Dépouillements

Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements / Thomas Schmidt in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1113-1121 Titre : Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements Type de document : texte imprimé Auteurs : Thomas Schmidt, Auteur ; Barbara Lothenbach, Auteur ; Michael Romera, Auteur Année de publication : 2010 Article en page(s) : pp. 1113-1121 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Sulfate  attack;  Ettringite;  Gypsum;  Thaumasite;  Temperature;Thermodynamic  modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na2SO4/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking. The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm). DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002063 [article] Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements [texte imprimé] / Thomas Schmidt, Auteur ; Barbara Lothenbach, Auteur ; Michael Romera, Auteur . - 2010 . - pp. 1113-1121. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1113-1121 Mots-clés : Sulfate  attack;  Ettringite;  Gypsum;  Thaumasite;  Temperature;Thermodynamic  modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na2SO4/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking. The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm). DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002063

Critical chloride content in reinforced concrete — A review / Ueli Angst in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1122-1138 Titre : Critical chloride content in reinforced concrete — A review Type de document : texte imprimé Auteurs : Ueli Angst, Auteur ; Bernhard Elsenerb, Auteur ; Claus K. Larsena, Auteur Année de publication : 2010 Article en page(s) : pp. 1122-1138 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Corrosion;  Durability;  Chloride;  Reinforcement Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Chloride induced corrosion as the major cause for degradation of reinforced concrete has been the subject of great research efforts over the last fifty years. The present literature review summarises the state of the art by presenting the concept of the critical chloride content, discussing influencing factors, and assessing available measurement techniques. A large number of published chloride threshold values together with the respective experimental details are collected. While today's experience is mostly based on Portland cement, more modern studies with non-traditional binders often reported contradictory results. The present literature evaluation highlights the strong need for a practice-related test method, and, in this regard, focuses especially on experimental procedures by discussing advantages and drawbacks of methods and setups. It clearly emerges that many of the setups used to determine critical chloride contents are not suited to give realistic results. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002099 [article] Critical chloride content in reinforced concrete — A review [texte imprimé] / Ueli Angst, Auteur ; Bernhard Elsenerb, Auteur ; Claus K. Larsena, Auteur . - 2010 . - pp. 1122-1138. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1122-1138 Mots-clés : Corrosion;  Durability;  Chloride;  Reinforcement Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Chloride induced corrosion as the major cause for degradation of reinforced concrete has been the subject of great research efforts over the last fifty years. The present literature review summarises the state of the art by presenting the concept of the critical chloride content, discussing influencing factors, and assessing available measurement techniques. A large number of published chloride threshold values together with the respective experimental details are collected. While today's experience is mostly based on Portland cement, more modern studies with non-traditional binders often reported contradictory results. The present literature evaluation highlights the strong need for a practice-related test method, and, in this regard, focuses especially on experimental procedures by discussing advantages and drawbacks of methods and setups. It clearly emerges that many of the setups used to determine critical chloride contents are not suited to give realistic results. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002099

