Concrete is the world’s most widespread construction material and the second most used resource after water. It consists of aggregates agglomerated by a binder, cement, which is itself produced by clinker, a mixture of limestone and clay. Concrete is the main material in the construction of our cities. From the point of view of cities wishing to reduce their environmental impact, how can we replace this resource?
One of the world’s most polluting materials
This artificial rock, which is easy to build and solid, is however an ecological aberration: it alone is responsible for 4% to 8% of the world’s CO2 emissions today, according to different sources. For every tonne of cement produced (between 10 and 15 tonnes of concrete), 750kg of CO2 are emitted, according to Peter A. Claisse, in Civil Engineering Materials, 2016.
Concrete is difficult to deconstruct, compared to other materials, and its reuse becomes complex because of its friability and non-modularity, as opposed to wood for example. However, some solutions do exist and we will come back to them later.
The ecological impact of concrete comes from its production, transport and the materials needed to make it: sand and rubble are most often extracted from the coast, damaging the coastline, salinising agricultural land and having a negative impact on the ecosystems and communities that depend on it.
This ecological balance sheet is increasingly impacting the real estate sector, especially investors and cities, which are interested in the resilience of buildings, but also in their socio-ecological impact.
In order to reduce these disastrous carbon emissions, Green Soluce has identified two solutions, and supports its private and public clients in the implementation of a strategy that combines these areas.
Saving or replacing concrete in construction
The first solution is first of all to save the concrete used in the construction process, in order to limit its production.
It is possible to completely replace concrete with other building materials. Many materials and solutions can be considered, but not all of them are applicable in cities and do not have the same uses: for example, stone, wood or mycelium can completely eliminate the need for concrete in urban construction.
Concrete is also one of the main contributors to heat islands in cities, which reach their peak temperatures at night. Urban heat comes from the buildings and the ground, which release the energy stored during the day: using materials other than concrete would improve the summer comfort of large cities, by reducing heat absorption with materials that retain less heat, increasing albedo, reducing the artificialization of the ground, etc.
Cities have specific needs for resilience and many constraints, linked to the existing situation and heritage. As the majority of works in large cities are renovations, the use of alternative materials is not systematically possible. On the other hand, less polluting concretes can be considered, such as Hoffman Green Cement Technology or Vertua® from CEMEX.
Reducing the overall impact of concrete
The second solution, which will probably be the preferred means for cities and investors wishing to make a transition, is to reduce the impact of concrete by all possible means.
There are several ways to reduce the impact of concrete. One way is to recycle and reuse concrete: by crushing it, concrete residues can be incorporated into new concrete, which can be used to build roads, for example, thus avoiding extra material extraction.
Reusing crushed concrete blocks in new construction ensures that some of the carbon impact of new construction is reduced, although the EN206 standard limits the amount of recycled concrete in new concrete to 30%.
Natural products or waste from certain industries can advantageously replace sand and rubble. An activity is being created around the replacement of concrete by other materials, and companies are trying to register copyrights around bricks made from hemp, coal ash or iron, wood fibres: HemCrete®, AshCrete® or TimberCrete®.
Plastic waste can indeed be collected, crushed, and replace part of the sand and rubble contained in traditional concrete. The same is true for wood, which can be used to make wood concrete, where all or part of the aggregates are replaced by treated wood fibers.
Reducing the impact also means extending the service life of concrete. The average lifespan of concrete today is theoretically about 100 years. External and building use conditions, as well as the reinforcement, often cause problems that shorten this lifespan. Chemical processes are being investigated to extend the service life by reducing the porosity and carbonation of the concrete, and to limit corrosion of the reinforcement.
It is in most cases more environmentally friendly to repair a concrete structure than to rebuild one. Many inspiring success stories are available here.
Many solutions exist, applicable in different cases, including so-called “carbon-negative” solutions, such as wood concrete: in each brick is stored more CO2 than is emitted for its production!
Written by Jeremy Pistien for Urban Chronicles™.
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