Hopefully there is concern and interest about the carbon footprint of everything these days. This critical spotlight should also be shined on our pavements, because their CO2 footprint can be large. Concrete generally gets a bad mark in this regard because its primary binder, Portland cement, is infamous for contributing around 8% of the total carbon emissions worldwide. However, pervious/ permeable/ porous concrete is different from ordinary concrete pavement in at least two significant ways and because of these, pervious concrete’s carbon footprint can be significantly lower than ordinary concrete and all other pavements as well.
Pervious concrete goes by one more name, ‘no-fines concrete’, because it is concrete made without sand and therefore water flows right through it. This feature contributes to multiple environmental advantages, and the name, ‘no-fines’, provides a clue to its potential low carbon footprint advantage over other concrete pavements. Pervious concrete mixes are usually designed to have a certain percentage of voids. Around 20% is a common void percentage goal and this amount will typically translate into over 600” of water per hour being able to be absorbed by the pavement, depending on installation techniques and other factors. One way to make pervious concrete stronger is to decrease the number of voids, to reduce the permeability. Decreasing the percentage of voids can be done in several ways. Adding more cement to the mix is one way, but it is more expensive and adds significantly to the carbon footprint.
Paying careful attention to the aggregate type and gradation or even adding some fines/ sand back into the pervious concrete mix generally can achieve similar strength gains as adding cement. By following this principle to a greater degree and aggressively substituting the remaining cement with scm’s (supplementary cementitious materials) like fly ash, slag and other pozzolans that are extremely low or no-carbon, we can reduce the carbon footprint of pervious concrete up to 70% lower than even today’s increasingly common ‘5 sack’ (4 sacks Portland cement + 1 sack fly ash per cubic yard of concrete) concrete pavement.
Concrete’s ability to sequester CO2 after it has been placed holds the clue for the other significant carbon footprint advantage that pervious concrete possesses. Around half of the CO2 that is released into the atmosphere during the production of Portland cement is due to a chemical reaction. This chemical reaction is partially reversed after concrete is placed and it is exposed to the air. With ordinary concrete pavement, very little of it is exposed to air until the end of its useful life and it is ground up to make baserock and other useful aggregates. However, because of pervious concrete’s open void structure, exponentially more of it is exposed to the air directly after placing, allowing it to sequester CO2 much earlier in its lifecycle. This amount is potentially up to 50% of its initial carbon footprint within a few years of placing.
A quick, very rough estimate of the lifetime carbon footprint of the two most common parking lot pavements gives us 10 lbs per square foot for concrete and 8 lbs per square foot for asphalt. It is possible to drop pervious concrete’s to below 3 lbs per square foot!