Carbonation 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: 23 - 34
Total Pages: 12
Language: English

Abstract: 
The rate of carbonation of concrete depends on many parameters such as water-cement ratio, type of cement, amount and type of mineral admixtures, age and humidity content of concrete, and of course CO₂ concentration of the environment. Carbonation is one of the most frequently observed damage processes of reinforced concrete structures. During the hydration of cement large amounts of Ca(OH)₂ are formed. Together with the dissolved alkali ions calcium hydroxide is at the origin of a high pH value of the pore water. As long as enough Ca(OH)₂ can be dissolved in the pore solution the pH value remains high enough to protect steel reinforcement from corrosion. When enough CO₂ has penetrated into the pore space of cement-based materials and when it has reacted with the pore solution to convert most of the dissolved Ca(OH)₂ into solid CaCO₃ the pH value decreases to values around 9. In this modified environment the reinforcement is not protected any longer.

Most of the parameters, which influence the rate of carbonation, have been studied in great detail in the past and most national and international codes are based on the results of these investigations. The primary process of carbonation is penetration of CO₂ gas into the pore space of concrete. For this reason gas permeability of concrete under different conditions has been studied by many authors. The pore volume as well as the pore size distribution, however, can be modified considerably by an applied mechanical load. This latter influence has been neglected for a long time but recently an increasing number of papers on the influence of an applied load on the rate of carbonation of concrete appear in journals and conference proceedings. Selected papers are included in this annotated bibliography.

It has been shown experimentally that a modest compressive load decreases the rate of carbonation. This effect is obviously due to partial closing of micro-cracks in the composite structure of cement based materials under an applied compressive stress. If the load is increased above 50 % of the ultimate load, however, the on-going closing of micro-cracks is overcompensated by formation of new cracks in the composite structure. These distributed cracks serve as new pathways for the penetration and the migration of CO₂. As a consequence the permeability and the rate of carbonation of the loaded material increase. Even a modest tensile stress opens new micro-cracks and therefore the rate of carbonation under tensile stress increases steadily with increasing applied load. In order to predict carbonation rate of concrete while exposed to an external load a correction factor can be introduced. When the rate of carbonation has been measured as function of time on concrete samples without load, the obtained values have to be multiplied with the correction factor to obtain the realistic carbonation rate of a loaded specimen in practice.

The influence of an applied load on the carbonation rate is well understood by now and it can be explained at least in a phenomenological way. The importance of this observation is given by the fact that service life prediction is not on the safe side in general if the influence of mechanical stress is neglected. Standardized methods have now to be developed to characterize the sensitivity of the rate of carbonation of a given type of concrete with respect to an applied tensile or compressive mechanical load. Results of these test series should be introduced as soon as possible into durability codes in order to make them more realistic.

It ought to be pointed out here that in most cases young concrete is exposed to a drying environment. As can be shown easily, the tensile stress originated by this process overcomes in many cases the tensile strength of concrete. That means that the concrete cover which should protect the steel reinforcement from corrosion is in most cases in practice seriously damaged right from the beginning, which may contribute to a significant shortening of the service life.

All chemical reactions are thermally activated. This means that the rate of carbonation also depends on the temperature of the environment. At least in extreme cases this has also to be taken into account in the context of service life prediction.

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