An ***IPRF*** Research Report

**Innovative Pavement Research Foundation**

Airport Concrete Pavement Technology Program

Report IPRF-01-G-002-02-1(G) Stabilized and Drainable

Base for Rigid Pavement

A Design and Construction Guide

![](media/image1.jpeg){width="6.0in" height="3.5in"}

**Programs Management Office**

**5420 Old Orchard Road**

**Skokie, IL 60077 October 2005**

**ACPA Document No. JP010P**

An ***IPRF*** Research Report

**Innovative Pavement Research Foundation**

Airport Concrete Pavement Technology Program

Report IPRF-01-G-002-021(G) Stabilized and Drainable

Base for Rigid Pavement

A Design and Construction Guide

**Principal Investigator**

Dr. Jim W. Hall, P.E., Applied Research Associates (ARA), Inc.

**Contributing Authors**

Mr. Jagannath Mallela, ARA, Inc.

Mr. Kelly L. Smith, ARA, Inc.

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height="0.8590277777777777in"}

505 W. University Avenue

Champaign, IL 61820-3915

\(217\) 356-4500

\(217\) 356-3088

**Programs Management Office**

**5420 Old Orchard Road**

**Skokie, IL 60077 October 2005**

This report has been prepared by the Innovative Pavement Research
Foundation under the Airport Concrete Pavement Technology Program.
Funding is provided by the Federal Aviation Administration under
Cooperative Agreement Number 01-G-002. Dr. Satish Agrawal is the Manager
of the FAA Airport Technology R&D Branch and the Technical Manager of
the Cooperative Agreement.

The Innovative Pavement Research Foundation and the Federal Aviation
Administration thanks the Technical Panel that willingly gave of their
expertise and time for the development of this report. They were
responsible for the oversight and the technical direction. The names of
those individuals on the Technical Panel follow.

**Mr. Rodney Joel, P.E. Federal Aviation Administration**

**Mr. Wouter Gulden, P.E. ACPA Southeast**

**Mr. Darin Larson, P.E. Post, Buckley, Schuh and Jernigan**

**Mr. Dan Owens Lamp, Ryneasron & Associates**

**Mr. Bill Stamper, P.E. Post, Buckley, Schuh and Jernigan**

**Mr. Matt Wenham, P.E. C&S Engineers**

**Dr. David Brill, P.E. FAA Technical Advisor**

The contents of this report reflect the views of the authors who are
responsible for the facts and the accuracy of the data presented within.
The contents do not necessarily reflect the official views and policies
of the Federal Aviation Administration. This report does not constitute
a standard, specification, or regulation.

Acknowledgements

This report was prepared by the following project team members:

[Principal Investigator]{.underline}

-   Dr. Jim Hall, P.E., ARA, Inc.

[Contributing Authors]{.underline}

-   Mr. Jagannath Mallela, ARA, Inc.

-   Mr. Kelly Smith, ARA, Inc.

The project team would like to acknowledge the invaluable insights and
guidance of the IPRF Program Manager (Mr. Jim Lafrenz), Technical Panel,
and project team expert consultants (Mr. Stan Herrin, Mr. John Rice, and
Mr. Randy Ahlrich). Their vast knowledge and experience in the area of
concrete airport pavement design and construction helped immensely in
developing both the vision and the technical matter for this document.

**THIS PAGE INTENTIONALLY LEFT BLANK**

Table of Contents

List of Figures vii

List of Tables viii

Chapter 1 Introduction 1

1.1 Background 1

1.2 Purpose of the Guide 2

1.3 Scope of the Guide 2

1.4 Organization and Use 3

1.5 Disclaimer 4

Chapter 2 Selection, Design, and Incorporation of Stabilized and
Permeable Bases in Rigid Airfield Pavements 5

2.1 Introduction 5

2.1.1 Stabilized and Permeable Base Functions 5

2.1.2 Consideration of Stabilized Bases in the FAA Rigid Pavement Design
Procedure 5

2.1.3 Permeable Bases in Rigid Pavement Design 7

2.1.4 Stabilized and Permeable Bases and Early-Age Rigid Pavement
Performance 8

2.2 Early-Age Cracking in Rigid Pavements 8

2.2.1 Types of Early-Age Cracking 8

2.2.2 Factors Contributing to Early-Age Cracking 9

2.3 Triggers Leading to Early-Age Cracking in PCC 10

2.3.1 Large Ambient Temperature Drops 10

2.3.2 Hot-Weather Paving 10

2.3.3 High Surface Evaporation Rate 11

2.4 Design, Construction, and Materials Variants Leading to Early-Age

Cracking in PCC 11

2.4.1 Design Variants 11

2.4.2 Materials Variants 16

2.4.3 Construction Variants 22

2.5 Interaction Between Triggers and Variants 27

2.6 Base Type Selection 30

2.6.1 Stabilized Bases 30

2.6.2 Permeable Bases 32

2.6.3 Decision Making -- Base Type Selection 33

Table of Contents (continued)

Chapter 3 Materials Selection and Mix Design 35

3.1 Base Layer Materials 35

3.1.1 Cement-Treated Base 35

3.1.2 Lean Concrete Base 38

3.1.3 Asphalt-Treated Base 40

3.1.4 Cement-Treated Permeable Base 42

3.1.5 Asphalt-Treated Permeable Base 44

3.2 Choke Stone Materials 45

3.2.1 Definition and Purpose 45

3.2.2 Materials 46

3.3 Separation Layer Materials 46

3.3.1 Definition and Purpose 46

3.3.2 Materials 48

Chapter 4 Stabilized and Permeable Base

construction 53

4.1 Pre-Construction Activities 53

4.1.1 Test Section Construction 53

4.2 Subgrade and Subbase Issues 54

4.3 Base Materials Construction 55

4.3.1 Cement-Treated Base 55

4.3.2 Lean Concrete Base 59

4.3.3 Asphalt-Treated Base 62

4.3.4 Cement-Treated Permeable Base 63

4.3.5 Asphalt-Treated Permeable Base 66

4.4 PCC Placement in Conjunction with Stabilized/Permeable Bases 68

4.4.1 Timing of PCC Placement in Relation to Stabilized/Permeable Base
Placement 68

4.4.2 Bond Breaker/Choke Stone Application 70

4.4.3 PCC Curing 70

4.4.4 PCC Slab Jointing 70

4.4.5 Hot- and Cold-Weather Paving 70

[]{.mark}

References 72

AppendiCES CD Inside Back Cover

Item P-304, Cement-Treated Base Course

Item P-306, Econocrete Base Course

Item P-ATPB, Asphalt-Treated Permeable Base Course

Item P-CTPB, Cement-Treated Permeable Base Course

Item-CS, Choke Stone Interlayer

List of Tables

[]{.mark}

[Page]{.underline}

Table 2-1. Factors that shorten the sawing window (after Voigt, 2002) 25

Table 2-2. Effect of Triggers and variants on pavement responses and
early-age cracking 28

Table 3-1. Aggregate gradation for CTB material 35

Table 3-2. Aggregate gradation for LCB material 38

Table 3-3. Aggregate gradations for ATB material 41

Table 3-4. VMA as a function of maximum aggregate size 42

Table 3-5. Aggregate gradation for CTPB material 43

Table 3-6. Aggregate gradation for choke stone material 46

List of Figures

[]{.mark}

[Page]{.underline}

Figure 2-1. Example illustrations of early cracking from recent airfield
paving projects 9

Figure 2-2. Coarseness factor chart (after Shilstone, 1990) 22

Figure 2-3. Shrinkage cracking in CTB 24

Figure 2-4. Joint sawing window of opportunity (Okamoto et al., 1991;
ACPA, 1994) 25

Figure 2-5. Slab liftoff tests with conventional curing compounds 27

Figure 2-6. Quantification of risk of early cracking in rigid airfield
pavement built over stabilized or permeable bases 29

Figure 3-1. Application of construction traffic to a 2-day old CTB
surface 37

Figure 3-2. Roles of a separation layer in a pavement system with a
permeable base 47

Figure 3-3. Plot of design envelop superimposed on base, subgrade, and
the dense-graded aggregate gradations (ARA, 1999) 49

Figure 3-4. Flowchart summarizing the soil retention, permeability, and
clogging criteria for selecting the properties of geotextile (ARA, 1999)
51

Figure 4-1. CTB placement with an asphalt paver 56

Figure 4-2. CTPB compaction with a static roller 65

Figure 4-3. Drainability and texture of finished ATPB 69

# CHAPTER 1. Introduction

## 1.1 Background

![](media/image3.jpeg){width="2.149212598425197in"
height="1.6070866141732283in"}The base layer plays an important role in
the short- and long-term performance of portland cement concrete (PCC)
pavements. Federal Aviation Administration (FAA) Advisory Circular (AC)
150/5320-6D, *Airport Pavement Design and Evaluation,* allows for the
use of various base types in concrete airfield pavement
design---granular, chemically stabilized (cement and asphalt),
pozzolanic, and mechanically stabilized layers. To ensure that the key
structural design requirements are satisfied, this FAA AC recommends
that stabilized bases---cement-treated base (CTB), lean concrete base
(LCB), or asphalt-treated base (ATB)---be provided when the pavement is
designed for gross aircraft loads of 100,000 lb (45,250 kg) or greater.
Various military agencies/departments (Army, Air Force, Navy, Marine
Corps) also allow the use of stabilized layers in thickness design of
rigid pavements.

Drainage layers in the form of an asphalt-treated permeable base (ATPB),
cement-treated permeable base (CTPB), or an untreated permeable base
(UPB) are also used routinely in typical sections of military airfield
pavements. Although not explicitly addressed in the FAA design AC, these
layers are also incorporated in the civilian airfield pavements.

Until recently, FAA AC 150/5370-10A, *Specifying Construction of
Airports,* provided guidance on materials, mix design, and construction
for various base layers including stabilized bases[^1]. Although neither
this FAA AC, nor its replacement FAA AC 150/5370-10B, cover permeable
layers, they are commonly used in commercial airfield pavements. Project
specifications for these layers on civilian airfields are typically
handled through modification of the existing specifications for
stabilized materials. For military airfields, the Unified Facilities
Guide Specification (UFGS) 02706---*Drainage Layer*, covers
specification aspects of the various drainage layers.

When specified, designed, and properly constructed, stabilized and
permeable bases have a positive impact on pavement performance. However,
to ensure success, the selection and specification of these layers
should be considered in the overall context of the rigid pavement design
and construction process. Stated in another way, mere inclusion of these
bases in the typical section is not adequate to guarantee pavement
performance under all situations. On the contrary, if careful attention
is not paid to how these layers alter the early-age and long-term
performance of pavements, they could even prove not as beneficial as
anticipated. Design details of the PCC layers (thickness, joint spacing,
etc.), PCC mixture properties, ambient conditions at the time of paving,
and curing and jointing of the PCC layer interact with the base layer's
as-constructed stiffness, thickness, and friction properties to create a
unique set of circumstances for each project which need to be carefully
accounted for to ensure a successful end product.

Deliberate decisions and decisive actions during design and construction
of rigid pavements with stabilized and permeable bases are necessary to
achieve the desired pavement performance goals---both early-age and
long-term.

## 1.2 Purpose of the Guide

There is emerging evidence suggesting that, under certain circumstances,
rigid pavements constructed over certain types of stabilized and
permeable bases have a higher risk of early-age, uncontrolled cracking
(even when constructed in accordance with standard specifications). This
is overshadowing the potential long-term benefits of these layers.
Early-age cracking is defined as cracking that occurs up to 90 days
after paving has been completed. In some cases, this has resulted in the
removal and replacement of up to 7 percent of the total number of slabs
on a project---an expensive proposition for owners and contractors.

Forensic investigations and engineering analyses performed on pavements
with such early failures have identified several plausible design,
materials, and construction factors that act either independently or in
concert with other factors, leading to the early-age cracking
phenomenon. Measures to control some of these factors can be addressed
by revising and updating the current specifications. However, additional
guidance is also needed for these materials from a design and
construction perspective, so that stabilized and drainable bases may be
more effectively and appropriately used in rigid pavement systems.

This document is intended to provide the engineer or constructor an
understanding of the materials used and the design and construction
criteria to apply when specifying stabilized or permeable base courses
in airfield rigid pavement structures. The criteria and techniques
described herein should result in a pavement system that will provide
the owner the expected level of service.

The purpose of this document is to supplement FAA AC 150/5320-5B, FAA AC
150/5320-6D, and FAA AC 150/5370-10B for guidance in designing and
constructing a PCC pavement. This Guide does not purport to address the
issues of rigid pavement design and construction in totality. Only those
aspects of pavement design and construction that interact with the
stabilized or permeable base that are critical to minimize the
probability of early-age cracking are addressed here.

## 1.3 Scope of the Guide

The scope of this design and construction Guide is limited to the
following base types:

-   Stabilized Layers

    -   Cement-treated base (CTB)

    -   Lean concrete base (LCB) or econocrete

    -   Asphalt-treated base (ATB)

-   Permeable Layers

    -   Cement-treated permeable base (CTPB)

    -   Asphalt-treated permeable base (ATPB)

Although unbound permeable base (UPB) layers offer excellent
drainability and have been used successfully in some rigid airfield
pavement structures, their ability to be stable under construction and
actual traffic, as well as their long-term durability as a base course,
has not been proven. Therefore, these layers are not recommended to be
used as base layers in rigid airfield pavement structures carrying
aircraft weighing 100,000 lb (45,250 kg) or greater. They may be used
deeper in the pavement structure. UPB layers are not covered in this
Guide.

Since rigid airfield concrete pavements typically are jointed plain
concrete (JPC) pavement, the discussion in the Guide is further
restricted to this pavement type.

## 1.4 Organization and Use

The Guide is a compilation of good design practices, materials
selection, construction, and inspection methods that should reduce the
risk of early-age cracking in rigid pavements built on stabilized and
permeable base layers. It is anticipated that this, in turn, will help
these pavement structures to realize their long-term pavement
performance goals. In addition to highlighting the best practices to
avoid early-age cracking, a discussion of design, materials selection,
and construction practices that are known to cause these problems is
also presented.

This Guide is divided into four chapters. This introductory chapter
provides the background and need for additional guidance when using
stabilized and permeable bases in rigid pavements.

Chapter 2 discusses the functions of a base layer, its role in ensuring
long-term pavement performance, and issues related to early-age cracking
in rigid pavements. This chapter also identifies the environmental
trigger factors and PCC and base layer design issues, material
deficiencies, and construction variants that interact to elevate the
risk of early-cracking.

Chapter 3 guides the user in selecting appropriate materials for use in
the various stabilized and permeable base layers, and provides
discussions related to mix design for each of these layers. This chapter
also discusses the choke stone layer and separation layers used in
conjunction with various stabilized and permeable base layers.

Finally, chapter 4 provides guidance on the construction of stabilized
and permeable bases including pre-construction activities, test strip
construction and evaluation, and base layer construction. In addition,
issues related to the placement of PCC surface layers on stabilized and
permeable bases are also discussed.

More detailed technical information and discussions concerning the
justification for the guidelines presented herein are available in the
final report of FAA IPRF Project FAA-01-002-02-1, *Design Guide and
Recommendations for the Use of Stabilized and Drainable Subbase in Rigid
Pavement Systems*.

## 1.5 Disclaimer

This manual is neither a construction guidance specification nor a
design tool. It does not provide detailed instructions on conducting
specific design or construction-related activities. It does not
constitute a standard, specification, or regulation. This manual should
not be used in lieu of a project specification. The specific
requirements of plans and specifications for a project have precedence.

#   {#section .unnumbered}

# CHAPTER 2. Selection, Design, and Incorporation of Stabilized and Permeable Bases in Rigid Airfield Pavements

## 2.1 Introduction

### 

### 2.1.1 Stabilized and Permeable Base Functions

The main functions of the base layer in a rigid pavement system include
providing a stable construction platform, providing uniform support,
preventing pumping, providing subsurface drainage (in the case of
permeable bases), and reducing detrimental frost effects.

![](media/image3.jpeg){width="1.8598425196850394in"
height="0.9854330708661417in"}

In a rigid pavement system, the PCC layer carries most of the applied
loads; therefore, high-strength bases are not required. In fact, a
durable base layer that provides reasonably uniform support over the
design life of the pavement structure is preferred over a high-strength
base.

A well-designed and constructed stabilized base layer increases the
foundation support, helps reduce stresses and deflections from aircraft
wheel loads, and improves load transfer across joints in the PCC slab.
All these lead to a reduction in the cracking and faulting potential of
the pavement.

Similarly, a well designed and constructed permeable base layer rapidly
removes water from within the pavement structure. This leads to a
mitigation of PCC durability-related distresses (e.g., D-cracking) and
helps increase resistance to joint faulting.

### 2.1.2 Consideration of Stabilized Bases in the FAA Rigid Pavement Design Procedure

#### 2.1.2.1 Structural Considerations

The FAA AC 150/5320-6D requires stabilized base layers (CTB, LCB, or
ATB) for all new rigid pavements designed to accommodate aircraft
weighing 100,000 lb (45,250 kg) or more. The structural benefit imparted
to a pavement section by a stabilized base is reflected in the increase
to the modulus of subgrade reaction (*k*) assigned to the foundation.
The foundation *k*-value is increased by a factor proportional to the
thickness of the base layer. An increased *k*-value translates to a
decreased slab thickness in the current design procedure due to the
decrease in load related stresses. The maximum *k*-value that can be
used in the AC is restricted to approximately 500 psi/in (1,840 kPa/mm);
this range represents the highest *k*-value that can be accurately
measured in the field.

The following aspects of the design procedure are noteworthy:

-   The minimum prescribed thickness of the stabilized base layer is 4
    in (102 mm).

-   The FAA design procedure considers load stresses alone in
    determining PCC layer thickness. The effects of temperature and
    moisture (curling and warping) on pavement thickness design are
    considered indirectly through field calibration of the theoretical
    fatigue model, application of a design "safety factor," and the
    guidance provided on joint spacing and slab length-to-width ratios.

-   In adjusting the foundation k-value to account for the presence of a
    stabilized base layer, the stiffnesses of all stabilized base layers
    are considered equal.

#### 2.1.2.2 Material Considerations

FAA AC 150/5370-10A provided standards for the construction of airports
in the United States until recently. The guide specifications for the
CTB, LCB, and ATB layers are referred to in this AC as Items P-304,
P-306, and P-401, respectively.

Salient aspects of the specifications for the base types contained in
FAA AC 150/5370-10A are discussed in this section; only items of
relevance to this Guide are highlighted. The information summarized in
this section is excerpted from the 1991 publication of the AC for P-304
and P-306 and the 1999 publication for P-401 (the last published changes
for these items).

##### Cement Treated Base -- Item P-304

The P-304 layer mixture design specifications are based on strength and
durability criteria. A minimum 7-day compressive strength of 750 psi
(5,171 kPa) is suggested. A durability criterion in the form of maximum
freeze-thaw weight loss is optionally applicable for areas subject to
considerable freeze-thaw cycles.

In the 1991 specification, a bond-breaking layer between the PCC layer
and CTB is not required (in AC 150/5370-10B, a bond-breaking layer is
recommended).

The acceptance of CTB layers is done on the basis of density and
smoothness. Contractor pay adjustments are made on the basis of density.

