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A Test Method to Characterize Flexural Creep Behaviourof Pre-cracked FRC Specimens

S.E. Arango & P. Serna & J.R. Mart-Vargas &

E. Garca-Taengua

Received: 26 April 2011 /Accepted: 13 September 2011# Society for Experimental Mechanics 2011

Abstract This paper presents a proposal of test setup andmethodology for testing the flexural creep behaviour of pre-cracked Fibre Reinforced Concrete (FRC) specimens,aimed at providing a basis for standardization. The designcriteria used to define the equipment and methodology are presented. A test results sheet and a curve are established to present the results of creep tests, and some experimentalresults are shown so that the test can be validated. Theequipment and methodology proposed make it possible toresearch the influence of factors such as concrete type,fibres type and content, applied load, and crack openingvalue.

Keywords Test method . Fibre reinforced concrete . Creep .Cracked state . Bending

Introduction

Fibre Reinforced Concrete (FRC) has been widely studied,this resulting in significant advances in knowledge regard-ing toughness and residual strength characterization. Inmost of its applications FRC elements are designed to besubjected to cracking, this way the material works in thecracked state, where residual strength gives the element further mechanical resistance and the capacity to control

crack propagation. For structural applications different codes (MC 2010 [ 1]; EHE-08 [ 2]; ACI 318 [ 3]) proposedesign methods that consider this possibility and assess theFRC contribution by means of the bending test (as the EN14651:2007 [ 4]) and the evaluation of parameters that define flexural behaviour.

Creep is a term used to define the tendency of materialsto develop increasing strains through time when under a sustained load, thus having an increase in deflection or elongation with time in relation to the short-term strain [ 5].Long-term deformations can be beneficial to some types of structures because they lead to a stress redistribution whichcan limit the extent of cracking. At the same time, in thecase of decreasing residual strength or significant deforma-tions the influence of creep will be negative.

Long-term behaviour of FRC has not been considered incodes yet. Studies on creep of FRC in compression indicatethat fibres restrain creep strains when compared to plainmortar and concrete [ 6, 7]. As FRC contribution to structuralload-bearing capacity is based on its flexural response, andmainly in the cracked state, the capacity of the material tokeep the crack opening values low enough to guarantee thereinforcement effectiveness should be assessed [ 8].

The knowledge of the concrete materials and propertiesis essential to better assess their structural applications, andthe development of measurement techniques and experi-mental methods is required [ 9, 10]. The analysis of flexuralcreep behaviour of cracked FRC elements is a relativelynew topic (it has attracted important research attention for the last 10 years only) which has not been entirelyresearched yet. There is no standardized method to assesssuch behaviour at this time.

Publications directly related to flexural creep behaviour of pre-cracked FRC beams are scarce [ 8, 11, 12]. Most of

S.E. Arango : P. Serna ( * ) : J.R. Mart-Vargas :E. Garca-Taengua ICITECH, Institute of Concrete Science and Technology,Universitat Politcnica de Valncia,Camino de Vera, s/n.,46022 Valencia, Spaine-mail: [email protected]

Experimental MechanicsDOI 10.1007/s11340-011-9556-2


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them analyze the contribution of plastic and, in some cases,of glass fibres to such behaviour, and consider steel fibresonly on a comparative basis [ 5, 13 18], not as one of their main objectives. Several serious differences are observed between such studies that compromise the possibility of considering their results as comparable: different method-ologies and test setups, different concrete and fibre types,fibre contents, crack openings or deflections considered,load levels, test specimen, standards, and procedures. As a result, some attempt to establish a methodology and a convenient test setup to study the flexural creep behaviour of pre-cracked FRC specimens was necessary.

Test Method Approach

It comes out of the definition of creep that an adequate test setup must guarantee the application of a constant load for a long period of time, a number of months or even years, andallow the increase of deformations through time.

Taking into account the methodologies and test setups proposed in previous studies [ 5, 8, 12 20] and the require-ments identified, a test methodology has been establishedand the corresponding equipment has been defined to carryout creep tests on cracked specimens under bending loads.

In the design process of the test equipment three parts have been defined: the creep frame and its components, themeasurement devices, and the data acquisition system (DAS).

