i

PREFACE

Coal fly ash is a coal combustion product that has numerous

applications in highway construction. Since the first edition of Fly

Ash Facts for Highway Engineers in 1986, the use of fly ash in

highway construction has increased and new applications have

been developed. This document provides basic technical

information about the various uses of fly ash in highway

construction.

Fly ash has been used in roadways and interstate highways since

the early 1950s. In 1974, the Federal Highway Administration

encouraged the use of fly ash in concrete pavement with Notice N

5080.4, which urged states to allow partial substitution of fly ash

for cement whenever feasible. In addition, in January 1983, the

Environmental Protection Agency published federal

comprehensive procurement guidelines for cement and concrete

containing fly ash to encourage the utilization of fly ash and

establish compliance deadlines. 

This document is sponsored by the U.S. Department of

Transportation, through the Federal Highway Administration, in

cooperation with the American Coal Ash Association and the

United States Environmental Protection Agency. The United

States Government assumes no liability for its contents or use.

The Federal Highway Administration endorses no products or

manufacturers. This publication does not constitute a standard,

specification, or regulation.

The United States Environmental Protection Agency supports the

beneficial use of coal combustion products as an important

priority and endorses efforts by the Federal Highway

Administration as described in this document.
iii

Fly Ash - An Engineering Material . . . . . . . . . . 1

Why Fly Ash? . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . 4

Quality of Fly Ash. . . . . . . . . . . . . . . . . . . . . . . 6

Highway Applications. . . . . . . . . . . . . . . . . . . . 9 

Fly Ash in Portland Cement Concrete . . . . . . . 9

Fly Ash in Stabilized Base Course. . . . . . . . . . 10

Fly Ash in Flowable Fill . . . . . . . . . . . . . . . . . 10

Fly Ash in Structural Fills/Embankments . . . . 11

Fly Ash in Soil Imprevements . . . . . . . . . . . . . 12

Fly Ash in Asphalt Pavements. . . . . . . . . . . . . 12

Fly Ash in Grouts for Pavement Subsealing . . 13

Fly Ash in Portland Cement Concrete. . . . . . . 15

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Mix Design and Specification Requirements. . 18

Fly Ash Properties . . . . . . . . . . . . . . . . . . . . . . 19

Other Constituents . . . . . . . . . . . . . . . . . . . . . 20

Construction Practices . . . . . . . . . . . . . . . . . . . 21

Fly Ash in Stabilized Base Course. . . . . . . . . . 25

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Mix Design and Specification Requirements. . 25

Testing to Determine Mix Proportions . . . . . . 26

Construction Practices. . . . . . . . . . . . . . . . . . . 28

Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Fly Ash in Flowable Fill . . . . . . . . . . . . . . . . . 31

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Mix Design and Specification Requirements. . 33 

Fly Ash in Structural Fills/Embankments . . . . 39

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Design and Specification Requirements . . . . . 39

Construction Practices. . . . . . . . . . . . . . . . . . . 43

CHAPTER 1

CHAPTER 2

CHAPTER 3

CHAPTER 4

CHAPTER 5

CHAPTER 6

CONTENTS
iv

CONTENTS

Fly Ash in Soil Improvement . . . . . . . . . . . . . . . . . . . . 47

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Soil Stabilization to Improve Strength . . . . . . . . . . . . . 48

Soil Stabilization to Control Shrink Swell. . . . . . . . . . . 50

Soil Modification to Reduce Water Content . . . . . . . . . 51

Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Fly Ash in Asphalt Pavement . . . . . . . . . . . . . . . . . . . . 53

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Mix Design and Specification Requirements . . . . . . . . 53

Fly Ash in Grouts for Pavement Subsealing . . . . . . . . . 57

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Mix Design and Specification Requirements . . . . . . . . 58

Construction Practices . . . . . . . . . . . . . . . . . . . . . . . . . 58

Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

Developments in Fly Ash Utilization . . . . . . . . . . . . . . 61

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Particle Size Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Carbon Reduction Technologies. . . . . . . . . . . . . . . . . . 62

Other Developments in Technology. . . . . . . . . . . . . . . 63

Conversion Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

Specifications and Recommended Practice Guidelines. . 69

American Association of State Highway and 

Transportation Officials (AASHTO. . . . . . . . . . . . . . . . 69

American Society for Testing and Materials (ASTM). . . . 69

American Concrete Institute (ACI). . . . . . . . . . . . . . . . . . 71

Other Test Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Design and Construction References . . . . . . . . . . . . . . 73

Portland Cement Concrete . . . . . . . . . . . . . . . . . . . . . . 73

Stabilized Base Course . . . . . . . . . . . . . . . . . . . . . . . . . 73

Flowable Fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Grouts for Pavement Subsealing. . . . . . . . . . . . . . . . . . 73

Soil Improvement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Structural Fills/Embankments. . . . . . . . . . . . . . . . . . . . 74

Asphalt Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

CHAPTER 7

CHAPTER 8

CHAPTER 9

CHAPTER 10

APPENDIX A

APPENDIX B

APPENDIX C
v

Method of fly ash transfer can be dry, wet or both

Fly ash particles at 2,000x magnification

Typical ash colors

Microscopic photographs of fly ash (left) and portland

cement (right)

Fly ash improves workability for pavement concrete

Typical strength gain of fly ash concrete

Permeability of fly ash concrete

Fly ash concrete is used in severe exposure applications

such as the decks and piers of Tampa Bay's Sunshine

Skyway Bridge

Fly ash concrete finishing

Full-depth reclamation of a bituminous road

Flowable fill used in a utility trench application

Flowable fill eliminates the need for manual compaction

Flowable fill can be used to backfill very narrow trenches

Bridge abutment backfill with flowable fill

Bridge replaced by culverts and flowable fill

Highway embankment with fly ash structural fill

Allowable moisture content range below optimum

moisture content

Spreading and compaction of fly ash structural fill

Mixing and shaping of fly ash stabilized soil

Compaction of fly ash stabilized soil

Mixing and compaction of fly ash into a plastic soil

Stone matrix asphalt

Fly ash grouts for pavement subsealing

LIST OF FIGURES

1-1

1-2

1-3

1-4

3-1

3-2

3-3

3-4

3-5

4-1

5-1

5-2

5-3

5-4

5-5

6-1

6-2

6-3

7-1

7-2

7-3

8-1

9-1
vi
vii

Fly ash production and use

Fly ash uses

Sample oxide analyses of ash and portland cement

Guidance documents used for fly ash quality assurance

Specifications for fly ash in PCC

High fly ash content mixes

Low fly ash content mixes

Flowable fill mixes with high air content

Combinations required for 95 percent of standard proctor

maximum dry density, AASHTO T 99

AASHTO M 17 specification requirements for mineral

filler use in asphalt paving mixtures

Typical mixes using ultra fine fly ash

Influence of the class of fly ash on concrete performance

Example mix designs incorporating Class C and Class F

blend

Example mix design for HVFAC

LIST OF TABLES

1-1

1-2

1-3

1-4

1-5

5-1

5-2

5-3

6-1

8-1

10-1

10-2

10-3

10-4
1

Fly Ash as an Engineering Material Chapter.01

CHAPTER.1

FLY ASH - AN ENGINEERING MATERIAL 

WHY FLY ASH?

What is fly ash? Fly ash is the finely divided residue that

results from the combustion of pulverized coal and is transported

from the combustion chamber by exhaust gases. Over 61 million

metric tons (68 million tons) of fly ash were produced in 2001. 

Where does fly ash come from? Fly ash is produced by

coal-fired electric and steam generating plants. Typically, coal is

pulverized and blown with air into the boiler's combustion

chamber where it immediately ignites, generating heat and

producing a molten mineral residue. Boiler tubes extract heat

from the boiler, cooling the flue gas and causing the molten

mineral residue to harden and form ash. Coarse ash particles,

referred to as bottom ash or slag, fall to the bottom of the

combustion chamber, while the lighter fine ash particles, termed

fly ash, remain suspended in the flue gas. Prior to exhausting the

flue gas, fly ash is removed by particulate emission control

devices, such as electrostatic precipitators or filter fabric baghouses

(see Figure 1-1).

Where is fly ash used? Currently, over 20 million metric tons

(22 million tons) of fly ash are used annually in a variety of

engineering applications. Typical highway engineering

applications include: portland cement concrete (PCC), soil and

road base stabilization, flowable fills, grouts, structural fill and

asphalt filler.

What makes fly ash useful? Fly ash is most commonly used

as a pozzolan in PCC applications. Pozzolans are siliceous or

siliceous and aluminous materials, which in a finely divided form

and in the presence of water, react with calcium hydroxide at

ordinary temperatures to produce cementitious compounds.

The unique spherical shape and particle size distribution of fly ash

make it a good mineral filler in hot mix asphalt (HMA) applications

and improves the fluidity of flowable fill and grout. The consistency

and abundance of fly ash in many areas present unique opportunities

for use in structural fills and other highway applications. 
2

Fly Ash Facts for Highway Engineers

Environmental benefits. Fly ash utilization, especially in

concrete, has significant environmental benefits including: (1)

increasing the life of concrete roads and structures by improving

concrete durability, (2) net reduction in energy use and greenhouse

gas and other adverse air emissions when fly ash is used to replace

or displace manufactured cement, (3) reduction in amount of coal

combustion products that must be disposed in landfills, and (4)

conservation of other natural resources and materials.

PRODUCTION

Fly ash is produced from the combustion of coal in electric utility

or industrial boilers. There are four basic types of coal-fired

boilers: pulverized coal (PC), stoker-fired or traveling grate,

cyclone, and fluidized-bed combustion (FBC) boilers. The PC

boiler is the most widely used, especially for large electric

generating units. The other boilers are more common at

industrial or cogeneration facilities. Fly ashes produced by FBC

boilers are not considered in this document. Fly ash is captured

Figure1-1. Method of fly ash transfer can be dry, wet or both.
3

Fly Ash as an Engineering Material Chapter.01

from the flue gases using electrostatic precipitators (ESP) or in

filter fabric collectors, commonly referred to as baghouses. The

physical and chemical characteristics of fly ash vary among

combustion methods, coal source, and particle shape.

As shown in Table 1-1, of the 62 million metric tons (68 million

tons) of fly ash produced in 2001, only 20 million metric tons (22

million tons), or 32 percent of total production, was used. The

following is a breakdown of fly ash uses, much of which is used in

the transportation industry.

HANDLING

The collected fly ash is typically conveyed pneumatically from the

ESP or filter fabric hoppers to storage silos where it is kept dry

pending utilization or further processing, or to a system where the

dry ash is mixed with water and conveyed (sluiced) to an on-site

storage pond.

The dry collected ash is normally stored and handled using

equipment and procedures similar to those used for handling

portland cement:

Table 1-1. 2001 Fly ash production and use.

Table 1-2. Fly ash uses
4

Fly Ash Facts for Highway Engineers

➤ Fly ash is stored in silos, domes and other bulk storage

facilities 

➤ Fly ash can be transferred using air slides, bucket conveyors

and screw conveyors, or it can be pneumatically conveyed

through pipelines under positive or negative pressure

conditions

➤ Fly ash is transported to markets in bulk tanker trucks, rail

cars and barges/ships

➤ Fly ash can be packaged in super sacks or smaller bags for

specialty applications 

Dry collected fly ash can also be moistened with water and

wetting agents, when applicable, using specialized equipment

(conditioned) and hauled in covered dump trucks for special

applications such as structural fills. Water conditioned fly ash can

be stockpiled at jobsites. Exposed stockpiled material must be

kept moist or covered with tarpaulins, plastic, or equivalent

materials to prevent dust emission.

CHARACTERISTICS

Size and Shape. Fly ash is typically finer than portland cement

and lime. Fly ash consists of silt-sized particles which are generally

spherical, typically ranging in size between 10 and 100 micron

(Figure 1-2). These small glass spheres improve the fluidity and

workability of fresh concrete. Fineness is one of the important

properties contributing to the pozzolanic reactivity of fly ash. 

Chemistry. Fly ash consists primarily of oxides of silicon,

aluminum iron and calcium. Magnesium, potassium, sodium,

titanium, and sulfur are also present to a lesser degree.

