We’ve discussed the carbon footprint of concrete previously. However, carbon dioxide and other greenhouse gases aren’t the only environmental effects of construction. Making the best use of scarce resources is increasingly important in our industry. CALGreen focuses on both greenhouse gases and efficient use of resources. One resource that’s beginning to attract more attention is water, so let’s take a look at the water footprint of concrete.
Determining the water footprint of concrete
The idea of water footprint for concrete is so new that there’s no agreement on how to calculate it. Does it include just the water for mixing and curing? What about the water we use to process the aggregates? Or the water embodied in the energy we use to process and transport the raw materials and get the concrete to the job site?
Some calculation methods distinguish among “blue water”, “green water”, and “gray water”. That is, “blue water” includes fresh surface and ground water. “Green water” falls as precipitation and doesn’t run off, but either evaporates or transpires through plants. “Gray water” is the water needed to dilute polluted water to acceptable concentrations.
Because agriculture is the sector that uses the most water, the Water Footprint Network method works well for it. However, in the construction industry, life cycle analysis is a more familiar method—it’s what we use for carbon footprint. Unfortunately the data for calculating a water footprint by any method are lacking.
Location, location, location
Another complication is that water use has a bigger environmental impact in some places than in others. Unlike the carbon footprint, which is the same for a pound of CO2 anywhere in the world, the water footprint depends on location. Even within the United States, fresh water is abundant near the Great Lakes and scarce in the West.
“The West begins,” Bernard DeVoto wrote, “where the average annual rainfall drops below twenty inches.” This is maybe the best definition of the West I have ever read, and it goes a long way toward explaining my own passion for seeing the water under control, but many people I know persist in looking for psychoanalytical implications in the passion. As a matter of fact I have explored, in an amateur way, the more obvious of these implications, and come up with nothing interesting. A certain external reality remains, and resists interpretation. The West begins where the average annual rainfall drops below twenty inches. Water is important to people who do not have it, and the same is true of control.—Joan Didion, “Holy Water”
If you can get all the water you need from surface water, your water use is sustainable. Precipitation replenishes water in aquifers, but only to an extent. Beyond that, drawing more water from an aquifer is like mining—it’s not sustainable.
Using water more sustainably
The construction industry is starting to make water use more sustainable. For example, ASTM C1602 permits reuse of water from concrete production. And ASTM C1798 permits reuse of returned fresh concrete. Local regulations may address the quality of runoff water from construction sites to keep rivers and lakes cleaner.
The water used directly in concrete—that is, the water for mixing and curing—is only a small fraction of the total. Controlling dust from crushing and storing aggregates, for example, can take much more water. To reduce the water for processing aggregates, you might select the aggregate source if you have a choice. Transporting aggregate from a great distance, however, requires more water as well as more fuel.
Portland cement takes more water than any other material in concrete, so minimize the portland cement content as much as you can. You can do that by appropriate use of supplementary cementitious materials. You can also reduce the overall cement paste content by efficiently grading the aggregates and by using high-range water reducers. Beton can help you specify the concrete you need or help you meet an existing specification.