Flexible Pavement Cracking Mechanisms

Hassan Baaj, Ph.D., P. Eng.

Associate Professor

Associate Director, Centre for Pavement and Transportation Technology (CPATT)

1

o About CPATT

o Introduction to pavement cracking mechanisms

o Some examples of common cracking modes in Canada

o Closing notes

o Questions

Presentation Outline

2

Centre of Pavement and

Transportation Technology

3

Canada Foundation for

Innovation

Ontario Innovation Trust

Regional Municipality of

Waterloo

McAsphalt Industries Limited

Ministry of Transportation

Ontario

Ontario Research and Development

Challenge Fund

Greater Toronto Airport Authority

Cement Association of Canada

Ontario Hot Mix Producers Assoc.

Stantec Consulting Ltd.

Several Universities

Public Sector Universities Private Sector

Partners

CPATT – History

Founded in 2004 at the University of Waterloo as a new initiative for

Pavement and Transportation Research in Canada

4

Centre of Pavement and Transportation

Technology

CPATT Goals

1. CPATT’s initiative involves an integrated program of field

and laboratory research.

2. Focus on emerging and innovative technologies.

3. State-of-the-art research infrastructure.

4. Train and educate next generation.

5. Sustained partnerships.

6. Provide national and international leadership.

5

Introduction

Failure

Flexible Pavements

are subjected to

Thermal

VariationsMoisture

Mechanical

loadings

Low

Temperature

cracking

Frost

Heave

FatigueRuttingStripping &

Ravelling

6

Cracking is more complex than this!!

o Thermally-induced cracking- Low temperature cracking - Frost-heave cracking- Reflective thermal cracking- Top-down longitudinal cracking (in joints and segregated

areas)

o Traffic loading-related cracking- Fatigue cracking - Top-down longitudinal cracking - Reflective cracking

o Combination of reasons

Introduction

7

Cracking is more complex than this!!

o Material related- Poorly-designed mix- Inadequate or poor binder- Binder-aggregates adhesion issues

o Construction related- Improper joint preparation- Thermal or physical segregation- Tack-coat problems- Compaction issues- Thin overlay over cracked pavement- Drainage issues (major problem)

o Structure related- Under-designed pavement- Unexpected traffic- Lack of frost protection

o Combination of different factors

Introduction

8

Tensile stress build up in the longitudinal

direction of the pavement

Low temperature cracking

9

Low temperature cracking

10

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250 300

Time (Min)S

tress

(M

Pa)

-35

-25

-15

-5

5

0 50 100 150 200 250 300

Stress at failure

Temperature at failure

Constant

Height

10°C/hour

Tem

pera

ture

(°C

)

TSRST

Low temperature cracking

11

Fatigue cracking

12

Fatigue cracking

13

Tension-Compression testDestructive Test

Temperatures: 10 °C

Frequency: 10 Hz

Fatigue cracking

14

For each tested specimen:

Determine the number of

cycles Nf corresponding to

failure.

Log Nf

Log e

Fatigue Line

(Wöhler )

|E*|

N

|E*0|

Nf

|E0|

2

e6

10+6

Fatigue cracking

15

Tack-coat issues

16

Tack-coat issues

17

Tack-coat issues

18

Segregation

19

Transverse joints preparation

20

Transverse joints preparation

21

Longitudinal joints

22

Patching

23

Ir regularit ies

24

Ir regularit ies

25

85.4%

196.990.0%

195.594.5%

218.689.0%

186.7

94.2%

223.7

*>252.7°F

*<68.0°F

80.0

100.0

120.0

140.0

160.0

180.0

200.0

220.0

240.0

Thermal segregation

26

Compaction issues

27

Compaction issues

28

Compaction issues

29

No skirts, it sticks !!!

Compaction issues

30

Frost heave of the pavement requires three conditions:

• The subgdare is frost susceptible

• Freezing conditions (very cold weather)

• A water table close to the subgrade (moisture source)

Frost heave

31

Source: http://www.mtq.gouv.qc.ca/en/reseau/chaussees/chaussees.asp

Frost heave

32

Ice lense

Frost heave

33

Frost

Non-frost

Frost depth tube

Frost heave

34

Impact of traffic loading

Traffic analysis

35

Traffic analysis

Which one of these two highways needs a

thicker and more robust pavement

structure?