Figure 1-2. Fly ash particles at 2,000x magnification.
5

Fly Ash as an Engineering Material Chapter.01

When used as a mineral admixture in concrete, fly ash is classified

as either Class C or Class F ash based on its chemical composition.

American Association of State Highway Transportation Officials

(AASHTO) M 295 [American Society for Testing and Materials

(ASTM) Specification C 618] defines the chemical composition of

Class C and Class F fly ash.

Class C ashes are generally derived from sub-bituminous coals

and consist primarily of calcium alumino-sulfate glass, as well as

quartz, tricalcium aluminate, and free lime (CaO). Class C ash is

also referred to as high calcium fly ash because it typically

contains more than 20 percent CaO.

Class F ashes are typically derived from bituminous and

anthracite coals and consist primarily of an alumino-silicate glass,

with quartz, mullite, and magnetite also present. Class F, or low

calcium fly ash has less than 10 percent CaO.

Color. Fly ash can be tan to dark gray, depending on its

chemical and mineral constituents. Tan and light colors are

typically associated with high lime content. A brownish color is

typically associated with the iron content. A dark gray to black

color is typically attributed to an elevated unburned carbon

content. Fly ash color is usually very consistent for each power

plant and coal source.

Table 1-3. Sample oxide analyses of ash and portland cement

Figure 1-3. Typical ash colors.
6

Fly Ash Facts for Highway Engineers

QUALITY OF FLY ASH

Quality requirements for fly ash vary depending on the intended

use. Fly ash quality is affected by fuel characteristics (coal), cofiring of fuels (bituminous and sub-bituminous coals), and various

aspects of the combustion and flue gas cleaning/collection

processes. The four most relevant characteristics of fly ash for use

in concrete are loss on ignition (LOI), fineness, chemical

composition and uniformity.

LOI is a measurement of unburned carbon (coal) remaining in the

ash and is a critical characteristic of fly ash, especially for concrete

applications. High carbon levels, the type of carbon (i.e.,

activated), the interaction of soluble ions in fly ash, and the

variability of carbon content can result in significant airentrainment problems in fresh concrete and can adversely affect

the durability of concrete. AASHTO and ASTM specify limits for

LOI. However, some state transportation departments will specify

a lower level for LOI. Carbon can also be removed from fly ash.

Some fly ash uses are not affected by the LOI. Filler in asphalt,

flowable fill, and structural fills can accept fly ash with elevated

carbon contents. 

Fineness of fly ash is most closely related to the operating

condition of the coal crushers and the grindability of the coal

itself. For fly ash use in concrete applications, fineness is defined

as the percent by weight of the material retained on the 0.044 mm

(No. 325) sieve. A coarser gradation can result in a less reactive

ash and could contain higher carbon contents. Limits on fineness

are addressed by ASTM and state transportation department

specifications. Fly ash can be processed by screening or air

classification to improve its fineness and reactivity. 

Some non-concrete applications, such as structural fills are not

affected by fly ash fineness. However, other applications such as

asphalt filler, are greatly dependent on the fly ash fineness and its

particle size distribution.

Chemical composition of fly ash relates directly to the mineral

chemistry of the parent coal and any additional fuels or additives

used in the combustion or post-combustion processes. The

pollution control technology that is used can also affect the
7

Fly Ash as an Engineering Material Chapter.01

chemical composition of the fly ash. Electric generating stations

burn large volumes of coal from multiple sources. Coals may be

blended to maximize generation efficiency or to improve the

station environmental performance. The chemistry of the fly ash

is constantly tested and evaluated for specific use applications. 

Some stations selectively burn specific coals or modify their

additives formulation to avoid degrading the ash quality or to

impart a desired fly ash chemistry and characteristics. 

Uniformity of fly ash characteristics from shipment to shipment

is imperative in order to supply a consistent product. Fly ash

chemistry and characteristics are typically known in advance so

concrete mixes are designed and tested for performance. 

Quality Assurance and Quality Control criteria vary for each

use of fly ash from state to state and source to source. Some states

require certified samples from the silo on a specified basis for

testing and approval before use. Others maintain lists of

approved sources and accept project suppliers' certifications of fly

ash quality. The degree of quality control requirements depends

on the intended use, the particular fly ash, and its variability.

Testing requirements are typically established by the individual

specifying agencies. 

Table 1-4. Guidance documents used for fly ash quality assurance

Figure 1-4. Microscopic photographs of fly ash (left) and portland

cement (right).
8

Fly Ash Facts for Highway Engineers

Notes: 

1 ASTM requirements are six percent

2 The density and fineness of individual samples shall not vary from the

average established by the 10 preceding tests, or by all preceding tests if

the number is less than 10, by more than the maximum percentages

indicated 

Table 1-5. Specifications for fly ash in PCC.
9

Highway Applications Chapter.02

CHAPTER.2

HIGHWAY APPLICATIONS

FLY ASH IN PORTLAND CEMENT CONCRETE

Overview. Fly ash is used in concrete admixtures to enhance the

performance of concrete. Portland cement contains about 65

percent lime. Some of this lime becomes free and available

during the hydration process. When fly ash is present with free

lime, it reacts chemically to form additional cementitious

materials, improving many of the properties of the concrete. 

Benefits. The many benefits of incorporating fly ash into a PCC

have been demonstrated through extensive research and countless

highway and bridge construction projects. Benefits to concrete

vary depending on the type of fly ash, proportion used, other mix

ingredients, mixing procedure, field conditions and placement.

Some of the benefits of fly ash in concrete:

➤ Higher ultimate strength 

➤ Improved workability

➤ Reduced bleeding

➤ Reduced heat of hydration

➤ Reduced permeability

➤ Increased resistance to sulfate attack

➤ Increased resistance to alkali-silica reactivity (ASR)

➤ Lowered costs

➤ Reduced shrinkage

➤ Increased durability

Cautions. Care should be taken when using fly ash in concrete

due to:

➤ Potential for decreased air entraining ability with high

carbon fly ash may reduce durability

➤ Reduced early strength

➤ Reduced heat of hydration in colder climates

These concerns can be accommodated using proper design and

construction practices. See Chapter 3 for additional information.
10

Fly Ash Facts for Highway Engineers

FLY ASH IN STABILIZED BASE COURSE

Overview. Fly ash and lime can be combined with aggregate to

produce a quality stabilized base course. These road bases are

referred to as pozzolanic-stabilized mixtures (PSMs). Typical fly

ash contents may vary from 12 to 14 percent with corresponding

lime contents of three to five percent. Portland cement may also

be used in lieu of lime to increase early age strengths. The

resulting material is produced, placed, and looks like cementstabilized aggregate base

Benefits. PSM bases have advantages over other base materials:

➤ Use of locally available materials

➤ Provides a strong, durable mixture

➤ Lower costs

➤ Autogenous healing

➤ Increased energy efficiency

➤ Suitable for using recycled base materials

➤ Can be placed with conventional equipment

Cautions. PSM bases require attention to:

➤ Seasonal limitations

➤ Traffic loading before complete curing

➤ Proper sealing and protection with asphalt or other surface

treatment is required to improve skid resistance

Chapter 4 provides a more thorough discussion of stabilized base

course.

FLY ASH IN FLOWABLE FILL

Overview. Flowable fill is a mixture of coal fly ash, water, and

portland cement that flows like a liquid, sets up like a solid, is selfleveling, and requires no compaction or vibration to achieve

maximum density. In addition to these benefits, a properly

designed flowable fill may be excavated later. For some mixes, an

optional filler material such as sand, bottom ash, or quarry fines, is

added. Flowable fill is also referred to as controlled low-strength

material, flowable mortar, or controlled density fill. It is designed

to function in the place of conventional backfill materials such as

soil, sand, or gravel and to alleviate problems and restrictions

generally associated with the placement of these materials.
11

Highway Applications Chapter.02

The benefits of using flowable fill include:

➤ Allows placement in any weather, even under freezing

conditions

➤ Achieves 100 percent density with no compactive effort

➤ Fills around/under structures inaccessible to conventional fill

placement techniques 

➤ Increases soil-bearing capacities

➤ Prevents post-fill settlement problems

➤ Increases the speed and ease of backfilling operations

➤ Decreases the variability in the density of the backfilled

materials

➤ Improves safety at the job site and reduces labor costs

➤ Decreases excavation costs

➤ Allows easy excavation later when properly designed

Cautions. When using flowable fill, care must be taken to:

➤ Anchor lighter weight pipes to prevent floating

➤ Provide confinement before initial set of the material

➤ Evaluate corrosion of metal pipe at interface of soil

Chapter 5 includes a detailed discussion of flowable fill

applications. 

FLY ASH IN STRUCTURAL FILLS/EMBANKMENTS

Overview. Fly ash can be used as a borrow material to construct

fills and embankments. When fly ash is compacted in lifts, a

structural fill is constructed that is capable of supporting highway

buildings or other structures. Fly ash has been used in the

construction of structural fills/embankments that range from small

fills for road shoulders to large fills for interstate highway

embankments.

Benefits. When used in structural fills and embankments, fly ash

offers several advantages over soil and rock:

➤ Cost-effective where available in bulk quantities

➤ Eliminates the need to purchase, permit, and operate a

borrow pit

➤ Can be placed over low bearing strength soils

➤ Ease of handling and compaction reduce construction time

and equipment costs
12

Fly Ash Facts for Highway Engineers

Cautions. Be aware that:

➤ State or local environmental regulations may require

consideration of the potential impacts to ground water at

adjoining properties

➤ Requires dust control and erosion prevention measures

Chapter 6 provides additional information.

FLY ASH IN SOIL IMPROVEMENT 

Overview. Fly ash is an effective agent for chemical and/or

mechanical stabilization of soils. Soil density, water content,

plasticity, and strength performance of soils. Typical applications

include: soil stabilization, soil drying, and control of shrink-swell.

Benefits. Fly ash provides the following benefits when used to

improve soil conditions:

➤ Eliminates need for expensive borrow materials

➤ Expedites construction by improving excessively wet or

unstable subgrade

➤ By improving subgrade conditions, promotes cost savings

through reduction in the required pavement thickness

➤ Can reduce or eliminate the need for more expensive natural

aggregates in the pavement cross-section

Cautions. The most important considerations for soil

improvement projects are:

➤ The rate of the hydration reaction upon exposure to water

➤ Soil moisture content at the time of compaction

➤ Fly ash with a sulfate content greater than 10 percent may

cause soils to expand more than desired

➤ In many cases, leaching tests may be required by local and

state agencies

Soil improvement is discussed more thoroughly in Chapter 7.

FLY ASH IN ASPHALT PAVEMENTS

Overview. Fly ash can be used as mineral filler in HMA paving

applications. Mineral fillers increase the stiffness of the asphalt

mortar matrix, improving the rutting resistance of pavements, and

the durability of the mix.
13

Highway Applications Chapter.02

Benefits. Fly ash will typically meet mineral filler specifications

for gradation, organic impurities, and plasticity. The benefits of

fly ash include:

➤ Reduced potential for asphalt stripping due to hydrophobic

properties of fly ash

➤ Lime in some fly ashes may also reduce stripping

➤ May afford a lower cost than other mineral fillers

Chapter 8 has additional information.

FLY ASH IN GROUTS FOR PAVEMENT SUBSEALING

Overview. Grouts are proportioned mixtures of fly ash, water,

and other materials used to fill voids under a pavement system

without raising the slabs (subsealing), or to raise and support

concrete pavements at specified grade tolerances by drilling and

injecting the grout under specified areas of the pavement.

Benefits. Fly ash grouts can:

➤ Be used to correct undermining without removing overlying

pavement

➤ Be accomplished quickly with minimum disturbance to

traffic

➤ Develop high ultimate strength

Cautions. Fly ash grouts:

➤ Require curing period before extremely heavy loading

because of low early strength

➤ Require confinement of the grout mixture under pavement

Chapter 9 presents more information on this topic.
14

Fly Ash Facts for Highway Engineers
15

Fly Ash in Portland Cement Concrete Chapter.03

CHAPTER.3

FLY ASH IN PORTLAND CEMENT CONCRETE

INTRODUCTION 

The use of fly ash in portland cement concrete (PCC) has many

benefits and improves concrete performance in both the fresh and

hardened state. Fly ash use in concrete improves the workability

of plastic concrete, and the strength and durability of hardened

concrete. Fly ash use is also cost effective. When fly ash is added

to concrete, the amount of portland cement may be reduced. 

