Code requirement issues are deeply embedded in the construction industry and a perpetual source of struggle. The intent behind using any material requirement is always the same: to create structures that are resilient, sturdy and safe. But the way we go about achieving those aims is the source of debate for a variety of reasons.
Arguments about prescriptive versus performance requirements for concrete, by far the most widely used construction material in the world, center on whether it’s best for specifications to delineate the composition of the concrete mix to be used or to delineate the performance characteristics and requirements.
There are valid arguments to be made for both sides and different motivations driving preference. Concrete has been around since the dawn of civilization, so there’s an incredible amount of institutional knowledge that can be leveraged for prescriptive specifications. Construction is also an industry that prizes stability and is, thus, resistant to change. For an infrastructure project such as a rural highway bridge, there’s probably little incentive to deviate from whatever specifications were used for similar bridges in the past—and those may be prescriptive in nature. If it worked before, the reasoning goes, it will work again.
Advocates of performance requirements will quickly note that prescriptions don’t necessarily serve the purpose for which they were created: chiefly, the performance of the concrete. Specified “recipes” are created to achieve certain material necessities such as strength per square inch (PSI) or permeability. But prescriptive specifications often outline the procedures and ingredients for what is desired, as opposed to specifying the actual desired result.
Concrete mixes with the same water-to-cementitious materials ratio can result in vastly different strength PSI results, for example, so why specify the mix ratio instead of simply specifying the necessary strength? There’s a tail-wags-dog failure in practical application.
The concrete producer laments that if the desired result is not achieved, even if the prescriptive specification was employed, the producer is on the hook for the failure regardless—be it curling or shrinkage or suboptimal PSI. But there’s no space in the prescriptive model for the producer to apply expertise that will ensure delivery of the desired result.
Just because we are a slow-moving industry doesn’t mean we haven’t moved or changed. The gradual introduction of chemical admixtures to concrete formulations over the past half-century has certainly extended the performance properties of concrete products immensely.
But many existing prescriptive standards predate their introduction, so those capabilities may go unaccounted for in existing orthodoxies. Prescriptions make no allowance for innovation. In circumstances where new parameters provide incentive, prescriptive methodologies are more apt to be abandoned.
The need for greater speed, efficiency and creativity under increasingly tighter project constraints—such as on dense urban developments or emerging sustainability targets in cutting-edge design—can motivate specifiers to seek efficiencies. If a high-rise structure has to be completed in a matter of months instead of years, you’re less apt to rely on a known model that takes years to deploy.
If you’re aiming to reduce the carbon footprint of your city or your housing development, you’ll be more open to embracing the use of CO2 mineralized concrete in your specifications. But the technology for creating that concrete did not even exist 20 years ago, so it can’t be used by following past prescriptions. How we specify materials has to make room for what we are still learning if we hope to enable the future that we want.
Embracing performance doesn’t mean abandoning collective knowledge. That still requires clearly defined functional requirements, quality control and testing and precertification to ensure that performance criteria are met or exceeded. But specifying performance also makes room for addressing new circumstances or introducing better methodologies. It opens the door to possibility— and that’s how we evolve.
Matthew Jetmore, central region general manager at Lauren Concrete in Austin, Texas, can be reached at CarbonCure@SterlingPR.com.
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