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Leaching 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: 105 - 117
Total Pages: 13
Language: English


Abstract: 
Whenever the surface of a concrete structural element is in permanent or at least long lasting contact with water the concentrated pore liquid is in direct contact with pure water or at least with a diluted aqueous solution. As a consequence diffusion of dissolved ions out of the pore space into the adjacent liquid will take place. This situation is common in case of concrete dams, concrete pipes, cement based renderings in water reservoirs and in particular in nuclear waste repositories because of the extremely long required service life. Pure water is able to remove alkali hydroxides, dissolve calcium hydroxide and decompose hydrated calcium silicate and aluminate phases. This well-known process finally disintegrates hardened cement paste, the binding matrix in concrete. If water, which is in contact with a concrete surface has a high concentration of CO₂ and a low pH value CaCO₃ will also be dissolved forming Ca²⁺ and 2HCO₃⁻ ions.

Leaching in fact is a rather complex chain of chemical reactions controlled by a set of equilibrium constants. In a simplified way it may be considered as a two-step process. First dissolved ions migrate out of the pore space by diffusion into the adjacent aqueous solution with lower concentrations. The concentration difference is then the driving force. As a second step hydration products are dissolved in order to reestablish the chemical equilibrium. As the density of hydration products is reduced porosity of concrete increases and strength as well as stiffness decrease. As calcium plays a dominant role in leaching the complex process is sometimes called decalcification. The rate of leaching depends essentially on the composition and the rate of flow of the surrounding water, the porosity of the hardened concrete and the composition of the hydration products. The rate of leaching increases significantly in presence of a solution of ammonium nitrate. This may lead to serious problems in concrete structures in agricultural environment.

It was found by numerous authors that leaching reduces tensile strength, the elastic modulus and fracture energy of concrete, while the porosity and creep increase considerably. That means that leaching combined with an applied tensile stress or under bending will facilitate crack formation. This leads necessarily to a reduction of service life. Accelerated creep of concrete due to leaching may lead to reduced service life of concrete structures by initiating tertiary creep under an applied load. The reduced strength and fracture energy weakens generally all concrete structures exposed to leaching.


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