The creep frame and its components have to be stiff enough to guarantee the application of a constant load and tokeep test conditions steady as long as the test lasts in spite of the deformation of the specimens. The best way to do so is by applying gravity loading directly on top of the test specimen. The relatively high loads to be applied (60 kN)lead to the need of combining this gravity loading with a lever system. A second class lever is employed (Fig. 1).

Because of time and space limitations, the possibility of working with at least three test specimens forming a column in every creep frame has been considered. Thestability of the test specimens (support and loadingconditions) in the column must be guaranteed, the inducedmoments must be prevented and it has to be possible tocontinue the test when one of the test specimens in thecolumn fails. Therefore, a set of elements to apply the loadand to support the test specimens has been designed.

Measurement devices are used to quantify crack openingand compressive strain values as well as the load appliedfor all the time the test takes. Additionally, environmentalconditions (humidity and temperature) must be measuredand registered.

Fig. 1 Second class lever scheme

Fig. 2 Scheme of the creepframe

Fig. 3 Creep frames at the time test (overview and test specimencolumn detail)

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The DAS has to be able to take readings from themeasurement devices with the proper reading cadence andto save the data in order to be analyzed later. Because of thelong time that creep tests take, the system is required to beable to auto-restart so that as few data as possible are lost incase of error.

Creep Frame and Components

The creep frame is made with steel profiles. Figure 2 showsschematics of a testing fixture and Fig. 3 shows generalview of a fixture during the testing.

& A support base (a) composed of two pairs of steel profiles: the two main ones are longitudinal, and theother two are shorter and disposed transversely to thelongitudinal ones.

& The column of test specimens is supported on twovertical profiles (b) welded to the longitudinal ones of the support base.

& The lever (c) is made up of two long profiles, each one placed at a side of the specimen s column and supportedon two vertical profiles (d) both placed at a lower height than the specimens. A plate (e) to hold the counter-weights is disposed on the lever, at the opposite end tothe support point. This plate and the two transversal profiles guarantee the parallelism of the two lever arms by avoiding any relative movement between them.

& The load is transmitted through a couple of screwed bars (f) placed between the support points and thecounterweights plate. Each one is connected to one of the lever long profile. To do that each lever profile hasholes dimensioned to allowing screwed bars traverse it without friction to avoid any parasite load application.

& This way the two screwed bars are placed vertically at same plane than the specimen s column central section.A transversal load plate (g) placed on top of the column

of test specimens connect both screwed bars and issupported over the upper specimen by means of theload transmission element (h). A nut and a washer at thetop and at the bottom of the screwed bars complete thesystem to transmit the load from the lever to thespecimen s column.

& In Fig. 3 a setup adapted for 3 specimens is shown,though this number may be increased by simplymodifying the screwed bars length. The lever, asdesigned, multiplies the applied force by 15. Frameand components detailed dimensions and specificationscan be found in [ 21].

Fig. 4 Positioning and loading elements (schemes and pictures)

Fig. 5 Displacement transducer placed on the test specimen (bottomsurface)

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& Positioning and loading (h and i) elements and thedimensions of the level hole have been designed toguarantee the adequate placement and stability of boththe load transducer and the test specimens. This wayload transmission is produced properly, isostatically andavoiding the transmission of induced efforts. Creep of

the load transmission bar would lead to the increase of its length and the lever slope would vary accordingly.The effect of this and of the creep of other framecomponents on the evolution of transmitted load has been evaluated and it is negligible due to their highstiffness when compared to that of concrete samples being tested. To this end, the load transmission element (h) (on top of the column of test specimens) consists of a rigid steel plate and two semicircular loading points at a distance of 150 mm. One of them allows transversalturning (to prevent the transmission of inducedmoments) and the other one is fixed (Fig. 4(a)). This

element has a slot on top to guarantee the right placement of the load transducer above.

& The support and load transmission element (i) (oneunder every specimen of the test specimens column) isformed by a rigid steel plate, two semicircular support points at a distance of 450 mm (top surface), and twosemicircular load points at a distance of 150 mm(bottom surface). One of the supports allows transversalturning (to prevent the transmission of inducedmoments) and the other one is fixed. On the top surfacetwo flanges have been disposed to preserve the displace-ment transducers in case of failure of some test specimen

in order to be able to continue the test (Fig. 4(b)).

