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Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures Tanabe et al. (eds) © 2009Taylor & Francis Group, London, ISBN 978-0-415-48508-1 Creep and shrinkage of self compacting concrete: Experimental behavior and numerical model C. Mazzotti & C. Ceccoli DISTARTStructural Engineering, University of Bologna, Bologna, Italy ABSTRACT: In the present paper, the results of an experimental campaign concerning the rheological properties of hardened self-compacting concrete are presented. Four mixes of self-compacting concrete have been cast, with different compressive strength. For each mix, compressive strength and shrinkage evolution with time have been monitored along a period of time of about one year. Creep tests have been performed at two different stress levels, 35% and 55% of compressive strength, and with two different ages at loading (7 and 28 days). Results have been compared with Model Code 1990 provisions. Finally, a simplified non linear creep model has been proposed which is able to properly take into account the effect of remarkable strength increase with time observed in young self compacting concretes. 1 INTRODUCTION The different methodology followed to obtain self- compacting concrete (SCC) in different countries (Ouchi et al. 2003) and the limited number of stud- ies concerning its long-term behaviour (Persson 2001, 2005, Poppe & De Shutter 2001, Seng & Shima 2005, Mazzotti et al. 2006) make still not clear if current International Standards apply successfully also for SCC (Klug & Holschemaker 2003, Vidal et al. 2005, Landsberger & F.-Gomez 2007). Moreover, it is not even assessed if long-term properties can be predicted with reference to conventional mechanical and phys- ical parameters only (like strength, w/c, ...) or the adoption of parameters concerning the mix design is needed. In the present paper, the results of an experimen- tal campaign concerning the rheological properties of hardened SCC are presented. Four mixes of SCC have been cast, with different compressive strengths (the main parameter adopted in European standards to identify concrete class), i.e. from C30/35 to C50/55. For each mix, compressive strength and elastic mod- ulus evolution with time have been monitored along a period of time of about one year (for further details, see Mazzotti et al. 2008). As for the creep tests, sustained stress levels (rang- ing from 35% to 70% of compressive strength at the age of loading) have been applied to cover the range of application from cast-in-place to prestressed struc- tures and to verify if conventional stress limits for linear viscoelasticity can still be applied to SCC ele- ments. Two different ages at loading (7 and 28 days) have been considered. Effect of sustained load and aging conditions on compressive strength has been also investigated. Long-term experimental data exhibits significant differences with Model Code 90 (MC90, CEB- FIP 1991) predictions. Hence, the study confirms that not only a new calibration process of parameters gov- erning creep laws is needed to cover the case of SCC but, probably, time evolution laws should be also mod- ified. For these reasons, an improved version of a previously proposed (Mazzotti et al. 2006) non linear creep model has been described, which is able to prop- erly take into account the effect of remarkable strength increase with time observed in young self compacting concretes. 2 MATERIAL PROPERTIES In order to compare the rheological properties of SCC with different strengths, a series of mixes ranging from normal to medium-high strength has been pre- pared. A detailed description of mix compositions adopted is reported in Table 1, whereas water/cement and water/binder ratios and fresh-state properties (flow cone, J-ring and V-Funnel tests) are reported in Tables 2 and 3, respectively. SCCs have been cast with different types of cement, same super plasti- cizer and viscosity-agent (coupled together) and they have slightly different w/b ratio. Experimental tests have been conducted on cylinders. After demolding, all specimens have been cured at RH = 60% and T = 20 C, except for mix 1 whose cylinders have been stored at RH = 98% until one day prior to tests. 667 © 2009 Taylor & Francis Group, London, UK

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  • Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures Tanabe et al. (eds) 2009 Taylor & Francis Group, London, ISBN 978-0-415-48508-1

    Creep and shrinkage of self compacting concrete: Experimental behaviorand numerical model

    C. Mazzotti & C. CeccoliDISTARTStructural Engineering, University of Bologna, Bologna, Italy

    ABSTRACT: In the present paper, the results of an experimental campaign concerning the rheologicalproperties of hardened self-compacting concrete are presented. Four mixes of self-compacting concrete havebeen cast, with different compressive strength. For each mix, compressive strength and shrinkage evolution withtime have been monitored along a period of time of about one year. Creep tests have been performed at twodifferent stress levels, 35% and 55% of compressive strength, and with two different ages at loading (7 and 28days). Results have been compared with Model Code 1990 provisions. Finally, a simplified non linear creepmodel has been proposed which is able to properly take into account the effect of remarkable strength increasewith time observed in young self compacting concretes.

