The Hoover Building in London

"The answer to everybody's prayer was re-alkalisation " project manager Bob Sanders said.

The Hoover factory, one of London's most loved landmarks, has finally been brought back to life after years of decay.

The factory, which was built in 1932 by the world famous USA originated Hoover company, became a grade II listed building in 1990. Hence when de long needed restoration was performed last year, its original appearance had to be kept. This was ensured by using the non-destructive technique of re-alkalisation.

The Hoover factory is known as the architect's, Wallis Gilbert & Partners', Art Deco masterpiece. Although the factory throughout the years has been extended several times, and today consists of three buildings, its appeal has always been maintained, and in 1990 it became a Grade II listed building. This means that any architectural or structural operation to the building must to be approved by English Heritage, a governmental body.

After the production has moved in the early 1980's, the buildings were left bare and unused for eight years. During these years vandalism, weather ad general neglect resulted in severe deterioration.

The new owner, Tesco, the large supermarket chain, bought the factory in 19489 and decided to convert it into a superstore. The design and building contractor Kyle Stewart won the multimillion pound design and construction contract, and the redevelopment started in January 1992. Previous fears that the office facade would be impaired proved to be unfounded, when 45 weeks later, the building re-emerged in all its former glory.

The three key areas that were restored were the faience, the windows and the concrete. The faience and windows were given back their original look. The concrete, which was in poor condition with extensive rebar corrosion caused by carbonation at depths up to 70 mm, could not be repaired in the conventional manner. Hacking out and replacing affected concrete would have resulted in serious impairment. Since English Heritage dictated that the original concrete had to be conserved, the answer became re-alkalisation.

Re-alkalisation is a non-destructive electro-chemical treatment in that only already cracked and spalled concrete has to be removed. Thereafter an alkaline solution is transported into the pores of the carbonated concrete resulting in pH conditions which effectively stop rebar corrosion. The corrosion protection provided will last for decades as the alkaline solution stays alkaline in normal atmospheric conditions.

The re-alkalisation was performed by the UK concrete repair specialist MAKERS Industrial Ltd who hold a license for the application of the re-alkalisation technique in England. MAKERS has performed many successful re-alkalisation projects, and in October 1992 the company received the British Construction Industry Award, Special Commendation for Technological Development, for the re-alkalisation of the Desborough Water Tower, which was performed in 1991, see adjacent article.

At the Hoover Building, with varying cover and average carbonation depth of about 30 mm, the average re-alkalisation time was 4 days. Areas up to 400 m² were treated in one go, and within 23 weeks, on a 28 week program, the total concrete area of 4500 m² was re-alkalised.

The effect of the treatment was documented on site by using the pH indicator phenolphthalein. Sprayed onto either re-alkalised concrete or noncarbonated concrete, the indicator colours the concrete red to purple. Sprayed onto carbonated concrete, there is no colour change. The red colour indicates a pH well above 10, which is sufficient to prevent corrosion.

After treatment, the alkaline fibre mixture was removed, anode mesh and leads dismantled, and the surface was high pressure water jetted and left to dry. A cementitious skim coat was then applied, and the concrete was finally given an elastomeric coating.

Re-alkalisation allowed the carbonated, but otherwise sound concrete, to be retained, thus saving time since large areas were treated in one go. This allowed all the concrete rehabilitation to be finished ahead of schedule and ensured completion within 45 weeks, which probably was the most demanding aspect of the redevelopment. The store could thus be opened before Christmas as planned. Hence, English Heritage is happy, as is contractor and owner.

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