Predicting rutting and fatigue cracking using the TSD

Christoffer P. Nielsen

Greenwood Engineering

Motivation

Rutting Fatigue (alligator) cracking

Pavement distress

Pothole

Can be detected using profilers and surface imaging systems

BUT: Can only detect damage that has already occurred

Can we do better?

Motivation

Rutting- Compressive strains in subgrade- Shear strains in asphalt

What happens under a moving load?

Rutting Fatigue (alligator) cracking

Pavement distress

Pothole

Pavement distress

Strains in pavement

Mechanistic model

Pavement responseExternal load

Non-destructive pavement testing

Falling Weight Deflectometer

One issue: FWD is slow- Costly- Disrupts traffic- Safety hazard

Vertical strain

Fatigue (alligator) cracking- Tensile strains at bottom of asphalt layer

The Traffic Speed Deflectometer

- Measures at traffic speed 5 km/h – 80 km/h- Uses laser Doppler vibrometers to measure pavement response

- 𝑆𝐶𝐼300 is related to tensile strains in asphalt- 𝐷0 is related to compressive strains in subgrade- 𝐵𝐷𝐼 is related to strains in the base layer

Using TSD data

Simple indices:𝐷0, 𝑆𝐶𝐼300, 𝐵𝐷𝐼

Model fit / Numerical integration

Pavement distress

Back-calculation

Static analysis Compare

Pavement model Simulated slopes Measured slopes

Modify

FWD: Time-dependent experimentStatic deflectionTSD: moving load

Dynamic modeling

Inertia

Varying load 𝐹 = 𝐹(𝑥, 𝑡)

Visco-elasticity 𝐸 = 𝐸(𝑡)

• Investigated in frequency domain• Complex modulus 𝐸 𝜔 = 𝐸′ 𝜔 + 𝑖𝐸′′(𝜔)

Newton’s 2nd law for displacement

Asphalt complex modulus

Visco-elastic models

Well-described by the Huet-Sayegh model

Asphalt complex modulus

Granularmaterials

Damping ⇐

Hysteretic damping model

Visco-elastic deflection basin

𝑣0

Time dependence: measurement examples

Road near Copenhagen.Three runs.

Large asymmetryDue to visco-elastic effects

Visco-elastic back-calculation

ℎ1 = 15 cm, ℎ2= 30 cm, ℎ3= ∞

Visco-elastic model fits

General finding: Measurements are well-described by the visco-elastic model

Relation to pavement distress

Pavement distress

Strains in pavement

Mechanistic model

Pavement responseExternal load

Back-calculation

Complex moduliStrains

Strains due to an actual driving truck!

Strains under driving truck

Related to compaction ruttingRelated to fatigue cracking

Related to shear flow rutting

Strains under driving truck

Related to fatigue crackingIdentify maximum strain

Damage prediction

• Complicated physical mechanisms

• Empirical relations between strain and damage

• Typically:• damage ∝ 𝜖4

• N ∝ 𝜖−420 % difference in strains causes a factor 2 difference in N

Important to have good strain estimates

Summary

• Traffic Speed Deflectometer

• Visco-elastic back-calculation

• Strains under moving truck

TSD slopesSimulated slopes

Predicting rutting and fatigue cracking using the TSD

Christoffer P. Nielsen

Greenwood Engineering