Experimental evidence of a moisture clog effect in cement-based materials under temperature / Xiao-Ting Chen in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1139-1148 Titre : Experimental evidence of a moisture clog effect in cement-based materials under temperature Type de document : texte imprimé Auteurs : Xiao-Ting Chen, Auteur ; Th. Rougelot, Auteur ; C.A. Davy, Auteur Année de publication : 2010 Article en page(s) : pp. 1139-1148 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Permeability;  Moisture  clog;  Temperature;  Saturation  level;  Scale  effect Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : This study is an original contribution to the understanding of the hydraulic behaviour of cement-based materials when subjected to temperature rises. Permeability is measured continuously during heating by injecting inert gas into a sample at homogeneous temperature. Using a confining cell especially designed in our laboratory, the sample is submitted to a constant heating rate, up to 200 °C, superimposed to hydrostatic pressure (at ca. 5 MPa). In parallel with a normalised CEM II mortar (water-to-cement ratio (W/C) of 0.5), a CEM V-cement-based concrete, used in nuclear waste storage applications, is studied. For normalised mortar, gas retention is evidenced, depending on the sample size (scale effect), water saturation level Sw, and heating rate. For dry normalised mortar, permeability may be divided by two during heating. In conjunction with thermo-gravimetry analysis (TGA) results, such evolution is attributed to the dehydration of C–S–H around 150 °C. Indeed, mass loss after heat cycling is substantially higher than that due to free water release solely: mortar loses structural, bound water during the process. For partially-saturated and long mortar samples, a gas retention phenomenon is recorded when heating at a rate of ca. 4.9 °C/min. Our analysis is that free water inside the macropores, as well as bound water released from the C–S–H, dilates or vaporizes, and obstructs the interconnected porous network. Due to moisture clogging, no more gas is allowed through the material pore network: a so-called gas retention phenomenon occurs. Most interestingly, although loosing structural water like normalised mortar, yet over a wider temperature range, dry CEM V concrete displays good temperature resistance, as its permeability remains constant during heating. For highly partially-saturated concrete, a gas retention effect is recorded. As a conclusion, observed phenomena at the laboratory scale testify of potentially strong gas retention effects upon engineering structures subjected to temperature gradients over time. Indeed, quite low temperature rises (and heating rates) are able to induce moisture clogging inside partially-saturated materials. It is also concluded that cement-based material composition, i.e. bound water release ability, is influential in gas transport phenomena under temperature. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900177X [article] Experimental evidence of a moisture clog effect in cement-based materials under temperature [texte imprimé] / Xiao-Ting Chen, Auteur ; Th. Rougelot, Auteur ; C.A. Davy, Auteur . - 2010 . - pp. 1139-1148. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1139-1148 Mots-clés : Permeability;  Moisture  clog;  Temperature;  Saturation  level;  Scale  effect Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : This study is an original contribution to the understanding of the hydraulic behaviour of cement-based materials when subjected to temperature rises. Permeability is measured continuously during heating by injecting inert gas into a sample at homogeneous temperature. Using a confining cell especially designed in our laboratory, the sample is submitted to a constant heating rate, up to 200 °C, superimposed to hydrostatic pressure (at ca. 5 MPa). In parallel with a normalised CEM II mortar (water-to-cement ratio (W/C) of 0.5), a CEM V-cement-based concrete, used in nuclear waste storage applications, is studied. For normalised mortar, gas retention is evidenced, depending on the sample size (scale effect), water saturation level Sw, and heating rate. For dry normalised mortar, permeability may be divided by two during heating. In conjunction with thermo-gravimetry analysis (TGA) results, such evolution is attributed to the dehydration of C–S–H around 150 °C. Indeed, mass loss after heat cycling is substantially higher than that due to free water release solely: mortar loses structural, bound water during the process. For partially-saturated and long mortar samples, a gas retention phenomenon is recorded when heating at a rate of ca. 4.9 °C/min. Our analysis is that free water inside the macropores, as well as bound water released from the C–S–H, dilates or vaporizes, and obstructs the interconnected porous network. Due to moisture clogging, no more gas is allowed through the material pore network: a so-called gas retention phenomenon occurs. Most interestingly, although loosing structural water like normalised mortar, yet over a wider temperature range, dry CEM V concrete displays good temperature resistance, as its permeability remains constant during heating. For highly partially-saturated concrete, a gas retention effect is recorded. As a conclusion, observed phenomena at the laboratory scale testify of potentially strong gas retention effects upon engineering structures subjected to temperature gradients over time. Indeed, quite low temperature rises (and heating rates) are able to induce moisture clogging inside partially-saturated materials. It is also concluded that cement-based material composition, i.e. bound water release ability, is influential in gas transport phenomena under temperature. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900177X

Modeling of diffusive mass transport in micropores in cement based materials / Yamaguchia Tetsuji in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1149-1154 Titre : Modeling of diffusive mass transport in micropores in cement based materials Type de document : texte imprimé Auteurs : Yamaguchia Tetsuji, Auteur ; Kumi Negishi, Auteur ; Seiichi Hoshino, Auteur Année de publication : 2010 Article en page(s) : pp. 1149-1154 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Microstructure  ;  Diffusion  ;  Long-term  performance  ;  Modeling  ;  Waste  management Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002257 [article] Modeling of diffusive mass transport in micropores in cement based materials [texte imprimé] / Yamaguchia Tetsuji, Auteur ; Kumi Negishi, Auteur ; Seiichi Hoshino, Auteur . - 2010 . - pp. 1149-1154. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1149-1154 Mots-clés : Microstructure  ;  Diffusion  ;  Long-term  performance  ;  Modeling  ;  Waste  management Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002257

