There is no doubt that concrete is a filthy substance to produce. Worldwide, it is the most widely-used construction material with over ten billion tons produced annually and it carries a massive CO2 burden – in total about 7% of global CO2 emissions come from concrete production.
On ‘round one’ of the Gorgeous Green House design we were rather shocked to learn how many truckloads of concrete would need to go into the building. We began looking at alternatives:
Fly-ash concrete is readily available in many countries. Fly-ash is a waste material – the inorganic residue – that remains after pulverized coal is burned in coal-burning power plants. Disposing of it can be a problem. Fortunately, the construction industry has discovered that fly-ash can produce a superior concrete with excellent finishing characteristics.
Recycled aggregates and lightweight aggregates can replace the usual sand and gravel in some concrete applications. This can consist of crushed concrete, brick, or crushed glass. Lightweight concrete is made by using expanded volcanic materials – pumice and perlite, for example – to replace some of the usual stone aggregate. Volcanic materials also add some thermal insulation value to the concrete. Unfortunately this is not yet available to us in South Africa.
There are also interesting products in R & D using for example waste products from oil refinement with huge CO2 emission reductions in the production process. Not yet available.
Traditional alternatives such as timber construction could also be considered.
So what did we do?
Firstly, we lost as many flat could roofs as we could to be replaced with timber trusses and metal sheeting.
A much larger percentage of concrete to be used though, is in the slab between the floors, the off-shutter wall and supporting the roof garden so we turned our attention here. The roof garden was non-negotiable as the structural support required doesn’t provide any options. We could have put timber flooring in elsewhere but it would be heavily treated for termites, noisy and aesthetically not in keeping with the contemporary design of the house.
The awesome British show Grand Designs provided inspiration. Polystyrene is being used in conjunction with cement to reduce the volumes required. Timber formwork is fiddly to make and a lot of it cannot be reused. Using polystyrene in place of timber effectively kills two birds with one stone: it acts as formwork to mould the concrete and is then left in place to provide insulation.
Polystyrene beads can also be added to a concrete mix but our engineer was not happy with the structural support this process would provide.
So our green guilt is somewhat reduced and we are also reminding ourselves that concrete lasts. This is the stuff the Romans and Egyptians built their empires with! It’s impermeable to air and wind-driven rain. And concrete is inedible, so bugs and vermin can’t gnaw at it. A building with exterior concrete walls can also be energy-efficient, especially in climates that have daily temperature fluctuations. Even though concrete provides little insulation, it creates thermal mass that can store warmth or cold, reducing indoor temperature fluctuation.
Check out our polystyrene process and stage one of the off-shutter wall. Concrete is being poured on Monday 05-08 Another significant milestone achieved. Very exciting!