Nuclear Engineering and Design 237 (2007) 20832089

Chemo-mechanical coupling behavioPart I: Experimental r

d,,

ole Naarnea-DeParisson, 9ry 20

Abstract

This pape adatieffects of th re hicement paste cs ofbeen chosen by using an ammonium nitrate solution instead. The specimens are immersed into a 6 mol/l ammonium nitrate solution with a controlledpH disposal. To quantify the leaching evolution, the degradation depth is then measured at certain time intervals by means of a phenolphthaleinsolution. The experimental results show the chemical degradation of the cement-based material and the important role of aggregate in the calciumleaching process of concrete. Compression tests of concrete samples are also performed. We observe that there is a strong coupling between thecalcium leaching and the mechanical behaviour; as leaching grows, a loss of stiffness and of strength are observed and a smoother post-peakbehaviour is 2007 Else

1. Introdu

Durabilcrete in londurabilityinduced bymay also dconcrete isand concrethe load-bethe level oscenario ofsite water.modificatioother thing

DOI of or Correspon

Cedex 15, FraE-mail ad

0029-5493/$doi:10.1016/jnoted.vier B.V. All rights reserved.

ction

ity will certainly be the key in future use of con-g-term structural applications. In the long term, the

of concrete is not exclusively affected by damagemechanical loads. The lifetime of such a materialepend on the environment. As a typical example,commonly employed in radioactive waste disposal

te containment structures that must therefore ensurearing capacity over extended periods depending onf radioactivity. In the lifetime of nuclear waste, theconcrete degradation is calcium leaching due to on-This leaching implies an increase in porosity, andn of the microstructure of concrete which, amongsts, influences the mechanical behaviour.

iginal article:10.1016/j.nucengdes.2007.02.012.ding author at: LCPC Paris, 58, Boulevard Lefebvre 75732 Parisnce. Tel.: +33 1 40 43 54 40; fax: +33 1 40 43 65 20.dress: jean-michel.torrenti@ponts.org (J.M. Torrenti).

The calcium leaching of the cementitious material is con-trolled by the chemical equilibrium of the hydration production.It depends on two main consecutive phenomena with differentkinetics (see Torrenti et al., 1999):

material transport by diffusion, resulting from concentrationgradients between the solid interstitial solution and the aggres-sive environment outside the cement samples;

dissolutionprecipitation chemical reactions induced by theconcentration variations brought about by diffusion.

The leaching process begins with a total dissolution ofportlandite, then ettringite and followed by a progressive decal-cification of C-S-H phase. Several authors have researchedthe chemical degradation on cement paste and mortar also(Adenot, 1992; Bourdette, 1994; Carde et al., 1996; Gerard,1996; Tognazzi, 1998; Le Bellego, 2001; Ulm et al., 2003)among many other references. Experimental data reveal that theleaching process timescale is governed by the diffusion process,as the dissolution is much faster, i.e. the leaching fluxes are

see front matter 2007 Elsevier B.V. All rights reserved..nucengdes.2007.02.013V.H. Nguyen a, H. Colina b, J.M. Torrenti c,a Laboratoire dAnalyse des Materiaux et Identication, Ec

Institut Navier, 6 et 8, Avenue Blaise Pascal, 77455 Mb ATILH, 7, Place de la Defense, 92974 Paris-L

c LCPC Paris, 58, Boulevard Lefebvre 75732d LMT, ENS Cachan, 61, Avenue du President Wil

Received 10 April 2006; received in revised form 5 Februa

r deals with concrete behaviour under chemical and mechanical degre calcium leaching process of concrete on its mechanical properties a, mortar and concrete samples are presented. Because of the slow kinetiur of leached concreteesultsC. Boulay c, B. Nedjar ationale des Ponts et Chaussees,la Vallee Cedex 2, Francefense Cedex, FranceCedex 15, France4235 Cachan, France07; accepted 14 February 2007

ons. Experimental investigations are described where theghlighted. The calcium leaching and mechanical tests onleaching under deionised water, an accelerated method has

2084 V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089

imposed by diffusion. In the solid phase assemblage, a sharpleaching frzones whements alsothe degrada(Carde et aagrees with2000).