    1 INTRODUCTION

    The different methodology followed to obtain self-compacting concrete (SCC) in different countries(Ouchi et al. 2003) and the limited number of stud-ies concerning its long-term behaviour (Persson 2001,2005, Poppe & De Shutter 2001, Seng & Shima 2005,Mazzotti et al. 2006) make still not clear if currentInternational Standards apply successfully also forSCC (Klug & Holschemaker 2003, Vidal et al. 2005,Landsberger & F.-Gomez 2007). Moreover, it is noteven assessed if long-term properties can be predictedwith reference to conventional mechanical and phys-ical parameters only (like strength, w/c, . . .) or theadoption of parameters concerning the mix design isneeded.

    In the present paper, the results of an experimen-tal campaign concerning the rheological propertiesof hardened SCC are presented. Four mixes of SCChave been cast, with different compressive strengths(the main parameter adopted in European standards toidentify concrete class), i.e. from C30/35 to C50/55.For each mix, compressive strength and elastic mod-ulus evolution with time have been monitored along aperiod of time of about one year (for further details,see Mazzotti et al. 2008).

    As for the creep tests, sustained stress levels (rang-ing from 35% to 70% of compressive strength at theage of loading) have been applied to cover the rangeof application from cast-in-place to prestressed struc-tures and to verify if conventional stress limits forlinear viscoelasticity can still be applied to SCC ele-ments. Two different ages at loading (7 and 28 days)have been considered. Effect of sustained load and

    aging conditions on compressive strength has beenalso investigated.

    Long-term experimental data exhibits significantdifferences with Model Code 90 (MC90, CEB-FIP 1991) predictions. Hence, the study confirms thatnot only a new calibration process of parameters gov-erning creep laws is needed to cover the case of SCCbut, probably, time evolution laws should be also mod-ified. For these reasons, an improved version of apreviously proposed (Mazzotti et al. 2006) non linearcreep model has been described, which is able to prop-erly take into account the effect of remarkable strengthincrease with time observed in young self compactingconcretes.

    2 MATERIAL PROPERTIES

    In order to compare the rheological properties of SCCwith different strengths, a series of mixes rangingfrom normal to medium-high strength has been pre-pared. A detailed description of mix compositionsadopted is reported in Table 1, whereas water/cementand water/binder ratios and fresh-state properties (flowcone, J-ring and V-Funnel tests) are reported inTables 2 and 3, respectively. SCCs have been castwith different types of cement, same super plasti-cizer and viscosity-agent (coupled together) and theyhave slightly different w/b ratio. Experimental testshave been conducted on cylinders. After demolding,all specimens have been cured at RH = 60% andT = 20C, except for mix 1 whose cylinders havebeen stored at RH = 98% until one day prior to tests.

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  • Table 1. Mix composition of SCC mixes n. 14.

    Component Type Unit Mix 1 Mix 2 Mix 3 Mix 4

    Cement 32.5 II AL kN/m3 3.55 3.60 Cement 42.5 II AL kN/m3 4.40 Cement 52.5 I kN/m3 4.40Filler kN/m3 1.99 1.73 1.10 1.10Fine sand 04 mm kN/m3 9.68 8.63 8.26 8.26Coarse sand 812 mm kN/m3 4.70 5.50 5.20 5.20Gravel 1225 mm kN/m3 1.82 1.81 2.40 2.40Glenium Sky lt/m3 7.00 9.60 7.50 7.50H20 lt/m3 173 205 204 209

    Table 2. Water/cement and water/binder ratio of mixes n.14.

