The Fatigue Performance of Asphalt Under Uniaxial Tensile Cyclic Loading

Fidez Oreta and Catherine Mills

Supervisors: Dr. Doug Wilson and Dr. Tam Larkin Project Number: 124

Group: Transport Infrastructure

Results A total of 44 samples were tested. One way analysis of variance was used to

compare the following fatigue characteristics:

Introduction Many structural aspects of asphalt pavements, especially

when utilising New Zealand aggregates, are not fully

understood. Fatigue cracking is a major distress mode in

asphalt and it is necessary to gain a better understanding

of fatigue behaviour before asphalt pavements become

widely implemented in New Zealand.

Crack Development Photographs were used to examine fatigue cracking in the

samples. The following crack behavior was observed:

Conclusions • AC20 60/70 mix had the longest fatigue life.

• AC14 60/70 had the largest strain at failure and shortest

fatigue life.

• 20mm nominal aggregate size had longer fatigue lives

compared to 14mm.

• No difference was observed between the fatigue life of

polymer modified binder and conventional 60/70 binder.

• Stiffness is constant at low stresses.

• Using crack initiation as the failure point produced shorter

fatigue lives compared to the 50% reduction in stiffness.

Tensile

Str

ain

Cycle

s

Cycle

s

Nominal Aggregate Size (mm)

Cycle

s

Binder Type

Two methods were used to determine fatigue life:

1) 50% reduction in initial stiffness;

2) Crack initiation.

The two methods produced similar trends. Results using the reduction in

stiffness method are shown above. Graphical techniques for identifying

failure are illustrated below.

• Fatigue life;

• Effect of nominal aggregate size;

• Strain at failure;

• Effect of polymer modified binder. 1. Crack development near elongated aggregates

2. Crack propagation along surfaces of large aggregate

3. Cracking through aggregate cleavage planes

Before testing During testing

Before testing During testing

Before testing During testing

Objectives • Explore the feasibility of cyclic uniaxial tensile testing as

a fatigue testing method.

• Compare fatigue behaviour between four asphalt mixes.

Methodology

A 60/70 binder and a polymer-

modified binder were mixed

with 20 and 14mm nominal

aggregate gradations.

Slabs were compacted using a kneading

compactor and cut with a diamond saw.

Samples were epoxied to

steel plates and stacked

on a jig to cure.

Samples were bolted to

the MTS via a pin

connection and a camera

was used to take photos of

the samples cracking.

0

50

100

150

200

0 1800 3600 5400 7200

Str

ess

(k

Pa

)

Number of Cycles

74 - 10 kPa (2Hz) 154 - 10 kPa (2Hz)

164 - 10 kPa (2Hz)

Samples were subject to a three-stage

controlled stress loading regime until

they fractured into two pieces.

From To Peak Trough

0 1800 10 74

1801 5400 10 154

5401 failure 10 164

Cyclic Range Stress (kPa)

Mix Variables AC14 AC20

60/70 AC146070 AC206070

DHD AC14DHD AC20DHD

DEPARTMENT OF CIVIL AND

ENVIRONMENTAL ENGINEERING

THE UNIVERSITY OF AUCKLAND

FACULTY OF ENGINEERING