The country is embracing infrastructure construction and foregoing earmarks and traditional approaches to invest the money wisely. But to build public structures as efficiently as possible, adopting technological advances is essential. Building information modeling (BIM) will become more prevalent along with advanced communications that allow collaboration among all the players on a construction project.
Concrete is a critical part of most infrastructure projects, but its carbon footprint is generally considered heavy because cement production is so energy-intensive. The federal government is expected to step up its demands for sustainability despite the sometimes conflicting desires for speed, efficiency, and cost control. But these goals are not mutually exclusive. The newest materials and techniques deliver high performance at lower cost and with a lower carbon footprint, allowing for better-engineered structures.USING PERFORMANCE-BASED SPECS
Cements and aggregates have evolved over the past 30 years, and new materials and methods allow for more efficient and cost-effective concrete production.
Specifications for infrastructure projects have ignored the hidden costs and carbon contribution of over-cemented mixes and not pursued value-engineering solutions that can reduce both cost and environmental impacts. Most specifications still have a conglomeration of prescriptive requirements combined with performance requirements that sometimes actually conflict, forcing the concrete producer to make a lower quality or higher cost mix than necessary.
For example, the Oklahoma DOT standard specification for Class AA concrete calls for 564 pounds of cement and a water-cement ratio of 0.25 to 0.44 to get a 6.5% air concrete with a 2-inch slump and a 28-day compressive strength of 4,000 psi. Although fly ash and high-range water reducers are allowed under certain circumstances, all of these requirements make it difficult for the producer to satisfy the prescribed minimums while keeping performance and workability in mind.
What the department really wants is a durable 4,000-psi compressive-strength mix with low permeability that's easy to pour under the existing environmental conditions. So why not just tell the contractor that and make it his responsibility to provide it?
In 2004, the U.S. Federal Highway Administration published “Performance Specifications Strategic Road-map: A Vision for the Future.” Although it acknowledged some difficulties with a pure performance-based approach, the task force that developed the document defined its vision, stating “that performance of highway facilities will improve through better translation of design intent and performance requirements into construction specifications.”
Celik Ozyildirim, principal research scientist with the Virginia Transportation Research Council, calls this approach End Result Specifications (ERS). “It's intended to provide innovation,” he says. “It also emphasizes process control. We've had good results when innovation by producers is introduced.”
The results from several pilot projects in Virginia have been durable pavements with very few defects at a lower overall price. “A percent-within-limits approach is included for strength and permeability,” Ozyildirim says. “Rideability and thickness pay adjustments are included as well. Pay adjustments will be discussed at the completion of the pilot projects.” (For more on performance-based pavements, see “Contracting for Performance” on page 33; PUBLIC WORKS April issue.)
Other engineers remain skeptical. “Most of my work is on water treatment plants,” says Bill Sherman, principal structural technologist in the Water Business Group at CH2M Hill. “Because of exposure to chemicals and durability considerations I get more nervous about whether we know enough and can do enough tests to be sure the concrete won't react with the water. That's why I'm hesitant.”
Whatever the project, buy concrete from a producer that has the knowledge and ability to consistently produce concrete that meets the required performance. Many lack that expertise — or they meet the specification by producing concrete that's more expensive than it should be.