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The importance of using the right type and quality of

aggregates cannot be overemphasized. The fine and

coarse aggregates generally occupy 60% to 75% of the

concrete volume (70% to 85% by mass) and strongly influ-

ence the concrete’s freshly mixed and hardened proper-

ties, mixture proportions, and economy. Fine aggregates

(Fig. 5-1) generally consist of natural sand or crushed

stone with most particles smaller than 5 mm (0.2 in.).

Coarse aggregates (Fig. 5-2) consist of one or a com-

CHAPTER 5

Aggregates for Concrete

bination of gravels or crushed stone with particles

predominantly larger than 5 mm (0.2 in.) and generally

between 9.5 mm and 37.5 mm (

⁄

in. and 1

⁄

in.). Some

natural aggregate deposits, called pit-run gravel, consist

of gravel and sand that can be readily used in concrete

after minimal processing. Natural gravel and sand are

usually dug or dredged from a pit, river, lake, or seabed.

Crushed stone is produced by crushing quarry rock, boul-

ders, cobbles, or large-size gravel. Crushed air-cooled

blast-furnace slag is also used as fine or coarse aggregate.

The aggregates are usually washed and graded at the

pit or plant. Some variation in the type, quality, cleanli-

ness, grading, moisture content, and other properties is

expected. Close to half of the coarse aggregates used in

portland cement concrete in North America are gravels;

most of the remainder are crushed stones.

Naturally occurring concrete aggregates are a mixture

of rocks and minerals (see Table 5-1). A mineral is a natu-

rally occurring solid substance with an orderly internal

structure and a chemical composition that ranges within

narrow limits. Rocks, which are classified as igneous, sedi-

mentary, or metamorphic, depending on origin, are gener-

ally composed of several minerals. For example, granite

contains quartz, feldspar, mica, and a few other minerals;

most limestones consist of calcite, dolomite, and minor

amounts of quartz, feldspar, and clay. Weathering and

erosion of rocks produce particles of stone, gravel, sand,

silt, and clay.

Recycled concrete, or crushed waste concrete, is a

feasible source of aggregates and an economic reality,

especially where good aggregates are scarce. Conven-

tional stone crushing equipment can be used, and new

equipment has been developed to reduce noise and dust.

Aggregates must conform to certain standards for

optimum engineering use: they must be clean, hard,

strong, durable particles free of absorbed chemicals, coat-

ings of clay, and other fine materials in amounts that could

affect hydration and bond of the cement paste. Aggregate

particles that are friable or capable of being split are unde-

sirable. Aggregates containing any appreciable amounts

of shale or other shaly rocks, soft and porous materials,

Fig. 5-1. Closeup of fine aggregate (sand). (69792)

Fig. 5-2. Coarse aggregate. Rounded gravel (left) and

crushed stone (right). (69791)

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should be avoided; certain types of chert should be espe-

cially avoided since they have low resistance to weather-

ing and can cause surface defects such as popouts.

Identification of the constituents of an aggregate

cannot alone provide a basis for predicting the behavior of

aggregates in service. Visual inspection will often disclose

weaknesses in coarse aggregates. Service records are

invaluable in evaluating aggregates. In the absence of a

performance record, the aggregates should be tested

before they are used in concrete. The most commonly used

aggregates—sand, gravel, crushed stone, and air-cooled

blast-furnace slag—produce freshly mixed normal-weight

concrete with a density (unit weight) of 2200 to 2400

kg/m

(140 to 150 lb/ft

). Aggregates of expanded shale,

clay, slate, and slag (Fig. 5-3) are used to produce struc-

tural lightweight concrete with a freshly mixed density

ranging from about 1350 to 1850 kg/m

(90 to 120 lb/ft

Other lightweight materials such as pumice, scoria,

perlite, vermiculite, and diatomite are used to produce

insulating lightweight concretes ranging in density from

about 250 to 1450 kg/m

(15 to 90 lb/ft

). Heavyweight

materials such as barite, limonite, magnetite, ilmenite,

hematite, iron, and steel punchings or shot are used to

produce heavyweight concrete and radiation-shielding

concrete (ASTM C 637 and C 638). Only normal-weight

aggregates are discussed in this chapter. See Chapter 18

for special types of aggregates and concretes.

Normal-weight aggregates should meet the require-

ments of ASTM C 33 or AASHTO M 6/M 80. These speci-

fications limit the permissible amounts of deleterious

substances and provide requirements for aggregate char-

acteristics. Compliance is determined by using one or

more of the several standard tests cited in the following

sections and tables. However, the fact that aggregates

satisfy ASTM C 33 or AASHTO M 6/M 80 requirements

does not necessarily assure defect-free concrete.

For adequate consolidation of concrete, the desirable

amount of air, water, cement, and fine aggregate (that is,

the mortar fraction) should be about 50% to 65% by

absolute volume (45% to 60% by mass). Rounded aggre-

gate, such as gravel, requires slightly lower values, while

crushed aggregate requires slightly higher values. Fine

aggregate content is usually 35% to 45% by mass or

volume of the total aggregate content.

CHARACTERISTICS OF AGGREGATES

The important characteristics of aggregates for concrete are

listed in Table 5-2 and most are discussed in the following

section:

Grading

Grading is the particle-size distribution of an aggregate as

determined by a sieve analysis (ASTM C 136 or AASHTO

Design and Control of Concrete Mixtures

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EB001

Table 5-1. Rock and Mineral Constituents in

Aggregates

Minerals Igneous rocks Metamorphic rocks

Silica Granite Marble

Quartz Syenite Metaquartzite

Opal Diorite Slate

Chalcedony Gabbro Phyllite

Tridymite Peridotite Schist

Cristobalite Pegmatite Amphibolite

Silicates Volcanic glass Hornfels

Feldspars Obsidian Gneiss

Ferromagnesian Pumice Serpentinite

Hornblende Tuff

Augite Scoria

Clay Perlite

Illites Pitchstone

Kaolins Felsite

Chlorites Basalt

Montmorillonites

Mica

Sedimentary rocks

Zeolite Conglomerate

Carbonate Sandstone

Calcite Quartzite

Dolomite Graywacke

Sulfate Subgraywacke

Gypsum Arkose

Anhydrite Claystone, siltstone,

Iron sulfide argillite, and shale

Pyrite Carbonates

Marcasite Limestone

Pyrrhotite Dolomite

Iron oxide Marl

Magnetite Chalk

Hematite Chert

Goethite

lmenite

Limonite

For brief descriptions, see “Standard Descriptive Nomenclature of

Constituents of Natural Mineral Aggregates” (ASTM C 294).

Fig. 5-3. Lightweight aggregate. Expanded clay (left) and

expanded shale (right). (69793)

T 27). The range of particle sizes in aggregate is illustrated

in Fig. 5-4. The aggregate particle size is determined by

using wire-mesh sieves with square openings. The seven

standard ASTM C 33 (AASHTO M 6/M 80) sieves for fine

aggregate have openings ranging from 150 µm to 9.5 mm

(No. 100 sieve to

⁄

in.). The 13 standard sieves for coarse

aggregate have openings ranging from 1.18 mm to 100 mm

(0.046 in. to 4 in.). Tolerances for the dimensions of open-

ings in sieves are listed in ASTM E 11 (AASHTO M 92).

Size numbers (grading sizes) for coarse aggregates

apply to the amounts of aggregate (by mass) in percent-

ages that pass through an assortment of sieves (Fig. 5-5).

For highway construction, ASTM D 448 (AASHTO M 43)

lists the same 13 size numbers as in ASTM C 33

Chapter 5

◆

Aggregates for Concrete

Table 5.2. Characteristics and Tests of Aggregate

Characteristic Significance Test designation* Requirement or item reported

Resistance to abrasion Index of aggregate quality; ASTM C 131 (AASHTO T 96) Maximum percentage of

and degradation wear resistance of floors and ASTM C 535 weight loss. Depth of wear

pavements ASTM C 779 and time

Resistance to freezing Surface scaling, roughness, ASTM C 666 (AASHTO T 161) Maximum number of cycles

and thawing loss of section, and ASTM C 682 or period of frost immunity;

aesthetics (AASHTO T 103 durability factor

Resistance to disintegration Soundness against ASTM C 88 (AASHTO T 104) Weight loss, particles

by sulfates weathering action exhibiting distress

Particle shape and Workability of fresh ASTM C 295 Maximum percentage of flat

surface texture concrete ASTM D 3398 and elongated particles

Grading Workability of fresh concrete; ASTM C 117 (AASHTO T 11) Minimum and maximum

economy ASTM C 136 (AASHTO T 27) percentage passing

standard sieves

Fine aggregate degradation Index of aggregate quality; ASTM C 1137 Change in grading

Resistance to degradation

during mixing

Uncompacted void content Workability of fresh ASTM C 1252 (AASHTO T 304) Uncompacted voids and

of fine aggregate concrete specific gravity values

Bulk density Mix design calculations; ASTM C 29 (AASHTO T 19) Compact weight and

(unit weight) classification loose weight

Relative density Mix design calculations ASTM C 127 (AASHTO T 85) —

(specific gravity) fine aggregate

ASTM C 128 (AASHTO T 84)

coarse aggregate

Absorption and surface Control of concrete quality ASTM C 70 —

moisture (water-cement ratio) ASTM C 127 (AASHTO T 85)

ASTM C 128 (AASHTO T 84)

ASTM C 566 (AASHTO T 255)

Compressive and flexural Acceptability of fine ASTM C 39 (AASHTO T 22) Strength to exceed 95% of

strength aggregate failing other tests ASTM C 78 (AASHTO T 97) strength achieved with purified

sand

Definitions of constituents Clear understanding and ASTM C 125 —

communication ASTM C 294

Aggregate constituents Determine amount of ASTM C 40 (AASHTO T 21) Maximum percentage allowed

deleterious and organic ASTM C 87 (AASHTO T 71) of individual constituents

materials ASTM C 117 (AASHTO T 11)

ASTM C 123 (AASHTO T 113)

ASTM C 142 (AASHTO T 112)

ASTM C 295

Resistance to alkali Soundness against ASTM C 227 Maximum length change,

reactivity and volume volume change ASTM C 289 constituents and amount

change ASTM C 295 of silica, and alkalinity

ASTM C 342

ASTM C 586

ASTM C 1260 (AASHTO T 303)

ASTM C 1293

* The majority of the tests and characteristics listed are referenced in ASTM C 33 (AASHTO M 6/M 80). ACI 221R-96 presents additional test

methods and properties of concrete influenced by aggregate characteristics.

aggregate proportions as well as cement and water

requirements, workability, pumpability, economy, poros-

ity, shrinkage, and durability of concrete. Variations in

grading can seriously affect the uniformity of concrete

from batch to batch. Very fine sands are often uneconomi-

cal; very coarse sands and coarse aggregate can produce

harsh, unworkable mixtures. In general, aggregates that

do not have a large deficiency or excess of any size and

give a smooth grading curve will produce the most satis-

factory results.

