DIGEST 2006 - Sabita

oad construction. As mentioned
previously, the weathering of the
aggregate is of utmost importance
to the stability of the aggregate,
and is considered to be one of the
most important aspects of raw
materials handling.
Steel slag asphalt mixes
The objective of investigating steel
slag asphalt mixtures was to
produce a rut resistant asphalt for
application where traffic loading is
high, vehicle speeds low, and
environmental conditions varying.
Trial steel slag asphalt mixes were
prepared for use during the N3
Toll Concession (N3TC) routine
maintenance contract between
Heidelberg and Cedara.
A mix design was completed using
the steel slag as an aggregate,
along with a filler to satisfy the
-0,075mm grading limits. The
grading specification used for the
mix design was according to TRH8
(Coarse). A 4% SBS modified
binder was used to enhance the
natural properties of the bitumen
by making the binder more elastic
at higher temperatures, which in
turn enables the binder to
withstand repeated traffic loading
and reduces rutting potential.
Results
For the mix adopted, the
specification required a stability
within the range 8,0 kN – 18,0 kN.
Test results indicated a stability in
the region of 15,0 kN. This can in
part be attributed to both the use
of steel slag as an aggregate, and
to the polymer modified binder.
The higher stability indicated that
the mix was ideally suited to the
requirements of a deformation
resistant asphalt surfacing. The
maximum flow recorded at the
various binder contents tested was
4mm compared to the recommended
range of 2 – 6mm.
The Indirect Tensile Strength (ITS)
of the mix is a measure of the
stiffness of an asphalt mix i.e. how
well it distributes loading to
underlying layers. In the Interim
guidelines for the design of hot
mix asphalt in South Africa 2001,
the minimum recommended
stiffness for a surfacing mix is
1000 kPa. The stiffness of the
steel slag mix design reported,
1280 kPa, is significantly higher
than this minimum requirement.
The above document also
recommends a minimum Voids in
Mineral Aggregate (VMA) of
15,0%. Low VMA asphalt mixes
tend to be sensitive to changes in
binder content and create tender
mixes that may be more inclined
to rutting and bleeding. The
minimum VMA recorded for the
binder contents tested – 16,3% –
exceeds the minimum requirement
and is suited to a rut resistant and
durable asphalt surfacing.
To optimise void content to
counter air and water permeability
on the one hand, and fattiness and
bleeding on the other, a target
Voids in Mix (VIM) of 4% was
adopted, as suggested by
Superpave 2001. The reported
results for the steel slag mix
design, was very close to the
optimum voids content of 4%, and
the mix met the requirement for
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