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:
There would be a qualification/certification system that
establishes the requirements for a quality control management system,
qualification of personnel and requirements for concrete production
facilities.
The specification would have provisions that clearly
define the functional requirements of the hardened concrete.
Producers and contractors will partner to ensure that the
right mix is developed, delivered and installed.
The submittal would not be a detailed list of mixture
ingredients but rather a certification that the mix will meet the
specification requirements, including pre-qualification test results.
After the concrete is placed, a series of field
acceptance tests would be conducted to determine if the concrete meets
the performance criteria.
A clear set of instructions
outlining what happens when concrete does not conform with the
performance criteria.
