Australian Society for Concrete Pavements
4th Concrete Pavements Conference
PERFORMANCE ENGINEERED MIXTURES PROGRAM
Peter Taylor, PhD
Director
National Concrete Pavement Technology Center
Iowa State University
ABSTRACT
The Performance Engineered Mixtures (PEM) program is designed to provide the tools for
agencies to specify, and contractors to deliver, concrete mixtures that reliably and
sustainably meet the needs for concrete infrastructure.
The PEM program will result in concrete pavements consistently achieving the performance
life of the design. The program is based on the concept of measuring and controlling the
concrete mixture around engineering properties that actually relate to performance:
• Identifying critical mixture properties for long-term durability specific to any climatic
environment
• Achieving these properties through measuring the performance-related engineering
parameters of the mixtures
• Developing a specification for mixtures
• Providing technical guidance and project-level support for preparing and delivering
concrete mixtures that meet the specification
This paper discusses the program and how it is being implemented in the USA.
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Introduction
The Performance Engineered Mixtures (PEM) Program seeks to provide the tools for
agencies to specify and, and contractors to deliver, concrete mixtures that reliably and
sustainably meet the needs for concrete infrastructure.
The program is built around the concept that there is a need to understand what critical
properties are required of a mixture in a given environment. Once these have been
identified, they can be called for using a specification that is based on effective test methods,
and appropriate limits. Finally, tools and guidance are provided which help suppliers
prepare and deliver mixtures that are constructible and will be accepted under the
specification.
Background
Concrete for pavements has historically been specified and field controlled around
acceptance criteria that do not relate well to durability, such as slump. Paving concrete
specifications need to be built upon engineering properties that directly relate to good field
performance. With the recent advancements in research knowledge on failure mechanisms,
and the parallel development of better tests, this is now possible.
A review of current and new specifications has found that they are still largely based on
strength, slump, and air, which provide poor correlation with the mechanisms of pavement
failure currently observed across the nation. Many local specifications also are
predominantly prescriptive, thus limiting innovation and not necessarily addressing current
materials, environments or construction methodologies.
The need for change in the way we specify concrete, especially concrete for paving
mixtures, is becoming increasingly apparent as mixtures become more complex with a
growing range of chemical admixtures and supplementary cementitious materials. Traffic
loadings continue to increase, very aggressive winter maintenance practices are
implemented, and demand increases to build systems more quickly, cheaply, and with
increased longevity.
Recent data are indicating that by improving the quality of the concrete pavement in place,
the potential life of a pavement can almost be doubled. There is therefore a need to provide
systems that are less prone to premature failure, while increasing their efficiency and
improving their constructability. It is also important to control the burden of testing, both for
the agency and for the contractor, recognizing that budgets are shrinking and the availability
of trained staff is reducing.
The Federal Highway Administration, through their Cooperative Agreement with the National
Concrete Pavement Technology Center, has been working with the 30 member-state
departments of transportation of the National Concrete Consortium and several universities
to identify the specification approach and key testing technologies that are needed for
paving concrete to have increased reliability and durability.
Testing technologies have been developed along with a provisional guide specification, and
the next critical activities are deployment of the new testing technologies, development of
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practical specifications and QA/QC recommendations, and correlating specification limits
with durable field performance.
Critical Properties
A fundamental part of the art of good engineering is delivering what is required, using a
minimum of resources, reliably. Traditionally, civil engineers have been taught that the
fundamental property of concrete that controls all others is compressive strength, followed
by workability. This may have been true when concrete systems were simpler and the only
adjustments that could be made to a mixture were the water and cement content, with water
affecting workability, and then enough cement being added to achieve strength.
Fundamentally, what was happening was that the w/c was being controlled, which in turn
governs most other properties such as permeability.
However, with the growing use of supplementary cementitious materials (with a range of
chemical compositions,) and a plethora of chemical admixtures, the old rules of thumb are
no longer valid. Almost any workability can be achieved over a range of w/cm, making
slump a poor indicator of concrete quality. Likewise, the ability to resist fluid transport is
influenced by the SCM’s included in the mixture, thus disconnecting strength and potential
durability.
Other changes are affecting the way we approach identifying concrete quality: the ability to
survive increasing aggressive environments; the ability to carry early construction traffic; and
the ability to maintain a smooth ride over time, are all becoming increasingly critical to
pavement owners.
