When I went on one of the guided walks to the Denge sound mirrors, one of the other vistors was a construction worker who was somewhat critical of the quality of some of the original concrete on the mirrors. I’m not qualified to comment, but Rowan Technologies has some experts on the conservation of concrete.
English Heritage has commissioned a series of investigative repairs on the mirrors – which are now scheduled as monuments (legal protection specifically for archaeological sites) – that will explore treatments that might be transferable elsewhere. The richly patinated surface of the mirrors is a complex amalgam of weathered aggregate and many varieties of lichen. Chris Wood from English Heritage’s building conservation team is also experimenting with yoghurt to encourage lichen reinstatement. The works were carried out by Rowan Technologies.
Source: The Architects’ Journal, 2008-11-24
Rowan Technologies has this to say:
Conservation Case Studies
The Listening Mirrors, Kent
Like-for-Like Concrete Repairs
The three early warning sound mirrors [at Denge] on the Kent coast were built using reinforced concrete in the late 1920s and the early 1930s to detect the distant sounds of enemy aircraft approaching from over the English Channel. The reinforced concrete has deteriorated in the marine environment and many parts of the structure are suffering from corrosion of the reinforcements and the delamination of the concrete cover.
Rowan Technologies undertook a series of trials of various repair and rehabilitation methods to assess their suitability for these monuments. This included ‘model’ patch repairs of the damaged concrete on a like-for-like basis, to achieving a similar texture and surface finish to the original
Source: Rowan Technologies Ltd