Impact of traffic loading

36

Traffic analysis Traffic (EASL)

Determine the number of expected repetitions of 18-kip (80 kN or

8.165 tons) Equivalent Single-Axle Load (EASL) applied to the

pavement on two sets of dual tires.

Equivalent Single-Axle

Impact of traffic loading

37

Equivalent Single Axle Load (ESAL)

Converts wheel loads of various magnitudes and repetitions ("mixed traffic") to an equivalent

number of "standard" or "equivalent" loads

Based on the amount of damage they do to the pavement

Commonly used standard load is the 18,000 lb. (80 kN) equivalent single axle load

Load Equivalency

Generalized fourth power approximation

Traffic analysis: ESAL and LEF

4

loadgroupaxlestandard

loadgroupaxlespecificFactoryEquivalencLoad

RAV 4

Average empty weight

= 1.2 tons LEF = 2 (0.6/8.2)4 = 0.000057

LEF = 2 (1.3/8.2)4 = 0.0013

Suburban Average

empty weight

= 2.6 tons

Impact of traffic loading

38

The standard axle weights for a standing-room-only loaded Metro articulated bus (60 ft. Flyer)

are:

Axle Empty Full

Steering 58 kN 75 kN

Middle 67 kN 89 kN

Rear 40 kN 62 kN

Using the 4th power approximation, determine the total equivalent damage caused by this bus

in terms of ESALs when it is empty. How about when it is full?

LEF calculation example

Empty

(58/80)4 = 0.276

(67/80)4 = 0.492

(40/80)4 = 0.063

Total = 0.831 ESALs

Full

(75/80)4 = 0.773

(89/80)4 = 1.532

(62/80)4 = 0.361

Total = 2.666 ESALs

Increase in total weight = 61 kN (about 80 people) or 37%Increase in ESALs is 3.2 times

37% load increase 320% Damage increase

165 kN 226 kN

Impact of traffic loading

39

Impact of traffic loading

40

Unladen (240 t) Laden (560 t)

- front 49% = 117.6 t 33% = 184.8 t

- rear 51% = 122.4 t 67% = 375.2 t

- LEF LEFU = ? LEFL = ?

Load Equivalency Factor: Generalized fourth power approximation would give an

approximate value of LEF (Rough estimation of ELF values).

4

loadgroupaxlestandard

loadgroupaxlespecificFactoryEquivalencLoad

Impact of traffic loading

Maximum payload capacity, kg 320,000

Unladen weight, kg 240,000

Gross weight, kg 560,000

41

Maximum payload capacity, kg 320,000

Unladen weight, kg 240,000

Gross weight, kg 560,000

Unladen (240 t) Laden (560 t)

- front 49% = 117.6 t

LEF = (117.6/8.2)4 = 42303

33% = 184.8 t

LEF = (184.8/8.2)4 = 257960

- rear 51% = 122.4 t

LEF = (122.4/8.2)4 = 49644

67% = 375.2 t

LEF = (375.2/8.2)4 = 4383246

- LEF LEFU ≈ 92 000 LEFL ≈ 4641200

Impact of traffic loading

Rough estimation of ELF values

42

Closing notes

Pavement cracking mechanisms are diverse

• External factors

o Environmental conditions

o Traffic loading

• Internal factors

o Materials

- Mix design & binder properties

o Construction

- Compaction, segregation, joints, etc.

o Pavement Structure- Under-designed pavement, unexpected traffic, lack

of frost protection

o Combination of different factors43

Closing notes

Pavement cracking could be avoided or reduced

o The identification of the reason of cracking is crucial to solve the problem and to avoid it in the future

o The cracking pattern could be a good indicator to identify the source of the problem

o Most of cracking could be avoided through:

- The right selection and design of construction materials

- Good pavement design

- Good construction practices

o Role of Pavement Management and Preservation Systems is important to increase the life of the pavement

44

THYANK YOU

45