##### Lean Concrete Base -- Item P-306

The mixture design of LCB in the FAA's guide specifications are based on
the following criteria:

-   Compressive strength---Minimum compressive strengths of 500 and 750
    psi (3,448 and 5,171 kPa) are specified at 7- and 28-days,
    respectively. Recognizing the detrimental effects of high strength
    bases, the P-306 guide specification suggests that an upper limit of
    1,200 psi (8,274 kPa) may be imposed on the compressive strength as
    an optional requirement.

-   Cement content---A minimum cement content of 200 lbs/yd^3^ (118
    kg/m^3^) is specified.

-   Slump---A slump of 1 to 3 in (25 to 76 mm) is specified.

-   Durability---As with CTB, a freeze-thaw weight loss based durability
    criterion is applicable for areas subject to considerable
    freeze-thaw cycles. For these areas, air contents between 6 and 10
    percent ±2 percent is also specified.

To prevent shrinkage cracks in the LCB layer from reflecting into the
PCC layer, joints are recommended when the compressive strength is
greater than 1,200 psi (8,274 kPa). However, this requirement is waived
if a good bond breaker is used between the LCB layer and the overlying
pavement.

The acceptance of the LCB layer is based on consistency (slump), air
content, thickness, strength, and grade. Contractor pay adjustments are
made as a function of thickness.

##### 

##### Asphalt Treated Base-- Item P-403

The Item P-401 guide specification, which is also used for the hot mix
asphalt (HMA) pavement surface layers, has been used until recently to
specify ATB layer construction. In AC 150/5370-10B, published by the FAA
in September, 2005, a new guide specification---Item
P-403---specifically for ATB and asphalt leveling courses was
incorporated.

The job mix formula for the ATB mixture in the P-403 specification is
established using the Marshall mixture design criteria for stability,
flow, air voids, and percent voids in mineral aggregate (VMA) for a
given number of blows (corresponding to the anticipated live load
expected on the pavement). In addition, a tensile strength ratio (TSR)
based criterion is used to ensure durability of the mixture. Performance
grade (PG) asphalt binders are encouraged to be used in the
specification, where available.

Mat density, joint density, thickness, smoothness, and grade are used
for contractor pay adjustment. In addition, the contractor is required
to run a quality control (QC) program to control the production and
construction processes. Variations in asphalt content and aggregate
gradation along the project are required to be plotted and maintained on
linear control charts.

### 2.1.3 Permeable Bases in Rigid Pavement Design 

Permeable base layers are not directly addressed in the current FAA
design procedure. However, they are allowed in airfield rigid pavement
construction.

The structural contribution of permeable base layers is ignored in the
design process since they are relatively weak. There is also no clear
consensus on the best location of these layers within the typical
section.

The construction specifications for these layers are typically developed
by modifying existing guide specifications, such as Items P-401 or 402
for ATPB and Item P-304 for CTPB, etc. However, the open-graded nature
of these materials prevents the application of conventional techniques
for performing mix designs and specifying their construction. For
example, the ATPB mix designs often are specified on the basis of a
gradation and percent binder content. Permeability---an important
consideration for this base type---is seldom specified or monitored.
Furthermore, field compaction of the mixtures is achieved using method
specifications. Acceptance of the mixture is done on the basis of
thickness. As can be noted, considerable empiricism is used to specify
and construct these mixes, some of which is unavoidable until further
research is done.

### 2.1.4 Stabilized and Permeable Bases and Early-Age Rigid Pavement Performance

There is ample evidence to support the notion that well designed and
constructed stabilized and permeable bases help rigid pavements achieve
their long-term performance goals. However, when the primary functions
of the base layer are not fully considered when incorporating them into
the pavement structure, short- and long-term performance deficiencies,
such as early cracking and base pumping, can occur. Examples of such
misapplications include:

-   Selecting the wrong base type for a given application. Certain base
    types are more effective than others for a given application. For
    example, permeable bases are most effective when there is a need to
    rapidly remove water from within a pavement structure; stabilized
    bases (e.g., CTB, LCB), when designed and constructed correctly, are
    effective in providing uniform, non-erodible support, etc. The
    selection of a base for a given paving project is governed by
    several factors, including agency policy, economics, availability of
    materials, and local experience. However, issues related to both
    short- and long-term performance should also be factored into the
    selection process.

-   Increasing stabilized base thickness to reduce PCC layer thickness.

-   Increasing stabilized base strength to achieve construction
    expediency.

-   Increasing permeability of permeable bases without properly
    balancing stability- or durability-related issues.

![](media/image3.jpeg){width="2.40748031496063in"
height="1.5456692913385828in"}Long-term performance deficiencies are
well known and understood. They manifest themselves in the form of
pavement distress---joint faulting, flexural fatigue cracking, corner
cracking, loss of smoothness, etc. However, there has been increasing
occurrence of early-age cracking in rigid pavements when constructed
over certain types of stabilized and permeable bases. Certain qualities
of these bases in combination with other rigid pavement design,
materials, and construction factors are primary causes for such
cracking. These causes need to be understood and addressed.

## 2.2 Early-Age Cracking in Rigid Pavements

### 2.2.1 Types of Early-Age Cracking

Early-age cracking in hardened PCC can take any of the following forms
(Kohn et al., 2003):

-   Random cracking (random orientation).

-   Longitudinal cracking (cracking parallel to the centerline of the
    feature being investigated).

-   Transverse cracking (cracking perpendicular to the centerline of the
    feature being investigated).

-   Corner cracking (cracking located at the PCC slab corner
    intersecting the longitudinal and transverse joints).

-   Pop-off cracks (cracking that happens just ahead of the sawing
    operation).

-   Later stage cracking (early-age slab bottom cracking propagating to
    the surface).

-   Sympathy cracks (cracking that occurs in adjacent slabs when joints
    between the slabs in questions are not aligned during new
    construction).

-   Settlement cracks over dowel or tie bars.

-   Re-entrant cracks.

Figure 2-1 illustrates several forms of early cracking photographed on
recently completed airfield paving projects (within the past 5 years).
Most early cracking occurs when the tensile stresses induced by slab
movements exceed the tensile strength of young concrete.

+------------------------+--------------------------+------------------+
| ![](med                | ![                       | ![]              |
| ia/image4.jpeg){width= | ](media/image5.jpeg){wid | (media/image6.jp |
| "2.1798611111111112in" | th="2.439583333333333in" | eg){width="1.620 |
| height="               | height                   | 1388888888888in" |
| 1.9590277777777778in"} | ="1.9097222222222223in"} | height="1.9631   |
|                        |                          | 944444444445in"} |
| \(a\) Longitudinal     | \(b\) Transverse         |                  |
| cracking               | cracking                 | \(c\) Diagonal   |
|                        |                          | cracking         |
+------------------------+--------------------------+------------------+

Figure 2-1. Example illustrations of early cracking from recent airfield
paving projects.

### 2.2.2 Factors Contributing to Early-Age Cracking

For early-age cracking to occur, two sets of factors need to be present
simultaneously on a given project: (1) driving forces that induce
movements in the young concrete pavement, and (2) factors that aggravate
the impact of these movements on the tensile or flexural stresses
developed in the pavement. The former set is referred to as trigger
factors, or simply *triggers*. Triggers are basically the forcing
functions that cause deformations in the PCC slabs. The latter set of
factors is termed as variant factors, or *variants*.

Triggers are associated primarily with ambient conditions accompanying
the placement of the PCC layer on top of the stabilized or permeable
base layer. Variants are key design, materials, and construction
properties of the stabilized/permeable base and PCC layer. When the
variants exceed their respective threshold levels, the risk of early
cracking is elevated.

## 2.3 Triggers Leading to Early-Age Cracking in PCC

Three types of environmental triggers---large ambient temperature drops,
hot-weather paving, and excessive surface evaporation---set off
early-age cracking in PCC, with the first two accounting for a majority
of early failures. A discussion on each of these triggers and how they
affect early-age behavior of PCC pavements is presented in this section.

### 2.3.1 Large Ambient Temperature Drops

![](media/image7.jpeg){width="2.65748031496063in"
height="1.8200787401574803in"}

A drop in ambient temperature of 25°F (14°C) or greater, shortly after
initial set of the concrete, is sufficient to elevate the risk of early
cracking significantly. This is particularly true when the temperature
falls to a level where the strength gain of the PCC is affected
significantly. For example, a temperature drop from 70 to 45°F (21 to
7°C) causes a greater risk of early cracking than a drop from 100 to
75°F (38 to 24°C).

A large ambient temperature drop imposes a negative thermal gradient
through the slab (where the top is cooler than the bottom). If the slab
is sufficiently hardened, this can lead to tensile stresses at the top
of the slab and a potential for top-down cracking. Large temperature
swings are typically prevalent in northern and northeastern climates
during late fall or spring construction. A sudden rain or snow event
shortly after PCC placement can also cause these swings.

### 2.3.2 Hot-Weather Paving

When ambient temperatures are in excess of 90°F (32°C), the risk of
early cracking is significantly elevated. Hot-weather concreting
practices must be followed when paving under these conditions. Further,
if the concrete is being placed on a dark-colored base, such as an ATB
or ATPB, care needs to be taken to cool the surface of the base prior to
PCC placement.

If hot-weather paving precautions are ignored, excessive drying
shrinkage can lead to warping and axial deformations. The effect of
warping is similar to that of a negative thermal gradient. Axial
deformations lead to stress build-up at locations of
restraint---slab/base interface, tie bars, etc. Cracking can be of any
orientation depending on the variants present.

![](media/image7.jpeg){width="2.133464566929134in"
height="1.3125984251968503in"}

Although hot-weather paving conditions are prevalent throughout the
country during the normal construction season (summertime), early
cracking problems due to this trigger factor occur more frequently in
the hotter parts of the western and southern United States.

### 

### 2.3.3 High Surface Evaporation Rate

Surface evaporation rates exceeding 0.1 lb/ft^2^ (0.5 kg/m^2^) elevate
the risk of early-age cracking in the PCC slabs. High evaporation rates
can occur due to a critical combination of high ambient and concrete
temperatures, high wind speeds, and low ambient relative humidities.
High surface evaporation rates generally result in plastic shrinkage
cracking. However, they can also lead to early-age cracking,
particularly due to their ability to cause differential volumetric
shrinkage through the slab (slab warping).

## 2.4 Design, Construction, and Materials Variants Leading to Early-Age

##  Cracking in PCC

Trigger conditions are necessary for the development of early cracking.
Several design, materials, and construction variants must also be
unfavorably aligned (i.e., exceed their threshold values) for early
cracking to occur. Variants include both stabilized/permeable base
properties as well as PCC layer related factors. The most important
variants and their disposition leading to early-age cracking include:

-   Design Variants

    -   Excess base thickness.

    -   PCC panels with large aspect ratios.

    -   Excessive restraint within the slab.

-   Materials Variants

    -   High PCC/base interface friction.

    -   Excessive base strength/stiffness.

    -   High PCC cement factor.

    -   Shrinkage susceptible PCC mixture.

-   Construction Variants

    -   Surface condition of base prior to paving.

    -   Shrinkage cracking in base.

    -   Late sawing or inadequate sawcut depth.

    -   Absence of bond-breaker.

    -   Inadequate PCC curing.

Detailed discussion of these factors is presented in the following
sections.

### 2.4.1 Design Variants

#### 2.4.1.1 Thick Stabilized or Permeable Bases

The FAA AC 150/5320-6D specifies a minimum stabilized base thickness of
4 in (102 mm). In this procedure, increasing the thickness beyond this
value increases the *k*-value of the foundation. While a higher
*k*-value is beneficial in reducing stresses imposed by wheel loads, its
effectiveness in doing that decreases with increasing thickness of the
PCC layer, as illustrated in Example Scenario 1.

+-----------------------------------------------------------------------+
| ***Example Scenario 1: Effect of Stabilized Base Thickness on PCC     |
| Stresses due to Wheel Loads*** The effect of stabilized base layer    |
| thickness on the maximum flexural stresses in a PCC pavement subject  |
| to a B-727 aircraft load is shown in the figure below. The range of   |
| k-values in the figure corresponds to stabilized base thicknesses     |
| ranging from 4 to 12 in (102 to 305 mm) resting on a 100 psi/in (28   |
| kPa/mm) subgrade. The ISLAB2000 finite element program (Khazanovich   |
| et al., 2000) was used to obtain the solutions.                       |
|                                                                       |
| ![](media/image8.wmf){width="3.576388888888889in"                     |
| height="2.438888888888889in"}                                         |
|                                                                       |
| It can be noted that wheel load stresses decrease with increasing     |
| stabilized base thickness. However, the ability of base thickness to  |
| reduce load stresses decreases with increasing PCC thickness. This is |
| no surprise since the primary load in a rigid pavement is the PCC     |
| layer itself. Further, both base thickness and stiffness contribute   |
| to the reduction in load stresses. If a stiffer base was chosen for   |
| this scenario, the impact of base thickness would have been further   |
| diminished and vice versa.                                            |
+-----------------------------------------------------------------------+

Further, for a given slab thickness and joint spacing, increasing the
foundation *k*-value can actually increase the curling and warping
stresses induced by temperature and moisture gradients, respectively.
This is illustrated in Example Scenario 2 for a typical airfield
pavement section. Therefore, load and curl stresses are affected in
opposite directions by an increasing k-value leading to a marginal
improvement in the combined stresses. In some cases, depending on the
slab thickness and joint spacing, the combined stresses for a higher
*k*-value can be higher than for a lower *k*-value.

+-----------------------------------------------------------------------+
| ***Example Scenario 2: Effect of Stabilized Base Thickness on PCC     |
| Curl Stresses***                                                      |
|                                                                       |
| The effect of stabilized base layer thickness on the PCC curl         |
| stresses for different positive (daytime) temperature gradients (top  |
| of slab is warmer than bottom) are shown below based on ISLAB2000     |
| analysis. It can be noted that higher k-values can actually lead to   |
| higher curling stresses at any given temperature gradient.            |
|                                                                       |
| ![](media/image9.wmf){width="3.8895833333333334in"                    |
| height="2.6590277777777778in"}                                        |
|                                                                       |
| Higher curling stresses can lead to higher combined stresses (load    |
| plus curl) in pavements. Therefore, while a thicker base can lower    |
| stresses due to wheel loads, this advantage is negated to a degree by |
| an increase in the curling stresses.                                  |
+-----------------------------------------------------------------------+

When a permeable base is used, its thickness should be that which is
required for it to serve as an adequate hydraulic channel, even though
large volumes of water are not expected in the pavement subsurface.
Beyond a certain practical limit, however, increasing the permeable base
thickness does not have a great impact on pavement drainability. In
fact, since a permeable base is an inherently weak layer, thicker layers
could actually be detrimental to pavement performance. Example Scenario
3 shows the relationship between base layer thickness and base
drainability for a typical CTPB layer. It can be noted that increasing
the permeable base thickness has a very small impact on the degree of
drainability of the pavement.

![](media/image3.jpeg){width="2.499212598425197in"
height="2.063779527559055in"}

Based on the discussion above, it is clear that thickness, in excess of
that required to achieve the desired goals of stabilized or permeable
base layers, is not necessary when designing rigid pavements. From
practical and economical considerations, a stabilized base thickness
between 6 and 8 in (152 and 203 mm) is sufficient for most applications.
Similarly, a permeable base between 4 and 6 in (102 and 152 mm) is
adequate. This thickness range allows for the base to be placed without
segregation and provides for an adequate hydraulic conduit.

+-----------------------------------------------------------------------+
| ***Example Scenario 3: Effect of Permeable Base Thickness on          |
| Drainability***                                                       |
|                                                                       |
| The effect of permeable base thickness on the time required to drain  |
| 50 percent of the drainable water from within the pavement structure  |
| after an initial storm saturates the base layer is shown below. The   |
| DRIP 2.0 program (Larson et al., 2001) was used to obtain the         |
| solutions presented in the figure. It can be noted that increasing    |
| the base thickness only has a marginal effect on drainability of the  |
| pavement system.                                                      |
|                                                                       |
| ![](media/image10.wmf){width="3.8in" height="2.5930555555555554in"}   |
+-----------------------------------------------------------------------+

#### 2.4.1.2 Large PCC Panel Sizes and Aspect Ratios

Long PCC panels affect the slab stresses in two ways: (1) the longer the
panel size, the higher the stresses in the slab due to a given
temperature or moisture gradient, and (2) the longer the joint spacing,
the higher the degree of movement of the slab edges with respect to the
fixed point in the slab (typically slab center), and therefore, the
higher the restraint stresses in the presence of slab/base friction. The
impact of long panel sizes is more pronounced in thinner PCC layers and
is exacerbated by the presence of stiff stabilized bases (which cannot
accommodate themselves to the curled or warped shape of the slab) and a
high coefficient of thermal expansion concrete.

![](media/image3.jpeg){width="2.749212598425197in"
height="2.4153543307086616in"}Another aspect of panel dimensions that
affects slab stresses is the slab length-to-width or aspect ratio.
Panels with higher aspect ratios have a skewed biaxial stress
distribution and can lead to cracking parallel to the shorter dimension.
Example Scenario 4 illustrates the how panel sizes and aspect ratios
affect curling stresses in the PCC slab. To prevent excessive stresses
due to curling and warping and to minimize restraint stresses due to
slab/base friction, it is desirable to keep the panel dimensions below 5
times the radius of relative stiffness (*l*). The *l* value is
determined as follows:

![](media/image11.wmf){width="1.3194444444444444in"
height="0.5416666666666666in"}

where:

E~PCC~ = PCC modulus of elasticity, psi.

h~PCC~ = PCC slab thickness, in.

*v*~PCC~ = PCC Poisson's ratio.

It is recommended that a maximum panel size of 20 ft (6.1 m) be adopted
for PCC slabs greater than 12 in (305 mm) thick. Further, it is
advisable to maintain the aspect ratio of the slab below 1.25, to avoid
long, narrow slabs.

+-----------------------------------------------------------------------+
| ***Example Scenario 4: Effect of Panel Dimensions and Aspect Ratio*** |
|                                                                       |
| The effect of slab size on the PCC curling stresses caused by a given |
| positive temperature differential in a PCC slab is presented in the   |
| figure below for various square panels (aspect ratio = 1.0) based on  |
| ISLAB2000 analysis. Clearly, longer panel dimensions not only         |
| increase the curling stresses for any given temperature differential, |
| but also increase the sensitivity of slab stresses to the applied     |
| thermal gradient.                                                     |
|                                                                       |
| ![](media/image12.wmf){width="3.704861111111111in"                    |
| height="2.5284722222222222in"}                                        |
|                                                                       |
| The impact of slab aspect ratio on PCC curling stresses is            |
| illustrated below as a function of the foundation *k*-value. Clearly  |
| panels with higher aspect ratios lead to higher curling stresses.     |
| Further, high aspect ratios lead to critical stress development over  |
| a wider portion of the slab span (not shown here), thereby increasing |
| the probability of cracking.                                          |
|                                                                       |
| ![](media/image13.wmf){width="3.8in" height="2.5930555555555554in"}   |
+-----------------------------------------------------------------------+

### 2.4.2 Materials Variants

#### 

#### 2.4.2.1 PCC Slab/Base Interface Friction

Like most materials, the nature of concrete is that expansion and
contraction occur as a function of the applied "through-thickness"
temperature or moisture variations or gradients. The degree of movement
and the associated tensile stresses developed as a result of these
changes are directly governed by several factors, including the applied
temperature and moisture loads, the thermal and mechanical properties of
concrete, self-weight of the concrete, slab geometry, and the restraint
provided at the slab-base interface.