The effect of a collection of various sizes in reducing

the total volume of voids between aggregates is illustrated

by the simple method shown in Fig. 5-7. The beaker on the

left is filled with large aggregate particles of uniform size

(AASHTO M 6/ M 80) plus an additional six more coarse

aggregate size numbers. Fine aggregate or sand has only

one range of particle sizes for general construction and

highway work.

The grading and grading limits are usually expressed

as the percentage of material passing each sieve. Fig. 5-6

shows these limits for fine aggregate and for one size of

coarse aggregate.

There are several reasons for specifying grading limits

and nominal maximum aggregate size; they affect relative

Design and Control of Concrete Mixtures

◆

EB001

Fig. 5-4. Range of particle sizes found in aggregate for use

in concrete. (8985)

Fig. 5-5. Making a sieve analysis test of coarse aggregate

in a laboratory. (30175-A)

oarse aggregate

ize N

o. 57

Fine sand

Optional,

see text

150

300

600

1.18 mm

2.36 mm

4.75 mm

12.5 mm

25 mm

37.5 mm

No. 100

No. 50

No. 30

No. 16

No. 8

No. 4

in.

1 in.

in.

Sieve sizes

Percent passing, by mass

100

Fine aggregate

Coarse sand

Fig. 5-6. Curves indicate the limits specified in ASTM C 33

for fine aggregate and for one commonly used size number

(grading size) of coarse aggregate.

Fig. 5-7. The level of liquid in the graduates, representing

voids, is constant for equal absolute volumes of aggre-

gates of uniform but different size. When different sizes are

combined, the void-content decreases. The illustration is

not to scale.

Video

and shape; the middle beaker is filled with an equal

volume of small aggregate particles of uniform size and

shape; and the beaker on the right is filled with particles

of both sizes. Below each beaker is a graduate with the

amount of water required to fill the voids in that beaker.

Note that when the beakers are filled with one particle size

of equal volume, the void content is constant, regardless of

the particle size. When the two aggregate sizes are

combined, the void content is decreased. If this operation

were repeated with several additional sizes, a further

reduction in voids would occur. The cement paste require-

ment for concrete is related to the void content of the

combined aggregates.

During the early years of concrete technology it was

sometimes assumed that the smallest percentage of voids

(greatest density of aggregates) was the most suitable for

concrete. At the same time, limits were placed on the

amount and size of the smallest particles. It is now known

that, even on this restricted basis, this is not the best target

for the mix designer. However, production of satisfactory,

economical concrete requires aggregates of low void

content, but not the lowest. Voids in aggregates can be

tested according to ASTM C 29 or AASHTO T 19.

In reality, the amount of cement paste required in

concrete is greater than the volume of voids between the

aggregates. This is illustrated in Fig. 5-8. Sketch A repre-

sents large aggregates alone, with all particles in contact.

Sketch B represents the dispersal of aggregates in a matrix

of paste. The amount of paste is necessarily greater than

the void content of sketch A in order to provide workabil-

ity to the concrete; the actual amount is influenced by the

workability and cohesiveness of the paste.

Fine-Aggregate Grading

Requirements of ASTM C 33 or AASHTO M 6/M 43 per-

mit a relatively wide range in fine-aggregate gradation,

but specifications by other organizations are sometimes

more restrictive. The most desirable fine-aggregate grad-

ing depends on the type of work, the richness of the

mixture, and the maximum size of coarse aggregate. In

leaner mixtures, or when small-size coarse aggregates are

used, a grading that approaches the maximum recom-

mended percentage passing each sieve is desirable for

workability. In general, if the water-cement ratio is kept

constant and the ratio of fine-to-coarse aggregate is chosen

correctly, a wide range in grading can be used without

measurable effect on strength. However, the best economy

will sometimes be achieved by adjusting the concrete

mixture to suit the gradation of the local aggregates.

Fine-aggregate grading within the limits of ASTM C

33 (AASHTO M 6) is generally satisfactory for most

concretes. The ASTM C 33 (AASHTO M 6) limits with

respect to sieve size are shown in Table 5-3.

Chapter 5

◆

Aggregates for Concrete

Fig. 5-8. Illustration of the dispersion of aggregates in co-

hesive concrete mixtures.

Table 5-3. Fine-Aggregate Grading Limits

(ASTM C 33/AASHTO M 6)

Sieve size Percent passing by mass

9.5 mm (

⁄

in.) 100

4.75 mm (No. 4) 95 to 100

2.36 mm (No. 8) 80 to 100

1.18 mm (No. 16) 50 to 85

600 µm (No. 30) 25 to 60

300 µm (No. 50) 5 to 30 (AASHTO 10 to 30)

150 µm (No. 100) 0 to 10 (AASHTO 2 to 10)

The AASHTO specifications permit the minimum

percentages (by mass) of material passing the 300 µm (No.

50) and 150 µm (No. 100) sieves to be reduced to 5% and

0% respectively, provided:

1. The aggregate is used in air-entrained concrete

containing more than 237 kilograms of cement per

cubic meter (400 lb of cement per cubic yard) and

having an air content of more than 3%.

2. The aggregate is used in concrete containing more

than 297 kilograms of cement per cubic meter (500 lb

of cement per cubic yard) when the concrete is not air-

entrained.

3. An approved supplementary cementitious material is

used to supply the deficiency in material passing

these two sieves.

Other requirements of ASTM C 33 (AASTHO M 6) are:

1. The fine aggregate must not have more than 45%

retained between any two consecutive standard

sieves.

2. The fineness modulus must be not less than 2.3 nor

more than 3.1, nor vary more than 0.2 from the typical

value of the aggregate source. If this value is ex-

given maximum-size coarse aggregate can be varied over

a moderate range without appreciable effect on cement

and water requirement of a mixture if the proportion of

fine aggregate to total aggregate produces concrete of

good workability. Mixture proportions should be changed

to produce workable concrete if wide variations occur in

the coarse-aggregate grading. Since variations are difficult

to anticipate, it is often more economical to maintain

uniformity in manufacturing and handling coarse aggre-

gate than to reduce variations in gradation.

The maximum size of coarse aggregate used in con-

crete has a bearing on the economy of concrete. Usually

more water and cement is required for small-size aggre-

gates than for large sizes, due to an increase in total aggre-

gate surface area. The water and cement required for a

slump of approximately 75 mm (3 in.) is shown in Fig. 5-9

for a wide range of coarse-aggregate sizes. Fig. 5-9 shows

that, for a given water-cement ratio, the amount of cement

required decreases as the maximum size of coarse aggre-

gate increases. The increased cost of obtaining and/or

ceeded, the fine aggregate should be rejected unless

suitable adjustments are made in proportions of fine

and coarse aggregate.

The amounts of fine aggregate passing the 300 µm

(No. 50) and 150, µm (No. 100) sieves affect workability,

surface texture, air content, and bleeding of concrete. Most

specifications allow 5% to 30% to pass the 300 µm (No. 50)

sieve. The lower limit may be sufficient for easy placing

conditions or where concrete is mechanically finished,

such as in pavements. However, for hand-finished

concrete floors, or where a smooth surface texture is de-

sired, fine aggregate with at least 15% passing the 300 µm

(No. 50) sieve and 3% or more passing the 150 µm (No.

100) sieve should be used.

Fineness Modulus. The fineness modulus (FM) of either

fine or coarse aggregate according to ASTM C 125 is calcu-

lated by adding the cumulative percentages by mass

retained on each of a specified series of sieves and divid-

ing the sum by 100. The specified sieves for determining

FM are: 150 µm (No. 100), 300 µm (No. 50), 600 µm (No.

30), 1.18 mm (No. 16), 2.36 mm (No. 8), 4.75 mm (No. 4),

9.5 mm (

⁄

in.), 19.0 mm (

⁄

in.), 37.5 mm (1

⁄

in.), 75 mm

(3 in.) and, 150 mm (6 in.).

FM is an index of the fineness of an aggregate—the

higher the FM, the coarser the aggregate. Different aggre-

gate grading may have the same FM. FM of fine aggregate

is useful in estimating proportions of fine and coarse

aggregates in concrete mixtures. An example of how the

FM of a fine aggregate is determined (with an assumed

sieve analysis) is shown in Table 5-4.

Degradation of fine aggregate due to friction and

abrasion will decrease the FM and increase the amount of

materials finer than the 75 µm (No. 200) sieve.

Coarse-Aggregate Grading

The coarse aggregate grading requirements of ASTM C 33

(AASHTO M 80) permit a wide range in grading and a

variety of grading sizes (see Table 5-5). The grading for a

Design and Control of Concrete Mixtures

◆

EB001

Table 5-5. Grading Requirements for Coarse Aggregates (ASTM C 33 and AASHTO M 80)

Amounts finer than each laboratory sieve, mass percent passing

Size Nominal size, sieves 100 mm 90 mm 75 mm 63 mm 50 mm

number with square openings (4 in.) (3

⁄

in.) (3 in.) (2

⁄

in.) (2 in.)

1 90 to 37.5 mm (3

⁄

to 1

⁄

in.) 100 90 to 100 — 25 to 60 —

2 63 to 37.5 mm (2

⁄

to 1

⁄

in.) ——100 90 to 100 35 to 70

3 50 to 25.0 mm (2 to 1 in.) ———100 90 to 100

357 50 to 4.75 mm (2 in. to No. 4) ———100 95 to 100

4 37.5 to 19.0 mm (1

⁄

in.) ————100

467 37.5 to 4.75 mm (1

⁄

in. to No. 4) ————100

5 25.0 to 12.5 mm (1 to

⁄

in.) —————

56 25.0 to 9.5 mm (1 to

⁄

in.) —————

57 25.0 to 4.75 mm (1 in. to No. 4) —————

6 19.0 to 9.5 mm (

⁄

in.) —————

67 19.0 to 4.75 mm (

⁄

in. to No. 4) —————

7 12.5 to 4.75 mm (

⁄

in. to No. 4) —————

8 9.5 to 2.36 mm (

⁄

in. to No. 8) —————

Table 5-4. Determination of Fineness Modulus of

Fine Aggregates

Percentage

of individual Cumulative

fraction Percentage percentage

retained, passing, retained,

Sieve size by mass by mass by mass

9.5 mm (

⁄

in.) 0 100 0

4.75 mm (No. 4) 2 98 2

2.36 mm (No. 8) 13 85 15

1.18 mm (No. 16) 20 65 35

600 µm (No. 30) 20 45 55

300 µm (No. 50) 24 21 79

150 µm (No. 100) 18 3 97

Pan 3 0 —

Total 100 283

Fineness modulus

= 283 ÷ 100 = 2.83

Video

handling aggregates much larger than 50 mm (2 in.) may

offset the savings in using less cement. Furthermore,

aggregates of different maximum sizes may give slightly

different concrete strengths for the same water-cement

ratio. In some instances, at the same water-cement ratio,

concrete with a smaller maximum-size aggregate could

have higher compressive strength. This is especially true

for high-strength concrete. The optimum maximum size

of coarse aggregate for higher strength depends on factors

such as relative strength of the cement paste, cement-

aggregate bond, and strength of the aggregate particles.

