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Aggregates

HMA Surface

Dr. Sherif El-Badawy

Aggregate

Definition:• A mass of crushed stone, gravel, sand, …. Etc.

• Composed of individual particles.

• May include clays and silts.

Uses: • Underlying materials for pavements:

• (Base & Subbase).

• Ingredient in PCC and AC

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Aggregates

In PCC: • 60% to 75% by volume

• 79% to 85% by weight

In AC: • Over 80% by volume

• 92% to 96% by weight

Aggregate SourcesNatural:

• Gravel pits. • River run deposits.• Rock quarries.

Manufactured: • Slag waste from steel mills and expanded shale

and clays Light weight agg.• Styrofoam Light weight agg.• Steel slugs and steel ball bearings Heavy

weight agg.

• Recycled Materials

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Gravel Pit in Germany

(From Wikipedia)

Quarry

Geological ClassificationIgneous -- cooled from molten magma

• Intrusive• High density granite, cooled slowly below ground

• Extrusive• Basalt (lava) cools rapidly above ground

• Large air pockets may form creating low density cinder &

pumice

Sedimentary• Disintegrated minerals moved by gravity, wind, water, or ice and

deposited as sediment in nearly horizontal layers

• These deposits may eventually be compressed into sandstone,

limestone, shale, conglomerate, gypsum

Metamorphic• Minerals are changed by heat and pressure

• Crystals partially melt & grow into quartzite, marble, slate, etc.

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Aggregate Terminology

• Aggregate

• Coarse Aggregate (CA)

• Fine Aggregate (FA)

• “Fines” (Mineral Filler: MF)

• Maximum Size

• Nominal Max. Size

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Sieve Designation• Sieve Opening: for sieves greater than ¼ in.

• Number of Openings per Linear Inch.

Sieve Designation Sieve Opening (in.)

75 mm 3 in. 3.0

37.5 mm 1 ½ in. 1.5

19.0 mm ¾ in. .75

12.5 mm ½ in. .5

6.3 mm ¼ in. .25

4.76 mm No. 4 .187

2.36 mm No. 8 .0937

1.18 mm No. 16 .0469

0.6 mm No. 30 .0234

0.3 mm No. 50 .0117

0.15 mm No. 100 .0059

0.074 mm No. 200 .0029

CA

FA

MF

Mineral Filler

(Source: AASHTO M17)

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Definition of Gravel and Crushed Stone

Gravel - Natural Particles No. 4

to 3 in. Size and the Particles tend to be Smooth and Rounded.

Crushed Stone - Artificially

Crushed Rock, Boulders, or Large Cobbles. Most or All of the Surfaces are from Crushing, and the Particle Edges tend to be Sharpand Angular.

• Gradation

• Particle Shape and Surface Texture

• Hardness

• Toughness

• Soundness

• Deleterious Materials

Aggregate Characteristics

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Maximum and Nom. Max. Aggregate Size

• Nominal Maximum Aggregate Size: The largest sieve that retains some of the aggregate particles but generally not more than 10% by weight.

• Superpave defines nominal maximum aggregate size as "one sieve size larger than the first sieve to retain more than 10 percent of the material“.

• Maximum Aggregate Size: The smallest sieve size through which 100% of the aggregate sample particles pass.

• Superpave defines the maximum aggregate size as "one sieve larger than the nominal maximum size“.

Grain Size Distribution Table

sieve #

Sieve size (mm)

% Passing

3/4 ″ 19.00 100

3/8 ″ 9.50 100

4 4.750 95

8 2.360 84

16 1.180 74

30 0.600 53

40 0.425 41

50 0.300 31

100 0.150 14

200 0.075 3.80

Pan --- 0.00

Maximum Aggregate Size = 9.5 mm

Nominal Maximum Aggregate Size =4.75 mm

Superpave defines nominal maximum aggregate size as "one sieve size larger than the first sieve to retain more than 10 percent of the material“.

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Maximum Size

• Maximum size can affect both AC and PCC in several ways.