Rough PCC slab/base interfaces promote a higher degree of friction,
which causes excessive axial restraint to volumetric shrinkage and to
thermal expansion and contraction. As horizontal forces developed by
either drying shrinkage or temperature differential pull the slab in one
direction, frictional resistance forces are developed in the opposite
direction. This type of friction has been researched the most with
regard to early-age cracking problems. Also, any forcing function (e.g.,
thermal and moisture stresses) imparted to the slab when the concrete is
still relatively young, can cause apparent adhesion, which can impact
the frictional restraint produced at the PCC slab/base interface. This
phenomenon is termed "contact friction."

![](media/image3.jpeg){width="2.749212598425197in"
height="2.4153543307086616in"}

Certain types of high-strength stabilized bases, such as CTB and LCB,
offer a high degree of restraint at the PCC slab/base interface due to
their rough finished texture. Permeable bases, such as CTPB, also offer
a high degree of restraint due to the near surface penetration of cement
paste into these bases (although PCC paste also penetrates the ATPB
layer, the lower stiffness of this layer offsets the development of high
restraint stresses). The relatively high stiffness of the CTB, LCB, and
CTPB layers, coupled with the high degree of restraint they offer,
increases the risk of early cracking in concrete pavements built over
these base types.

In addition, the bond developed at the interface of the PCC slab/base
increases the effective slab thickness rendering planned sawing depths
inadequate in some instances. Therefore, extra precautions need to be
taken to ensure that uncontrolled cracking does not happen in the field
when using these base types. These can include a variety of measures,
such as the use of bond breakers, more favorable slab geometry (e.g.,
panel sizes and aspect ratios), and better material selection and
construction practices to reduce slab movements.

Example Scenario 5 demonstrates the impact of PCC slab/base interface
friction on the maximum tensile stresses developed in a slab.

+-----------------------------------------------------------------------+
| ***Example Scenario 5: Effect of PCC Slab/Base Interface Friction***  |
|                                                                       |
| The effect of PCC slab/base interface friction is demonstrated in     |
| this example. The details of the pavement structure analyzed are      |
| shown in the figure below. The PCC modulus of elasticity, E~PCC~, was |
| estimated at different ages based on the compressive strength, which  |
| was varied from 70 to 4,300 psi (483 to 29,650 kPa) to simulate aging |
| of the concrete over the first 72 hours after placement.              |
|                                                                       |
| The loading is assumed to be in the form of a nighttime temperature   |
| gradient with the PCC surface at 35°F (2°C) and the PCC bottom at     |
| 85°F (29°C), thereby simulating a thermal shock soon after PCC        |
| placement (the high bottom temperatures are assumed to be from the    |
| heat of hydration).                                                   |
|                                                                       |
| Two bond conditions---full bond and no bond---were simulated using    |
| the ISLAB2000 program. The maximum tensile stresses developed at the  |
| top of the slab are reported for these two interface conditions for   |
| the first 72 hours of the pavement's life are shown in the figure     |
| below along with a projection of PCC tensile strength.                |
|                                                                       |
| ![](media/image14.wmf){width="4.3875in" height="2.63125in"}           |
|                                                                       |
| It can be seen from the figure that although the projected PCC        |
| tensile strength is greater than the calculated stress for the most   |
| part (except immediately after placement) for both the full bond and  |
| no bond cases, clearly, the risk of early cracking (probability that  |
| the maximum tensile stresses developed will exceed the tensile        |
| strength of the PCC) due to any fluctuations in the projected         |
| strength (brought about by weather conditions or other causes), is    |
| greater for the former case. If longer panel dimensions (e.g., 25 ft  |
| x 25 ft \[7.6 m x 7.6 m\] slabs) had been used, the computed stresses |
| would have exceeded the tensile strength.                             |
+-----------------------------------------------------------------------+

#### 

#### 2.4.2.2 Excessive Base Strength/Stiffness

##### High Strength Stabilized Bases

Often, strength is misused as a surrogate for durability. High strength
stabilized bases, such as CTB and LCB, are typically designed for a
minimum 7-day compressive strength of 750 psi (5,171 kPa). This strength
requirement was established in the current FAA guidance because, at this
strength level, the long-term durability of these layers when subject to
repeated cycles of wetting and drying or freezing and thawing was deemed
to be virtually assured (PCA, 1971). However, the selection of this
strength level is conservative and somewhat arbitrary since the strength
level corresponding to adequate durability characteristics ranges from
300 to 800 psi (2,070 to 5,516 kPa).

As stated earlier, strong stabilized bases are not necessarily better
bases. While a high strength stabilized base increases the *k*-value of
the foundation, thus reducing load stresses and improving joint load
transfer efficiency, it can also lead to higher curling stresses (see
Example Scenario 2). In addition, a very stiff base does not conform
easily to changes in the curvature of the PCC slab (particularly at the
corners), resulting in an increased tendency for top-down cracking,
corner breaks, and high corner deflections.

![](media/image3.jpeg){width="3.091732283464567in"
height="2.7570866141732284in"}

For construction purposes, a compressive strength of 350 psi (2,413 kPa)
for CTB and LCB layers is adequate for supporting construction traffic
used to place the overlying PCC layers. From a mix design standpoint, a
minimum 7-day compressive strength of 500 psi (3,448 kPa) and a maximum
strength of 1,000 psi (6,895 kPa) are desirable for these layers. When
the 7-day strengths of CTB or LCB exceed 1,000 psi (6,895 kPa), the risk
of early-age cracking in the PCC slabs built over them is significantly
increased.

Using compressive strength as a criterion for designing CTB and LCB
mixes is adequate in most situations. Freeze-thaw testing can be
performed as a supplemental durability test when local experience
indicates that it is an important consideration in the performance of
the pavements. As far as long-term durability is concerned, since the
CTB and LCB continue to gain strength well beyond 7 days, the strength
required for durability purposes is reached soon after initial
construction has been accomplished and the pavement is in service.

##### Permeable Bases

![](media/image3.jpeg){width="2.749212598425197in"
height="2.4153543307086616in"}

For permeable bases, the ability of it to hold up under construction
traffic, its long-term durability in terms of erosion potential, and its
drainability are the important concerns. Balancing the need for these
three items are the keys to success of these layers. Permeability values
in the range of 500 to 1,500 ft/day (152 to 458 m/day) are adequate for
these layers in most situations. The effect of permeability on the time
required to drain a base from an initially saturated condition to where
50 percent of the drainable water has been removed is demonstrated in
Example Scenario 6.

+-----------------------------------------------------------------------+
| ***Example Scenario 6: Effect of Permeability on Drainability***      |
|                                                                       |
| The effect of permeability on the time required to drain 50 percent   |
| of the drainable water from a base that is saturated from a rain      |
| event is demonstrated in this case study below. The drainage          |
| calculations were made with the DRIP 2.0 computer program.            |
| ![](media/image15.wmf){width="5.992361111111111in"                    |
| height="2.984027777777778in"}                                         |
|                                                                       |
| The requirement to drain 50 percent of the permeable water is a       |
| conservative one, since studies have shown that bringing the          |
| saturation level to an 85% level is adequate to decrease the harmful  |
| effects of water. However, even for this case, it can be seen that    |
| the base with the least amount of permeability takes just over a day  |
| to drain the water. It can also be noted that increasing the          |
| permeability beyond approximately 800 ft/day (244 m/day) has          |
| practically negligible effect on the time required to drain the       |
| pavement for this case.                                               |
+-----------------------------------------------------------------------+

A minimum cement content of 250 lb/yd^3^ (148 kg/m^3^) for CTPB and
asphalt content between 2.0 and 3.5 percent by weight of total mixture
for ATPB are recommended to ensure uniform particle coating (related to
long-term durability) and adequate strength. The final dosage rate of
the binder should be optimized based on the permeability requirements
for a given mix.

#### 2.4.2.3 High Cement Factor and Shrinkage Susceptible PCC Mixes

A poor concrete mix design can aggravate the problem of premature
cracking in PCC pavements. The main factors that influence premature
cracking include (Shilstone, 1990; Lafrenz, 1997):

-   Mixtures with higher water demand---These mixes have an increased
    potential for volumetric shrinkage. Factors that increase water
    demand include higher cement factor concrete and concrete made with
    fine sand.

-   Gradation of combined aggregates---This affects the workability of
    concrete mixtures, which, in turn, may affect performance.

-   Type of coarse aggregate---This can influence the temperature
    sensitivity of concrete.

In addition, certain types of chemical admixtures (e.g., accelerators
such as calcium chloride and water reducers which contain an
accelerator) can increase drying shrinkage. Another problem that has
come to the forefront lately is the issue of self-desiccation of
concrete, which results in autogenous shrinkage brought about by a
combination of low water-cement ratios, high cement factors, and fine
cements being employed in paving mixtures. Some of these variants are
discussed below.

##### Cementitious Material

![](media/image16.wmf){width="2.0in" height="1.7153543307086614in"}

Mixtures with higher cement factors (quantities of cement and/or
pozzolanic and slag additions) require more mixing water, even if the
water-cementitious materials ratio is minimized, and consequently have a
higher potential to shrink. When shrinkage is resisted by internal
restraints in the PCC slab or external restraints (PCC slab/base
friction), random cracking can develop. Mixes with cement factors
greater than 500 lb/yd^3^ (295 kg/m^3^) have a risk of early cracking.

Conversely, mixtures with high contents of pozzolans or
ground-granulated blast furnace slag, or lower contents of cement may
experience delayed early-age strength development in cooler weather.
Depending on the air, base, and concrete temperatures, this could delay
the concrete set time and the ability to saw without excessive raveling
(ACPA, 2002a and 2002b). In the end, the considerations for early-age
cracking need to be balanced with requirements of strength, workability,
and durability.

##### Sand

FAA specifications, as implemented on several projects, require that the
sand for the PCC meet the ASTM C 33 specification. ASTM C 33 provides a
gradation band for material passing the ⅜ in (9.5 mm) sieve to the No.
100 (150 µm) sieve and stipulates the following acceptability
characteristics for the concrete sand gradation:

-   No more than 45 percent of material is retained on any one sieve.

-   Fineness modulus between 2.3 and 3.1.

![](media/image3.jpeg){width="2.175196850393701in"
height="1.910236220472441in"}When applied indiscriminately, this
specification can lead to a mix design that is susceptible to
uncontrolled cracking (even when criteria noted above are satisfied),
due to the possibility of the production of gap-graded mixtures with
excessive fine sand contents. The presence of fine sand (excessive minus
No. 50 \[300 µm\] sieve material) increases the bulking potential
dramatically and thereby the potential for volumetric shrinkage and
early cracking.

To circumvent this problem, the use of coarse sand and a minimum cement
factor is encouraged. Both of these mix components directly control the
water demand. In general, concrete with a high cement factor, such as
those used in airfield pavement construction, should include coarse
sand. ASTM C 33 allows for a reduction of the portion of the sand
passing the No. 50 and No. 100 (300 µm and 150 µm) sieves to 5 and 0
percent, respectively, for pavement grade concrete.

If attention is paid to these guidelines and coarse sand with fineness
modulus values in the range of 3.1 to 3.4 is used in pavement concrete,
excellent results can be obtained from a volumetric shrinkage
standpoint. If sand with a well-graded character and fineness modulus
values above 3.1 is not available, manufactured sand may need to be used
(ACPA, 2002b).

##### Combined Aggregate Gradation

Examination of the combined aggregate gradation provides insight into
the workability and segregation potential of concrete mixtures. Mixtures
prone to segregation are also prone to early distress. Shilstone (1990)
provided a tool in the form of a coarseness factor chart (CFC) to
evaluate concrete mixture workability and the risk of problems such as
uncontrolled cracking, which was validated by the United States Air
Force (see Figure 2-2). The factors considered in evaluating a given
mixture include the workability factor and the coarseness factor. The
workability factor is simply the percent passing the No. 8 (2.36 mm)
sieve for the combined aggregate gradation. The coarseness factor is
expressed as a fraction of the percentage of aggregate retained on the ⅜
in (9.5 mm) sieve to that retained on the No. 8 (2.36 mm) sieve,
multiplied by 100.

![](media/image17.wmf){width="4.040277777777778in"
height="2.939583333333333in"}

Figure 2-2. Coarseness factor chart (after Shilstone, 1990).

Generally speaking, a potentially optimized concrete mix with the least
risk of premature cracking should be made with a combined aggregate that
falls within Zone II of the CFC shown in Figure 2-2. A well-graded,
optimized concrete mixture will reduce water demand and drying shrinkage
potential and provide better workability and improved early strength
development (ACPA, 2002b).

##### Total Water Content and Mortar Volume

![](media/image3.jpeg){width="2.083464566929134in"
height="1.8299212598425196in"}Being within the workability box defined
by the workability and coarseness factors does not alone reduce the risk
of excessive shrinkage. The total water in the mixture and the total
mortar volume (combined volume of the cementitious materials, sand
\[passing the No. 8 {2.36 mm} sieve on the combined aggregate
gradation\], water, and entrapped air content) also contribute to this
risk and must be kept within recommended limits. Total water content
less than 250 lb (113 kg) and a paste volume below 60 percent are
desirable for most paving mixes. Note that these numbers are to be used
as guidelines and the final concrete constituents should be selected
based on an optimization of other concrete properties (e.g., durability,
strength).

##### Coarse Aggregate

![](media/image3.jpeg){width="2.083464566929134in"
height="1.704724409448819in"}The type of coarse aggregate used directly
controls the uniaxial expansion and contraction and curling deformations
in PCC slabs. Generally, limestone, granite, and basalt have lower
coefficients of thermal expansion (CTE) than quartz, sandstone, and
siliceous gravel (Kosmatka et al., 2002; Mallela et al. 2005). This
means that concrete made with the former materials will be more
insensitive to ambient conditions and will perhaps exhibit lower
tendencies to crack at early and later ages. While aggregate
types/sources cannot be changed economically on a given paving project,
recognition of their impact on pavement performance can lead to
optimization of other design and materials parameters that can
counteract the effect of these variables. For example, PCC slabs built
with shorter joint spacing (less than 20 ft \[6.1 m\]) can mitigate the
adverse impact of a high CTE aggregate in the PCC mix significantly.

### 2.4.3 Construction Variants

#### 

#### 2.4.3.1 Surface Condition of Base Prior to Paving

##### Texture of the Finished Base 

High strength stabilized bases, such as CTB and LCB, tend to have a
rough surface texture. In addition, CTB and ATB layers are often trimmed
to meet grade requirements, creating a rough surface texture. Rough
textured bases cause high PCC slab/base friction, which increases the
restraint to the slab during curing. To minimize high friction slab/base
interfaces, it is advisable to use bond breakers or meet specified grade
tolerances the first time the base is constructed. If trimming is
unavoidable, bond breakers or leveling courses are recommended.

![](media/image7.jpeg){width="2.1251968503937007in"
height="1.6874015748031497in"}Even in the case of permeable bases, the
possibility of excessive restraint at the PCC slab/base interface exists
due to the penetration of the PCC paste into the "near-surface" portion
of these bases. This penetration causes an interlocking of the slab and
base, which increases the restraint stresses in the presence of
trigger-induced deformations. If the base layer is stiff and resists
these deformations, early-age cracking can occur. This tendency is
greater in relatively stiffer bases, such as CTPB, than in ATPB. Another
aspect of the concrete penetration into the stiff permeable bases is
that the effective slab thickness increases as a result, and the planned
sawcut depths may not be adequate to cause controlled cracking. Deeper
sawcuts may be needed in this situation.

##### Surface Temperature

![](media/image7.jpeg){width="2.042913385826772in"
height="1.2007874015748032in"}An important consideration for
dark-colored bases, such as ATB and ATPB, is the capacity of these bases
to absorb heat. In summer conditions particularly, the surface of these
layers can reach 140°F (60°C), which impacts both strength gain and
shrinkage rate of fresh concrete (Kohn et al., 2003). Therefore, these
layers should be whitewashed with a lime-water solution prior to
concrete placement to depress their surface temperature.

#### 2.4.3.2 Shrinkage Cracking in Base

![](media/image7.jpeg){width="2.3744094488188976in"
height="1.911023622047244in"}In addition to restraint issues, other
aspects of base layer surface conditions can lead to a heightened risk
of early cracking in PCC slabs. For example, CTB layers tend to develop
shrinkage cracks when high cement contents are used and adequate water
for hydration is not available or inadequate curing is performed. An
example of this is shown in Figure 2-3.

Although there is evidence that not all shrinkage cracks present in a
base propagate into the PCC layer above it, there is a valid concern for
increased stress concentrations (just above the cracks in the base
layer) in the PCC surface layer during strength gain and the possibility
of reflection cracking. This is particularly true when the PCC has not
developed adequate strength. When wide (\>0.5 in \[13 mm\]) shrinkage
cracks are visible in the base, it is advisable to treat them with a
reflection crack relief layer, such as a medium to high-density
geotextile fabric.

![](media/image18.jpeg){width="4.479166666666667in"
height="3.3506944444444446in"}

Figure 2-3. Shrinkage cracking in CTB.

#### 2.4.3.3 Late or Inadequate Sawing

##### Timing of Sawcut

One of the biggest variants contributing to early cracking is late or
inadequate initial sawing. To derive the anticipated benefit of
jointing, there is an optimum window for sawcutting joints, as
illustrated in Figure 2-4 (after Okamoto et al., 1991; ACPA, 1994). This
window typically occurs a few hours after the concrete placement, but
the exact timing is variable. The window begins when concrete strength
is acceptable to operate the saw equipment without excessive raveling at
the joints. The window ends when the concrete's volume reduces
significantly (from drying shrinkage or temperature contraction) and
restraint of the reduction induces tensile stresses greater than the
tensile strength. If sawing is performed after this point, pop-off
cracks (i.e., cracks just ahead of the sawing operation) can occur
(Voigt, 2002).

![](media/image7.jpeg){width="2.25in" height="1.4956692913385827in"}The
paving contractor typically is provided with guidance that the saws
should be operated on the pavement at the earliest possible time to
provide the initial sawcut, without excessively raveling the slab.
Typically, the sawing window is long enough and affords adequate time
for the paving contractors to make a decision as to when to saw.
However, the combination of certain design, materials, and
weather-related factors can considerably shorten the window. In extreme
conditions, the window can be so short as to be impracticable for crack
control (ACPA, 2002b). Table 2-1 presents a listing of factors and the
way each factor influences the sawing window (Voigt, 2002), which can be
used as a basis for making adjustments to the sawing operation as
necessary.

![](media/image19.wmf){width="4.450694444444444in"
height="3.265972222222222in"}

Figure 2-4. Joint sawing window of opportunity (Okamoto et al., 1991;
ACPA, 1994).

Table 2-1. Factors that shorten the sawing window (after Voigt, 2002).

