3- Highway Materials

Aggregates

Highway Materials/ Aggregates

Aggregates are granular mineral particles that are widely used for highway bases, subbases, and backfill.

Aggregate are also used in combination with cementing materials (Portland cement and asphalt) to form concretes for bases, subbases, wearing surfaces, and drainage structures.

Aggregate Sources

Natural deposits of sand and gravel.

Pulverized concrete and asphalt

pavements.

Crushed stone

Blast furnace slag

Aggregate processing

Excavations

Transportation

Crushing

Sizing

Stockpiling

Properties of Aggregates

1. Particle size and gradation.

2. Hardness or resistance to wear.

3. Durability or resistance to weathering.

4. Specific gravity & absorption.

5. Chemical stability

6. Particle shape and surface texture.

7. Freedom from deleterious particles or substances.

Particle Size & Gradation

Gradation: Blend of particle sizes in the mix.

Gradation affects: Density; Strength; Economy of pavement structure.

Particles are separated by sieve analysis.

Sieve analysis: Determination of particle size distribution of fine and coarse aggregates by sieving, expressed as %.

Grain size analysis data are plotted on aggregate gradation chart.

Using the gradation chart engineer can determine a preferred aggregate gradation that meet spec..

Coarse : Retain # 4 ……. Fine : pass # 4 Retain # 200 & fines : pass # 200

Aggregate Size Definitions

Nominal Maximum Aggregate

Size

one size larger than the first sieve to

retain more than 10%

Maximum Aggregate Size

one size larger than nominal

maximum size

100 99 89 72 65 48 36 22 15 9 4

100 100 91 72 65 48 36 22 15 9 4

0.45 Power Chart

k0.45

%P

by w

t

max size

100%

0%

SIEVE NO. (log scale)

TO

TA

L P

ER

CE

NT

PA

SS

IN

G (

arit

hm

eti

c)

Typical Gradations

Dense or well-graded. Refers to a gradation that is near maximum density. The most common HMA mix designs in the U.S. tend to use dense graded aggregate.

Gap graded. Refers to 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. These mixes can be prone to segregation during placement.

Open graded. Refers to a gradation that contains only a small percentage of aggregate particles in the small range. This results in more air voids because there are not enough small particles to fill in the voids between the larger particles. The curve is flat and near-zero in the small-size range.

Uniformly graded. Refers to a gradation that contains most of the particles in a very narrow size range. In essence, all the particles are the same size. The curve is steep and only occupies the narrow size range specified.

Types of Gradations

* Uniformly graded

- Few points of contact - Poor interlock (shape dependent)

- High permeability

* Well graded - Good interlock

- Low permeability

* Gap graded

- Only limited sizes - Good interlock - Low permeability

Sieve Analysis Example

Sieve No.

Sieve

Size

Wt. Retained

(g) % Retained

Cumulative Retained

%

Com. %

Passing

inch mm (wt. ret./ Total) 100% Sum % Retained 100 - Com. Ret.

1" 25 10 0.5 0.5 99.5

3/4" 19 50 2.5 3 97

1/2" 12.5 140 7 10 90

3/8" 9.5 250 12.5 22.5 77.5

# 4 4.75 340 17 39.5 60.5

# 8 2.36 50 2.5 42 58

# 16 1.18 450 22.5 64.5 35.5

# 30 0.6 200 10 74.5 25.5

# 50 0.3 175 8.75 83.25 16.75

# 100 0.15 225 11.25 94.5 5.5

# 200 0.075 100 5 99.5 0.5

Pan Pan 10 0.5 100 0

Total 2000 100

Sieve Analysis Example Plot

Sieve Analysis Example

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Sieve Size (0.45 Power)