Effect of carbonation on the hydro-mechanical properties of Portland cements / A. Fabbria in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp.1156-1163 Titre : Effect of carbonation on the hydro-mechanical properties of Portland cements Type de document : texte imprimé Auteurs : A. Fabbria, Auteur ; J. Corvisiera, Auteur ; A. Schubnel, Auteur Année de publication : 2010 Article en page(s) : pp.1156-1163 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Characterization  ;  Microcraking  ;  Carbonation  ;  Elastic  moduli  Permeability Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : We evaluate experimentally the effect of carbonation on the hydro-mechanical properties of Portland cement. Samples were carbonated at 90 °C and 28 MPa under wet supercritical CO2. Two types of carbonation features were achieved, either the samples were homogeneously carbonated or they displayed sharp carbonation fronts. Using a tri-axial apparatus, the static elastic moduli and the mechanical strength were measured at in-situ pressure conditions (28 MPa) and showed a degradation of the mechanical properties of the samples where a carbonation front prevailed. Water and gas permeabilities were measured and showed that the samples with a carbonation front exhibit a stress sensitive permeability. P and S elastic wave velocities were measured to evaluate dynamic (ultrasonic range, 1 MHz) elastic moduli. The use of an effective medium theory approach enabled us to characterize the density and distribution of cracks within the samples. This approach outlines that the samples which developed a carbonation front were damaged. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002014 [article] Effect of carbonation on the hydro-mechanical properties of Portland cements [texte imprimé] / A. Fabbria, Auteur ; J. Corvisiera, Auteur ; A. Schubnel, Auteur . - 2010 . - pp.1156-1163. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp.1156-1163 Mots-clés : Characterization  ;  Microcraking  ;  Carbonation  ;  Elastic  moduli  Permeability Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : We evaluate experimentally the effect of carbonation on the hydro-mechanical properties of Portland cement. Samples were carbonated at 90 °C and 28 MPa under wet supercritical CO2. Two types of carbonation features were achieved, either the samples were homogeneously carbonated or they displayed sharp carbonation fronts. Using a tri-axial apparatus, the static elastic moduli and the mechanical strength were measured at in-situ pressure conditions (28 MPa) and showed a degradation of the mechanical properties of the samples where a carbonation front prevailed. Water and gas permeabilities were measured and showed that the samples with a carbonation front exhibit a stress sensitive permeability. P and S elastic wave velocities were measured to evaluate dynamic (ultrasonic range, 1 MHz) elastic moduli. The use of an effective medium theory approach enabled us to characterize the density and distribution of cracks within the samples. This approach outlines that the samples which developed a carbonation front were damaged. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002014

Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties / F. Cassagnabèrea in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1164-1173 Titre : Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties Type de document : texte imprimé Auteurs : F. Cassagnabèrea, Auteur ; M. Mouret, Auteur ; G. Escadeillas, Auteur Année de publication : 2010 Article en page(s) : pp. 1164-1173 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Cement/slag/MK  binder  ;  Steam  curing  ;  Early  age;  Hydration  product Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : High strength can be obtained at early ages for precast concrete elements by the use of CEMI 52.5R cement (OPC) and thermal treatment (steam curing). To compensate for the announced withdrawal of CEM I cements because of high CO2 emissions during their production and the ecotax that this will imply, one attractive alternative is the use of composed cements resulting from the combination of clinker with mineral admixtures. In steam curing conditions, previous studies have shown an increase in the compressive strength at one day of age for mortars incorporating an OPC/blast furnace slag (GGBS)/metakaolin (MK) combination, in comparison with mortars incorporating OPC only. The present study investigates the connection between the compressive strength, at one day of age, of steam cured mortars made with various binders and the hydration of these binders. The progress of the hydration was characterised by means of XRD, thermal and microprobe analyses. The results indicate that the increase in compressive strength when MK is incorporated (OPC/MK or OPC/MK/GGBS) can be explained by an increase in the amount of C-S-H, C-A-H, C-A-S-H phases, a decrease in the amount of CH and a change in the chemical nature of the matrix (decrease in C/S ratio). The decrease in compressive strength of OPC/slag-based material can be explained by a reduction in the amount of hydrated phases (particularly C-S-H) and compactness. These are promising results for precast concrete manufacturers who are concerned about preserving the environment. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001859 [article] Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties [texte imprimé] / F. Cassagnabèrea, Auteur ; M. Mouret, Auteur ; G. Escadeillas, Auteur . - 2010 . - pp. 1164-1173. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1164-1173 Mots-clés : Cement/slag/MK  binder  ;  Steam  curing  ;  Early  age;  Hydration  product Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : High strength can be obtained at early ages for precast concrete elements by the use of CEMI 52.5R cement (OPC) and thermal treatment (steam curing). To compensate for the announced withdrawal of CEM I cements because of high CO2 emissions during their production and the ecotax that this will imply, one attractive alternative is the use of composed cements resulting from the combination of clinker with mineral admixtures. In steam curing conditions, previous studies have shown an increase in the compressive strength at one day of age for mortars incorporating an OPC/blast furnace slag (GGBS)/metakaolin (MK) combination, in comparison with mortars incorporating OPC only. The present study investigates the connection between the compressive strength, at one day of age, of steam cured mortars made with various binders and the hydration of these binders. The progress of the hydration was characterised by means of XRD, thermal and microprobe analyses. The results indicate that the increase in compressive strength when MK is incorporated (OPC/MK or OPC/MK/GGBS) can be explained by an increase in the amount of C-S-H, C-A-H, C-A-S-H phases, a decrease in the amount of CH and a change in the chemical nature of the matrix (decrease in C/S ratio). The decrease in compressive strength of OPC/slag-based material can be explained by a reduction in the amount of hydrated phases (particularly C-S-H) and compactness. These are promising results for precast concrete manufacturers who are concerned about preserving the environment. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001859