+------------------------------------------------+---------------------+
| **Factor**                                     | **Affects**         |
+------------------------------------------------+---------------------+
| Weather-related factors                        |                     |
+------------------------------------------------+---------------------+
| Sudden temperature drop or rain shower         | End of Window       |
|                                                |                     |
| Sudden temperature rise                        | End of Window       |
|                                                |                     |
| High winds and low humidity                    | Start of Window     |
|                                                |                     |
| Cool temperatures & cloudy                     | End of Window       |
|                                                |                     |
| Hot temperatures & sunny                       | End of Window       |
+------------------------------------------------+---------------------+
| Base-related factors                           |                     |
+------------------------------------------------+---------------------+
| High friction between the underlying subbase   | End of Window       |
| and concrete slab                              |                     |
|                                                | End of Window       |
| Bond between the underlying subbase and        |                     |
| concrete slab                                  | Start of Window     |
|                                                |                     |
| Dry surface                                    | Start of Window     |
|                                                |                     |
| Porous aggregate subbase materials             |                     |
+------------------------------------------------+---------------------+
| PCC mixture-related factors                    |                     |
+------------------------------------------------+---------------------+
| Rapid early strength                           | End of Window       |
|                                                |                     |
| Slow early strength                            | Start of Window     |
|                                                |                     |
| Retarded set                                   | Start of Window     |
|                                                |                     |
| Coarse aggregate                               | Start and End of    |
|                                                | Window              |
+------------------------------------------------+---------------------+
| Miscellaneous factors                          |                     |
+------------------------------------------------+---------------------+
| Paving against or between existing lanes       | End of Window       |
|                                                |                     |
| Saw blade selection                            | Start of Window     |
|                                                | (False)^1^          |
| Delay in curing protection                     |                     |
|                                                | Start of Window     |
+------------------------------------------------+---------------------+

> ^1^ Inadequate equipment or blunt saw blades give a false indication
> regarding the start of the sawing window.

##### Depth of Sawcut

![](media/image7.jpeg){width="2.0in" height="1.2543307086614173in"}The
initial sawcut depth, along with the sawcut timing and the equipment
used, has a significant impact on the performance of the contraction
joint. The issue of sawcut depth is further aggravated when concrete is
placed over stabilized or permeable bases. When the PCC surface layer
bonds with the base, its effective thickness is increased and the depth
of the initial sawcut (which may otherwise be adequate) becomes
ineffective in forcing a controlled crack at the desired locations,
thereby increasing the likelihood of random cracking. Therefore, for
rigid airfield pavements constructed on stabilized and permeable bases,
an initial sawcut depth of one-third the PCC thickness should be
employed for sawing longitudinal and transverse contraction joints to
decrease the potential for random, uncontrolled cracking. The use of
early entry saws can also minimize the potential for uncontrolled
cracking provided the deepest initial sawcut that is practical is made.

#### 2.4.3.4 Bond Breakers

![](media/image7.jpeg){width="2.4326771653543307in" height="1.6in"}A
variety of bond breakers are available to reduce the friction between
the PCC slab and base layer. The most effective bond breaker for a CTB,
LCB, and CTPB layer is a thin layer of choke stone[^2] broadcast over
its surface just prior to the placement of the PCC layer. In addition to
being a bond breaker, the choke stone, in the case of the CTPB, also
prevents interlocking of the PCC layer due to paste penetration. For ATB
and ATPB layers, no bond breakers are necessary since the lower
stiffnesses of these layers mitigate the impact of frictional restraint.
The only exception is when large areas (\> 15 yd^2^ \[12.5 m^2^\]) of
the ATB layer are milled to achieve grade tolerance. In such an event, a
single-coat of asphalt emulsion should be applied to the milled areas to
reduce friction.

Conventional bond breakers, such as asphalt emulsion and wax-based
liquid membrane forming curing compound (LMFCC), have variable success.
In some cases, these materials do not break the bond between the PCC
surface and the base layer at all. Figure 2-5 shows photos of slab
liftoff tests that were performed to verify the effectiveness of the
conventional bond breakers, such as asphalt emulsion and double-coat of
wax-based LMFCC on CTB.

+-----------------------------------+-----------------------------------+
| ![](media/image20.j               | ![](med                           |
| peg){width="2.1326388888888888in" | ia/image21.wmf){width="2.04375in" |
| height="1.5916666666666666in"}    | height="1.5590277777777777in"}    |
|                                   |                                   |
| \(a\) Slab liftoff -- PCC/asphalt | \(b\) Hole resulting from slab    |
| emulsion bond breaker/CTB         | liftoff -- CTB with asphalt       |
|                                   | emulsion bond breaker.            |
+-----------------------------------+-----------------------------------+
| ![](media/image22                 | ![](media/image23.                |
| .wmf){width="2.084722222222222in" | wmf){width="1.9979166666666666in" |
| height="1.5673611111111112in"}    | height="1.507638888888889in"}     |
|                                   |                                   |
| \(a\) Slab liftoff -- PCC/wax     | \(b\) Hole resulting from slab    |
| base LMFCC bond breaker/CTB       | liftoff -- CTB with wax-based     |
|                                   | LMFCC bond breaker.               |
+-----------------------------------+-----------------------------------+

Figure 2-5. Slab liftoff tests with conventional curing compounds.

#### 2.4.3.5 Curing

Curing is the maintenance of adequate moisture and temperature regimes
in freshly placed concrete (Kohn et al., 2003). Curing is essential for
proper hydration, reduction of the potential for shrinkage, reduction in
built-in thermal and moisture gradients, etc., and it has an impact on
long-term performance. When PCC layers are placed over stabilized or
permeable bases, the impact of shrinkage and curling and warping
gradients due to improper curing will be exaggerated. Therefore, special
care should be taken to ensure that adequate curing is provided to the
PCC layers in the presence of certain stabilized and permeable bases
(namely, CTB, LCB, and CTPB). As far as curing is concerned, enforcement
of specifications seems to be a bigger factor than guidance.

## 2.5 Interaction Between Triggers and Variants

As stated earlier, both triggers and variants need to be present to
cause early-age cracking. It is the interaction between the
trigger-induced deformations with various design, materials, and
construction variants on any given construction project. Table 2-2
presents an overview of the relationship between two key trigger
conditions, the variants that combine with them to cause early-age
cracking, and the modes of cracking resulting from this combination.
This table helps illustrate the "cause and effect" relationship that
exists between triggers and variants.

Table 2-2. Effect of triggers and variants on pavement responses and
early-age cracking.

+------------+--------------------+-----------------------------------+
| **Trigger  | **Effect on        | **Aggravating Variants and        |
| Factor**   | Pavement Response  | Interactions**                    |
|            | & Potential        |                                   |
|            | Distress Modes**   |                                   |
+------------+--------------------+-----------------------------------+
| Large      | -   Imposes a      | -   Late sawing or inadequate     |
| t          |     negative       |     sawcut depth.                 |
| emperature |     thermal        |                                   |
| dr         |     gradient       | -   Long PCC slab panels or high  |
| op-induced |     though the     |     slab aspect ratios.           |
| thermal    |     slab (top      |                                   |
| shock      |     cooler than    | -   Very thick or stiff base.     |
| caused by  |     bottom).       |                                   |
| an         |                    | -   Improper timing of PCC        |
| a          | -   If the slab is |     placement with respect to the |
| pproaching |     sufficiently   |     timing of thermal shock       |
| cold front |     hardened, this |     (e.g., placing it when the    |
| or a       |     can lead to    |     heat of hydration is maximum  |
| s          |     tensile        |     at the time of steepest temp. |
| ignificant |     stresses at    |     drop).                        |
| rain/snow  |     the top of the |                                   |
| event.     |     slab and a     | -   Excessive restraint at the    |
|            |     potential for  |     slab/base interface.          |
|            |     top-down       |                                   |
|            |     cracking.      | -   Inadequate planning or        |
|            |                    |     execution of cold weather     |
|            |                    |     paving plans.                 |
+------------+--------------------+-----------------------------------+
| Hot        | -   Causes         | -   Hot concrete temperatures (\> |
| weather    |     excessive      |     85°F \[29°C\]).               |
| paving     |     drying         |                                   |
| conditions |     shrinkage      | -   Inadequate or late curing.    |
| caused by  |     through the    |                                   |
| high       |     slab leading   | -   Late sawing or inadequate     |
| ambient    |     to warping and |     sawcut depth.                 |
| tem        |     axial          |                                   |
| peratures, |     deformations.  | -   Excessive restraint at the    |
| high solar |                    |     slab/base interface.          |
| radiation, | -   The effect of  |                                   |
| low        |     drying         | -   High cement factor concrete   |
| relative   |     shrinkage is   |     without supplementary         |
| humidity,  |     similar to     |     admixtures.                   |
| and high   |     that of a      |                                   |
| wind       |     negative       | -   Shrinkage susceptible PCC     |
| speeds.    |     thermal        |     mixture.                      |
|            |     gradient.      |                                   |
|            |     Axial          | -   Certain types of chemical     |
|            |     deformations   |     admixtures (e.g., high-range  |
|            |     cause stress   |     water reducers).              |
|            |     build-up at    |                                   |
|            |     locations of   | -   Placing PCC in a way that the |
|            |     restraint      |     maximum heat from hydration   |
|            |     (e.g.,         |     occurs during the hottest     |
|            |     slab/base      |     part of the day.              |
|            |     interface, tie |                                   |
|            |     bars).         | -   Placing PCC on a hot base     |
|            |     Cracking can   |     layer.                        |
|            |     be of any      |                                   |
|            |     orientation    | -   Inadequate planning/execution |
|            |     depending on   |     of hot weather paving plans.  |
|            |     variants       |                                   |
|            |     present.       |                                   |
+------------+--------------------+-----------------------------------+

Each trigger has a threshold value that, when exceeded, contributes to
the risk of premature cracking. It is important to note that if the
trigger factors do not exceed their thresholds, the chances of premature
cracking are minimal, regardless of the alignment of parameters. For
this reason, proper planning and execution of the construction to
anticipate and account for adverse climate conditions is the first and
foremost defense against early-age cracking. However, since it is
difficult to always exercise direct control over climatic factors, a
good way to build insurance against early-age cracking is to control the
variants.

The ranking of the key variants that have the most influence on the
development of early-age cracking is as follows (ranked in decreasing
order of importance):

-   Base strength/stiffness.

-   Sawing.

-   Panel sizes and aspect ratios.

-   PCC/base interface friction.

-   PCC cement factor.

-   Presence or absence of bond-breaker.

-   PCC curing.

-   Shrinkage susceptibility of PCC mixes.

-   Base thickness.

-   Presence of shrinkage cracking in base.

-   Internal slab restraint (dowel bars, tie bars, etc.).

Figure 2-6 summarizes the various triggers and their threshold values,
and quantifies the risk of early cracking as combination of these
factors. Using this figure, the risk of early cracking for any given
project can be assessed and mitigated. Note that the presence of one
trigger factor is adequate to cause early-age cracking. However, the
greater the number the triggers that are unfavorably aligned, the higher
the risk of early cracking. Also, as the number of trigger factors
increases, fewer variants need to be unfavorably aligned for the risk of
early cracking to increase.

As suggested in Figure 2-6, the value of favorably aligning multiple
variants to mitigate the risk of premature cracking over stabilized
bases is demonstrated in Example Scenario 7. In this case study, the
impact of variants, such as the panel size, PCC slab/base interface
friction, and stabilized base stiffness on the tensile stresses
developed in PCC slabs, is illustrated.

Figure 2-6. Quantification of risk of early cracking in rigid airfield
pavement built over stabilized or permeable bases.

## 

+-----------------------------------------------------------------------+
| ***Example Scenario 7: Effect of Controlling Multiple Variants on     |
| Early-Age Cracking Risk***                                            |
|                                                                       |
| The effect of panel size, base friction, and base stiffness on        |
| tensile stress development in PCC slabs in the first 72 hours after   |
| placement is demonstrated in this example. The details of the         |
| pavement structure analyzed are shown in the figure. The PCC modulus  |
| of elasticity, E~PCC~, was estimated at different ages based on the   |
| compressive strength given in Example Scenario 5. The loading is also |
| similar to that assumed in Example Scenario 5.                        |
|                                                                       |
| The maximum tensile stresses developed at the top of the slab for     |
| various combinations of slab sizes, friction, and stiffness, are      |
| reported during the first 72 hours of the pavement's life in the      |
| figure below along with a projection of PCC tensile strength. Stress  |
| calculations were performed using ISLAB2000.                          |
|                                                                       |
| ![](media/image24.wmf){width="5.958333333333333in"                    |
| height="2.967361111111111in"}                                         |
|                                                                       |
| The curve representing the 25 ft x 25 ft (7.6 m x 7.6 m) slabs built  |
| on a stiff base layers with a high degree of friction develops the    |
| highest tensile stresses due to the application of the thermal shock. |
| As some of the variants are brought within acceptable ranges with     |
| their desired threshold parameters values, the stresses developed     |
| begin to drop. However, the lowest developed stresses, and therefore  |
| the least risk of cracking at an early age is for the scenario where  |
| all three variants (i.e., panel size, interface friction, and base    |
| stiffness) are within the guidance suggested.                         |
+-----------------------------------------------------------------------+

## 2.6 Base Type Selection

### 2.6.1 Stabilized Bases

#### 2.6.1.1 Cement-Treated Base

CTB has several advantages that make it a good choice for a base under
rigid airfield pavements. For example, it offers considerable resistance
to erosion and pumping, improves load transfer efficiency, and improves
foundation support (increased *k*-value) and thereby the load carrying
capacity of the pavement structure. From a construction standpoint,
these materials can be processed and placed easily and allow for the
construction of the overlying pavement layers relatively quickly. CTB
can improve the long-term performance of pavements.

One of the highest incidences of early-age cracking in rigid airfield
pavements in recent years has been found on pavements that include a
CTB. The reasons cited include excessive stiffness and high friction. To
realize the long-term benefits of this material and to mitigate the
risks of early-age cracking, the guidance provided in this chapter with
regard to base related variants can be utilized during design and
construction. Other non-base related variants should also be controlled;
specifically, the bond breaker, PCC joint spacing, and PCC slab aspect
ratio. Generally, avoiding bases with excessive thickness and stiffness,
using shorter PCC joint spacing, more favorable PCC slab aspect ratios,
and an effective bond breaker is preferred when using this base type.

#### 2.6.1.2 Lean Concrete Base

The advantages LCB offers to the pavement structure are similar to those
offered by CTB. They have been successfully incorporated in rigid
airfield pavement structures albeit to a lesser extent than CTB layers.
From a construction standpoint, LCB layers are more cumbersome to place
than CTB layers since they generally require slipform equipment and a
longer curing period.

Like CTB, LCB elevates the risk of early-age cracking in rigid airfield
pavements. Among the primary reasons are the excessive stiffness of the
base and the high friction at the PCC slab/base interface. The friction
is particularly an issue with this layer since, unlike CTB, finishing
generally is not performed on these layers and the aggregate structure
can be relatively coarse in some instances. To realize the long-term
benefits of this material and to mitigate the risks of early-age
cracking, LCB thickness and LCB stiffness should not be excessive. LCB
layers may also be notched to prevent uncontrolled cracking in the LCB.
Other variants similar to those discussed for CTB should also be
controlled.

#### 2.6.1.3 Asphalt Concrete Base

ATB layers offer the same advantages as a CTB or LCB layer in terms of
erosion resistance, improved joint load transfer, and improved
foundation support. The key difference is that the stiffness of ATB
layer is, on average, lower than that of the CTB and the LCB layers. The
PCC slab/base interface friction is not an issue with ATB since a smooth
surface texture can be obtained as a product of construction. Further,
the lower stiffness of the layer offers less restraint to axial and
curling deformations. Therefore, a bond breaker is not required unless
the surface of the layer is milled for grade control.

The lower stiffness of the ATB layer also allows it to conform to the
shape of the slab and provide more uniform support. From a construction
standpoint, ATB layers are easy to place and can expedite the pavement
construction schedule. There are relatively fewer instances of reported
early-age cracking in rigid airfield pavements constructed over ATB
layers when compared to CTB and LCB layers.

There are relatively few disadvantages of using ATB layers. A
significant issue that needs attention during construction of ATB in hot
weather is managing the surface temperatures prior to concrete
placement. A drawback is that a mobilization cost for the asphalt paving
operation will be required.

### 2.6.2 Permeable Bases

#### 2.6.2.1 Cement-Treated Permeable Base

Due to the open-graded nature of the CTPB mixtures, these layers tend to
be weaker than the stabilized bases. However, they must be constructed
strong enough to withstand construction traffic without deformation.
They improve the durability of PCC layers (e.g., D-cracking) because of
positive drainage and provide reasonably uniform support to the surface
layer. The primary purpose of these layers, however, is to provide
positive drainage. Hence, they should be used only when there is a need
for drainage (e.g., high rainfall, perched water table, low permeability
subgrade soils, heavy traffic). If designed and constructed properly,
CTPB layers can improve the long-term performance of pavements. CTPB is
being used successfully in rigid airfield pavement structures,
particularly in the mid-western United States.

CTPB layers do suffer from some of the issues that CTB and LCB layers
do. A significant issue is the considerable surface penetration of PCC
paste into the void structure of these bases. This penetration increases
the PCC slab/base interface restraint and also the effective thickness
of the PCC slab. Also, since a CTPB layer is inherently weak, its
thickness should be restricted to a maximum value so that the stability
of the structure is not compromised. Segregation during placement can be
a significant issue with this layer.

To realize the long-term benefits of CTPB and to mitigate the risks of
early-age cracking, the guidance provided in this Guide with regard to
base-related variants such as thickness, the need for drainability with
permeability, aggregate gradation, and durability, should be referenced.
Other variants---such as use of an appropriate bond breaker, PCC joint
spacing, PCC slab aspect ratio, and non-shrinkage susceptible PCC
mixtures---should also be controlled.

#### 2.6.2.2 Asphalt-Treated Permeable Base

With the exception of ATPB layers being generally weaker (less stiff)
than CTPB layers, they offer a similar set of advantages when used in
rigid airfield pavement structures. Because they are less stiff, the
restraint created from PCC cement paste penetration at the surface of
the ATPB is not as great. Thus, ATPB layers are able to accommodate more
unfavorably aligned variants. A bond breaker is not considered essential
for this base type.

Just as with CTPB, the primary purpose of an ATPB is to provide positive
drainage. Significant concerns when using ATPB include the overheating
of the base in hot-weather paving conditions (similar to ATB) and
stripping and stability problems. Also, similar to ATB, the use of an
ATPB adds an extra mobilization cost.

To realize the long-term benefits of this material and mitigate the
risks of early-age cracking, the guidance provided in this Guide with
regard to base-related variants such as thickness, balancing the need
for drainability with permeability, aggregate gradation, ATPB surface
temperature management, and durability, should be referenced during
design and construction. Other non-base related variants should also be
controlled particularly PCC joint spacing and PCC slab aspect ratio.

### 2.6.3 Decision Making -- Base Type Selection 

The selection of a base type for a given project should be based on (1)
the intended role of the base layer within the pavement structure, (2)
its effect on both short- and long-term performance, (3) economics, and
(4) local experience.