Measurement Devices

The creep test requires to control and measure: the appliedload, crack openings, concrete strains, temperature andhumidity.

To control and measure the load applied to the test specimens load cells are used with a loading head accessoryto guarantee an adequate load transmission and load

measurement. The load cell is placed between the load plate (g) and the load transmission element (h) (Fig. 4(a)).

To measure crack openings displacement transducers are placed under the test specimen bridging the notch, wherecracking occurs (Fig. 5).

To measure concrete strains strain gauges are placed on

top of the test specimens, i.e. the compression surface.Creep tests have been carried out inside a chamber where temperature and humidity are controlled: temperatureis kept at 20C, and relative humidity at 50%. A transducer is used to monitor temperature and humidity levels duringthe test.

Data Acquisition System (DAS)

The data acquisition system consists of a field point and a computer, required to read and save the readings from theload cells, displacement transducers, strain gauges, and the

temperature and humidity transducer. The computer has been configured to continuously save data and, if there is a power failure, it is able to auto-restart and to continuesaving data. This way only the data corresponding with the power failure time are loss. The software used, which has

Fig. 6 Scheme bending test concerning to EN 14651 [ 4] (a )and presents creep frame ( b )

( k N )

(mm)

Fig. 7 Pre-cracking process scheme

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been specifically developed for this test by the ICITECHtechnicians, measures and saves the readings in twodifferent files: one with slow cadence (one reading every3600 s), and the other one with fast readings (one readingevery 5 s), to simplify further analysis.

Test Operative Procedure

The creep test is performed by applying a constant bendingload on a 150150600 mm pre-cracked notched speci-men and by controlling the load-crack opening evolutionfor all the time the test takes.

Once the specimens have been prepared, a creep test consists of three stages:

Specimen pre-cracking up to a nominal crack openingvalue wn .

Creep test on the pre-cracked specimens under anapplied load level IF a .

Final bending test until failure of the specimen that hasexperienced the creep process.

Specimens Casting, Curing and Preparation

Specimens are casted, cured, and prepared by following therecommendations of the EN 14651 standard [ 4] (bendingtest) and other standards and recommendations [ 22 25].

Compaction shall be carried out in order to avoid anyinfluence on the fibre orientation along the control section.In this case compaction was produced by external vibration.

Prior to the test, specimens are rotated by 90 aroundtheir longitudinal axis and then a 25 mm notch is executed

on them by sawing through the width of the test specimenat mid span.

A series of concrete characterization tests must bedeveloped to control the actual concrete mechanical proper ties, in particular: compressive strength, elast icmodulus, and flexural behaviour. The bending test resultswill be used to define the applied load for the creep test.

Bending Test Methodology

The creep test procedure is based on the principles of theEN 14651:2007 [ 4] standard. Nevertheless, as it is not possible to dispose a test specimens column with one point loading system, to guarantee the stability of the column of specimens the test setup requires some aspects to bemodified. For this reason specimens for the pre-cracking,creep test and final bending test stages are tested as shownin Fig. 6: with a 450 mm span and two load points placed at a distance of 150 mm.

Figure 6 comparing schemes of load according to EN14651:2007 [ 4] (a) and proposed on the testing fixture (b).

Test Specimens Pre-cracking

Test specimens pre-cracking is carried out by following the bending test methodology at the controlled crack openingrate according with the EN 14651:2007 [ 4] standardrequirements, but when the nominal crack opening value wn is reached the test is stopped and the specimenunloaded. The load crack opening evolution during theloading, unloading and recovery process is registered.Figure 7 presents a scheme of idealization of the pre-cracking process, where the following parameters are

Fig. 8 Placement of first support element over neoprene(a ) and load transmissionelement and load cell placement ( b )

Fig. 9 Lever arm lifted due to preload ( a ) and counterweights placed on lever arm creepframe working ( b )

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identified: F L-first crack load, w p-maximum crack openingvalue at the pre-cracking process, F w-load at w p, w pr -residual crack opening value at the pre-cracking process.

Creep Test

After the pre-cracking process the test specimens are movedto the test chamber where the creep frames are located. Thespecimens are left inside the chamber for at least two hoursto acclimatize before the creep tests are set up.

