The Importance of Cryo-shrinkage In the Frost Resistance of Porous Materials
Author(s): Paulo J.M. Monteiro
Paper category: Proceedings
Book title: 3rd International RILEM Conference on Microstructure Related Durability of Cementitious Composites
Editor(s): Changwen Miao, Wei Sun, Jiaping Liu, Huisu Chen, Guang Ye and Klaas van Breugel
Publisher: RILEM Publications SARL
Publication year: 2016
Total Pages : 7
Language : English
Abstract: The present paper reviews the microscopic characterization of air voids using lowtemperature scanning electron microscopy and directional cooling microscopy. These results clearly showed that a) air voids effectively reduce frost damage by acting as a hydraulic sink for supercooled water and b) air voids may act as an efficient cryo-pump. Massive ice crystals were observed in in air voids that were originally at atmospheric pressure, consequently liquid water diffused towards the air voids rather than towards the larger already frozen pores, resulting in a beneficial lowering of the in-pore crystal pressure.
When cooled below the freezing point, the porous cementitious matrix undergoes a cryodeformation as a result of various coupled and contrary actions. These include: i) the difference of density between the liquid water and the ice crystal; ii) the drainage of the liquid water expelled from the freezing sites; iii) the interfacial effects between the different constituents, which govern the crystallization process in connection with the pore access radius distribution; iv) the cryo-suction process, which drives liquid water towards the already frozen pores; and v) the thermo-mechanical coupling between the deformable porous matrix,
the liquid water, and the ice crystal. Here it is shown experimentally and theoretically that air voids act as macroscopic cryopumps, resulting in cryo-shrinkage and partial melting of the ice already formed when the cooling suddenly stops. A numerical simulation shows that, at low cooling rates, the cryoexpansion is lower for narrow-banded distributions of the access pore radius than for wide distributions. The values are negative range for incipient freezing in the former case as a result of the thermal contraction. At larger cooling rates, the cryo-expansion suddenly increases for narrow-banded distributions, over-passing the cryo- expansion obtained for wide distributions. This is due to the compared availability of pores that are capable of freezing at the same time, and to the compared density of simultaneous microscopic cryo-pumps. The liquid water transforms largely to ice when the temperature is equal to the crystallization temperature suited to the pore radius the most frequently encountered.
Online publication : 2016
Publication type : full_text
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