The functions of the base layer in a rigid pavement system were
previously defined, and the designer should keep these in mind when
selecting a base type. For example, permeable bases should be used only
when there is a need for drainage in the first place. Generally
speaking, if there is adequate vertical drainage (i.e., subgrade
permeability is greater than 10 ft/day \[3 m/day\]), permeable bases are
not needed. Similarly, one of the main functions of a base layer in a
rigid pavement system is to provide a uniform and non-erodible support.
This is often misinterpreted as a "strong" support. An LCB layer is not
necessary when an ATB layer can do the job.

During the selection process, the performance of the base type should
also be weighted properly. Since a majority of the base types discussed
in this Guide are viable candidates for inclusion in a rigid pavement
structure from a long-term performance standpoint, their impact on
short-term performance should be evaluated thoroughly. Certain types of
bases, such as the CTB and LCB, bring a greater inherent "risk" of
early-age cracking in certain construction seasons when compared to ATB.
If local design guidance, policy, and/or materials permit accommodation
of an ATB without compromising long-term pavement performance, it should
be given preference. If a CTB or LCB is used, it is advisable to
thoroughly understand the risks involved so that they can be mitigated
during the design and construction process. The intent here is not to
discourage the use of any given base type. Rather, recognition of the
issues involved will help incorporate good design and construction in a
more effective manner.

Availability of materials, equipment, and crew to place the material of
choice dictates if a given base can be used on a given project. Local
experience with a given base type should be used to guide the final
selection.

# 

**THIS PAGE INTENTIONALLY LEFT BLANK**

# CHAPTER 3. materials selection and mix design

## 3.1 Base Layer Materials

This section describes the materials, mix proportioning requirements,
and attainable properties of CTB, LCB, ATB, CTPB, and ATPB layers.

### 3.1.1 Cement-Treated Base

CTBs are composed of mineral aggregate and cementitious materials
uniformly blended and mixed with water. They are generally of a higher
quality than a soil-cement base (Item P-301) and are produced in a
central mixing plant. Since the consistency of CTB material is on the
drier side, materials need to be compacted with rollers after placement,
usually with a paver.

#### 3.1.1.1 Materials for CTB 

##### Aggregate Materials

CTB includes select aggregate material comprised of crushed or uncrushed
gravel and/or stone or recycled crushed and graded PCC. The use of hard,
durable particles of accepted quality, free from an excess of soft,
flat, elongated, or disintegrated pieces, and matter deleterious to
reaction with cement is a principal requirement for strength and
durability.

The aggregate used should conform to one of the two gradations shown in
Table 3-1. The final aggregate blend should be well graded from coarse
to fine within the limits designated in the table and should not vary
from the low limit on one sieve to the high limit on adjacent sieves, or
vice versa, within the gradation bands. The portion of final aggregate
blend passing the No. 40 (425-µm) sieve should have a liquid limit less
than 25 and a plasticity index less than 6.

#### 

Table 3-1. Aggregate gradation for CTB material.

  ----------------------- ----------------------- -----------------------
  **Sieve Size**          **Percentage by Weight  
                          Passing Sieves**        

                          **Gradation A**         **Gradation B**

  2 in (51 mm)            100                     100

  No. 4 (4.75 mm)         45 - 100                55 - 100

  No. 10 (1.80 mm)        37 - 80                 45 - 100

  No. 40 (450 μm)         15 - 50                 25 - 80

  No. 80 (210 μm)         0 - 25                  10 - 35
  ----------------------- ----------------------- -----------------------

![](media/image25.jpeg){width="1.7751968503937008in"
height="1.3665354330708661in"}

When the CTB is placed under a PCC surface course, the maximum size of
the aggregate should be restricted to 1 in (25 mm) to permit accurate
grading of the base course and to ensure that a closed texture can be
obtained by compaction.

The percentage of wear of the crushed aggregate retained on the No. 4
(4.75 mm) sieve is restricted to 40 percent. The sodium sulfate
soundness loss of this material should not exceed 10 percent, or the
magnesium sulfate soundness loss should not exceed 13 percent, after
five cycles. Additionally, all crushed material to be used in CTB should
be evaluated and tested for alkali-aggregate reactivity using ASTM C
1260. The test results should have a measured expansion equal to or less
than 0.10 percent in 16 days.

If recycled crushed and graded PCC are used, they should meet the
requirements for virgin aggregate for gradation, LA abrasion resistance,
and sulfate soundness. Guidance is available in IPRF Report
01-G-002-03-5.

##### Cementitious Materials 

##### *Several types of portland or blended hydraulic cements may be used to produce CTB. These include the following:*

-   ASTM C 150 Type I, II, III, IV, or V. Type II cements can be
    specified in areas with a history of sulfate reaction with the
    selected aggregate.

-   ASTM C 595 Type IS, IS-A, IP, IP-A, P, or PA.

In addition, certain pozzolanic materials and ground granulated blast
furnace (GGBF) slag may also be added to the CTB mix as a partial
replacement for cement. Pozzolanic materials should meet the
requirements of ASTM C 618, Class C, F, or N with the exception of loss
of ignition, where the maximum must be less than 6 percent for Class F
or N. GGBF slag should conform to ASTM C 989, Grade 80, 100, or 120.

Typical cement contents used in CTB are between 4 and 5 percent by
weight. The final cement content used should be based on the mix
proportioning requirements.

##### Water

Water used in CTB mixture production and curing should be clean and free
of oil, salt, acid, alkali, sugar, vegetable, or other deleterious
substances injurious to the finished product. Water known to be of
potable quality may be used without testing. Seawater has also been
successfully used to produce CTB when fresh water was not available
(PCA, 1995).

#### 3.1.1.2 CTB Mix Design Requirements

![](media/image25.jpeg){width="2.33740157480315in" height="1.8in"}

The mix proportioning of CTB can be based on strength criteria alone.
Freeze-thaw and wet-dry durability testing should be restricted to areas
where past experience has demonstrated that the long-term durability of
the CTB layer and long-term performance of the pavement is compromised
by wet-dry and/or freeze-thaw of base materials. When wet-dry and
freeze-thaw durability testing is required, the weight loss for each
type of test must not exceed 14 percent after 12 cycles. However, if a
7-day compressive strength of 750 psi (5,171 kPa) is achieved, the
wet-dry and freeze-thaw tests are not necessary. When considering
durability testing, it should be noted that base materials are subject
to lesser freeze-thaw/ wet-dry cycles than pavement surfaces.

The CTB mixture to be placed under a PCC surface layer should utilize a
cement content that, when tested in the laboratory in accordance with
ASTM D 1633, produces a 7-day compressive strength meeting the following
requirements:

-   A minimum 7-day compressive strength of 500 psi (3,448 kPa).

-   A maximum 7-day compressive strength of 1,000 psi (6,895 kPa).

An estimate of the cement mixture proportioning is determined from table
1, chapter 2, of the *Soil-Cement Laboratory Handbook*, published by the
Portland Cement Association (1971). In designing the CTB mixture, cement
contents above and below the initial estimated amount should be tested
to determine the minimum quantity of cement needed to achieve the
required strength (or strength and durability where freeze-thaw
resistance is deemed necessary).

![](media/image25.jpeg){width="1.683464566929134in"
height="1.2956692913385828in"}

Construction and other traffic can be allowed on the CTB layer when a
minimum compressive strength of 350 psi (2,413 kPa) is achieved. The
laboratory mix proportioning study can be used to estimate the time
required for strength to be achieved. Figure 3-1 shows proof rolling of
a CTB layer (with approximately 5 percent cement content) using a
track-type asphalt paver 2 days after placement. No significant
deformations were noticed on the CTB (right side paver tracks in Figure
3-1 are on CTB) when compared to the compacted granular material (left
side paver tracks).

![](media/image26.wmf){width="4.251388888888889in"
height="3.323611111111111in"}

Figure 3-1. Application of construction traffic to a 2-day old CTB
surface.

### 3.1.2 Lean Concrete Base

Lean concrete consists of aggregate and cementitious materials uniformly
blended together and mixed with water. The term econocrete, sometimes
used interchangeably with lean concrete, implies that the materials used
in producing this material are of marginal quality when compared to
those used to produce conventional concrete. The material is
manufactured in a central mixing plant and usually contains 200 to 300
pounds of portland cement per cubic yard. This material is slip formed
or placed using side forms just as PCC layers.

#### 3.1.2.1 Materials for LCB 

##### Aggregate Materials

![](media/image25.jpeg){width="2.6838582677165355in"
height="2.0665354330708663in"}

The use of lean concrete is usually proposed to overcome shortages in
the supply of quality aggregates and to be able to use local materials
and still produce a high-quality paving layer. A wide range of
aggregates can be used in the production of lean concrete. The coarse
aggregate fraction in lean concrete can include crushed stone, crushed
or uncrushed gravel, crushed recycled PCC pavement, or a combination
thereof. The fine aggregate fraction may be part of the natural
aggregate blend as obtained from the borrow source or it may be natural
sand added at the mixer.

The aggregate should be hard, durable particles, free from an excess of
flat, elongated, soft, or disintegrated pieces, or other deleterious
matter that inhibits reaction with cement. The aggregate blend should
conform to one of the gradations shown in Table 3-2.

Table 3-2. Aggregate gradation for LCB material.

  ----------------------- ----------------------- -----------------------
  **Sieve Size**          **Percentage by Weight  
                          Passing Sieves**        

                          **1½ in (38 mm)         **1 in (25 mm)
                          Maximum**               Maximum**

  2 in (51 mm)            --                      --

  1½ in (38 mm)           100                     --

  1 in (25 mm)            70 - 95                 100

  ¾ in (19 mm)            55 - 85                 70 - 100

  No. 4 (4.75 mm)         30 - 60                 35 - 65

  No. 40 (425 µm)         10 - 30                 15 - 30

  No. 200 (75 µm)         0 - 15                  0 -15
  ----------------------- ----------------------- -----------------------

![](media/image25.jpeg){width="2.0062992125984254in"
height="1.353543307086614in"}

Aggregate proposed to be used in lean concrete should be tested for
alkali-aggregate reactivity using ASTM C 1260. The test data should have
an expansion less than 0.10 percent at 16 days.

##### Cementitious Materials

The types of portland and blended hydraulic cements allowed for the
production of CTB can also be used to produce lean concrete. Further,
pozzolanic materials and GGBF slag may be added to the lean concrete mix
as a partial replacement for cement. The pozzolanic materials allowed in
lean concrete must, however, meet the requirements of ASTM C 618, Class
F fly ash. Allowable slag types are similar to those proposed for CTB.

##### Water

Water used in LCB mixture production and curing should be clean and free
of oil, salt, acid, alkali, sugar, vegetable, or other deleterious
substances injurious to the finished product. Water known to be of
potable quality may be used without testing.

#### 3.1.2.2 LCB Mix Proportioning Requirements

The mix proportioning of LCB should be based on strength requirements.
Workability and compatibility with placement-related issues must be
acknowledged. Freeze-thaw durability testing should not be used unless
past experience has demonstrated that the long-term durability of the
LCB layer and long-term pavement performance is compromised by
freeze-thaw cycles.

![](media/image25.jpeg){width="2.802755905511811in"
height="2.158267716535433in"}Cement contents used in LCB are greater
than those used in CTB but less than that used in PCC mixes. Typical
cement contents range from 6 to 10 percent of the LCB mix (roughly 2 to
3 bags of cementitious material) by weight. The final cement content
used should be based on the mix design requirements.

The LCB mixture should include enough cementitious material to obtain a
compressive strength not less than 500 psi (3,448 kPa) nor greater than
800 psi (5,516 kPa) at 7 days, when tested in accordance with ASTM C 192
and ASTM C 39. If the maximum strength is exceeded, the LCB should be
notched to match planned joints in the PCC layer. Alternatively, a new
mix design that satisfies the strength requirements can be proportioned.

When freeze-thaw durability testing is performed, it should be performed
in accordance with ASTM D 560. The weight loss for each type of test
must not exceed 14 percent after 12 cycles. However, if a 7-day
compressive strength of 750 psi (5,171 kPa) is achieved, the wet-dry and
freeze-thaw tests are not necessary. When considering durability
testing, it should be noted that base materials are subject to lesser
freeze-thaw/ wet-dry cycles than pavement surfaces.

The water-to-cementitious material ratio is typically higher for LCB
than for conventional concrete and is generally around 1.0 or higher.
The percentage of air entrainment in LCB is usually at 6 percent to
promote workability. The use of fly ash, water reducing admixtures,
extra fines, workability agents, or a combination thereof can also be
used.

Similar to CTB, construction and other traffic can be allowed on the LCB
layer when a minimum compressive strength of 350 psi (2,413 kPa) is
achieved.

### 3.1.3 Asphalt-Treated Base

ATB material consists of a bituminous plant mix composed of a mixture of
well graded aggregate, filler, anti-stripping agent if required, and
bituminous material.

#### 3.1.3.1 Materials for ATB

##### Aggregate Materials

The quality of coarse and fine aggregate used in the ATB mixes is
similar to that of a high-quality HMA layer. Aggregates used in the ATB
typically consist of crushed stone, crushed gravel, or crushed slag with
or without natural sand or other inert finely divided mineral aggregate.
Normal weight, air cooled, blast furnace slag can be used.

![](media/image25.jpeg){width="1.800787401574803in"
height="1.2153543307086614in"}

Recycled HMA can consist of reclaimed asphalt pavement (RAP), coarse
aggregate, fine aggregate, mineral filler, and asphalt cement. The RAP
should be of a consistent gradation and asphalt content. The amount of
RAP is limited to 30 percent for base courses as long as the resulting
recycled mix meets all requirements that are specified for virgin
mixtures.

The gradation or gradations of the mineral aggregates for any given
project should conform to the requirements given in Table 3-3. The
selected aggregate from within the ranges suggested should be well
graded from coarse to fine aggregate and should not vary from the low
limit on one sieve to the high limit on the adjacent sieve, or vice
versa.

Table 3-3. Aggregate gradations for ATB material.

+--------------------+-----------+-----------+-------------+-----------+
| **Sieve Size**     | **P       |           |             |           |
|                    | ercentage |           |             |           |
|                    | by Weight |           |             |           |
|                    | Passing   |           |             |           |
|                    | Sieves**  |           |             |           |
+--------------------+-----------+-----------+-------------+-----------+
|                    | **1½ in** | **1 in**  | **¾ in**    | **½ in**  |
|                    |           |           |             |           |
|                    | **(38 mm) | **(25 mm) | **(19 mm)   | **(13 mm) |
|                    | max**     | max**     | max**       | max**     |
+--------------------+-----------+-----------+-------------+-----------+
| 1‑½ in. (38 mm)    | 100       | --        | --          | --        |
+--------------------+-----------+-----------+-------------+-----------+
| 1 in. (25 mm)      | 86 ‑ 98   | 100       | ‑‑          | --        |
+--------------------+-----------+-----------+-------------+-----------+
| ¾ in. (19 mm)      | 68 ‑ 93   | 76 ‑ 98   | 100         | --        |
+--------------------+-----------+-----------+-------------+-----------+
| ½ in. (13 mm)      | 57 ‑ 81   | 66 ‑ 86   | 79 ‑ 99     | 100       |
+--------------------+-----------+-----------+-------------+-----------+
| ⅜ in. (9.5 mm)     | 49 ‑ 69   | 57 ‑ 77   | 68 ‑ 88     | 79 ‑ 99   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 4 (4.75 mm)    | 34 ‑ 54   | 40 ‑ 60   | 48 ‑ 68     | 58 ‑ 78   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 8 (2.36 mm)    | 22 ‑ 42   | 26 ‑ 46   | 33 ‑ 53     | 39 ‑ 59   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 16 (1.18 mm)   | 13 ‑ 33   | 17 ‑ 37   | 20 ‑ 40     | 26 ‑ 46   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 30 (0.600 mm)  | 8 ‑ 24    | 11 ‑ 27   | 14 ‑ 30     | 19 ‑ 35   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 50 (0.300 mm)  | 6 ‑ 18    | 7 ‑ 19    | 9 ‑ 21      | 12 ‑ 24   |
+--------------------+-----------+-----------+-------------+-----------+
| No. 100 (0.150 mm) | 4 ‑ 12    | 6 ‑ 16    | 6 ‑ 16      | 7 ‑ 17    |
+--------------------+-----------+-----------+-------------+-----------+
| No. 200 (0.075 mm) | 3 ‑ 6     | 3 ‑ 6     | 3 ‑ 6       | 3 ‑ 6     |
+--------------------+-----------+-----------+-------------+-----------+
| Asphalt percent:   | 4.5 ‑ 7.0 | 4.5 ‑ 7.0 | 5.0 ‑ 7.5   | 5.5 ‑ 8.0 |
|                    |           |           |             |           |
| Stone or gravel    | 5.0 ‑ 7.5 | 5.0 ‑ 7.5 | 6.5 ‑ 9.5   | 7.0 ‑     |
|                    |           |           |             | 10.5      |
| Slag               |           |           |             |           |
+--------------------+-----------+-----------+-------------+-----------+

##### Bituminous Material

The asphalt cement used in the ATB layer can conform to either a
performance grade (PG), viscosity grade, or a penetration grade. For
optimum performance, the grade of the bituminous material should be
selected in accordance with geographical location and climatic
conditions. Where possible, the use of PG graded binders is encouraged.

##### Mineral Filler

If filler, in addition to that naturally present in the aggregate, is
necessary, it should meet the requirements of ASTM D 242.

##### Anti-stripping Agent

An anti-stripping agent may be needed for some mixes based on durability
requirements of the mix (tensile strength ratio). If needed, a heat
stable anti-stripping agent that does not modify the asphalt cement
viscosity beyond specifications is recommended for use.

#### 3.1.3.2 ATB Mix Design Requirements

The job mix formula (JMF) for ATB mixtures is currently established on
the basis of the Marshall method of mixture design. The mix design
criteria for a 1-in (25-mm) maximum size ATB mixture intended for use in
pavements designed for aircraft gross weights of 60,000 lbs (27,120 kg)
or more or tire pressures of 100 psi (690 kPa) or more are as follows:

-   Number of blows: 75

-   Stability, lbs: 1,800

-   Flow, 0.01 in: 8 - 16

-   Air voids, percent: 2 - 5

-   Voids in Mineral Aggregate (VMA), percent: See Table 3-4 (expressed
    as a function of maximum aggregate size).

Table 3-4. VMA as a function of maximum aggregate size.

  --------------------------------- -------------------------------------
  **Maximum Particle Size**         **Minimum VMA, percent**

  ½ in (13 mm)                      16

  ¾ in (19 mm)                      15

  1 in (25 mm)                      14

  1½ in (38 mm)                     13
  --------------------------------- -------------------------------------

An air void content of 4 percent is typically used to determine the
asphalt content based on the criteria above.

![](media/image25.jpeg){width="2.060236220472441in"
height="1.3901574803149606in"}

A mixture durability requirement in the form of a tensile strength ratio
(TSR) is also typically imposed. The TSR value should be greater than 75
(80 in areas with a history of stripping \[e.g., wet climates and local
aggregates that are not limestone or dolomite\]). An anti-stripping
agent can be added to the ATB mixture, as necessary, to meet the TSR
requirements.