Study of ion cluster reorientation process of geopolymerisation reaction using semi-empirical AM1 calculations / Zhang Yunsheng in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1174-1175 Titre : Study of ion cluster reorientation process of geopolymerisation reaction using semi-empirical AM1 calculations Type de document : texte imprimé Auteurs : Zhang Yunsheng, Auteur ; Jia Yantao, Auteur ; Wei Sun, Auteur Année de publication : 2010 Article en page(s) : pp. 1174-1175 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Geopolymeric  cement;  Ion  clusters  reorientation;  Semi-empirical  AM1  calculations Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : In this paper all the possible reaction pathways involved in ion clusters reorientation process of metakaolin based Geopolymeric cement were systemically investigated according to thermodynamic theory. The reaction energy of every possible reorientation pathway was also calculated using computational chemistry method-semi-empirical AM1 calculation. The optimum reorientation pathway was analyzed based on the energy-minimized principle. The calculation results showed that Si–Al hybrid reorientation should be primary reorientation pathway of Geopolymeric cement made with NaOH or KOH activated metakaolin. This viewpoint was also verified by Nuclear Magnetic Resonance Spectroscopy equipped with Magic Angle Spinning technique (MAS-NMR). In addition, the semi-empirical AM1 calculations revealed that the reorientation reaction is easier and stronger in NaOH solution than in KOH solution. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001823 [article] Study of ion cluster reorientation process of geopolymerisation reaction using semi-empirical AM1 calculations [texte imprimé] / Zhang Yunsheng, Auteur ; Jia Yantao, Auteur ; Wei Sun, Auteur . - 2010 . - pp. 1174-1175. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1174-1175 Mots-clés : Geopolymeric  cement;  Ion  clusters  reorientation;  Semi-empirical  AM1  calculations Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : In this paper all the possible reaction pathways involved in ion clusters reorientation process of metakaolin based Geopolymeric cement were systemically investigated according to thermodynamic theory. The reaction energy of every possible reorientation pathway was also calculated using computational chemistry method-semi-empirical AM1 calculation. The optimum reorientation pathway was analyzed based on the energy-minimized principle. The calculation results showed that Si–Al hybrid reorientation should be primary reorientation pathway of Geopolymeric cement made with NaOH or KOH activated metakaolin. This viewpoint was also verified by Nuclear Magnetic Resonance Spectroscopy equipped with Magic Angle Spinning technique (MAS-NMR). In addition, the semi-empirical AM1 calculations revealed that the reorientation reaction is easier and stronger in NaOH solution than in KOH solution. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001823

