Archive for the ‘Energy Conservation’ Category

How to Reduce the Amount of Cement in Your Hardscape

Monday, September 27th, 2010

Following up on my last post concerning the amount of cement in pervious concrete and all concrete.  There is a very easy way to reduce the amount of cement in any concrete flatwork.  Simply pour it thinner, as thin as 1 1/2″  and add control joints about 12″ on center.

We call this type of concrete work ‘thin concrete’ and for us it is a little more involved than simply adding a bunch of control joints.  Besides adding all those control joints can be a little too much work!  We have a whole process we implement, which you can learn more about in the energy conservation section of our website.

First I want to address cement alternatives, and I’ll forewarn you that I am a little cynical these days.  A lot of hoopla has been made about finding a replacement for portland cement or more commonly replacing portions of portland cement in concrete.  While it is true that fly ash and slag in particular can replace various amounts of cement and still leave the concrete with the same strengths or even greater than it would have had with 100% portland cement, I feel the environmental benefits that are touted for doing this are overrated.  One big factor that is overlooked is the amount of transportation that is involved in getting both of these products to us here in the Bay Area.  I believe most of the fly ash that is used locally comes from Wyoming and the slag generally comes all the way from Asia.  In addition as the EPA grapples with how toxic of a substance to define fly ash as, I think there ought to be concerns about the long term ramifications of using these products in our concrete.  For instance, what will become of it when it is crushed up to be re-used as base rock 50 years from now?

The cynical part of me questions why & by whom these cement alternatives are being pushed so hard?  I also wonder why such elegant, immediately implementable solutions such as pouring concrete thinner which could easily save 50% of the cement (and 50% embodied energy and 50% less pollution), aren’t discussed and encouraged?


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.