### 3.1.4 Cement-Treated Permeable Base

A CTPB consists of durable, high-quality mineral aggregate with
negligible fines, blended with sufficient amounts of cement and water to
coat the particles uniformly. The gradation of a CTPB allows for a large
void structure so that moisture that seeps into the pavement is drained
away rapidly and efficiently.

#### 3.1.4.1 Materials for CTPB

![](media/image25.jpeg){width="2.504724409448819in"
height="1.6901574803149606in"}*Aggregate Materials*

The aggregate used in the CTPB should consist of clean, sound, hard,
durable, angular particles of crushed stone that meets the specification
requirements. The aggregate should be free from clay balls, organic
matter, and other deleterious substances. Guidance on deleterious
materials should be developed in accordance with ASTM C 33.

The aggregate should contain not more than 15 percent, by weight, of
flat or elongated pieces, as defined in ASTM D 693. The use of crushed
aggregate that has at least 90 percent by weight of particles with at
least two fractured faces and 100 percent with at least one fractured
face is highly encouraged. The percentage of wear of the crushed
aggregate retained on the No. 4 (4.75 mm) should not be greater than 50
percent. The sodium sulfate soundness loss should not exceed 10 percent,
or the magnesium sulfate soundness loss should not exceed 13 percent,
after five cycles, when tested in accordance with ASTM C 88.

The CTPB gradation should conform to one of the aggregate gradations
shown in Table 3-5 when tested in accordance with ASTM C 136. The
gradations can easily be obtained by blending aggregates of different
sizes. The permeability of the CTPB can be expected to increase slightly
from Gradation A to Gradation C.

Table 3-5. Aggregate gradation for CTPB material.

+--------------+-----------------+-----------------+-------------------+
| **Sieve      | **Percentage by |                 |                   |
| Size**       | Weight Passing  |                 |                   |
|              | Sieves**        |                 |                   |
+--------------+-----------------+-----------------+-------------------+
|              | **Gradation A** | **Gradation B** | **Gradation C**   |
|              |                 |                 |                   |
|              | **(¾ in \[19    | **(1 in \[25    | **(1½ in \[38     |
|              | mm\] max.)**    | mm\] max.)**    | mm\] max.)**      |
+--------------+-----------------+-----------------+-------------------+
| 1½ in (38    | --              | --              | 95 - 100          |
| mm)          |                 |                 |                   |
+--------------+-----------------+-----------------+-------------------+
| 1 in (25 mm) | --              | 95 - 100        | 72 - 82           |
+--------------+-----------------+-----------------+-------------------+
| ¾ in (19 mm) | 95 - 100        | 77 - 87         | 60 - 70           |
+--------------+-----------------+-----------------+-------------------+
| ½ in (13 mm) | 67 - 77         | 53 - 63         | 40 - 50           |
+--------------+-----------------+-----------------+-------------------+
| ⅜ in (9.5    | 50 - 60         | 41 - 51         | 30 - 40           |
| mm)          |                 |                 |                   |
+--------------+-----------------+-----------------+-------------------+
| No. 4 (4.75  | 19 - 29         | 15 - 25         | 10 - 20           |
| mm)          |                 |                 |                   |
+--------------+-----------------+-----------------+-------------------+
| No. 8 (2.36  | 0 - 6           | 0 - 6           | 0 - 6             |
| mm)          |                 |                 |                   |
+--------------+-----------------+-----------------+-------------------+

#### 3.1.4.2 CTPB Mix Proportioning Requirements

![](media/image25.jpeg){width="2.802755905511811in"
height="2.158267716535433in"}

The mix design establishes the percentage of dry weight of aggregate
passing each specified sieve size and the percentage of cement required
based upon the weight of the total mix.

The criterion to establish the amount of cement required for a selected
gradation from Table 3-5 is to have sufficient paste volume to
adequately coat the aggregate particles without creating excess that can
plug the pore structure. The recommended way to determine this is
through visual examination in the laboratory. It is recommended that the
mix proportioning contain about 250 lbs/yd^3^ (148 kg/m^3^) to meet this
requirement. The cement content can be varied from this minimum amount
to satisfy the desired mix characteristics.

![](media/image25.jpeg){width="2.7279527559055117in"
height="2.100787401574803in"}A mix prepared, placed, and compacted in
the field should have a permeability of not less than 500 ft/day (152
m/day) nor more than 1,500 ft/day (458 m/day) when tested in the
laboratory in accordance with ASTM D 2434/AASHTO T 215 (Constant Head
Permeability Test). The permeability requirements may be verified in the
field by taking cores from the test section. If the test section
permeability is significantly less than that stated above, a new mix
design with a coarser gradation or lower cement content may be needed.
If the test section permeability is greater than that stated above,
additional rolling may be permitted to close-up the void structure, as
long as such rolling does not crush the aggregates or deform the
surface. Alternatively, a mix design with a finer aggregate gradation
may be proposed.

### 3.1.5 Asphalt-Treated Permeable Base

ATPB consists of durable, high quality mineral aggregate with negligible
amount of fines. The aggregates are blended with an adequate amount of
bituminous material to coat the particles uniformly. An anti-stripping
agent should be used.

#### 3.1.5.1 Materials for ATPB

*Aggregate Materials*

The aggregate used in ATPB should consist of clean, sound, hard,
durable, angular particles of crushed stone. The aggregate should be
free from clay balls, organic matter, and other deleterious substances.

The aggregate should contain not more than 10 percent by weight of flat
or elongated pieces and should have at least 90 percent by weight of
particles with at least two fractured faces. The percentage of wear of
the crushed aggregate retained on the No. 4 (4.75 mm) sieve is
restricted to 40 percent when tested in accordance with ASTM C 131. The
sodium sulfate soundness or magnesium sulfate loss should be less than
10 and 13 percent, respectively, after five cycles.

![](media/image25.jpeg){width="1.6007874015748031in"
height="1.080708661417323in"}The quality requirements for the aggregates
used for ATPB are more stringent than those for CTPB, due to the
increased importance of aggregate interlock during construction and
resultant long-term performance of these layers.

The gradations proposed for the CTPB (see Table 3-5) can be used for
ATPB mixtures as well.

*Bituminous Materials*

The asphalt cement binder should be selected based on geographical
location and climatic conditions to improve layer performance. The
asphalt grade used for P-401 material for the project location or a
stiffer grade (to prevent drain down of the asphalt cement) is adequate.

*Anti-stripping Agent*

![](media/image25.jpeg){width="1.7980314960629922in"
height="1.213779527559055in"}

By design, ATPB allows water to drain through it. In such situations,
stripping of the asphalt is a major concern. Therefore, to improve the
durability of ATPB, an anti-stripping agent in the form of hydrated lime
is highly recommended, particularly if the coarse aggregate is not
limestone or dolomite. Lime should be added at a dosage rate of rate of
0.5 to 1.0 percent by weight.

#### 3.1.5.2 ATPB Mix Proportioning Requirements

The JMF establishes the percentage of dry weight of aggregate passing
each required sieve, a percentage of asphalt cement to be added, and a
temperature for the mixture.

The most important criterion is to establish the amount of asphalt
binder required for a selected gradation that provides adequate coating
of the aggregates and a permeability value between 500 and 1,500 ft/day
(152 and 458 m/day). To prepare the specimen for permeability testing,
the component aggregates are blended together, mixed with the specified
amount of asphalt cement at a temperature of 250°F (121°C) (note: higher
mixing temperatures may be required when modified asphalts are used),
and compacted on one-side at 150°F (66°C) with 35 blows of a standard
Marshall hammer. This level of compaction roughly correlates with field
compaction of ATPB layers. The JMF should have a minimum asphalt binder
content of 2.0 percent by weight, which can be adjusted upward to 3.5
percent to provide stability under rollers during construction and for
durability of the mix.

The permeability requirements may be verified in the field by taking
cores from the test section. If the test section permeability is
significantly less than that stated above, a new mix design with a
coarser gradation and/or lower asphalt content may be needed. If the
test section permeability is greater than that stated above, additional
rolling may be permitted to close-up the void structure as long as such
rolling does not crush the aggregates or deform the surface.
Alternatively, a new mix design with a finer aggregate gradation may be
proposed.

## ![](media/image25.jpeg){width="2.1188976377952757in" height="1.4307086614173228in"}3.2 Choke Stone Materials

### 3.2.1 Definition and Purpose

> A choke stone is a small-size stone layer (generally 0.5 to 1.0 in
> \[13 to 25 mm\] thick). It is used in conjunction with stabilized
> bases, such as CTB and LCB, to serve as a bond-breaking layer and with
> permeable bases, such as CTPB, to prevent excessive penetration of the
> PCC paste and to guard against yield loss of the PCC surface. The bond
> prevention qualities of the choke stone are superior to that of a
> wax-base liquid membrane forming curing compound or asphalt emulsion.
> These latter materials are commonly specified in project
> specifications as bond breakers but were found to be largely
> ineffective in a field trial conducted as part of FAA IPRF Project
> FAA-01-002-02-1.

### 3.2.2 Materials

> A choke stone course is comprised of crushed mineral aggregate. The
> aggregate should consist of clean, sound, hard, durable, angular
> particles of crushed stone free from clay balls, organic matter, and
> other deleterious substances.
>
> The aggregate should conform to the gradation shown in Table 3-6 or
> the gradation requirements for ASTM No. 89 stone.

Table 3-6. Aggregate gradation for choke stone material.

  ------------------------------- ---------------------------------------
  **Sieve Size**                  **Percentage by Weight Passing Sieves**

                                  **Choke Stone**

  ½ in (13 mm)                    100

  ⅜ in (9.5 mm)                   80 - 100

  No. 4 (4.75 mm)                 10 - 100

  No. 8 (2.36 mm)                 5 - 50

  No. 16 (1.18 mm)                0 - 10
  ------------------------------- ---------------------------------------

## 3.3 Separation Layer Materials

### 3.3.1 Definition and Purpose

A separation layer is the soil, fabric, or other material that is placed
between a permeable base and the underlying layer (subgrade or subbase)
to preserve the integrity of the permeable base and to improve the
efficiency of the entire drainage system. An effective separation layer
is required to maintain the design thickness and permeability of the
permeable base to enable it to perform efficiently.

As Figure 3-2 shows, the separation layer specifically (a) maintains
separation between the permeable base and subgrade, preventing them from
intermixing, and (b) forms an impermeable barrier that deflects water
from the permeable base horizontally toward the pavement edge. The
separation layer can also provide support to construction traffic when
an adequate construction platform has not been constructed. The
separation layer is not required to act as a filter (i.e., allowing
groundwater through, while holding back soil material). However, when
the drainage layer is placed to intercept ground water, a separation
layer will still be needed but it can be designed to act as a filter.

![](media/image27.wmf){width="4.6819444444444445in"
height="1.6736111111111112in"}

\(a\) Separation (Holtz et al., 1998).

![](media/image28.wmf){width="4.395138888888889in"
height="2.240972222222222in"}

\(b\) Facilitation of horizontal drainage (ERES, 1998).

![](media/image29.wmf){width="2.8409722222222222in"
height="1.4083333333333334in"}

\(c\) Construction support (FHWA, 1995).

Figure 3-2. Roles of a separation layer in a pavement system with a
permeable base.

### ![](media/image25.jpeg){width="2.6110236220472443in" height="2.010236220472441in"}3.3.2 Materials

Separation layers can be dense-graded aggregate, dense-graded HMA, or
geotextile materials having certain characteristics. Lime- or
cement-treated subgrades and cement-treated layers are not acceptable as
separation layers over fine-grained soils.

#### 3.3.2.1 Aggregate Separation Layers

Dense-graded aggregate layers can fulfill the requirements of a
separation layer. A minimum thickness of 4 in (102 mm) is recommended
for this layer. The thickness may be increased up to 8 in (203 mm) if
the layer is placed directly over the subgrade to meet design
requirements. The following physical properties are desirable of an
aggregate separation layer (FHWA 1994):

-   It should be a durable, crushed angular aggregate material. As a
    minimum, the aggregate should have at least two fractured faces, as
    determined by the material retained on the No. 4 (4.75 mm) sieve;
    preferably, it should consist of 98 percent crushed stone.

-   The percentage of wear of the crushed aggregate retained on the No.
    4 (4.75 mm) sieve is restricted to 50 percent when tested in
    accordance with ASTM C 131.

-   The sodium sulfate soundness loss should not exceed 10 percent, or
    the magnesium sulfate soundness loss should not exceed 13 percent,
    after five cycles, when tested in accordance with ASTM C 88.

-   A California Bearing Ratio (CBR) of 50 or greater.

-   The gradation should allow a maximum permeability of approximately
    15 ft/day (5 m/day).

The gradation of the aggregate separation layer is engineered to satisfy
uniformity and separation requirements at both its bottom interface
(subgrade/subbase) and top interface (permeable base). The aggregate
separation layer gradation design is a three-step process (ARA, 2004),
as described below.

*Step 1: Check for Aggregate Separation Layer--Subgrade/Subbase Layer
Interface Requirements*

The gradation of the separation layer must meet the interface
requirements listed below:

-   Separation requirement: D~15~ (separation layer) \< 5D~85~
    (subgrade/subbase layer).

-   Uniformity requirement: D~50~ (separation layer) \< 25D~50~
    (subgrade/subbase layer).

D~X~ represents the particle size that x percent of the material is
smaller than by weight.

*Step 2: Check for Aggregate Separation Layer--Permeable Base Interface
Requirements*

Similar requirements must be applied to the separation layer--permeable
base interface, as listed below.

-   Separation requirement: D~15~ (permeable base) \< 5D~85~ (separation
    layer).

-   Uniformity requirement: D~50~ (permeable base) \< 25D~50~
    (separation layer).

*Step 3: Other Requirements*

The following requirements ensure that the dense-graded aggregate
separation layer does not have too many fines and is well-graded:

-   Maximum percentage of material passing the No. 200 (75 µm) sieve
    should not exceed 12 percent, by weight.

-   Coefficient of uniformity (D~60~/D~10~) should be greater than 20;
    preferably greater than 40.

The first criterion limits the amount of fines in the aggregate
separation layer, and the second provides guidance for developing a
well-graded aggregate base.

The results of these checks are typically plotted on a gradation chart
to develop a design envelop through which the gradation of the aggregate
separation layer must pass. A sample plot of a gradation that satisfies
the design checks is shown in Figure 3-3. Also plotted in this figure is
the design envelop developed from the criteria discussed above for
sample permeable base and subgrade gradations.

An asphalt prime coat can be applied to the dense-graded separation
layer to reduce the risk of erosion of fines at its surface.

![](media/image30.wmf){width="4.447222222222222in"
height="2.9138888888888888in"}

Figure 3-3. Plot of design envelop superimposed on base, subgrade, and
the dense-graded aggregate gradations (ARA, 1999).

#### 3.3.2.2 Dense-Graded HMA Separation Layers

Dense-graded HMA mixtures, such as those specified in Item P-403 of FAA
AC 150/5370-10B, satisfy all the requirements of a separation layer.

#### 3.3.2.3 Geotextile Separation Layers

Geotextile separation layers are typically used when a construction
platform (e.g., a stabilized layer, a granular layer, or a subgrade with
a CBR greater than 10) exists. Both woven and non-woven geotextiles have
been used for the separation application.

The important design criteria to be considered when specifying the
properties of geotextile as a separator layer are divided into four
categories, namely:

-   Soil retention.

-   Permeability.

-   Clogging.

-   Survivability and endurance.

The design guidelines for the soil retention, permeability, and clogging
criteria are summarized in the flowchart in Figure 3-4. In addition to
these criteria, to ensure that the geotextile will survive the
construction process, certain strength and endurance properties are
required. Geotextile fabrics satisfying the AASHTO M 288 Survivability
Class 2 criteria (either woven or non-woven fabrics) tend to have
adequate strength and durability to survive both construction and
long-term use. A relatively heavy (weight-to-area ratio of 0.1 oz/ft^2^
\[30.5 gm/m^2^\]), non-woven geotextile is recommended for separation
layer applications.

![](media/image31.wmf){width="6.777777777777778in"
height="5.245833333333334in"}

> Figure 3-4. Flowchart summarizing the soil retention, permeability,
> and clogging criteria for selecting the properties of geotextile (ARA,
> 1999).

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# Chapter 4. Stabilized and Permeable Base Construction 

## 4.1 Pre-Construction Activities

### 4.1.1 Test Section Construction

![](media/image7.jpeg){width="1.6251968503937009in" height="1.25in"}

Depending on the type of material to be used on a given project, a test
section should be constructed to evaluate constructability issues
related to producing, hauling, placing, and finishing the material. Test
section construction should closely mimic the design details of the
project and the anticipated construction processes and conditions.

The test section should be of sufficient size to allow the contractor to
fully demonstrate the paving operation. The Engineer should be able to
fully evaluate each aspect of the construction work. The size best
represented is dual lane paving that extends for either (a) 500 ft (152
m) in length or (b) 1 hour of production, whichever transpires first.

Specific items that should be evaluated in the construction of the test
section are:

-   Pre-paving Inspection Activities

    -   Subgrade/subbase condition---Check grade, stability, general
        surface conditions.

    -   Grade control adequacy---Check grade stakes, pins, forms to be
        used in placing base material.

    -   Equipment---Check adequacy of equipment used to mix, haul,
        place, compact, and finish the material.

-   Mix Production Inspection Activities

    -   Conformity of mix with approved job mix formula (JMF)/mix
        design.

    -   Mix uniformity---Check for consistency within and among mix
        batches and throughout continuous mix production. Check for
        improper sequencing of materials, improper feed/metering rates,
        inadequate mixing times, changes in stockpile moisture.

    -   Mix temperature (for asphalt-treated base materials and
        cement-treated base materials that require heating).

    -   Production rate adequacy---Check for acceptable stockpile
        management plans, efficient loading operations, acceptable
        mixing times based on mix uniformity testing.

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-   Mix Hauling Inspection Activities

    -   For cement-treated base materials, check time between start of
        moist mixing and delivery of the material on-site to verify that
        the material can be produced, transported, and delivered within
        specified time limits (typically 30 minutes for non-agitating
        trucks and 45 minutes for transit mixers).

    -   For asphalt-treated base materials, check temperature of
        delivered material to verify that the material can be produced,
        transported, and delivered within the specified temperature
        limits (typically 250°F \[121°C\] minimum for non-permeable
        mixes and between 200 and 250°F \[93 and 121°C\] for permeable
        mixes).

    -   For all materials, make sure there is adequate equipment to
        prevent stop-and-go paving.

    -   For all materials, material transfer devices or dumping
        procedures ensure that segregation is not occurring.

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-   Mix Placement Inspection Activities

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-   Spreading and laydown of material---Check for damage to separation
    layer (if used) caused by trucks and/or laydown equipment, adequacy
    of spreading across paving width, adequacy of coating of aggregate
    particles by cement paste or asphalt binder, mix segregation, proper
    depth of placement taking into account roll-down, speed of laydown.

-   Compaction and finishing of material---Check for stability (firm and
    unyielding), surface texture (uniform, with no crushing of
    aggregate), and depth (using probes, string lines and ruler) during
    rolling. Test for surface tolerance, grade, mat and joint density
    (note: nuclear gauge is unsuitable for use on permeable bases), and
    permeability of permeable bases (1 gal/minute \[3.8 L/minute\] water
    pour), as specified by the project. Check for depth following
    roll-down and undue displacement, cracking, or shoving caused by
    rollers or improper roller speeds.