Naturalhundred yewater is noseveral cenessary to leprincipal wture (KamaGerard, 19ent solutiothe interstimajority otious materThe deionammoniumCarde et aTognazzi,2003).

The inflbehaviour oby severalBellego etof the expestiffness ofthe total dition of C-Sa linear vain porositysound crosductility ofthe microsdegradationrelation bethe elastici(Torrenti etBellego (20ties of mortequal to 48mens is abof cement sdependencdue to the ials frictionsee (Heukafor calciumconcrete leferences becontent, sizprocess anin the pre a

The paper is organised as follows. In Section 2, we outlineerimental setup and results on cement paste, mortar andte specimens. In Section 3, the mechanical properties ofte are investigated on cylindrical concrete samples sub-to uniaxial compression tests after accelerated leaching.ean stressmean strain diagrams at different degradationare presented. Finally, the conclusions and perspectiveswn in Section 4.

ign and setup of the accelerated leachingure

aimlea

(cemuenhisg de

ater

con

C cepre

ompcemcemnentur e

entm hilindeers w

etes war tot at t

ealis

his sen used w

expgradhe p

ition

ents

s sands sands sandiliceoCEMont is observed experimentally. These fronts separatere mineralogy is constant (Adenot, 1992). Experi-show that the leaching fluxes and the position oftion front are proportional to the square root of time

l., 1996; Tognazzi, 1998; Torrenti et al., 1998), whichtheoretical considerations (Mainguy and Coussy,

leaching is a very slow process (a few centimetres perars). For laboratory experiments, the use of deionisedt an optimum choice for concrete for which we needtimetres of degradation. Accelerated leaching is nec-ach the samples in a reasonable time. There are threeays to accelerate calcium leaching: using tempera-li, 2003), using an electrical field (Saito et al., 1992;96) and by replacing deionised water with a differ-n agent to increase concentration gradients betweential solution and the aggressive environment. Thef the experiments on calcium leaching of cementi-ial samples are performed by using the last method.ised water is replaced by a strongly concentrated

nitrate solution (Goncalves and Rodrigues, 1991;l., 1996; Carde and Francois, 1997; Gerard, 1996;1998; Le Bellego, 2001; Ulm et al., 2003; Kamali,

uence of calcium leaching on the mechanicalf the cement paste and mortar has been investigatedauthors (Carde et al., 1996; Ulm et al., 1999; Leal., 2001; Heukamp et al., 2001). In their analysisrimental results, Carde et al. (1996) show that thethe material specimens is significantly reduced afterssolution of portlandite and the progressive dissolu--H. Both mechanical and water porosity tests showriation between the loss of strength and the growthin relation to the ratio between the degraded and thes-sections. Compared with the sound material, thethe chemically degraded material is larger because

tructure is modified. The influence of the chemicalon mechanical behaviour has been presented by a

tween the calcium concentration in solid phase andty modulus, as in (Gerard, 1996) or the strength as inal., 1998) by using micro-hardness experiments. Le01) has shown recently that the mechanical proper-ar decrease as leaching grows. For degradation ratios, 59 and 74%, the loss of stiffness of mortar speci-

out 23, 36 and 53%, respectively. From triaxial testspecimens subjected to accelerated leaching, a strong

e of the mechanical properties on the pore pressurencreased pore space and the reduction of the materi-al performance of the leached cement paste is noted,mp et al., 2001, 2003). However, experimental result

leaching of concrete and post-peak behaviour ofached are not yet available in literature data. The dif-tween cement paste or mortar and concrete (cemente of aggregates, . . .) should influence the leaching

d the mechanical behaviour of the leached concretend post-peak regimes.

the expconcre

concre

jectedThe mlevelsare dra

2. Desproced

Thecalciumscalesthe inflcrete. Tleachin

2.1. M

Thean OPused iscrete cwaterwatercompo

In oof cem100 mfull cycylindof diamsamplein ordeand no

2.2. R

In thas bedeionilightedThe deusing t

Table 1Compos

Compon

SiliceouSiliceouSiliceouCrotoy sCementWaterof the experimental campaign is to determine theching kinetic of the cementitious phase at differentent paste, mortar and concrete). It allows to observe

ce of the aggregate on the calcium leaching of con-section presents design considerations for the calciumvice and its practical application and results.

