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Reflecting on the past

Concrete Restoration

Neglected concrete sentinels of a forgotten technology are being preserved for posterity. Damon Schunmann reports from Kent.

An eerie quiet now blankets the three preradar early warning sound mirrors at Greatstone-on-Sea on the UK's Kent coast, built in the late 1920s and early 1930s to pick up the distant sounds of enemy bomber fleets approaching across the Channel. In the end it was early radar that detected the Luftwaffe, but the acoustic detectors were some of the most advanced concrete structures of their time, and their restoration is presenting an ongoing challenge.

Work has already started on the listening devices, which consist of 6m and 9m dishes, as well as a 61m parabolic wall.

Seven decades of exposure to a corrosive coastal environment have taken a harsh toll. Before local contractor O&L Construction began repairs in 2002, buttresses had started to collapse and ruptured concrete was clearly visible. This was particularly noticeable on the wall where lateral reinforcement bars had burst through along the seams between construction lifts.

All three mirrors are cantilevered gravity structures with reinforced mass concrete foundations. These were shuttered to form the listening rooms. The faceted shuttered front faces were hand finished with dense concrete.

According to consultant Cameron Taylor Bedford conservation and heritage specialist Alan Wright: 'The design is good. The construction sequence is what has caused the problems.'

The wall was built using 1,140mm lifts, but instead of scabbling between lifts, a surface of grout was trowelled flat and two lateral reinforcing bars were laid on top. When the next lift was started, grout was poured in to wet the surface and then concrete was placed and hand tamped. The lack of scabbling has resulted in failures between the lifts and consequent water ingress has done the rest.

Wright says: 'The way it's been built has left it like a whole load of concrete fence panels.'

However, there has been one benefit resulting from the lateral movement along the lift seams.

Because this has introduced a flexibility of sorts the wall has not cracked elsewhere.

With the structures listed as ancient monuments, modern repair methods have not been an option.

'We had to use like materials, although it will be visible to an experienced eye that repairs have been undertaken, ' Wright says.

One challenge was to match the concrete used in the original design specifications. This required additive-free white cement, sand and aggregate in a classic 1:2:4 volumetric ratio mix that produced a strong, dense concrete. Density, and hence low surface porosity, was desired for two main reasons. Most obvious was that it offers protection against the ingress of aggressive chlorides in a very corrosive marine environment.

Equally important was that this made it less penetrable to incoming sound waves that needed to be reflected from the wall. Microphones facing the mirrors picked these up and the sounds were then passed along to operators wearing stethoscopes.

The larger of the two dishes has suffered around the sump hole that allows rainwater to drain from its basin. The absence of an effective lip around the top edge of the dish has accelerated the deterioration of the wall.

Different problems have beset the 6m dish. Two big cracks in the top half needed to be stitched to the lower section with reinforcement bars. These were drilled through and grouted into position.

Yet despite these failings, the mirrors have stood the test of time remarkably well, says Wright:

'The designs were certainly towards the front end of technology.' This is reflected in the low levels of carbonation that were found in the concrete, which had penetrated no more than 17mm. This compares favourably to typical 1960s buildings where depths of 50mm to 60mm are common. In addition, the levels of wind-driven sea-salt were found to be minimal due to the quality of the concrete and the aggregate used appears to have been either thoroughly washed, or to have come from a non-marine environment.

To help prevent future deterioration, wind or solar powered cathodic protection is being considered in tandem with an electrolyte. As contentious as this may sound, the remote location of the mirrors might make them a suitable test case. It could be implemented if research indicates that the two methods do not conflict with one another.

Experience gained from the Greatstone-on-Sea site is now being used on a prototype mirror located along the coast at Hythe.

This dish is in a far greater state of deterioration and was built to different technological standards.

Here, after the foundations and buttresses had been completed, a steel frame was erected. This was shuttered on the back face and an expanded metal lath was put in. A base coat of concrete was laid on with reinforcement bars added, and successive layers of concrete became finer and finer up to a final hand finished surface. The need for the concrete to be pushed through the lath meant a very low aggregate content was used.

Once the shutter was removed, a back coat was put on and hand finished.

The dilapidated state of this dish makes it difficult to repair. It may be necessary to pin the underside together with reinforcing bars. A layer of concrete could then be laid against the back surface with a non-shrink additive to prevent this support from coming away from the original.

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