The Listening Mirrors

Abstract

Between the World Wars use of a concave profiled concrete surface (a sound mirror) to reflect sound waves was thought to be the answer to the early detection of aerial attack. Ultimately, this technology was superseded by radar and the experimental work on sound reflection and collection was abandoned. As a result, the south and east coast of England is scattered with the remains of such forgotten concrete structures. The only extant collection of these structures is at Greatstone in Kent, where three sound mirrors remain and have been listed as scheduled monuments. One of the largest projects funded by the Aggregate Levy Sustainability Fund and managed by English Heritage was concerned with stabilizing these structures and undertaking research into their repair.This paper aims to outline the conservation approach to the project and to detail the concrete repair techniques trialled. It also highlights some pointers for the repair of twentieth-century concrete based on the advice of a master mason and a concrete repair contractor. Finally, the long-term monitoring that is in place for the carbonation inhibitors and cathodic protection systems that are installed on these structures are detailed.