Question

What is the difference between a prescriptive specification and a performance standard

for the production of wood structural panels?

Answer 1

What is a Prescriptive Specification?

A prescriptive specification is one that includes clauses for means and methods of construction and composition of the concrete mix rather than defining performance requirements. Many times intended performance requirements are not clearly indicated in project specifications and the prescriptive requirements may conflict with the intended performance. The producer is always called on when the mix does not perform even though this is in conflict with the basic premise of a prescriptive specification as clearly indicated in ASTM C 94, Specification for Ready Mixed Concrete. For example, a low water-cementitious materials (w/cm) ratio at high paste content might increase the potential for shrinkage and cause more curling in a concrete floor while the intent was to reduce it. This might also cause a stiff mix that will adversely affect placing and finishing.

Many project specifications include prescriptive limits on w/cm ratio as a surrogate for durability. The intent is to reduce permeability of the concrete. There are many ways to achieve this with the use of supplementary cementing materials and admixtures. The typical process of working with a w/cm limit is to start with water content as required for a target slump and the local aggregates. The cement content is then calculated. Conforming to a low w/cm ratio generally drives the cement content higher which leads to higher costs. Alternatively, admixtures can be used to reduce the paste content in the mix.

What is a Performance Specification?

A performance specification is a set of instructions that outlines the functional requirements for hardened concrete depending on the application. The instructions should be clear, achievable, measurable and enforceable. For example, the performance criteria for interior columns in a building might be compressive strength and weight since durability is not a concern. Conversely, performance criteria for a bridge deck might include strength, permeability, scaling, cracking and other criteria related to durability since the concrete will be subjected to a harsh environment.

Performance specifications should also clearly specify the test methods and the acceptance criteria that will be used to verify and enforce the requirements. Some testing may be required for pre-qualification and some might be for jobsite acceptance. The specifications should provide flexibility to the contractor and producer to provide a mix that meets the performance criteria in the way they choose. The contractor and producer will also work together to develop a mix design for the plastic concrete that meets additional requirement for placing and finishing such as flow and set time while ensuring that the performance requirements for the hardened concrete are not compromised. Performance specifications should avoid requirements for means and methods and should avoid limitations on the ingredients or proportions of the concrete mixture.