A group of experts was convened to discuss the parameters that were, indeed, critical to
long-term performance of concrete pavements. The final list of properties agreed upon
were:
• Transport properties
• Aggregate stability
• Strength
• Cold weather resistance
• Shrinkage
• Workability
These are discussed in more detail in the following sections.
Transport properties
This property (also referred to as permeability) refers to the ability of a given mixture to resist
the passage of water, solutions and gasses from the surface into the deeper layers of the
system. This is critical because all chemical based failure mechanisms involve the presence
of water; therefore reducing the ability of water to penetrate the system will slow down the
deleterious reactions (ref). Such mechanisms include chloride penetration, sulfate attack,
carbonation, alkali silica reaction, and freezing and thawing.
Measurement and control of this property has long been a challenge, and the focus of many
research projects. Permeability is fundamentally controlled by the connectivity of pores
within the microstructure, primarily of the hydrated cementitious paste and the interfacial
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zone around the aggregate particles. It has long been accepted that permeability is most
easily controlled by:
• w/cm, because any unreacted water remaining in the system will eventually evaporate,
leaving behind capillary pores, and the greater the volume of these pores the greater the
probability that they will be interconnected through the system. It is generally accepted
that a w/cm of 0.38 to 0.42 is reasonable to ensure a minimum of capillaries in a well-
hydrated system.
• SCM type and dose, because modern portland cements contain a higher amount of C3S
than historically recorded. Hydration of C3S is generally faster, leading to improved
early strengths, but it also generates three times more CaOH than C2S, and CaOH is
more soluble and permeable, and less able to resist cracking than calcium-silicate-
hydrate (C-S-H). The silica in SCMs in a mixture will convert CaOH to C-S-H hence
improving long-term transport performance. Other effects of SCMs are to slow initial
hydration, thus slowing setting and early properties, but the pozzolanic reaction
continues for longer leading to significantly reduced permeability over time.
• Time, because cementitious hydration is a slow reaction requiring presence of water for
enough time for sufficient impermeability to be achieved.
• Temperature, because the higher the temperature of a system, the faster will be the rate
of reaction. The negative side-effect of this, though, is that systems that hydrate rapidly
early on tend to be more permeable in the long run
A number of test methods have been used over time assess this property, all with different
strengths and weaknesses. The largest barrier has been that all of the methods are
sensitive to the moisture state of the sample at the time of testing, as well as the degree of
hydration of the mixture.
A method commonly used in specifications is the so-called rapid chloride penetrability test
(RCPT) in which the electrical current pushed through a sample under a 60V potential is
monitored over a number of hours. The scatter, length of test and efforts required to
precondition the sample make it a relatively expensive procedure.
A recent approach based on measuring the resistivity (Figure 1) across a sample has shown
good correlation with the RCPT and is more cost effective. The thought process behind the
method is that it is easier to conduct electricity through fluid filled pores than through solids,
therefore the higher the resistivity, the lower the permeability of the system.
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Figure 1 Resistivity Measurement
Resistivity is affected by the following factors:
• Moisture state
• Temperature
• Geometry
• Curing conditions
• Ionic concentration of the pore solution
• Formation Factor
The first four can be controlled, while the pore solution can be determined either by
measurement or by calculation from the chemistry of the cementitious system. The
remaining parameter, the formation factor, is a fundamental property of the paste describing
the amount and connectivity of the pores – this is the property we are really looking for.
Thus, by controlling storage, curing and preparation of a sample, the formation factor can be
calculated and used for acceptance testing, even at relatively young ages (AASHTO PP 84).
The resistivity can be measured either using a 4-pin array applied to the side of a cylinder, or
using current applied through the length of a cylinder (AASHTO TP 119). The test is non-
destructive, therefore the same sample can be tested at various ages to monitor the
development of hydration product over time.
Aggregate stability
This property refers to the ability of an aggregate particle to avoid chemical reactions that
may cause it to expand inside the concrete. Deleterious reactions that most commonly
occur include
• Alkali silica reaction – a reaction between alkali hydroxides from the cementitious
system, stressed silica in the aggregate, and water, to form a gel that expands when it
imbibes water. The expansion will take several years to exhibit and can range in severity
from negligible to severely compromising serviceability of a pavement.
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• D-cracking occurs in some dolomitic limestones that have a pore system that attracts
water into the aggregate particles, but prevents drying out. If such a saturated aggregate
particle freezes, it will expand and cause cracking, normally near joints in a slab.
