EURAD D16.3 Selected experiments for assessing the evolution of concrete, their mechanical safety functions and performance targets

Cementitious materials are used in disposal facilities for radioactive waste as a construction material to ensure a safe operation of this facility for example the lining. These materials can also be used to contribute to the containment of radionuclides in the post-closure phase. The required mechanical strength class of concrete is determined by its function. The strength of concrete is determined by the strength of millimetre sized aggregates and the attachment of these aggregates with the cementitious phase in concrete. Only samples of concrete with aggregates are therefore included in this report since Task 2 of EURAD Work Package MAGIC is dedicated to study the chemo-mechanical behaviour at the macro scale i.e. for centimetre and decimetre sized samples. 
The required strength class for a construction material is usually higher than the required strength for a containment material especially if unreinforced concrete is used. It may take several trials to obtain a concrete recipe that can achieve the required strength after 28 days of hardening. The concrete also needs to maintain the majority of this strength during the operation of the disposal facility. The service life of a disposal facility can be 100 years or more for many countries. The hardening process may continue during the operational life by which the strength of concrete increases. This increase in strength has been measured for the Czech, Dutch and French samples. For well cured concrete, there can also be an additional contribution in strength by the suction force of empty pores within the concrete as have been determined by the studies on the Dutch samples. Concrete may also need to maintain its strength for a required period in the post-closure phase. The strength of concrete is highly related to the permeability of concrete i.e. a higher strength of concrete is also less permeable hence the rate of ingress of species that can alter the chemistry of concrete is smaller. Some alterations may result into a decrease in strength of concrete.
Luckily, a lot of knowledge has been generated by civil engineering and this knowledge has been translated into standards. Two examples with this known decrease in strength are highlighted: 
•    Reaction between siliceous aggregates and cementitious phase by which the strength of the aggregates is reduced;
•    Formation of a sulphate mineral within the cementitious phase by which the aggregates become less strongly hold together by which the strength of the concrete is reduced.
A too early loss in the mechanical strength of concrete by a reaction between the siliceous aggregates and the cementitious phase in concrete can be prevented by following these standards although irradiation of cementitious materials by the radionuclides contained in a waste form may introduce some special considerations. The reaction is caused by a specific polymorph of silica and a high alkali content of cement. Quartz is a polymorph of silica with such a low reactivity with this cementitious phase that the strength of the aggregate is not reduced. Irradiation can however change the polymorph of quartz into polymorph of silica that is more reactive with such high alkali content cementitious phase. Prevention in the framework of the handling radioactive waste can then take place by choosing a concrete recipe by which quartz aggregates are replaced by non-siliceous aggregates or replacing the cement with a high alkali content for a lower alkali content. Consequently, these considerations are known and this detrimental reaction can therefore be prevented by a careful selection of the ingredients for the concrete recipe to manufacture concrete. The reaction with siliceous aggregates can be excluded for most of the samples studied in Task 2 of MAGIC. 
Delayed ettringite formation (DEF) is another familiar process in which the strength of concrete is too early lost by ingress of dissolved sulphate reacting with a calcium aluminium mineral into ettringite. DEF can also be prevented by a careful selection of the type of cement used to manufacture concrete and considering the exposure environment of concrete. This carefulness is especially required for saline environments for example the pore water in crystalline and clay formations. The pore water chemistries in these formations are country specific. There are countries that consider geological formations with fresh water. DEF can considered to be negligible in those environments. 