    Properties Mix 1 Mix 2 Mix 3 Mix 4

    w/c 0.48 0.57 0.46 0.47w/b 0.31 0.39 0.37 0.38

    Table 3. Fresh properties of SCC mixes n. 14.

    Test Unit Mix 1 Mix 2 Mix 3 Mix 4

    Slump flow cm 79 79 78 66V-funnel s 7.6 5.2 5.0J-ring cm 75.5 77.5 62

    3 STRENGTH EVOLUTION WITH TIME

    Strength evolution with time of all mixes hasbeen investigated by compressive tests on 100 200 mm( h) cylinders at different aging times.Experimental results on strength variation with timehave then been compared with provisions by ModelCode 90 according to the equation:

    fc(t0) = fci exp[s(1

    28/t)], (1)

    where t0 is the age of concrete (days), s is a param-eter depending on type of cement (s = 0.25 in thepresent case) and fci is the 28-day cylindrical com-pressive strength (mean value obtained from tests, i.e.fci = fcm (28)). Comparison reported in Figures 1a,b, for mixes 1, 2 and 3, 4, respectively, shows thatthe MC90 formulation, originally defined for standardconcretes, is able to reproduce quite well the strengthincrease of all mixes up to 90 days; after that time, codeprovision suggests a negligible strength increase (5%from 3 to 6 months), whereas some mixes (1 and 3)

    Figure 1. Time variation of compressive cylindricalstrength of (a) mixes n. 1, 2, and (b) mixes n. 3, 4 comparedwith provision by CEB MC90.

    have been subject to a strength increase significantlyhigher (about 15%). This is probably due to the poz-zolanic activity of the filler adopted in the mix. Eventhough the number of tests is very limited, analogousresults can be found in the literature (Persson 2001).Strength increase of SCC after 3 months can modifyalso creep effects, as will be explained in the follow-ing. Moreover, as far as strength increase with time ofmix 4 is concerned, results follow more regularly codeprovisions and after few months of aging, strengthincrease is significantly reduced; early attainment ofhigh strength values is a well known consequence ofthe adoption of 52.5 type cement. Results concern-ing elastic modulus evolution with time of the sameconcretes can be found in Mazzotti et al. (2008).

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  • 4 SHRINKAGE AND CREEP DEFORMATION

    Delayed deformations have been investigated withtests on cylinders with 2 different sizes ( h), i.e.,98 200 mm and 122 250 mm, respectively. Foreach mixture, four cylinders (two for each diameter)have been used for creep tests at two different ages atloading and five ( h) 98 200 mm for shrinkagetests. Tests have been conducted in a climate room at20C and 60% RH.

    4.1 Experimental set-up and instrumentation

    After a wet curing period of 2 days, creep specimenshave been exposed to RH = 60% and loaded at age of7 and 28 days from casting, for one year at least, byusing a steel loading frames. Two different diametersof cylinders have been adopted in order to prescribe,within the same steel frame, two different stress lev-els: about 36% and 55% of compression strength atthe loading time fcm(t0) for all mixes. The first stresslevel can be thought to produce creep strains withinthe framework of linear viscoelasticity, whereas in thesecond case a non linear behaviour is expected. Twocylinders for each stress level have been used.

    Creep strains have been measured by using couplesof longitudinal electrical strain gauges; the mean strainvalue is recorded. One specimen for each stress levelhas been instrumented with two additional transversestrain gauges. Specimens subject to shrinkage testshave been instrumented analogously; autogenous anddrying shrinkage have been measured starting fromtwo days from casting for all mixes but mix 2, whoseshrinkage has been measured starting only after sevendays from casting. Autogenous contribution has beenobtained by using specimens sealed with plastic sheets.

    4.2 Test results

    In order to reduce experimental data scattering, themean values of the experimental tests obtained fromtwo specimens are reported in the following.

    Mean strains caused by longitudinal total shrinkagefrom all mixes are reported in Figure 2. Mixes 3 and 4show remarkably higher values of total shrinkage withrespect to mixes 1 and 2; this can be due to the highercement content. The reduced shrinkage strain values ofmix 2 (with respect to other mixes with smaller watercontent) can be also explained by considering that thetest started only at 7 days from casting. Moreover, aftermore than one year from loading, the rate of increaseis almost negligible for mix 1, while mix 2 still showsa significant rate of increase.