ials and samples

crete was composed of siliceous aggregates andment (CEM 1 52,5). The composition of concretesented in Table 1, where the ratio between the con-onents is: cement:sand:gravel = 1:1.82:2.8, with a

ent ratio of 0.6. The cement paste has the sameent ratio equal to 0.6 and the ratio between the mortars is: water:cement:sand = 0.6:1:1.82.xperimental program, the samples used for the casepaste and mortar are cylinders 32 mm diameter andgh. Two types of specimens are used for concrete:rs with dimensions 110 mm 220 mm and hollowith the same external dimensions and a centred hole

r = 27 mm. The top and the bottom of the concretes protected from leaching by means of a silicon resinhave only leaching in the central part of the sampleshe level of loaded surfaces.

ation of the calcium leaching process

tudy, a degradation under ammonium nitrate solutionsed. The equivalence of the leaching process underater and under ammonium nitrate solution is high-

erimentally by Carde et al. (1996), Tognazzi (1998).ed state of specimens may be determined easily byH indicators.

of a 1 m3 of concrete

Quantity (kg)S28 (0.20.5 mm) 281S30 (0.41.0 mm) 193S36 (1.03.15 mm) 210

us gravel (4.012.5 mm) 1050I 375

225

V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089 2085

Fig. 1. Schemnitrate solutio

After 5 mimmersed iequal to 6 mcalcium leaare recordewas chosenavoid renewcalcium thaammoniuming procesbetween thsive solutioinduce a dsolution towof time (36of concretethe chemic

2.3. Result

To avoidplaned to bage. But limwe presentstone. Thisleaching prnique used

Table 2Experimental results of chemical degradation of limestone

Length (cm) Diameter (cm) Weight (g) PorositySound state

0.79 2.85 2.70 28.6After 142 days of degradation

0.60 2.70 1.40 32.3

degradation, we show that the specimens of limestone have alimited increase in porosity and mainly a significant reductionin volume and mass (see Table 2 and Fig. 2).

This result shows that limestone is unstable in the ammo-nium nitrate solution. This explains the choice to use siliceousaggregates for our tests. For real storage structures limestone

stabcalcof sdegon s

entinviroH vnt. Wuishhaleiic soer, tuese exer, b

haleihaset ca

mea

.17 e

cros

erveenolpoloure of the calcium leaching test of concrete under 6 M ammoniumn.

onths of curing (water storage), the specimens weren an ammonium nitrate solution with a concentration

ol/l (6 M). The experimental setup for acceleratedching is presented in Fig. 1. The pH and temperatured by an acquisition system. The volume of solutionto avoid renewal: while pH is lower than 8.2 we caning (Le Bellego, 2001). Knowing the quantity of

t would be leached we can estimate the quantity ofnitrate we need to respect this condition. The leach-

s results from the high gradients of concentrationse pore solution in the cement paste and the aggres-n that surrounds the samples. These gradients then

iffusion process of the calcium present in the poreards the environment. Subsequently, at each period

, 57, 105, 152, 163, 197, 274 and 547 days for the case), specimens were extracted to measure the depth ofal degradation and test their mechanical behaviour.

s

alkali-aggregate reaction, limestone aggregates aree used for concrete in the case of nuclear waste stor-estone could be leached in our solution. That is why

firstly the result of the chemical degradation of lime-

will beties ofinstead

Thethaleinof cembasic ewith pronme

distingnolphtin basHowevpH valgive thHowevnolpht2001)depthephenol

et = 1In thebe obsthe phgrey cexperiment has been performed to characterise theoperties of limestone as well as to check the tech-and precautions for the later test. After 142 days of

