Chloride Penetration and Mechanical Load

Author(s):
Paper category: Bibliography
Book title: Publications on Durability of Reinforced Concrete Structures under Combined Mechanical Loads and Environmental Actions: An Annotated Bibliography
Editor(s): Yao Yan, Wang Ling, Wittmann Folker
Print ISBN: 978-3-942052-03-0
Publisher: Aedificatio Publishers
Pages: 1 - 22
Total Pages: 22
Language: English

Abstract: 
Cement-based materials such as mortar and concrete are porous materials. Under common conditions part of the pore space is filled with the aqueous pore solution. Depending on the pore volume, the pore size distribution and the degree of saturation concrete can absorb a certain amount of water or aqueous salt solutions by capillary action whenever the surface gets in contact with the liquid. The pore space of cement-based materials depends on the water-cement ratio, the duration of hydration, the type of cement, the type and percentage of mineral admixtures and the aggregate concentration. by capillary absorption aggressive ions dissolved in water such as chloride, sulphate and ammonia can be transported deep into the pore space by convection. Once the dissolved ions have entered the pore space they may migrate deeper into the porous material by diffusion. The diffusion coefficient depends on the pore space and the degree of saturation. The migration of dissolved ions does not depend on the diffusion coefficient exclusively. The diffusion is in most cases associated with chemical reactions and sorption processes in the gel pores. In addition initially nearly water saturated concrete undergoes drying, which may last for several decades. The water movement due to the long lasting drying process leads to reverse ion transport by convection. Hence, diffusion is one migration mechanism only among others. In practice changes of temperature and air pressure of the surrounding also contribute to the complex transport of ions in the pore space.

The original pore space of cement-based materials is not stable. It changes with on-going hydration and it can be changed significantly by application of a mechanical load. Under moderate compressive load micro-pores can be closed and under high compressive load or under tensile load new micro-cracks, which then act as new pathways for diffusion of ions can be formed. Moderate compressive load also compensates the comparatively high tensile stress in the outer layers of a drying specimen. This effect alone reduces capillary absorption and diffusion of ions. The changes in the microstructure can be observed directly if the pore space or the stiffness of the loaded material are measured. Based on these time and stress dependent modifications of the porous structure of cement-based materials it may be expected that the different transport processes depend on the actual state of stress and the total stress history. Damage induced into the composite structure of concrete by an applied load can be reduced again by self-healing. Self-healing has been observed and described in a number of papers, but it is difficult to be quantified.

From the results of relevant papers published so far it can be deduced, that the pore space is modified by an applied load. Capillary absorption under compressive load decreases. Chloride diffusion under moderate compressive load is slowed down but it increases if the applied load overcomes one third or half of the ultimate load. Under tensile load both the capillary absorption and the diffusion coefficient increase steadily with increasing applied load. The diffusion coefficient can also be increased by cyclic loading.

Results published so far clearly show the influence of an applied load on capillary absorption and on chloride diffusion. But we must not forget that transport of water and diffusion of ions are complex processes as outlined above. What we can determine with simple experiments is an apparent diffusion coefficient, which is influenced by chemical reactions, by sorption processes and by convection.

It has also been shown that transport mechanisms in fibre reinforced cement-based materials such as SHCC are less sensitive with respect to an imposed strain. Self-healing of induced damage could also be observed experimentally in SHCC. In addition it could be shown that surface impregnation with a water repellent agent reduces capillary absorption substantially and as a consequence chloride penetration is slowed down significantly. The changes of the pore space under the action of an applied load are well understood by now and they can be explained in a phenomenological way at least. Based on the wealth of experimental data available already a test method is to be developed to characterize the sensitivity of a given type of concrete with respect to chloride penetration under the influence of an applied load. Results of this test method will then serve as input into a more realistic service life design of reinforced concrete structures placed in an aggressive environment. As there is no chance at this moment to take all influencing parameters into consideration adequately a comparatively high safety margin has to be respected.

Online publication: 2014
Publication Type: full_text
Public price (Euros): 0.00