    With reference to creep tests, specific creep func-tion C = (v/) for all mixes, loaded at 7-day agefrom casting at low and high stress levels (0.35 and0.55 fcm), are reported in Figures 3a, b, respectively;

    Figure 3. Specific creep of all mixes at (a) low and (b) highstress levels for 7 days age at loading.

    Table 4. Compressive strength of all concrete mixes at thetime of loading t0.

    Compressive strength (MPa)

    Mix 1 Mix 2 Mix 3 Mix 4

    t0 = 7 days 32.20 26.50 33.50 49.63t0 = 28 days 42.62 34.34 40.05 57.85

    in Table 4, the concrete strength for each mix at thetime of loading is reported. Shrinkage strain has beensubtracted in order to retain creep contribution only.After about one year of loading, mixes 1 and 4 showsimilar creep strains at both stress levels; with moredetails, mix 4 shows an higher rate of increase for thewhole test duration and the change in convexity ofspecific creep curve is detected only at a later stage

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  • Figure 4. Specific creep of all mixes at (a) low and (b) highstress levels for 28 days age at loading.

    with respect to mix 1. Differences between slopesof curves can be explained considering the differentstrength evolution with time of two different mixes(see Figure 1). Mix 1, in fact, is subject to a moreprogressive strength increase, reducing delayed creepstrains due to the constant load application causingconcrete internal compaction (Bazant & Kim 1979);on the contrary, mix 4 attains very rapidly its finalcompressive strength, reducing the beneficial effect ofconcrete compaction. From an analytical point of view,this behaviour can be predicted by considering the ratio(t) = 0/fcm(t) as a reducing factor of the creepcompliance. Specific creep curve for other mixes isqualitatively similar but with higher values, probablydue to the type of cement and the higher w/c ratio.

    In Figures 4a, b, similar behaviour can be observedfor specimens loaded at an age of 28 days. In thiscase, non linear amplification of creep function C athigher stress levels is more pronounced with respectto younger concrete: for mix 1, non linear amplifica-tion is about 1.3 for specimens loaded at 7 days and1.5 for specimens loaded at 28 days; the smaller non-linear creep behaviour at high stress levels for youngconcretes can be considered as an effect of concretecompaction (Mazzotti et al. 2005). Non linear creepamplification is smaller for mix 3 because the twoloading levels (36% and 44%) are much closer thanfor other mixes.

    As for the non-linear creep behaviour, Figures 5a, bshow creep strain at increasing time under loading vsthe applied stress level for mix 2 and 4, respectively;moreover, in these Figures, two extrapolation lines are

    Figure 5. Creep strain vs applied stress at different timesafter initial loading for mixes (a) n. 2 and (b) n. 4, with 7-dayage at loading.

    also reported; they have an initial linear behaviourfollowed by a non linear creep strain increase andcorrespond to very short (0.2 days) and medium(7 months) time intervals under loading. Starting fromstress level /fcm of about 0.35 0.40 an apprecia-ble non linear behaviour can be observed; consideringthe young age at loading of two concretes under test(t0 = 7 days), this result is in agreement with thewell recognized stress limit of linear viscoelasticity forstandard concretes. Mix 4 shows higher non-linearity,probably due to its higher compression strength.

    4.3 Comparison with internationalrecommendation (MC90)

    Experimental results have been compared with MC90predicting curves, considering T = 20C, RH = 60%and 2A/u = 50. Ability of MC90 to describe timeevolution of delayed strains for SCC with differentstrength is investigated.