The terminology used to specify size of coarse aggre-

gate must be chosen carefully. Particle size is determined

by size of sieve and applies to the aggregate passing that

sieve and not passing the next smaller sieve. When speak-

ing of an assortment of particle sizes, the size number (or

grading size) of the gradation is used. The size number

applies to the collective amount of aggregate that passes

through an assortment of sieves. As shown in Table 5-5,

the amount of aggregate passing the respective sieves is

given in percentages; it is called a sieve analysis.

Because of past usage, there is sometimes confusion

about what is meant by the maximum size of aggregate.

ASTM C 125 and ACI 116 define this term and distinguish

it from nominal maximum size of aggregate. The maxi-

mum size of an aggregate is the smallest sieve that all of a

particular aggregate must pass through. The nominal

maximum size of an aggregate is the smallest sieve size

through which the major portion of the aggregate must

pass. The nominal maximum-size sieve may retain 5% to

15% of the aggregate depending on the size number. For

example, aggregate size number 67 has a maximum size of

25 mm (1 in.) and a nominal maximum size of 19 mm (

⁄

in.). Ninety to one hundred percent of this aggregate must

pass the 19-mm (

⁄

-in.) sieve and all of the particles must

pass the 25-mm (1-in.) sieve.

The maximum size of aggregate that can be used

generally depends on the size and shape of the concrete

member and the amount and distribution of reinforcing

steel. The maximum size of aggregate particles generally

should not exceed:

Chapter 5

◆

Aggregates for Concrete

3/8

4.75

9.5

12.5

19.0

25.0

37.5

50.0

75.0

112.0

1/2 3/4 1 1

233/16

Non-air-entrained concrete

Air-entrained concrete

Non-air-entrained concrete

Air-entrained concrete

Slump approximately 75 mm (3 in.)

w/c ratio: 0.54 by mass

Slump approximately 75 mm (3 in.)

w/c ratio: 0.54 by mass

Maximum nominal size of aggregate, mm

Maximum nominal size of aggregate, in.

300

200

100

450

400

350

300

250

200

500

400

300

200

100

700

600

500

400

Cement, kg/m

Water, kg/m

Cement, lb/yd

Water, lb/yd

Fig. 5-9. Cement and water contents in relation to maximum

size of aggregate for air-entrained and non-air-entrained con-

crete. Less cement and water are required in mixtures having

large coarse aggregate (Bureau of Reclamation 1981).

37.5 mm 25.0 mm 19.0 mm 12.5 mm 9.5 mm 4.75 mm 2.36 mm 1.18 mm

⁄

in.) (1 in.) (

⁄

in.) (

⁄

in.) (

⁄

in.) (No. 4) (No. 8) (No. 16)

0 to 15

—

0 to 5

—————

0 to 15

—

0 to 5

—————

35 to 70

0 to 15

—

0 to 5

——

—

35 to 70

—

10 to 30

—

0 to 5

——

90 to 100 20 to 55 0 to 15 — 0 to 5 ———

95 to 100 — 35 to 70 — 10 to 30 0 to 5 ——

100 90 to 100 20 to 55 0 to 10 0 to 5 ———

100

90 to 100

40 to 85

10 to 40

0 to 15

0 to 5

——

100 95 to 100 — 25 to 60 — 0 to 10 0 to 5 —

— 100 90 to 100 20 to 55 0 to 15 0 to 5 ——

— 100 90 to 100 — 25 to 55 0 to 10 0 to 5 —

——100 90 to 100 40 to 70 0 to 15 0 to 5 —

— ——100 85 to 100 10 to 30 0 to 10 0 to 5

• The optimum mixture has the least particle interfer-

ence and responds best to a high frequency, high

amplitude vibrator.

However, this optimum mixture cannot be used for

all construction due to variations in placing and finishing

needs and availability. Crouch (2000) found in his studies

on air-entrained concrete that the water-cement ratio

could be reduced by over 8% using combined aggregate

gradation. Shilstone (1990) also analyzes aggregate grada-

tion by coarseness and workability factors to improve

aggregate gradation.

Gap-Graded Aggregates

In gap-graded aggregates certain particle sizes are inten-

tionally omitted. For cast-in-place concrete, typical gap-

graded aggregates consist of only one size of coarse

aggregate with all the particles of fine aggregate able to

pass through the voids in the compacted coarse aggregate.

Gap-graded mixes are used in architectural concrete to

obtain uniform textures in exposed-aggregate finishes.

They can also used in normal structural concrete because

of possible improvements in some concrete properties, and

to permit the use of local aggregate gradations (Houston

1962 and Litvin and Pfeifer 1965).

For an aggregate of 19-mm (

⁄

-in.) maximum size, the

4.75 mm to 9.5 mm (No. 4 to

⁄

in.) particles can be omit-

ted without making the concrete unduly harsh or subject

to segregation. In the case of 37.5 mm (1

⁄

in.) aggregate,

usually the 4.75 mm to 19 mm (No. 4 to

⁄

in.) sizes are

omitted.

Care must be taken in choosing the percentage of fine

aggregate in a gap-graded mixture. A wrong choice can

result in concrete that is likely to segregate or honeycomb

because of an excess of coarse aggregate. Also, concrete

with an excess of fine aggregate could have a high water

demand resulting in a low-density concrete. Fine aggregate

is usually 25% to 35% by volume of the total aggregate. The

lower percentage is used with rounded aggregates and the

higher with crushed material. For a smooth off-the-form

finish, a somewhat higher percentage of fine aggregate to

total aggregate may be used than for an exposed-aggregate

finish, but both use a lower fine aggregate content than

continuously graded mixtures. Fine aggregate content also

depends upon cement content, type of aggregate, and

workability.

Air entrainment is usually required for workability

since low-slump, gap-graded mixes use a low fine aggre-

gate percentage and produce harsh mixes without

entrained air.

Segregation of gap-graded mixes must be prevented

by restricting the slump to the lowest value consistent with

good consolidation. This may vary from zero to 75 mm (to

3 in.) depending on the thickness of the section, amount of

reinforcement, and height of casting. Close control of grad-

ing and water content is also required because variations

might cause segregation. If a stiff mixture is required, gap-

1. One-fifth the narrowest dimension of a concrete

member

2. Three-quarters the clear spacing between reinforcing

bars and between the reinforcing bars and forms

3. One-third the depth of slabs

These requirements may be waived if, in the judg-

ment of the engineer, the mixture possesses sufficient

workability that the concrete can be properly placed with-

out honeycomb or voids.

Combined Aggregate Grading

Aggregate is sometimes analyzed using the combined

grading of fine and coarse aggregate together, as they exist

in a concrete mixture. This provides a more thorough

analysis of how the aggregates will perform in concrete.

Sometimes mid-sized aggregate, around the 9.5 mm (

⁄

in.) size, is lacking in an aggregate supply, resulting in a

concrete with high shrinkage properties, high water

demand, poor workability, poor pumpability, and poor

placeability. Strength and durability may also be affected.

Fig. 5-10 illustrates an ideal gradation; however, a

perfect gradation does not exist in the field—but we can

try to approach it. If problems develop due to a poor

gradation, alternative aggregates, blending, or special

screening of existing aggregates, should be considered.

Refer to Shilstone (1990) for options on obtaining optimal

grading of aggregate.

The combined gradation can be used to better control

workability, pumpability, shrinkage, and other properties

of concrete. Abrams (1918) and Shilstone (1990) demon-

strate the benefits of a combined aggregate analysis:

• With constant cement content and constant consis-

tency, there is an optimum for every combination of

aggregates that will produce the most effective water

to cement ratio and highest strength.

Design and Control of Concrete Mixtures

◆

EB001

37.5

25.0

19.0

12.5

9.5

4.75

2.36

1.18

0.600

0.300

0.150

0.075

0.045

Percent retained

Sieve size in mm

Fig. 5-10. Optimum combined aggregate grading for

concrete.

Video

graded aggregates may produce higher strengths than

normal aggregates used with comparable cement contents.

Because of their low fine-aggregate volumes and low

water-cement ratios, gap-graded mixtures might be

considered unworkable for some cast-in-place construc-

tion. When properly proportioned, however, these con-

cretes are readily consolidated with vibration.

Particle Shape and Surface Texture

The particle shape and surface texture of an aggregate

influence the properties of freshly mixed concrete more

than the properties of hardened concrete. Rough-textured,

angular, elongated particles require more water to

produce workable concrete than do smooth, rounded,

compact aggregates. Hence, aggregate particles that are

angular require more cement to maintain the same water-

cement ratio. However, with satisfactory gradation, both

crushed and noncrushed aggregates (of the same rock

types) generally give essentially the same strength for the

same cement factor. Angular or poorly graded aggregates

can also be more difficult to pump.

The bond between cement paste and a given aggre-

gate generally increases as particles change from smooth

and rounded to rough and angular. This increase in bond

is a consideration in selecting aggregates for concrete

where flexural strength is important or where high

compressive strength is needed.

Void contents of compacted fine or coarse aggregate

can be used as an index of differences in the shape and

texture of aggregates of the same grading. The mixing

water and cement requirement tend to increase as aggre-

gate void content increases. Voids between aggregate par-

ticles increase with aggregate angularity.

Aggregate should be relatively free of flat and elon-

gated particles. A particle is called flat and elongated

when the ratio of length to thickness exceeds a specified

value. See ASTM D 4791 for determination of flat, and/or

elongated particles. ASTM D 3398 provides an indirect

method of establishing a particle index as an overall meas-

ure of particle shape or texture, while ASTM C 295

provides procedures for the petrographic examination of

aggregate.

Flat and elongated aggregate particles should be

avoided or at least limited to about 15% by mass of the total

aggregate. This requirement is equally important for coarse

and for crushed fine aggregate, since fine aggregate made

by crushing stone often contains flat and elongated parti-

cles. Such aggregate particles require an increase in mixing

water and thus may affect the strength of concrete, particu-

larly in flexure, if the water-cement ratio is not adjusted.

