Failure of coastal protection concrete armour units due to alkali–silica reaction

Abstract

The 1993 Clacton Coastal Defence Works included a 1·4 km length of sloping revetment formed of precast concrete armour units. The revetment was designed for a service life of greater than 50 years, but severe cracking of the armour units necessitated its entire replacement after only 22 years. This paper describes an investigation that determined the primary cause of the armour unit deterioration to be alkali–silica reaction (ASR), which appeared to have been exacerbated by the choice of cement, leaching action and the geometry of the units. In addition, sustainable reuse of the concrete units within the rock armour fishtailed groynes of a replacement coastal protection scheme is described. This case study is of particular importance as it is notable that the failed concrete armour units complied with the current UK guidelines for prevention of ASR. As such, it demonstrates the importance of considering the potential action of leaching and the associated alkali migration when designing concrete structures to resist alkali–aggregate reactions. In particular, designers of marine/coastal structures should be aware of the potential leaching and alkali migration implications of specifying hollow concrete armour units with relatively low surface area to volume ratios.