FATIGUE TESTING ON BITUMINOUS MIXTURES

PRESENTED BY-VINOD KUMAR

INTRODUCTION Fatigue cracking is consider a major distress in asphalt

pavement. The repetitive stresses and strain cause cyclic loading in

asphaltic layers which leads to fatigue cracking. Fatigue characteristics of asphalt is an important factor

that affect the structural design. This is accepted that fatigue cracking generally initiate at

the bottom of the asphalt layer and then propagate upward to the surface layer (Pell 1975).

The tensile strain consider as an important parameter controlling fatigue cracking in mechanistic approach of design.

LITERATURE REVIEW

Brovelli, Claudio, et al.i. Different additives namely cellulose and synthetic fibers,ethyl vinyl

acetate (EVA), amorphous polyolefin are used.ii. The fatigue test was performed using IDT under control stress mode of

loading.iii. The test shows that modified mix has more fatigue life than

conventional mix. Button et al.i. Controlled force tests does not account the crock propagation. Pell et al.i. Controlled strain tests include number of cycles for failure as well as

no. of cycles of crack propagation.

LITERATURE REVIEW

IRC RECOMMENDATIONS: According to IRC 37-2012  Bituminous mix design needs to be considered an integral part of pavement design exercise with a view to provide  i. Fatigue resistant mixes in the bottom bituminous layer to

eliminate bottom-up cracking. ii. Rut resistant bituminous layers of high tensile strength to

eliminate rutting and surface cracking

Methodology

PREPARATION OF SAMPLE

FIND OUT THE OPTIMUM BITUMEN CONTENT

CONDUCT THE TESTS ON SAMPLE

ANALYSIS OF RESULTS

CALCULATION OF FATIGUE LIFE, FATIGUE FACTOR, AND SHIFT FACTOR

PREPARATION OF SPECIMEN

The Marshall stability and flow test provides the performance prediction measure for the Marshall mix design method.

Load is applied to the specimen till failure, and the maximum load is designated as stability.

Loading rate is 50.8mm/min. The flow value is recorded in 0.25 mm increments at the

same time when the maximum load is recorded. Approximate 1200gm of aggregate and filler is used and trial

percentage of bitumen starting from 3.5% is taken. The thickness of specimen is 63.5mm and diameter

101.6mm.

PREPARATION OF SPECIMEN Theoretical specific gravity of the mix Gt:

Bulk specfic gravity of mix Gm: The bulk specific gravity or the actual specific gravity of the mix Gm is the specific gravity considering air voids and is found out by: 

Gm = Wm / (Wm - Ww) Air voids percent Vv: Air voids Vv is the percent of air voids by

volume in the specimen and is given by: Vv =(Gt - Gm)*100/Gt  Percent volume of bitumen Vb: The volume of bitumen Vb is

the percent of volume of bitumen to the total volume and given by:

PREPARATION OF SPECIMEN Voids in mineral aggregate VMA:  Voids in mineral aggregate VMA is

the volume of voids in the aggregates, and is the sum of air voids and volume of bitumen, and is calculated from,

VMA = Vv + Vb Voids filled with bitumen VFB: Voids filled with bitumen V FB is the

voids in the mineral aggregate frame work filled with the bitumen, and is calculated as:

VFB = (Vb * 100) / VMA Determine optimum bitumen content: Determine the optimum binder

content for the mix design by taking average value of the following three bitumen contents found form the graphs obtained in the previous step.

i. Binder content corresponding to maximum stability.ii. Binder content corresponding to maximum bulk specific gravity (Gm).iii. Binder content corresponding to the median of designed limits of percent

air voids (Vv) in the total mix (i.e. 4%).

DIFFERENT TYPES OF TEST

Indirect tensile test Four point bending test Two point banding fatigue test Three point banding fatigue test Wheel tracking fatigue test

CONTROLLED STRAIN TEST

This test is also known as controlled displacement test. In this test the displacement amplitude is maintained constant and

force required to maintain the initial strain decreases gradually. The failure or termination point for this test is when the strain

reaches to twice the initial strain or the stiffness of the specimen falls half to the initial stiffness.

In controlled strain test the fatigue life is generally longer because the crack propagation is included in the record.

CONTROLLED STRESS TEST

This test is also known as controlled force test. In this test the force amplitude is maintained at the same level as

the initial force. As a result of which the displacement amplitude increases until it

reaches the twice the initial amplitude. This test does not account the both crack initiation and

propagation (Button et al. 1987). The failure point is defined by the fracture of the sample.

12