Research and building code changes provide pathway to performance-based specifications for concrete

Used since the early days of construction, modern concrete technology has evolved into an advanced science. Most modern concrete producers have laboratories that incorporate rigorous quality control and product development programs, and expert concrete technologists to develop innovative concrete mixtures for any application—from houses to high-rises and sidewalks to superhighways. High-performance concrete incorporates fly ash, slag, and silica fume along with a variety of chemical admixtures to provide solutions to increasingly demanding applications. High-early strength concrete, self-consolidating concrete, and concretes designed to last for 100 years in harsh environments are just a few examples. Design methodology and construction practices have also achieved much higher levels of innovation and efficiencies since the early days.

Unfortunately, building codes and project specifications in the United States have not kept pace with current progress. Most are prescriptive in nature and stifle innovation by limiting the types and quantities of ingredients and material proportions, design methods, or construction means and methods. Prescriptive codes and specifications are often overly conservative, which can lead to higher costs and unexpected negative results. A shift to performance-based specifications is the next logical step to take full advantage of high-performance concrete technology.

Performance-based specifications provide details of required results such as strength and other mechanical properties along with requirements for durability and serviceability. The results are verifiable through measurement or testing to ensure the product meets the desired requirements. And finally, performance-based specifications are free of process limitations such as mixture proportions and construction methods.

A well-written, performance-based specification encourages a team approach where all members of the design and construction team participate based on their area of expertise. For example, architects and engineers focus on design and code compliance; contractors focus on construction means and methods; and concrete producers focus on designing and delivering concrete in compliance with specifications.

Performance-based specifications encourage innovative products and construction methods along with rigorous quality management systems that lead to superior structures.

Research

In a recent research study conducted by the National Ready Mixed Concrete Association (NRMCA), plastic and hardened properties of concrete mixtures developed to meet typical prescriptive specifications were compared to mixtures that were optimized for intended performance. In all cases, the concretes designed to performance targets performed better than those designed to the prescriptive limitations in the specifications.

In one series of tests, concretes designed for a bridge deck application (similar to concretes that would be used for parking decks) were compared. The prescriptive specification was taken from an actual State Department of Transportation (DOT) specification for bridge decks located in a Northern climate. The performance property targets was set using the best known performance criteria to ensure low permeability and low shrinkage, two desirable qualities for concretes exposed to freeze-thaw cycles and deicing chemicals. Both specification criteria are shown in Table 1.

The prescriptive provisions in the DOT specification dictate specific cementitious materials content, places limits on quantities of supplementary cementitious materials, and prescribe the water-to-cementitious materials ratio (w/cm). Specifying these criteria together could lead to undesirable results such as high paste fractions and subsequent cracking due to thermal effects and drying shrinkage. Mixture BR-1 was designed to the prescriptive specification and three alternatives, BR-2, BR-3, and BR-4, were designed to selected target performance properties. A comparison of the mixture proportions used for the four mixtures is shown in Table 2.

Test results—The compressive strengths at 28 days of all the laboratory mixtures were much higher than specified: 6800 to 8970 pounds per square inch (psi). The permeability of the performance mixtures, as measured using the Rapid Chloride Permeability Test (RCPT), ASTM C 1202, easily complied with the specified value of 1,500 coulombs after 45 days. The prescriptive mixture had an RCPT value of 1,593 coulombs and did not meet the target property. The 180-day RCPT values for both the prescriptive and performance mixtures were all extremely low.

Using another measure of permeability as a comparison, the 60-day Rapid Migration Test (RMT), AASHTO TP 64, results varied from 0.018 mm/(v-hr) for BR-2 to 0.023 mm/(v-hr) for BR-4. The 180-day RMT results varied from 0.0045 for BR-2 to 0.0058 for BR-1. All of these values are very low and would indicate very low migration of chloride ions within the concrete.

Low shrinkage is another desirable quality for durable concrete. The target shrinkage of 0.04 percent at 28 days was achieved for all the mixtures. The drying shrinkage of the prescriptive mixture was nearly twice that of the performance mixtures.

The test results indicate that all of the performance mixtures had similar or better performance than the prescriptive mixtures for permeability, drying shrinkage, workability (stickiness), and strength. And because the performance mixes used less cementitious material they provide substantial material cost savings. It should be noted, however, that even the prescriptive mixture would have most likely performed well in the field. This research, however, illustrates that performance-based specifications allow concrete producers to optimize concrete mixtures to provide better performance at potentially a lower cost.

Changes to ACI 318-08

One of the key steps toward performance-based specifications is to eliminate prescriptive limitations or allow performance-based alternatives in the building codes. The 2005 version of The American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318-05), places several prescriptive limits on materials for concrete subjected to severe environments or service conditions. To address these limitations, revisions to Chapter 4 that include a significant restructuring of the durability provisions have been approved by ACI Committee 318 and will be published in its 2008 document.