Benefits to Fresh Concrete. Generally, fly ash benefits fresh

concrete by reducing the mixing water requirement and

improving the paste flow behavior. The resulting benefits are

as follows:

Improved workability. The spherical shaped particles of fly ash

act as miniature ball bearings within the concrete mix, thus

providing a lubricant effect. This same effect also improves

concrete pumpability by reducing frictional losses during the

pumping process and flat work finishability. 

Figure 3-1. Fly ash improves workability for pavement concrete.
16

Fly Ash Facts for Highway Engineers

➤ Decreased water demand. The replacement of cement by

fly ash reduces the water demand for a given slump. When

fly ash is used at about 20 percent of the total cementitious,

water demand is reduced by approximately 10 percent.

Higher fly ash contents will yield higher water reductions.

The decreased water demand has little or no effect on drying

shrinkage/cracking. Some fly ash is known to reduce drying

shrinkage in certain situations.

➤ Reduced heat of hydration. Replacing cement with the

same amount of fly ash can reduce the heat of hydration of

concrete. This reduction in the heat of hydration does not

sacrifice long-term strength gain or durability. The reduced

heat of hydration lessens heat rise problems in mass concrete

placements. 

Benefits to Hardened Concrete. One of the primary benefits of

fly ash is its reaction with available lime and alkali in concrete,

producing additional cementitious compounds. The following

equations illustrate the pozzolanic reaction of fly ash with lime to

produce additional calcium silicate hydrate (C-S-H) binder:

➤ Increased ultimate strength. The additional binder

produced by the fly ash reaction with available lime allows

fly ash concrete to continue to gain strength over time.

Mixtures designed to produce equivalent strength at early

ages (less than 90 days) will ultimately exceed the strength of

straight cement concrete mixes (see Figure 3-2). 
17

Fly Ash in Portland Cement Concrete Chapter.03

➤ Reduced permeability. The decrease in water content

combined with the production of additional cementitious

compounds reduces the pore interconnectivity of concrete,

thus decreasing permeability. The reduced permeability

results in improved long-term durability and resistance to

various forms of deterioration (see Figure 3-3)

Figure 3-2. Typical strength gain of fly ash concrete.

Figure 3-3. Permeability of fly ash concrete

➤ Improved durability. The decrease in free lime and the

resulting increase in cementitious compounds, combined

with the reduction in permeability enhance concrete

durability. This affords several benefits:
18

Fly Ash Facts for Highway Engineers

-Improved resistance to ASR. Fly ash reacts with available

alkali in the concrete, which makes them less available to react

with certain silica minerals contained in the aggregates. 

-Improved resistance to sulfate attack. Fly ash induces three

phenomena that improve sulfate resistance:

·Fly ash consumes the free lime making it unavailable to react

with sulfate

·The reduced permeability prevents sulfate penetration into

the concrete

·Replacement of cement reduces the amount of reactive

aluminates available 

-Improved resistance to corrosion. The reduction in

permeability increases the resistance to corrosion.

MIX DESIGN AND SPECIFICATION REQUIREMENTS 

Procedures for proportioning fly ash concrete (FAC) mixes

necessarily differ slightly from those for conventional PCC. Basic

guidelines for selecting concrete proportions are contained in the

American Concrete Institute (ACI) Manual of Concrete Practice,

Section 211.1. Highway agencies generally use variations to this

procedure, but the basic concepts recommended by ACI are

widely acknowledged and accepted. There is very little on

proportioning in ACI 232.2.

Fly ash is used to lower the cost and to improve the performance

of PCC. Typically, 15 percent to 30 percent of the portland

Figure 3-4. Fly ash concrete is used in severe exposure

applications such as the decks and piers of Tampa Bay's

Sunshine Skyway Bridge.
19

Fly Ash in Portland Cement Concrete Chapter.03

cement is replaced with fly ash, with even higher percentages used

for mass concrete placements. An equivalent or greater weight of

fly ash is substituted for the cement removed. The substitution

ratio for fly ash to portland cement is typically 1:1 to 1.5:1.

A mix design should be evaluated with varying percentages of fly

ash. Time versus strength curves can be plotted for each

condition. To meet specification requirements, curves are

developed for various replacement ratios and the optimum

replacement percentage ratio is selected. A mix design should be

performed using the proposed construction materials. It is

recommended that the fly ash concrete being tested incorporates

local materials in performance evaluation. 

Cement Factors. Because fly ash addition contributes to the total

cementitious material available in a mix, the minimum cement

factor (portland cement) used in the PCC can be effectively

reduced for FAC. The ACI acknowledges this contribution and

recommends that a water/ (cement plus pozzolan) ratio be used for

FAC in lieu of the conventional water/cement ratio used in PCC. 

Fly ash particles react with free lime in the cement matrix to

produce additional cementitious material, and thus, to increase

long-term strength.

FLY ASH PROPERTIES 

Fineness. The fineness of fly ash is important because it affects

the rate of pozzolanic activity and the workability of the concrete.

Specifications require a minimum of 66 percent passing the 0.044

mm (No. 325) sieve. 

Specific gravity. Although specific gravity does not directly

affect concrete quality, it has value in identifying changes in other

fly ash characteristics. It should be checked regularly as a quality

control measure, and correlated to other characteristics of fly ash

that may be fluctuating.

Chemical composition. The reactive aluminosilicate and

calcium aluminosilicate components of fly ash are routinely

represented in their oxide nomenclatures such as silicon dioxide,

aluminum oxide and calcium oxide. The variability of the

chemical composition is checked regularly as a quality control
20

Fly Ash Facts for Highway Engineers

measure. The aluminosilicate components react with calcium

hydroxide to produce additional cementitious materials. Fly

ashes tend to contribute to concrete strength at a faster rate when

these components are present in finer fractions of the fly ash. 

Sulfur trioxide content is limited to five percent, as greater

amounts have been shown to increase mortar bar expansion.

Available alkalis in most ashes are less than the specification limit

of 1.5 percent. Contents greater than this may contribute to

alkali-aggregate expansion problems.

Carbon content. LOI is a measurement of unburned carbon

remaining in the ash. It can range up to five percent per

AASHTO and six percent per ASTM. The unburned carbon can

absorb air entraining admixtures (AEAs) and increase water

requirements. Also, some of the carbon in fly ash may be

encapsulated in glass or otherwise be less active and, therefore,

not affect the mix. Conversely, some fly ash with low LOI values

may have a type of carbon with a very high surface area, which

will increase the AEA dosages. Variations in LOI can contribute

to fluctuations in air content and call for more careful field

monitoring of entrained air in the concrete. Further, if the fly ash

has a very high carbon content, the carbon particles may float to

the top during the concrete finishing process and may produce

dark-colored surface streaks.

OTHER CONSTITUENTS

Aggregates. As with any concrete mix, appropriate sampling

and testing are needed to ensure that the aggregates used in the

mix design are of good quality and are representative of the

materials that will be used on the project. Aggregates containing

reactive silica may be used in the FAC.

Cement. Fly ash can be used effectively in combination with all

types of cements: portland cement, performance cement, and

blended cements. However, special care should be taken when

using fly ash with high early strength or pozzolanic cements.

Appropriate mix design and testing should be conducted to

evaluate the impact of fly ash addition on the performance of high

early strength concrete. Blended or pozzolanic cements already

contain fly ash or other pozzolan. Additional cement replacement
21

Fly Ash in Portland Cement Concrete Chapter.03

would affect early strength development. Characteristics of

cement vary, as do fly ashes, and not all combinations produce a

good concrete. The selected portland cement should be tested and

approved on its own merit, as well as evaluated in combination

with the specific fly ash to be used.

Air Entraining Admixtures (AEAs). The higher the carbon

content in the fly ash, the more difficult it is to control the air

content. Further, if the carbon content varies, air content must be

closely monitored and admixture dosage rates changed to insure

proper levels of air entrainment. 

Retarders. Adding fly ash should not appreciably alter the

effectiveness of a chemical retarder. Some fly ashes may delay

the time of set and may reduce the need for a retarder.

Water reducers. Fly ash concrete normally requires less water,

but it can be further improved with the use of a water-reducing

admixture. The effectiveness of these admixtures can vary with

the addition of fly ash.

CONSTRUCTION PRACTICES

Fly ash concrete mixes can be developed to perform essentially

the same as PCC mixes with minor differences. When mixing

and placing any FAC, some minor changes in field operation may

be desirable. The following general rules-of-thumb will be useful:

Plant Operations. Fly ash requires a separate watertight, sealed

silo or holding bin for storage. Take care and clearly mark the

loading pipe for fly ash to guard against cross-contamination

when deliveries are made. If a separate holding bin cannot be

provided, it may be possible to divide the cement silo. If

available, use a double-walled divider to prevent crosscontamination. Due to its particle spherical shape, dry fly ash is

more flowable than dry portland cement. The angle of repose of

fly ash is typically less than that of cement. 

As with any concrete mix, mixing time and conditions are critical

to producing quality concrete. The increase in paste volume and

concrete workability (ball bearings effect) associated with the use

of fly ash typically improve mixing efficiency. 
22

Fly Ash Facts for Highway Engineers

Field Practices. Beginning with the first concrete delivery to the

job site, every load should be checked for entrained air until the

project personnel are confident a consistent air content is being

obtained. After that, periodic testing should continue to ensure

consistency. Concrete should be placed as quickly as possible to

minimize entrained air loss by extended agitation. Normal

practices for consolidation should be followed. Excessive

vibration should be avoided to minimize the loss of in-place 

air content.

FAC mix workability characteristics allow it to be placed easily.

Many contractors report improved smoothness of FAC pavements

over those constructed with conventional PCC. FAC contains

more paste than conventional PCC, which is beneficial to the

finishing. The slower early strength development of FAC may

also result in longer moisture retention.

Troubleshooting. First-time users of fly ash in concrete should

evaluate the performance of proposed mixes prior to construction.

All concrete ingredients must be tested and evaluated to develop

the desired mix design. 

Air content. The fineness of fly ash and the improved

workability of FAC make it naturally more difficult to develop

and hold entrained air. Also, residual unburned carbon in ash

adsorbs some of the air entraining agent and make it more

challenging to develop the desired air content. Higher carbon

content ashes naturally require higher AEA contents. Quality

assurance and quality control testing of ash at the source must

Figure 3-5. Fly ash concrete finishing
23

Fly Ash in Portland Cement Concrete Chapter.03

ensure that the fly ash used maintains a uniform carbon content

(LOI) to prevent unacceptable fluctuations in entrained air. New

technologies and procedures to address unburned carbon in fly

ash are described in Chapter 10.

Lower early strength. Fly ash concrete mixes typically result in

lower strengths at early ages. The slower strength gain may

require forms to be strengthened to mitigate hydraulic loads. It

should be noted that form removal and opening to traffic may be

delayed due to the slower strength gains. Lower early strengths

can be overcome by using accelerators.

Seasonal limitations. Construction scheduling should allow time

for FAC to gain adequate density and strength to resist de-icing

applications and freeze-thaw cycling prior to the winter months.

Strength gain of FAC is minimal during the colder months.

Although pozzolanic reactions are significantly diminished below

4.4 degrees C (40 degrees F), strength gain may continue at a

slower rate resulting from continued cement hydration. Chemical

admixtures can be utilized to off-set seasonal limitations.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.
24

Fly Ash Facts for Highway Engineers
25

Fly Ash in Stabilized Base Course Chapter.04

CHAPTER.4

FLY ASH IN STABILIZED BASE COURSE

INTRODUCTION

Fly ash stabilized base courses are proportioned mixtures of fly

ash, aggregate, and an activator (cement or lime) that, when

properly placed and compacted, produce a strong and durable

pavement base course. Fly ash stabilized base courses are costeffective substitutes for properly engineered full-depth asphalt,

cement-treated, and crushed stone base courses. Fly ash stabilized

base course is suitable for both flexible and rigid pavements.

