Characterisation of material properties and behaviour of cold bituminous mixtures for road pavements
Date
2008-03
Authors
Ebels, Lucas-Jan
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
The cold bituminous mixtures, which are the subject of this study, are obtained by
mixing mineral aggregate with either bitumen emulsion or foamed bitumen at
ambient temperatures. These techniques are frequently used in Cold In-Place
Recycling whereby typically the top 150 – 250 mm of the existing pavement is
reworked, as a rehabilitation measure when structural maintenance is required. To
differentiate from the cold mixes for surfacing layers the term Bitumen Stabilised
Materials (BSM’s) is adopted here.
The increased use of BSM’s, shortcomings in the existing design guidelines and
manuals and ongoing developments in the concepts and understanding of these
materials require further research into the fundamental properties and behaviour of
BSM’s. Achieving a better understanding of the fundamental performance properties
of BSM’s is the main objective of this study, with a view to using the extended
knowledge for improvements to current mix design and structural design practices.
The state-of-the-art of bitumen emulsion and foamed bitumen techniques is reviewed
in a literature study. Current best practices in the design of BSM’s and pavements
incorporating such materials is also included in this literature study. Shortcomings
and areas for further improvement of the design practice have been identified. With
new environmental legislation that recently came into effect in South Africa, the
importance of BSM technology as an environmentally-friendlier and more
sustainable construction technique is set to increase in the coming years.
A laboratory testing programme was set-up to study the properties and behaviour of
BSM’s and to establish links with the compositional factors, i.e. the type of binder
used, the percentage of Reclaimed Asphalt Pavement (RAP) in the mix and the
addition of a small dosage of cement as active filler. The mineral aggregates used
were sourced in the USA and consisted of crushed limestone rock and RAP millings.
These were blended in two different proportions of crushed rock : RAP, i.e. 3:1 (with
3.6 % residual binder) and 1:3 (with 2.4 % residual binder). Tri-axial testing
(150 mm diamter) was carried out to determine shear parameters, resilient modulus
and permanent deformation behaviour, while four-point beam testing was carried out
to determine strain-at-break, flexural stiffness and fatigue behaviour.
It was found that the process of bitumen stabilisation improves the shear strength of
the material, particularly in case 1 % of cement is added as active filler. This increase
in shear strength is entirely the result of increased cohesion. There is a good
correlation between the shear strength and the resilient modulus of BSM’s. The
resilient modulus of BSM is stress-dependent and the Mr-θ model is adequate to
model the resilient modulus of the blends with a low percentage of RAP. For the
blends with a higher percentage of RAP this model cannot be applied and the
resilient modulus reduces in stiffness at higher deviator stress ratios.
A considerable part of the efforts of this study were dedicated to characterise and
model the permanent deformation behaviour. The General Permanent Deformation Law as originally developed by Francken applies also to BSM’s. An improved nonlinear
method to converge at a solution for the model parameters that describe the
tertiary flow part of this deformation law was developed as part of this study.
Parameters that can be derived from the first stage of the permanent deformation test,
i.e. initial strain and initial strain rate as defined in this study, were found that
correlate well with the model parameters that describe the first linear part of the
deformation law. Critical deviator stress ratios for the several mixes tested were
determined. When BSM’s are subjected to loading below these ratios, tertiary flow is
unlikely to occur.
A high variability was generally found in the four-point beam test results, especially
for the strain-at-break. Specimen preparation protocols and the quality of the beam
specimens are of utmost importance when performing four-point beam tests on
BSM’s. This limits the practical applications of the strain-at-break test. Trends
observed in the strain-at-break were also inconsistent and sometimes not in line with
the other type of tests.
BSM’s exhibit a visco-elastic behaviour, which was determined by flexural stiffness
testing, however, to a lesser extent than HMA. Phase angles and Black Diagrams
were developed for the BSM’s tested, which also made it possible to determine the
parameters of the Burgers Model, which is a mechanical model describing viscoelastic
behaviour. Fatigue relationships were also developed for the BSM’s tested.
The fatigue performance of these mixes is lower than for selected HMA mixes. The
foamed BSM generally showed better fatigue life than emulsion BSM, however, the
lower initial stiffness of the foamed BSM’s may contribute to a perceived longer
fatigue life. For the mixes tested, the flexural stiffness of foamed BSM’s is generally
also lower than that of emulsion BSM’s
It is recommended that the mix design of BSM’s be split into two phases. During the
first phase the usually large number of variables could be reduced to a selected few
by means of UCS and ITS indicator testing. Subsequently, more fundamental
parameters should be determined during the second phase, such as shear strength and
resilient modulus, as well as permanent deformation behaviour. The fact that
commercial laboratories in South Africa do not have tri-axial testing facilities is
currently a practical limiting factor. Initiatives currently underway to develop
“simple” shear tests are welcomed in this regard. It is proposed that classification of
BSM is based on shear strength.
There are indications that shear failure in BSM is more critical than failure as a result
of fatigue. The effect of curing resulting in an increase in BSM stiffness in the period
after construction, i.e. typically 6 to 18 months, is currently ignored in structural
design models. The rapid stiffness reduction of BSM’s during the first period after
construction in the current structural design models and also found during
Accelerated Pavement Testing is not being observed in Long-Term Pavement
Performance (LTPP). On the contrary, an increase in stiffness is observed in LTPP.
This would indicate that stiffness reduction as a result of fatigue does not occur or is
overshadowed by the effect of curing and that fatigue as a failure mechanism of
BSM’s is currently over-emphasized.
Description
Thesis (PhD (Civil Engineering))--Stellenbosch University, 2008.
Keywords
Bitumen stabilised materials, Shear strength, Mix design, Dissertations -- Civil engineering, Theses -- Civil engineering