News Detail

A Whole Lotta Performance Goin' On

4/1/2011

Performance-based requirements are a hot topic in the concrete industry. The idea of specifying concrete on the basis of "end performance" instead of prescribed "means and methods" is catching on for some applications, and ACI is moving rapidly to explore, evaluate, and develop appropriate model specs.

The fundamental premise of performance-based requirements is that if you tell folks what you really want, you dramatically improve your chances of getting it, as demonstrated in the following fairly common gift-giving ritual. In advance of the event, the giver asks the receiver, "What do you really want this year?" The receiver responds, "Oh, you know what I like. You always give me something I like!" When the great day comes, the receiver hides mild disappointment, privately whining that "Nobody ever gives me what I really want," and then starts planning how to take the gift back to the store or otherwise extract some cash out of the situation.

This ritual is periodically repeated on concrete construction sites: the specs call for slump, air, cylinder strength, and minimum cement (always nice gifts), but don't always ask for color, texture, maximum shrinkage, limits on cracking, elastic modulus, scaling resistance, low permeability, time-to-corrosion, or whatever the owner really, truly wants. If the owner doesn't get the hoped-for results, the consequences can range from disappointment to refusal to relinquish the retainer. And regardless of contractual relationships, "if the owner ain't happy, ain't nobody happy."

This all sounds great—but do we always know what we really want, and can we describe it in quantifiable, measureable terms? If we did create a laundry list of concrete performance objectives, do we have reliable tests to determine if we reached our goals? And if we had those tests, are they more expensive, complicated, or time-consuming than our current quality-control tools? And should we be specifying only end-product, hardened concrete performance properties, or can we reliably project hardened properties from the key characteristics of the fresh concrete as it was batched or delivered? And while we're thinking about it, can we "prequalify" concrete mixtures on the basis of their proven ability to achieve specified performance criteria? What do we do if the performance criteria aren't achieved?

Would the move to performance change any of our traditional roles and responsibilities? While changes in responsibility seem likely, there's some thought that a contractor or concrete producer whose retainer is being withheld over failure to achieve a nonspecified color, texture, uniformity, tolerance, or durability is already responsible for end-product performance, even under a prescriptive spec. If a concrete producer spent considerable time and money engineering a mixture and documenting its performance, would that mixture then be proprietary? If so, wouldn't that producer quite reasonably want to limit access to the proportions?

There are obviously plenty of questions, but the potential benefits make it worth our while to find useful answers. Performance specs might eliminate some unintended barriers to innovation and take advantage of advanced materials technology, with benefits for better, more sustainable, and more economical concrete construction. Performance specs could motivate advances in materials and testing and help us to concentrate on what we really want. It's also likely that performance specs will be most useful for certain types of projects or for certain teams of owners/designers/builders/suppliers. It's also likely that the prescriptive specs that have purposefully evolved over many years will remain the backbone for many years to come.

So where are we? Many private specifiers and public agencies have been developing performance-oriented specs for over a decade, and the transition started many years ago. (As early as 1920, the emerging ACI code had a performance-based strength requirement.) In 2004, our friends at NRMCA kicked off their Prescription-to-Performance initiative that began with a survey of international practice and the state of the technology and ended with a guide to writing a performance spec for concrete.

With the encouragement of our Strategic Development Council, ACI took up the charge with ITG-8 (report recently released), and that led to our brand-new ACI Committee 329 on performance specifications. At the same time, ACI's Responsibility for Concrete Construction Committee began to explore relationships among responsibility, authority, and control over the end product. The Tampa convention will feature five technical sessions on the topic, and this summer we will enjoy a series of articles right here in Concrete International. To paraphrase The King, "There's a whole lotta performance goin' on."

Kenneth C. Hover
American Concrete Institute
kch7@cornell.edu

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