MIX DESIGN AND SPECIFICATION REQUIREMENTS

Mix design. The stabilization of aggregate road bases with fly

ash has a long and successful history. This application, termed

pozzolanic-stabilized mixture (PSM) uses several materials and

material combinations to construct stabilized aggregate bases.

Class C fly ash can be used as a stand-alone material. Class F fly

ash can be used when blended with lime, portland cement or

cement kiln dust (CKD). Typical proportions for the Class F fly

ash lime blends are two to eight percent lime blended with 10 to

15 percent Class F fly ash. Also, 0.5 to 1.5 percent Type I

portland cement can be blended with Class F fly ash to produce

the stabilizing agent. The stabilization of aggregate bases provides

several advantages:

➤ Adds significant strength and durability

➤ Allows the use of marginal or low quality aggregates

➤ Permits better use of open graded base courses

➤ Reduces project cost

Strength. Closely controlled curing conditions are important as

both time and temperature significantly affect strength. Use

standard proctor-sized specimens; normal curing for lime/fly

ash/aggregate mixtures is at plus 38 degrees C (100 degrees F) for

7 days. Some states use different curing times and temperatures. 

Durability. It is important to ensure that adequate resistance to

freeze-thaw cycling is achieved before the onset of colder months.

The vacuum saturation test is normally used per ASTM C 593.
26

Fly Ash Facts for Highway Engineers

TESTING TO DETERMINE MIX PROPORTIONS

The following steps summarize the procedures for a laboratory

determination of mix proportions:

➤ Obtain a representative sample of aggregate. Determine the

particle size distribution of the aggregate. Screen the

aggregate through a three-quarter-inch sieve, and use the

portion passing the three-quarter-inch sieve for testing.

➤ Use proctor-size molds for all test samples. Add fly ash to

the aggregate in five different proportions, starting at the

lower limit (10 percent for coarse aggregate) and proceed in

convenient increments to the upper limit (20 percent for

coarse aggregate). Mold one test specimen at each fly ash

content in accordance with ASTM C 593 compaction

procedures at an estimated optimum moisture content.

➤ Determine the molded dry density of each aggregate-fly ash

blend. Plot the test results to identify a peak value or

maximum dry density.

➤ Select an optimum matrix content at least two percent above

the matrix content found at the maximum dry density. Then

determine the optimum moisture content and maximum dry

density for that blend.

➤ Determine the most suitable proportions of activator to fly ash.

Use five different activator-to-fly ash combinations at the

optimum matrix content. The five combinations should span

the recommended range of ratios for each activator. The

typical range of activator to fly ash ratio is 1:3 to 1:4 using

lime or portland cement; with either lime kiln dust or cement

kiln dust as an activator, the typical range is 1:1 to 1:2.

➤ Prepare six proctor-size specimens for each combination in

accordance with the compaction procedures in ASTM C

593. Cure all six test specimens for seven days in sealed

containers. For lime or kiln dust activators, cure at 37.8

degrees C (100 degrees F). For portland cement as the

activator, cure in a moist room at ambient conditions of 22.8

degrees C (73 degrees F) and 100 percent relative humidity.
27

Fly Ash in Stabilized Base Course Chapter.04

➤ Test three specimens for compressive strength and test the

other three specimens for durability at the end of the sevenday curing period, as described in ASTM C 593. Some

agencies utilize the ASTM D 560 freezing and thawing test,

which incorporates a brushing procedure and related

performance criteria developed by the Portland Cement

Association for soil-cement mixtures. In areas with virtually

no freezing and thawing, durability testing may be waived in

accordance with local practices.

➤ Plot a curve of compressive strength as a function of

activator percentage for each of the five activator-to-fly ash

combinations. Only test mixtures with a seven-day

compressive strength exceeding 2,760 kPa (400 psi) and

acceptable durability should be considered as a potential

PSM for field use.

➤ Select the most economical (lowest percentage activator)

mixture that exceeds the compressive strength and durability

requirements. The PSM actually used in the field should

contain a higher percentage of activator (a 0.5 percent

increase for lime or portland cement; and a one percent

increase for lime kiln dust or cement kiln dust) than the most

economical mixture identified in the laboratory. This assures

an adequate factor of safety for placement techniques

available in the field

Control of Materials

Lime. Hydrated lime is the most popular form used, although

quick lime and other products containing lime (kiln dust, etc.) can

be used successfully with appropriate precautions. Type 1 portland

cement has also been used successfully as a reactant when higher

early strength requirements or reactant market conditions dictate.

Determine actual lime content from samples using approved

titration methods (ASTM D 2901, AASHTO T 232).

Fly ash. Unconditioned (dry) or conditioned (water added) fly

ash can be used successfully. Check the reactivity of fly ash with

cement in accordance with ASTM C 593 and for comparison and

mix design results. Reactivity and fineness are the fly ash

characteristics that most directly affect PSM quality.
28

Fly Ash Facts for Highway Engineers

Aggregates. Aggregates must be sound and resist deterioration

from environmental elements. They may include sands, gravels,

or crushed stones. Gradation should be such that the final

mixture is mechanically stable and highly compactable

CONSTRUCTION PRACTICES

Blending of materials. Central plant mixing provides the best

quality, although in-place mixing has also been successful. Most

plants use a continuous pugmill, but central mix concrete plants

also work well. When unconditioned (dry) fly ash is used, a silo

and surge bin are needed for lime or cement and fly ash. When

belt feeding, drop dry fly ash on top of the aggregate to keep it

from rolling down the belt during pugmill loading. Conditioned

Class F fly ash can be routinely added through an aggregate.

In-place mixing. In-place mixing involves the use of portable

pulverizing and mixing equipment to blend granular soil or

aggregate materials with PSM reagent material and water in predetermined proportions at the project site. Class F fly ash is

usually added in conditioned form, although it may also be added

dry. The reagent materials (lime and/or portland cement) are

usually added after the fly ash and are most often introduced in a

dry form, although they may be added in a slurry form in order

to minimize dusting. Water is usually sprayed on the mixture as

needed just prior to in-place mixing.

Full depth reclamation. When deteriorated asphalt pavements

are recycled in place, a technique known as full-depth reclamation

can be used. The flexible pavement and a pre-determined

portion of the underlying base material are milled and pulverized

to a depth that can range from 150 to 300 mm (6 to 12 inches) or

more. The pulverized material is mixed within the reclaiming

machine while stabilization reagents (such as lime or portland

cement and fly ash) and water are introduced and blended with

the pulverized recycled paving aggregate. The reclaimer is then

followed by grading, spreading, and compaction equipment

working in the same manner and sequence as if plant-mixed PSM

material were delivered and placed at the project site.

Spreading. A PSM can be placed with spreader boxes or

asphalt laydown machines. Equipment with automated grade

control is highly recommended. Layers are normally spread to a
29

Fly Ash in Stabilized Base Course Chapter.04

thickness of 15 to 30 percent greater than the desired compacted

thickness. Maximum lift thickness is 200 to 250 mm (8 to 10 in).

Place the second lift on the same day or take appropriate

measures to ensure adequate sealing and subsequent bonding of

additional lifts.

Compaction. Achieving a high degree of compaction is crucial

to the successful performance of PSM roadbases. Final density

should be reached as quickly as possible to achieve the highest

ultimate strengths. This is especially the case when using Class C

fly ash as the stabilization reagent, since nearly all Class C fly

ashes are rapid setting and will not achieve the desired density

unless compacted immediately following spreading. Compacting

this noncohesive material with steel-wheel pneumatic, and

vibratory rollers has been successful. The PSM surface should be

kept moist throughout compaction. PSM moisture should be on

the low side of optimum to achieve the best field compaction. If

placed with a spreader box, the final surface should be fine

graded with a motor grader before final rolling with a steelwheeled roller. When fine grading, take care not to fill in the low

spots because the feathering-in will tend to reduce bonding at that

location, thus creating a potential trouble spot. If equipment with

graded control is used, fine grading is not required.

Curing. Compacted layers should be quickly sealed to prevent

drying. Apply a prime coat of 0.45 to 0.90 liters per square

meter (0.1 to 0.2 gal/sy) of cut back or emulsified asphalt to the

moist surface within 24 hours of final compaction. Multiple

applications of lighter coats tend to produce better penetration

and improve adhesion.

CAUTIONS

Ash quality. Fly ashes, which contain sulfur in excess of 5.0

percent as SO3 or contain scrubber residues, should be carefully

evaluated with specific project soils to evaluate the expansion

potential of the materials combination. 

Seasonal limitations. PSMs often require several weeks of

warmer weather to develop adequate strength to resist freezethaw cycling of the first winter. If late season placements are

necessary, add portland cement in lieu of some of the lime to

increase early strength.
30

Fly Ash Facts for Highway Engineers

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.

Figure 4-1. Full-depth reclamation of a bituminous road
31

Fly Ash in Flowable Fill Chapter.05

CHAPTER.5

FLY ASH IN FLOWABLE FILL

GENERAL

Flowable fill includes a family of products usually consisting of

water, fly ash, portland cement, and sometimes, coarse or fine

aggregates, or both. Flowable fill is an engineered, strength

controlled, fill material that is self-leveling, self-compacting and

non-settling. This material is also known as:

➤ Controlled density fill (CDF)

➤ Controlled low strength material (CLSM)

➤ Flowable fly ash

➤ High slump fly ash grout

➤ Lean concrete slurry

➤ Lean mix backfill

➤ Unshrinkable fill

Flowable fill mixtures make up a class of engineering materials

having characteristics and uses that overlap those of a broad

range of traditional materials including compacted soil, soilcement, and concrete. Consequently, flowable fills are

proportioned, mixed, and delivered in a form that resembles a

very workable concrete; and they provide for an in-place

product that is equivalent to a high-quality compacted soil

without the use of compaction equipment and related labor.

Virtually any coal fly ash can be used in flowable fill mixes.

The fly ash does not have to meet AASHTO M 295 (ASTM C

Figure 5-1. Flowable fill used in a utility trench application
32

Fly Ash Facts for Highway Engineers

618) specification requirements as a concrete admixture to be

suitable for use in flowable fill, even fly ash with high LOI or

carbon content is suitable. The individual specifying agencies

may have applicable specifications or regulations regarding fly

ash flowable fill. Regardless of the type of handling practices,

fly ash for flowable fill can be used in a dry or moisture

conditioned form. Fly ash recovered from storage ponds has

been used successfully. Flowable fill mixes using high-calcium

fly ash may not require any cement. Mix design and

performance testing are typically prepared to determine the

suitability of a fly ash and other ingredients for the specific

flowable requirements.

Upon comparison, flowable fill materials usually offer an

economic advantage over the cost of placing and compacting

earthen backfill materials. Depending on the job conditions

and costs involved, significant savings are possible. The closer

the project location is to the source of the flowable fill, the

greater the potential cost savings. Flowable fill also becomes

more economical than conventional earthen backfill if shoring

and/or sloping of the trench is necessary for worker safety

within the excavated area. With flowable fill, workers need not

be in the excavation, resulting in cost savings from less

excavation and no shoring.

Figure 5-2. Flowable fill eliminates the need for manual compaction
33

Fly Ash in Flowable Fill Chapter.05

MIX DESIGN AND SPECIFICATION REQUIREMENTS

Flowable fill mixes typically contain fly ash, portland cement and

water. Other cementing compounds (e.g., Class C ash, cement

kiln dust, etc.) may be used in lieu of portland cement in certain

applications. Filler materials such as bottom ash, sand (including

some foundry sands) or other aggregates can also be used in the

mix. The flowable character of these mixtures derives from the

spherical particle shape of fly ash or from a distribution of

spherical and irregular particle shapes and sizes in fly ash and

sand combinations when mixed with enough water to lubricate

the particle surfaces.