The DAS is set to recording mode and then thecreep stage begins: the first support element is placedon top of the vertical support profiles using neoprene toimprove contact and stability (Fig. 8(a)), then the first test specimen is placed over this first support element.Subsequently a support and load transmission element is placed on top of the test specimen previously placed, andso on, until the last test specimen in the column is placed,always minding the transducers and assuring the right

placement of all tes t specimens. Once the las t tes t specimen has been placed, the setup procedure continues by placing the load transmission element on top of the last test specimen, then the load cell is located in the slot left on the top surface of the load transmission element followed by the load plate placed on top of the head of the load cell (Fig. 8(b)).

The free side of the lever arm is placed at a height of 0.35 m from the ground by using a temporarywooden support, and the nuts are tightened (on the

screw bars) by controlling the horizontality of the platewith a bubble level. A 4 kN preload (by tightening thenuts) is applied to free the lever arm (Fig. 9(a)). Thenthe temporary support is removed, the counterweightsneeded to reach the nominal load are placed on the freeend of the lever arm, and the actual applied load value iscontrolled with the load cells (Fig. 9(b)). The loading must be carr ied out very carefully, avoiding sudden loadincreases.

The load to be applied F a is related to the previouslydefined applied load level IF a by this expression:

IF a F a = F w % 1

Once the creep test is set up and thus initialized, it continues without interruption until it is time to unload thespecimens according to the lapse of time previously definedfor the test.

The unloading process takes place by using a manualhydraulic jack to lift the free end of the lever arm

(Fig. 10(a) ), then the nuts that hold the load plate areuntightened, and the load plate, load cell, load transmis-sion element, and counterweights are removed (Fig. 10(b) ). Test specimens remain unloaded but the DASremains activated for 2 weeks in order to record therecovery of deformations. The test ends when specimensare removed from the creep frame to be tested in bendinguntil failure.

Figure 11 presents a scheme of idealization of the crack opening parameters to be obtained from transducer meas-

Fig. 10 Unloading process by using a hydraulic manual jack (a ), and unloadedcreep frame ( b )

L o a

d ( k N )

(min)

( m m

)

(mm)

Fig. 11 Definition of crack opening parameters on idealizedcurves: crack opening relation at the time and load relation at the crack opening (creeploading stage)

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urements during the creep test loading stage. The param-eters of this process are:

F a , kN applied load.wci , mm initial crack opening (at the beginning of thecreep test after stabilizing the load for 1 min).w j cd , mm deferred crack opening at time j .w j ct , mm total crack opening at time j , expressed as:

wct j wci wcd j 2

Figure 12 presents an idealization of the parameters to beobtained from measurements during the post-creep unload-ing and recovery stage.

The parameters of this process are:

wtf ct , mm total crack opening at the time of unloading(final time t f ).wcui , mm instantaneous recovery crack opening at the beginning of the unloading stage, after stabilizing the

load for minute.wcud , mm deferred recovery crack opening after unloading at the recovery stage (crack opening accu-mulated after stabilizing the test specimens unloadedfor minute).wtt cr , mm deferred crack opening at the end of thecreep test (time t t ), after the recovery stage.

wcut , mm total recovery crack opening after theunloading and recovery stage, expressed as:

wcut wtf ct wtt cr wcui wcud 3

Post-creep Bending Test Until Failure

When the creep test is over (loading, unloading andrecovery stages), the test specimens are subject to post-creep bending tests until their failure by following the bending test methodology exposed above, reaching crack opening values higher than 4 mm.

Test Results Presentation

The results of the creep test are presented in a curve of load

relation and crack opening and a test summary sheet.

Test Results Diagram

Each test specimen, during the creep test process, issubjected to: an initial pre-cracking process, a loading, anunloading, and a strains recovery at the creep frame, and to

L o

a d ( k N )

(min)

( m m

)

(mm)

Fig. 12 Definition of crack opening parameters on idealizedcurves: crack opening relationat the time and load relationat the crack opening(post-creep unloading andrecovery stage)

Fig. 13 Complete test resultsdiagram

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Fig. 14 Test results sheet

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a final bending test until failure. Assembling the experi-mental results of all the aforementioned stages and plottingdiagram load crack opening, the evolution of the wholetest is shown in a single curve (Fig. 13), thus making possible to establish a common origin for all crack openingvalues.

