دانلود رایگان مقاله دیفیوزیویتی پورتلند سیمان مبتنی بر مواد معمولی اشباع شده

Abstract

This paper provides a comprehensive overview of existing experimental and modelling approaches to determine effective diffusion coefficients of water saturated ordinary Portland cement-based materials. A dataset for diffusivity obtained from different experimental techniques have been presented for cement paste, mortar and concrete. For cement paste at low porosities, diffusivity reported by different authors varies up to a factor of five and electrical resistivity measurements for low capillary porosity are up to one order of magnitude higher compared to other techniques. Experimental data of mortar and concrete reveals predominant influence of increasing tortuosity due to aggregates and limited influence of interface transition zone. Hence, a particular emphasis has been placed on assessing predictability of diffusivity models for cement paste on a larger dataset collected in this paper. It has been observed that all predictive models have similar level of accuracy and fail to predict electrical resistivity data at low capillary porosity as these models are not calibrated using electrical resistivity data.

4. Conclusions

This paper reviews different experimental techniques and modelling approaches for ordinary Portland cement based material under saturated conditions. A database of existing measured relative diffusivities obtained for cement paste, mortar and concrete serves as a basis to critically assess different measurement techniques and model predictions. For electrical resistivity and through-diffusion experiments, all the collected data for cement paste lie within a factor 5-bounds and factor of 4-bounds, respectively. For electro-migration experiments, the range is smaller with all data for cement paste falling within factor 2- bounds. This scatter between the values can arise from several reasons such as differences in cement composition, curing, maturity, experimental protocols and use of different tracers for through-diffusion experiments. It has been also observed that the relative diffusivities for cement paste, mortar and concrete obtained at low to moderate w/c ratio or capillary porosities obtained by electrical resistivity measurements are always larger (up to one order) than those obtained with the other methods. Electro-migration and through-diffusion techniques give similar diffusivity values for cement paste, however very few data points exists for cement paste for w/c ratio in the range of 0.25 to 0.3. For mortar/concrete, data points collected in the w/c ratio range from 0.25 to 0.3 reveals 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 falls within a factor 1.5-bounds. This reveals that the relative diffusivity of mortar/concrete can be directly determined from the diffusivity of cement paste diffusivity and that the ITZ might have a limited to no influence on diffusion.