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Số người truy cập: 38,046,597

 An overview on diffusivity measurement in saturated cementitious materials – Introducing a novel method based on dissolved gases
Tác giả hoặc Nhóm tác giả: Quoc Tri Phung, Norbert Maes, Elke Jacops, Ravi Patel
Nơi đăng: 5th International workshop on the testing of low permeability materials, Belgium; Số: -;Từ->đến trang: -;Năm: 2016
Lĩnh vực: Khoa học công nghệ; Loại: Báo cáo; Thể loại: Quốc tế
TÓM TẮT
Diffusion is an important property for characterizing concrete durability because it governs the penetration of aggressive substances (Cl-, SO42-, O2, CO2) responsible for degradation. However, data on the diffusion of substances (other than Cl-) in concrete are very scarce due to time and resource consuming measurements. This work aims at providing a comprehensive overview of experimental approaches to determine effective diffusion coefficients of saturated cement-based materials. A database of existing measured relative diffusivities obtained for cement paste, mortar and concrete served as a basis to critically assess different measurement techniques including (i) through-diffusion based on measuring fluxes, (ii) in-diffusion based on measuring concentration profiles in the sample, (iii) electro-migration experiments, either by through- or in-diffusion, in which ion diffusion is accelerated by an electric field, and (iv) techniques in which proxy variables are used to determine diffusivity, for example, electrical resistivity techniques. For electrical resistivity and through-diffusion experiments, all the collected data for cement paste lie within a factor of five and four, respectively. For electro-migration experiments, the range is smaller with all data for cement paste falling within a factor of two of a fitted empirical relation. It was also observed that the relative diffusivities for cement paste, mortar and concrete obtained at low to moderate water/cement (w/c) ratio or capillary porosities obtained by electrical resistivity measurements are always larger than those obtained with the other methods. Electro-migration and through-diffusion techniques gave similar diffusivity values for cement paste, however very few data points exists for cement paste for w/c in the range of 0.25 to 0.3. For mortar/concrete, data points collected in the w/c range from 0.25 to 0.3 revealed that relative diffusivity measured by electro-migration might be lower than by through-diffusion. The increase in relative diffusivity with w/c ratio and capillary porosity is obvious for cement paste. For mortar and concrete, increase in diffusivity with w/c ratio and decrease of diffusivity with an increase in the amount of aggregates for a given w/c ratio is observed, the latter being the consequence of an increased tortuosity. The mortar and concrete diffusivities, when normalized with cement paste diffusivity and plotted with respect to fraction of cement paste in the mortar/concrete, fall on a straight line irrespective of the study or the measurement method. All the data fall within a factor of 1.5. This reveals that the relative diffusivity of mortar/concrete can be directly determined from the diffusivity of cement paste diffusivity and that the interface transition zone (ITZ) might have a limited influence on diffusion. In the second part, a new method to determine the diffusion coefficients of dissolved gases in saturated cement-based materials in order to study the effects of degradation on the transport properties is described. The proposed method is adapted from the technique used to determine the diffusivity of clay materials. The method is based on a through-diffusion methodology and allows simultaneous determination of diffusivities of two dissolved gases diffusing in opposite directions. A cement plug is mounted between two water reservoirs pressurized by two different gases at equal pressure (~ 1 MPa) to avoid advection. The changes in the dissolved gas concentration (at the opposite sides) are measured indirectly via gaseous phases which are in equilibrium with aqueous phases according to Henry's law. Additionally, a simple 1-D diffusive transport model (based on 1st and 2nd Fick’s laws) is developed to interpret the experimental data. The concentrations at outlet and inlet are used as inputs for the model, and the diffusivity is obtained by a fitting procedure. Experiments performed on intact and carbonated cement paste samples with a gas combination: He and Xe, using the proposed method confirmed that the proposed method is providing reliable results within a reasonable experimental time and is useful to study the effects of degradation on the transport properties of cementitious materials.
ABSTRACT
Diffusion is an important property for characterizing concrete durability because it governs the penetration of aggressive substances (Cl-, SO42-, O2, CO2) responsible for degradation. However, data on the diffusion of substances (other than Cl-) in concrete are very scarce due to time and resource consuming measurements. This work aims at providing a comprehensive overview of experimental approaches to determine effective diffusion coefficients of saturated cement-based materials. A database of existing measured relative diffusivities obtained for cement paste, mortar and concrete served as a basis to critically assess different measurement techniques including (i) through-diffusion based on measuring fluxes, (ii) in-diffusion based on measuring concentration profiles in the sample, (iii) electro-migration experiments, either by through- or in-diffusion, in which ion diffusion is accelerated by an electric field, and (iv) techniques in which proxy variables are used to determine diffusivity, for example, electrical resistivity techniques. For electrical resistivity and through-diffusion experiments, all the collected data for cement paste lie within a factor of five and four, respectively. For electro-migration experiments, the range is smaller with all data for cement paste falling within a factor of two of a fitted empirical relation. It was also observed that the relative diffusivities for cement paste, mortar and concrete obtained at low to moderate water/cement (w/c) ratio or capillary porosities obtained by electrical resistivity measurements are always larger than those obtained with the other methods. Electro-migration and through-diffusion techniques gave similar diffusivity values for cement paste, however very few data points exists for cement paste for w/c in the range of 0.25 to 0.3. For mortar/concrete, data points collected in the w/c range from 0.25 to 0.3 revealed that relative diffusivity measured by electro-migration might be lower than by through-diffusion. The increase in relative diffusivity with w/c ratio and capillary porosity is obvious for cement paste. For mortar and concrete, increase in diffusivity with w/c ratio and decrease of diffusivity with an increase in the amount of aggregates for a given w/c ratio is observed, the latter being the consequence of an increased tortuosity. The mortar and concrete diffusivities, when normalized with cement paste diffusivity and plotted with respect to fraction of cement paste in the mortar/concrete, fall on a straight line irrespective of the study or the measurement method. All the data fall within a factor of 1.5. This reveals that the relative diffusivity of mortar/concrete can be directly determined from the diffusivity of cement paste diffusivity and that the interface transition zone (ITZ) might have a limited influence on diffusion. In the second part, a new method to determine the diffusion coefficients of dissolved gases in saturated cement-based materials in order to study the effects of degradation on the transport properties is described. The proposed method is adapted from the technique used to determine the diffusivity of clay materials. The method is based on a through-diffusion methodology and allows simultaneous determination of diffusivities of two dissolved gases diffusing in opposite directions. A cement plug is mounted between two water reservoirs pressurized by two different gases at equal pressure (~ 1 MPa) to avoid advection. The changes in the dissolved gas concentration (at the opposite sides) are measured indirectly via gaseous phases which are in equilibrium with aqueous phases according to Henry's law. Additionally, a simple 1-D diffusive transport model (based on 1st and 2nd Fick’s laws) is developed to interpret the experimental data. The concentrations at outlet and inlet are used as inputs for the model, and the diffusivity is obtained by a fitting procedure. Experiments performed on intact and carbonated cement paste samples with a gas combination: He and Xe, using the proposed method confirmed that the proposed method is providing reliable results within a reasonable experimental time and is useful to study the effects of degradation on the transport properties of cementitious materials.
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