Pavement Design

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Material Characterization

University of Florida

Topic 3 – Material Characterization – Soils and Granular Bases

Classification provides some insights on soil characteristics and expected response in terms of: • Modulus • Strength • Bearing capacity • Particle structure (gradation) • Moisture susceptibility • Drainage • Erosion potential

1 Soils

Why is it important to classify soils?

1.1 Soil Classification

Topic 3 – Material Characterization – Soils and Granular Bases

Subgrade performance generally depends on:

• Load bearing capacity; subgrade must be able to support loads passed on from the pavement structure

• Moisture content; it tends to affect a number of subgrade properties including load bearing capacity

• Shrinkage and/or swelling; some soils shrink or swell depending upon their moisture content

1.1 Soil Classification (cont.)

Topic 3 – Material Characterization – Soils and Granular Bases

Remedies for poor subgrade conditions:

• Remove and replace; subgrade soil can be removed and replaced with higher quality fill

• Stabilization; adding an appropriate binder – such as lime, Portland cement or emulsified asphalt – can increase subgrade stiffness and/or reduce water susceptibility

• Additional base layers; include a subbase, or increase thickness of base

1.1 Soil Classification (cont.)

Topic 3 – Material Characterization – Soils and Granular Bases

1.1 Soil Classification (cont.)

No pavement construction should proceed without a soil report which should include the following:

• Soil boring logs of the subgrade material along the roadway alignment at designated intervals with a specified maximum depth of testing (usually 5 feet below the surface)

• Sieve analysis and Atterberg’s limit test to facilitate soil classification

• Determination of the modulus of the subgrade and base material

• Analysis of the water table fluctuations

Topic 3 – Material Characterization – Soils and Granular Bases

1.1 Soil Classification (cont.)

Sieve Analysis

Atterberg’s Limits

AASHTO or USCS Classification System

Topic 3 – Material Characterization – Soils and Granular Bases

USCS Soil Classification

Unified Soil Classification System (USCS) (ASTM D 2487)

Major Divisions Typical Names

Course-Grained Soils

≥50% retained on 0.075 mm (No. 200) sieve

Gravels ≥50% of course fraction retained

on 4.75 mm (No. 4) sieve

Clean Gravels GW Well-graded gravels and gravel-sand mixtures, little or no fines

GP Poorly-graded gravels and gravel-sand mixtures, little or no fines

Gravels with Fines

GM Silty gravels, gravel-sand-silt mixtures

GC Clayey gravels, gravel-sand-clay mixtures

Sands ≥50% of course fraction passes

4.75 mm (No. 4) sieve

Clean Sands SW Well-graded sands and gravelly sands, little or no fines

SP Poorly-graded sands and gravelly sands, little or no fines

Sands with Fines

SM Silty sands, sand-silt mixtures

SC Clayey sands, sand-clay mixtures

Fine-Grained Soils

≥50% passes 0.075 mm (No. 200) sieve

Silts and Clays LL ≤ 50%

ML Inorganic silts, very fine sands, rock four, silty or clayey fine sands

CL Inorganic clays of low to medium plasticity, gravelly/sandy/silty/lean clays

OL Organic silts and organic silty clays of low plasticity

Silts and Clays LL > 50%

MH Inorganic silts, micaceous or diatomaceous fine sands or silts, elastic silts

CH Inorganic clays or high plasticity, fat clays

OH Organic clays of medium to high plasticity

Highly Organic Soils PT Peat, muck, and other highly organic soils

Topic 3 – Material Characterization – Soils and Granular Bases

USCS Soil Classification (cont.)

