 45 days of salt sprayexposure, the corresponding initial viscous stress values are 80, 96 and 105 MPaindicating increasing tendency to buckling. In the case, however, of 90 days, the initialviscous stress value, at around 120 MPa, becomes independent to strain range indicatingthat the response of the rebars to the first loading cycle is common. This could beattributed to the fact that after 90 days of exposure the rebars have developed severestress concentrations (pitting) which in turn has induced an additional geometric affectto their buckling behaviour (Apostolopoulos and Papadakis, 2007).

    Figure 6.Partitioning of hystereticloop according to Cottrell(1953)

    R

    Elastic part X

    semax

    semin

    e

    smaxs

    sV

    Notes:The parameters smax

    , semax

    and semin

    are the peak stress and the upper and

    lower bound of the elastic part of half the hysteretic loop, respectively

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    In all cases, the reader can identify that there is a value of viscous stress atapproximately 150 MPa over which the viscous stress tends to saturation. Once thisvalue is reached the ability of the rebar to dissipate energy via positioning andmovement of dislocations has been exhausted (ductility exhaustion) whilemicro-cracking is eminent. The development of micro-cracking will gradually relaxthe viscous stress up to the point where the density of micro-cracks is such as to lead tolocal interaction effects (strain amplification) and crack coalescence (failure eminent). Itis therefore reasonable to assume the viscous stress value of 150 MPa is critical for theremaining energy dissipation capacity of S500s. To further reinforce the above rationalit is imperative to note that:

    . The value is independent to corrosion exposure, same value is obtained also forthe as-received and pre-corroded rebars.

    . The value is independent to strain range. The latter is found barely to control thenumber of cycles to critical value.

    Fractographic analysisTo better understand the progression of damage and the effect of porosity found in theas-received rebar fractographic analysis was performed. Figure 8(a) shows the fracture

    Figure 7.Behaviour of viscous

    stress with strain rangeand exposure time

    Cycles to failure

    1 10 100 1,000

    Viscousstress(MPa)

    40

    60

    80

    100

    120

    140

    160

    180

    (a)

    Cycles to failure

    1 10 100 1,000

    (b)

    Viscousstress(MPa)

    40

    60

    80

    100

    120

    140

    160

    180

    200

    S500s, = 8%, as-receivedS500s, = 5%, as-receivedS500s, = 2%, as-received

    S500s, = 2%, 45 daysS500s, = 5%, 45 daysS500s, = 8%, 45 days

    Cycles to failure

    1 10 100 1,000

    (c)

    Viscousstress(M

    Pa)

    20

    40

    60

    80

    100

    120

    140

    160

    180

    200

    S500s, = 2%, 90 daysS500s, = 5%, 90 daysS500s, = 8%, 90 days

    Notes:(a) As-received; (b) 45 days; (c) 90 days; the arrows indicate the transition towards brittle

    fracture; the rectangular section is used to indicate a bound of values where the viscous stress is

    independent to strain range and corrosion exposure

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    surface of the as-received rebar at D1 2 per cent. Herein, the black arrows indicatemultiple-crack initiation. The white arrow indicates rupture parallel to loading direction.

    The rupture can be attributed to the presence of pores found ahead of the crack tip. Similarbehaviour is shown in the case of the as-received D1 5 per cent, Figure 8(b). Herein, therole of porosity is further reinforced by the presence of ruptures found away from thecracks. In this case, the ruptures were the result of the higher strain range. Similarbehaviour was found in the case of as-received rebars at D1 8 per cent. In the case of45 days at D1 5 per cent, failure was due to a synergistic effect of cracking/ruptures,Figure 8(c) as well as embrittlement zones, Figure 8(d). Extensive embrittlement was alsofound in the case of 90 days at D1 2 per cent, Figure 8(e). Embrittlement was never

    Figure 8.Indicative images takenfrom the fracture surfaceof as-received andcorroded S500s

    (a) (b)

    (c) (d)

    (e)

    Rupture

    parallel to

    loading

    Fatigue cracking

    Ruptures

    Embrittlement

    Crack

    Ruputure

    Crack

    Ruputure

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    observed in the as-received state. In every case, embrittlement zones were observed at amaximum depth of 0.34 millimeter and 0.8 millimeter for the case of 45 and 90 days,respectively, (martensite layer). The embrittlement shall be related to the production ofhydrogen during the iron dissolution in the pitting process (Broomfield, 2007).

    ConclusionsIn this work, the inelastic cyclic behaviour of ribbed S500s tempcore steel reinforcementis presented. In the as-received state the material was found to contain extensiveporosity potentially due to trapped oxygen. Cu residues were also found at the poresrelated to the scrap metal. Rebars were subjected to inelastic fully reversed loading atdifferent strain ranges and comparison of the hysteretic loops was performed into thoserepresenting the as-received state and after 45 and 90 days of slat-spray exposure.

    The results indicate that only in the case of D1 2 per cent there is a cleardegradation due to corrosion in terms of cycles to failure. In the case ofD1 5 and 8 percent, the total cycles to failure appear to be independent to corrosion time. Breakdown ofthe hysteretic loops and examination of the viscous stress indicate that there is a criticalvalue of 150 MPa after which failure of the S500s is eminent due to rupturing.Progression towards such critical value was found to be the main controlling parametertowards the ability of S500s to dissipate energy with time. Fractographic analysisrevealed that failure in the as-received state was due to the synergistic effect of crackprogression and porosity ruptures. In the corroded state, hydrogen embrittlement zoneswere also identified as an additional damage mechanism. Yet, the critical value of theviscous stress was found to be independent to hydrogen embrittlement predominantlydue to its limited depth of penetration.

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    Corresponding authorCh. Alk. Apostolopoulos can be contacted at: [email protected]

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