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.