Casagrande’s chart:

Topic 3 – Material Characterization – Soils and Granular Bases

AASHTO Soil Classification

General Classification Granular Materials (35% or less passing the 0.075 mm

sieve) Silt-Clay Materials (>35%

passing the 0.075 mm sieve)

Group Classification

A-1

A-3

A-2

A-4 A-5 A-6

A-7

A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7 A-7-5 A-7-6

Sieve Analysis, % passing

2.00 mm (No. 10) 50 max … … … … … … … … … …

0.425 (No. 40) 30 max 50 max 51 min … … … … … … … …

0.075 (No. 200) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min

Characteristics of fraction passing 0.425 mm (No. 40)

Liquid Limit … … 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min

Plasticity Index 6 max N.P. 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min 1

Usual types of significant constituent materials

stone fragments, gravel and sand

fine sand

silty or clayey gravel and sand silty soils clayey soils

General rating as a subgrade excellent to good fair to poor

Topic 3 – Material Characterization – Soils and Granular Bases

• Penetration test (strength)

• Used for granular and fine-grained soils

• Load (pressure) is recorded

• Take the ratio to the bearing capacity of a standard rock

• Range: 0 (worst) – 100 (best)

1.2 California Bearing Ratio (CBR)

�� = �� 0.1" �� ℎ� ��

�� 0.1" ��

Topic 3 – Material Characterization – Soils and Granular Bases

1.2.1 California Bearing Ratio (CBR) Laboratory

Piston

Deflection dial (loading)

Deflection dial (swelling)

Proctor

Ø = 6”

Sample 4.5”

Surcharge (confinement)

• ASTM D 1883: Performed in the laboratory

Topic 3 – Material Characterization – Soils and Granular Bases

• ASTM D 4429: Performed in the field

1.2.2 California Bearing Ratio (CBR) Field

Load

Deflection measurement

Loading Plate (Φ=10”)

Topic 3 – Material Characterization – Soils and Granular Bases

• Determination of the bearing capacity (strength)

• Laboratory test @ different moisture contents

• Used for limestone, stabilized soils and materials encountered in Florida

• Load (pressure) is recorded

• Take the ratio to the bearing capacity of a standard rock

• Range: 0 (worst) – 100 (best)

1.3 Limestone Bearing Ratio (LBR)

�� = �� 0.1" �� ℎ� ��

800 �� (�� )

�� ≈ 0.8 � ��

Topic 3 – Material Characterization – Soils and Granular Bases

1.4 Stabilometer (R-value)

Pressure Gauge Testing Head

Bottom Plunger

Sample

Fluid under pressure

(ph)

Apply a vertical pressure and then measure the resulting horizontal pressure (reaction)

Topic 3 – Material Characterization – Soils and Granular Bases

• Resistance (internal friction) value determined by stabilometer

• Laboratory test for granular materials and asphalt mixtures

• Apply vertical pressure

• Measure horizontal pressure induced in the fluid

• Range: 0 (worst) – 100 (best)

1.4 Stabilometer (R-value)

pv & ph = Vertical and horizontal pressure respectively D2 = displacement of stabilometer fluid to increase ph from 5 to

100 psi, measured in revolutions of a calibrated pump handle

� = 100 − 100

2.5 ��

� ��

− 1 + 1

Topic 3 – Material Characterization – Soils and Granular Bases

1.5 Triaxial Test

Sample

= Confining Pressure (σ2, σ3)

= Deviator Stress (σd)

σ1

σ2

σ3

In Triaxial cell (cylinder): σ2=σ3

Deviator Stress: Axial stress in excess of the confining pressure in Triaxial cell

σ3

σd σ1

�� = �� − ��

• Laboratory test for granular materials (including asphalt mixtures) and fine-grained soils

Topic 3 – Material Characterization – Soils and Granular Bases

1.5 Triaxial Test (AASHTO 307-ASTM D5311)

εp,2

Topic 3 – Material Characterization – Soils and Granular Bases

1.5.1 Triaxial Test on Granular Soils

• Effect of confinement on modulus

Triaxial test #1 (σ3,1)

D e v ia

to r

st re

ss (

σ d )

Triaxial test #2 (σ3,2)

