Reducing embodied carbon of high-rise modular residential buildings by systematic structural and material optimization

Abstract

Buildings significantly contribute to carbon emissions. Embodied carbon (EC), which is mainly generated from the before-use stages of buildings, obtains high carbon intensity in a short period. Modular Integrated Construction (MiC) has been greatly promoted as an innovative construction approach, and concrete MiC is expected to be largely adopted in high-rise residential buildings for housing supply in Hong Kong (HK). However, there has been a lack of systematic EC assessment for concrete MiC high-rise residential buildings. Few studies have examined the system boundaries for EC assessment and reduction. Besides, low carbon structural design (LCSD) has yet to be systematically examined, especially for concrete MiC high-rise residential buildings. The guiding framework for conducting LCSD, practical LCSD strategies, and their EC reduction potentials are still to be investigated.

This research aims to systematically examine the life cycle EC emissions and reduction through structural design and material use measures of concrete MiC high-rise residential buildings in HK. First, a multi-level spatiotemporal EC assessment model was developed for concrete MiC high-rise residential buildings. A real-life MiC building case was selected for data collection and empirical validation of the model. Second, detailed structural design measures for EC reduction and the LCSD of buildings were examined based on system boundary theory. The data were collected and analyzed against each system boundary, providing the general EC reduction potentials of different LCSD measures. Third, a multi-level LCSD model was proposed to guide a systematic LCSD of concrete MiC high-rise residential buildings from the module design, global structural design, and low-carbon material use. The same MiC building case was adopted for the empirical validation.

The results show that the developed multi-level model can achieve a systematic EC assessment of concrete MiC high-rise residential buildings. The EC of the MiC building case was quantified as 569.3 kgCO2e/m2, of which over 90% was from the production and transportation stage. Concrete and reinforcement generated the most EC at all five spatial levels. Structural modules generated more EC than non-structural modules. The modularized flat area had more EC than that of the public area. The proposed twelve system boundaries can effectively examine the LCSD of buildings. Seven LCSD measures were identified from the component, layout, and system scales, of which five measures were identified feasible for concrete MiC high-rise residential buildings. The multi-level LCSD model can effectively achieve the EC reduction purpose, by which up to 40% of the EC was reduced in the MiC building case. If considering non-structural component optimization in LCSD, more EC reduction can be achieved.

This research extends the existing knowledge body about the EC assessment and reduction of concrete MiC high-rise residential buildings. It contributes to the theoretical knowledge with innovative solutions for EC assessment and LCSD addressing the system thinking approach. It also contributes to the practical knowledge with detailed EC results, practical LCSD measures, and quantified EC reduction potentials for concrete MiC high-rise residential buildings. It should provide a good guidance for low-carbon building research and design practices in other high-rise cities.