Wood you believe it?

Oh dear, the New Scientist is the latest publication to fall under the spell of ‘timber can save the
world’ mantra by replacing the evil that is concrete (The New Age of Wood, 16th March 2019).

The article argues that we live in the “hydrocarbon age” which makes possible the materials that ‘define our
civilisation: steel, concrete and plastic’. It goes on to claim that “everything that is made from fossil-based materials today, can be made from a tree tomorrow’. While some examples are obvious, e.g. timber buildings, others require new technologies. Apparently, a timber ‘tougher and stronger’ than
high performance steel can be made from soft wood. Obviously, it must be processed first, which involves ‘chemically removing half of the lignin then brutally compressing what is left at high temperature’. No mention is made of how much carbon is emitted in this process. I wonder what
solution they will claim can replace concrete paving or asphalt roads. Perhaps they’ll transform
timber decking into something that can survive being run over by all the articulated lorries that will
be needed to haul all that imported timber and timber products around the country. Better hope it doesn’t rain, it might get a bit slippery!

I have rehearsed the arguments about the carbon content of concrete before and how it is a victim
of its own success and talked about the disadvantages of CLT, e.g. the poor acoustic qualities that required a school to ban pupils from talking in the corridors and the peeling layers that
means it can add fuel to a fire.

Let’s consider some of the other issues that get glossed over.
Where are you going to put all these trees? Has anybody worked out how many trees would need
to be planted to replace all concrete, steel and plastic and would there be any land left over to
provide food for the world’s growing population or house them?

Apparently, one cubic metre of timber stores one tonne of CO2, which contrasts positively to
cement where one tonne of cement creates getting on for one tonne of CO2. However, concrete is a
low carbon material because not much cement is used in its production (and that cement is often partly replaced by low CO2 products like slag and fly ash). What happens at the end of life? The timber will probably be burnt to produce energy also known as releasing all that stored CO2 back into the atmosphere. So, when the New Scientist claim “switching to timber would immediately wipe a billion tonnes off global carbon emissions”, what they fail to add is that in 50-100 years time much of it will still end up in the atmosphere. Concrete by contrast, reabsorbs CO2 throughout its life by a
process of carbonation. At the end of its life, if it is crushed up to produce recycled aggregate, the
increase in surface area of the particles will accelerate the carbonation, increasing the amount of
reabsorbed CO2.

Don’t get me wrong, I’m not saying don’t use timber, or develop new technologies. What I am
saying is that concrete is a wonderfully adaptable, durable, cheap and locally available product. Let’s
look at ways to improve further the sustainability of concrete, e.g. by sequestration of CO2, rather
than trying to create a concrete-free fantasy land.

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