Fly ash can be a major ingredient in a flowable fill mix. It can be

less expensive than sand. Flowable fills can use non-concrete

grade fly ash, which may be obtained at a reduced cost. However,

when sand is more economical, fly ash can be limited to 300 or

fewer kilograms per cubic meter (500 or fewer pounds per cubic

yard). Water requirements for mixture fluidity will depend on the

surface parameters of all solids in the mixture, however, a range

of 250 to 400 liters per cubic meter (50 to 80 gallons per cubic

yard) would satisfy most materials combinations. Portland cement

is added, typically in quantities from 30 to 60 kilograms per cubic

meter (50 to 100 pounds per cubic yard) to provide a weak

cementitious matrix. 

The two basic types of flowable fill mixes are high fly ash content

and low fly ash content. The high fly ash content mixes typically

contain almost all fly ash with a small percentage of portland

cement and enough water to make the mix flowable. The low fly

ash content mixes contain a high percentage of fine aggregate or

filler material (usually sand), a low percentage of fly ash sufficient

Figure 5-3. Flowable fill can be used to backfill very narrow trenches
34

Fly Ash Facts for Highway Engineers

to help the sand particles flow, a small percentage of portland

cement (similar to that used in high fly ash content mixes), and

enough water to make the mix flowable.

ACI Committee 229 has designated low fly ash content mixes

that contain high percentages of fine aggregate as CLSM.

According to the ACI definition, CLSM has an upper

compressive strength limit of 8,300 kPa (1,200 psi), however,

strengths can be designed as low as 345 kPa (50 psi). Most

flowable fill mixes are designed to achieve a maximum strength of

1,000 to 1,400 kPa (150 to 200 psi) so as to allow for excavation at

a later time. It is important to remember that flowable fill mixes

with an ultimate strength in the 345 to 480 kPa (50 to 70 psi)

range have at least two to three times the bearing capacity of wellcompacted earthen backfill material.

Usually, flowable fill mix designs are proportioned based on the

percentage of fly ash, on a dry weight basis. The high fly ash

content mixes normally contain 95 percent fly ash and 5 percent

portland cement. In some areas, self-cementing fly ash accounts

for 100 percent of the cementitious material. Because the low fly

ash content mixes contain an additional ingredient (sand or filler),

there is a much broader range of mix proportions. Some typical

mix designs for high and low fly ash content mixes are included

in Tables 5-1 and 5-2. The most important physical characteristics

of flowable fill mixtures are: strength development, flowability,

hardening time, and bleeding/subsidence.

Table 5-1. High fly ash content mixes.

Table 5-2. Low fly ash content mixes.
35

Fly Ash in Flowable Fill Chapter.05

Strength development. Strength development in flowable fill

mixtures is directly related to the amount of cementitious material

and water content. With low CaO (Class F) fly ashes, the cement

content and water content relate directly to strength development.

With high CaO (Class C) fly ashes, no cement may be required

and strength is related directly to the fly ash and water content.

Most high fly ash content mixes only require from three to five

percent portland cement by dry weight of the fly ash to develop

28-day compressive strengths in the 345 to 1,000 kPa (50 to 150

psi) range. Long-term strength may gradually increase beyond

the 28-day strength. Water content of mix also influences strength

development. Water is added to achieve a desired flowability or

slump. At a given cement content, increased water content

usually results in a slight decrease in compressive strength

development over time. With high air (greater than 20 percent)

flowable fill mixtures, the water content is reduced and strength is

limited by the presence of air voids.

Flowability. Flowability is basically a function of the water and

entrained air content. The higher the water and air content, the

more flowable the mix. It is usually desirable to make the mix as

flowable as practical in order to take advantage of the selfcompacting qualities of flowable fill. Typical flowable fill mixes

with high air content are shown in Table 5-3.

Hardening time. Hardening time is directly related to the

cement content. Typical high fly ash flowable fill mixes

containing five percent cement achieve a sufficient set to support

the weight of an average person in about three to four hours,

depending on the temperature and humidity. Within 24 hours,

construction equipment can move across the surface without any

apparent damage. Some low fly ash flowable fill mixes containing

high calcium fly ash have reportedly set sufficiently to allow street

patching within one to two hours after placement.

Table 5-3. Flowable fill mixes with high air content.
36

Fly Ash Facts for Highway Engineers

Bleeding and subsidence. Bleeding and subsidence are possible

in high fly ash flowable fill mixes with relatively high water

contents (corresponding to a 254 mm/10-in slump). Evaporation

of the bleed water and absorption into the surrounding soil often

results in a subsidence of approximately 11 mm/m (1/8-in/ft) of

depth of the fill. This shrinkage may occur laterally as well as

vertically, but no additional shrinkage or long-term settlement of

flowable fill occurs after initial set. Prior to hardening, flowable

fill mixes are self-leveling.

Because the flowable fill is commonly obtained from ready-mixed

concrete producers, quality control of the fill is easily maintained

with the materials scales and metering devices already in use at

the concrete plant. Delivery is usually by conventional ready-mix

trucks. Flowable fill can also be pumped or placed by bucket,

conveyor, or hose. It does not usually segregate even if dropped

from considerable heights or pumped for long distances.

Applications of flowable fills include, but are not limited to:

backfill for bridge abutments, culverts, and trenches; fill for

embankments, bases, and subbases; bedding for slabs and pipes;

insulating fill; fill for caissons and piles; and fill for abandoned

storage tanks, shafts, and tunnels.

An important requirement for flowable fills in many applications

is that they can be removed with ordinary excavation equipment.

This means that compressive strength should be limited to 700 to

1,400 kPa (100 to 200 psi) with mechanical equipment and 345

kPa or less (50 psi) for manual excavation. Because of the

combined concrete/soils technology associated with flowable

mixtures, a variety of control tests have been applied to their use,

including flowability as measured by a concrete slump cone, a

flow cylinder, or a mortar flow cone and unit weight, as well as

measures of compressive strength, bearing capacity, or penetration

resistance (ACI 229R).
37

Fly Ash in Flowable Fill Chapter.05

As with any construction application, quality control and quality

assurance (QC/QA) of the materials and the mix are extremely

important. Good QC/QA will take maximum advantage of the

benefits of flowable fill.

Corrosion. Test methods developed to evaluate the potential for

corrosion of metals in soil backfills can be used to evaluate the

corrosion potential of flowable fills. ACI 229R describes several

test methods specific to the material in contact with flowable fill.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.

Figure 5-4. Bridge abutment backfill with flowable fill.

Figure 5-5. Bridge replaced by culverts and flowable fill.
38

Fly Ash Facts for Highway Engineers
39

Fly Ash in Structural Fills/Embankments Chapter.06

CHAPTER.6

FLY ASH IN STRUCTURAL FILLS/EMBANKMENTS

INTRODUCTION

Specifications for fly ash structural fills and embankments are

similar to specifications for engineered soil fills. Proper placement

and compaction of fly ash fills is required to achieve the desired

strength and compressibility characteristics assumed for design.

DESIGN AND SPECIFICATION REQUIREMENTS

Ash sources. Fly ash for structural fill applications can be

obtained as conditioned fly ash from a silo or as ponded fly ash

from a pond or a stockpile. Silo fly ash can be delivered with

close controls on moisture content and grain size distribution.

Conversely, the moisture content or grain size distribution of

ponded or stockpiled ash can vary considerably depending upon

its location within the pond or stockpile. Therefore, using ponded

Figure 6-1. Highway embankment with fly ash structural fill.
40

Fly Ash Facts for Highway Engineers

or stockpiled ash may require several series of laboratory tests. If

fly ash from more than one source is being used on the project at

the same time, it is preferable to place and compact the ashes

separately. Because of its self-hardening properties, high calcium

ash is typically stored dry in silos and hauled to the construction

site in pneumatic tank trucks, or is removed from a pond or

temporary stockpile. This procedure may not be necessary if the

site is close enough to the plant to allow the ash to be hauled in

the moistened condition.

Site conditions. As with any embankment project, standard

geotechnical techniques are used to evaluate subsurface soil and

groundwater conditions. The two most important subsurface

characteristics affecting embankment construction and

performance are shear strength and compressibility of the

foundation soils. ASTM E 1861 provides additional guidance

concerning technical and environmental considerations.

Physical, engineering, and chemical properties of the ash.

The physical and engineering properties of fly ash that will

determine the behavior of the embankment are grain-size

distribution, moisture-density relationships, shear strength,

compressibility, permeability, capillarity, and frost susceptibility.

Laboratory tests designed for testing soil properties may be applied

to testing fly ash. The chemical characteristics of the fly ash affect

the physical behavior as well as the quality of the leachate

produced by the ash. The utility company or its marketing agent

can provide information on the physical, engineering, and

chemical composition of the ash and leachate characteristics.

Design issues. The mechanical behavior and compaction

characteristics of fly ash are generally similar to those of silt. For

this reason, fly ash also shares some of the difficulties that are

characteristic of silt such as dusting, erosion and frost

susceptibility. These difficulties can be properly addressed during

the design of the embankment. For example, ice lenses grow in

silt-sized soils by wicking water up from a shallow groundwater

table. Such ice lenses expand during the winter and melt during

the spring causing unstable and soft embankment conditions. The

problem can be avoided by controlling upward seepage with a

layer of coarse-grained material or geotextile at the base of the

embankment. In general, avoid using fly ash as a fill material
41

Fly Ash in Structural Fills/Embankments Chapter.06

below the groundwater table or when the embankment design

cannot provide adequate drainage.

Environmental impacts. The trace element concentrations in

many fly ashes are similar to those found in naturally occurring

soils. Although the leachates of some fly ashes may contain trace

element concentrations that exceed drinking water quality

standards, this is also true of certain soils. State environmental

regulatory agencies can guide you through applicable test

procedures and water quality standards. The amount of leachate

produced can be controlled by assuring adequate compaction,

grading to promote surface runoff, and daily proof-rolling of the

finished subgrade to impede infiltration. When construction is

finished, a properly seeded soil cover will reduce infiltration. For

highway embankments, the pavement may be an effective barrier

to infiltration.

Erosion and dust control. To prevent wind and surface water

erosion of the fly ash embankment, use the same sediment and

erosion control techniques common during earthwork operations.

This includes wetting down exposed surfaces and installing silt

fences or straw bales around construction areas. Dusting will

likely occur when compacted fly ash is placed in dry, windy, or

freezing weather, or during traffic disturbance. During

construction, the surface should be kept moist, covered, or

stabilized with lime or bitumen. To prevent erosion and dusting

after completion of the embankment, protect the fly ash with

topsoil and vegetation or by sealing with bituminous emulsion.

Specifications/quality control. There are two types of

specifications used for the construction of fly ash embankments:

performance specifications and method specifications. The

construction quality control effort required depends greatly on the

selected specification approach. Additional information on

specifications and procedures can be found in ASTM E 1861. 

Performance specifications. Performance specifications

designate the required degree of compaction and the allowable

moisture content range. For road embankments, a typical

requirement is to compact the fly ash to 95 to 100 percent of

maximum dry density, as determined by AASHTO T 99 (ASTM

D 698). The allowable range for the moisture content is
42

Fly Ash Facts for Highway Engineers

determined by plotting the laboratory moisture-density

relationship. Figure 6-2 shows the dry density and moisture

content relationship. Fly ash may be variable enough that several

curves are required. It is preferable to place the ash at less than

optimum moisture because it will be difficult to impossible to

obtain compaction above optimum moisture contents. 

Method specifications. On some projects, method specifications

are preferable over performance specifications. The method

specification is based on the results of field compaction tests on

trial strips. The required lift thickness, weight of compaction

equipment, speed of compaction equipment, and the number of

passes are thus specified so that the necessary degree of

compaction is achieved. If vibratory compaction equipment is

being used, the resonant frequency of each compactor in use is also

established in the field. The method specification allows for

simpler quality control because the compaction procedures can be

monitored visually. Compactive efforts to achieve 95 percent of the

standard proctor maximum dry density are shown in Table 6-1. 

Figure 6-2. Allowable moisture content range below optimum

moisture content
43

Fly Ash in Structural Fills/Embankments Chapter.06

CONSTRUCTION PRACTICES

General. Recommended construction procedures have been

developed as the result of experience gained with trial embankments

and construction projects. Adjustments to these standard procedures

may be necessary, depending on actual field conditions.