The first part of the diagram corresponds to the pre-cracking stage. The curve begins at zero with an ascendinglinear branch until the first crack occurs (A). After this, thetest specimen is gradually loaded until the pre-crackingload value previously defined (B) is reached, and then it istotally unloaded, thus recording its capacity for recovery.Point (C) stands as the starting point for the creep test.

The second part of the diagram is the creep test: it startswith an ascending line (CD) corresponding to the creeploading process, which is followed by a horizontal branch(DE) corresponding to the increasing deferred deformationsthat end up when the test specimen is unloaded. A newdescending line (EF) describes the post-creep unloadingand recovery stage.

The third part of the diagram is the post-creep bendingtest, which begins with an ascending line (FG) andcontinues with the curve which informs of the residual performance of the test specimen (GH).

Test Results Sheet

For every creep test that is performed, a test results sheet must be filled in, this sheet containing the most relevant data (Fig. 14): General data: identifying concrete casting data, and

creep process, including: Creep test basic information, with the series, batch and

concrete identification; test specimen position in thespecimens column (1: top, 3: bottom), and nominal test parameters ( wn pre-cracking, IF n creep load level).

Test agenda. Concrete compressive strength, and elastic modulus. The characterization bending test results, the actual

creep test parameters for each specimen ( F a appliedload F a , and IF a creep load level), and the results of the creep test, as defined in this paper (pre-cracking,loading, unloading, and recovery stages at the creepframes, and final bending test until failure).

If the failure of any test specimens occurs duringthe creep test, some elements are loaded, unloadedand reloaded; the test results sheet must include thenumber of reloading processes for such specimens(incidences).

To make the analysis of all results easier, the test results

sheet evaluates creep behaviour through three groups of parameters:

1. 8 c

wj creep coefficient in the creep stage at different

times, evaluated as the ratio between the deferred crack opening at time j (w j cd ) and the initial crack openingduring the creep test ( wci ).

8 cwj w

j cd =wc 4

2. 8 o

wj creep coefficient referred to the origin at different

times, evaluated as the ratio between the deferred crack opening at time j (w j cd ) and the crack opening at the beginning of the creep test in the complete curve ( woci).

8 owj w

j cd =w

oci 5

Where:

woci w pr wci 6

3. COR t1-t2 crack opening rate for different time periods,evaluated as the ratio between the crack openingincrease during the period t 1 to t 2 (w

t2cd

wt1 cd ) andthe lapse of time t 2 t 1.

COR t 1 t 2 wt 2cd wt 1cd = t 2 t 1 7

The proposed times to evaluate the crack openingrate are: 14, 30, 90, 365 days, and final creep test time.

Compressive strains in the top specimen surface may beevaluated if a strain gage is used to measure them as it has been aforementioned (see Measurement Devices ). The parameters previously defined for crack opening values tofacilitate the analysis of results can be employed for compressive strains as well. This way the strain rate for different time periods ( SRt1 t2 ) is evaluated as the ratio

Table 1 Concretes and test parameters for the creep tests presented

Concrete Cement Fibre Pre-crack opening (mm)

Nominal appliedload level ( IF a )

Type Amount (kg/m 3 ) Type Amount (kg/m 3 )

I-80/35-70-10 CEM I 52.5R 375 Dramix RC80/35BN 70 0.5 80

II-50/30-40-10 CEM I 42.5SR 325 F-DUE 50/30 40 0.5 60

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between the strain increase during the period t 1 to t 2( t2 cd

t1cd ) and the lapse of time t 2 t 1.

SRt 1 t 2 "t 2cd "t 1cd = t 2 t 1 8

If so, the test results sheet (Fig. 14) would have toinclude the same parameters for strains as those defined for crack opening values.

