Feature

Hybrid concrete: Best of both worlds

The benefits of going hybrid

Jenny Burridge explains how using precast and in-situ concrete together can often be the most buildable option

The government is currently championing the use of off-site construction as a way to improve productivity and reduce the number of hours required on site. Purely precast concrete systems are available, but there are also structural systems that use a combination of precast and in-situ concrete, with each type playing its part. This is known as hybrid concrete construction (HCC). 

HCC can deliver very significant cost savings, through economic structures, increased prefabrication, faster construction and consistent performance. Although the structural frame of a building represents only 10% of the total construction cost, the choice of material has dramatic consequences for subsequent processes. Hybrid construction can reduce frame costs by using precast concrete for the repetitive elements, or to act as permanent formwork. In-situ concrete is more cost-effective for large volumes (due to reduced transport costs), for tying the frame together and for bespoke areas. Using the two together maximises cost efficiency.

Case study: The Ray, London, 2019

Hybrid concrete construction was used in the construction of The Ray, the UK home of social networking company LinkedIn. The building, by architect Allford Hall Monaghan Morris (AHMM), steps back in order to address the varying scale of the neighbouring buildings, and to create accessible terraces and allow natural daylight to flood all floorplates. Concrete was chosen as the frame material to provide thermal mass to reduce the servicing required. The structural frame is formed from post-tensioned slabs, with spans of up to 10.5m. The setbacks are achieved through either cantilevering the posttensioned slabs or by transferring the edge columns. The hybrid section is in the lift and stair cores, which were formed using twinwall (see figure 2, type 1). Precast concrete sandwich panels were used for the brick facade.

The concrete slab soffit is exposed throughout the building, contributing to the project’s environmental strategy, while the use of ground granulated blast-furnace slag (GGBS) as cement replacement in the concrete mixes reduces the embodied carbon associated with the structural frame’s construction, further enhancing the building’s sustainability credentials.

The key advantage of HCC is its buildability. Because precast and in-situ concrete are used where each is most appropriate, construction becomes relatively simple and logical and important decisions are resolved at design stage. This means, for example, that precast elements can be manufactured, stored at the factory and delivered just-in-time to site. They can then be lifted from delivery truck to final position in a single crane movement, eliminating the need for site storage and reducing crane hook time.

HCC also offers all of the other advantages of off-site construction, improving both speed of construction and safety. By taking a proportion of work into the factory, it reduces the duration of on-site operations critical to the programme. The precast process takes place in a controlled environment, unaffected by weather and with no need for working at height. Rigorous inspection before installation removes causes of delay on site, while better buildability helps provide safer working conditions. HCC can reduce the potential for accidents by providing successive work platforms and a tidier site. If precast spandrel beams are used, they can provide immediate edge protection.

Some HCC techniques can reduce or eliminate following trades – for example, installing ceilings and finishes. If precast concrete is used for those areas of visual concrete, the workmanship required takes place under factory conditions. This enables even faster programme times but requires greater coordination and care in detailing and protection on site. The Town House at Kingston University (see Inspiration article in this issue) is an excellent example of the use of HCC to provide beautiful visual concrete, while enabling many of the add-on finishes to be eliminated.

Figure 1: Sustainability of hybrid options

Figure 2: Hybrid options explained

Figure 3: Relevant precast concrete product standards

Design

As both precast concrete and HCC are very wellestablished forms of construction, there is plenty of guidance for the designer. Eurocode 2 covers the design of in-situ, precast and hybrid concrete construction. There are also a number of product standards for precast concrete, which also cover the precast elements of HCC, some of which are listed in figure 3.

The use of precast and in-situ concrete may well lead to individual elements being designed by different companies. Therefore, it is essential that there should be a single named designer or engineer who retains overall responsibility for the stability of the structure and the compatibility of the design and details of the parts and components, even where some or all of the design, including details, of those parts and components are not carried out by the named designer. This is particularly important for hybrid structures, where there is greater scope for misunderstandings. 

It is the responsibility of the named designer, before incorporating any proprietary system as part of the structure, to ensure that the assumptions made in the design and construction of the system are compatible with the whole. This should include:

  • an adequate specification for that part
  • ensuring that any standard product designed and detailed by the precast manufacturer is suitable for that particular structure
  • reviewing the design of any such part to ensure that it satisfies the design intent and is compatible with the rest of the structure.

The design of each component should include consideration of:

  • its performance in the permanent condition
  • the construction method and loading
  • any temporary supports required during construction.

The design should be carried out following the requirements of Eurocode 2 Part 1-1, Cl. 1.3, which assumes:

  • structures are designed by appropriately qualified and experienced personnel
  • adequate supervision and quality control are provided in factories, in plants and on site
  • construction is carried out by personnel having the appropriate skill and experience
  • the construction materials and products are used as specified in Eurocode 2 or in the relevant material or product specifications
  • the structure will be adequately maintained
  • the structure will be used in accordance with the design brief
  • the requirements for execution and workmanship given in BS EN 13670 are complied with.

The designer should state the design assumptions, which should generally include the following construction-related information:

  • sequence of construction
  • exposure requirements
  • pour sizes assumed (if appropriate)
  • concrete strength at time of striking formwork and back-propping requirements
  • breakdown of loading including allowance for construction loads loading history assumed.

For more information, see Design of Hybrid Concrete Buildings by The Concrete Centre.

Photos Rob Parrish; Timothy Soar