Site preparation. Preparing the site for fly ash placement is

similar to requirements for soil fill materials. The site must be

cleared and grubbed. Topsoil should be retained for final cover.

Special attention should be given to draining the site and to

preventing seeps, pools, or springs from contacting the fly ash.

Delivery and on-site handling. Fly ash is usually hauled to the

site in covered dump trucks or pneumatic tanker trucks. Adjust

the water content of the ash to prevent dusting. In the case of

lagoon ash, reduce the water through temporary stockpiling

and/or mixing with drier silo ash to prevent road spillage during

hardening properties of high calcium ash, it is stored dry in silos

or pneumatic tanker trucks. Low calcium ash can be stockpiled

on-site if the ash is kept moist and if the ash is covered to prevent

dusting and erosion.

Table 6-1. Combinations required for 95 percent of the standard

proctor maximum dry density, AASHTO Method T 99
44

Fly Ash Facts for Highway Engineers

Spreading. Fly ash is usually spread and leveled with a dozer,

grader, or other equipment in loose lifts of 150 to 300 mm (6 to 12

in) thick. The lift is then tracked with the dozer or other

equipment for initial compaction.

Compaction equipment. Begin compaction as soon as the

material has been spread and is at the proper moisture content.

The most successful compaction results have been achieved with

self-propelled, pneumatic-tired rollers and self-propelled or towed

vibratory rollers. Vibratory rollers operated at the resonant

frequency of the fly ash will compact the ash more effectively and

in fewer passes than non-vibratory rollers. Table 6-1 lists the types

of compaction equipment that have been tested for use with fly ash.

Moisture control. Control of the required range of moisture is

an important consideration in the compaction procedure. Fly ash

can be conditioned with water at the plant silo to achieve the

desired moisture content. Be sure to compare the alternatives of

hauling fly ash moistened to the desired water content at the

plant, or adding water at the site. Hauling moist fly ash to the site

means higher transportation costs, while adding water on-site

sacrifices productivity in field placement.

Weather restrictions. Fly ash can often be placed during

inclement weather. In the winter months, frost usually penetrates

only the upper layer of the compacted ash, which can be recompacted upon thawing. During compaction, if the water

freezes too fast, the operation should be suspended until the

temperature rises. Construction can also proceed during wet

Figure 6-3. Spreading and compaction of fly ash structural fill.
45

Fly Ash in Structural Fills/Embankments Chapter

weather even if the moisture content of the ash is too high.

However, the equipment may bog down and it may be difficult to

achieve proper compaction.

Insensitivity to moisture variations. Because water is added to

low calcium fly ash during unloading from storage silos, fly ash

can be obtained at any moisture content that is desired. Although

the optimum moisture content is greater than that of silty soils, the

compaction behavior of low calcium fly ash is relatively

insensitive to variations in moisture content when placed dry of

optimum. High calcium fly ash, however, will self-harden when

water is added and becomes difficult to handle if not placed in a

timely manner.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.
46

Fly Ash Facts for Highway Engineers
47

Fly Ash in Soil Improvement Chapter.07

CHAPTER.7

FLY ASH IN SOIL IMPROVEMENT

INTRODUCTION

Soil stabilization is the alteration of soil properties to improve the

engineering performance of soils. The properties most often

altered are density, water content, plasticity and strength.

Modification of soil properties is the temporary enhancement of

subgrade stability to expedite construction.

Class C fly ash and Class F-lime product blends can be used in

numerous geotechnical applications common with highway

construction:

➤ To enhance strength properties

➤ Stabilize embankments

➤ To control shrink swell properties of expansive soils

➤ Drying agent to reduce soil moisture contents to permit

compaction

Class C fly ash can be used as a stand-alone material because of its

self-cementitious properties. Class F fly ash can be used in soil

stabilization applications with the addition of a cementitious agent

(lime, lime kiln dust, CKD, and cement). The self-cementitious

behavior of fly ashes is determined by ASTM D 5239. This test

provides a standard method for determining the compressive

strength of cubes made with fly ash and water (water/fly ash weight

ratio is 0.35), tested at seven days with standard moist curing. The

self-cementitious characteristics are ranked as shown below:

Very self-cementing > 500 psi (3,400 kPa)

Moderately self-cementing 100 - 500 psi (700 - 3,400 kPa)

Non self-cementing < 100 psi (700 kPa)

It should be noted that the results obtained from ASTM D 5239

only characterizes the cementitious characteristics of the fly ashwater blends and does not alone provide a basis to evaluate the

potential interactions between the fly ash and soil or aggregate.

The use of fly ash in soil stabilization and soil modification may

be subject to local environmental requirements pertaining to

leaching and potential interaction with ground water and

adjacent water courses.
48

Fly Ash Facts for Highway Engineers

SOIL STABILIZATION TO IMPROVE SOIL STRENGTH

Fly ash has been used successfully in many projects to improve

the strength characteristics of soils. Fly ash can be used to

stabilize bases or subgrades, to stabilize backfill to reduce lateral

earth pressures and to stabilize embankments to improve slope

stability. Typical stabilized soil depths are 15 to 46 centimeters (6

to 18 inches). The primary reason fly ash is used in soil

stabilization applications is to improve the compressive and

shearing strength of soils. The compressive strength of fly ash

treated soils is dependent on:

➤ In-place soil properties

➤ Delay time

➤ Moisture content at time of compaction

➤ Fly ash addition ratio

Delay time. Delay time is the elapsed time measured between

when the fly ash first comes into contact with water and final

compaction of the soil, fly ash and water mixture. Compressive

strength is highly dependent upon delay time. Both densities and

strength are reduced with increasing delay to final compaction.

Delay time is critical due to the rapid nature of the tricalcium

aluminate (C3A) reaction that occurs when Class C fly ash is

mixed with water. Densities and strengths are reduced because a

portion of the compactive energy must be used to overcome the

bonding of the soil particle by cementation and because a portion

of the cementation potential is lost. Maximum strength in soil-fly

ash mixtures is attained at no delay. Typically, a one-hour

compaction delay is specified for construction purposes. 

Figure 7-1. Mixing and shaping of fly ash stabilized soil.
49

Fly Ash in Soil Improvement Chapter.08

Moisture content. The water content of the fly ash stabilized

soil mixture affects the strength. The maximum strength realized

in soil-fly ash mixtures generally occurs at moisture contents

below optimum moisture content for density. For silt and clay

soils the optimum moisture content for strength is generally four

to eight percent below optimum for maximum density. For

granular soils the optimum moisture content for maximum

strength is generally one to three percent below optimum

moisture for density. Therefore, it is crucial that moisture content

be controlled during construction. Moisture content is usually

measured using a nuclear density measurement device.

Addition Ratios. Typical fly ash addition rates are 8 percent to

16 percent based on dry weight of soil. The addition rate depends

on the nature of the soil, the characteristics of the fly ash and the

strength desired. The addition rate must be determined by

laboratory mix design testing. In general the higher the addition

rate the higher the realized compressive strength. Fly ashes for

state department of transportation projects are usually specified to

meet AASHTO M 295 (ASTM C 618), even though the

requirements of this specification are not necessary for this

application and may increase the ash supply costs. Increasingly

non-AASHTO M 295 compliant materials are being successfully

used. It should be noted that virtually any fly ash that has at least

some self-cementitious properties can be engineered to perform in

transportation projects.

Soil Properties. The plasticity of soils treated with Class C or

other high-calcium fly ash is influenced by the types of clay

minerals present in the soil and their adsorbed water. Soils

Figure 7-2. Compaction of fly ash stabilized soil.
50

Fly Ash Facts for Highway Engineers

containing more than 10 percent sulfates have been prone to swell

excessively in some applications. Also, organic soils are difficult

to stabilize using fly ash.

SOIL STABILIZATION TO CONTROL SHRINK SWELL

Many clay soils (plastic soils) undergo extensive volumetric

changes when subjected to fluctuating moisture contents. These

volumetric changes if not controlled can lead to movements in

structures and impose loads which can cause premature failure.

The plasticity of soils has historically been quantified by the

plasticity index, as determined by ASTM D 4318. Typically

specifications limit the plasticity index of a soil to no more than

10-12 to ensure a stable material. In general terms, the higher the

plasticity index, the higher the potential to shrink or swell as the

soil undergoes moisture content fluctuations.

Historically, plastic soils have been treated with quick lime (CaO)

or hydrated lime [Ca (OH)2] to lower their plasticity. The lime

chemically reacts with the soil particles, effectively changing the

soil grains from clay size (less than 0.002 mm) to silt size (0.05 to

0.002 mm). The determination of the plasticity index is geared

towards measuring this chemical change in the soil.

Fly ash reduces the potential of a plastic soil to undergo

volumetric expansion by a physical cementing mechanism, which

cannot be evaluated by the plasticity index. Fly ash controls

shrink-swell by cementing the soil grains together much like a

portland cement bonds aggregates together to make concrete. By

bonding the soil grains together, soil particle movements are

restricted. Typical addition rates based on dry weight of soil are

12 to 15 percent.

Figure 7-3. Mixing and compaction of fly ash into a plastic soil
51

Fly Ash in Soil Improvement Chapter.07

It should be noted that a Class C fly ash may contain 15 percent

or more calcium expressed as calcium oxide, but very little of this

calcium is in the form of free calcium oxide (CaO). Therefore,

the determination of the plasticity limits is inappropriate when

evaluating the effect of fly ash on the shrink-swell characteristics

of a soil. To properly evaluate a fly ash for shrink-swell control,

the soil-fly ash blends should be tested with a soil expansion test

such as ASTM D 4829 or ASTM D 1883.

The swell potential of fly ash treated soils is typically less than 0.5

percent under confining pressures of 48 kPa (100 psf) even when

compacted two to four percent below optimum moisture content

for maximum density.

SOIL MODIFICATION TO REDUCE WATER CONTENT

Soils must be compacted to their maximum practical density to

provide a firm base for overlying structures. For soils to be

compacted the moisture content must be controlled because of

the relationship between soil density and moisture content. If the

soil to be compacted is either to wet or too dry, the moisture

content must be adjusted to near optimum to achieve maximum

density. If a soil is too dry, moisture is simply added. If a soil is

too wet, the moisture content of the soil must be lowered. Class

C fly ash and other high lime fly ash have been found to be very

effective drying agents, capable of reducing soil moisture content

by 30 percent or more.

The fly ash dries the soil by two basic mechanisms, chemical

reactions that consume moisture in the soil and by simple

dilution. Class C fly ashes contain tricalcium aluminate (C3A),

which is highly reactive with water. C3A is the chemical

compound present in ordinary portland cement which is

responsible for early strength. The C3A present in fly ash reacts

with the water, lowering the overall moisture content of the soil.

The drying effect of fly ash in wet soil is very rapid and

immediate, permitting the contractor to quickly proceed with

construction. In addition to the speeding up of the construction

process the use of fly ash provides several other benefits, such as

making the soil more resistant to additional water infiltration,

provides additional support for traffic, creates a more stable work

platform and reduces dusting from construction traffic.
52

Fly Ash Facts for Highway Engineers

CAUTIONS

Some state or local environmental agencies may require a

leaching test of the ash prior to use.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.
53

Fly Ash in Asphalt Pavement Chapter.08

CHAPTER.8

FLY ASH IN ASPHALT PAVEMENTS

INTRODUCTION

Fly ash can be used as a cost-effective mineral filler in hot mix

asphalt (HMA) paving applications. Where available locally, fly

ash may cost less than other mineral fillers. Also, due to the

lower specific gravity of fly ash, similar performance is obtained

using less material by weight, further reducing the material cost of

HMA. Mineral fillers increase the stiffness of the asphalt mortar

matrix, improving the rutting resistance of pavements. Mineral

fillers also help reduce the amount of asphalt drain down in the

mix during construction, which improves durability of the mix by

maintaining the amount of asphalt initially used in the mix.

Fly ash will normally meet mineral filler specification

requirements for gradation, organic impurities and plasticity.

Also, fly ash is hydrophobic (non-water wettable) , reducing the

potential for asphalt stripping; the presence of lime in some fly

ashes may also reduce stripping.

