Tiny Tuesday: Looking at Life Cycles

What’s in a cubic yard of concrete? About two tons of gravel, 300 pounds of water, and 600 pounds of Portland cement, the gluey substance that binds it all together.

That’s the simple answer. But Portland cement doesn’t occur naturally: it’s a refined mixture of gypsum and something called clinker. The clinker in turn is made from bauxite, limestone, petroleum, sand, clay, and more gravel. It takes significant energy to produce the concrete and its constituents: 4 kilowatt-hours of electricity and 30 btu of diesel and other oils, plus the fuel required to mine and transport raw materials. All these processes use copious volumes of water as well, to the tune of four tons per cubic yard. And let’s not forget all the byproducts that get released into the waste stream and the atmosphere: 30 pounds of solid waste, 1000 btu of waste heat, and some 600 pounds of carbon dioxide.

Figures taken from this life cycle analysis I did awhile back.

Figures taken from this life cycle analysis I did awhile back.

What I just did is known as life cycle analysis. To measure the full impact of a product on the environment, you have to consider every input and output of the processes that make it. When concrete begins its life (during construction) it uses up lots of raw materials, expends lots of fuel, and generates lots of waste. When concrete ends its life (during demolition) there’s another energy expenditure, plus the waste of all those raw materials. In general, concrete can’t be recycled in a structural application, though some classmates and I once tried to invent one. Thus, the life cycle of concrete consists of a few decades being a useful part of a building… with a long and humbling list of side effects.

Throwback: a concrete-earthbag construction project from graduate school. Not realistic for widespread application.

Throwback: a concrete-earthbag construction project from graduate school. Not realistic for widespread application.

Colin’s not-so-big house contains 80 cubic yards of concrete, so imagine how much goes into a skyscraper. And then think about all the water and oil and coal that construction eats up, and all the heat and carbon dioxide that get released. The lesson here is obvious: use less! (This logic applies to virtually every building material.) Construction has a greater impact on the environment than any other sector of the economy, and by looking at the life cycle it’s clear how much we can gain by building smaller.

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