The chemo-mechanical evolution of concrete is a very slow process, especially when the existing standards for civil engineering have been followed and blended cements have been used to manufacture concrete. Mainly concrete from existing experiments have therefore been selected. Concrete made with blended cements usually has a smaller permeability than concrete made with Ordinary Portland Cement. The samples studied within MAGIC are frequently manufactured with blended cements. All samples in Task 2 of MAGIC are being exposed or have been exposed to disposal representative conditions for several years. These conditions are frequently limited to chemical representative conditions; microbiological conditions are unknown. The chemical exposure may result into a gaseous carbonation of concrete by which the pH of the concrete pore water is reduced or a complex chemical alteration of cement minerals in the cementitious phase by exposure of concrete to a multi-ionic solution. This multi-ionic solution can be present in clay. The ingress of dissolved species can be smaller for concrete exposed to clay compared to concrete exposed to a solution. All samples are made of plain concrete except in the French programme that also considers reinforced concrete, knowing that corrosion of the steel bars is favoured by initial defects and that this corrosion is responsible in the long term for the mechanical degradation of a structure such as a lining. Not all linings are manufactured with reinforced concrete for example unreinforced concrete has been for the lining in the underground research laboratory in Mol (Belgium). 
There are three types of exposures to concrete samples within MAGIC:
1.    Clay and ventilation air in underground research laboratories i.e. lining;
2.    Air with a natural carbon dioxide concentration of 0.04% or far larger concentrations of CO2;
3.    Underground pore water only.
The exposed concrete in a lining in underground laboratories have the combined effect of the mechanical loading of concrete and ingress of gaseous and dissolved species. Belgian and French samples have this chemo-mechanical combination. Samples of concrete need to be taken by drilling or sawing and the number and size of samples is limited in order to preserve the construction. Both, the size of the samples and the number of samples are however very important to measure reliably the strength of concrete with the current available standards. It is therefore strongly advised to make many concrete samples with the same concrete recipe from which the lining has been made by which the detrimental effects of the exposure environment to the strength of concrete can be monitored for example sequentially in decades.
The difference between a strength measurement from mechanically loaded samples and with ingress of dissolved species and strength measurement from a mechanically unloaded sample and with ingress of dissolved species is yet unknown. In Task 2, the strength of concrete is mainly measured from samples that have been exposed to dissolved species in a mechanically unloaded state.
Sample preparation and the early age treatment have a high impact on the measured strength. The mixing of ingredients, curing conditions and sawing samples from larger concrete specimens can all have an influence on the measured strength. After 3 years of exposure, the studies performed on the Dutch samples showed that the samples made by sawing of hardened concrete have a significantly smaller strength than only casted samples. 
Following the standards, carbonation of concrete is considered as a process that is detrimental to the strength of reinforced concrete. Ingress of dissolved magnesium can be detrimental to the strength of reinforced and unreinforced concrete. A range in dissolved magnesium concentrations is considered in MAGIC. The solutions with dissolved magnesium also have other dissolved species that can be found in saline water. The dissolved concentration of magnesium ranges from as saline as brackish water till more than saline as seawater in Task 2 of MAGIC. Concrete in MAGIC is either exposed to these saline solutions or fresh solutions in which the dissolved concentration of magnesium is negligible. The solutions can be present in clay to which concrete is exposed. 
Only for the Dutch samples, preliminary results of the macro scale impact of a multi-ionic solution with dissolved magnesium is available. The studies on these samples that were exposed to a solution as saline as seawater for 5 years showed a decrease in compressive strength for the lower strength class samples. After 5 years of exposure, the measured compressive strength was not decreased for the higher strength samples. This difference between results in strength class is currently attributed to the higher porosity and larger size in pores of the lower strength concrete by which the ingress rate of detrimental dissolved species such as magnesium was larger for the lower strength concrete than the higher strength concrete. No microbial studies have yet been performed.
Microbial activity can have an impact on the rate of precipitation of secondary minerals for example biogenic calcite followed by an alteration of the dissolution of cement minerals inside concrete. This activity is envisaged to be negligible by space restriction especially within higher strength concrete that usually have a small porosity and refined pore structure. This activity cannot yet be excluded for concrete with cracks and lower strength concrete with a higher porosity and large size in pores. Apart from ingress of species, also the additional impact of microbial activity is therefore studied in Task 2 of MAGIC.