Hydration of calcium sulfoaluminate cement by a ZnCl2 solution: Investigation at early age / Stéphane Bergera in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1180-1187 Titre : Hydration of calcium sulfoaluminate cement by a ZnCl2 solution: Investigation at early age Type de document : texte imprimé Auteurs : Stéphane Bergera, Auteur ; Céline Cau Dit Coumes, Auteur ; Patrick Le Bescop, Auteur Année de publication : 2010 Article en page(s) : pp. 1180-1187 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Sulfialuminate;  Hydration;  Zinc;  Waste  management;  Calorimetry Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Hydration of calcium sulfoaluminate cement at early age was investigated as a function of the gypsum content of the binder, the thermal history of the material, and the ZnCl2 concentration in the mixing solution. Early hydration was strongly accelerated by the presence of gypsum, but lower percentages of reaction were reached after 24 h. The slowing down effect induced by ZnCl2, even at a concentration as high as 0.5 mol/L, was moderated compared to OPC but had a greater intensity in the absence of gypsum. Unlike what would have been expected for Portland cement, it was shown that the delay of a gypsum-free calcium sulfoaluminate cement resulted from the strong retardation caused by chloride anions, which was partly compensated by the accelerating effect of Zn2+ cations. The mineralogical observations revealed the precipitation of chloro–AFm phases such as Friedel's and Kuzel's salts, but no crystallized zinc-containing phases could be identified by XRD. The thermal history of the samples proved to be a key parameter. Applying a thermal cycle which reproduced the temperature rise and decrease occurring in a massive mortar block accelerated the rate of hydration and mainly modified the proportion of AFt versus AFm hydrates, especially when the binder had a gypsum content below 20%. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900204X [article] Hydration of calcium sulfoaluminate cement by a ZnCl2 solution: Investigation at early age [texte imprimé] / Stéphane Bergera, Auteur ; Céline Cau Dit Coumes, Auteur ; Patrick Le Bescop, Auteur . - 2010 . - pp. 1180-1187. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1180-1187 Mots-clés : Sulfialuminate;  Hydration;  Zinc;  Waste  management;  Calorimetry Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Hydration of calcium sulfoaluminate cement at early age was investigated as a function of the gypsum content of the binder, the thermal history of the material, and the ZnCl2 concentration in the mixing solution. Early hydration was strongly accelerated by the presence of gypsum, but lower percentages of reaction were reached after 24 h. The slowing down effect induced by ZnCl2, even at a concentration as high as 0.5 mol/L, was moderated compared to OPC but had a greater intensity in the absence of gypsum. Unlike what would have been expected for Portland cement, it was shown that the delay of a gypsum-free calcium sulfoaluminate cement resulted from the strong retardation caused by chloride anions, which was partly compensated by the accelerating effect of Zn2+ cations. The mineralogical observations revealed the precipitation of chloro–AFm phases such as Friedel's and Kuzel's salts, but no crystallized zinc-containing phases could be identified by XRD. The thermal history of the samples proved to be a key parameter. Applying a thermal cycle which reproduced the temperature rise and decrease occurring in a massive mortar block accelerated the rate of hydration and mainly modified the proportion of AFt versus AFm hydrates, especially when the binder had a gypsum content below 20%. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900204X

Durability of concrete exposed to leaching and external sulphate attacks / E. Rozière in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1188-1189 Titre : Durability of concrete exposed to leaching and external sulphate attacks Type de document : texte imprimé Auteurs : E. Rozière, Auteur ; R. El Hachem, Auteur ; F. Grondin, Auteur Année de publication : 2010 Article en page(s) : pp. 1188-1189 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Durability;  Sulfate  attack;  Leaching;  Sulfate  Resistant  Cements;Expansion Résumé : Leaching and external sulphate attack on concrete lead to dissolution of hydration products, mainly portlandite, and in case of ingress of sulphate ions to formation of expansive products such as gypsum and ettringite. Durability of concrete exposed to these environments is still based on prescriptive specifications. The aim of the study is to provide experimental data to design performance based tests and specifications for a comparative approach. Tests have been developed to study these degradations in controlled conditions at constant pH. A leaching test has been performed on concrete. Indicators can be deduced from the leaching test and they are sensitive to the variations of potential durability. An accelerated test using a high sulphate concentration has been performed on mortars. Expansion results are sensitive to mortar mix proportions and the test could be used to qualify cements or binders in a comparative approach. Concrete specimens are exposed to external sulphate attack at two levels of sulphate concentration to provide long-term data that can be used to study the relevance of the accelerated tests and models which are being developed. Results from both series of tests show a correlation between the resistance to leaching and the resistance to external sulphate attack. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001811 [article] Durability of concrete exposed to leaching and external sulphate attacks [texte imprimé] / E. Rozière, Auteur ; R. El Hachem, Auteur ; F. Grondin, Auteur . - 2010 . - pp. 1188-1189. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1188-1189 Mots-clés : Durability;  Sulfate  attack;  Leaching;  Sulfate  Resistant  Cements;Expansion Résumé : Leaching and external sulphate attack on concrete lead to dissolution of hydration products, mainly portlandite, and in case of ingress of sulphate ions to formation of expansive products such as gypsum and ettringite. Durability of concrete exposed to these environments is still based on prescriptive specifications. The aim of the study is to provide experimental data to design performance based tests and specifications for a comparative approach. Tests have been developed to study these degradations in controlled conditions at constant pH. A leaching test has been performed on concrete. Indicators can be deduced from the leaching test and they are sensitive to the variations of potential durability. An accelerated test using a high sulphate concentration has been performed on mortars. Expansion results are sensitive to mortar mix proportions and the test could be used to qualify cements or binders in a comparative approach. Concrete specimens are exposed to external sulphate attack at two levels of sulphate concentration to provide long-term data that can be used to study the relevance of the accelerated tests and models which are being developed. Results from both series of tests show a correlation between the resistance to leaching and the resistance to external sulphate attack. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609001811