When there are difficulties in mix uniformity, temperature, or
production rate, changes to the plant, the mixing process, the JMF/mix
design, and/or the production testing procedure should be considered.
Similarly, if deficiencies in the field operations are observed that
impact the desired quality of the constructed base, changes should be
made to the process. Construction of test sections should be continued
until a base is placed successfully.

## 4.2 Subgrade and Subbase Issues

Depending on the project, the stabilized and/or permeable base will be
placed on top of a subgrade or subbase, with or without the inclusion of
an aggregate or geotextile separation layer. As pointed out by Kohn et
al. (2003), uniformity and stability of the prepared subgrade or subbase
affect both the long-term performance of the pavement and the
construction process. Moreover, proper grade control of the underlying
layer(s) will help minimize the potential for deficient base and/or
concrete slab thickness.

Kohn et al. (2003) covers in great detail the key issues concerning the
grading, stabilizing, and compacting of subgrade materials and the
placement and compaction of granular subbase materials. When a
separation layer is to be included on top of the prepared
subgrade/subbase, the importance of achieving a solid working
platform---with the proper grade and surface tolerance---cannot be
over-emphasized. In addition, great care must be exercised in the
allowance of construction traffic on the prepared subgrade/subbase, so
as to ensure that the layer(s) is free of ruts, depressions, and bumps
prior to the placement of the separation layer or stabilized and/or
permeable base.

Separation layers should not be placed on soft, muddy, or frozen
subgrade/subbase and caution should be exercised to prevent
incorporating subgrade/subbase or shoulder material into the aggregate
or HMA separation layer material. If a geotextile is used for the
separation layer, the underlying subgrade/subbase layer should be free
of all sharp objects (sticks, stumps, metal debris) and large stones
that could puncture the fabric.

The geotextile should be placed in accordance with the requirements and
standard details provided in the project plans, as well as the
manufacturer's recommendations. The fabric should be draped taut over
the prepared subgrade/subbase, so as to avoid wrinkles, but it should
not be stretched so tight as to produce tearing. Fabric overlaps and
seams should be made as specified and securing of the fabric to the
subgrade/subbase should be achieved through sufficient pinning and/or
placement of soil.

Construction traffic should only be allowed on the secured geotextile
fabric after it has been demonstrated that the material can hold up
under the loads and movements of the traffic. Any tears, holes, or other
damage done to the fabric during installation should be repaired via
patches or replacement. Routine checks of vertical and horizontal
displacement of the fabric are recommended.

## 4.3 Base Materials Construction

### 4.3.1 Cement-Treated Base

#### 4.3.1.1 CTB Mix Production 

CTB mixes should be produced in a central plant, continuous flow pugmill
mixer to ensure uniform quality. Batch pugmill or rotary drum mixers may
be used on smaller jobs (say less than 2,500 yd^2^ \[2,090 m^2^\]). The
continuous flow plants should be equipped with feeders to proportion
aggregates and bulk cement, by weight, automatically. The plant site,
layout, equipment, and provisions for transporting material should
assure a continuous supply of material to the work. The plant should be
calibrated before mixing and placing operations begin to ensure that the
correct proportion of material will be discharged.

Aggregate stockpiles should be constructed in a manner that prevents
segregation and intermixing of deleterious materials. Aggregates that
are segregated or mixed with earth or foreign material should not be
accepted.

Aggregate and cement may be proportioned either by weight or volume, and
must be mixed sufficiently to prevent the forming of cement balls after
water is added. The mixing time should be that which is required to
secure a uniform mixture of aggregate, cement, water, and pozzolan (if
used). This time varies based on the material feed rates, belt speed,
pugmill tilt, and paddle pitch of the mixer; therefore, prescriptive
mixing times are not recommended. The minimum mixing time should be
based on the uniformity and consistency of the mixture produced based on
visual examination. Typically, a minimum mixing time of 30 seconds can
be expected (PCA, 1995).

#### 4.3.1.2 CTB Mix Hauling and Placement 

##### Hauling

The mixed CTB material should be transported from the plant to the job
site in trucks or other hauling equipment having beds that are smooth,
clean, and tight. Truck bed covers should be used to protect the CTB
from rain. CTB material that becomes wet by rain during transport or
placement should be examined and rejected if there is evidence of any
alteration in the consistency and uniformity of the mix.

##### Placement

The CTB mixture should be deposited on moistened subgrade or subbase and
spread into a uniform layer. The layer should be of such width and
thickness that, following compaction to the required density and
trimming, it conforms to the required grade and cross-section.
Mechanical spreaders or equipment capable of receiving, spreading, and
shaping the mixture (without segregation) into a uniform layer/lift can
be used. The equipment should be equipped with a strike-off plate
capable of being adjusted to the specified layer thickness. It should
also be equipped with two end gates or cut off plates, so that the CTB
may be spread in widths varying up to lane width. Figure 4-1 illustrates
CTB placement using an asphalt laydown machine.

![](media/image32.wmf){width="4.746527777777778in"
height="2.9479166666666665in"}

Figure 4-1. CTB placement with an asphalt paver.

A single spreader may be used over the entire width, provided it is
capable of placing a uniform, full-depth layer of material across the
full width of the base in one pass. Otherwise, two or more spreaders
will be required. When using two spreaders in adjacent lanes, they
should operate in a staggered position and the adjacent lanes should be
paved such that there is not more than a 30-minute gap between the
placing times at any given point in time (PCA, 1995).

The CTB material should not be mixed or placed while the air temperature
is below 40°F (4°C) or when conditions indicate that the temperature may
fall below 35°F (2°C) within 24 hours. The CTB should not be placed on
frozen ground. Further, CTB should not be placed when rainfall is
occurring. If an unexpected rain event occurs during placement, the
layer should be quickly compacted and protected. CTB material that
becomes wet by rain during transport or placement should be examined and
rejected if excess water in the mixture changes its consistency and
uniformity.

#### 4.3.1.3 Compaction

![](media/image7.jpeg){width="1.75in"
height="1.330708661417323in"}Immediately upon completion of the
spreading operations, the CTB material should be compacted. Compaction
can be accomplished using one or a combination of the following pieces
of equipment: tamping or grid roller, steel-wheeled roller, vibratory
roller, pneumatic-tire roller, vibrating plate compactor (for areas
inaccessible to rollers). The number, type, and weight of rollers and/or
compactors must be sufficient to compact the mixture to the required
density.

In-place density can be monitored using a nuclear density gage. At the
start of compaction, the moisture content should be within 2 percentage
points of the specified optimum moisture. For summertime construction
conditions, a moisture content of +2 percent is recommended (Kohn et
al., 2003).

The CTB layer may be placed in single or multiple compacted lifts;
however, each compacted lift must be at least 4 in (102 mm) thick and no
greater than 8 in (203 mm) thick. Greater lift thicknesses (up to 12 in
\[305 mm\]) may be permitted if equipment is available to achieve the
desired compaction. In multi-lift construction, the surface of the
compacted lift should be kept moist until covered with the covering
lift. Successive lifts should be placed and compacted so that the
required total depth of the CTB layer is completed within a 12-hour
period. While forming construction joints, the material at the joint
should be compacted adequately and the joints finished level with the
remainder of the layer.

#### 4.3.1.4 Finishing

After compaction, the surface of the CTB layer should be shaped to the
specified lines, grades, and cross-section. Final trimming of the
compacted CTB to meet surface requirements should be accomplished using
a self-propelled trimming machine, with a mold board cutting edge, which
is at least 12 ft (3.7 m) wide and is automatically controlled by
sensors in conjunction with an independent grade control from a taut
stringline. Stringline will be required on both sides of the sensor
controls for the pilot lane. For all other lanes, a single stringline on
the outside and grade matching with previously completed adjacent lanes
is permissible. Since CTB is a rigid material, it is not practical to
re-grade after compaction. Thus, care needs to be taken to achieve the
specified grade tolerances the first time (Kohn et al., 2003).

![](media/image7.jpeg){width="1.5in" height="1.1251968503937009in"}In
hot weather, the surface should be kept moist by means of fog-type
sprayers. Compaction and finishing should be done in such a manner as to
produce a smooth, dense surface, free of ruts, cracks, ridges, and loose
material. All placement, compaction, and finishing operations should be
completed within 2 hours from the start of mixing. Material not
completed within the 2-hour time limit should be removed and replaced.

CTB layer limits that extend beyond the edges of the new PCC surface
course should be rolled down or shaped in such a manner that the
drainage is away from the edge of the proposed PCC surface layer.

#### 4.3.1.5 Jointing

A placement plan should be developed that will minimize the number of
longitudinal and transverse joints. At the end of each day\'s
construction, or when continuing placement is interrupted for more than
60 minutes, a transverse construction joint should be formed that is a
true vertical face (perpendicular to the centerline) free of loose
material.

![](media/image7.jpeg){width="1.6251968503937009in"
height="1.250787401574803in"}

Longitudinal construction joints (parallel to the centerline) should be
a consistent well-defined "near vertical" edge that is free of loose
material. The longitudinal joints should be located such that there is a
2-ft (0.6-m) minimum offset from planned joints in any overlying layer.

#### 4.3.1.6 Curing

The compacted and finished CTB should be cured as soon as possible, no
later than 2 hours after completion of the finishing operations. The
layer should be kept moist using a moisture-retaining cover or a light
application of water until the curing material is applied (care should
be taken to not oversaturate the surface of CTB with water).

A LMFCC is recommended for use as a curing agent. The entire surface of
the CTB layer should be uniformly sprayed with the compound at the rate
of 1 gal to not more than 200 ft^2^ (1 L to not more than 4.9 m^2^).

The curing seal should be maintained and protected until the PCC surface
layer is placed above it. Should the surface of the finished CTB and/or
the curing seal become damaged, additional curing material should be
applied at the time it is damaged or when the damage is first observed.

#### 4.3.1.7 Acceptance

The CTB layer should be tested for density, thickness, grade, and
surface tolerance on a lot basis. The following are the target values
for each of these criteria:

-   Density---A density of 98 percent or greater of the maximum density
    for full pay.

-   Thickness---Thickness within 0.5 in (13 mm) of the specified
    thickness.

-   Grade---The completed surface should not be 0.5 in (13 mm) above the
    plan grade.

-   Surface Tolerance---Tolerance should vary by no more than 0.38 in
    (9.5 mm) when tested with a 16-ft (4.9-m) straightedge.

#### 4.3.1.8 Bond-Breaker Application

A single uniform layer of choke stone, no thicker than 0.5 in (13 mm),
should be broadcast onto the CTB layer prior to the placement of the PCC
pavement. The choke stone should be worked into the surface of the CTB
using two additional passes of a vibratory roller.

### 4.3.2 Lean Concrete Base

#### 4.3.2.1 LCB Mix Production

Lean concrete production is similar to that of conventional concrete. It
may be produced in a stationary mixer, at a central batch plant, or in a
truck mixer.

The mixing time must be adequate to produce a mix that is uniform in
appearance, with all ingredients evenly distributed. If mixing in a
plant, the mixing time should not be less than 50 nor greater than 90
seconds. If mixing in a truck, the mixing time should not be less than
70 nor more than 125 truck-drum revolutions at a mixing speed of not
less than 6 nor more than 18 truck-drum revolutions per minute.

Retempering lean concrete by adding water is not permitted, except when
delivered in truck mixers. With truck mixers, additional water may be
added to the batch materials and additional mixing performed to allow
proper placement of the material, provided (a) the addition of water is
performed within 45 minutes after the initial mixing operations and (b)
the water/cementitious ratio specified in the mix design is not
exceeded.

#### 4.3.2.2 LCB Mix Hauling and Placement

##### Hauling

Lean concrete mixes can be hauled from the plant to the job site in an
agitator truck, a truck mixer operating at agitating speed, or a
non-agitating truck. When truck mixers are used to mix lean concrete,
they may be transported to the job site in the same truck operating at
agitating speeds, truck agitators, or a non-agitating truck. The bodies
of non-agitating trucks should be smooth, metal containers and must be
capable of discharging the concrete at a controlled rate without
segregation.

The elapsed time from the addition of cementitious material to the mix
until the lean concrete is deposited in place at the work site should
not exceed 45 minutes when the lean concrete is hauled in non-agitating
trucks, nor 90 minutes when it is hauled in truck mixers or truck
agitators.

##### Placement

Similar to conventional concrete, lean concrete can be placed by both
machine and hand paving. Machine paving should be used for mainline
paving, connecting taxiway sections, and other areas large enough to
accommodate a paving machine. However, areas too small for machine
paving will need hand paving. Slipform pavers, bridge deck pavers, or
fixed form pavers may be used, provided they are capable of handling the
amount of lean concrete required for the full-lane width specified, and
consolidating it full depth. Rotating pipe and tube floats are not
considered suitable for placing lean concrete.

Lean concrete should not be placed when the ambient temperature is below
40°F (4°C) or when conditions indicate that the temperature may fall
below 35°F (2°C) within 24 hours. Under no circumstances should the LCB
be placed on frozen underlying courses or mixed when the aggregate is
frozen. During periods of warm weather, when the maximum daily air
temperature exceeds 85°F (30°C), the forms and/or the underlying
material should be sprinkled with water immediately before placing the
LCB.

All mixing and batching operations should be halted during rain showers
and any plastic LCB placed should be covered immediately. The LCB should
be kept covered with plastic sheeting or other waterproof material until
such time that the rain does not make any surface indentation on the LCB
layer. Areas damaged by rain should be refinished.

#### 4.3.2.3 Placement and Consolidation

Regardless of the method of paving, hauled lean concrete material should
be discharged onto the prepared underlying course such that segregation
of the mix is minimized and minimum handling of the mix is needed.
Placement of the econocrete material should be continuous between
construction joints.

##### Side Form Construction

For side form placement, the lean concrete should be spread uniformly
between the forms. Necessary hand spreading can be done with shovels,
not rakes.

The spreading should be followed immediately by thorough consolidation
using vibrating screeds or spud vibrators. The vibrators may be either
the surface pan type for layers less than 8 in (203 mm) thick or the
internal type with either immersed tube or multiple spuds for the full
width of the slab. In no case should the vibrator be operated longer
than 20 seconds in any one location, nor should the vibrators be used to
move the lean concrete. Hand finishing will not be permitted except in
areas where the mechanical finisher cannot operate.

![](media/image7.jpeg){width="2.1728346456692913in"
height="1.3645669291338582in"}

##### Slipform Construction

The slipform paver should spread, consolidate, and shape the freshly
placed econocrete in one complete pass of the machine. The slipform
paver should vibrate the lean concrete mix for the full width and depth
of the strip of pavement being placed. The number, spacing, frequency,
and eccentric weights of vibrators should be provided as necessary to
achieve an acceptable consolidation and finishing quality.

Adequate power to operate all vibrators at the weight and frequency
required for a satisfactory finish must be available on the paver. The
internal vibrators may be supplemented by vibrating screeds operating on
the surface of the LCB. The frequency of each of the individual
vibrators should be monitored continuously using electronic means during
paving. Also, the consolidation process should be carefully monitored to
avoid honeycombing or over-consolidation.

#### 4.3.2.4 Finishing

Finishing the lean concrete surface is not necessary since it is not an
exposed layer in the rigid pavement system. The surface produced after
screeding or strikeoff is adequate.

#### 4.3.2.5 Curing

Just as with conventional concrete, immediately after the placing
operations are complete, within 2 hours of placement, the entire surface
and edges of the econocrete should be sprayed uniformly with white
pigmented LMFCC. The curing material should be applied using mechanical
sprayers under pressure at the rate of 1 gal to not more than 200 ft^2^
(1 L to not more than 4.9 m^2^). The layer should be kept moist using a
moisture-retaining cover or a light application of water until the
curing material is applied. Excessive delays in applying the curing
compound can result in uncontrolled shrinkage cracking. Hand spraying of
odd widths or shapes and econocrete surfaces exposed by the removal of
forms is permitted.

Should the film of curing material become damaged from any cause,
including sawing operations, the damaged portions should be repaired
immediately with additional compound or other approved means as quickly
as practical.

#### 4.3.2.6 Jointing

![](media/image7.jpeg){width="2.3909448818897636in"
height="1.3751968503937009in"}

There are primarily two types of joints in the LCB layer---construction
joints and contraction joints. Construction joints separate adjacent
construction placed at different times, at the end of a day's placement,
or between paving lanes. The placing plan should minimize construction
joints as much as possible.

Contraction joints control the location of LCB cracking. These joints
should match within 3 in (76 mm) of the planned joints of the concrete
surface.

#### 4.3.2.7 Acceptance

The LCB should be tested for air content, compressive strength,
thickness, grade, and surface tolerance on a lot basis. The following
are the target values for each of these criteria:

-   Air content---Test results should be between 4 and 8 percent for (a)
    the first three truckloads of lean concrete produced at the start of
    operations each day and (b) the first three truckloads produced
    after any scheduled or non-scheduled shutdown.

-   7-day compressive strength---Minimum lot average of 500 psi (3,448
    kPa), with no more than 20 percent of individual cylinders tested
    within a lot having a 7-day strength greater than 800 psi (5,516
    kPa). When greater than 20 percent of the individual cylinders in a
    given lot have 7-day strengths in excess of 800 psi (5,516 kPa), and
    transverse joints have not been notched, a choke stone layer must be
    used as a bond breaker.

-   Thickness---Thickness should be within 0.5 in (13 mm) of the
    specified thickness.

-   Grade---Completed surface will not be more than 0.5 in (13 mm) above
    the plan grade.

-   Surface tolerance---Tolerance should vary by no more than 0.38 in
    (9.5 mm) when tested with a 16-ft (4.9-m) straightedge.

#### 4.3.2.8 Bond Breaker

When lean concrete is placed directly beneath PCC pavement, a bond
breaker must be used. A second application of the curing compound can be
as a bond breaker when the 7-day compressive strength values satisfy the
minimum and maximum requirements. This application should be made at
least 8 hours and not more than 24 hours prior to beginning the
placement of the PCC surface layer. The rate of application should be
the same as that specified for the curing application. After application
of the bond breaker coat, traffic will be limited to that required for
the placement of the overlying pavement layer.

If the maximum 7-day compressive strength values exceed the maximum
strength requirements, choke stone must be used as a bond breaker.

### 4.3.3 Asphalt-Treated Base

The production, placement, control, and acceptance of ATB are similar to
that of a high-quality asphalt paving layer. Guidance on HMA layer
construction is provided in other documents, such as FAA AC
150/5370-14A, *Hot-Mix Asphalt Paving Handbook* (2000). Therefore, only
salient points specific to the construction of ATB under rigid pavements
are presented here. Much of the guidance here has been excerpted from
new Item P-403 in AC 150/5370-10B and Kohn et al. (2003).

-   The ATB mix should be produced in a central mixing plant.

-   A test section is required prior to full production to determine
    that mix can be satisfactorily produced, placed, and compacted with
    the proposed equipment on the project. The test section should be a
    minimum of 300 ft (92 m) long and 20 to 30 ft (6.1 to 9.2 m) wide,
    and it should include a longitudinal cold joint and have the same
    thickness as the planned ATB layer. The test section affords the
    opportunity to determine the quality of the mixture in place, as
    well as performance of the plant and laydown equipment. Any
    adjustments needed to the JMF should be made at this stage.

