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.

For example, the two concrete mixtures shown above have the same w/cm ratio. The one on the right has a higher paste content that will probably result in higher heat of hydration and higher shrinkage and associated problems such as cracking. The mixes probably have similar permeability and strength or they might be significantly different. Clearly one (or both) of the mixtures may not be optimized for the intended performance.

For each set of materials there is a unique relationship between the strength and w/cm ratio. A different set of materials has a different relationship as illustrated by the plots of compressive strength versus w/cm ratio for the three different mixes in the graph on the left. A 0.45 w/cm ratio for these three mixtures have strengths of 3800, 5000 and 6000 psi respectively. Clearly specifying a w/cm ratio requirement does not ensure certain strength will be achieved.

For the most part, maximum w/cm ratio is included in prescriptive specification to ensure durability, which is generally affected by the permeability of concrete. Generally, as w/cm ratio decreases, an electrical charge passed through hardened concrete (a measure of permeability) decreases. Alternatively, cementitious components of the mix can also be varied to decrease permeability. Different combinations of portland cement and supplementary cementitious materials (SCM) such as fly-ash, slag, silica fume and other pozzolans can drastically affect permeability.

For example, the four different mixes shown in the graph on the right have different permeability at the same w/cm ratio ranging from very low permeability for the ternary mix (portland cement plus two SCMs) to very high permeability for the portland cement only mix. Even though a producer furnishes a mix at 0.45 w/cm ratio, there is no guarantee that the mix will have low permeability.

This is not to say that w/cm ratio is not important. It is a parameter concrete producers use to design concrete mixtures.  It should not however be a specification requirement. Furthermore, w/cm ratio cannot be measured or enforced on the jobsite by a reliable test and a specification requirement that cannot be enforced is not effective.

Prescriptive specifications also lead to higher costs. The bidder with the lowest overhead—which usually means lowest investment in quality control, research and development—is often the one that can bid the lowest and profit the most at the lowest bid. An engineer might think he has established a level playing field with a prescriptive mix, but in fact he is encouraging low quality. For this reason, engineers often revert to more prescriptive specifications that are extremely conservative (over-designed) to compensate for low quality leading to higher costs. If the engineer specifies the desired performance and relies on the expertise of the concrete contractor and concrete producer to deliver an optimized mix, it can often be delivered at lower cost and higher quality.

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. 

The general concept of how a performance-based specification for concrete would work is as follows: