SEAUPG 2002 CONFERENCE 1 Top-Down Cracking: Causes and Potential Solutions Dr. Rey Roque, P.E. University of Florida 352-392-9537 [email protected]Introduction Surface-initiated longitudinal wheel path cracking: Common mode of failure Recently identified Mechanisms now better understood Occurs primarily under critical conditions Existing design methods are inadequate Understanding leads to more resistant pavements Background In the Past, Cracking Assumed to Always Start at Bottom of AC Layer (Structure has a Strong Effect) Have evaluated effects of factors on response at this location CRACKING IN WHEEL PATHS CORE EXTRACTED FROM FIELD Objectives To Summarize Findings Related to Top-Down Cracking - Mechanisms for Initiation and Propagation - Key Factors Dominating the Mechanisms - Implications for Mitigation and Design (improved guidelines for mixture and pavement design)
Common mode of failure Recently identified Mechanisms now better understoodOccurs primarily under critical conditionsExisting design methods are inadequateUnderstanding leads to more resistant pavements
BackgroundIn the Past, Cracking Assumed to AlwaysStart at Bottom of AC Layer (Structure has a Strong Effect)
Have evaluatedeffects of factorson response atthis location
CRACKING IN WHEEL PATHS
CORE EXTRACTED FROM FIELDObjectives
To Summarize Findings Related to Top-Down Cracking- Mechanisms for Initiation and Propagation- Key Factors Dominating the Mechanisms- Implications for Mitigation and Design (improved guidelines for mixture and pavement design)
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Pavement Surface Stresses
Tire-Pavement Interface Stresses- Including lateral stressesThermal StressesPavement Structure Effects
Layer Stiffness and Thickness
Measured Tire-Pavement Interface Stresses
(Smithers Scientific Services, Inc.)
16 Transducers
Bed Motion
Tire RollingDirection
Bed
σz δz
σy δyσx δx
Tire
Transducer Detail
Transverse Contact Stresses
. . . . .
Radial (R24.5)Truck Tire
Transverse Stress, σxx
Tension
Compression
Transverse Surface Stress Distribution
-10-8-6-4-202468
10
0 50 100 150 200 250 300Transverse Distance, X (mm)
Tran
sver
se S
tres
s,
xx (
MPa
)
Specified Load/Pressure
Tension
Compression
1 3 4 52Tread
Thermal Stress
-1-0.8-0.6-0.4-0.2
00.20.40.60.8
1
0 24 48 72Time (hr)
Ther
mal
Str
ess
(MPa
) 0 mm Depth50 mm Depth100 mm Depth
Aged AC-30
Findings for Crack Initiation
Primary Contributors to Surface-Initiated Longitudinal Wheel Path Cracking- Transverse contact stresses induced by
radial truck tires- Thermal stresses- Age-Hardening near surface
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Propagation of Top-Down Cracks
Surface Contact Stresses (Tire or Thermal) Cannot Explain Propagation of Top-Down Cracks- Continued work to identify mechanisms and factors controlling propagation
RANGE OF FACTORS
• Load Spectra Positioning with respect to crackRealistic tire contact stresses (vertical and lateral)
• Tension >> Shear failure at crack tip• Higher stiffness ratio increases tension at crack tip
E1:E2= 60
E1:E2= 27E1:E2= 40
E1:E2= 18
Load Position/Crack Length
Undeformed Pavement
CRACK
LOAD
CRACK
LOAD
Deformed Pavement
Transverse Stress DistributionAlong Surface of PavementTensile
Zone (+)
(-)Compressive
Zone
(+)(-)
PARAMETRIC STUDY - LOADING
0
25
50
75
100
125
150
175
200
225
250
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Crack Length (in)
Mod
e I F
ract
ure,
KI (
psi-(
in)0.
