One of the cornerstones of the Obama administration's economic stimulus package (www.recovery.gov) is investment in U.S. infrastructure, including roads, bridges, ports, airports, and water management systems from the federal level all the way to small-town, USA. Fortunately for the concrete industry, concrete plays a major role in almost all aspects of the nation's infrastructure. Concrete producers and all their suppliers are eagerly awaiting the roll out of projects demanding millions of cubic yards of concrete. However, this opportunity brings a responsibility—to make certain that the investment in our future lasts a lifetime. Performance specifications can help make certain that our current problems result in a long-term solution.
Background
Traditionally concrete specifications have focused on the “how to” of construction. The low-tech approach to concrete production resulted in an environment where designers felt the need to protect themselves by telling the concrete producers what they can and can't do during the production and delivery of the concrete. Even today, there are some producers who are not fully aware of what is happening with their concrete and how it impacts the constructed project. Iowa DOT conducted a study of roads produced from 1920 to 2005 and found that many of their pavements constructed from 1955 through 1992 were failing prematurely, sometimes after only 5 to 10 years. Older roads were outperforming newer modern roads built with more modern technology and materials. The new technology wasn't predicting concrete performance accurately.
The design side also had its share of problems. Specifications for maximum allowable water-cement (w/c) ratios have been lowering, based on the “fact” that a lower w/c ratio is “better.” But w/c ratio isn't the entire story in regard to concrete quality. A large concrete bridge project was designed using a ductile (or flexible) design concept. This design required concrete that could bend slightly and that wasn't brittle. However, when the concrete specifications were produced, a maximum w/c ratio of 0.35 was specified. This would have resulted in a very brittle concrete that wouldn't be suitable for the ductile design. Designers are pushing the limits of concrete capabilities, structurally and architecturally. Concrete is being squeezed on one side by demands for increased performance and on the other by limitations in its capabilities.
Compounding the problem is the fact that the average designer has trouble keeping up with the rapidly changing concrete technology. Superplasticizers and self-consolidating concrete make traditional slump limits obsolete. Simply specifying an air content at the end of the truck chute doesn't guarantee freeze/thaw durability. Long chain molecule admixtures, which don't meet any ASTM specification, can improve concrete performance. Things are changing rapidly enough that it is difficult for concrete specialists to keep up, much less a specifier who must deal with 15 other divisions in the specifications.
Performance specifications
Traditional prescriptive specifications focus on the means and methods of how concrete is to be produced. What materials are allowed? How should they be proportioned? How should they be mixed and delivered to the jobsite? Everything is focused on how to make a good product.
Performance specifications ignore the means and methods and concentrate on the results, typically in measurable terms. Minimum strength limits, such as 4000 psi (27.6 MPa), are typical and represent a performance specification. Other examples of performance characteristics include modulus of elasticity, shrinkage, permeability, freezing and thawing resistance, and sulfate soundness.
Other factors, such as combined aggregate grading limits, percentage of air content, w/c ratio, and chloride limits on materials blur the line between prescriptive specs and performance specs. The National Ready Mixed Concrete Association's (NRMCA) P2P Program (Prescriptive to Performance) tries to define performance specifications as those that can be measured over a short-term period (usually less than one year) without imposing restrictions on material combinations. Air content and w/c ratio are two material restrictions that are permitted in a new model specification the NRMCA has written pending the development of simple, reliable tests that can be used to measure properties related to those restrictions.
Time restrictions on the type of tests allowed are important because some durability tests can take years to complete. ASTM C-1293, “Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction,” takes 12 to 24 months to complete. The shorter version of the test, ASTM C-1260, “Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)” takes two weeks to complete, but is known for its false positives and negatives.
