New model improves evaluation of long-term ageing in concrete structures

Olli-Pekka Kari, Licentiate of Science (Technology) developed a model that can be used to evaluate the durability of concrete structures when they remain in use for hundreds of years.

Modern type of concrete has only been used in construction for a few decades. However, new concrete structures may be designed with a planned service life of several hundred years.

'Long-term ageing phenomena have traditionally been assessed by means of methods that are based on short-term tests and often on single factors. They do not take into account all of the physical and chemical phenomena that affect ageing during a long period of use and which can also change the properties of the concrete,' says Kari.

Storage of nuclear waste

The model presented in the dissertation was applied to Finnish concept for final disposal of low- and intermediate-level nuclear waste. The aim is to dispose of the waste in concrete silos built in rock caverns at a depth of 50–100 meters.

According to a Government decision, the technical engineered barriers must be serviceable for at least 500 years.

Environmental conditions in the rock caverns vary significantly according to the service life of the repository. Nuclear waste will be delivered to the repository for one hundred years. During this time, there will be normal air in the repository. After the operational phase is complete, the rock cavern will be sealed and filled with groundwater.

During the first phase, the concrete will age as a result of carbonation caused by carbon dioxide in the air.  Carbonation causes changes in the concrete microstructure and also affects the properties of the concrete in the period following closure of the rock caverns. As it progresses, carbonation can lead to corrosion of the steel reinforcement in the concrete.

After the rock cavern is sealed, the groundwater and the ions contained in it that are harmful to concrete structures penetrate the concrete. At the same time, the concrete components can also leach out into the groundwater. Interactions between the ions and concrete impact on the progress of the process. Chloride in the groundwater can be considered the single most harmful factor increasing the possibility of steel corrosion.

The structure simulated in the dissertation was produced from sulphate-resistant concrete.

'The simulation shows that although changes do occur during the time period, they are clearly limited to a depth of less than 50 millimetres from the concrete surface, which is the typical depth for installing concrete steel reinforcement. This is very small in comparison to the overall thickness of the concrete structure and other technical barriers. Calculations indicate that even the damage to the surface section is not severe. However, the possibility of steel reinforcement corrosion cannot be completely ruled out,' explains Kari.

A tool for designers

The feasibility of the model presented in the dissertation was verified by a large number of laboratory tests. Concrete specimens that had been stored for 13 years in controlled conditions in air or in solution were used for the tests. The specimens were exposed to either carbonation caused by air or to penetration of harmful ions contained in the solution.

'The test results demonstrated that the model was a credible and feasible method for evaluating the ageing of concrete structures, especially when the planned service life is several hundred years,' states Kari.

According to Kari, the model can improve identification of latent factors that affect the deterioration of concrete already in the design phase. Thus, the model assists designers in selecting the proper concrete mix.

The model can be directly applied to similar uses and different concrete mixes by specifying the prevailing environmental conditions and the material-specific parameters for the concrete being examined prior to exposure.

Olli-Pekka Kari, Licentiate of Science (Technology) defended his doctoral dissertation on 27 February 2015 at the Aalto University School of Engineering's Department of Civil and Structural Engineering. The dissertation was entitled Long-term ageing of concrete structures in Finnish rock caverns as application facilities for low- and intermediate-level nuclear waste.

Web address for the dissertation: https://aaltodoc.aalto.fi/handle/123456789/15114

Further information:
Olli-Pekka Kari, tel. +358 50 384 1646, [email protected]