    In Figure 6a, total shrinkage (drying+autogenous)has been compared with provisions by MC90 formixes 1 and 4, exhibiting the highest and the smallestshrinkage values during long-term tests. A remarkableunderestimation of strain prediction has been observedin both cases (sh = 60 200%). The long-termrate of increase of curves is also poorly described byMC90: it is overestimated for mix 1 and underes-timated for mix 4. Finally, creep strains have beenalso compared with MC90 predictions. Coefficient = (Cexp CMC90)/Cexp after 300 days of loading for

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    Figure 7. Strength variation due to aging and load condi-tions.

    all mixes has been reported in Figure 6b. A systematicunderestimation of creep strains (about 3060%)givenby the code provisionwith respect to experimental datacan be observed both at 7- and 28-day age at loading.

    4.4 Effect of aging and sustained load on strength

    At the end of creep and shrinkage tests, the strengthof all specimens under failure loading from mixes 1and 4 has been obtained in order to evaluate the effectof sustained loading and of different aging condi-tions (drying or sealed specimens) on the compressive

    strength of different SCCs. Figure 7 shows the com-parison between compression strengths of specimenssubject to different loading and aging conditions,expressed as a percentage of results obtained understandard wet curing (T = 20, RH = 98%), afterabout 400 days from casting.

    5 A SIMPLIFIED NON-LINEAR MODEL

    The experimental results showed that in a number ofcases concrete strength increase continues also afterseveral months (see Figs. 1a, b); this is particularlytrue if reactive o partially reactive filler has been used.Moreover, creep strain strongly depends on appliedstress level /fcm(t), so that an appreciable variationwith timeof concrete strength can substantiallymodifycreep strain rate.

    Furthermore, instantaneous stress-strain behaviourobserved during the loading phases of creep testsshowed an appreciable non linear behaviour also whenthe medium stress level is considered (0.30 to 0.40of fcm).

    In order to take into account all these experimentalfindings and to describe the delayed behaviour alsofor medium-high stress levels, a simplified non lin-ear creep model has been developed. The analyticalformulation is based on the standardMC90 creep com-pliance law, extended in the non linear range followingthe non linear creep amplification function proposedby Bazant & Prasannan (1989):

    C(t, t0) = (t, t0)Ec,28

    F[(t)]. (2)

    Furthermore, the creep coefficient has been alsomodified in order to take the mix-design parametersinto account, so obtaining the following expression:

    (t, t0) = RH (fcm) (t0) mix (t t0). (3)The first two functions in Eq. (3) are the same

    appearing in the MC90, while the other functionshave been modified or introduced according to thefollowing expressions:

    (t0) = 10, 1 + (t0) , (4)

    c(t t0) =[

    (t t0)H + (t t0)

    ]1c/p, (5)

    with fcm mean compressive strength, t0 age at load-ing and H function of relative humidity (the readeris addressed to MC90 for further details). Coeffi-cients and 1 must be calibrated while c/p is the

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  • cement/powder ratio. The new term mix is defined asa function of mix parameters:

    mix = Km[( c

    G

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    p

    )3]4, (6)

    where c/G is the ratio between cement and aggregates(sand, gravel, . . .) and Km, 2, 3, 4 are the param-eters to be calibrated. Finally, the non linear creepamplification function can be defined as:

    F[(t)] = 1 + 2 s (t, t0)5

    1 , (7)

    where 1 and 2 and 5 are additional parameters tobe obtained from a least square minimization proce-dure starting from experimental data; moreover, thestress function s(t, t0) is the actual stress/strength ratio,being:

    s (t, t0) = (t0)fcm (t)

    , (8)

    in the case of constant applied load. In Eq. (7), numer-ator and denominator indicate the effect of sustainedload and the effect of a damage level due to instan-taneous loading. If 1 < 1, Eq. (7) shows that, forlow load levels, F[(t)] may be smaller than one.Creep strain obtained from Eq. (2) is then smallerthan predicted by linear theory excluding the effect ofconcrete compaction. The law fcm(t) representing theevolution with time of compression strength, reportedin Figure 1, has been defined by modifying MC90proposal according to expression:

    fcm(t) = f c,28 exp{s[1 (28/t)n]}, (9)

    where parameters s and n have been specifically cali-brated for each SCC concrete mix by using experimen-tal results previously described (see Fig. 1). Accordingto the available data, parameters s and n range from0.2 and 0.6, and 0.280.35, respectively. The adoptionof function s(t, t0) allows for variable rate of increaseof mechanical properties be taken into account, par-ticularly important for concretes loaded at early ages.Finally, the non linear behaviour during the load appli-cation has been introduced in Eq. (7) according to theconventional scalar damage index = 1 E/E0,where E is the secant stiffness at the end of loadingand E0 is the initial tangent stiffness. Usually dam-age index is about 0.100.15 or 0.220.35 for low(0.35 fcm(t)) or medium (0.55 fcm(t)) applied stresslevels, respectively.