The restructuring involves the definition of various exposure categories that are subdivided into exposure classes depending on the severity of the exposure. This is essentially the method used by most other international codes to address concrete durability.

ACI 318-08 will define the following exposure categories:
* Category F for exposure to freezing and thawing cycles,
* Category S for exposure to sulfate solutions in soil or water in contact with concrete,
* Category P for concrete in contact with water that requires a low permeability, and
* Category C for conditions that require protection of reinforcement from corrosion.

Each category is subdivided into exposure classes with higher numbers for more severe exposure as shown in Table 3. Requirements for concrete for each exposure class are detailed in Table 4.

Category F—Exposure to freezing and thawing—For concrete members exposed to freezing and thawing cycles, the exposure classes are defined in Table 3a and the requirements for concrete subject to different levels of freezing and thawing are summarized in Table 4a. The requirements for air entrainment are summarized in Table 5 and the limitations on cementitious materials for structural elements subject to scaling due to deicer chemical application are summarized in the new provisions.

Category S—Exposure to sulfates—The exposure classes for concrete members in contact with sulfates in soil or water are defined based on the concentration of the water-soluble sulfates in soil or water as shown in Table 3b. Requirements for concrete mixtures exposed to water-soluble sulfates are summarized in Table 4b.
Alternative cementitious combinations of materials to those in the Table 4 are permitted if they are qualified by ASTM C 1012, Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution, and meet the expansion criteria. The use of cementitious materials with documented good service records is also permitted.

Category P—Concrete members in contact with water requiring a low permeability—Presumably this exposure condition for structural members in buildings is one where the other exposure conditions are not pertinent but you still need low permeability for structural members in contact with water. One example might be a water tank in a warm climate. The exposure classes are defined in Table 3c and the requirements are summarized in Table 4c.

Category C—Conditions needing corrosion protection of reinforcement—The defined exposure classes that would require the design professional to pay particular attention to avoid corrosion are in Table 3d and the requirements for concrete are shown in Table 4d. The chloride ion limit in Table 4d refers to water-soluble chloride ion expressed in terms of percent by weight of cement. It is measured on crushed concrete samples by ASTM C 1218 at an age between 28 and 42 days. This requirement protects against using concrete ingredients, such as admixtures or marine aggregate that will incorporate chlorides in the concrete mixture. For Class C2 the design professional is notified to pay attention to the clear cover of concrete over the reinforcing steel. For prestressed concrete, the code specifically requires that the cover be increased by 50 percent.

Simplified specifications for concrete

This revision to the ACI 318 code lends more clarity to the durability provisions for both the engineer and the concrete producer. The new code provisions will require that the design professional identify the exposure classes for each application of concrete used on the project. The clear definition of the exposure classes and the parallel requirements for concrete eliminates reasons for the design professional to impose restrictions on concrete if they are not needed. For instance, there is no reason to impose a maximum w/cm on an interior building column where the primary performance function is strength. In addition, specifying exposure classes allows the design professional to focus on performance rather than mixture proportions.

The new format simplifies the specification for concrete. An example specification for different applications of concrete in a building might look like that shown in Table 5. The more critical requirements of the strength or the durability exposure would apply. For example for the foundations and slabs on grade, even though ƒ´c required for loads is 3,000 psi, the ƒ´c required for exposure class F2, 4,500 psi, would govern.

The P2P Initiative

The concrete industry recognizes that customer needs will best be served through innovative concrete technology and improved quality—which are the outcomes of performance-based specifications. Led by NRMCA, the domestic ready-mixed concrete industry has established the P2P Initiative (Prescriptive to Performance Specifications for Concrete) to promote performance specifications as an alternative to traditional prescriptive specifications. NRMCA staff and members have collaborated with industry stakeholders, including concrete contractors, material suppliers, engineers, and architects to develop a set of goals and strategies to implement performance-based specifications.

These changes to the building code requirements are the first step in the process of changing the way concrete is specified and designed in the United States. Although the challenges are many and the effort involved will be extensive, the concrete industry feels the change is necessary to ensure improved quality and innovation. NRMCA has established a website, www.nrmca.org/P2P, to disseminate information about the initiative and provide resources for designers, contractors, and producers.

Lionel Lemay, P.E., S.E., is senior vice president of Technical Resources at the National Ready Mixed Concrete Association (NRMCA). He can be reached at llemay@nrmca.org. Also with the NRMCA, Colin Lobo, Ph.D., is senior vice president of engineering, and Karthik Obla, Ph.D, P.E., is managing director of research and materials engineering.