Alternative building materials
Making concrete accounts for 5% of GLOBAL carbon emissions, so is there a better alternative? Chris Smith laid the foundations with Darshil Shah from Cambridge University...
Darshil - Well, there is a founded industry around steel already and also steel has high strength and stiffness, therefore load bearing capacity and you can form it into complex shapes and that enables load transfer efficiently. I think one of the advantages of concrete is that the material is available abundantly in many places and that you can form it into different shapes of all sizes.
Chris - So it is a good material but it's not as good as a natural material, potentially?
Darshil - Yes, in different ways though. The three mainstream construction materials are concrete, steel and timber. So, for example, up to 25% of new homes in the U.K. are based on light frame timber and the advantage of timber, in this respect, is that it is a carbon sink...
Chris - Because it's soaking up carbon dioxide from the atmosphere to turn it into wood in the tree?
Darshil - Yep. The production process itself can be carbon neutral if you employ a closed loop cycle where the fuel is generated through the offcuts of the wood that is produced...
Chris - That's why I asked you about natural materials because on the one hand, if we were to build buildings with wood, that sounds good because they're going to soak up CO2 from the atmosphere. They will be a strong material that we know from thousands of years of construction appears to work. But how quickly do we need to renew buildings made of wood because there's obviously going to be an energy cost in building the building, building it with wood sounds good but then if I have to do it every 20 years - not so good? So how does the maths and the equation balance?
Darshil - So the design life that we tend to work is between 50 to 100 years and that matches quite well with the harvest cycle or the rotation cycle of timber. So once you have a new harvest, once you have a new set of wood, you can create new homes and replace the existing ones.
Chris - Now one of the projects you're working on is to look at or explore the possibility of using GM wood or bamboo in order to better serve the requirements of architects and engineers - tell us about that?
Darshil - I think we first need to understand wood as a material. We are looking at the cellular level through plants like arabidopsis thaliana, which are the mice of the plant world whose genetic sequence is well known. And our host biochemist, Professor Paul Dupree's group, is able to modify these plants and change the polysaccharide constituents of the cell wall and the microstructure confirmation of the stem. The interesting thing is that the wood formation is these tiny little stems (1mm in diameter, or even smaller) is similar to that in hardwoods, in poplar for example. Therefore, by studying these genetically modified stems and looking at how maybe a small change in the xylan content, or the cellulose content, and even small functional group changes and how the interaction between the polysaccharide changes affects the properties of the material. And therefore, we can then work our way towards genetically modified poplar and better understanding of how the properties of wood come about and what we can do, perhaps, to improve the properties of wood.
Chris - Right, so we have this project here where you're actually going from the cell right up to the mature plant, organism, tree, whatever. So you can actually, potentially, generate a tree that will generate the wood that will have the properties that you need for a certain building project - I mean is that what you're doing?
Darshil - In a way, yes. I think that's an important point because trees weren't made to be used in a dry state. In nature, trees are quite wet and it's not really a building material and we are using it as a building material so, if we want to use it as such, we may have to engineer it for those properties.
Chris - Right, so because we dry the wood out to turn it into a plank, it then loses some of the naturally evolved properties it had as wet wood?
Darshil - Yes.
Chris - Are you there, have you solved the problem, are you getting close?
Darshil - It's not only a multiscale project from the cellular level to the building level and even to the city level but also this work is at a fundamental level. So we are still trying to understand, at the cell wall scale, how the properties are derived and at the other extreme, we are trying to develop concepts and products that can actually be implemented for the designs of actual buildings. You have to keep in mind that there's a life cycle in growing a tree and it's only after 50 years or so you'll be able to harvest that tree. So you have to do a lot of long term planning with these sort of projects and we are, I think, a long way away from it but it's a strong start in the right direction.
Chris - In other words you can make sure that what you plant to be the sustainable wood source of the future because, at the end of the day, we want to reassure people here that you're not talking about going to the nearest forest, chopping down a whole load of trees, getting wood and turning those into houses? You're talking about let's have sustainable wood supplies but let's plant something now with 50 years hence in mind, which is actually going to be fit for purpose?
Darshil - Yes, absolutely. Deforestation is still a problem, although deforestation rates have been reducing. Importantly for us, substantial amounts of wood can be harvested sustainably without depleting or degrading forests. And since the 1990s, on average, Asia and the developed countries, so North America and Oceania, Europe have extracted about 3.8 billion metres cubed of wood annually and yet forest cover has increased annually by 1 million hectares. So you can extract wood resources efficiently and sustainably.
Chris - Now given that the Mckinsey report we referred to earlier said that there is this very pressing need, just by 2025, of 440 million households in really quite poor conditions or needing better quality housing. Are you going to get there with this work fast enough, taking into account it's going to take 50 years to grow this sustainable timber source?
Darshil - By 2050 we will have an additional 30% of wood resources, just new growth resources to use. It is possible it will be hard but if we think carefully about it and think about how we are actually designing homes, because using timber we will have to think about redesigning them using these new materials rather than basing designs on steel construction or concrete construction. So the efficient use of resources is the key and like in the energy industry, you want a diverse mix of energy, you would want to use a diverse mix of materials efficiently.
Chris - And I suppose if it does need a bit of DIY then you can always grow the raw material in the garden - couldn't you?