A number of automated test machines are available

for rapid determination of the particle size distribution of

aggregate. Designed to provide a faster alternative to the

standard sieve analysis test, these machines capture and

analyze digital images of the aggregate particles to deter-

mine gradation. Fig. 5-11 shows a videograder that meas-

ures size and shape of an aggregate by using line-scan

cameras wherein two-dimensional images are constructed

from a series of line images. Other machines use matrix-

scan cameras to capture two-dimensional snapshots of the

falling aggregate. Maerz and Lusher (2001) developed a

dynamic prototype imaging system that provides particle

size and shape information by using a miniconveyor

system to parade individual fragments past two orthogo-

nally oriented, synchronized cameras.

Bulk Density (Unit Weight) and Voids

The bulk density or unit weight of an aggregate is the

mass or weight of the aggregate required to fill a container

of a specified unit volume. The volume referred to here is

that occupied by both aggregates and the voids between

aggregate particles.

The approximate bulk density of aggregate com-

monly used in normal-weight concrete ranges from about

1200 to 1750 kg/m

(75 to 110 lb/ft

). The void content

between particles affects paste requirements in mix design

(see preceding sections, “Particle Shape and Surface

Texture” and “Grading”). Void contents range from about

30% to 45% for coarse aggregates to about 40% to 50% for

fine aggregate. Angularity increases void content while

larger sizes of well-graded aggregate and improved grad-

ing decreases void content (Fig. 5-7). Methods of deter-

mining the bulk density of aggregates and void content

are given in ASTM C 29 (AASHTO T 19). In these stan-

dards, three methods are described for consolidating the

aggregate in the container depending on the maximum

size of the aggregate: rodding, jigging, and shoveling. The

measurement of loose uncompacted void content of fine

aggregate is described in ASTM C 1252.

Relative Density (Specific Gravity)

The relative density (specific gravity) of an aggregate is the

ratio of its mass to the mass of an equal absolute volume of

Chapter 5

◆

Aggregates for Concrete

Fig. 5-11. Videograder for measuring size and shape of

aggregate. (69545)

Video

aggregates in order to accurately meet the water require-

ment of the mix design. If the water content of the concrete

mixture is not kept constant, the water-cement ratio will

vary from batch to batch causing other properties, such as

the compressive strength and workability to vary from

batch to batch.

Coarse and fine aggregate will generally have absorp-

tion levels (moisture contents at SSD) in the range of 0.2%

to 4% and 0.2% to 2%, respectively. Free-water contents will

usually range from 0.5% to 2% for coarse aggregate and 2%

to 6% for fine aggregate. The maximum water content of

drained coarse aggregate is usually less than that of fine

aggregate. Most fine aggregates can maintain a maximum

drained moisture content of about 3% to 8% whereas coarse

aggregates can maintain only about 1% to 6%.

Bulking. Bulking is the increase in total volume of moist

fine aggregate over the same mass dry. Surface tension in

the moisture holds the particles apart, causing an increase

in volume. Bulking of a fine aggregate (such as sand)

occurs when it is shoveled or otherwise moved in a damp

condition, even though it may have been fully consoli-

water. It is used in certain computations for mixture

proportioning and control, such as the volume occupied by

the aggregate in the absolute volume method of mix

design. It is not generally used as a measure of aggregate

quality, though some porous aggregates that exhibit accel-

erated freeze-thaw deterioration do have low specific grav-

ities. Most natural aggregates have relative densities

between 2.4 and 2.9 with corresponding particle (mass)

densities of 2400 and 2900 kg/m

(150 and 181 lb/ft

Test methods for determining relative densities for

coarse and fine aggregates are described in ASTM C 127

(AASHTO T 85) and ASTM C 128 (AASHTO T 84), respec-

tively. The relative density of an aggregate may be deter-

mined on an ovendry basis or a saturated surface-dry (SSD)

basis. Both the ovendry and saturated surface-dry relative

densities may be used in concrete mixture proportioning

calculations. Ovendry aggregates do not contain any

absorbed or free water. They are dried in an oven to con-

stant weight. Saturated surface-dry aggregates are those in

which the pores in each aggregate particle are filled with

water but there is no excess water on the particle surface.

Density

The density of aggregate particles used in mixture propor-

tioning computations (not including voids between parti-

cles) is determined by multiplying the relative density

(specific gravity) of the aggregate times the density of

water. An approximate value of 1000 kg/m

(62.4 lb/ft

) is

often used for the density of water. The density of aggre-

gate, along with more accurate values for water density,

are provided in ASTM C 127 (AASHTO T 85) and ASTM

C 128 (AASHTO T 84). Most natural aggregates have

particle densities of between 2400 and 2900 kg/m

(150

and 181 lb/ft

Absorption and Surface Moisture

The absorption and surface moisture of aggregates should

be determined according to ASTM C 70, C 127, C 128, and

C 566 (AASHTO T 255) so that the total water content of

the concrete can be controlled and correct batch weights

determined. The internal structure of an aggregate particle

is made up of solid matter and voids that may or may not

contain water.

The moisture conditions of aggregates are shown in

Fig. 5-12. They are designated as:

1. Ovendry—fully absorbent

2. Air dry—dry at the particle surface but containing

some interior moisture, thus still somewhat absorbent

Saturated surface dry (SSD)—neither absorbing water

from nor contributing water to the concrete mixture

4. Damp or wet—containing an excess of moisture on

the surface (free water)

The amount of water added at the concrete batch

plant must be adjusted for the moisture conditions of the

Design and Control of Concrete Mixtures

◆

EB001

Ovendry Air dry

Saturated,

surface dry

Damp

or wet

State

None Less than

potential

absorption

Equal to

potential

absorption

Greater

than

absorption

Total

moisture:

Fig. 5-12. Moisture conditions of aggregate.

0 5 10 15 20

Percent of moisture added by mass

to dry, rodded fine aggregate

Fine grading

Medium

grading

Coarse

grading

Percent increase in volume over dry,

rodded fine aggregate

Fig. 5-13. Surface moisture on fine aggregate can cause

considerable bulking; the amount varies with the amount of

moisture and the aggregate grading (PCA Major Series 172

and PCA ST20).

Video

dated beforehand. Fig. 5-13 illustrates how the amount of

bulking of fine aggregate varies with moisture content and

grading; fine gradings bulk more than coarse gradings for

a given amount of moisture. Fig. 5-14 shows similar infor-

mation in terms of weight for a particular fine aggregate.

Since most fine aggregates are delivered in a damp condi-

tion, wide variations can occur in batch quantities if batch-

ing is done by volume. For this reason, good practice has

long favored weighing the aggregate and adjusting for

moisture content when proportioning concrete.

Resistance to Freezing and Thawing

The frost resistance of an aggregate, an important charac-

teristic for exterior concrete, is related to its porosity,

absorption, permeability, and pore structure. An aggre-

gate particle may absorb so much water (to critical satura-

tion) that it cannot accommodate the expansion and

hydraulic pressure that occurs during the freezing of

water. If enough of the offending particles are present, the

result can be expansion of the aggregate and possible

disintegration of the concrete. If a single problem particle

is near the surface of the concrete, it can cause a popout.

Popouts generally appear as conical fragments that break

out of the concrete surface. The offending aggregate parti-

cle or a part of it is usually found at the bottom of the void.

Generally it is coarse rather than fine aggregate particles

with higher porosity values and medium-sized pores (0.1

to 5 µm) that are easily saturated and cause concrete dete-

rioration and popouts. Larger pores do not usually be-

come saturated or cause concrete distress, and water in

very fine pores may not freeze readily.

At any freezing rate, there may be a critical particle size

above which a particle will fail if frozen when critically

saturated. This critical size is dependent upon the rate of

freezing and the porosity, permeability, and tensile strength

of the particle. For fine-grained aggregates with low perme-

ability (cherts for example), the critical particle size may be

within the range of normal aggregate sizes. It is higher for

coarse-grained materials or those with capillary systems

interrupted by numerous macropores (voids too large to

hold moisture by capillary action). For these aggregates the

critical particle size may be sufficiently large to be of no

consequence, even though the absorption may be high. If

potentially vulnerable aggregates are used in concrete

subjected to periodic drying while in service, they may

never become sufficiently saturated to cause failure.

Cracking of concrete pavements caused by the freeze-

thaw deterioration of the aggregate within concrete is

called D-cracking. This type of cracking has been ob-

served in some pavements after three or more years of

service. D-cracked concrete resembles frost-damaged

concrete caused by paste deterioration. D-cracks are

closely spaced crack formations parallel to transverse and

longitudinal joints that later multiply outward from the

joints toward the center of the pavement panel (Fig. 5-15).

D-cracking is a function of the pore properties of certain

types of aggregate particles and the environment in which

the pavement is placed. Due to the natural accumulation

of water under pavements in the base and subbase layers,

the aggregate may eventually become saturated. Then

Chapter 5

◆

Aggregates for Concrete

Mass of fine aggregate and water in one

unit of volume measured loose in air

Moisture in fine aggregate, percent by mass

02468101214161820

Bulk density, kg/m

Bulk density, lb/ft

2480

2280

2080

1880

1680

1480

1280

150

140

130

120

110

100

Bulking, percent by volume

Fig. 5-14. Bulk density is compared with the moisture con-

tent for a particular sand (PCA Major Series 172).

Fig. 5-15. D-cracking along a transverse joint caused by

failure of carbonate coarse aggregate (Stark 1976). (30639)

ovendried and the percentage of weight loss calculated.

Unfortunately, this test is sometimes misleading.

Aggregates behaving satisfactorily in the test might

produce concrete with low freeze-thaw resistance;

conversely, aggregates performing poorly might produce

concrete with adequate resistance. This is attributed, at

least in part, to the fact that the aggregates in the test are

not confined by cement paste (as they would be in

concrete) and the mechanisms of attack are not the same as

in freezing and thawing. The test is most reliable for strat-

ified rocks with porous layers or weak bedding planes.

An additional test that can be used to evaluate aggre-

gates for potential D-cracking is the rapid pressure release

method. An aggregate is placed in a pressurized chamber

and the pressure is rapidly released causing the aggregate

with a questionable pore system to fracture (Janssen and

Snyder 1994). The amount of fracturing relates to the

potential for D-cracking.

Wetting and Drying Properties

Weathering due to wetting and drying can also affect the

durability of aggregates. The expansion and contraction

coefficients of rocks vary with temperature and moisture

content. If alternate wetting and drying occurs, severe

strain can develop in some aggregates, and with certain

types of rock this can cause a permanent increase in

volume of the concrete and eventual breakdown. Clay

lumps and other friable particles can degrade rapidly with

repeated wetting and drying. Popouts can also develop

due to the moisture-swelling characteristics of certain

aggregates, especially clay balls and shales. While no

specific tests are available to determine this tendency, an

experienced petrographer can often be of assistance in

determining this potential for distress.