-   During production, careful consideration should be given to
    controlling the mix by ensuring that it satisfies the JMF through
    periodic sampling. Frequent plant inspections are also recommended
    to check batch proportions, temperatures, etc.

-   Acceptance of the mix should be based on mat density, joint density,
    thickness, smoothness (same as surface tolerance), and grade.

-   Contractor quality control testing should include monitoring
    aggregate gradation, asphalt content, moisture content of aggregate
    and mixtures, in-place density, and temperatures (at the dryer, the
    bitumen in the storage tank, the mixture at the plant, and the
    mixture at the job site).

-   When ATB is placed, care should be taken to meet the surface
    tolerance and grade requirements the first time. Milling of the
    layer over large areas (\> 15 yd^2^ \[12.5 m^2^\]) to establish
    evenness and grade can result in bonding of PCC to ATB and can lead
    to early cracking of the PCC. If milling is necessary, a thin
    leveling course of the same material, a coat of asphalt emulsion, or
    a medium- to heavy-duty geotextile fabric should be applied over the
    affected areas just prior to the placement of the PCC.

-   Temperatures at the surface of the ATB in hot weather paving
    conditions, which can be as high as 140°F (60°C), will affect the
    hydration of the PCC. Early cracking is possible unless temperatures
    of the ATB are controlled. The ATB should be whitewashed using a
    lime-water solution before concrete placement to reduce its surface
    temperature.

### 4.3.4 Cement-Treated Permeable Base

#### 4.3.4.1 CTPB Mix Production 

The CTPB material should be mixed in a stationary mixer at a central
batch plant. The mixing time should be adequate to produce CTPB that is
uniform in appearance, with all ingredients evenly distributed.

#### 4.3.4.2 CTPB Test Section 

A test section is necessary. Use the guidance presented at the beginning
of this chapter.

#### 4.3.4.3 CTPB Mix Hauling and Placement

##### Hauling

![](media/image7.jpeg){width="1.891732283464567in" height="1.5in"}The
mixed CTPB material should be transported from the plant and delivered
to the spreader in trucks having smooth, clean beds. The nature of the
CTPB mix makes segregation a potential problem during transport;
therefore, hauling time should be minimized, and haul routes should be
smooth. The elapsed time between the start of moist mixing and the time
the CTPB is deposited in-place at the work site should not exceed (a) 30
minutes when the CTPB is hauled in non-agitating trucks, or (b) 45
minutes when the CTPB is hauled in transit mixers. Retempering the CTPB
material by adding water or by other means should not be permitted.

##### Placement

The CTPB material can be placed using a mechanical spreader. An asphalt
paving machine with dual tamping bars can be used, but care must be
taken when using this machine to not fracture the aggregates. If a
spreader is used, the equipment should be capable of placing the
material, without segregation, into a uniform layer or lift meeting the
required grade and cross-section. The equipment should be equipped with
a strike-off plate capable of being adjusted to the specified layer
thickness and two end gates or cut-off plates such that the CTPB may be
spread in varying widths.

CTPB layers can be installed in single or multiple compacted lifts;
however, each compacted lift must be at least 4 in (102 mm) thick but
not greater than 6 in (152 mm) thick to ensure compaction.

CTPB material must not be mixed or placed while the air temperature is
below 40°F (4°C) or when conditions indicate that the temperature may
fall below 35°F (2°C) within 24 hours. The CTPB must not be placed on
frozen underlying courses or mixed when aggregate is frozen. The CTPB
may also not be placed when rainfall is occurring or where rain is
imminent. Any CTPB material that has become excessively wet by rain
during transport and/or placement should be rejected.

#### 4.3.4.4 Compaction

Immediately after the spreading operations are completed, the CTPB
material should be compacted using the approved compaction equipment and
roller pattern/sequence, as determined in the
![](media/image7.jpeg){width="1.891732283464567in"
height="1.5in"}approved test section. There must be a sufficient number
of rollers to match the output of the plant. A preferred method of
compaction of CTPB is with a static roller (see Figure 4-2). Density
testing with a nuclear gage has limited value for this material. In
places not accessible to rollers, the CTPB material can be compacted
with hand-operated tampers.

Field compaction should begin no more than 30 minutes from the start of
moist mixing. The surface of the CTPB should be kept lightly moist prior
to compaction. In addition, field compaction should be completed within
60 minutes.

![](media/image33.wmf){width="4.497916666666667in"
height="3.3097222222222222in"}

Figure 4-2. CTPB compaction with a static roller.

#### 4.3.4.5 Curing

There is no consensus on a suitable method for curing CTPB. The CTPB can
be covered with polyethylene sheeting for 3 to 5 days subsequent to a
fine water mist cure applied on the day after the base is placed. Other
schools of thought say not to cure CTPB at all because test data show
that there is no significant difference in strength of cured and uncured
CTPB. The use of wax-based LMFCCs is not recommended since they can plug
the void structure of the CTPB. Also, when mist curing or fog spraying
the CTPB, adequate care should be taken not to flush the cement paste
into the void structure.

#### 4.3.2.6 Acceptance

The CTPB layer is accepted based on thickness, grade, and surface
tolerance on a lot basis. The following are the target values for each
of these criteria:

-   Thickness---Thickness within 1 in (25 mm) of the planned thickness.

-   Grade---The completed surface should not be more than 0.5 in (13 mm)
    above or below the plan grade.

-   Surface tolerance---Tolerance should vary by no more than 0.5 in
    (13 mm) when tested with a 16-ft (4.9-m) straightedge.

#### 4.3.4.7 Bond Breaker

When the CTPB is placed beneath PCC pavement, a single uniform layer of
choke stone, no thicker than 0.5 in (13 mm), should be broadcast onto
the CTPB layer shortly prior to the placement of the PCC pavement. The
choke stone should be worked into the surface of the CTPB using two
passes of a vibratory roller. This layer prevents excessive PCC paste
penetration into the CTPB and reduces slab restraint.

Completed portions of the CTPB layer, following placement of the choke
stone, can be opened immediately to low-speed traffic and to
construction equipment, provided the CTPB does not ravel or loosen under
such traffic. The CTPB must be protected from freezing.

### 4.3.5 Asphalt-Treated Permeable Base

#### 4.3.5.1 ATPB Mix Production 

The ATPB material may be produced either batch type or continuous mixing
type plants. The mixing time should be adequate to produce ATPB that is
uniform in appearance, with all ingredients evenly distributed.

The aggregate for the ATPB mixture should be dried and heated prior to
mixing with asphalt. The resulting mix should have a combined aggregate
moisture content (weighted according to the composition of the blend)
less than 0.25 percent for aggregate blends with water absorption less
than or equal to 2.5 percent, or less than 0.50 percent for aggregate
blends with water absorption greater than 2.5 percent.

At the time of mixing, the temperature of the aggregate must be within
the range specified in the JMF. The maximum temperature and rate of
heating should be such that no damage occurs to the aggregates.

The dried aggregates should be combined in the mixer to meet the
gradation requirements for the mix design. The asphalt cement should be
weighed or metered and introduced into the mixer in the amount specified
by the JMF. The combined materials must be mixed until the aggregate
obtains a uniform coating of asphalt binder and is thoroughly
distributed throughout the mixture.

The temperature at the discharge from the plant or surge and storage
bins should be maintained between 275 and 325°F (135 and 163°C),
depending on the viscosity of the binder.

#### 4.3.5.2 ATPB Test Section 

A test section is required for this layer. See the introduction to this
chapter.

#### 4.3.5.3 ATPB Mix Hauling and Placement

##### Hauling

Trucks used for hauling the ATPB mixture from the plant to the job site
should have clean and smooth beds. To prevent the mixture from adhering
to the truck beds, they could be lightly coated with a minimum amount of
concentrated hydrated lime-water solution. The truck beds should be
raised to drain any excess solution before loading the mixture in the
trucks. Each truck should have a suitable cover to protect the mixture
from adverse weather or long hauls.

The ATPB mixture should be transported to the job site and delivered to
the asphalt paver for placement. Any truck causing excessive segregation
of the ATPB mixture should be removed from the work until such
conditions are corrected. Hauling over freshly placed material should
not be permitted until the material has been compacted, as specified,
and allowed to cool to atmospheric temperature (typically a period of 24
hours is recommended).

##### Placement

![](media/image7.jpeg){width="1.891732283464567in"
height="1.5570866141732282in"}The ATPB material can be placed using an
asphalt lay-down machine that is self-contained, power-propelled, and
equipped with an activated screed or strike-off assembly (heated as
necessary). The laydown machine should be capable of spreading (without
segregation) and finishing courses of ATPB material that will meet the
specified thickness, smoothness, and grade. An alternative method for
placement that provides compaction of the ATPB material is with a large
asphalt paving machine with dual tamping bars (care must be taken when
using this machine to not fracture the aggregates).

The ATPB should be spread at a temperature between 200 and 250°F (93 and
121°C), as measured in the hopper of the paving machine. It should be
placed to the full width by the asphalt paver in a uniform layer of such
depth that, when compacted, it is of the required thickness and conforms
to the grade and contour indicated. The speed of the paver must be
regulated to eliminate pulling and tearing of the ATPB mat.

ATPB can be placed in a single or multiple compacted lifts; however,
each compacted lift must be at least 4 in (102 mm) thick and no greater
than 6 in (152 mm) thick. If multiple lifts are used, the longitudinal
joint in one lift should offset the longitudinal joint in the lift
immediately below by at least 1 ft (0.3 m); however, the joint in the
surface lift should be at the centerline of crowned pavements.
Transverse joints in one lift should be offset by at least 2 ft (0.6 m)
from transverse joints in the previous lift. Transverse joints in
adjacent strips should be offset a minimum of 10 ft (3.1 m).

In placing adjacent strips of ATPB, the screed of the paving machine
should overlap the previously placed strip 3 to 4 in (76 to 102 mm) and
should be sufficiently high so that compaction will produce a smooth,
dense joint. The ATPB material placed on the edge of the previously
placed strip by the paver should be pushed back to the edge of the strip
being placed. Excess material should be removed and wasted.

In areas where machine spreading is impractical, the ATPB material can
be spread using hand tools, shovels, and lutes; rakes should not be
allowed. The material should be spread uniformly in a loose layer to
prevent segregation. The material should conform to the required grade
and thickness after compaction.

#### 4.3.5.4 Compaction

Compaction of the ATPB material should begin when the temperature of the
mix has cooled to 150°F (66°C) and should be completed before the
temperature falls below 100°F (38°C). The ATPB material should be
compacted using the approved compaction equipment and roller
pattern/sequence as determined in the test section. Each roller should
operate at a speed no greater than 1.5 mi/hr (2.4 km/hr). The use of
pneumatic rollers is preferred. Vibratory rollers should be used with
caution since they can degrade the aggregate and cause liquefaction of
underlying unbound materials if they are wet.

If the designated rolling pattern/sequence deviates from that approved
in the test section, or if crushing of the aggregate is observed, work
should be stopped until the cause(s) can be determined and corrections
are made. Generally speaking, static steel wheel rollers or pneumatic
rollers are preferable and give best results. Vibratory rollers should
be used with care for the same reasons as mentioned for CTPB. In all
places not accessible to the rollers, the ATPB material should be
compacted with approved mechanical hand-operated tampers.

#### 4.3.5.5 Curing

Curing of ATPB layers is not required.

#### 4.3.5.6 Acceptance

The ATPB layer is generally accepted based on thickness, grade, and
surface tolerance on a lot basis. The following are the target values
for each of these criteria:

-   Thickness---Thickness within 1 in (25 mm) of the specified
    thickness.

-   Grade---The completed surface should not be more than 0.5 in (13 mm)
    above the plan grade.

-   Surface tolerance---Tolerance should vary by no more than 0.5 in
    (13 mm) when tested with a 16-ft (4.9-m) straightedge.

The finished texture and drainability of the completed ATPB is
illustrated in Figure 4-3. The small spread of water in the figure
illustrates the quality of drainage.

![](media/image34.wmf){width="4.51875in" height="2.933333333333333in"}

Figure 4-3. Drainability and texture of finished ATPB.

#### 4.3.5.7 Bond Breaker

A bond breaker is not required.

#### 4.3.5.8 Whitewashing

The surface temperatures of the ATPB layers can reach 140°F (60°C)
during summer time construction. When hot-weather paving conditions
exist, these layers should be whitewashed using a lime-water solution to
reduce their surface temperature immediately prior to PCC placement.

## 4.4 PCC Placement in Conjunction with Stabilized/Permeable Bases

When PCC layers are placed over high strength/stiffness stabilized or
permeable bases, certain aspects of the construction process need to be
dealt with more carefully to avoid the risk of early cracking. Some of
these items are discussed below.

### 4.4.1 Timing of PCC Placement in Relation to Stabilized/Permeable Base Placement

When a stabilized layer, such as a CTB or LCB, is placed several months
in advance of the PCC layer (common occurrence in large, multi phase
projects), its strength and stiffness increases significantly over the
design values due to the continued hydration of cement. Higher strengths
and stiffnesses increase the curling stresses in fresh concrete and,
hence, increase the risk of early-age cracking. Therefore, careful
attention needs to be paid to factors that aggravate the movements in
new PCC slabs, including panel sizes, PCC mixes, curing (or lack
thereof) and factors that cause restraint stresses. If possible, PCC
slab related design and materials variants need to be altered or
construction variants need to be more vigilantly kept under control.

Stabilized layers may need to be trimmed/milled to establish grade
control prior to the PCC placement. If this is done, the surface of the
base layer will have a rougher texture that will affect the early-age
performance. The design, materials, and construction variants should be
adjusted to mitigate the risk of early cracking.

### 4.4.2 Bond Breaker/Choke Stone Application

A bond breaker should be applied on top of the stabilized bases prior to
the placement of the PCC. When the bond breaker is a wax-based LMFCC,
the application should be between 8 and 24 hours prior to paving. When
choke stone layers are used, they can be applied any time prior to PCC
placement, provided contamination of this layer with fines due to
construction traffic is minimized. This is particularly important when
choke stone layers are used for CTPB layers.

### 4.4.3 PCC Curing

An important aspect of PCC curing that is often overlooked is that it is
not only important for the hydration of concrete but it also helps
reduce shrinkage of concrete (Aitcin, 1998; Kovler and Jensen, 2005).
External curing helps reduce the drying shrinkage portion of the total
shrinkage which accounts for a bulk of the shrinkage in mixes with
water-to-cementitious ratios above 0.4 (Kovler and Jensen, 2005). When
ambient or PCC material factors point to the potential for excessive
shrinkage, extra care should be taken to ensure that adequate curing is
provided while the concrete has still not developed adequate strength.
If possible, changes need to be made to the project specifications to
allow for more effective curing techniques as needed. Besides decreasing
the likelihood of early cracking, effective curing also improves the
long-term pavement performance by improving concrete strength and
durability as well as mitigating the effects of factors such as built-in
curling and warping that are unquantifiable during pavement design.

### 4.4.4 PCC Slab Jointing

A significant factor that affects early-age concrete performance on
stabilized and permeable bases is the timing of the initial sawcut, as
well as the depth of sawcut. The variants that particularly sensitize
the issue are base stiffness and the PCC slab/base restraint. The
restraint increases the effective slab thickness and makes an otherwise
adequate sawcut inadequate. While depth of initial sawcutting can be
mandated in design, the timing to perform the sawcut cannot be
specified.

The timing of the initial sawcut is a function of a variety of factors,
including environmental conditions, equipment used, mixture properties,
and temperature at time of PCC placement. There is no substitute to
experience and vigilance when it comes to sawcutting.

### 4.4.5 Hot- and Cold-Weather Paving

The most common trigger situation leading to early-age cracking in rigid
pavements built over stabilized or permeable bases is a large ambient
temperature drop caused by an approaching cold front or a sudden rain
shower during PCC placement. This is followed by hot-weather paving
associated with high evaporation losses. Proper planning and execution
of the construction to account for adverse climate conditions is a key
to good performance. Most project specifications include provisions to
deal with potential trigger conditions, but to achieve consistent
success, an understanding of the consequences of trigger factors for a
given set of variants and enforcement of the provisions is imperative.

**REFERENCES**

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England.

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Concrete Pavements*, TB004.02P, ACPA, Skokie, Illinois.

ACPA. 2002a. "Early Cracking of Concrete Pavement---Causes and Repairs,"
*Concrete Pavement Technology*, ACPA, Skokie, Illinois.

ACPA. 2002b. "Stabilized Subbases and Airfield Concrete Pavement
Cracking," *R&T Update Number 3.06*, Concrete Pavement Research and
Technology, ACPA, Skokie, Illinois.

Applied Research Associates (ARA), Inc. 1999. "Pavement Subsurface
Drainage Design---Reference Manual," Federal Highway Administration
(FHWA), Washington D.C.

ARA Inc. 2004. "Guide for Mechanistic-Empirical Design of New and
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FHWA. 1995. "Geosynthetics Design and Construction Guidelines
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Washington, D.C.

Holtz, R.D., B.R. Christopher, and R.R. Berg. 1998. *Geosynthetic Design
and Construction Guidelines, Participants Notebook*, Publication No.
FHWA-HI-95-038, FHWA, Washington, D.C.

Kohn, S.D., S. Tayabji, P. Okamoto, R. Rollings, R. Detwiller, R.
Perera, E. Barenberg, J. Anderson, M. Torres, H. Barzegar, M. Thompson,
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Foundation (IPRF), Washington, D.C.

Kosmatka, S.H., B. Kerkhoff, and W.C. Panarese. 2002. " Design and
Control of Concrete Mixtures," Portland Cement Association (PCA),
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Kovler, K. and O.M. Jensen. 2005. "Novel Techniques for Concrete
Curing," *Concrete International*, Vol. 27, No. 9.

Lafrenz, J. 1997. "Aggregate Gradation Control for PCC Pavements,"
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University of Texas, Austin, Texas.

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Using New Pavement Design Procedures for Hot Mix Asphalt Mixtures
Modified with Hydrated Lime," National Lime Association (NLA),
Arlington, Virginia.

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2005. "Measurement and Significance of Coefficient of Thermal Expansion
of Concrete In Rigid Pavement Design," CD-ROM publication, *84^th^
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Wu, and S.D. Tayabji. 1991. *Guidelines for Timing Contraction Joint
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Document Available From:

**American Concrete Pavement Association**

**5420 Old Orchard Road, Suite A100**

**Skokie, IL 60077 USA**

**(847) 966-2272**

***www.pavement.com* JP010P**

[^1]: AC 150/5370-10A was replaced by AC 150/5370-10B by the FAA in
    September, 2005. However, the guidance provided herein is on the
    basis of empirical observations of pavements built in accordance
    with the former AC. For the base types discussed here, the
    differences between these two ACs are minimal. Therefore, the
    guidance provided herein is very relevant.

[^2]: A choke stone layer is a small-size stone layer (generally 0.5 to
    1.0 in \[13 to 25 mm\] thick). A new specification for this layer
    was developed under FAA IPRF Project DOT/FAA-01-G-002-02-1.