% P

assin

g

Com. %

Passing

Sieve

Size

^.45

in mm

1" 25 99.5 4.2567

3/4" 19 97 3.7622

1/2" 12.5 90 3.1161

3/8" 9.5 77.5 2.7541

# 4 4.75 60.5 2.0161

# 8 2.36 58 1.4717

# 16 1.18 35.5 1.0773

# 30 0.6 25.5 0.7946

# 50 0.3 16.75 0.5817

# 100 0.15 5.5 0.4258

# 200 0.075 0.5 0.3117

Superpave Aggregate Gradation

100

0 .075 .3 2.36 12.5 19.0

Percent Passing

Design Aggregate Structure

Sieve Size (mm) Raised to 0.45 Power

Gradation Considerations

Max. size < 1/2 AC lift thickness Larger max size Increases strength

Improves skid resistance

Increases volume and surface area of agg which decreases required AC content

Improves rut resistance

Increases problem with segregation of particles

Smaller max size Reduces segregation

Reduces road noise

Decreases tire wear

Blending of Aggregates

Reasons for Blending

Obtain desirable gradation

Single natural or quarried material not

enough

Economical to combine natural and process

materials

Blending Stockpiles

Plot individual gradations

Plot specification limits

Can be used for initial assessment

Can blend be made from available

materials?

Identification of critical sieves

Est. trial proportions

Blending Stockpiles

0

10

20

30

40

50

60

70

80

90

100

0.075 0.3 1 .18 4.75 9.5 12.5 19

Sieve Size, mm

Percent Passing, %

Control points for

12. 5 nominal max. size

Gradation A

Gradation B

Blending Stockpiles

0

10

20

30

40

50

60

70

80

90

100

0.075 0.3 1 .18 4.75 9.5 12.5 19

Sieve Size, mm

Percent Passing, %

Control points for

12. 5 nominal max. size

Gradation A Gradation B

Combined Specific Gravities

G =

1

P1 + P2 + ……. Pn

100 G1 100 G2 100 Gn

Blending of Aggregates

P = Aa + Bb + Cc + ….

Where: P = % of material passing a given sieve for the

blended aggregates A, B, C, …

A, B, C, … = % material passing a given sieve

for each aggregate A, B, C, …..

a, b, c, …. = Proportions (decimal fractions)

of aggregates A, B, C, … to be used in

Blend

Blending of Aggregates

Agg. #2 Agg. #1

Blend Target

Material

%

Passing

%

Passing

% Used

U.S. Sieve %

Batch

%

Batch

No. 4

No. 8

No. 16

No. 30

No. 50

No. 100

No. 200

3/8 “

80 - 100

65 - 100

40 - 80

20 - 65

7 - 40

3 - 20

2 - 10

100

45

15

3.5

1.5

0.5

0

0

100

50

50

44

23.5

16

12

5

50

50 % 50 %

90

30

7

3

1

0

0

50

95

65

47.5

25

16.5

12

5

100

100

100

88

47

32

24

10

100

Aggregate Properties

Resistance to Wear

Material should be hard & resist wear due to:

1. The loading from compaction equipments.

2. The polishing effect of traffic.

3. Internal abrasive effects of repeated loading.

Measure used for hardness of aggregate is

Los Angelos (LA) abrasion test.

L A Abrasion Test

Insert aggregate sample in a drum that rotates 30 – 33 rpm for 500 revolutions with steel spheres inside as an abrasive charge.

Sample removed & sieved @ #12 sieve.

Retained material are washed and dried.

Difference between original mass and final mass expressed as percentage of original mass is reported as %wear.

%wear = [(Original – Final)/ Original] 100%

LA Abrasion Test

- Approx. 60% loss for soft limestones and sandstones

- Approx. 10% loss for extremely hard igneous rocks

Durability & Resistance to

Weathering (Soundness Test)

Soundness Test AASHTO T104, ASTM C88

Measures the resistance of aggregate to disintegration in a saturated solution of sodium or magnesium sulfate (Na2SO4, MgSO4).

It simulates the weathering of aggregates that occur in nature.

Simulates freeze/thaw action by successively wetting and drying aggregate in sodium sulfate or magnesium sulfate solution

One immersion and drying is considered one cycle

Durability & Resistance to

Weathering (Soundness Test)

It measures resistance to breakdown

due to crystal growth.

