Prescriptive vs. Performance Based Specifications for Concrete

By Tim Duncan

Progress in Ready Mix Industry

The ready mixed concrete industry in Fargo-Moorhead, as well as the rest of the state, has progressed to a highly professional industry from its roots back in the 1930's when it was primarily an industry that "trucked" material. Our industry manufactures a very versatile product to a volume exceeding 400 million cubic yards annually nationwide. Ready mixed concrete can be customized for a variety of applications from very low strength to extremely high strength and enhanced durability; and to accommodate a significant range of placement procedures that enhances the productivity of its contractor customers. Batch modifications are necessary and possible in real time to accommodate variations in ingredient materials, the number of which are ever increasing, environmental conditions, placement methods and customers needs.

Specifications Have Not Kept Up

Specifications for ready mixed concrete have not evolved at the same pace as innovations in the concrete industry. For the most part specifications today still have prescriptive provisions such as specifying the types and quantities of the mixture ingredients, limits on cementitious materials, water-cement ratios, aggregate grading, etc. The general premise is that "if a problem occurred on a previous job, I will write a more detailed spec for the next job to solve my problem". This is probably the historical basis for prescriptive specifications and is not necessarily appropriate for the broad requirements and challenges for concrete construction today. This problem is compounded when the specification calls for prescriptive and performance criteria that conflict.

Prescriptive specifications inhibit innovation and professionalism in the concrete industry. It also limits the competitiveness, profitability, economy and assignment of responsibility for concrete construction. The ready mixed concrete producer is intimately familiar with the product and its ingredients and should be responsible for designing and modifying the mixture for the performance needed by the customer. Some producers not only have fully equipped concrete laboratories, they also have experienced engineers directing product research and development. If they don't, excellent testing lab personnel with concrete expertise are available. The architect or design engineer has certain design criteria and the contractor has certain constructability criteria. They, however, may not be qualified to design a concrete mixture or discern what is relevant in a submittal that details the composition of the mixture.

Mixture Submittals

Submittals are an unavoidable process of a construction project. Mixture submittals for concrete generally require details on the mixture ingredients and quantities. Frequently these are based on trial batches performed by commercial testing laboratories, even though industry documents, such as the Building Code for Structural Concrete, ACI 318, emphasize the preference of submittals based on previous job mixtures. The details of a new concrete mixture submittal may not ensure the success to the contractor or the design engineer. The current submittal process needs significant modification.

Proprietary Info Can Be Maintained

The mixture proportions should be considered proprietary to the concrete supplier. The submittal should only qualify that the producer will conform to the requirements of the specification and in broad terms indicate that prescriptive limitations, when included out of necessity in the specification, will be abided by. Failure to do so will result in liability to the concrete producer.

Summary of Specification Goals

In summary, the goals of the ready mix producers in the Fargo area (and other locations) can be stated as follows:

1. Requirements for concrete mixtures in specifications for concrete construction should be performance-based for constructibility and in-place properties. The concrete producer should be empowered to optimize mix designs for the intended performance without many of the normally seen prescriptive restrictions.

2. The producer is qualified to make economical mixes as effective as the engineer needs while maintaining accountability for the product.

3. The submittal process should be significantly simplified and the concrete producer should be able to make real time adjustments to mixtures while retaining the intellectual property of the mixture composition.

Note: The North Dakota Ready Mix & Concrete Products Association will be at the forefront in completing a roadmap to achieving these goals. We hope all engineers and architects will embrace the performance specification concept on a welcome basis. If you have any opinions on this topic, please let us know. The next issue will include a simplified example of a performance specification