Development of a modular system for circular timber highway bridges

Abstract

The infrastructure industry currently deals with two issues: the deterioration of (highway) bridges, and the urge to reduce emissions by the construction sector. Rijkswaterstaat must repair or replace hundreds of bridges in the upcoming decades. At the same time, the impact of human behaviour on climate change becomes more visible and must be reduced. The most structural solution to diminish the human impact on the environment is switching from a linear economy to a circular economy (CE).
This research aims to develop a system for circular highway bridges by both constructing with timber and extending the lifespan. The first contributes to a lower environmental impact as timber is a renewable material that captures carbon during growth. For an outdoor timber structure, protection is crucial to prevent the timber from deteriorating due to weather influences. The lifespan extension is obtained by applying three circular principles: (1) Design for Material Efficiency (DfME), (2) Design for Adaptability (DfA), and
(3) Design for Disassembly (DfD). Variant studies on typology, connections and material optimise for material efficiency. A flexible structure enables DfA: converting in function and expansion in length and width is possible. Furthermore, a modular system is developed to include DfD: connections between modules are demountable to disassemble, adapt and reuse the system. In summary, four design strategies are defined: efficient, protected, adaptable and demountable.
This research aims to provide an alternative for concrete highway bridges with a lower environmental impact.The timber bridge, a circular concrete bridge and a traditional concrete bridge are compared. The comparison includes the production (A1‐A3), construction (A4‐A5) and end of life (C1‐C4) stages. Timber results in a lower carbon footprint for all reference periods and lifespans. The study considers the third scenario most reliable, as the technical lifespans are most substantiated. In this scenario, reductions of 18% and 47% for respectively the circular and traditional concrete bridge are established.
This study concludes that Dutch highway bridges suit replacement with a modular timber alternative. The developed modular system covers 58% of the highway bridges stock owned by Rijkswaterstaat that were built in 1950‐1980. Moreover, timber shows substantial environmental benefits compared to concrete. The alternative reduces the carbon footprint by 47% compared to a traditional concrete bridge. Furthermore, this study observes additional benefits when one considers carbon emissions in time, as the structure
sequestrates carbon during use. Consequently, building with timber lowers the carbon concentration in the atmosphere, reducing the global warming effect. This study recommends Rijkswaterstaat to invest in the development of circular timber highway bridges. By changing the ’business as usual’ from concrete to a more sustainable alternative, the circular and climate goals can be obtained. Furthermore, the recommendation is made to clarify the regulations on quantification of environmental impact, both on data and methodology level.