Asphalt Review - Volume 29 Number 2 (June / July 2010)

ASPHALT REVIEW
Foamed bitumen
The quantities of residual bitumen emulsion or foamed
bitumen added do not typically exceed 3% by mass of dry
aggregate. In many situations, active filler in the form of
cement or hydrated lime is also added to the mix. The cement
content should not exceed 1%, and should also never exceed
the percentage of the bitumen stabiliser. If more cement than
bitumen is added, then the material should be considered a
cement treated material and the relevant guidelines for such
materials followed.
The addition of bitumen emulsion or foamed bitumen to
produce a BSM results in an increase in material strength
and a reduction in moisture susceptibility as a result of the
manner in which the bitumen is dispersed amongst the finer
aggregate particles. The “non-continuous” binding nature
of the individual aggregate particles makes BSMs different
from all other pavement materials. The dispersed bitumen
changes the shear properties of the material by significantly
increasing the cohesion value whilst effecting little change to
the internal angle of friction. A compacted layer of BSM will
have a void content similar to that of a granular layer, not
an asphalt. BSMs are therefore granular in nature and are
treated as such during construction. The failure mechanism
for BSMs in a pavement structure subjected to repeated axle
loads is permanent deformation, not fatigue cracking. BSMs
are therefore more closely related to granular materials than
to asphalt concrete.
The behaviour of BSMs, relative to other pavement materials
is illustrated in Figure 1.
Condensed version - full version available from Conference
papers 2009 - aapa@asn,com.au
TG2, SECOND EDITION
The research feeding into this publication took five years and
was officially launched in May 2009 at a series of seminars
held in the main cities around South Africa. The attendance
of almost 300 delegates at these seminars was an indication
of the level of interest shown by industry and subsequent
feedback suggested that this technology will be adopted more
widely in future, mainly as a result of a better understanding
of material behaviour (especially the failure mechanism),
coupled with the new simplified design procedures.
Also appreciated was the decision taken to house the
technology on Asphalt Academy’s website, allowing the 136-
page manual to be downloaded in pdf format at no cost.
All new and non-standard laboratory test methods for mix
designs are also housed on the website rather than included
in the manual.
Another feature of the website is the inclusion of the
programs for material classification and pavement design.
Although these programs cannot be downloaded, users can
access the website at any time, log on and use the software,
free of charge. Input data and results can, however, be stored
on the individual’s computer.
It is envisaged that this website will receive ongoing
development as more users provide feedback and additional
information is obtained from research initiatives.
The manual introduces the abbreviation BSM for a “bitumen
stabilised material” and incorporates both bitumen emulsion
treated material as “BSM-emulsion” and foamed bitumen
treated material as “BSM-foam”. Regardless of which form
of treatment is adopted, the resulting BSM is regarded as a
generic product from a design and performance perspective.
However, the most important feature of the new design
approach is the direct relationship between the engineering
properties of the materials in the various layer components
and pavement performance.
The new mix design procedures
Figure 2 illustrates the three levels of tests that have been
adopted for classifying the product into one of three BSM
classes. The level of testing required is dictated by the design
traffic (structural capacity requirement); the higher the level
of testing, the greater the level of confidence achieved.
Figure 2. Mix design flowchart Condensed version - full
version available from Conference papers 2009 - aapa@
asn,com.au
TG2 Second Edition classifies BSMs into three classes,
depending on the quality of the parent material, the effectiveness
of stabilisation and the design traffic. The three classes are:
BSM1: This material has a high shear strength
BSM2: This material has moderately high shear strength
BSM3: This material typically consists of soil-gravel and/or
sand stabilised with higher bitumen contents.
Table 1 summarises the boundary values for the various test
results that are the primary indicators used to classify BSMs into
one of three classes. Also shown are the CF values for each test.
The new approach to pavement design
The structural design of pavements in TG2 Second Edition
utilises a knowledge-based approach that uses a Pavement
Number (PN) that is based on AASHTO’s Structural Number
concept. However, the shortcomings of the Structural Number
method have been addressed in developing this PN method.
This method was based on a plethora of data collected from
numerous in-service pavements where the type and detail of the
data suggested that a relatively simplistic method be adopted
(it also precluded the use of a Mechanistic-Empirical design
approach).
Rules relating to pavement systems/ Rules relating to specific
pavement layers:
Condensed version - full version available from Conference
papers 2009 - aapa@asn,com.au
BSMs are assumed to act in a similar way to coarse granular
materials but with a higher cohesive strength. Although not
confirmed, the cohesive strength is presumed to reduce as a
consequence of repeated loading and thus some softening may
30 ROADS JUNE 2010/JULY 2010