Other less common mechanisms may include alkali carbonate reaction, pyrite and pyrrhotite
related expansions.
ASR can be controlled by inclusion of sufficient SCM in the mixture, the dosage required
being dependent on the alkali content and calcium/silica ratio of the cementitious system. D-
cracking aggregates can be mitigated by limiting maximum size or by limiting their volume in
a mixture. These will only delay eventual distress.
Evaluation of alkali reactivity of aggregates, and the mixtures they are used in, is a
challenge. The more reliable test (ASTM C1293) takes up to 2 years to run, while the
shorter term (ASTM C 1260/1567) method is report give both false positive and false
negative results about half of the time. AASHTO PP65 presents a protocol that guides users
in how to assess their materials and mixtures.
There is no nationally accepted approach to assessing the risk of D-cracking. The State
DOTs in locations where this is a problem have developed a number of different approaches
that seem to work for their locations.
Strength
Compressive and flexural strength have been used for decades as the primary form of
acceptance testing.
In general, a mixture that meets typical durability requirements will have more than enough
strength. It is not recommended that strength be used for determining bonuses because the
actions taken to increase strength may not be beneficial to the more critical durability related
properties.
Measurement of compressive strength using cylinders or cubes is more cost effective than
measuring flexural strength of beams, although most pavement design approaches are
based on knowing the flexural strength. One approach may be to develop a calibration
between, compressive and flexural strengths for a given mixture, to prequalify the mixture
based on flexure and run acceptance testing using compression.
Maturity approaches can be used to evaluate time to opening to traffic.
Cold weather resistance
The ability of a mixture to resist cold weather covers three aspects:
• Freezing and thawing
• Effects of deicing salts
• Salt scaling
Water within the microstructure of a paste system will expand when it freezes, potentially
setting up cracking depending on the degree of saturation of the voids including capillary
and air voids. Adding entrained air will significantly slow the rate of saturation. The primary
characteristic needed from the air void system is that there are a number of small bubbles
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close together. The standard ASTM pressure pot does not report bubble size or spacing,
but the Super Air Meter (SAM) (AASHTO TP 118) does report a number that correlates well
with lab and field freeze thaw testing (Figure 2).
Figure 2 Correlation between SAM data and freeze-thaw performance (Ley 2016)
It is acknowledged that only locations that undergo an appreciable number of freeze-thaw
cycles need be concerned about this parameter, but that does cover about 70% of the USA
land mass.
Some Anti- and De-icing salts will chemically attack the CaOH in the paste to form
expansive calcium oxychloride compounds. This compound is only stable at about 35 to
50°F meaning that distress occurs during spring rather than in the dead of winter. The risk
can be reduced by including enough SCMs to significantly lower the CaOH in the system,
typically about 35% SCM by mass of cementitious. This means that extra care has to be
taken to prevent early age cracking if paving in cold weather. Test methods are being
developed to assess the resistance of mixtures to this form of distress.
Salt scaling is not necessarily a cold weather phenomenon. Solutions of salts that penetrate
the microstructure, and later dry out, will leave the salts behind, potentially causing
expansion and cracking near the surface. It is most common in streets in cold weather
locations because salt is used to melt ice, but is observed in tropical marine and industrial
settings. Conventional wisdom is that entrained air will help reduce damage, but there is a
growing body of opinion that impermeability and workmanship (related to working or trapping
bleed water at the surface) may be more critical.
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Shrinkage
Shrinkage is considered critical because it is related to the risk of cracking and the extent of
warping, both of which will reduce ride quality and longevity of a pavement.
Early age random cracking is a response to a number of factors:
• Increasing shrinkage due to moisture loss will increase the strains in a system.
• Restraint imposed on the system due to end connections or base friction will limit
movement, turning strain into stress.
• Increasing stiffness increases the stress for a given system. Unfortunately, stiffness
develops faster, initially, than strength.
• When stresses exceeds strength, cracking occurs.
The standard approach to addressing this is to limit shrinkage, primarily by limiting the total
paste content of the mixture. Shrinkage can be measured using a prism test (ASTM C 157)
although readings for this test only begin 24 hours after casting, potentially losing useful
data. The restrained ring test (AASHTO T 334) provides an assessment of the cracking risk
for a mixture but may take several weeks to complete. A dual ring test has also been
proposed (AASHTO PP 84) that can be completed in 7 days.