Result is total percent loss over various

sieve intervals for a prescribed number

of cycles

specify max % loss after X cycles typical 10-20% after 5 cycles

Soundness Test

Before

After

Specific Gravity & Absorption

Required for the design of concrete & bituminous mixes.

S.G. : Ratio of the mass to volume of an object to that of water at the same temperature.

Mass Solid

Volume

Mass Water

Volume

Specific Gravity & Absorption

Due to permeable voids in aggregates, three types of S. G. are defined

apparent (Gsa)

bulk (oven-dry) (Gsb)

effective (Gse)

Density is the unit weight of a material

lb/ft3 or kg/m3

Unit weight = g w G

g w = 1.000 g/cm3 = 62.4 lb/ft3

Ws Wssd

Fine Aggregate

OD (Ws) or SSD (Wssd) Container filled Container + agg + H20 aggregate with H2O

Wpyc+agg+w2 Wpyc+w1

Coarse Aggregate Wsub

S.G. Cont.

Apparent Specific Gravity (Gsa) Computed based on net volume of the aggregates

impermeable voids (ip) permeable voids (pp)

wips

ssa

VV

WG

g)(

21 waggpycswpyc

s

subs

ssa

WWW

W

WW

WG

Bulk Specific Gravity (Gsb)

Computed based on total volume of the aggregates

impermeable voids (ip) permeable voids (pp)

wppips

ssb

VVV

WG

g)(

21 waggpycssdwpyc

s

subssd

ssb

WWW

W

WW

WG

Effective Specific Gravity (Gse)

Aggregate absorb some asphalt cement (AC).

Gsa assumes all PP absorb AC (Vab = Vpp)

Gsb assumes no PP absorb AC (Vab = 0)

Neither is correct - Gse defined based on

overall volume exclusive of those that absorb

AC

Effective Specific Gravity (Gse), Cont.

impermeable voids (ip) permeable voids (pp)

wapppips

sse

VVVV

WG

g)(

Calculated from mixture information

mix wtby for 100

100

b

b

b

mm

bse P

G

P

G

PG

Specific Gravity Relationships

Gsb < Gse< Gsa

Chemical Stability

Aggregate surface chemistry affects bonding to asphalt cement.

Aggregates that have affinity to water are not desirable in the asphalt mixes.

Stripping

Hydrophobic Agg.: Water-hating such as limestone and dolomites have a positive surface charge. Work well in asphalt concrete (show little or no strength reduction)

Hydrauphilic Agg.: Water-loving such as gravels and silicates (acidic) have a negative surface charge (show reduce strength).

Gravels may tend to create a weaker interfacial zone in concrete than lime-stone aggregates.

Surface coating (dust of clay, silt, gypsum).tend to reduce bond strength.

Immersion stripping test

Chemical Stability

ITS = 2p / t d

Aggregate Shape & Surface Texture

Results from Processing

Shape: rounded, sub-rounded, flat,

elongated.

Angularity: sub-angular, angular

surface texture: very rough, rough,

smooth, polished

Percent Crushed Fragments in Gravels

0% Crushed 100% with 2 or More

Crushed Faces

Flat and Elongated Particles

ASTM D4791 Maximum

Minimum

Disintegration/ Cleanliness

Clay Lumbs & Friable Particle (ASTM C142, AASHTO T112).

Specify max (typical 0.2 - 10%).

Dries a given mass of agg., then soaks for 24, hr., and each particle is rubbed. A washed sieve is then performed over several screens, the aggregate dried, and the percent loss is reported as the % clay or friable particles.

Cleanliness of Aggregates/ SE

sand equivalent (ASTM D2419)

Used to estimate the relative proportions of

fine agg. and clay-like or plastic fines and

dust

specify min

SE = Sand Reading Clay Reading *100

Sand

Reading

Clay Reading

Flocculating

Solution

Suspended

Clay

Sedimented

Aggregate

Things to Remember

Aggregates should be clean, tough,

durable, and free from :excess flat and

elongated particles, dust, clay lumbs,

and any other objectionable materials.