Offsite Concrete - Unique Benefits
6 May 2020
Offsite construction is in demand. But rather than highlight the much-repeated benefits of design for manufacture and assembly (DFMA) I have considered here the significant benefits that concrete can bring to offsite construction that can save carbon and cost as well as contribute to health, wellbeing and our adaptation to climate change.
Concrete is of course a hugely versatile material, which means that it can be designed and constructed to deliver whole structures as well as individual elements, for inside or outside a building, below ground or water level, giving almost a limitless potential application in the built environment. It is also readily available from the local supply chain, manufactured using local, responsibly sourced materials.
Concrete’s resilience to fire and water and good acoustic isolation are all essential performance criteria and are often well understood, it is also important to understand sustainability credentials and environmental performance as well as aesthetics… fortunately precast concrete provides benefits here too.
Non-combustibility is of course a major benefit of concrete and masonry, especially as regulations change to address the failings shown by the Grenfell tragedy and a number of cladding fires. Concrete cannot burn and so does not contribute fuel to a fire. It also provides fire resistance and insulation, limiting fire spread. It can be used as a structural material as well as for external walls and cladding and is the most risk free choice for buildings, both during construction and over the life of building. As the fire resistance is built in there is no reliance on long term essential maintenance of additional fire linings or coatings.
Durability and resilience
The durability of concrete facilitates the long service life of a building and its potential re-use, and for precast concrete this includes the potential for disassembly - providing resource for future construction.
Part of the durability of concrete comes from its resilience to water damage, whether from weather, or more commonly water leakage. Concrete offers a simple solution to flood resilience and our adaptation to climate change.
Concrete’s resilience to water damage offers benefits on site too. Prefabricated concrete elements are typically stored, delivered and erected without the need for wrappings and covers for protection from the elements, saving resource and wastage and risk of delays caused by damage in delivery.
Acoustics and aesthetics
During discussions with architects and other construction professionals there is a growing appreciation for the role concrete can play to address concerns often associated with offsite construction. The ‘solidity’ provided by concrete is especially important in multi-occupancy residential buildings, such as flats, student accommodation, hotels, retirement homes etc. Concrete offers a simple means of offering the required acoustic requirements of contemporary residential buildings and a less ‘lively’ feel through vibration control. I have heard on numerous occasions “If the floors are not concrete you can tell’.
Concerns on aesthetic due to the standardisation of offsite construction can also be addressed with concrete. Precast cladding can vary in colour and texture and can be designed to resemble stone, often referred to as ‘recon’ (short for reconstituted stone). There also many recent examples of brick-faced precast cladding that can provide considerable programme savings in achieving this desirable aesthetic.
Energy savings and avoiding overheating
Improved thermal performance requirements have led to an evolution in the detailing of buildings. When the mass housing of the 60’s and 70’s was constructed, there were no requirements for insulation, airtightness or the avoidance of cold bridging. Today designers and manufacturers need to be pay close attention to the joints and junctions of the building envelope and contemporary concrete offsite construction can achieve the highest levels of thermal performance.
In fact the first Passivhaus accredited building in the UK was constructed using a prefabricated concrete structure, as was the first UK’s Passivhaus zero carbon school which has a concrete crosswall structure and insulated concrete sandwich panel façade.
Since concrete itself is effectively airtight, a precast concrete inner leaf offers a simple and durable solution for long-term airtightness, facilitated by the clean solid, straight edges around window and door openings, which can also be delivered to site pre-installed and tested. Cold bridges can be simply avoided by extending insulation over the ends of floor slabs together with the use of low conductivity connectors to tie the two leaves of insulated concrete sandwich cladding panels.
Key to achieving excellent thermal performance is a construction solution that is detailed, manufactured and assembled properly. Concrete can achieve this and brings the added benefit of high thermal mass. Concrete’s thermal mass, as part of a low energy strategy combining ventilation, can improve thermal comfort, reduce the risk of overheating and save energy as air-conditioning can be avoided, or greatly reduced. Saving energy can save cost and carbon and reduce fuel poverty and addressing overheating can minimise the associated risks to health.
Saving carbon and sustainable manufacture
Designers can significantly reduce the whole life carbon footprint of their buildings by optimising the use of concrete’s thermal mass. As well as operational energy savings there are also carbon savings from the avoidance of emissions related to installation, repair and replacement of finishes and air conditioning over the life of a building, as well as the material usage avoided from the extended design life of the structure itself.
But what of embodied carbon – here all those inherent benefits of concrete help again, as a concrete solution does not rely on many additional materials, fixings and treatments to meet performance requirements. In the recent “Embodied Carbon Primer” from LETI, for example, brick-faced precast concrete cladding was shown as one of the lowest embodied carbon solutions.
In assessing carbon and environmental performance accurate data is key and the precast concrete sector has resources to help designers. Working with sustainability consultants Thinkstep, British Precast, the trade body for UK precast concrete manufacturers, have developed a lifecycle assessment tool to produce verified Environmental Product Declarations (EPDs), both for members, and for a series of generic products. These generic EPD’s have been third-party verified and are available at www.britishprecast.org
This initiative is part of the sector commitment to sustainable manufacture. British Precast has a Sustainability Charter that is a prerequisite for membership and the sector reports performance annually and has targets in place to reduce energy use, carbon emissions, waste and increase the use of low carbon fuels. The sector is also part of the Concrete Industry Sustainable Construction Strategy that provides annual reporting for the UK concrete industry – see www.sustainableconcrete.org.uk
But that is just part of the early action of the industry, and in the coming months the UK concrete industry will publish the roadmap to net zero carbon and beyond by 2050.
Significant carbon savings can be achieved now through the design decisions of architects and engineers, in collaboration with precast manufacturers. Material efficient structure can be optimised to use less to save carbon. Low carbon concretes can be used, for example using 50% GGBS, to reduce embodied carbon. And as with all concrete, the natural process of carbonation means that concrete will absorb carbon throughout its lifecycle.
The Concrete Centre has recently published Offsite Concrete Construction, and this gives more information about the benefits and structural options, including case studies that demonstrate the high structural quality and environmental standards that contemporary offsite buildings can achieve using concrete, benefiting construction and occupation today, and also in the future.
See the new Concrete Compass: offsite concrete construction page bringing together all resources and articles available on this topic.