Concrete Considerations for Offsite Construction
1 Mar 2016
The versatility of concrete means that it can be designed and constructed to deliver high performance buildings through a spectrum of solutions ranging from onsite cast concrete, onsite build using factory made blocks, through to offsite manufactured two dimensional panels and volumetric units. When considering offsite solutions there are additional benefits offered by the use of precast concrete for offsite construction compared to other systems.
By Elaine Toogood
The robustness of concrete, for example means that it can be delivered and erected without the need for wrappings and covers to protect it from the elements. Care is required when handling prefabricated elements made of any material to prevent damage, but with robustness comes less risk of damage. Quite apart from the resource and wastage saved, the structural integrity of concrete is not compromised by weather, a significant advantage for construction. Less durable structures will be particularly vulnerable to delayed installation of building enclosure.
Another advantage during construction is concrete’s inherent resistance to fire, avoiding the need for special measures to be installed to reduce the risk of fire spreading during build programmes. As architect or client this provides an immediate solution to meet the health and safety responsibilities set out by recent HSE endorsed guidance for reducing fire in construction. Statistics show that, during occupation, fires are also more likely to be contained in buildings with non-combustible structure, limiting the extent of fire damage to the property. Fire detailing and correct installation of fire stops and barriers are critical, but if major building components have inherent fire resistance the reliance on appropriate detailing and workmanship is reduced.
Precast concrete for housing in the UK historically suffered from an image problem, due in part to the poorly maintained ‘brutal aesthetic’ of much of the mass housing put up in the 60’s and 70’s. While new housing is still more likely to be built from concrete than any other material, the concrete is likely to be hidden on the inside of the building, with concrete constructed to higher structural and quality standards. Furthermore, external precast concrete panels, with a range of potentially different aesthetics, are often designed to look more like stone than concrete, hence the name ‘Recon’. The elaborate and elegant examples of precast concrete cladding used for much of the East Village Stratford (formerly the athletes’ village for London Olympics) are a good example and I am sure that many residents are unaware that their buildings are clad in concrete and not stone. The range of finishes available in concrete is very broad, so it is wise to establish the required standard of finish and tolerances required with manufacturers before tender in order to align the clients and designers expectations with the right product or specification.
Improved thermal performance requirements have also lead to an evolution in the detailing of housing. To achieve the low air permeability required of current building regulations or passiv haus standard, designers and constructors now need to be pay closer attention to the joints and junctions of a buildings enclosure. This is facilitated by the solid, straight edges of precast panels, around window openings for example. Windows can also be pre-installed and tested in the panels. Since concrete itself is effectively airtight a precast concrete inner leaf offers a simple and durable solution for long term airtightness. Greater thermal insulation is provided and large cold bridges can be simply avoided by continuing insulation over the ends of floor slabs. In the manufacture of insulated sandwich cladding panels, low conductivity connectors are used to tie the two leaves together.
While much of the precast concrete used in construction is part of the structure, and is not on show it can be manufactured to be left exposed as a final finish, either inside or outside a building. Since structure tends to be located inside of the insulation and waterproofing layers, a concrete structure provides the opportunity to ‘design out’ the use of additional internal finishes such as suspended ceilings, wall linings or carpets without compromising the fire, structural and often acoustic requirements. There is a reduction in waste generated by avoiding these subsequent trades.
Exposed concrete on the inside of a building also offers opportunities to utilise its thermal mass. When used as part of a low energy strategy, with adequate ventilation, concrete can help improve internal thermal comfort; reduce the risk of overheating and lower energy bills. This saving in energy can also significantly lower the carbon footprint of the building over its life time. When taking into account the savings in embodied CO2 of the avoided finishes and air conditioning and their periodic repair and replacement over the life of the building this adds up to a low whole life carbon solution for buildings using concrete. The Concrete Centre will be launching a new publication at Ecobuild this year, explaining the CO2 saving of concrete through every stage of construction, including manufacture and demolition.
Using thermal mass is an excellent way to optimise the use of precast concrete for sustainable construction, but there are also other many sustainability issues worth considering. These include long life, climate change adaption, flood resilience, recyclability and recycled content, local manufacture and local, responsibly sourced materials. For additional information on the inherent sustainable credential of precast concrete refer to ‘Sustainability Matters’ an annual publication by British Precast.
So concrete is a great starting point when considering material selection for new buildings. It provides advantages during construction as well as offering long term performance benefits for occupants and building owners.