Posts Tagged ‘carbon dioxide’

Amount of Cement in Pervious Concrete

Monday, September 20th, 2010

During a conversation with one of my colleagues at the Bay-Friendly Conference, she reminded me of an earlier environmental concern I had when I first started working with pervious concrete several years ago. The concern was regarding the amount of cement in pervious concrete, or for that matter in any concrete. Cement, the main binder used in concrete, is energy intensive to produce, emitting approximately one ton of Carbon Dioxide for every ton of cement that is created. Considering that cement accounts for approximately 14% of the volume of concrete and knowing how much concrete can be used even on a single job, in this light it would appear that concrete has a large environmental footprint.

I am not saying it doesn’t. I’d just like to make a couple key points that I have learned over these past 8 years or so that I have been dedicated to finding and creating more environmental hardscapes. In that oft-sited statistic about one ton cement = one ton CO2, what they don’t tell you, is that half of the carbon dioxide that is emitted is as a result of a chemical reaction in the limestone as it is being turned into Portland Cement. It is well known that once concrete is placed it recaptures some of that carbon where it can, which is typically only on the uppermost exposed surface. So in the case of pervious(porous, permeable) concrete, with voids throughout the entire matrix, there is a tremendous amount of surface area exposed to the air and therefore sequestering more of that carbon dioxide that was lost during the limestone’s transition into cement.

I do not know what percentage of carbon dioxide is recaptured. I believe it is fairly significant as there have been studies showing how when concrete is crushed at the end of its lifecycle to be re-used, it actually carbonates then, and within the first couple of months re-captures up to 40%(exact # needs to be verified) of the carbon emitted during production.

The second point I would like to make is that most cement is produced locally. In our case it is literally produced within walking distance of our office in Cupertino. The old Kaiser plant is one of the largest cement kilns in the country. I believe that most other products aren’t produced this close to home, and I believe that fact is usually minimized in any discussion of how ‘green’ a product may be – that transportation is never given its full environmental weight.

I realize concrete is not environmentally benign, but hopefully I have been able to offer a more objective view of its true environmental footprint.

At the end of the day, generally the best thing for the environment is no hardscape or at least less of it, unfortunately this is not always practical. Gravel and decomposed granite/fines and others can be more environmental hardscape alternatives, unfortunately they generally have lots of maintenance issues. Due primarily to this reason, I think we are going to be stuck with a little concrete for some time to come. And if we have to put in concrete it might as well be pervious, so water can stay onsite and the soil can breathe underneath.