The thicthe measurdegraded d

Fig. 2. Photo of the limestone sample on the sound state and ale, because on-site water contains sufficient quanti-ium. Calcareous aggregates could therefore be usediliceous ones for nuclear waste storage.raded depth is determined by using the phenolph-ectioned samples. The pH value in the pore solutiontious materials is higher than 12.5, creating a verynment. Consequently, an ammonium nitrate solution

alues below this level characterises the acid envi-e can use the pH indicator like phenolphthalein tobetween the sound zone and the degraded zone. Phe-n turns from colourless in acidic solutions to pinklutions with the transition occurring around pH 9.he dissolution of portlandite occurs as long as thedrop below 12. Therefore, phenolphthalein does notact position of the dissolution front of portlandite.y comparison between the measurement by phe-n and by SIMS microprobe analysis (Le Bellego,shown that for the cement paste the total degradedn be determined by correcting the degraded depthsured by phenolphthalein with the formula:

phenol (1)s-section of the specimens, two distinct zones mayd: a sound zone, i.e. with the pink colour caused byhthalein solution, and a degraded zone, i.e. with the(see Figs. 39).

kness of this degraded zone can be measured by usingements at 16 points around the specimen. The totalepth is the mean of these measurements.fter 142 days of degradation.

2086 V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089

Fig. 3. Degradation state of the cement cylinder after 14 days. The pH indicatoris phenolphthalein (pink when pH > 12.5). (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of thearticle.)

The relaof time

t

paste, mortthat the kinmaterial isusing Fick

Compartar, it is intealmost theand the expdifference.ence betwe15% (see Fof the aggrlogical geo

Fig. 4

Fig. 5. Degradation state of the concrete hollow cylinder after 36 days.

g. 6. Degradation state of the concrete full cylinder after 36 days.tion between the degraded depth and the square rootshows a linear evolution for the three cases (cementar and concrete) (see Figs. 10 and 11). This meansetics of the chemical degradation of the cementitiousgoverned by a diffusion process and can be describeds law.ing the results obtained with cement paste and mor-resting to note that the degraded depth evolutions are

same for the two cases. A comparison of these resultserimental results of concrete highlights a significantFor example, after 25 days of degradation, the differ-en the degraded depth in concrete and mortar is aboutigs. 10 and 11). These results highlight the influenceegates in concrete. Its volume fraction and morpho-metries obviously play an important role. This effect Fi. Degradation state of the mortar cylinder after 14 days. Fig. 7. Degradation state of the concrete hollow cylinder after 197 days.

V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089 2087

Fig. 8. D

is accounteet al., 2006

Comparwe note thinternal degexternal. Tin the holenot possibl

3. Mechan

3.1. Exper

At eachspecimensthe compleand strengtimum capaplaced betwsometer isdisplacemebase lengththree LVDTthis displac

Fig. 9. De

Degrd mor

fnescurve

esult

mean strain is the relative variation of the base length l0 ofensometer (110 mm). This is only a mean strain becauselocalisation process in the softening range (Torrenti et al.,

mean compressive stress is evaluated by dividing theby the area of the cross-section of specimen S. It is a meanbecause, due to the heterogeneity of the chemical degra-egradation state of the concrete full cylinder after 197 days.

d for by using the homogenisation method (Nguyen).ing the results obtained with full and hollow cylinder,at the external degraded depth is the same. But theraded depth of the hollow cylinder is lower than the

his seems to be due to a different boundary condition: the calcium concentration should be higher (it wase to check this assumption).

ical behaviour of leached concrete

imental setup

time interval of the chemical degradation, concreteare subjected to compression tests in order to measurete mean stressmean strain curve, the mean stiffnessh. The device used is a MFL-5000 press with a max-city of 5000 kN. An extensometer and three LVDTeen the platens are used (see Fig. 12). The exten-

clamped directly on the specimen and measures the

Fig. 10.paste an

the stifstrain

3.2. R

Thethe extof the1993).

Theload Fstressnt of the central part of the sample on a 110 mm(Boulay and Colson, 1981). The mean value of theis used to control the test. Loading is controlled by

ement between the platens with cycles to determine

gradation state of the concrete hollow cylinder after 163 days.Fig. 11. Degrspecimens.aded depth evolution vs. the square root of time for the cementtar specimens.

s, the mean strength, the complete mean stressmeans and irreversible deformations.

saded depth evolution vs. the square root of time for the concrete

2088 V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089

Fig. 12. Experimental setup for the compressive test of concrete.

dation, we have different local mechanical properties (Youngsmodulus for instance) conducting to different local stresses inthe samples.

