Carbonation and Freeze-Thaw Attack

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: 43 - 47
Total Pages: 5
Language: English

Abstract: 
Carbonation is among the most common deteriorating processes of reinforced concrete structures. In harsh climate reinforced concrete structures are exposed to numerous freeze-thaw cycles in addition. Both carbonation and frost attack have been investigated in great detail experimentally and theoretically in the past. There is wide agreement on the deteriorating mechanisms of carbonation and freeze thaw attack among most colleagues. Results obtained so far have also been integrated into national and international codes. But the two processes have generally been considered separately up to now. In case there are synergetic effects if both processes take place simultaneously or successively, however, durability of reinforced concrete structures may be significantly overestimated.

Different mechanisms of carbonation of cement-based materials have been studied in great detail for many years. by now it is well understood that the pore solution in young concrete is saturated with Ca(OH)₂ and contains among other ions a certain percentage of dissolved sodium and potassium. This is the reason why the pH value of the pore solution is above 12.5. Under these conditions the surface of the steel reinforcement will be covered with a stable protective layer, which prevents steel from corrosion. When CO₂ from the environment penetrates into the pore space Ca(OH)₂ reacts and CaCO₃ is formed. Due to this chemical reaction the pH value of the pore solution is reduced to values below 9. Under these conditions the passivation layer on the steel reinforcement is dissolved and corrosion may start. The rate of carbonation depends on the humidity content, the temperature, the type of cement, the CO₂ content of the environment and before all of the water-cement ratio. All of these influencing parameters have been studied in detail and therefore the time-dependent thickness of the carbonated layer can be predicted in a realistic way.

Research on mechanisms of frost damage of concrete can be traced back to the beginning of the 20th century. It has been found that freezing of water in concrete depends on the salt concentration and even more on the diameter of the pores. by now sophisticated theories such as the formation of micro-ice lenses and hydrostatic pressure in micro pores have been developed and verified on the basis of results obtained in the laboratory and of observations in practice. These theoretical concepts help us to explain frost damage in concrete under well-defined conditions.

In practice, however, reinforced concrete structures are generally not exposed to one single action. Carbonation takes place in nearly all reinforced concrete structures. Some structures built in severe climate are exposed at the same time to a certain number of freeze thaw cycles, while the number of cycles and the lowest temperature depends on the region. Then the question arises, is there a synergetic effect between carbonation and freeze-thaw cycles or can these two deteriorating processes be considered to be independent. Recently a number of publications in professional journals and in conference proceedings have been published, which show that carbonation and frost damage must not be considered separately.

All transport processes in porous materials depend on the porosity, the tortuosity and the pore size distribution. It has been shown that under the influence of freeze-thaw cycles new micro-cracks and cracks are formed in the composite structure of concrete. These cracks will serve as additional pathways for the transport of CO₂ into the pore space. As a consequence migration of CO₂ is facilitated and the rate of carbonation will increase under the influence of freeze-thaw cycles. This is just one typical example for reduced durability under combined actions. In papers listed in this section details of this synergetic effect are described. The carbonation rate under given environmental conditions has to be determined in order to make service life prediction more realistic. As the porosity and the pore size distribution are modified by carbonation there is also a risk that frost resistance will be changed after carbonation.

Experimental results presented in the contributions, which are collected in this section, clearly demonstrate that the combined action of carbonation and frost attack has to be taken into consideration in future and more realistic service life design. But it is also obvious that many more studies will be necessary in order to provide a solid basis for the establishment of new and more reliable codes. Well-structured international cooperation will certainly speed up progress in this field. RILEM may be helpful to coordinate the necessary test series and the development of relevant theoretical concepts. Needless to say, this work is of utmost importance. It will have serious economic and ecological consequences if we continue to overestimate the real service life of reinforced concrete structures.

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