Over the past twenty
years; road traffic (both passenger and freight) has grown
significantly and loading is progressively getting worse due to the
introduction of newer and more powerful trucks with heavier wider
bodies in Pakistan. Consequently, premature rutting in the form of
shear flow in flexible pavements has been observed during high
ambient temperatures. National Highway Authority (NHA), Pakistan has
been facing serious threats like, frequent pavement failures, poor
riding quality and high maintenance cost. Modifications of Asphalt
cement with polymers and rigid pavement design trends, being the end
solutions, have increased the construction cost, even more than four
times than that of conventional design. In order to cater for the
growing axle load demand and to increase the performance of asphalt
concrete mixes, true prediction and accurate estimation of probable
behavior of mixes need to be investigated.
A comprehensive laboratory study was carried out using NHA aggregate
gradation Class “A” for asphaltic wearing course, which has commonly
been used in the field. The main objective of the research work was
to evaluate the effects of temperature and loading on the permanent
deformation behavior of mixes, designed with the same aggregate
gradation and three commonly available asphalt cement types (A.C).
Two gradations i.e. “01” and “02”, within the envelope of the same
gradation, were chosen for this study. Three asphalt cement (A.C.)
types i.e. two neat A.C with penetration grade “60/70”, “40/50” and
one modified A.C. (base “60/70” pen. grade with Elvaloy Terploymer)
were selected. Six mixes ranging from finer to coarser aggregate
gradation were therefore designed at optimum filler contents, in
order to get better mix cohesion, resistant to rutting and to
improve serviceability. Three percentages of mineral fillers (i.e.
2.4%, 3.4%. & 4.4%) were trialed in order to determine the optimum
filler content for asphalt mixes.
Mix design properties i.e. optimum asphalt content, percentage air
voids, voids in mineral aggregates, voids filled with asphalt, flow
and stability were determined using Marshall Method of Mix Design.
Asphalt cement consistency i.e. penetration grade, ductility,
softening point and rheological properties i.e. phase angle &
complex shear modulus were also measured. The temperature influence
on rheological properties were determined at constant frequency of
10 Hz using Dynamic Shear Rheometer.
Two performance tests i.e. uniaxial repeated loading strain & wheel
tracking tests were chosen to measure the permanent deformation of
asphalt mixes. Relationship between rutting and factors effecting
rutting were developed. Domains of intercept and slope coefficient
and permanent deformation coefficients alpha (α) and mu (µ) were
also determined. New relationships have been developed to predict
the impact of Hot Mix Asphalt properties on its permanent
deformation behaviour. It has been revealed that mixes with coarser
gradation and polymer modified asphalt showed better resistance
against rutting. Permanent deformation coefficients, such as α and µ
indicate influence of mix properties on mix rut potential. It was
further observed that temperature has no effect on α but influences
significantly on µ.
This study presents relationships developed to correlate temperature
and rut values of mixes, tested under wheel tracker (WT). It also
presents a comparison between uni-axial repeated load strain tests
(or repeated creep test) and wheel tracker tests in terms of
regression constants, shift factors and a correlation between both
the test methods. Intercept coefficient varies in a narrow range.
Based on intercept coefficient, uniaxial repeated load strain test
(repeated creep test) did not provide clear ranking of mixes at
specified temperatures and stress conditions. The slope of data line
between both the tests reduces with increase in temperature and
stress levels, which means permanent deformation increases with an
increase in temperature and stress. An average value has been
determined for approximate shift of creep test data to WT test data.
Furthermore a statistical estimation of Mechanistic-Empirical Model
has been formulated that relates plastic to elastic strain ratio
with different variable that influenced in the mix rut development.
Using 54 variables, comprising six mixes designed at low asphalt
contents, three temperatures (25, 40 and 550C) and three stress
levels (100, 300 and 500 kPa) under repeated load test, a
mathematical model was developed to assess the magnitude of plastic
to elastic strain ratio. It was observed that plastic to elastic
strain ratio is a function of number of load repetitions,
temperature, stress levels, shear complex modulus of asphalt cement
and aggregate gradations respectively. Despite few limitations i.e.
single source of aggregate, single method for mix design, two test
methods, certain parameters that help to estimate the permanent
deformation in the asphalt layers of flexible pavements were
successfully captured in the model.
Keywords: Flexible Pavement, Hot Mix Asphalt, Uniaxial Repeated
Loading, Wheel Tracker, Permanent Deformation.