A Prescriptive Specification1 is like a recipe for completing a project. Prescriptive specifications describe the products that can be used, details of the construction procedures, workmanship required, and other details. Typically, there is an intent for a desired end result but it may not be precisely defined.
The responsibility of the designer in writing prescriptive specifications include: determining the end product and the primary properties needed, identify which equipment is best suited for construction, developing an outline for construction procedure, and establish the extent of testing and what party will perform these tests. Because these specifications are how-to instructions, achieving the end result is directly related to the adequacy of the specification. A contractor cannot be faulted if the desired result is not achieved, and thereby, failures are deemed to be the responsibility of the specifier.
A Performance Specification1 describes the end product that is desired. It does not describe how to achieve the result. The contractor can most often choose products, equipment, and construction procedures from various available alternatives. The ability to choose the best of these alternative for the specific needs of a project can lead to innovation, improved quality, and optimized products and construction procedures.
The responsibility of the designer is to adequately define the end result to ensure that the intended outcome is achieved. Clear testing and inspection criteria should be defined. The contractor now bears responsibility to research different alternatives, develop needed data and development work, and propose the products and process to the designer. This may require a longer lead time during the bidding and submittal process.
Combining prescriptive and performance, referred to as hybrid specifications, is quite common. This may not create problems if these are not combined for the same product or construction process. As an example, concrete mixtures may be specified for a strength requirement and other constraints applied to the mixture materials and proportions. It should be ensured that this does not impose conflicting requirements for developing the concrete mixture or adversely impact constructability.
1Paraphrased from: Preparation of Construction Specifications for Civil Projects, Committee on Specifications of the Construction Institute of the American Society of Civil Engineers.
Specific to concrete: Performance of concrete is based on performance indicators measured by standard test methods with defined acceptance criteria stated in contract documents and with no restrictions on the parameters of concrete mixture proportions. (NRMCA P2P)
When requirements for concrete are prescriptive based on industry standards for specific reasons and there are no alternative performance-based criteria, it can be considered to be performance-based. An example is the durability requirements for concrete for assigned durability exposure classes in ACI 318, Building Code for Structural Concrete. There may be performance-based alternatives to these prescriptive requirements that would be deemed to satisfy.
The P2P Initiative
P2P is an acronym for Prescription to Performance specifications. The P2P Initiative is led by the National Ready Mixed Concrete Association (NRMCA) advocating for the ready mixed concrete industry to promote a shift from traditional prescriptive specifications to performance specifications for concrete. Initiatives are addressed through the NRMCA Research Engineering and Standards (RES) Committee.
It is recognized that project specifications that are prescriptive stifle innovation by limiting the types and quantities of ingredients and material proportions. These are often overly conservative resulting in higher costs, lacks appropriate responsibility, and does not support sustainable construction.
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 assure the product meets the desired requirements. These specifications are free of process limitations such as restrictions on materials, mixture proportions, and construction methods. Performance-based specifications encourage innovative and optimized products and construction methods along with rigorous quality management systems that lead to superior products and satisfied customers. There is a strong synergy between performance specifications and goals for sustainable construction.
Goals of the P2P Initiative
The primary goal of the P2P Initiative is to improve the quality and competitive position of concrete by using optimized products that are cost competitive, ensure concrete structures last for designed service life with minimal maintenance, and to support sustainable construction.
- Promote performance specifications as an alternative to current prescriptive specifications through education and communication.
- Leverage the expertise of all stakeholders in the construction industry to improve quality and reliability of concrete construction.
- Assist architects and engineers to address concrete specifications by minimizing prescriptive requirements and establishing applicable performance requirements; thereby allowing concrete suppliers and contractors flexibility on the details of concrete mixtures and construction means and methods.
- Elevate the performance level and credibility of the ready mixed concrete industry through training and certification.
- Foster innovation and acceptance of new technology at a faster pace through research and development.
There is no intent to eliminate prescriptive specifications that work well in some market areas and in some established relationships. However, there should be an improvement in prescriptive specifications whereby some of the inherent conflicts are eliminated and responsibilities are more clearly defined.
Resources for the P2P Initiative
Below are some of the resources available to the industry and to designers for information on revising specifications for concrete construction to more performance based:
- Guide to Improving Specifications for Ready Mixed Concrete (uses the MasterSpec format of Section 033000 for Cast in Place Concrete with advisory information to applicable clauses)
- Recorded Webinar – Performance-based Specifications – State of the Industry and Way Forward
- Copy of Presentation with notes and supporting documents (will download)
Specifications in Practice – brief discussions on 5 most restrictive prescriptive requirements identified by the industry
SIP 2 – Limits on Water-Cementitious Materials Ratio (w/cm)
Published Articles and Papers
- Specifying Sustainable Concrete (NRMCA, 2019)
- Specifying Requirements for Concrete Mixtures (published in Structure magazine 2019)
- Should Minimum Cementitious Contents for Concrete Be Specified? (published in Transportation Research Record, 2017)
- Specifying for Performance (published in NRMCA Concrete InFocus magazine 2017)
- Prescriptive Specifications – A Reality Check (published in ACI Concrete International magazine 2015)
- Acceptance Criteria for Durability Tests (published in NRMCA Concrete inFocus magazine 2007)
- Performance Specifications for Durable Concrete – current practice and limitations (published in ACI Concrete International magazine 2006)
- The P2P Initiative – A Shift to Performance Specifications for Concrete Focuses on Innovation, Quality and Customer Satisfaction (NRMCA 2005)
Reports – Funded by the RMC Research & Education Foundation
- An Evaluation of Performance-Based Alternatives to the Durability Provisions of the ACI 318 Building Code – Report of research performed at the NRMCA Research Laboratory (Report from the RMC Research & Education Foundation, 2015)
- Preparation of a Performance-based Specification for Cast-in-Place Concrete (Report from the RMC Research & Education Foundation, 2008)
- Quality Management System for Ready Mixed Concrete Companies – Preparation Guidelines for Quality Manual; Sample Quality Manual; External Audit Checklist (Report from the RMC Research & Education Foundation, 2008)
- Guide to Specifying Concrete Performance – Phase II Report (Report from the RMC Research & Education Foundation, 2006)
- Experimental Case Study Demonstrating Advantages of Performance Specifications – Report of research performed at the NRMCA Research Laboratory (Report from the RMC Research & Education Foundation, 2006)