Experimental Test Method Validation

To validate the creep test setup and methodology presentedherein, an ambitious experimental program has beendeveloped at ICITECH laboratories comprising 9 different concretes, one column per concrete with 3 specimens each[21]. The variables considered were: concrete matrix (typeand amount of cement, water/cement ratio, maximum

aggregate size), type and amount of steel fibres, and test parameters (pre-crack opening value and applied load levelduring the creep test stage). As an example, the results fromtwo columns of test specimens are presented. In order toexpose the test method adaptability those results wereobtained from two concretes designed to have a verydifferent structural behaviour. Concrete mix designs and

testing parameters for the creep test considered aredescribed in Table 1.

The Fibre Reinforced Concrete typified as I 80/35-70-10 was made with a 56.1 MPa compressive strengthconcrete matrix replicating a concrete designed for struc-

tural precasting concrete purposes. 70 kg/m3

of 35 mmlength steel cold drawn fibres with hooked ends were used.Fibres length/diameter ratio was 80. With this concrete, inthe characterizations bending tests, a clear hardening post-crack behaviour was obtained.

On the other hand the Fibre Reinforced Concrete typified as II 50/30-40-10 was made with a 41.3 MPa compressivestrength concrete matrix replicating a general purpose con-crete. 70 kg/m 3 of 40 mm length steel cut sheet fibres wereused. Fibres length/diameter ratio was 50. With this concretea clear softening post-crack behaviour was obtained.

Creep curves for both concretes showing the crack

opening experienced in loading, creep, and unloading andrecovery stages are presented in Figs. 15 and 16 for thethree specimens in a column.

Creep test assembled curves including those of pre-cracking, creep test and bending test to failure stages areshown in Figs. 17 and 18. The characterization bending test is included.

specimen 1 specimen 2 specimen 3

Fig. 16 Creep curves for II-50/30-40


Fig. 15 Creep curves for I-80/35-70

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As it can be seen in Figs. 16, 17 and 18, the test methodis capable of performing flexural creep tests on pre-crackedFRC beams satisfactorily, registering the behaviour of thespecimens when being loaded, at the sustained load period(creep stage) and during the unloading and recovery process. Though only the results for two concretes have been presented in this paper, the test method has proved to be equally valid in the remaining 13 concretes that were produced and tested.

In some cases (when high load levels were applied), thefailure of one out of the three test specimens in the columnoccurred, but this did not damage the other specimens inthe same test frame nor affected their curves.

Conclusions

& An experimental methodology to study flexural creep behaviour of cracked FRC specimens has been conceived.

& The creep frame designed optimizes the load applica-tion capacity, guarantees a constant load for whatever lapse of time desired and offers the possibility of testingmore than one element at the same time in one frame.Besides it has been complemented with load transmis-sion and support elements to avoid the transmission of undesirable induced moments and to allow creep teststo continue without any interruption even when thefailure of some specimen occurs.

& The proposed methodology allows studying the influ-ence on FRC cracked beams of variables such as: typeof concrete, type and amount of fibres, applied load and pre-crack opening values.

& This methodology for creep testing is susceptible of being standardized. Standardization criteria can bedefined in two directions: to analyze creep behav-iour in a determined set of condi tions ( for a determined concrete type, mix design, load level,etc.), or to characterize the behaviour of a fibretype in standard conditions (materials and concretemix design, fibre content, pre-cracking and loadlevel, etc.).

& The feasibility of the application of this creep test setupand methodology has been verified in an experimental program to evaluate long-term flexural behaviour of FRC elements in cracked state. Experimental results for two different concretes have been shown to support thisstatement.

& Specific terminology and analysis parameters for thedifferent stages of the test process (pre-cracking, loading,unloading and recovery at the creep test, and a following bending test until failure) have been proposed.

& Experimental results obtained by using the equipment and methodology proposed herein allows analyzing theflexural creep behaviour of cracked FRC specimens, based on the measurement of crack opening values andcompressive strains.


Fig. 18 Creep test assembled curve for II-50/30-40


Fig. 17 Creep test assembled curve for I-80/35-70

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Acknowledgements The present study was supported by theSpanish Universitat Politcnica de Valncia (UPV), Valncia, Spain; by the project BIA 2009 12722 of the Spanish Ministry; and by the project HABITAT 2030 [PS-380000-2008-11] funded by both theSpanish Ministry of Science and Innovation and the EuropeanRegional Development Found. The authors would like to expresstheir gratitude for this support.

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