σ3,2 > σ3,1 MR,1

MR,2

εp,1 εr,1

εr,2

εr,2 < εr,1

��,� = σ� ��,�

MR,2 > MR,1

Topic 3 – Material Characterization – Soils and Granular Bases

• For granular soils: – MR = function of confinement

• Triaxial tests are performed at certain levels of confining pressure and vary the deviator stress (σd) – k1 & k2 experimentally determined values

log θ or s3

lo g M

xk1 MR = k1 · s3

MR = k1 · q k2

1.5.1 Triaxial Test on Granular Soils

State of confinement defined by the first stress invariant: θ = σ1 + σ2 + σ3

Topic 3 – Material Characterization – Soils and Granular Bases

MR = k1 · θ k2

MR =3960 · θ 0.35

AI suggests: • σ3=σ2=2 psi • σd=σ1-σ3=6 psi

θ=12 psi

• Example:

1.5.1 Triaxial Test on Granular Soils

k1 = 3960

k2 = 0.35

MR =9450 psi

Topic 3 – Material Characterization – Soils and Granular Bases

• Effect of deviator stress on modulus

D e v ia

to r

st re

ss (

σ d )

MR,1 MR,2 < MR,1

1.5.2 Triaxial Test on Fine-grained Soils

MR,2

Topic 3 – Material Characterization – Soils and Granular Bases

1.5.2 Triaxial Test on Fine-grained Soils

• For fine-grained (cohesive) soils: – MR = function of deviator stress (σd)

• Run triaxial tests at certain values of deviator stress and vary the confining pressure – k1, k2, k3 & k4 experimentally determined values

�� = �� + �� − ��

�� = �� − �� − ��

∀ �� ≤ ��

∀ �� > ��

σd

M R

Topic 3 – Material Characterization – Soils and Granular Bases

• Example:

1.5.2 Triaxial Test on Fine-grained Soils

�� = �� − �� − �� = 5600 − 388 � 6 − 5.2 = 5290 ��

AI suggests: • σ3=σ2=2 psi • σd=σ1-σ3=6 psi

σd > k2

Topic 3 – Material Characterization – Soils and Granular Bases

1.6 Design Resilient Modulus (Soils)

1.6.1 Correlations

Maybe there is information already available

Asphalt Institute Conversions: • MR=1500·(CBR) • MR=1155+555·(R)

Obtaining MR from a series of different test types

Topic 3 – Material Characterization – Soils and Granular Bases

Source: Guide for Mechanistic Empirical Design of New and Rehabilitated Pavement Structures Appendix CC-1 (ARA, 2001)

1.6.1 Correlations (cont.)

Topic 3 – Material Characterization – Soils and Granular Bases

1.7 Plate Loading Test

Reaction (Steel Beam)

Pressure Gauge

Deflection Dial @ 1/3 Points Hydraulic Jack

� = �

∆

• Deflection measurements are used to estimate modulus

• Field test

• Used for granular and fine-grained soils

• Best way to obtained a single value for design

• Modulus of subgrade reaction:

Plate (Φ=30”)

Topic 3 – Material Characterization – Soils and Granular Bases

1.7 Plate Loading Test (cont.)

Modulus of subgrade reaction: � = �

∆

Resilient modulus (rigid plate): �� = �

1 − ν� ��

∆

Substituting Δ from MR into k: � = 2

�

�� 1 − ν� �

For a 30-in plate (ν=0.45): � ��/�� = �� /��

�

18.8

1.7.1 Modulus of subgrade reaction-resilient modulus relationship

• Penetration test (strength)

• Field test

• Used for sands and fine-grained soils

• Drop a hammer and record penetration vs. #blows

Topic 3 – Material Characterization – Soils and Granular Bases

1.8. Dynamic Cone Penetrometer

Topic 3 – Material Characterization – Soils and Granular Bases

1.8.1 DCP Index conversion to CBR

Source: Predicting California Bearing Ratio from Trafficability Cone Index Values (Shoop et al., 2008)

Topic 3 – Material Characterization – Soils and Granular Bases

1.9 Water Table Elevation

Installation of a piezometer in the soil with a perforated pipe after a continuous flight auger drills a hole into the ground