Abrasion and Skid Resistance

The abrasion resistance of an aggregate is often used as a

general index of its quality. Abrasion resistance is essential

when the aggregate is to be used in concrete subject to

abrasion, as in heavy-duty floors or pavements. Low abra-

sion resistance of an aggregate may increase the quantity

of fines in the concrete during mixing; consequently, this

may increase the water requirement and require an adjust-

ment in the water-cement ratio.

The most common test for abrasion resistance is the

Los Angeles abrasion test (rattler method) performed in

accordance with ASTM C 131 (AASHTO T 96) or ASTM C

535. In this test a specified quantity of aggregate is placed

in a steel drum containing steel balls, the drum is rotated,

and the percentage of material worn away is measured.

Specifications often set an upper limit on this mass loss.

However, a comparison of the results of aggregate abra-

sion tests with the abrasion resistance of concrete made

with the same aggregate do not generally show a clear

with freezing and thawing cycles, cracking of the concrete

starts in the saturated aggregate (Fig. 5-16) at the bottom

of the slab and progresses upward until it reaches the

wearing surface. This problem can be reduced either by

selecting aggregates that perform better in freeze-thaw

cycles or, where marginal aggregates must be used, by

reducing the maximum particle size. Also, installation of

effective drainage systems for carrying free water out

from under the pavement may be helpful.

The performance of aggregates under exposure to

freezing and thawing can be evaluated in two ways: (1)

past performance in the field, and (2) laboratory freeze-

thaw tests of concrete specimens. If aggregates from the

same source have previously given satisfactory service

when used in concrete, they might be considered suitable.

Aggregates not having a service record can be considered

acceptable if they perform satisfactorily in air-entrained

concretes subjected to freeze-thaw tests according to

ASTM C 666 (AASHTO T 161). In these tests concrete spe-

cimens made with the aggregate in question are subjected

to alternate cycles of freezing and thawing in water.

Deterioration is measured by (1) the reduction in the

dynamic modulus of elasticity, (2) linear expansion, and

(3) weight loss of the specimens. An expansion failure

criterion of 0.035% in 350 freeze-thaw cycles or less is used

by a number of state highway departments to help indi-

cate whether or not an aggregate is susceptible to D-crack-

ing. Different aggregate types may influence the criteria

levels and empirical correlations between laboratory

freeze-thaw tests. Field service records should be made to

select the proper criterion (Vogler and Grove 1989).

Specifications may require that resistance to weather-

ing be demonstrated by a sodium sulfate or magnesium

sulfate test (ASTM C 88 or AASHTO T 104). The test

consists of a number of immersion cycles for a sample of

the aggregate in a sulfate solution; this creates a pressure

through salt-crystal growth in the aggregate pores similar

to that produced by freezing water. The sample is then

Design and Control of Concrete Mixtures

◆

EB001

Fig. 5-16. Fractured carbonate aggregate particle as a

source of distress in D-cracking (magnification 2.5X) (Stark

1976). (30639-A)

correlation. Mass loss due to impact in the rattler is often

as much as that due to abrasion. The wear resistance of

concrete is determined more accurately by abrasion tests

of the concrete itself (see Chapter 1).

To provide good skid resistance on pavements, the

siliceous particle content of the fine aggregate should be at

least 25%. For specification purposes, the siliceous particle

content is considered equal to the insoluble residue

content after treatment in hydrochloric acid under stan-

dardized conditions (ASTM D 3042). Certain manufac-

tured sands produce slippery pavement surfaces and

should be investigated for acceptance before use.

Strength and Shrinkage

The strength of an aggregate is rarely tested and generally

does not influence the strength of conventional concrete as

much as the strength of the paste and the paste-aggregate

bond. However, aggregate strength does become important

in high-strength concrete. Aggregate stress levels in concrete

are often much higher than the average stress over the entire

cross section of the concrete. Aggregate tensile strengths

range from 2 to 15 MPa (300 to 2300 psi) and compressive

strengths from 65 to 270 MPa (10,000 to 40,000 psi).

Different aggregate types have different compressibil-

ity, modulus of elasticity, and moisture-related shrinkage

characteristics that can influence the same properties in

concrete. Aggregates with high absorption may have high

shrinkage on drying. Quartz and feldspar aggregates,

along with limestone, dolomite, and granite, are consid-

ered low shrinkage aggregates; while aggregates with

sandstone, shale, slate, hornblende, and graywacke are

often associated with high shrinkage in concrete (Fig. 5-17).

Resistance to Acid and

Other Corrosive Substances

Portland cement concrete is durable in most natural envi-

ronments; however, concrete in service is occasionally

exposed to substances that will attack it.

Most acidic solutions will slowly or rapidly disinte-

grate portland cement concrete depending on the type and

concentration of acid. Certain acids, such as oxalic acid, are

harmless. Weak solutions of some acids have insignificant

effects. Although acids generally attack and leach away the

calcium compounds of the cement paste, they may not

readily attack certain aggregates, such as siliceous aggre-

gates. Calcareous aggregates often react readily with acids.

However, the sacrificial effect of calcareous aggregates is

often a benefit over siliceous aggregate in mild acid expo-

sures or in areas where water is not flowing. With calcare-

ous aggregate, the acid attacks the entire exposed concrete

surface uniformly, reducing the rate of attack on the paste

and preventing loss of aggregate particles at the surface.

Calcareous aggregates also tend to neutralize the acid,

especially in stagnant locations. Acids can also discolor

concrete. Siliceous aggregate should be avoided when

strong solutions of sodium hydroxide are present, as these

solutions attack this type of aggregate.

Acid rain (often with a pH of 4 to 4.5) can slightly etch

concrete surfaces, usually without affecting the perform-

ance of exposed concrete structures. Extreme acid rain or

strong acid water conditions may warrant special concrete

designs or precautions, especially in submerged areas.

Continuous replenishment in acid with a pH of less than

4 is considered highly aggressive to buried concrete, such

as pipe (Scanlon 1987). Concrete continuously exposed to

liquid with a pH of less than 3 should be protected in a

manner similar to concrete exposed to dilute acid solu-

tions (ACI 515.1R).

Natural waters usually have a pH of more than 7 and

seldom less than 6. Waters with a pH greater than 6.5 may

be aggressive if they contain bicarbonates. Carbonic acid

solutions with concentrations between 0.9 and 3 parts per

million are considered to be destructive to concrete (ACI

515.1R and Kerkhoff 2001).

A low water-cement ratio, low permeability, and low-

to-moderate cement content can increase the acid or corro-

sion resistance of concrete. A low permeability resulting

from a low water-cement ratio or the use of silica fume or

other pozzolans, helps keep the corrosive agent from

penetrating into the concrete. Low-to-moderate cement

contents result in less available paste to attack. The use of

sacrificial calcareous aggregates should be considered

where indicated.

Certain acids, gases, salts, and other substances that

are not mentioned here also can disintegrate concrete.

Acids and other chemicals that severely attack portland

cement concrete should be prevented from coming in

contact with the concrete by using protective coatings

(Kerkhoff 2001).

Chapter 5

◆

Aggregates for Concrete

Sandstone Slate Granite Limestone Quartz

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Drying shrinkage after 1 year in percent

Fig. 5-17. Concretes containing sandstone or slate produce

a high shrinkage concrete. Granite, limestone and quartz,

are low shrinkage-producing aggregates (ACI 221R).

Fire Resistance and Thermal Properties

The fire resistance and thermal properties of concrete—

conductivity, diffusivity, and coefficient of thermal expan-

sion—depend to some extent on the mineral constituents

of the aggregates used. Manufactured and some naturally

occurring lightweight aggregates are more fire resistant

than normal-weight aggregates due to their insulating

properties and high-temperature stability. Concrete

containing a calcareous coarse aggregate performs better

under fire exposure than a concrete containing quartz or

siliceous aggregate such as granite or quartzite. At about

590°C (1060°F), quartz expands 0.85% causing disruptive

expansion (ACI 216 and ACI 221). The coefficient of ther-

mal expansion of aggregates ranges from 0.55 x 10

-6

per °C

to 5 x 10

-6

per °C (1 x 10

-6

per °F to 9 x 10

-6

per °F). For

more information refer to Chapter 15 for temperature-

induced volume changes and to Chapter 18 for thermal

conductivity and mass concrete considerations.

POTENTIALLY HARMFUL MATERIALS

Harmful substances that may be present in aggregates

include organic impurities, silt, clay, shale, iron oxide, coal,

lignite, and certain lightweight and soft particles (Table

5-6). In addition, rocks and minerals such as some cherts,

strained quartz (Buck and Mather 1984), and certain dolo-

mitic limestones are alkali reactive (see Table 5-7). Gypsum

and anhydrite may cause sulfate attack. Certain aggre-

gates, such as some shales, will cause popouts by swelling

(simply by absorbing water) or by freezing of absorbed

water (Fig. 5-18). Most specifications limit the permissible

amounts of these substances. The performance history of

an aggregate should be a determining factor in setting the

limits for harmful substances. Test methods for detecting

harmful substances qualitatively or quantitatively are

listed in Table 5-6.

Aggregates are potentially harmful if they contain

compounds known to react chemically with portland

cement concrete and produce any of the following: (1)

significant volume changes of the paste, aggregates, or

both; (2) interference with the normal hydration of

cement; and (3) otherwise harmful byproducts.

Organic impurities may delay setting and hardening

of concrete, may reduce strength gain, and in unusual

cases may cause deterioration. Organic impurities such as

peat, humus, and organic loam may not be as detrimental

but should be avoided.

Materials finer than the 75-µm (No. 200) sieve, espe-

cially silt and clay, may be present as loose dust and may

form a coating on the aggregate particles. Even thin coat-

ings of silt or clay on gravel particles can be harmful

because they may weaken the bond between the cement

paste and aggregate. If certain types of silt or clay are pres-

ent in excessive amounts, water requirements may

increase significantly.

