Should High Performance Concrete Specifications be Prescriptive or Performance-Based?

The purpose of this article is simply to offer some food for thought when considering best practices for specifying potentially complex concrete mixtures, such as High Performance Concrete (HPC).

HPC is often broadly defined as concrete that attains mechanical, durability, or constructability properties exceeding those of conventional concrete. High-Strength Concrete (HSC) and Ultra-High Performance Concrete (UHPC) are a few examples of the types of concrete that fall under the HPC umbrella, though, by its definition, a concrete need not possess exceptionally high strength to be considered HPC.

In the construction industry, specifications are mandatory-written, legally enforceable documents typically written by Architects or Engineers on behalf of Owners. Whether done so via a bid-build or design-build basis, the purpose of specifications is to communicate to the Contractor the guidelines necessary to ensure that the materials, products, procedures and services employed in the work satisfy the intent of the design, and therefore, the Owner’s expectations. Satisfactory concrete construction and favorable in-service performance requires concrete with specific properties.

For most concretes, off-the-shelf universally applicable, or “boilerplate” specifications are undesirable, cost inefficient, and in many cases, inhibit the ability to achieve the properties most critically needed. In terms of HPC, boilerplate specifications will almost guarantee less than desirable performance. For example, a bridge deck in a cold weather region exposed to saturated conditions, freeze-thaw cycling, de-icing salts and anti-icing agents needs to effectively resist chloride ion penetration in order to delay the onset of corrosion of embedded reinforcing bars or prestressing strands. In addition to having an effectively low permeability to the intrusion of chloride ions, other measures, including high freeze-thaw durability would also be necessary in a long-term protection regime for the deck. Note also that additional protective measures, such as sealers, membranes, or exceptionally low permeability toppings might also be necessary given the conditions of exposure, specified design life and planned maintenance schedule. Depending on the structural requirements, the concrete will invariably need to have some minimum compressive strength; however, a compressive strength that is too high will have a proportionally higher modulus of elasticity that could increase the predisposition of the bridge deck to cracking when subjected to service loads. Cracking is detrimental to durability, particularly in environments conducive to corrosion. Cracking increases maintenance needs and costs, and can decrease service life. In such cases, designing deck elements with a lower design strength may be more appropriate from a long-term serviceability perspective and the inclusion of additional protective measures may be warranted. In this example, a boilerplate specification could not address the project-specific requirements for such a concrete.

Balancing mechanical, durability and constructability properties can be challenging for all structural concretes, but particularly with many HPC mixtures, and placing too much focus on a single property, such as compressive strength can throw a concrete mixture out of balance and promote less than desirable performance. A fundamental key with many HPC mixtures is effectively balancing compressive strength with chloride ion permeability, and very careful consideration is necessary. In many cases involving HPC, durability is the governing property in the selection of the constituent materials and mixture proportions needed, not strength. 

Specifications pursuant to HPC should always be written in project-specific terms with the particular design and constructability requirements in mind. Neglecting even a few requisite properties could set the stage that the concrete will not perform as intended. Concrete specifications of an appropriate nature are crucially important for a project to be successful. Specifications must be written such that they are technically sound, understandable, and achievable, and they must be strictly followed by the Contractor. Once a project is awarded, again, specifications become legally enforceable contract documents.

Specifications for concrete can fundamentally be written in one of three ways: (1) purely prescriptive-based, (2) purely performance-based, or (3) a hybrid combination of prescription and performance. Arguably, there are advantages and disadvantages with both purely prescriptive and purely performance-based specifications, therefore, hybrids also have the potential for being problematic. Pure prescriptive-based specifications stipulate the materials and mixture proportions to be used along with the production, delivery, placement, consolidation and curing methods to be followed. Prescriptive specifications typically include stated values for minimum cementitious materials content, maximum aggregate size, permissible slump or spread range, air content or air-void parameters (if applicable), and maximum ratio of water to cement or cementitious materials. On the other side of the spectrum, pure performance-based specifications only stipulate the ‘end result’ properties necessary to satisfy the design criteria, without prescribing the manner in which they are to be attained. Hybrid specifications contain both prescriptive and performance requirements, that is, along with the material, proportioning, or procedural requirements which may be included, end result properties are also stipulated.

To successfully produce and deliver HPC requires intimate knowledge of the concrete constituents, mixture proportions, material variations, and the manner in which the constituents interact. The source of such knowledge is usually with the producers of the concrete. Performance-based specifications may not be suitable for everyone, but may be highly desirable for technically astute concrete producers and contractors. Untying an astute producer's hands from prescriptive limitations can enhance the chances that the concrete will perform as intended.

Because the properties of constituent materials are subject to variations, it would be difficult to consistently produce HPC using purely prescriptive specifications. The quality of constituent materials, which drives mixture proportions, varies from market-to-market and day-by-day. Variations in constituent material quality, even subtle in nature can have a pronounced effect with the performance of HPC. Without due consideration given to constituent material compatibility, unanticipated problems are significantly more likely to occur, particularly in light of the fact that HPC can be a more complex material, utilizing more types of constituents and having greater performance demands. 

Specifications calling for the achievement of end results in a prescriptive manner can establish barriers in the ability to achieve favorable end results. Prescriptions can be extremely problematic when high performance properties are needed. For example, a producer’s ability to supply HSC without the use of supplementary cementing materials (SCM’s) such as slag or fly ash would be both challenging and cost inefficient. Similarly, the use of SCM’s with arbitrarily-established prescriptive limits can reduce the chances for successfully achieving favorable strength performance and economy.

Specifications prescribing the manner in which performance is to be achieved can become very problematic, particularly as the performance requirements of the material increases. For example, numerous disastrous outcomes have resulted when attempts were made to produce HSC in a prescriptive manner. Prescriptive specifications may not allow for the use of more suitable material types, quantities, or proportions available in order to satisfy the project requirements and in a more cost-effective manner. 

Since both prescriptive-based and performance-based specifications can have their pros and cons, the answer to this question is not a simple one, particularly when complex concretes are involved.

The principal concern is not how the concrete is specified, but what is too often neglected; that due consideration is given on the part of the specifier to ensure that project specifications apply to the concrete to be used, that they take into account all important properties needed to ensure a successful end result, that they are technically sound, understandable and achievable, and that they are diligently adhered to through the course of the work.