Mineral fillers have become more necessary as mixture gradations

have become coarser. Asphalt pavements with coarse gradations

are increasingly being designed because they perform well under

heavy traffic conditions. Some mixtures require higher dust to

asphalt ratios than can be gained through the recycling of

baghouse fines alone.

MIX DESIGN AND SPECIFICATION REQUIREMENTS

Fly ash must be in a dry form when used as mineral filler.

Typically, fly ash is handled in a similar manner to hydrated

lime - it is transported to the HMA plant in pneumatic tankers;

stored in watertight silos at the plant; and metered into the

HMA using an auger.

Engineering Properties. The physical requirements for 

mineral filler in bituminous paving are defined in AASHTO

M 17, Table 8-1.
54

Fly Ash Facts for Highway Engineers

Organic impurities. Although no standard for carbon content

or LOI is specified for fly ash used as mineral filler, laboratory

asphalt mortar evaluations incorporating fly ashes with LOIs up to

10 percent perform satisfactorily.

Plasticity. Fly ash is a non-plastic material.

Gradation. Most fly ashes typically fall within a size range of 60

to 90 percent passing the 75 μm (No. 200 sieve). 

Fineness. There is no fineness standard for mineral filler beyond

the AASHTO M 17 gradation requirements; however, when

Stone Matrix Asphalt (SMA) was first introduced to the U.S. in

1990, often a requirement for a maximum percent passing the 20

μm (No. 635) sieve was specified. Typically, fly ash has 40 to 70

percent passing the 20 μm sieve and performs well in mortar

testing and field performance.

Specific gravity. The specific gravity of fly ash varies from

source to source; it is typically 2.0 to 2.6. Most "non-fly ash"

mineral fillers have a specific gravity ranging from 2.6 to 2.8;

therefore, HMA designed with fly ash will usually require a lower

Figure 8-1. Stone matrix asphalt.

Table 8-1. AASHTO M 17 specification requirements for mineral

filler usein asphalt paving mixtures.
55

Fly Ash in Asphalt Pavement Chapter.08

percentage by weight to obtain the same performance (e.g., voids

in mineral aggregate, stiffness, drain down, etc.).

Rigden voids. Research indicates that mineral fillers with more

than 50 percent voids as determined using the modified Rigden's

voids test tend to overly stiffen the asphalt binder. Most fly ashes

have a Rigden void of less than 50 percent.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.
56

Fly Ash Facts for Highway Engineers
57

Fly Ash in Grouts for Pavement Subsealing Chapter.09

CHAPTER.9

FLY ASH GROUTS FOR PAVEMENT SUBSEALING

INTRODUCTION

Fly ash grouts are used for sealing voids beneath pavement slab

sections. The unconfined compressive strength requirements for a

grout mixture will usually exceed 8,300 kPa (1,200 psi) at 28 days

and will range between 4,100 and 5,500 kPa (600 and 800 psi) at

7 days. This strength profile for grouts differentiates them from

flowable fill materials inasmuch as the flowable fill definition (or

controlled low strength material) sets an unconfined compressive

strength limit of 8,300 kPa (1,200 psi) at 28 days. Furthermore,

the relatively small size of the void filled with the grout mixture

further differentiates this application from the larger voids where

the lower strength flowable fills are utilized.

The principal requirements for a slab stabilization material is that

it can flow to fill very small voids and still have adequate strength

to support the slab. A good stabilization material should remain

insoluble, incompressible, and not erode after it has been placed

and hardened. It should also have sufficiently low internal

friction to flow into very small voids and water channels. If the

material is too stiff, it will create a seat below the grout hole and

not fill all voids. If it is too wet, it will not have enough strength

to support the slab and may have a large amount of shrinkage.

Finally, it should have sufficient body to displace free water from

under the slab and develop adequate strength and durability. 

Figure 9-1. Fly ash grouts for pavement subsealing
58

Fly Ash Facts for Highway Engineers

MIX DESIGN AND SPECIFICATION REQUIREMENTS

Mix design. A typical cement-fly ash (Class F) mix design calls

for 1 part cement (Type I, II, or III), 3 parts fly ash, and enough

water for flowability, usually about 1.5 to 3.0 parts water. It may

be possible to reduce the cement component if a Class C ash is

used. Water content is determined using a flow cone and ASTM

C 939. The flow cone measures the flowability of the grout

mixture. The time of efflux is the flow time in seconds required

to empty the cone. A time of efflux in the range of 10 to 16

seconds gives the best flowability and strengthens cement-fly ash

grout slurries. The determination of initial set time of the grout in

laboratory tests is useful in comparing various mixes. Usually, the

Proctor Needle Test (AASHTO T 197/ASTM C 403) is used.

Typical set times with these tests are 11⁄2 to 2 hours.

Additives. It may be necessary to use additives or admixtures in

fly ash-cement grouts to achieve the required goals of flow, set

time, shrinkage and strength. 

Types of additives may include accelerators such as calcium

chloride to reduce set times; retarders to increase set time;

expanding materials such as powdered aluminum to offset

shrinkage; friction reducers or pumping aids to ease pumping,

increase flow, and aid in cleanup; wetting and dispersing agents to

get a more uniform mixture; and water-reducing agents to lower

the water content.

Strength. A minimum strength requirement is normally used to

ensure the durability of the grout. A typical value is 4,100 kPa

(600 psi) at 7 days measured by the standard mortar cube test,

AASHTO T 106 (ASTM C 109). A pozzolan-cement grout is

typically designed to achieve a 7-day compressive strength of

4,100 to 5,500 kPa (600 to 800 psi). The ultimate strength of the

grout will be much higher (10 to 28 mPa/1,500 to 4,000 psi). 

CONSTRUCTION PRACTICES

Equipment. In the past, most grouting/stabilization contractors

used batch mixers and bagged materials. Today's contractors

use very mobile, self-contained units that carry all the

equipment and materials needed for slab stabilization. The dry

materials are packaged in either uniform-volume bags or

measured by bulk weight.
59

Fly Ash in Grouts for Pavement Subsealing Chapter.09

A colloidal mixer is needed to produce the cement-fly ash grouts.

Grout mixes made by these mixers stay in suspension and resist

dilution by free water. The two most common types of colloidal

mixers are the centrifugal pump and the shear blade. The first

pulls the grout through a high-pressure centrifugal pump at high

velocity. The shear blade slices through the grout at speeds

between 800 rpm and 2000 rpm. The high shearing action and

subsequent pressure release of these mixers de-aerate the solid

particles, which allows them to be wetted to make a more

homogeneous mixture.

It is not recommended to use paddle-type drum mixers to

produce cement-fly ash grouts. Grout should never be mixed

using a screw conveyor, a mortar mixer, or a ready mix truck.

Mixes made with these devices will not be homogenous and will

require more water to promote flowability.

Injection holes. Grout injection holes are drilled in a pattern

determined by the contracting agency in consultation with the

contractor. They are typically no larger than 50 mm (2 in) in

diameter, drilled vertically and to a depth sufficient to penetrate

any stabilized base and into the subgrade material to a depth of

no more than 76 mm (3 in). The holes can be washed with water

or blown with air to create a small cavity to better intercept the

voids beneath the pavement. 

Upon completion of the subsealing, all drilled holes are sealed

flush with the pavement surface with a fast-setting concrete or

other patching material approved by the engineer.

Pumping and vertical grade control. String lines are

established above the pavement to monitor movement during

subsealing. An expanding rubber packer or other approved

device is lowered into the drilled holes. The discharge end of the

packer or hose must not extend below the lower surface of the

concrete pavement.

The pressure in the grout must be monitored by an accurate

pressure gauge in the grout line that is protected from the grout

slurry. Continuous grout pressures to 1,400 kPa (200 psi) are

typical, with pressures to 2,100 kPa (300 psi) allowed only for

short periods. In the event the pavement is bonded to the
60

Fly Ash Facts for Highway Engineers

subbase, brief pressure rises (10 seconds or fewer) to 4,100 kPa

(600 psi) are not unusual. Allow the water displaced from

pavement system voids by the grout to flow freely. Take

appropriate measures to prevent excessive loss of the grout

through cracks and joints or in the shoulder area.

Opening to traffic. The time allowed before traffic can get back

on the grouted slabs varies considerably. Deflection

measurements taken after slab stabilization have shown that the

deflections are reduced over a period of 30 minutes to 3 hours.

Grout hardening depends on the temperature, degree of

confinement, and grout properties.

CAUTIONS

Pavement movements above the specified tolerances may require

grinding or even removal and replacement of the pavement.

Existing cracks in the pavement should be marked prior to

subsealing operations. New cracks radiating diagonally through

the grout injection holes typically will be presumed to have been

caused by improper injection techniques and could result in

penalties to the contractor or even removal and replacement of

the pavement.

When the ambient air temperature is between 2 degrees C and 10

degrees C (35 degrees F and 50 degrees F), add an accelerator to

the grout mix. When calcium chloride is used, it must be

thoroughly pre-mixed with the water before the addition of dry

ingredients. Cement-fly ash grouts should not be used when the

ambient temperature is below 2 degrees C (35 degrees F) or when

the subgrade is frozen.

DESIGN AND CONSTRUCTION REFERENCES

See Appendix C.
61

Developments in Fly Ash Utilization Chapter.10

CHAPTER.10

DEVELOPMENTS IN FLY ASH UTILIZATION

INTRODUCTION

Several new and important technologies are being

commercialized in the areas of fly ash beneficiation and utilization

to bring fly ash into conformance with current AASHTO and

ASTM specifications for use in concrete. As the fly ash utilization

industry has matured, quality control, quality assurance, and

improved product performance have increasingly become

important. Technologies have been commercialized to improve

and assure fly ash quality for conventional concrete applications.

Also, fly ash utilization technologies have been developed to

produce high performance concrete products. 

Changes in boiler operations or alteration of air emissions control

systems at power plants will alter the quality of fly ash produced.

Factors that may impact ash quality in this way include:

➤ A reduction in the pozzolanic reactivity caused by increased

proportion of coarse particles

➤ The presence in the fly ash of excessive unburned carbon

(UBC)

➤ Chemical residuals from post-combustion emission control 

PARTICLE SIZE CONTROL

Screening. In mineral processing, it is common practice to use

screens to remove coarse particles from powdered products. In

general, dry screening of powders is not economically feasible at

below 45 μm (325 screen mesh size). Typical fly ash has a large

proportion of the particles (typically more than 50 percent) finer

than 45 μm. The use of coarse screens (100 or 80 mesh), might be

effective for the removal of most of the coarse particles, many of

which comprise UBC. The ability to remove carbon by this method

depends on the degree to which the carbon-rich particles are

discrete (liberated), and the size and shape of the carbon particles.

As such, screening may be effective as part of a general ash

processing scheme to reduce coarse particle content, reduce carbon

content, reduce variability, and improve concrete workability.
62

Fly Ash Facts for Highway Engineers

Air classification. Classification systems that use air to separate

particles by size and weight are also used to retain the finer ash

fraction. Air classification may be performed on ash for the

removal of coarse particles or the selective concentration of fine

particles. In some instances, the products differ not only in

particle fineness, but also in carbon content. The type of carbon

removed by air classification is comprised of the coarse unburned

coal particles. 

CARBON REDUCTION TECHNOLOGIES

Carbon removal by combustion. A simple approach to

remove carbon is to re-burn it, producing a pozzolan quality fly

ash. Carbon removal by combustion is commercially available. 

Electrostatic separation. Electrostatic separation exploits the

forces acting upon charged particles in an electrical field. This

technology basically involves passing fly ash through a high

voltage electric field, thus inducing opposite charges on the

mineral fly ash particles and residual carbon. The fly ash is

separated into a low carbon (less than three percent) pozzolan

fraction and a carbon rich reject. Electrostatic separation is

commercially available. 

TREATMENT PROCESSES

Chemical treatment/carbon modification. Residual carbon in

fly ash, as represented by the LOI value, is not the only factor

that may affect the performance of AEAs in concrete. Adsorption

of AEA due to the level and type of unburned carbon and

conductance due to the interaction of soluble ions from the trace

mineral components of the fly ash are also factors that interfere

with the performance of AEAs. Fly ash can be treated using a

chemical reagent to passivate the carbon adsorptive properties. In

this technology, carbon is not removed, but its effect on air

entrainment is minimized. The chemical treatment of fly ash for

carbon passivation is commercially available. 