    From best fitting of experimental results obtainedat different sustained stress levels and with specimens

    Table 5. Values of the parameters of the proposed non linearcreep model obtained from best fitting of experimental data.

    1 2 Km 1 2 3 4 5

    0.05 0.90 1.80 13 2.2 1.28 0.55 1.28 2.10

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    from all the different batches (mixes 1 to 4) the valuesreported in Table 5 are obtained.

    Hence, being 1 < 1, for low to medium stresslevels Eqs. (6, 7) predict concrete compaction.

    Figure 8a shows the comparison between specificcreep function obtained from experimental results atmedium and medium-high stress levels (0.36 and 0.55of f c mix 2 specimens) and numerical predictionfrom Eq. (2); good agreement can be observed both interms of qualitative behavior and quantitative values.Not only creep values, but also the slope of curves after400 days of loading are well predicted. Hence, the nonlinear model is expected to be able to correctly predictcreep strain also for longer times. Finally, in Figure 8b,creep strain experimental values of all consideredspecimens after different times under constant loading(1, 10, 100 and at the end of test) are plotted versus thecorresponding numerical predictions given by Eq. (2).

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  • Very good prediction capability can be observed bothat medium and medium-high stress level.

    ACKNOWLEDGEMENTS

    The authors would like to thank SAPABA and BASFfor providing materials for tests. The financial sup-ports of (italian) MIUR (PRIN 2006 Grant: Structuralapplication of self compacting concrete) is gratefullyacknowledged.

    REFERENCES

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    CEB-FIP. Model Code 1990, Tomas Telford Publ., 1990.Klug, Y. & Holschemaker, K. 2003, Comparison of the hard-

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    Landsberger, G.A. & Fernandez-Gomez J. 2007, Evalua-tion of creep prediction models for self consolidatingconcrete, in 5th RILEM Symp. on SCC, Pro 54, Ghent,V. 2: 605610.

    Mazzotti, C., Savoia, M. & Ceccoli, C. 2005, A comparisonbetween long term properties of self compacting con-cretes and normal vibrated concretes with same strength,in CONCREEP 7, Proc. Int. Conf., Nantes: 523528.

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    Mazzotti, C., Savoia, M. & Ceccoli, C. 2008, Compar-ison between long term properties of self compactingconcretes with different strength, in 3rd North AmericanConference on the Design and Use of Self-ConsolidatingConcrete, SCC 2008, Proc. Int. Conf., Chicago: 16(on CD).

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    Persson, B. 2001, A comparison between mechanical prop-erties of SCC and the corresponding properties of normalconcrete, Cem. Conc. Res. 31(2): 193198.

    Persson, B. 2005, Creep of self compacting concrete, inCONCREEP 7, Proc. Int. Conf., Nantes: 535540.

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    Seng, V. & Shima, H. 2005, Creep and shrinkage of self-compacting concrete with different limestone powdercontents, in 4th RILEM Symposium on self-compactingconcrete, Proc. Int. Conf., Chicago: 981987.

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    Chapter 88: Creep and shrinkage of self compacting concrete: Experimental behavior and numerical model1 INTRODUCTION2 MATERIAL PROPERTIES3 STRENGTH EVOLUTION WITH TIME4 SHRINKAGE AND CREEP DEFORMATION4.1 Experimental set-up and instrumentation4.2 Test results4.3 Comparison with international recommendation (MC90)4.4 Effect of aging and sustained load on strength

    5 A SIMPLIFIED NON-LINEAR MODELACKNOWLEDGEMENTSREFERENCES