How much CO2 is embodied in concrete?
A 16 foot stretch of sidewalk (about four of those squares) produces enough CO2 to fill a 20 x 20 room with nothing but CO2. Why’s that matter? Because the earth over the next several years is able to ONLY absorb about 40% of this emission, and the rest simply accumulates and builds-up in the atmosphere.
Concert is a mixture of water, glue, and aggregate (sand or pebble) According the concrete experts, its 6% air (yes that’s right), 10% cement (the glue), 18% water (the sauce), 25% sand, and 41% gravel. All except the concrete is natural for the most part – excluding the processing and cleaning of the aggregates. Its the manufacturing of the cement that consumes most of the energy. The other materials are mined, crushed, processed, stored, and delivered.
Concrete has anywhere between 7% and 15% cement by weight, depending on the performance requirements for the concrete – super strong or just normal. The average quantity of cement is around 250 kg/m3 (420 lb/yd3). One cubic meter (m3) of concrete weighs approximately 2400 kg (1 cubic yard (yd3) weighs approximately 3800 lb). As a result, approximately 100 to 300 kg of CO2 is embodied for every cubic meter of concrete (170 to 500 lb per yd3) produced or approximately 5% to 13% of the weight of concrete produced, depending on the mix design.1
We picked the mid-point – 350 pounds of CO2 per cubic yard (yd3). The size of the CO2 cube for 350 pounds is 15x15x15 cube. Or in terms of a typical room thats a 20 x 20 room, basically for visual comparison purposes a good size bedroom or living room. In terms of how large of a concrete walk that produces, that’s about 16 running feet of a typical 4 foot wide walkway.
The point is that although concrete is an incredibly strong and durable product, its comes at cost to the environment and where possible wherever it can be avoided or minimized and a more natural product used in its place – the better. For instance, plans for a garden walk to side walk might suggest a wide concrete walkway. The amount of material and associated CO2 can be minimized simply by reducing the width or other dimensions. Or, better yet, a natural paving stone used instead.
1Concrete CO2 Fact Sheet – National Ready Mixed Concrete Association – Marceau, Medgar L., Nisbet, Michael A., and VanGeem, Martha G., Life Cycle Inventory of Portland Cement Concrete, SN3011, Portland Cement Association, Skokie, IL, 2007, 121.