The other factor related to moisture loss based movements is warping, the tendency for the
corners of a slab to displace vertically due to differential moisture profiles through the
thickness of the section. Warping can lead to faulting and corner breaks, especially if the
base system is very rigid and not able to absorb curvature of the slab (Figure 3). Again,
warping may be influenced by the stiffness of the foundation system, the mixture paste
content, curing practices and to a lesser extent the chemistry of the cementitious system.
Figure 3 Typical faulting
Data are showing that poor initial ride due to warping will shorten lifetime as well as cause
significant discomfort for users.
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This factor is less of a concern in moist environments but in warm, dry locations is a
significant part of design and construction practice.
Workability
Typically, the owner of a pavement should not be concerned about the workability of a
mixture, but experience has shown that contractors will tend over-vibrate and wrestle a
poorly workable mixture into place, potentially compromising the air void system as well as
impairing the smoothness.
A challenge has been that the historical gold standard for measuring workability, the slump
test, does not inform about how a mixture will respond to vibration, which is the property
critical to slipform paving operations. It is desirable to have a mixture that flows readily
under vibration, then stands up straight after the paving machine has moved on. Recently
two test methods have been proposed that better indicate this property.
The first is the VKelly test in which a small vibrator is clamped to a Kelly ball. The rate at
which the ball sinks when the vibrator is running gives a good measure of the response to
vibration, even for mixtures with similar slump. It has been shown that the combined
aggregate gradation and the paste content of the mixture both affect the VKelly Index. This
is intended to be a lab test to identify desirable mixtures during the qualification stage of a
project (Figure 4).
The Box test, compromises a 12” cubic mold that is partially filled with unconsolidated
concrete. A vibrator is inserted for 6 seconds before the form is removed. The edges of the
concrete are examined and visually rated for edge slump and for honeycomb. This is also
intended to be a qualification tool.
The slump test is still of value to the contractor as a QC tool because it will identify changes
between loads.
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Figure 4 VKelly Test
Guide Specification
A provisional guide specification has been published by AASHTO as PP84 in April 2017.
The specification is structured around the critical properties discussed above.
Not every parameter is critical in every location, meaning, for instance, that cold weather
related properties need not be regulated in warm locations or shrinkage emphasized in moist
environments. As it is a guide specification, agencies are able to select the parameters that
are critical to their location. It also means that they are able to add in language that
addresses local problems, such as the presence pyrrhotite in some aggregates in the north
east.
The heart of the specification is illustrated in Table 1.
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Table 1 Summary of critical parameters in the specification and how they are
assessed
Property Specified Test Specified Value Acceptance Selection Details
Transport Properties
Water to Cementitious
Ratio
— ≤0.45 or ≤0.50 — Yes
Choose only one Formation Factor Table 1 ≥500 or ≥1000 — Yes
Ionic Penetration, F Factor Appendix X2 25 mm 30 year Yes
Concrete Strength
Flexural Strength T 97 4.1 MPa 600 psi Yes
Compressive Strength T 22 24 MPa 3500 psi Yes
Durability of Hydrated Cement Paste for Freeze–Thaw Durability
Water to Cementitious
Ratio
— 0.45 — Yes
Choose only one Fresh Air Content T 152, T 196, TP 118 5 to 8 % Yes
Fresh Air Content/SAM T 152, T 196, TP 118 ≥4% air; ≤0.2 %, psi Yes
Time of Critical Saturation “Bucket Test”
Specification
30 yr
Deicing Salt Damage — 35% SCM Yes
Choose only one
Deicing Salt Damage M 224 — Topical
treatment
Yes
Calcium Oxychloride Limit Test sent to
AASHTO
<0.15 g CaOXY/g paste
Reducing Unwanted Slab Warping and Cracking Due to Shrinkage (if cracking is a concern)
Volume of Paste — 25%
Choose only one
Unrestrained Volume
Change
ASTM C157 420 με At 28 days
Restrained Shrinkage T 334 Crack free At 180
days
Restrained Shrinkage Dual Ring As specified
Aggregate Stability
D Cracking T 161, ASTM C1646 — —
Alkali Aggregate Reactivity R 80 — —
Workability
Box Test Appendix X3 <6.25 mm, <30%
surface void
V-Kelly Test Appendix X4 15–30 mm/root s
Innovative concepts in this specification include:
• Test methods are now available that allow agencies to measure these properties rather
than depend on prescriptive surrogates.