Grain Size Distribution Curve

• Semi-logarithmic PCC and Soils

• Fuller Curve HMA (hot mix asphalt

aggregate gradation)

Pi = 100(di/D)n

Pi = % passing a sieve of size didi = sieve size iD = Maximum size of aggregaten = 0.5 (Fuller), n = 0.45 FHWA (0.45

gradation chart)

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Semi-logarithmic Grain Size Dis. Curve

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10

Grain Size (mm)

Percent Pa

ssing

(by we

ight)

Silica Sand

Piedmont Silt

Plastic Kaolin

CLAY SIZE SILT SIZE SAND SIZE GRAVEL

0.075 mm

Fine-Grained Soils Coarse-Grained Soils

Grain Size (mm)

Grain Size Distribution Curve (0.45 Power Gradation Table)

sieve # Sieve size (d) (mm)

% P of the aggregate

(d)0.45

pi = 100(di/D)0.45

(FHWA Max Density Line), %

3/8 9.5 100 2.754 100(9.5/9.5)0.45 =100

4 4.75 95 2.016 100(4.75/9.5)0.45 =73

8 2.36 84 1.472 53

16 1.18 74 1.077 39

30 0.6 53 0.795 29

40 0.425 41 0.680 25

50 0.3 31 0.582 21

100 0.15 14 0.426 15

200 0.075 3.80 0.312 11.3

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(FHWA 0.45 Power Gradation Chart)

0

10

20

30

40

50

60

70

80

90

100

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Sieve Size (mm)

% P

assin

g

9.5

0

4.7

5

2.3

6

1.1

8

0.6

00

0.0

75

0.1

50

0.3

00

0.4

25

Max Density Line (FHWA 0.45 Power Curve)

d0.45

Aggregate Gradation) Max Size

Source: www.pavementinteractive.org

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Source: www.pavementinteractive.org

Gradation Types

Uniform (poor)

Well (Dense) graded

Gap Graded

Open Graded

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Gradation Types

• Dense (well-graded): a gradation that is near the FHWA’s 0.45 Power Curve for maximum density.

• Gap Graded: a gradation that contains only a small percentage of aggregate particles in the mid-size range. The curve is flat in the mid-size range.

• Open graded: a gradation that contains only a small percentage of aggregate particles in the small range. This results in more air voids. The curve is near vertical in the mid-size range, and flat and near-zero in the small-size range.

• One Sized: a gradation with the majority of aggregates passing one sieve (vertical line).

• Uniformly Graded. a gradation that contains most of the particles in a very narrow size range. (almost vertical line).

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Particle Shape

Angular Rounded Flaky

Elongated Flaky & Elongated

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Particle Shape

Rounded

Angular

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Percent Fractured Faces

0% Crushed 2 or More Fractured Faces

Surface Texture

Specs defines minimum percentage of aggregates with fractured faces

Uncompacted Voids in Coarse Aggregate (Coarse Aggregate Angularity)

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Uncompacted Voids in Coarse Aggregate

Uncompacted Voids in Coarse Aggregate

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Soundness and Durability

• Soundness or durability is the resistance to weathering (freeze/thaw).

• Most common methods• Sodium or magnesium sulfate

• AASHTO T104

Soundness Test Method

• Aggregates soaked then transferred to oven to dry• 1 cycle = one soak + one dry

• 5 cycles to 30 cycles used• 5 to 10 most common

• Conduct sieve analysis to determine change in gradation due to weathering

Before After

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Deleterious Materials in Aggregates

Voids and Moisture Absorption

Oven Dry (Bone Dry) Air Dry

SSD Moist

Permeable Voids

Solid

Moisture

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AbsorptionOven Dry (Bone dry)

• Zero moisture in voids

Air Dry

• Some moisture in voids (equal to atmospheric

humidity)

Saturated-Surface Dry (SSD)

• Voids totally filled with moisture but surface of

particles are dry

Moist

• Voids are filled and some surface moisture exists

• We want SSD for concrete so that mix water is not absorbed

and is available for workability and hydration

• We want dry aggregate for asphalt

• Some asphalt absorption is good for bonding and locking

• But, this uses a little more asphalt in the mix which is

slightly more costly

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Adhesion of Aggregate to Bitumen

• Some aggregates have an affinity for water over asphalt (hydrophilic). • These aggregates tend to be acidic and suffer from stripping after

exposure to water.

• Some aggregates have an affinity for asphalt over water (hydrophobic). • These aggregates tend to be basic and do not suffer from stripping

problems.

• Mineralogy and chemical composition of the aggregate are important contributing factors to the stripping problem.

• Additionally, an aggregate’s surface charge when in contact with water will affect its adhesion to asphalt cement and its susceptibility to moisture damage.

Stripping•Loss of bond between aggregates and asphalt binder•Typically begins at the bottom of the HMA layer and progresses upward.

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Stripping

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Cairo – Alexandria Road, 2012

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Raveling•Loss of bond between aggregates and asphalt binder.•Typically begins at the surface of the HMA layer and progresses downward.

Elsalwa Road,

KSA, 2007

Desirable Properties of Rocks for HMA

Source:Pavementinteractive