By Alan Sparkman, Contributing Author
Sustainability is a term so overused that it is difficult to know what someone means when they use the word.
For the construction industry, the current conversation about sustainability revolves primarily around the initial embodied carbon (EC) in the materials we use to construct projects. That is the extraction, transportation, and manufacture of the materials.
The perception that concrete is a high embodied carbon (EC) material creates a challenge. This comes from a failure to distinguish concrete from Portland cement. While Portland cement has high EC, cement is one of several ingredients in concrete, and those other ingredients have a much lower EC.
In fact, Portland cement accounts for nearly 90% of the EC in a typical concrete mix, but compared to other common road and bridge construction materials, it has a low to moderate amount of EC per unit of material.
Often accused of being a major contributor to pollution, concrete accounts for up to 10% of carbon emissions in any given year. This may well be true, but it is also misleading.
One of the reasons that concrete accounts for such a high percentage of annual carbon emissions is the fact that it is by far the most utilized construction material on the planet.
The real question is: What would happen if we were to substitute other materials for concrete in our construction process?
This would reduce concrete’s contribution to EC , but we would find that overall EC on a given project often would go up. The reason concrete is so heavily utilized is that it delivers economy and value across a wide range of applications, and it turns out that it also delivers lower EC in most of those applications than any other alternative.
Low Hanging Fruit
For concrete, sustainability is often short for the embodied carbon (EC) in a cubic yard, and this is directly related to the amount of Portland cement used in a particular concrete mix.
Luckily, there are many ways to reduce the amount of Portland cement in concrete while maintaining the properties needed for excellent performance in the hardened concrete. This is low hanging fruit to increase concretes’ sustainability profile and can be implemented immediately to lower the EC, or carbon footprint, of concrete on many projects.
The amount and strength of concrete needed for certain applications could be categorized as low hanging fruit, as well. The basic rule here is to not use more than you need for the desired performance.
For example: specifications that call for concrete sidewalks at any thickness more than 4 inches. In light duty pavements, such as parking lots, 4 inches of concrete is sufficient. Using 6 inches of concrete for pedestrian sidewalks doesn't make sense, either. The same thing can often be applied to specifying more compressive strength than is needed for a particular application.
The specification of strength is a bit more complicated than thickness since some applications require higher compressive strengths for durability rather than for structural capacity, and the time needed to develop strength often has a big impact on the project’s schedule. Still, if your application doesn’t call for 5000 psi concrete, don’t use it.
Beyond Low Hanging Fruit
In the long term, there are many exciting developments in progress that offer the possibility of dramatic reductions in the EC of in-place concrete. These include alternative binders, alternatives to both coarse aggregate and fine aggregate in a concrete mixture, and a wide variety of chemical and mineral admixtures that can be incorporated to reduce the cement content in many types of concrete.
Here's a quick look at some of these and an estimate of their impact in lowering concrete’s EC:
- Portland Limestone Cement (PLC), or Type 1L Cement: Soon to be the primary cement across the country. Reduction of EC (versus existing Type 1 cements) is about 5-10%, with little to no impact on the performance of concrete. Note that PLC is not new and is well vetted.
- CO2 Mineralization: At scale and in use now, it is nearly a 5% reduction of EC. This technology is virtually transparent to the end user regarding strength and set times.
- CO2 Injection and Curing: At early industrial scale, it is likely best for block and precast plants, and it could produce ‘carbon negative’ concrete products by completely offsetting the EC in the concrete.
- Fly Ash Closure by Harvesting: I cite this term based on my involvement with ‘harvesting’ fly ash from old impoundments at power plants. While estimates vary, there are at least a couple of decades of fly ash that could be harvested and beneficially re-used in concrete across the country. Barriers here are regulatory and bureaucratic, and initial capital cost, but there are mature technologies in use today in some areas. Fly ash replaces 20-30% of cement, thus providing significant EC reduction. Regional utilities are key decision makers as they own the impoundments.
- Ground Glass Pozzolan: At industrial scale now. Recycles all types of glass to produce a pozzolan for concrete that will be an important alternative to Class F and C fly ash. Plus, it solves a nasty recycling issue for cities and states.
- Cultured Aggregate: “Grown” using recaptured CO2, it’s at industrial scale but has very limited availability. Potential reduction of concrete’s EC to zero, or below, for mixes that use this as the primary aggregate.
- Synthetic Sand: A relatively new arrival, this technology is at industrial scale and expanding. This process takes nearly all plastics as raw material and produces a lightweight sand suitable for use as a partial substitute for regular sand in concrete. Likely a negligible impact on EC in concrete but an exciting solution to recycle rather than landfill plastic waste.
- Nano Scale Materials: Many products fit under this broad umbrella and can lower EC in concrete by reducing cement contents. They sometimes bring other important benefits and can generally be used in conjunction with other strategies described here to produce additional reduction of EC.
- In Place Carbonation of Concrete: Concrete is the only construction material that absorbs CO2 from the atmosphere while in service. Estimates vary, and experts argue, about how much CO2 can be re-absorbed but it appears that 10–20% of original EC can be re-absorbed during the service life of the concrete.
Sustainability is a critical and complex challenge. EC is one dimension of the challenge, but there are many others. Concrete is improving (and reducing) it’s carbon footprint rapidly and additional improvements are in the pipeline. Concrete brings many other crucial benefits to the sustainability of the built environment that other road and bridge construction materials can’t match – things like resilience, long life cycles, life safety and more.
As concrete people we must be in the conversation to make sure our material is accurately represented. R&B
Alan Sparkman, CAE, LEED AP, CCPf, has served as the Executive Director of the Tennessee Concrete Association since 1998.