Design and Control of Concrete Mixtures

◆

EB001

Effect on Test

Substances concrete designation

Organic Affects setting ASTM C 40 (AASHTO T 21)

impurities and hardening, ASTM C 87 (AASHTO T 71)

may cause

deterioration

Materials finer Affects bond, ASTM C 117 (AASHTO T 11)

than the 75-

m increases water

(No. 200) sieve requirement

Coal, lignite, Affects durability, ASTM C 123 (AASHTO T 113)

or other may cause

lightweight stains and

materials popouts

Soft particles Affects durability ASTM C 235

Clay lumps Affects work- ASTM C 142 (AASHTO T 112)

and friable ability and

particles durability, may

cause popouts

Chert of less Affects ASTM C 123 (AASHTO T 113)

than 2.40 durability, may ASTM C 295

relative density cause popouts

Alkali-reactive Causes ASTM C 227

aggregates abnormal ASTM C 289

expansion, ASTM C 295

map cracking, ASTM C 342

and popouts ASTM C 586

ASTM C 1260 (AASHTO T 303)

ASTM C 1293

Table 5-6. Harmful Materials in Aggregates

Table 5-7. Some Potentially Harmful Reactive

Minerals, Rock, and Synthetic Materials

Alkali-carbonate

Alkali-silica reactive substances* reactive substances**

Andesites Opal Calcitic dolomites

Argillites Opaline shales Dolomitic limestones

Certain siliceous Phylites Fine-grained dolomites

limestones Quartzites

and dolomites Quartzoses

Chalcedonic cherts Cherts

Chalcedony Rhyolites

Cristobalite Schists

Dacites Siliceous shales

Glassy or Strained quartz

cryptocrystalline and certain

volcanics other forms

Granite gneiss of quartz

Graywackes Synthetic and

Metagraywackes natural silicious

glass

Tridymite

* Several of the rocks listed (granite gneiss and certain quartz for-

mations for example) react very slowly and may not show evi-

dence of any harmful degree of reactivity until the concrete is over

20 years old.

** Only certain sources of these materials have shown reactivity.

Video

There is a tendency for some fine aggregates to

degrade from the grinding action in a concrete mixer; this

effect, which is measured using ASTM C 1137, may alter

mixing water, entrained air and slump requirements.

Coal or lignite, or other low-density materials such as

wood or fibrous materials, in excessive amounts will affect

the durability of concrete. If these impurities occur at or

near the surface, they might disintegrate, pop out, or

cause stains. Potentially harmful chert in coarse aggregate

can be identified by using ASTM C 123 (AASHTO T 113).

Soft particles in coarse aggregate are especially objec-

tionable because they cause popouts and can affect dura-

bility and wear resistance of concrete. If friable, they could

break up during mixing and thereby increase the amount

of water required. Where abrasion resistance is critical,

such as in heavy-duty industrial floors, testing may indi-

cate that further investigation or another aggregate source

is warranted.

Clay lumps present in concrete may absorb some of

the mixing water, cause popouts in hardened concrete,

and affect durability and wear resistance. They can also

break up during mixing and thereby increase the mixing-

water demand.

Aggregates can occasionally contain particles of iron

oxide and iron sulfide that result in unsightly stains on

exposed concrete surfaces (Fig. 5-19). The aggregate

should meet the staining requirements of ASTM C 330

(AASHTO M 195) when tested according to ASTM C 641;

the quarry face and aggregate stockpiles should not show

evidence of staining.

As an additional aid in identifying staining particles,

the aggregate can be immersed in a lime slurry. If staining

particles are present, a blue-green gelatinous precipitate

will form within 5 to 10 minutes; this will rapidly change

to a brown color on exposure to air and light. The reaction

should be complete within 30 minutes. If no brown gelat-

inous precipitate is formed when a suspect aggregate is

placed in the lime slurry,

there is little likelihood of

any reaction taking place in

concrete. These tests should

be required when aggregates

with no record of successful

prior use are used in archi-

tectural concrete.

ALKALI-AGGREGATE

REACTIVITY

Aggregates containing certain

constituents can react with

alkali hydroxides in concrete.

The reactivity is potentially

harmful only when it produces significant expansion

(Mather 1975). This alkali-aggregate reactivity (AAR) has

two forms—alkali-silica reaction (ASR) and alkali-carbonate

reaction (ACR). ASR is of more concern than ACR because

the occurrence of aggregates containing reactive silica

minerals is more common. Alkali-reactive carbonate aggre-

gates have a specific composition that is not very common.

Alkali-silica reactivity has been recognized as a poten-

tial source of distress in concrete since the late 1930s

(Stanton 1940 and PCA 1940). Even though potentially

reactive aggregates exist throughout North America, ASR

distress in structural concrete is not common. There are a

number of reasons for this:

• Most aggregates are chemically stable in hydraulic-

cement concrete.

• Aggregates with good service records are abundant in

many areas.

• Most concrete in service is dry enough to inhibit ASR.

• Use of certain pozzolans or slags can control ASR.

• In many concrete mixtures, the alkali content of the

concrete is low enough to control harmful ASR.

• Some forms of ASR do not produce significant delete-

rious expansion.

To reduce ASR potential requires understanding the

ASR mechanism; properly using tests to identify poten-

tially reactive aggregates; and, if needed, taking steps to

minimize the potential for expansion and related cracking.

Alkali-Silica Reaction

Visual Symptoms of Expansive ASR. Typical indicators

of ASR might be any of the following: a network of cracks

(Fig. 5-20); closed or spalled joints; relative displacements

of different parts of a structure; or fragments breaking out

of the surface of the concrete (popouts) (Fig. 5-21). Because

ASR deterioration is slow, the risk of catastrophic failure is

low. However, ASR can cause serviceability problems and

can exacerbate other deterioration mechanisms such as

those that occur in frost, deicer, or sulfate exposures.

Chapter 5

◆

Aggregates for Concrete

Fig. 5-18. A popout is the breaking away of a small frag-

ment of concrete surface due to internal pressure that

leaves a shallow, typically conical depression. (113)

Fig. 5-19. Iron oxide stain

caused by impurities in the

coarse aggregate. (70024)

Video

Mechanism of ASR. The alkali-silica reaction forms a gel

that swells as it draws water from the surrounding cement

paste. Reaction products from ASR have a great affinity

for moisture. In absorbing water, these gels can induce

pressure, expansion, and cracking of the aggregate and

surrounding paste. The reaction can be visualized as a

two-step process:

1. Alkali hydroxide + reactive silica gel → reaction prod-

uct (alkali-silica gel)

2. Gel reaction product + moisture → expansion

The amount of gel formed in the concrete depends on

the amount and type of silica and alkali hydroxide

concentration. The presence of gel does not always coin-

cide with distress, and thus, gel presence does not neces-

sarily indicate destructive ASR.

Factors Affecting ASR. For alkali-silica reaction to occur,

the following three conditions must be present:

1. reactive forms of silica in the aggregate,

2. high-alkali (pH) pore solution, and

3. sufficient moisture.

If one of these conditions is absent, ASR cannot occur.

Test Methods for Identifying ASR Distress. It is impor-

tant to distinguish between the reaction and damage

resulting from the reaction. In the diagnosis of concrete

deterioration, it is most likely that a gel product will be

identified. But, in some cases significant amounts of gel are

formed without causing damage to concrete. To pinpoint

ASR as the cause of damage, the presence of deleterious

ASR gel must be verified. A site of expansive reaction can

be defined as an aggregate particle that is recognizably

reactive or potentially reactive and is at least partially

replaced by gel. Gel can be present in cracks and voids and

may also be present in a ring surrounding an aggregate

particle at its edges. A network of internal cracks connect-

ing reacted aggregate particles is an almost certain indica-

tion that ASR is responsible for cracking. A petrographic

examination (ASTM C 856) is the most positive method for

identifying ASR gel in concrete (Powers 1999).

Petrography, when used to study a known reacted

concrete, can confirm the presence of reaction products

and verify ASR as an underlying cause of deterioration

(Fig. 5-22).

Control of ASR in New Concrete. The best way to avoid

ASR is to take appropriate precautions before concrete is

placed. Standard concrete specifications may require

modifications to address ASR. These modifications should

be carefully tailored to avoid limiting the concrete pro-

ducer’s options. This permits careful analysis of cementi-

tious materials and aggregates and choosing a control

strategy that optimizes effectiveness and the economic

selection of materials. If the aggregate is not reactive by

historical identification or testing, no special requirements

are needed.

Design and Control of Concrete Mixtures

◆

EB001

Fig. 5-20 (top and bottom). Cracking of concrete from alkali-

silica reactivity. (69549, 58352)

Fig. 5-21. Popouts caused by ASR of sand-sized particles.

Inset shows closeup of a popout. (51117, 51118)

Fig. 5-21. Popouts caused by ASR of sand-sized particles.

Inset shows closeup of a popout. (51117, 51118)

Identification of Potentially Reactive Aggregates. Field

performance history is the best method of evaluating the

susceptibility of an aggregate to ASR. For the most defini-

tive evaluation, the existing concrete should have been in

service for at least 15 years. Comparisons should be made

between the existing and proposed concrete’s mix propor-

tions, ingredients, and service environments. This process

should tell whether special requirements are needed, are

not needed, or whether testing of the aggregate or job

concrete is required. The use of newer, faster test methods

can be utilized for initial screening. Where uncertainties

arise, lengthier tests can be used to confirm results. Table

5-8 describes different test methods used to evaluate

potential alkali-silica reactivity. These tests should not be

used to disqualify use of potentially reactive aggregates,

as reactive aggregates can be safely used with the careful

selection of cementitious materials as discussed below.

Materials and Methods to Control ASR. The most effec-

tive way of controlling expansion due to ASR is to design

mixtures specifically to control ASR, preferably using

locally available materials. The following options are not

listed in priority order and, although usually not neces-

sary, they can be used in combination with one another.

In North America, current practices include the use of

a supplementary cementitious material or blended cement

proven by testing to control ASR or limiting the alkali

content of the concrete. Supplementary cementitious mate-

rials include fly ash, ground granulated blast-furnace slag,

silica fume, and natural pozzolans. Blended cements use

slag, fly ash, silica fume, and natural pozzolans to control

ASR. Low-alkali portland cement (ASTM C 150) with an

alkali content of not more than 0.60% (equivalent sodium

oxide) can be used to control ASR. Its use has been success-

ful with slightly reactive to moderately reactive aggre-

gates. However, low-alkali cements are not available in all

areas. Thus, the use of locally available cements in combi-

nation with pozzolans, slags, or blended cements is prefer-

able for controlling ASR. When pozzolans, slags, or

blended cements are used to control ASR, their effective-

ness must be determined by tests such as ASTM C 1260

(modified) or C 1293. Where applicable, different amounts

of pozzolan or slag should be tested to determine the opti-

mum dosage. Expansion usually decreases as the dosage

of the pozzolan or slag increases (see Fig. 5-23). Lithium-

based admixtures are also available to control ASR. Lime-

stone sweetening (the popular term for replacing

approximately 30% of the reactive sand-gravel aggregate

with crushed limestone) is effective in controlling deterio-

ration in some sand-gravel aggregate concretes. See

AASHTO (2001), Farny and Kosmatka (1997), and PCA

(1998) for more information on tests to demonstrate the

effectiveness of the above control measures.