We obsestrength desponding toby (Ulm etmodulus. F

In additipression tetimes are shbetween thThe soundleaching grimportant stimes highwere obtain

In the cathe existenThese defoan evolutiomaterial. T

Fig. 13. Evolhollow cylind

Fig. 14. Evolution of the strength vs. the degradation time for full and hollowcylinders.

Mean stress vs. mean strain curves at different degradation times (hol-nders).

that leached C-S-H is a cohesive incompressible mate-d that the pores created by the calcium leaching providesfor the incompressible solid during compressive loadingamp et al., 2003).ntually, one should note that in only two cases ourould be considered homogeneous: for plain concreter totally leached concrete. In these cases, the constitu-rve that the mean stiffness and the mean compressivecrease with the degradation time until values corre-a totally degraded concrete are stabilised. As found

al., 2002), we observe residual strength and Youngsigs. 13 and 14 illustrate these evolutions.on, and by way of illustration, the results of the com-sts for the hollow cylinders at different degradationown in Fig. 15. We note that there is a strong couplinge calcium leaching and the mechanical behaviour.concrete has an almost brittle behaviour, while as theows, concrete becomes more and more ductile withtrains: the mean strain at peak stress is almost threeer when concrete is totally leached. Similar resultsed by (Le Bellego, 2001) with mortar.se of cyclic loading, the experimental results reveal

ce of inelastic deformations (see Figs. 16 and 17).rmations are larger with the leaching time. There isn when concrete is leached towards a more plastichis is coherent with Heukamps results who have

Fig. 15.low cyli

shownrial anspace(Heuk

Evetests cand foution of the mean stiffness vs. the degradation time for full anders.

Fig. 16. Meanof degradationstress vs. mean strain curves under cyclic loading after 197 days(hollow cylinders).

V.H. Nguyen et al. / Nuclear Engineering and Design 237 (2007) 20832089 2089

Fig. 17. Meadays of degra

tive behaviintermedianon-homog

4. Conclu

We havemechanicatest on cemics of calcshown cleaprocess ofmechanicacoupling bbehaviour.is observeding irreversa plastic-likconcrete.

Acknowled

FinanciaENPC-IRSThis suppo

Reference

Adenot, F., 19cessus phUniversite

Bourdette, B.transitionchimiques

Boulay, C., Colson, A., 1981. Un extensome`tre eliminant linfluence desdeformations transversales sur la mesure des deformations longitudinales.Materiaux et Constructions 14 (79), 3538 (in French).

Carde, C., Francois, R., 1997. Effect of the leaching of calcium hydroxide fromcement paste on mechanical and physical properties. Cem. Concr. Res. 27(4), 539550.

Carde, C., Francois, R., Torrenti, J.M., 1996. Leaching of both calcium hydrox-yde and C-S-H from cement paste: modeling the mechanical behaviour. Cem.Concr. Res. 26 (8), 12571268.

Gerard, B., 1996. Contribution des couplages mecaniques-chimie-transfert dansla tenue a` long terme des ouvrages de stockage de dechets radioactifs. Ph.D.Thesis. ENS Cachan et Universite Laval, Quebec, Canada.

es, Ate attreal,p, F.

ium-lepressp, F.

ium-le.S., 2

ronnean (igo, CueesCach

go, Crete s.), Fre, pp.y, M.,ride p, V.H.ogeniput. M., Nake det

. Des.i, C.riauxouse, J.Mnsc, J.M.n coneedinema,

, J.M.on s

, 2369ns stress vs. means strain curves under cyclic loading after 679dation (hollow cylinders).

our could be obtained directly from the test. For allte states, an inverse analysis that take into account aeneous degraded state should be considered.

sions

presented an experimental program of the chemo-l behaviour of leached concrete. Calcium leachingent paste, mortar and concrete highlighted the kinet-ium leaching of cementitious materials and haverly the important role of aggregates on the leachingconcrete. The mechanical tests show the chemo-

l behaviour of leached concrete: there is a strongetween the calcium leaching and the mechanicalAs leaching grows, a loss of strength and stiffness, and a smoother post-peak behaviour with increas-ible straining is to be noted. Cyclic loading suggestse constitutive behaviour for the completely leached

gement

l support for this research was provided by theN collaboration under project no. IRSN/2002-03827.rt is gratefully acknowledged.

