australian society for concrete pavements - ascp … papers/paper 04... · ascp 4th concrete...

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.

ASCP 4th Concrete Pavements Conference 2 Performance Engineered Mixtures Program, Peter Taylor

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


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


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.


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.


• 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


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.


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.


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.


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.


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.


• 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).


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


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.


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

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