Physico-chemical characterization of EVA-modified mortar and porcelain tiles interfaces / Alexandra A.P. Mansur in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1199-1208 Titre : Physico-chemical characterization of EVA-modified mortar and porcelain tiles interfaces Type de document : texte imprimé Auteurs : Alexandra A.P. Mansur, Auteur ; Otávio Luiz do Nascimento, Auteur ; Herman S. Mansura, Auteur Année de publication : 2010 Article en page(s) : pp. 1199-1208 Note générale : Génie Civil Langues : Anglais (eng) Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : EVA (poly(ethylene-co-vinyl acetate)) redispersable powders with different contents of copolymers were tested in order to evaluate their effects on the complex interactions developed at mortar/tile interface. EVA addition has increased the bond strength almost 40% when compared with unmodified mortar. Based on EVA hydrolysis, a novel method named as ‘Extension of Hydrolysis (EH)’ was suggested to monitor the reaction mechanism which occurred during the interface layer formation. Thus, higher EH values were related to a significant improvement of adhesion at interface observed as a changing on the failure mode from mostly cohesion to adhesion. Calculated EH parameter of 2.64 was associated with a cohesive component of rupture in the mortar of 50% while for an EH of 0.92, the rupture in the mortar was only 7%. The enhancement on adhesion was attributed to the hybrid ceramic–polymer interface formed based on hydrogen-bonds between silanol groups from tile surface and hydroxyl groups from hydrolyzed EVA. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900180X [article] Physico-chemical characterization of EVA-modified mortar and porcelain tiles interfaces [texte imprimé] / Alexandra A.P. Mansur, Auteur ; Otávio Luiz do Nascimento, Auteur ; Herman S. Mansura, Auteur . - 2010 . - pp. 1199-1208. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1199-1208 Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : EVA (poly(ethylene-co-vinyl acetate)) redispersable powders with different contents of copolymers were tested in order to evaluate their effects on the complex interactions developed at mortar/tile interface. EVA addition has increased the bond strength almost 40% when compared with unmodified mortar. Based on EVA hydrolysis, a novel method named as ‘Extension of Hydrolysis (EH)’ was suggested to monitor the reaction mechanism which occurred during the interface layer formation. Thus, higher EH values were related to a significant improvement of adhesion at interface observed as a changing on the failure mode from mostly cohesion to adhesion. Calculated EH parameter of 2.64 was associated with a cohesive component of rupture in the mortar of 50% while for an EH of 0.92, the rupture in the mortar was only 7%. The enhancement on adhesion was attributed to the hybrid ceramic–polymer interface formed based on hydrogen-bonds between silanol groups from tile surface and hydroxyl groups from hydrolyzed EVA. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S000888460900180X