Alkali-Carbonate Reaction

Mechanism of ACR. Reactions observed with certain

dolomitic rocks are associated with alkali-carbonate reac-

tion (ACR). Reactive rocks usually contain large crystals of

dolomite scattered in and surrounded by a fine-grained

matrix of calcite and clay. Calcite is one of the mineral

forms of calcium carbonate; dolomite is the common

name for calcium-magnesium carbonate. ACR is relatively

rare because aggregates susceptible to this reaction are

usually unsuitable for use in concrete for other reasons,

such as strength potential. Argillaceous dolomitic lime-

stone contains calcite and dolomite with appreciable

amounts of clay and can contain small amounts of reactive

silica. Alkali reactivity of carbonate rocks is not usually

dependent upon its clay mineral composition (Hadley

1961). Aggregates have potential for expansive ACR if the

following lithological characteristics exist (Ozol 1994 and

Swenson 1967):

Chapter 5

◆

Aggregates for Concrete

Fig. 5-22. Polished section view of an alkali reactive aggregate

in concrete. Observe the alkali-silica reaction rim around the

reactive aggregate and the crack formation. (43090)

Control

20 30 56 35 50 65 7.5 10 12.5

0.05

0.1

0.15

0.2

0.25

0.3

14-Day expansion, %

% Fly ash % Slag % Silica fume

0.10% expansion limit for evaluating

effectiveness of supplementary

cementitious materials against AAR

Sudbury aggregate

Fig. 5-23. Influence of different amounts of fly ash, slag,

and silica fume by mass of cementing material on mortar

bar expansion (ASTM C 1260) after 14 days when using

reactive aggregate (Fournier 1997).

Design and Control of Concrete Mixtures

◆

EB001

Table 5-8. Test Methods for Alkali-Silica Reactivity (Farny and Kosmatka 1997)

Test name Purpose Type of test Type of sample

ASTM C 227, To test the susceptibility of Mortar bars stored over At least 4 mortar bars of

Potential alkali-reactivity cement-aggregate com- water at 37.8°C (100°F) standard dimensions:

of cement-aggregate binations to expansive and high relative 25 x 25 x 285 mm

combinations reactions involving humidity (1 x 1 x 11

⁄

in.)

(mortar-bar method) alkalies

ASTM C 289, To determine potential Sample reacted with Three 25-gram samples

Potential alkali-silica reactivity of siliceous alkaline solution at 80°C (176°F) of crushed and sieved

reactivity of aggregates aggregates aggregate

(chemical method)

ASTM C 294, To give descriptive Visual identification Varies, but should be

Constituents of nomenclature for the representative of entire

natural mineral more common or impor- source

aggregates tant natural minerals—an

aid in determining their

performance

ASTM C 295, To outline petrographic Visual and microscopic Varies with knowledge of

Petrographic examination procedures examination of prepared quarry: cores 53 to 100 mm

examination of for aggregates—an aid in samples—sieve analysis, in diameter (2

⁄

to 4 in.)

aggregates determining their per- microscopy, scratch or 45 kg (100 lb) or 300

for concrete formance acid tests pieces, or 2 kg (4 lb)

ASTM C 342, To determine the poten- Mortar bars stored in Three mortar bars per

Potential volume tial ASR expansion of water at 23°C (73.4°F) cement-aggregate

change of cement- cement-aggregate combination of standard

aggregate combinations dimensions: 25 x 25 x 285 mm

combinations (1 x 1 x 11

⁄

in.)

ASTM C 441, To determine effectiveness Mortar bars—using Pyrex At least 3 mortar bars

Effectiveness of mineral of supplementary cementing glass as aggregate— and also 3 mortar bars

admixtures or GBFS in materials in controlling stored over water at of control mixture

preventing excessive expansion from ASR 37.8°C (100°F) and

expansion of concrete high relative humidity

due to alkali-silica reaction

ASTM C 856, To outline petrographic Visual (unmagnified) At least one 150-mm

Petrographic examination procedures and microscopic exam- diameter by 300-mm long

examination of for hardened concrete— ination of prepared core (6-in. diameter by

hardened concrete useful in determining samples 12-in. long)

condition or performance

ASTM C 856 To identify products of Staining of a freshly- Varies: core with lapped

(AASHTO T 299), ASR in hardened exposed concrete surface, core with

Annex uranyl-acetate concrete surface and viewing broken surface

treatment procedure under UV light

Los Alamos staining To identify products of Staining of a freshly-exposed Varies: core with lapped

method ASR in hardened concrete surface with two surface, core with

(Powers 1999) concrete different reagents broken surface

ASTM C 1260 To test the potential for Immersion of mortar At least 3 mortar bars

(AASHTO T 303), deleterious alkali-silica bars in alkaline

Potential alkali-reactivity reaction of aggregate solution at 80°C (176°F)

of aggregates (mortar- in mortar bars

bar method)

ASTM C 1293, To determine the Concrete prisms stored Three prisms per cement-

Determination of length potential ASR expansion over water at 38°C (100.4°F) aggregate combination of

change of concrete of cement-aggregate standard dimensions:

due to alkali-silica combinations 75 x 75 x 285 mm

reaction (concrete (3 x 3 x 11

⁄

in.)

prism test)

Accelerated concrete To determine the Concrete prisms stored Three prisms per cement-

prism test (modified potential ASR expansion stored over water aggregate combination of

ASTM C 1293) of cement-aggregate at 60°C (140°F) standard dimensions:

combinations 75 x 75 x 285 mm

(3 x 3 x 11

⁄

in.)

Chapter 5

◆

Aggregates for Concrete

Duration of test Measurement Criteria Comments

Varies: first measure- Length change Per ASTM C 33, maximum Test may not produce signif-

ment at 14 days, then 1,2, 0.10% expansion at 6 months, icant expansion, especially

3,4,6,9, and 12 months; or if not available for a 6- for carbonate aggregate.

every 6 months after month period, maximum of Long test duration.

that as necessary 0.05% at 3 months Expansions may not be

from AAR.

24 hours Drop in alkalinity Point plotted on graph falls Quick results.

and amount of in deleterious or potentially Some aggregates give low

silica solubilized deleterious area expansions even though

they have high silica content.

Not reliable.

Short duration—as long Description of type Not applicable These descriptions are used

as it takes to visually and proportion of to characterize naturally-

examine the sample minerals in aggregate occurring minerals that make

up common aggregate

sources.

Short duration—visual Particle character- Not applicable Usually includes optical

examination does not istics, such as shape, microscopy. Also may include

involve long test size, texture, color, XRD analysis, differential

periods mineral composition, thermal analysis, or infrared

and physical condition

spectroscopy—see ASTM C 294

for descriptive nomenclature.

52 weeks Length change Per ASTM C 33, unsatis- Primarily used for aggregates

factory aggregate if expan- from Oklahoma, Kansas,

sion equals or exceeds Nebraska, and Iowa.

0.200% at 1 year

Varies: first measure- Length change Per ASTM C 989, minimum Highly reactive artificial

ment at 14 days, then 1,2, 75% reduction in expansion aggregate may not represent

3,4,5,9, and 12 months; or 0.020% maximum expan- real aggregate conditions.

every 6 months after sion or per ASTM C 618, com- Pyrex contains alkalies.

that as necessary parison against low-alkali

control

Short duration—includes Is the aggregate known See measurement—this Specimens can be examined

preparation of samples to be reactive? examination determines with stereomicroscopes,

and visual and micro- Orientation and if ASR reactions have polarizing microscopes,

scope examination geometry of cracks. taken place and their metallographic microscopes,

Is there any gel effects upon the concrete. and scanning electron

present? Used in conjunction with microscope.

other tests.

Immediate results Intensity of Lack of fluorescence Identifies small amounts of

fluorescence ASR gel whether they cause

expansion or not.

Opal, a natural aggregate,

and carbonated paste can

Immediate results Color of stain Dark pink stain corresponds glow—interpret results

to ASR gel and indicates an accordingly.

advanced state of Tests must be supplemented

degradation by petrographic examination

and physical tests for deter-

mining concrete expansion.

16 days Length change If greater than 0.10%, Very fast alternative to C 227.

go to supplementary test Useful for slowly reacting

procedures; if greater than aggregates or those that

0.20%, indicative of poten- produce expansion late in

tial deleterious expansion the reaction.

Varies: first measure- Length change Per Appendix X1, potentially Requires long test duration

ment at 7 days, then 28 deleteriously reactive if ex- for meaningful results.

and 56 days, then 3,6,9, pansion equals or exceeds Use as a supplement to C 227,

and 12 months; every 0.04% at one year C 295, C 289, and C 1260.

6 months as after that as Similar to CSA A23.2-14A.

necessary

3 month (91 days) Length change Potentially deleteriously Fast alternative to C 227.

reactive if expansion Good correlation to ASTM

equals or exceeds 0.04% C 227 for carbonate and

at 91 days sedimentary rocks.

minerals and water proportioned to have a relative density

(specific gravity) less than that of the desirable aggregate

particles but greater than that of the deleterious particles.

The heavier particles sink to the bottom while the lighter

particles float to the surface. This process can be used when

acceptable and harmful particles have distinguishable rela-

tive densities.

Jigging separates particles with small differences in

density by pulsating water current. Upward pulsations of

water through a jig (a box with a perforated bottom) move

the lighter material into a layer on top of the heavier mate-

rial; the top layer is then removed.

Rising-current classification separates particles with

large differences in density. Light materials, such as wood

and lignite, are floated away in a rapidly upward moving

stream of water.

Crushing is also used to remove soft and friable parti-

cles from coarse aggregates. This process is sometimes the

only means of making material suitable for use. Unfortun-

ately, with any process some acceptable material is always

lost and removal of all harmful particles may be difficult

or expensive.

HANDLING AND STORING AGGREGATES

Aggregates should be handled and stored in a way that

minimizes segregation and degradation and prevents

contamination by deleterious substances (Fig. 5-24). Stock-

piles should be built up in thin layers of uniform thickness

to minimize segregation. The most economical and

acceptable method of forming aggregate stockpiles is the

truck-dump method, which discharges the loads in a way

that keeps them tightly joined. The aggregate is then re-

claimed with a front-end loader. The loader should re-

move slices from the edges of the pile from bottom to top

so that every slice will contain a portion of each horizon-

tal layer.

• clay content, or insoluble residue content, in the range

of 5% to 25%;

• calcite-to-dolomite ratio of approximately 1:1;

• increase in the dolomite volume up to a point at

which interlocking texture becomes a restraining

factor; and

• small size of the discrete dolomite crystals (rhombs)

suspended in a clay matrix.

