What’s holding back concrete sustainability?

By Christie Gamble

Concrete is the most abundant man-made material in the world. In fact, the only material more abundant than concrete is water. This ubiquitous substance that lines our streets and holds up our bridges and envelops our buildings keeps us safe, warm and dry. It is one of the most important materials developed by humanity to enable modern day standards of living. 

Unfortunately, due to concrete’s sheer abundance, the production of concrete has a significant impact on the environment. Cement – the key ingredient that gives concrete its strength—creates 7% of the world’s CO₂ greenhouse gas emissions. The cement and concrete industries have been working diligently for decades to reduce their carbon impacts while continuing to meet the infrastructure and building construction demands from a growing global population. 

Many innovations to improve concrete sustainability have emerged in these past decades. For example, the use of Supplementary Cementitious Materials (SCMs), industrial byproducts that have cementing properties such as fly ash (coal byproduct) and slag (steel byproduct), have become standard practice in most concrete applications. The use of Portland-Limestone Cement, a staple in most European construction, is becoming more predominant in North America as a replacement for General Use cement. Promising innovative materials are emerging such as natural pozzolans, recycled glass pozzolans, and aggregates made from recycled materials. 

CarbonCure is leading a rapidly growing industry practice to utilize waste CO2 in concrete manufacturing in a process known as CO2 mineralization. CO2 mineralized concrete has been used in a wide variety of commercial applications, including airports, roadways, and high-rise buildings; the process continues to gain incredible momentum as a viable solution to reduce embodied carbon in building and infrastructure developments. 

While innovations within the concrete and cement manufacturing processes are flourishing, concrete producers continue to run into a significant roadblock to implementing these innovative solutions and improving the sustainability profile of their products: prescriptive concrete specifications. 

When designing a building or infrastructure project, structural engineers employ two basic methods of specification-writing: performance or prescriptive. Performance-based specifications define the performance qualities required of the concrete to meet the engineering needs of that particular application. Most significantly, a performance specification defines the required strength, alongside other durability or work-ability related criteria. Prescriptive specifications meanwhile define a set of criteria for how the concrete is made, such as the quantity of cement required, the maximum volume of SCMs allowed, or the maximum water-to-cementing materials ratio. 

Prescriptive specifications create barriers than inhibit innovation and performance. The practice of writing prescriptive specifications, in particular a minimum cement content requirement, originated decades ago, when concrete production quality control standards were not as robust as they are today. Modern quality control testing practices enable structural engineers to confidently ensure that the defined concrete performance criteria (such as strength) have been met. 

The National Ready Mixed Concrete Association (NRMCA) and America Concrete Institute (ACI) assert that 85% of concrete specifications include unnecessary restrictions on SCM quantity, 73% of concrete specifications include unnecessary or over-designed maximum water-to-cementing materials requirements, and 46% of specifications invoke a minimum cementitious content requirement. The NRMCA and ACI have conducted considerable research to suggest that improper use of these prescriptive requirements often results in poorer concrete quality, poorer sustainability impacts, and higher costs. The ACI clearly states, “prescription does not assure performance.”

Why then, are prescriptive specifications so prevalent in commercial building and infrastructure design today? Is it perhaps due to inertia; specifications practices continue because that’s the way it’s always been done? Are there perhaps not enough resources available to structural engineers to provide clear guidelines on how to write performance-based specifications? Concrete specifications are large documents filled with incredible quantities of dry technical information; is it possible that they’ve been overlooked?

Thankfully, while prescriptive specifications remain a considerable barrier to sustainability, the tide is changing among the structural engineering community. The Structural Engineering Institute recently unanimously endorsed the Structural Engineers 2050 Challenge to specify structures with zero embodied carbon by the year 2050. The first step to meeting this challenge is for structural engineers to adopt performance-based specifications. More resources are becoming available for firms to adapt their specifications to enable sustainable innovation, without compromising structural quality.

Concrete design is also now becoming more of a collaborative effort between structural engineers and concrete producers. It is becoming a more common place for structural engineers to turn to their local concrete experts – many of whom are engineers themselves – for technical recommendations. One of America’s most iconic buildings – One World Trade Center – was built to high performance standards using innovative concrete specifications and collaborative efforts. Concrete producers who may have historically been left out of the conversation in early design stages are finding it far easier than expected to make recommendations on concrete mix designs; they simply need to initiate the conversation with the structural engineers and provide data to back up their recommendations.

There is more than enough evidence to suggest that structural engineering firms should consider moving away from prescriptive-based specifications and begin incorporating performance-based specification practices into their standard operating procedures. Concrete producers also need to supplement technological and manufacturing innovation with education to their design and construction community.

Prescriptive specifications continue to remain a barrier holding concrete sustainability back. It’s time to break down silos, engage in collaboration between designers and producers, and move forward.

Christie Gamble is the Senior Director of Sustainability at CarbonCure Technologies.