Prediction of the plastic viscosity of self-compacting steel fibre reinforced concrete / Akbar Ghanbari in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1209-1216 Titre : Prediction of the plastic viscosity of self-compacting steel fibre reinforced concrete Type de document : texte imprimé Auteurs : Akbar Ghanbari, Auteur ; Bhushan L. Karihaloo, Auteur Année de publication : 2010 Article en page(s) : pp. 1209-1216 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Rheology;  Workability;  Fresh  concrete;  Fibre  reinforcement;Self-compacting  concrete Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Micromechanical constitutive models are used to predict the plastic viscosity of self-compacting steel fibre reinforced concrete (SCFRC) from the measured plastic viscosity of the paste. The concrete is regarded as a two-phase composite in which the solid phase is suspended in a viscous liquid phase. The liquid matrix phase consists of cement, water and any viscosity modifying agent (VMA) to which the solids (fine and coarse aggregates and fibres) are added in succession. The predictions are shown to correlate very well with available experimental data. Comments are made on the practical usefulness of the predicted plastic viscosity in simulating the flow of SCFRC. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002312 [article] Prediction of the plastic viscosity of self-compacting steel fibre reinforced concrete [texte imprimé] / Akbar Ghanbari, Auteur ; Bhushan L. Karihaloo, Auteur . - 2010 . - pp. 1209-1216. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1209-1216 Mots-clés : Rheology;  Workability;  Fresh  concrete;  Fibre  reinforcement;Self-compacting  concrete Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Micromechanical constitutive models are used to predict the plastic viscosity of self-compacting steel fibre reinforced concrete (SCFRC) from the measured plastic viscosity of the paste. The concrete is regarded as a two-phase composite in which the solid phase is suspended in a viscous liquid phase. The liquid matrix phase consists of cement, water and any viscosity modifying agent (VMA) to which the solids (fine and coarse aggregates and fibres) are added in succession. The predictions are shown to correlate very well with available experimental data. Comments are made on the practical usefulness of the predicted plastic viscosity in simulating the flow of SCFRC. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002312

Multiple cracking and strain hardening in fiber-reinforced concrete under uniaxial tension / Alessandro P. Fantilli in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1217-1229 Titre : Multiple cracking and strain hardening in fiber-reinforced concrete under uniaxial tension Type de document : texte imprimé Auteurs : Alessandro P. Fantilli, Auteur ; Hirozo Mihashi, Auteur ; Paolo Vallini, Auteur Année de publication : 2010 Article en page(s) : pp. 1217-1229 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Bond  properties;  Tensile  properties;  Fiber  reinforcement;High  Performance  Concrete;  Modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Fiber-reinforced concrete (FRC) showing strain hardening after cracking is commonly defined as High Performance Fiber-Reinforced Cementitious Composite (HPFRCC). In the post-cracking stage, several cracks develop before complete failure, which occurs when tensile strains localize in one of the formed cracks. As is well known, multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why, in such situations, high mechanical performances are obtained, an analytical model is herein proposed. It is based on a cohesive interface analysis, which has been largely adopted to investigate the mechanical response of FRC or the snubbing effects produced by inclined fibers, but not the condition of multiple cracking and strain hardening of HPFRCC. Through this approach, all the phenomena that affect the post-cracking response of FRC are evidenced, such as the nonlinear fracture mechanics of the matrix, the bond–slip behaviour between fibers and matrix, and the elastic response of both materials. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, leads to a new design criterion for HPFRCC and can eventually be used to enhance the performances of cement-based composites. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002269 [article] Multiple cracking and strain hardening in fiber-reinforced concrete under uniaxial tension [texte imprimé] / Alessandro P. Fantilli, Auteur ; Hirozo Mihashi, Auteur ; Paolo Vallini, Auteur . - 2010 . - pp. 1217-1229. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1217-1229 Mots-clés : Bond  properties;  Tensile  properties;  Fiber  reinforcement;High  Performance  Concrete;  Modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Fiber-reinforced concrete (FRC) showing strain hardening after cracking is commonly defined as High Performance Fiber-Reinforced Cementitious Composite (HPFRCC). In the post-cracking stage, several cracks develop before complete failure, which occurs when tensile strains localize in one of the formed cracks. As is well known, multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why, in such situations, high mechanical performances are obtained, an analytical model is herein proposed. It is based on a cohesive interface analysis, which has been largely adopted to investigate the mechanical response of FRC or the snubbing effects produced by inclined fibers, but not the condition of multiple cracking and strain hardening of HPFRCC. Through this approach, all the phenomena that affect the post-cracking response of FRC are evidenced, such as the nonlinear fracture mechanics of the matrix, the bond–slip behaviour between fibers and matrix, and the elastic response of both materials. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, leads to a new design criterion for HPFRCC and can eventually be used to enhance the performances of cement-based composites. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002269