GoncalvnitraMon

Heukamcalcpore

Heukamcalc1173

Kamali,enviCach

Le BelleattaqENS

Le Belleconc

(EdsLiss

Mainguchlo

NguyenhomCom

Saito, Hon thEng

TognazzmateToul

Torrentibeto

Torrentiing iProcBalk

Torrentitions(12)s

92. Durabilite du beton: Caracterisation et modelisation des pro-ysiques et chimiques de degradation du ciment. Ph.D. Thesis.dOrleans (in French).

, 1994. Durabilite du mortier: Prise en compte des aureoles dedans la caracterisation et modelisation des processus physiques etdalteration. Ph.D. Thesis. INSA de Toulouse (in French).

Ulm, F.J., Toleaching i

Ulm, F.J., Hecement-ba211, 516

Ulm, F.J., Lemof leachin70, 8718., Rodrigues, X., 1991. The resistance of cements to ammoniumack. In: Durability of Concrete, second International Conference,Canada.H., Ulm, F.J., Germaine, J.T., 2001. Mechanical properties ofached cement pastes. Triaxial stress states and influence of theures. Cem. Concr. Res. 31, 767774.H., Ulm, F.J., Germaine, J.T., 2003. Poroplastic properties ofached cement-based materials. Cem. Concr. Res. 33, 1155

003. Comportement et simulation des materiaux cimentaires enment agressifs: lixiviation et temperature. Ph.D. Thesis. ENS-n French).., 2001. Couplages chimie-mecanique dans les structures en betonpar leau: etude experimental et analyse numerique. Ph.D. Thesis.an (in French).., Gerard, B., Pijaudier-Cabot, G., 2001. Mechanical analysis oftructures submitted to an aggressive water. In: de Borst, R., et al.acture Mechanics of Concrete Structures. Swets and Zeitlinger,239246.Coussy, O., 2000. Propagation fronts during calcium leaching andenetration. J. Eng. Mech. 126 (3), 250257., Nedjar, H., Colina, B., Torrenti, J.M., 2006. A separation of scaleszation analysis for the modelli ng of calcium leaching in concrete.

ethods Appl. Mech. Eng. 195, 71967210.ane, S., Ikari, Fujiwara, A., 1992. Preliminary experimental study

erioration of cementitious materials by acceleration method. Nucl.138, 151155.

, 1998. Couplage fissuration-degradation chimique dans descimentaires: caracterisation et modelisation. Ph.D. Thesis. INSA

(in French).., Didry, O., Ollivier, J.P., Plas, F., 1999. La degradation desouplage fissuration-degradation chimique, Herme`s., Mainguy, M., Adenot, F., Tognazzi, C., 1998. Modelling of leach-crete. In: de Borst, R., Bicanic, N., Mang, H., Meschke, G. (Eds.),g of Euro-C 98, Computational Modelling of Concrete Structure.Rotterdam, The Netherland, pp. 531538., Benaija, E.H., Boulay, C., 1993. Influences of boundary condi-train softening in concrete compression test. J. Eng. Mech. 1192384.rrenti, J.M., Adenot, F., 1999. Chemoporoplasticity of calciumn concrete. J. Eng. Mech. (ASCE) 125 (10), 12001211.ukamp, F.H., Germaine, J.T., 2002. Residual design strength ofsed materials for nuclear waste storage systems. Nucl. Eng. Des.0.archand, E., Heukamp, F.H., 2003. Elements of chemomechanics

g of cement-based materials at different scales. Eng. Frac. Mech.89.

Chemo-mechanical coupling behaviour of leached concreteIntroductionDesign and setup of the accelerated leaching procedureMaterials and samplesRealisation of the calcium leaching processResults

Mechanical behaviour of leached concreteExperimental setupResults

ConclusionsAcknowledgementReferences