Systematic embodied carbon assessment and reduction of prefabricated high-rise public residential buildings in Hong Kong

Buildings contribute significantly to carbon emissions worldwide. Prefabrication is regarded as a cleaner production and sustainable construction approach, which has been utilized for public residential building construction in Hong Kong to achieve more productive, safer and higher-quality construction process with less negative impacts on the environment. Previous studies explored the life cycle carbon assessment of prefabricated buildings, but no unified embodied carbon assessment model or reporting format has been established in Hong Kong. This paper aims to contribute a better systematic understanding of the embodied carbon of prefabricated high-rise public residential buildings at different levels of unit of analysis and to identify appropriate measures for effective embodied carbon reduction. The research was carried out through a case study using a typical 30-storey public residential building in Hong Kong. The embodied carbon was calculated and reported at five levels of unit of analysis, i.e. material, component, assembly, flat and building. The cradle-to-end of construction embodied carbon of the case building was found to be 561 kg CO2/m2. Off-site activities accounted for 27.3% of the total embodied carbon, most of which were contributed by the production of façades (6.9%), bathrooms (6%), kitchens (4.5%) and slabs (4.2%). Embodied carbon from the manufacturing of materials was mainly attributed to concrete (59.2%) and steel (20.1%). The results reveal that the embodied carbon could be reduced significantly by adopting low carbon concrete such as replacing the ordinary Portland cement with blast furnace slag cement (with 22.8% reduction potential) and utilizing cement substitutes (25% pulverized fly ash (PFA)) (with 9.8% reduction potential). However, other measures were revealed to only impose marginal effect, such as decreasing the thickness of walls (with 1.9% reduction potential) and optimizing prefabrication rate (with 1.5% reduction potential), which was believed to be hindered by the high transportation carbon to and in Hong Kong. The developed five-level analytical framework paves the foundation for future standardization and benchmarking of prefabricated buildings’ embodied carbon and the findings provide strategies for the most effective carbon reduction of prefabricated buildings in cities.