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Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar / Xiao-Ting Chen in Cement and concrete research, Vol. 39 N° 3 (Mars 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 3 (Mars 2009) . - pp. 195–205 Titre : Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar Type de document : texte imprimé Auteurs : Xiao-Ting Chen, Auteur ; C.A. Davya, Auteur ; F. Skoczylas, Auteur Année de publication : 2009 Article en page(s) : pp. 195–205 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Mortar;  Poro-elasticity;  Thermal  treatment;  Hydrostatic  stress;Micro-cracking Résumé : This contribution aims at identifying experimentally the poro-elastic properties of a cement-based material under different levels of confining pressure, and after a heat-treatment up to 400 °C. The model material used is a normalized mortar, with a (W/C) ratio of 0.5. After a given heating/cooling cycle, drained bulk modulus Kb, solid matrix bulk modulus Ks and Biot's coefficient b are measured at different confining pressure levels (with a maximum of 25 MPa). Results show that under drained conditions, mortar stress–strain relationship evolves with increasing heat-treatment temperature from linear elastic with brittle failure (up to 105 °C heat treatment) to plastic and ductile (from 200 °C and above). Plastification testifies of material degradation under gradual confining pressure. At the microstructure scale, this is attributed to thermal damage after heat treatment above 105 °C, which consists mainly in various micro-cracking. This leads to easier failure of solid skeleton bridges (or trabecules), and to pore network collapse. Concomitantly to this, at given confining pressure Pc, secant drained bulk modulus Kb decreases monotonously, for heat-treatment temperatures above 105 °C. On the opposite, at given heat-treatment temperature above 105 °C, secant drained bulk modulus Kb increases when confining pressure is increased. This testifies of a solid matrix rigidification in the elastic domain, and it is attributed to increased skeleton compactness linked with pore network collapse. This is directly attributable to heat treatment followed by confinement. At given confining pressure Pc, matrix bulk modulus Ks and Biot's coefficient b increase with heat-treatment temperature above 105 °C. The increase in b means that mortar becomes less and less cohesive and more and more of a granular nature. Moreover, Biot's coefficient b and solid matrix bulk modulus Ks are independent of confining pressure Pc for intact mortar, whereas they decrease for heat-treated mortars when Pc increases. From literature analysis alone, it was quite unexpected that after heat treatment, Ks should vary under confinement. This is interpreted as the closure, under confining pressure, of micro-void connections and of micro-cracks created by heat-treatment. Therefore, increasing confinement induces more and more occluded pores in the solid matrix, whereby Ks diminishes. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884608002202 [article] Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar [texte imprimé] / Xiao-Ting Chen, Auteur ; C.A. Davya, Auteur ; F. Skoczylas, Auteur . - 2009 . - pp. 195–205. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 3 (Mars 2009) . - pp. 195–205 Mots-clés : Mortar;  Poro-elasticity;  Thermal  treatment;  Hydrostatic  stress;Micro-cracking Résumé : This contribution aims at identifying experimentally the poro-elastic properties of a cement-based material under different levels of confining pressure, and after a heat-treatment up to 400 °C. The model material used is a normalized mortar, with a (W/C) ratio of 0.5. After a given heating/cooling cycle, drained bulk modulus Kb, solid matrix bulk modulus Ks and Biot's coefficient b are measured at different confining pressure levels (with a maximum of 25 MPa). Results show that under drained conditions, mortar stress–strain relationship evolves with increasing heat-treatment temperature from linear elastic with brittle failure (up to 105 °C heat treatment) to plastic and ductile (from 200 °C and above). Plastification testifies of material degradation under gradual confining pressure. At the microstructure scale, this is attributed to thermal damage after heat treatment above 105 °C, which consists mainly in various micro-cracking. This leads to easier failure of solid skeleton bridges (or trabecules), and to pore network collapse. Concomitantly to this, at given confining pressure Pc, secant drained bulk modulus Kb decreases monotonously, for heat-treatment temperatures above 105 °C. On the opposite, at given heat-treatment temperature above 105 °C, secant drained bulk modulus Kb increases when confining pressure is increased. This testifies of a solid matrix rigidification in the elastic domain, and it is attributed to increased skeleton compactness linked with pore network collapse. This is directly attributable to heat treatment followed by confinement. At given confining pressure Pc, matrix bulk modulus Ks and Biot's coefficient b increase with heat-treatment temperature above 105 °C. The increase in b means that mortar becomes less and less cohesive and more and more of a granular nature. Moreover, Biot's coefficient b and solid matrix bulk modulus Ks are independent of confining pressure Pc for intact mortar, whereas they decrease for heat-treated mortars when Pc increases. From literature analysis alone, it was quite unexpected that after heat treatment, Ks should vary under confinement. This is interpreted as the closure, under confining pressure, of micro-void connections and of micro-cracks created by heat-treatment. Therefore, increasing confinement induces more and more occluded pores in the solid matrix, whereby Ks diminishes. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884608002202

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