Split in the Asphalt
Eine Semesterarbeit des Instituts für Landschaft und Urbane Studie der ETH Zürich zeigt, wie Biel durch entsiegelte Wege und grüne Korridore den Wandel zur Klimastadt gestalten könnte. (Englisch)
A semester project with the Chair of Landscape Architecture of Martina Voser at the ETH Zurich began as an effort to understand specific topographical qualities of Biel/Bienne in a project of our own choosing. A regional portrait of geology and material extraction emerged in parallel with a site in question: the Schüss river of the Bernese Jura. From its origins in the Saint-Imier valley to where it enters Biel/Bienne, the Schüss flows through a stretch of the Jura Massif abundant with limestone. North of the city’s Bözingen neighborhood, and past the Gorges du Taubenloch, the Ciments Vigier produces 500’000 tons of cement each year.
Upon entering the city, the Schüss forks into three paths, each with a distinctive relationship to the surrounding neighborhoods and streets. The third fork, the Madretsch-Schüss, winds its way furtively through plots of industrial sheds, schoolyards, and private residences. From time to time it is culverted beneath slabs of concrete or cut off from view entirely behind densely overgrown vegetation and chain-linked fences. Amidst a backdrop of factory demolitions and various vacant lots, it paints a unique picture to the nature of the city’s deindustrialization, and prompts one to ask what might a new relationship to the riverfront look like?
The design redirects attention to the ground, and looks to the region’s geology as a starting point. Several streets terminate along the river’s embankments as it winds its way through the Madretsch neighborhood, and some high-speed roads amputate pedestrian connections to the water altogether. In reducing traffic flows through these streets, by de-sealing and de-paving the asphalt surfaces adjacent to the Schüss, pedestrian and cycling paths could be built and open new connections to the water. In splitting and removing impermeable surfaces, the old pavement could then be milled, crushed, and sorted as a form of ‘urban mining.’
The extracted materials would then be bonded with lime-cement, locally re-sourced from Jura, for new permeable paving, critically with gaps between the pavers. Extended tree pits and planting beds might further buffer roads and offer points of transition, creating green corridors that expand connections to Schüss. While the lifespan of an asphalt road varies, a surface course typically needs replacement every 20-25 years in Canton Zurich. Other factors, including climate-induced freeze/thaw cycles and the installation of cables, pipes, and subsurface infrastructure means that these repairs occur often. The proposal seeks to insert a notion of reuse into the maintenance and waste patterns, whereby asphalt and other demolition materials might be reincorporated, in a form of cyclopean masonry. Unbonded surface paving could lessen the waste streams of asphalt that is poured and later demolished in perennial cycles. Perhaps most importantly, the usage of “waste materials” such as coal fly ash in the formulation of this concrete mixture could lessen the dependency on conventional cement by up to 70%.
Bituminous materials rely on the extraction and refinement of crude oils and pollution-intensive processes, are immensely damaging in extracting and persist with uncertain hazards once they enter into waste streams. In incorporating bituminous material and other rubble into an envelope of lime-cement, the material is rendered inert. It adapts a logic of porosity and respiration offered by permeable surfaces, signaling a transition from oil-based materials and sealed surfaces.
Modern construction practices often exude attitudes of permanence, often without regard to cycles of maintenance or potential for repair. The infrastructures of staking claims to mineral resources carry profound implications to our relationships with landscapes, what they’ve been, and what solutions rooted in place might be.
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* Francisco Coch is studying for a Master's degree in Landscape Architecture at the Institute for Landscape and Urban Studies at ETH Zurich.The lime-concrete experiments would not have been possible without the time and help of many people. Lila Siewczyk and the team at the RAP Lab (ETH) provided the space and materials in the early stages of concrete casting, and Rösli Duft and team at Kalkfabrik Netsal AG generously supplied the quicklime for the hydraulic concrete experiments.
Limestone (CaCO3) as a building material is heated during calcination to form quicklime (CaO). Calcium oxide as a powder is extremely reactive, and when slaked with water, undergoes an exothermic reaction to produce hydrated lime (Ca(OH)2). The concrete used by the ancient Romans, opus caementicium, is a mixture of hydraulic mortar with aggregates of stone or building rubble, often bricks. The hydraulic mortar itself sets when calcium oxide comes into contact with water, and in the presence of silicate or alumina particles of volcanic ash, creates a cementitious mix. This reaction is referred to as a pozzolanic reaction, and reoccurs as cracks form and water seeps into the cured concrete. The calcium alumino-silicate hydrates, or C-A-S-H, help the concrete to self-heal, a quality famously attributed to roman concrete. It essentially builds on the lime cycle, with pozzolanic additives preventing further decomposition. In material experiments, ratios of quicklime to sand and coal-fly ash, a residual and ‘waste’ byproduct of coal-combustion high in alumino-silicate contents, were carefully calibrated to produce a recipe for self-healing concrete with little to no cement used at all. Whereas Ordinary Portland Cement (OPC) releases up to 1 metric ton of CO2 emissions per metric ton of produced material, the inclusion of pozzolans may help to significantly reduce the need for cement in concrete mixtures. The self-healing properties stand to further extend the longevity and durability of concrete, and perhaps reduce the process of material turnover.