5 )
Crack Located Under W ide Rib
Crack Located 20" Away From Center of Load
Crack Located 25" Away From Center of Load
Crack Located 30" Away From Center of Load
E1:E2 = 800:20 ksi
Load position – most critical factor for tension at crack tipSTIFFNESS GRADIENTS
Case 1: Uniform temperature distribution (i.e., no stiffness gradient) mean pavement temperature computed at 1/3-depth when
temperature conditions = warm
Case 2: Sharpest temperature gradient near surface - temperatures at 7 PM represent this condition
Case 3: Highest temperature differential between surface & bottomof asphalt concrete layer - temperatures at 5 AM representthis condition
Case 4: Rapid cooling near the surface represent case of suddenrain showers
Temperature differentials, age-hardening, sudden rains induce sub-layers of variable stiffness within asphalt concrete
0
25
50
75
100
125
150
175
200
225
250
275
300
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Crack Length, a (in)
Mod
e I F
ract
ure,
K 1
(psi
-(in)
0.5 )
8in AC, Case 1 - Uniform Stiffness E1 = 800 ksi
8in AC, Case 2 - Sharp Gradient
8in AC, Case 3 - Highest TemperatureDifferential8in AC, Case 4 - Sudden Rain
Base Stiffness E2=20ksi
Major increase in tension – key factor in propagation of cracks
STIFFNESS GRADIENTS STAGES OF CRACK GROWTHShort Cracks, a = 0.25”,0.5”
Intermediate Cracks, a = 0.75”,1.0”,1.5”
aWide Rib Load 20”-30”
8” ACd
aLoad 20”-30”
8” ACd
Load 7”
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MPLICATION OF LOAD SPECTRA
Load Wander & Magnitude Instrumental in Mechanism- Critical position not always directly in wheel path on top of crack
- Depends on crack length, pavement structure,type of stiffness gradient
- Need to determine how many loads induce tension @ crack tip
Key FindingsTop-Down Cracks Develop Even in Thick Pavements (Previous Field Studies)Mechanisms Cannot be Captured Without Considering- Realistic Contact Stresses- Temperature Gradients- Load Wander- Presence of Cracks (Fracture Mechanics)
Most Recent Field Test Sections5 pairs of poor and good performing sections throughout Florida
•Location:
Performance:
SR 16-4C
SR 16-6U
US 19-1U
US 19-2C
TPK 1U
TPK 2C
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NW 39-1C
NW 39-2U
NW 39-2U
NW 39-1C
SamplingCoring
Section WP BWP Total
SR 16-4C 3 4 7
SR 16-6U 4 4 8
US 19-1U 18 18 36
US 19-2C 18 18 36
SR 375-1U 12 0 12
SR 375-2C 12 0 12
TPK-1U 12 12 24
TPK-2C 8 12 20
NW 39-1C 18 18 36
NW 39-2U 18 18 36
Total 227
Sample selection: closest to average BSGCrack depth:
SR 16-6U SR 16-4C US 19-1U US 19-2C SR 375-1U SR 375-2C
TP K 1U TP K 2C NW 39-2U NW 39-1C
Section
DC
SE N
f
7000 9000 11000
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
SR 16-6U SR 16-4C US 19-1U US 19-2C SR 375-1U SR 375-2C
TP K 1U TP K 2C NW 39-2U NW 39-1C
Section
FE N
f
7000 9000 11000
•Propagation
0
2,00 0
4 ,00 0
6 ,00 0
8 ,00 0
10 ,00 0
12 ,00 0
SR 16 -6 U SR 16-4C US 19-1U US 19 -2C SR 375-1U SR 3 75-2C TPK 1U TPK 2C NW 39-2U NW 3 9-1C
Section
Nf t
o pr
opag
atio
n
7000 9000 11000
Individual Analysis of the SectionsSR 16: high air voids, high m-value, low FEpossible effect of friction course
US 19: stiffness overwhelmed FE, low AC content and film thickness
SR 375: low FE, high m-value, high air voids
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• TPK: low base stiffness, high m-value, lowfilm thickness
• NW 39: high E1/E2 ratio, low compliance, lowtensile strength, low FE, possible effect of friction course
Summary of Findings• Structure has effects on surface-initiated cracking
• Load + Thermal stresses plays significant role
• High MR + Low Compliance / Load and thermal σ
• UF Cracking model properly explained performance differences
• No single property will assure adequate mixtureperformance. Need of conditions and model
Key Findings (Continued)
Design Approach Using Averaged Pavement and Load Conditions Inadequate- Critical Conditions Must Be IdentifiedMitigation Should be Addressed Primarily Through- More Crack-Resistant Surface Materials- Use of Less Damaging Tires
Implications for Design
Focus on Improving Surface MaterialsDialogue With Tire DesignersPavement Design/Management- Realistic contact stresses- Load spectra (magnitude and position)- Temperature gradients- Cracks/discontinuities (fracture mechanics)
