301
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Triaxial testing of asphalt
Essais triaxiaux de l'asphalte
Airey D.
University of Sydney, Australia
Prathapa R.
Roads and Maritime Services, New South Wales, Australia
ABSTRACT: The response of asphalt in conventional triaxial tests has received very little attention in the soil mechanics or pavement
literature. As a result, the relative importance of the aggregate and the asphalt binder are not well understood, and in particular how
they contribute towards the resistance to permanent deformation. This paper describes the results of a series of conventional drained
and undrained triaxial tests on two types of asphalt, stone mastic asphalt (SMA) and dense asphaltic concrete (DAC). Tests without
the asphalt binder have been conducted for each asphalt type. Results show that the DAC, which is well-graded and has a bitumen
content of about 5%, behaves similarly to the aggregate without the bitumen at conventional soil mechanics rates of loading.
However, significant strain rate effects are observed as the rate of loading is increased. For the SMA mixture, which has a higher
bitumen content of about 6%, the asphalt appears to affect the frictional resistance of the mixture, and it is also far more significantly
affected by strain rate. It is also noted that the asphalt binder has the effect of reducing the effects of strain localisation and allows
uniform and barrelling type deformations despite the very dense aggregate.
RÉSUMÉ : La réponse de l’asphalte dans les essais à triaxiaux conventionnels a suscité peu d’intérêt dans la littérature de la
mécanique des sols et des chaussées. Par conséquence l’importance relative de l’agrégat et du liant asphaltique n’est pas bien
comprise, notamment comment ils participent à la résistance à la déformation permanente. Cet article décrit les résultats d’une série
d’essais triaxiaux drainés et non drainés sur deux types d’asphalte, l’asphalte coulé gravillonné (SMA) et le béton asphaltique dense
(DAC). Des essais sans liant asphaltique ont été effectués pour chaque type d’asphalte. Les résultats montrent que le DAC, qui à une
granulométrie continue et une teneur en bitume d’environ 5 %, se comporte de manière similaire à un agrégat sans bitume pour les
taux de chargement conventionnels en mécanique des sols. Cependant, on observe des effets significatifs de taux de contrainte à
mesure que le taux de chargement augmente. Pour le mélange de SMA qui a une teneur plus élevée en bitume d’environ 6 %,
l’asphalte semble affecter la résistance à la friction du mélange, qui est beaucoup affectée par le taux de contrainte. On note également
que le liant asphaltique a l’effet de réduire les effets de la localisation de la contrainte et permet les déformations de type uniforme et
en tonneau, malgré la très grande densité de l’agrégat.
KEYWORDS: aggregate, asphalt, triaxial tests.
1 INTRODUCTION
Analaysis of asphalt materials has focused on their response
under traffic loading, and in particular on the effects of
temperature and stress level on the resilient stiffness and
permanent deformation (Antes et al, 2003, Li et al, 2010).
However, there has been much less interest in the behaviour of
asphalt under slow rates of loading, and of the role of the
aggregate skeleton (Muraya et al, 2009). The latter can be
important when a pavement is subject to subsidence, for
example caused by underground mining. In this situation the
role of the aggregate controls the asphalt behaviour and the
stiffness can be as much as 2 orders of magnitude lower than the
resilient modulus determined from conventional asphalt testing.
To capture this behaviour elastic-visco-plastic models of asphalt
have been developed, but there is little data available to
determine the parameters for these more sophisticated models.
The triaxial tests described in this paper are part of a study
designed to provide the basic soil mechanics framework for the
aggregate and to enable the effects of different aggregate
gradations and bitumen contents to be more rigorously
interpreted. The paper will describe a series of triaxial tests
performed on two types of asphalt, stone mastic asphalt (SMA)
and dense asphaltic concrete (DAC). Tests of the two materials
without the bitumen have also been performed. The gradations
of the materials, their method of preparation and the results of
the triaxial tests are presented.
1.1
Specimen preparation
Asphalt test specimens for these tests were provided by the
Roads and Maritime Services (RMS) and were manufactured in
the Fulton Hogan (FH) asphalt laboratory in Sydney. The
majority of the aggregates in the asphalt mix were comprised of
crushed basalt from the Bass Point quarry.
Dense graded asphalt mixes with 14mm nominal aggregate
size conforming to RTA Roadworks specifications R116 (DAC)
and R121 (SMA) were used in testing. The particle size
distributions are outlined in Table 1, and the materials and their
proportions used to achieve these gradations are shown in
Table 2.
Samples of at least 200 kg of each SMA and DAC mix were
taken from a plant production run and were delivered to the FH
asphalt laboratory. Care was taken during handling to avoid any
segregation of the mixes. In the laboratory the asphalt mixes
were reheated and compacted in a shear box compactor to
produce prismatic compacted specimens with dimensions of
450 mm x 150 mm x 185 mm. Approximately 10 cylindrical
specimens with diameter of 70 mm and length of 140 mm were
cored from each of the compacted rectangular prisms. The