Dedolomitization. Dedolomitization, or the breaking

down of dolomite, is normally associated with expansive

ACR (Hadley 1961). Concrete that contains dolomite and

has expanded also contains brucite (magnesium hydrox-

ide, Mg(OH)

), which is formed by dedolomitization. De-

dolomitization proceeds according to the following

equation (Ozol 1994):

CaMgCO

(dolomite) + alkali hydroxide solution →

MgOH

(brucite) + CaCO

(calcium carbonate) +

(potassium carbonate) + alkali hydroxide

The dedolomitization reaction and subsequent crystalliza-

tion of brucite may cause considerable expansion.

Whether dedolomitization causes expansion directly or

indirectly, it’s usually a prerequisite to other expansive

processes (Tang, Deng, Lon, and Han 1994).

Test Methods for Identifying ACR Distress. The three

test methods commonly used to identify potentially alkali-

carbonate reactive aggregate are:

• petrographic examination (ASTM C 295);

• rock cylinder method (ASTM C 586); and

• concrete prism test (ASTM C 1105).

See Farny and Kosmatka (1997) for detailed information.

Materials and Methods to Control ACR. ACR-suscepti-

ble aggregate has a specific composition that is readily

identified by petrographic testing. If a rock indicates ACR-

susceptibility, one of the following preventive measures

should be taken:

• selective quarrying to completely avoid reactive ag-

gregate;

• blend aggregate according to Appendix in ASTM C

1105; or

• limit aggregate size to smallest practical.

Low-alkali cement and pozzolans are generally not very

effective in controlling expansive ACR.

AGGREGATE BENEFICIATION

Aggregate processing consists of: (1) basic processing—

crushing, screening, and washing—to obtain proper grada-

tion and cleanliness; and (2) beneficiation—upgrading

quality by processing methods such as heavy media sepa-

ration, jigging, rising-current classification, and crushing.

In heavy media separation, aggregates are passed

through a heavy liquid comprised of finely ground heavy

Design and Control of Concrete Mixtures

◆

EB001

Fig. 5-24. Stockpile of aggregate at a ready mix plant. (69552)

Video

When aggregates are not delivered by truck, accept-

able and inexpensive results can be obtained by forming

the stockpile in layers with a clamshell bucket (cast-and-

spread method); in the case of aggregates not subject to

degradation, spreading the aggregates with a rubber-tire

dozer and reclaiming with a front-end loader can be used.

By spreading the material in thin layers, segregation is

minimized. Whether aggregates are handled by truck,

bucket loader, clamshell, or conveyor belt, stockpiles

should not be built up in high, cone-shaped piles since this

results in segregation. However, if circumstances necessi-

tate construction of a conical pile, or if a stockpile has

segregated, gradation variations can be minimized when

the pile is reclaimed; in such cases aggregates should be

loaded by continually moving around the circumference

of the pile to blend sizes rather than by starting on one

side and working straight through the pile.

Crushed aggregates segregate less than rounded

(gravel) aggregates and larger-size aggregates segregate

more than smaller sizes. To avoid segregation of coarse

aggregates, size fractions can be stockpiled and batched

separately. Proper stockpiling procedures, however,

should eliminate the need for this. Specifications provide

a range in the amount of material permitted in any size

fraction partly because of segregation in stockpiling and

batching operations.

Washed aggregates should be stockpiled in sufficient

time before use so that they can drain to a uniform mois-

ture content. Damp fine material has less tendency to

segregate than dry material. When dry fine aggregate is

dropped from buckets or conveyors, wind can blow away

the fines; this should be avoided if possible.

Bulkheads or dividers should be used to avoid

contamination of aggregate stockpiles. Partitions between

stockpiles should be high enough to prevent intermingling

of materials. Storage bins should be circular or nearly

square. Their bottoms should slope not less than 50

degrees from the horizontal on all sides to a center outlet.

When loading the bin, the material should fall vertically

over the outlet into the bin. Chuting the material into a bin

at an angle and against the bin sides will cause segregation.

Baffle plates or dividers will help minimize segregation.

Bins should be kept as full as possible since this reduces

breakage of aggregate particles and the tendency to segre-

gate. Recommended methods of handling and storing

aggregates are discussed at length in Matthews (1965 to

1967)

, NCHRP (1967), and Bureau of Reclamation (1981).

MARINE-DREDGED AGGREGATE

Marine-dredged aggregate from tidal estuaries and sand

and gravel from the seashore can be used with caution in

some concrete applications when other aggregate sources

are not available. Aggregates obtained from seabeds have

two problems: (1) seashells and (2) salt.

Seashells may be present in the aggregate source.

These shells are a hard material that can produce good

quality concrete, however, a higher cement content may

be required. Also, due to the angularity of the shells, addi-

tional cement paste is required to obtain the desired work-

ability. Aggregate containing complete shells (uncrushed)

should be avoided as their presence may result in voids in

the concrete and lower the compressive strength.

Marine-dredged aggregates often contain salt from the

seawater. The primary salts are sodium chloride and

magnesium sulfate and the amount of salt on the aggregate

is often not more than about 1% of the mass of the mixing

water. The highest salt content occurs in sands located just

above the high-tide level. Use of these aggregates with

drinkable mix water often contributes less salt to the

mixture than the use of seawater (as mix water) with salt-

free aggregates.

Marine aggregates can be an appreciable source of

chlorides. The presence of these chlorides may affect the

concrete by (1) altering the time of set, (2) increasing drying

shrinkage, (3) significantly increasing the risk of corrosion

of steel reinforcement, and (4) causing efflorescence.

Generally, marine aggregates containing large amounts of

chloride should not be used in reinforced concrete.

Marine-dredged aggregates can be washed with fresh

water to reduce the salt content. There is no maximum

limit on the salt content of coarse or fine aggregate;

however, the chloride limits presented in Chapter 9

should be followed.

RECYCLED-CONCRETE AGGREGATE

In recent years, the concept of using old concrete pave-

ments, buildings, and other structures as a source of aggre-

gate has been demonstrated on several projects, resulting

in both material and energy savings (ECCO 1999). The

procedure involves (1) breaking up and removing the old

concrete, (2) crushing in primary and secondary crushers

(Fig. 5-25), (3) removing reinforcing steel and other embed-

Chapter 5

◆

Aggregates for Concrete

Fig. 5-25. Heavily reinforced concrete is crushed with a

beamcrusher. (69779)

adequate compressive strength. The use of recycled fine

aggregate can result in minor compressive strength reduc-

tions. However, drying shrinkage and creep of concrete

made with recycled aggregates is up to 100% higher than

concrete with a corresponding conventional aggregate.

This is due to the large amount of old cement paste and

mortar especially in the fine aggregate. Therefore, consid-

erably lower values of drying shrinkage can be achieved

using recycled coarse aggregate with natural sand

(Kerkhoff and Siebel 2001). As with any new aggregate

source, recycled-concrete aggregate should be tested for

durability, gradation, and other properties.

Recycled concrete used as coarse aggregate in new

concrete possesses some potential for alkali-silica-reaction

if the old concrete contained alkali-reactive aggregate. The

ded items, (4) grading and washing, and (5) finally stock-

piling the resulting coarse and fine aggregate (Fig. 5-26).

Dirt, gypsum board, wood, and other foreign materials

should be prevented from contaminating the final product.

Recycled concrete is simply old concrete that has been

crushed to produce aggregate. Recycled-concrete aggre-

gate is primarily used in pavement reconstruction. It has

been satisfactorily used as an aggregate in granular

subbases, lean-concrete subbases, soil-cement, and in new

concrete as the only source of aggregate or as a partial

replacement of new aggregate.

Recycled-concrete aggregate generally has a higher

absorption and a lower specific gravity than conventional

aggregate. This results from the high absorption of porous

mortar and hardened cement paste within the recycled

concrete aggregate. Absorption values typically range

from 3% to 10%, depending on the concrete being recy-

cled; this absorption lies between those values for natural

and lightweight aggregate. The values increase as coarse

particle size decreases (Fig. 5-27). The high absorption of

the recycled aggregate makes it necessary to add more

water to achieve the same workability and slump than for

concrete with conventional aggregates. Dry recycled

aggregate absorbs water during and after mixing. To

avoid this, recycled aggregate should be prewetted or

stockpiles should be kept moist.

The particle shape of recycled-concrete aggregate is

similar to crushed rock as shown in Fig. 5-28. The relative

density decreases progressively as particle size decreases.

The sulfate content of recycled-concrete aggregate should

be determined to assess the possibility of deleterious

sulfate reactivity. The chloride content should also be

determined where applicable.

New concrete made from recycled-concrete aggregate

generally has good durability. Carbonation, permeability,

and resistance to freeze-thaw action have been found to be

the same or even better than concrete with conventional

aggregates. Concrete made with recycled coarse aggre-

gates and conventional fine aggregate can obtain an

100

Design and Control of Concrete Mixtures

◆

EB001

Fig. 5-26. Stockpile of recycled-concrete aggregate. (69813)

Water absorption in % by mass

150 µm to

4.75 mm

(No. 100 to

No. 4)

2.36 mm to

9.5 mm

(No. 8 to

3/8 in.)

2.36 mm to

9.5 mm

(No. 8 to

3/8 in.)

2.36 mm to

9.5 mm

(No. 8 to

3/8 in.)

4.75 mm to

19 mm

(No. 4 to

3/4 in.)

Recycled Natural Lightweight

Aggregate size

Fig. 5-27. Comparison of water absorption of three different

recycled aggregate particle sizes and one size of natural and

lightweight coarse aggregate. (Kerkhoff and Siebel 2001)

Fig. 5-28. Recycled-concrete aggregate. (69812)

alkali content of the cement used in the old concrete has

little effect on expansion due to alkali-silica-reaction. For

highly reactive aggregates made from recycled concrete,

special measures discussed under “Alkali-Silica Reaction”

should be used to control ASR. Also, even if expansive

ASR did not develop in the original concrete, it can not be

assumed that it will not develop in the new concrete if

special control measures are not taken. Petrographic

examination and expansion tests are recommended to

make this judgment (Stark 1996).

Concrete trial mixtures should be made to check the

new concrete’s quality and to determine the proper

mixture proportions. One potential problem with using

recycled concrete is the variability in the properties of the

old concrete that may in turn affect the properties of the

new concrete. This can partially be avoided by frequent

monitoring of the properties of the old concrete that is

being recycled. Adjustments in the mixture proportions

may then be needed.

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Chapter 5

◆

Aggregates for Concrete

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103

Chapter 5

◆

Aggregates for Concrete

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