Self-consolidating concrete subjected to high temperature: Mechanical and physicochemical properties / Hanaa Fares in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1230-1238 Titre : Self-consolidating concrete subjected to high temperature: Mechanical and physicochemical properties Type de document : texte imprimé Auteurs : Hanaa Fares, Auteur ; Albert Noumowe, Auteur ; Sébastien Remond, Auteur Année de publication : 2010 Article en page(s) : pp. 1230-1238 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Self-consolidating  concrete;Temperature  ;  Mechanical  properties  ;  Physical  properties  ;  Permeability Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : This paper presents an experimental study on the performance of self-consolidating concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete were tested. These concrete mixes were developed in the French National Project B@P. Mechanical and microstructural properties were studied at ambient temperature and after heating. We studied compressive strength, flexural strength, bulk modulus of elasticity, porosity and permeability. For each test, the specimens were heated at a rate of 1 °C/min up to different temperatures (150, 300, 450 and 600 °C). In order to ensure a uniform temperature throughout the specimen, the temperature was held constant at the target temperature for 1 h before cooling. In addition, the specimen mass was measured before and after heating in order to determine the loss of water during the test. The results allowed us to analyze the degradation of SCC and vibrated concretes due to heating. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002026 [article] Self-consolidating concrete subjected to high temperature: Mechanical and physicochemical properties [texte imprimé] / Hanaa Fares, Auteur ; Albert Noumowe, Auteur ; Sébastien Remond, Auteur . - 2010 . - pp. 1230-1238. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1230-1238 Mots-clés : Self-consolidating  concrete;Temperature  ;  Mechanical  properties  ;  Physical  properties  ;  Permeability Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : This paper presents an experimental study on the performance of self-consolidating concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete were tested. These concrete mixes were developed in the French National Project B@P. Mechanical and microstructural properties were studied at ambient temperature and after heating. We studied compressive strength, flexural strength, bulk modulus of elasticity, porosity and permeability. For each test, the specimens were heated at a rate of 1 °C/min up to different temperatures (150, 300, 450 and 600 °C). In order to ensure a uniform temperature throughout the specimen, the temperature was held constant at the target temperature for 1 h before cooling. In addition, the specimen mass was measured before and after heating in order to determine the loss of water during the test. The results allowed us to analyze the degradation of SCC and vibrated concretes due to heating. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002026

Revealing the temperature history in concrete after fire exposure by microscopic analysis / E. Annerel in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1239-1249 Titre : Revealing the temperature history in concrete after fire exposure by microscopic analysis Type de document : texte imprimé Auteurs : E. Annerel, Auteur ; L. Taerwe, Auteur Année de publication : 2010 Article en page(s) : pp. 1239-1249 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Temperature;  SEM  ;EDX;  X-Ray  Diffraction;  Petrography Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Concrete structures behave in most cases very well during a fire, after which it is often possible to repair or strengthen the structure to a certain level. This could result in important economic benefits, as costs for demolition and rebuilding can be avoided and the building can be reused faster. In this paper three methods for determining the maximum temperature to which a concrete structure was submitted during a fire are studied. Knowledge of the temperature distribution is necessary to assess the overall damage of a concrete structure. First, the physico-chemical transformations of heated concrete are investigated with scanning electron microscopy (SEM). Secondly, the features visible under the polarising and fluorescent microscope (PFM) are discussed. And third, the influence of heat on the colour of the aggregates is analysed. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002324 [article] Revealing the temperature history in concrete after fire exposure by microscopic analysis [texte imprimé] / E. Annerel, Auteur ; L. Taerwe, Auteur . - 2010 . - pp. 1239-1249. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1239-1249 Mots-clés : Temperature;  SEM  ;EDX;  X-Ray  Diffraction;  Petrography Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : Concrete structures behave in most cases very well during a fire, after which it is often possible to repair or strengthen the structure to a certain level. This could result in important economic benefits, as costs for demolition and rebuilding can be avoided and the building can be reused faster. In this paper three methods for determining the maximum temperature to which a concrete structure was submitted during a fire are studied. Knowledge of the temperature distribution is necessary to assess the overall damage of a concrete structure. First, the physico-chemical transformations of heated concrete are investigated with scanning electron microscopy (SEM). Secondly, the features visible under the polarising and fluorescent microscope (PFM) are discussed. And third, the influence of heat on the colour of the aggregates is analysed. DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002324

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