Topic 3 – Material Characterization – Soils and Granular Bases

1.10 Subgrade Seasonal Variations

Normal MR

50,000 psi Frozen MR

Thaw MR

Freeze Time

Thaw Time

Recovery Time

Normal Time

Total Time = 12 Months

Time

2 Asphalt mixtures

2.1 Components

Topic 3 – Material Characterization – Asphalt mixtures

2.2 Superpave Performance Grade System for Asphalt Binders

• Asphalt binder: black sticky substance composed of high- molecular-weight hydrocarbons; results from distillation of crude oil

• Rheological characterization tests:

Topic 3 – Material Characterization – Asphalt mixtures

2.2 Superpave Performance Grade System for Asphalt Binders

• Grading is based on climate:

PG 64—22Performance Grade

Average 7-day maximum pavement temperature

(@20 mm below surface)

Minimum pavement temperature (@ surface)

Topic 3 – Material Characterization – Asphalt mixtures

2.3 Purpose of Asphalt Mixtures

• Riding surface:

- Smooth

- Safe (friction)

• Distribute stresses (protect subgrade)

• Resist loads without excessive permanent deformation

• Remove water

- Impermeability

- Specialty mixtures (OGFC)

Topic 3 – Material Characterization – Asphalt mixtures

2.4 Resilient Modulus (MR)

Time

S tr

e ss

( σ )

Time

S tr

a in

( ε )

εr = resilient (recoverable)

εp = permanent

Type and duration of loading is supposed to simulate that occurring in the field

• Typical response of materials under repeated loading:

�� = σ

��

Topic 3 – Material Characterization – Asphalt mixtures

2.5 Dynamic Modulus (|E*|)

Time

• Specimens subjected to cyclic (sinusoidal) loading:

�∗ = σ� ��

σ0

ε0

Δt

� = ∆�

� � 2�

2.6 Fatigue testing

εt Why 3rd-point loading?

To have an even distribution of M; we know the value of M, no matter where the specimen fails

Topic 3 – Material Characterization – Asphalt mixtures

�� = � 1

��

�� 1

��

2.6 Fatigue testing (cont.)

Topic 3 – Material Characterization – Asphalt mixtures

2.6.1 Constant Stress (Load) Fatigue Test

• Apply constant stress (load) • Failure occurs when the material fractures

σ0 S tr

e ss

, σ

Number of Cycles, N

S tr

a in

, ε

Number of Cycles, N

ε0

Topic 3 – Material Characterization – Asphalt mixtures

• More representative of pavements with thick asphalt layer

2.6.2 Constant Strain (Deformation) Fatigue Test

• Apply constant strain (rate of deformation) • Failure occurs when E=½E0

S tr

e ss

, σ

Number of Cycles, N

S tr

a in

, ε

Number of Cycles, N

ε0

σ0

Topic 3 – Material Characterization – Asphalt mixtures

• More representative of pavements with thin asphalt layers

2.6.3 Fatigue Test Analysis

• Plot the strain VS number of repetitions to failure on log scales • C1 & C2 curves for different material/temperature

S tr

a in

, L o g ε

Number of Cycles, Log Nf

Which curve has the highest stiffness?

Check: • Select a strain level • Find the corresponding Nf • Higher stiffness will have less

number of cycles to failure

Nf1Nf2

Low

High

From the graph: • Stiffness of the material will depend on time of the year (temperature) • εt depends on the material properties (E) • So, the cycles to failure Nf will also depend on the temperature

Must use cumulative damage approach to evaluate failure

Topic 3 – Material Characterization – Asphalt mixtures

What material properties do we used for design?

• Most paving materials are non-linear and experience some permanent deformation (not elastic) after each load application

• If the load is small compared to the strength of the material and is repeated for a large number of cycles, the deformation is almost fully recoverable and the response can be considered elastic

Topic 3 – Material Characterization – Summary

3 Summary