• Each property is addressed with traditional metrics (such as w/cm) or performance tests.
Agencies, however, are encouraged to consider moving toward the performance tests.
• Guidance is provided as to when each test should be conducted. This is based on the
premise that not all tests are needed for acceptance. Some properties can be evaluated
at the mixture qualification stage, such as aggregate stability, while others, such as air
void system, should be assessed regularly as part of the daily acceptance process.
• The document also lays out the minimum activities that should be part of the Quality
Control process, thus ensuring that materials variability or problems with the construction
process can be caught early, so minimizing their impacts on the contractor and the
agency.
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• Requirements for ingredients in a mixture refer back to existing materials specifications.
The document is a provisional specification, meaning that
• As tests are improved or replaced, the changes can be incorporated
• Precision and bias statements can be developed
• The efficacy of the current pass/fail limits can be reviewed
The document does not address PWL factors as this is considered to be a local activity.
Mixture Proportioning
An integral part of the program includes assisting contractors with tools on how to develop
mixtures that will meet the needs of the specification using local materials as efficiently as
possible.
Conventional approaches to mixture proportioning were developed before chemical
admixtures and supplementary cementitious materials became common. The availability of
computing resources has also changed the way that calculations can be performed.
A tool has been developed that takes the following approach:
• The gradation of the combined aggregate system is determined. It is recommended that
the gradation fit as close as possible to the middle of the tarantula curve to attain good
workability with a minimum paste content.
• The quality of the paste is then selected, largely based on the specification, in terms of
w/cm, SCM type and dose, and air content.
• Finally, the volume of paste is estimated to achieve workability by filling all of the voids
between the aggregates, and then adding a bit more (typically about 50% to 100%more)
to separate the aggregate particles (Figure 5).
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Figure 5 Illustration of paste content (grey) at about double the volume of the voids
between the consolidated aggregates (blue)
A spreadsheet has been developed that performs these calculations. It is in use in at least
one state and appears to be providing effective guidance, with the caveat that trial batches
are still essential.
Implementation
Implementing the program is dependent on significant education and assistance for all levels
of staff across the community from the design office through the construction site. Work will
also include construction of shadow projects and demonstration projects that can be
monitored over time.
It is planned to develop and present a suite of training materials targeting different audience
levels for both agency and contractor staff. This will provide guidance on:
• What parameters are required for the location of the project
• What tools to use to monitor compliance
• How to run the new test methods
• Guidance on quality related activities
• How to develop mixtures that will meet requirements while improving ruggedness and
reducing financial and environmental impacts
In addition, the CPTech Center team along with FHWA Mobile Concrete Trailer (MCT) will
be available to visit states to assist DOTs with shadow testing of the specification on projects
to help field personnel gain firsthand experience with the PEM and associated quality plan
requirements. The team will also be available to assist contractors with reviewing their
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mixtures to ensure compliance with the specification while remaining efficient and cost
effective.
Demonstration, pilot and parallel sites will also be monitored over time in order to develop
models that can correlate field test data with long-term performance of the system.
Acknowledging that this task has a longer horizon than the program, it is also planned to
mine data from the LTPP records collected over the last 30 years to obtain longer term
information. Where possible, materials from the LTPP Materials Reference Library will be
re-evaluated using the newer tests to support the historical records and improve the models.
Another ongoing activity will be to continue to develop effective tests that better assess the
critical properties.
Summary
The primary goal of the work is to improve the reliability of concrete mixtures in order to
ensure that when a 40-year pavement is promised, it is actually delivered. At the same time,
the intent is to rationalize the way in which concrete is assessed by measuring the things
that actually matter, taking into account the challenges imposed by the fact that it is often
fabricated on site, using highly variable ingredients, and that it changes, slowly due to
hydration over a long period.
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Acknowledgements
Thanks go to the Federal Highway Administration for their support in developing and
implementing this program. The bulk of the hard work has been carried out by the team:
• Jason Weiss, Oregon State University
• Tyler Ley, Oklahoma State University
• Tom Van Dam, NCE
• Cecil Jones, Diversified Engineering Services
• Tom Cackler, Woodland Consulting
References
1. AASHTO PP 84-17 Standard Practice for Developing Performance Engineered Concrete
Pavement Mixtures, April 2017