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Reusing concrete structures: one step closer to a circular economy

It’s good to see a growing momentum in favour of reusing existing building structures and extending their lives, as the construction industry considers ways to reduce the amount of materials it consumes. Concrete lends itself well to this approach – as can be seen by the number of buildings that have successfully been given a new lease of life. This year’s Pritzker Prize was won by Lacaton & Vassal partly for the transformation of three social housing buildings at Grand Parc in Bordeaux, where the concrete frames were upgraded and generous flexible spaces added to each unit.

The case study of 160 Old Street also shows how well an older concrete frame can be adapted to form the structure for a new building. In this case, 76% of the original structure was retained, reducing life-cycle emissions by 2,850 tonnes, while increasing the net lettable area of the building by 70%.

This kind of project is one approach to a circular economy, where the element that is repurposed is not individual beams or columns, but the entire frame. It is possible because concrete is a very durable material – when used internally, designing for 50 years will give at least 100 years of useable life. Reuse is necessarily always a bespoke solution, and the decision in every case will depend on the unique circumstances of a specific building and the current requirements of the client and the market. But for structural engineers involved in potential reuse projects, there are several steps that will be key to a successful outcome.

 
  Above Lacaton & Vassal won the Mies van der Rohe award for the renovation of three social housing blocks in Bordeaux, which were transformed through the addition of a 3m-deep structural facade.

Work out what’s there

When conducting an initial investigation into the potential for reuse, a first step is to look for record drawings. This will make a huge difference to the design of the modifications – and will act as a useful reminder that the drawings and information we produce for new buildings or extensions today should be kept safe for future engineers. With a copy of the drawings, the checks required to analyse the frame for the new use and loadings becomes much easier. If the old drawings no longer exist, it will be necessarily to conduct investigations to check what is there. In particular, the following should be analysed: 

  • Concrete strength, using cores taken from the concrete and analysed using BS EN 13791. Samples should be taken from around the building and include slabs, beams and columns
  • Cover to the reinforcement and the distribution of the reinforcement 
  • Element sizes.

Concrete continues to increase in strength as it ages, so even with the drawings to hand it is worth checking the concrete strengths. Another resource for checking existing concrete frames might be archive editions of Concrete Quarterly, which go back to 1947 and contain many detailed case studies. The full archive is available in PDF form here.

Assessing capacity

The concrete frame can then be backanalysed to check its capacity and to assess whether this will be sufficient for the new use. The codes of practice that were in use when the frame was built are very helpful in this respect (see table 1), but it is possible to use modern methods of analysis on an old frame. Strut-and-tie modelling is particularly helpful in this regard as it can show many possible load paths within the concrete (for more information, refer to Strut-and-tie Models, The Concrete Centre, 2015).

Table 1: Historic design codes for concrete structures

Year

Code

1915

Reinforced Concrete Regulations of the London County Council

1934

Code of Practice for the use of reinforced concrete in buildings

1948

CP114, The structural use of normal reinforced concrete in buildings

1957

CP114 revised version

1959

CP115, The structural use of prestressed concrete in buildings

1965

CP116, The structural use of precast concrete

1969

CP114, CP115 and CP116 Part 2 – metric units

1970

Addendum to CP116 to cover large-panel structures

1972

CP110 Code of practice for the structural use of concrete (first design code to include limit states)

Broadly speaking, design concrete strengths have increased over the years. In 1934, the ordinary grade concretes had strengths of 16–20MPa. By 1985, BS 8110 assumed minimum strengths of 20MPa for reinforced concrete, 25MPa for precast concrete and 30MPa for prestressed concrete. Concretes in the 1980s were typically a cube strength of 30MPa for normal reinforced concrete frames.

Reinforcement grades and types have also changed with time. It is advisable to test reinforcement in buildings dating from before 1960, as it tended to be quite variable. Generally, plain round mild steel bars had a yield strength of 250MPa, with high yield deformed bars 415–485MPa depending on the age and diameter. The Concrete Society’s 2020 publication TR70 Historical approaches to the design of concrete buildings and structures is a very useful guide for the designer.

 
  Above Mæ Architects’ refurbishment of an estate of 1960s slab blocks at Hillington Square, King’s Lynn, involved demolishing elevated walkways, internal replanning and upgrades to the facades. Above The 500-flat Kleiburg in Amsterdam. Its refurbishment by NL Architects focused on renovating the structure, leaving the users to finish the apartments.

If a concrete structure does need to be strengthened, various methods can be adopted, from replacement of the over-stressed element to strengthening with additional reinforcement or carbon fibre. The 2012 Concrete Society publication TR55 Design guidance for strengthening concrete structures using fibre composite materials gives guidance on using fibre composites to strengthen both bending elements and columns. The confinement provided by wrapping the column with carbon fibre increases the capacity of the concrete significantly. 

Jenny Burridge is head of structural engineering at The Concrete Centre

Photos Will Scott, Philippe Ruault, Mæ Architects & Marcel van der Burg