Ammonia removal processes. Certain air emissions control

systems may deposit excess ammonia on the fly ash. Technologies

under development to deal with ammoniated fly ash include: heat

treatment, wet washing/stripping, and chemical treatment. Low

concentrations of ammonia have no impact on concrete properties,

however, a strong ammonia odor may be emitted. 
63

Developments in Fly Ash Utilization Chapter.10

OTHER DEVELOPMENTS IN TECHNOLOGY

Ultra fine fly ash. As compared to typical fly ash, with a mean

particle diameter ranging from 20-30 micrometers, ultra fine fly

ash can be produced with a mean particle diameter of 1-5

microns. The reduced particle size means that the pozzolanic

reaction, which is normally a slow process is accelerated. Further,

the finer particles may more completely react than the coarser

particles of fly ash. So, the durability and strength benefits that

one observes with a typical fly ash at a late age (more than one

year) can be attained at a much earlier age (less than 90 days) and

with a smaller dosage of an ultra fine fly ash. Table 10-1 shows

typical mix designs containing ultra fine fly ash. 

Typically, ultra fine fly ash is used at a replacement rate of 5 to 15

percent of the cement weight. At these dosage levels, it has been

demonstrated that ultra fine fly ashes contribute more to concrete

strength gain and permeability reduction than common AASHTO

M 295 (ASTM C 618) fly ash, and will perform comparable to

highly reactive pozzolans such as silica fume. Concrete durability

properties such as resistance to alkali-silica reaction, sulfate attack,

and corrosion are also enhanced by ultra fine fly ash.

Table 10-1. Typical mixes using ultra fine fly ash.
64

Fly Ash Facts for Highway Engineers

Fly ash blends (Class C and Class F). Both AASHTO M 295

(ASTM C 618) Class C and Class F fly ashes have their own

specific advantages when used as a cementitious material in

concrete. Table 10-2 summarizes some general properties of

concrete made with Class C and Class F fly ashes. It may be ideal

to have a fly ash with a low to moderate LOI and that can be used

to prepare a concrete that is very effective in resisting ASR, sulfate

attack, and at the same time have high early strengths. One way

to achieve that is to blend Class C and Class F fly ashes. The

exact ratio of the blend will depend upon the specific fly ashes and

their desired behavior in concrete. An example of fly ash blend

mix design is shown in Table 10-3. Blended fly ashes are being

marketed on a small scale in the United States. 

High volume fly ash concrete (HVFAC). HVFAC refers to

concrete where the fly ash comprises more than 30 percent of the

total cementitious materials. HVFAC has a lower cost and is

more durable than conventional concrete, and affords improved

resistance to ASR and sulfate attack. Several successful field

applications have been completed. Adequate early strengths and

set times are obtained by using high range water reducers to

achieve a very low water/cement ratio. Allowable cement

substitution rates are currently limited by state transportation

department specifications. An example of an HVFAC mix design

is shown in Table 10-4. 

Table 10-2. Influence of the class of fly ash on

concrete performance.
65

Developments in Fly Ash Utilization Chapter.10

Table 10-4. Example mix design for HVFAC.

Table 10-3. Example mix designs incorporating Class C and

Class F blend.
66

Fly Ash Facts for Highway Engineers
67

Conversion Factors 

APPENDIX.A

CONVERSION FACTORS

2.54 cm

0.305 m

0.436 kg

0.907 tons

(metric)

6.895 kPa

0.479 kPa

4.527 liters/m3

5/9 (° F - 32°)

9/5 (° C + 32°)

1 inch

1 foot

1 pound

1 ton

1 psi

1 psf

1 gal/sy

° C

° F

APPENDIX.A
68

Fly Ash Facts for Highway Engineers
69

Specifications and Recommended Practice Guidelines APPENDIX.B

American Association of State Highway and Transportation

Officials (AASHTO)

American Society for Testing and Materials (ASTM)

Specification for Portland Cements

Standard Specification for Mineral Filler for

Bituminous Paving Mixtures 

Specification for Blended Cements

Standard Specifications for Coal Fly Ash and

Raw or Calcined Natural Pozzolan for Use as a

Mineral Admixture in Concrete

Standard Method of Test for Quality of Water to

be Used in Concrete

Standard Method of Test for Moisture-Density

Relation of Soils Using a 2.5 kg (5.5 pound)

Rammer and 305 mm (12 inch) Drop

Standard Method of Test for Compressive

Strength of Hydraulic Cement Mortar (Using 50

mm or 2 inch) Cube Specimens

Standard Method of Test for Time of Settling of

Concrete Mixtures by Penetration Resistance

Standard Method of Test for Cement Content of

Freshly Mixed Soil-Cement

AASHTO M 85

AASHTO M 172

AASHTO M 240

AASHTO M 295

AASHTO T 26

AASHTO T 99

AASHTO T 106

AASHTO T 197

AASHTO T 232

Test Method for Unit Weight and Voids in

Aggregates 

Specification for Concrete Aggregates

Test Method for Compressive Strength of

Cylindrical Concrete Specimens

Test Method for Flexural Strength of Concrete

(Using Simple Beam with Third-Point Loading)

Specification for Ready Mixed Concrete

Test Method for Compressive Strength of

Hydraulic Cement Mortars (Using 2-in. or 50-

mm Cube Specimens)

ASTM C 29

ASTM C 33

ASTM C 39

ASTM C 78

ASTM C 94

ASTM C 109

APPENDIX.B

SPECIFICATIONS AND RECOMMENDED PRACTICE GUIDELINES 
70

Fly Ash Facts for Highway Engineers

Specification for Portland Cement

Test Method for Time of Setting of Concrete

Mixtures by Penetration Resistance

Specification for Chemical Admixtures for Concrete

Fly Ash and Other Pozzolans for Use with Lime

Specification for Blended Hydraulic Cements

Test Method for Drying Shrinkage of Mortar

Containing Portland Cement

Specification for Coal Fly Ash and Raw or

Calcined Natural Pozzolan for Use as a Mineral

Admixture in Portland Cement Concrete

Test Method for Resistance of Concrete to Rapid

Freezing and Thawing

Test Method for Scaling Resistance of Concrete

Surfaces Exposed to Deicing Chemicals

Specification for Concrete Made by Volumetric

Batching and Continuous Mixing

Test Method for Evaluation of Cement Strength

Uniformity From a Single Source

Specification for Packaged, Dry, Rapid-Hardening

Cementitious Materials for Concrete Repairs

Test Method for Flow of Grout for PreplacedAggregate Concrete (Flow Cone Method)

Performance Specification for Hydraulic Cement

Test Method for Laboratory Compaction

Characteristics of Soil Using Standard Effort

(12,400 ft-lbf/ft3 (600 kN-m/m3))

Test Method for CBR (California Bearing Ratio) of

Laboratory-Compacted Soils

Test Method for Cement Content of Freshly Mixed

Soil-Cement

Test Method for Liquid Limit, Plastic Limit, and

Plasticity Index of Soils

Test Method for Expansion Index of Soils

Practice for Characterizing Fly Ash for Use in Soil

Stabilization

Guide for Use of Coal Combustion By-Products in

Structural Fills

ASTM C 150 

ASTM C 403 

ASTM C 494

ASTM C593

ASTM C 595

ASTM C 596

ASTM C 618

ASTM C 666

ASTM C 672

ASTM C 685

ASTM C 917

ASTM C 928

ASTM C 939

ASTM C 1157

ASTM D 698

ASTM D 1883

ASTM D 2901

ASTM D 4318

ASTM D 4829

ASTM D 5239

ASTM E 1861
71

Specifications and Recommended Practice Guidelines APPENDIX.B

ACI 211.1 

ACI 211.4R

ACI 212.3R

ACI 214

ACI 226.1R

ACI 229R

ACI 232.2R

ACI 301

ACI 305R

ACI 306R

ACI 318

ACI 363R

ACI 363.2R

Standard Practice for Selecting Proportions of

Normal, Heavyweight, and Mass Concrete

Guide for Selecting Proportions for High-Strength

Concrete with Portland Cement and Fly Ash

Chemical Admixtures for Concrete

Recommended Practice for Evaluation of

Strength Results of Concrete Strength Testing

Ground Granulated Blast Furnace Slag as A

Cementitious Constituent in Concrete

Controlled Low Strength Materials (CLSM)

Use of Fly Ash in Concrete

Specifications for Structural Concrete for

Buildings

Hot Weather Concreting

Cold Weather Concreting

Building Code Requirements for Reinforced

Concrete

State-of-the-Art Report on High-Strength Concrete 

Guide to Quality Control and Testing of High

Strength Concrete

Other Test Methods

ANSI/AWWA C105/A21.5 American National Standard for

Polyethylene Encasement for DuctileIron Pipe Systems

American Concrete Institute (ACI)
72

Fly Ash Facts for Highway Engineers
73

Design and Construction References APPENDIX.C

PORTLAND CEMENT CONCRETE

ACI Manual of Concrete Practice, American Concrete Institute, 

Farmington Hills, Michigan.

Fast Track Concrete Pavements, Technical Bulletin 004P, American

Concrete Pavement Association, Skokie, Illinois, 1994.

STABILIZED BASE COURSE

Coal Fly Ash in Pozzolanic Stabilized Mixtures for Flexible Pavement 

Systems (Flexible Pavement Manual), American Coal Ash 

Association, Washington, DC.

Guidelines and Guide Specifications for Using Pozzolanic Stabilized 

Mixture (Base Course or Subbase) and Fly Ash for In-Place 

Subgrade Soil Modifications, AASHTO Task Force Report 

28, Washington, DC.

Soil and Pavement Base Stabilization with Self-Cementing Coal Fly Ash, 

American Coal Ash Association, Alexandria, Virginia, 

May 1999.

FLOWABLE FILL

ACI 229R, Controlled Low Strength Materials, American Concrete 

Institute, Farmington Hills, Michigan.

NRMCA Flowable Fill Pamphlet, National Ready Mixed 

Concrete Association.

GROUTS FOR PAVEMENT SUBSEALING

Slab Stabilization Guidelines for Concrete Pavements, Technical Bulletin 

018P, American Concrete Pavement Association, Skokie, 

Illinois, 1994.

SOIL IMPROVEMENT

Soil and Pavement Base Stabilization with Self-Cementing Coal Fly Ash, 

American Coal Ash Association, Alexandria, Virginia, 

May 1999.

Fly Ash for Soil Improvement, Geotechnical Special Publication No. 

36, American Society of Civil Engineers, New York, New 

York, 1993.

APPENDIX.C

DESIGN AND CONSTRUCTION REFERENCES
74

Fly Ash Facts for Highway Engineers

Guidelines and Guide Specifications for Using Pozzolanic Stabilized 

Mixture (Base Course or Subbase) and Fly Ash for In-Place 

Subgrade Soil Modifications, AASHTO Task Force Report 

28, Washington, DC.

STRUCTURAL FILLS/EMBANKMENTS

ASTM E 1861, Structural Guide for the Use of Coal Combustion ByProducts in Structural Fills, American Society for Testing 

and Materials, West Conshohocken, Pennsylvania.

Technical Advisory T 5080.9, Use of Coal Ash in Embankments 

and Bases, U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC, May 1988.

ASPHALT PAVEMENTS

American Association of State Highway Transportation Officials. 

Standard Method of Test, Mineral Filler for Bituminous 

Paving Mixtures, AASHTO Designation M17-83, Part 1 

Specifications, 14th Edition, 1986.

L. Allen Cooley, Jr. and Michael H. Huner. Evaluation of Fly Ash 

Sources for Use as Mineral Filler in Hot Mix Asphalt, 

Proceedings: 14th International Symposium on 

Management and Use of Coal Combustion Products, 

Volume 2, Palo Alto, California, January 2001.

Guidelines for Materials, Production, and Placement of Stone Matrix 

Asphalt, National Asphalt Pavement Association, 

Information Series 118, August 1994.