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Repair mortars for historic masonry; Effects of the binder choice on durability

Caspar Groot
Delft University of Technology, the Netherlands

Factors affecting the design of repair mortars for historic masonry are: the type of masonry, the condition of the masonry and the exposure conditions. Especially in case of low-strength masonry exposed to heavy rain and high salt contents the design of a repair mortar may be a challenge. The most important problem is obtaining adequate insight into and feeling for the requirements, that play a role with regard to the mortar design. To this end the paper concentrates on materials behaviour of repair mortars in historic masonry. This is done in the context of the analysis of several types of attack on mortar durability: freeze-thaw cycling, salts, thermal movement, rain penetration. From this analysis relevant materials characteristics and technical requirements are derived. The main conclusions are

the binder choice may significantly affect the durability of repairs,
a basic and determining condition is a good execution technique.

Consequently, thorough insight into the characteristics and behaviour of the various available binders is then an important tool to the repair mortar designer. And, practical field experience with the possibilities and limitations of on-site execution practices may be very helpful to ensure good quality of the repair work.