concrete and its alternatives

Concrete is a wonderfully versatile material, which has been in use since Roman times and with it we can build amazing structures that would otherwise be impossible. Unfortunately it also carries a huge environmental cost, caused mostly in the cement binding used. Whilst gravel extraction is also an issue, for that there are some alternatives such as recycled, crushed concrete.

The production of modern cement (ordinary portland cement) is a hugely polluting process and produces around 10% of the world's carbon emissions - five times that of the aviation industry! Whilst the Romans used a volcanic dust called pozzolona as their binding agent, until the invention of OPC, we used Lime, in various forms. Lime too has to be processed by heat and so is not without it's environmental cost, but it uses less energy to produce than cement and lime concrete/mortar reabsorbs carbon over its lifetime (if exposed to the air), offsetting some of the initial pollution. Moreover, because it is softer, materials can be disassembled easily, for re-use.

In a landscape, there will still be much use of concrete in various structures - walls, paving, sub-bases, ponds etc. Much of this concrete use could be reduced or avoided altogether. Such thinking may not be much to the liking of the paving manufacturers, but if sustainability is to be given top priority - as it must - then the use of cement has to be reduced as much as possible. This is quite a challenge to us all, as it is not always easy to find an alternative and I don't suggest for one minute that it should never be used, but it should be reserved for essential structural use only. Even then there may be alternatives. I have used concrete in most, if not all, of my own designs and so this challenge is personal - but then aren't all such sustainability issues!

Let's look at the different areas of the garden where cement is used:

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Walling

Many walls use cement, either in mortar or in concrete blocks as well as in cement renders. In this garden (left) we build retaining walls using hollow concrete block construction on concrete footings with cement render and mineral pigments for the final colouring, with concrete paving slabs as coping. As the site was on a chalk hillside, it would have been ideal for construction using rammed chalk or earth. Indeed, we had a surplus of chalk on the site from the house construction and it would have made perfect eco-sense to have used this to build the walls! The pigmented finish is very popular so could have been done using lime based render, and the coping could have been stone. As the chalk hillside is inherently quite stable, rammed chalk could possibly have been used for the footings too.

The cost of rammed earth building would probably have been cheaper than the conventional method used, both in time to do and in material purchase, especially when the removal off site of subsoil is taken into account.

The picture at right shows rammed earth walls in a sustainable landscape I designed for Grand Designs Live, ExCel, 2006. Rammed earth walls are made by compacting subsoil inside timber or mettal formwork. The soil must be kept dry so it needs some kind of footing and capping, here provided by cast copings made with recycled aggregates and partial cement substitute (furnace slag). The gravel to the main paving area is a recycled brick/concrete mixt, laid through a geo-grid membrane made from recycled plastic. The aim here is to be as carbon neutral as possible. You can also eat day=lilly flowers!

Walling alternatives to concrete include:

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Paving

Modern paving is often made from concrete and in some instances it is quite hard to find realistic substitutes. Stone paving is the obvious one but can be expensive. There are many cheaper stone imports available these days, such as Indian Sandstone, which are cost-effective but which carry a high transport and pollution price tag. Reclaimed York paving is highly desirable and of course, reused, so environment friendly (new is now very scarce or nonexistent as we have worked out many of our traditional quarries) but will cost 2-3 times that of cast concrete or Indian Sandstone. Brick is also traditional as paviors but as for walling, still carries a high energy/pollution cost in production, plus is expensive to lay.

For larger areas and drives, self-binding gravels may be one of the better answers, as these form a firm surface, once compacted, without the use of cement. Recycled or local gravels are also acceptable and can be held in place using recycled plastic grids/membranes.

Paving is one of those areas which highlights the question of just what is sustainable. is it better to use imported sandstone with its high transport miles but relatively low energy production, or locally made concrete paving with cement and aggregate use, but low transport miles? In either case, the (financial) costs may be similar.

Timber decking can be a viable alternative, especially on sites with changes of levels. I want to experiment with casting my own slabs using lime based mortars and hope to report back on this at a future date. Meanwhile, i tend to feel that local, recycled materials are the best option, where available. Possible alternatives:

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Ponds

Even natural looking ponds may use concrete in their construction and formal ponds may be dominated by it. Again choice here may be limited but I am doing research on alternatives.

If you read my pond construction notes on this site, you will see that concrete is often used to retain rocks and place protective pads over liners, etc. In many places it may be possible to do without concrete pads, which are often used in a “belt-and-braces” approach to protecting a liner. Felt geomembranes are used as standard as a layer underneath and often inside the liner (this may in fact be degraded by contact with concrete) and in many cases, use of additional layers of felt may eliminate the need for concrete. Felt is usually a synthetic, petroleum based product, so also has its environmental considerations (as do liners!). I have successfully used old carpet as replacement for felt, under the pond (I wouldn't recommend it inside) but be VERY wary of tacks that may have been used!. Non-synthetic carpet will degrade over time, which must be born in mind. Of course, before the advent of felt, liners were simply bedded on a layer of sand.

Where rocks and stones create waterfalls, they are used to form spillways and to prevent stones moving when climbed over or stepped upon (an important safety consideration). It may be possible to use hydraulic lime based concretes for this, although I haven't tried this out yet. Perhaps well laid rocks, without the use of cement are the key, which would require a careful re-thinking of the construction method. Nature doesn't use concrete, but then natural streams don't retain water within an artificial environment (liner)! A more holistic approach to water features, where the roof water and household grey water are recycled and the pond design doesn't hold a fixed level or amount of water, may be the long term answer.

For formal ponds with rills and straight walls, I am investigating, amongst other things, the use of rammed earth stabilised with hydraulic lime, and lime based concretes.