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
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
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
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
What is a Performance Specification?
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
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.
would have provisions that clearly define the functional requirements of the
contractors will partner to ensure that the right mix is developed, delivered
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.
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