Geoscience Reference
In-Depth Information
contrast, asphalts have a low content of single-sized
(open graded) aggregate, with a high bitumen content
and a high filler/fines content. These provide a dense,
strong, and stiff layer with the load being transmitted
through the binder. Due to the low aggregate content,
bitumen-coated stone chippings are often rolled into the
upper (trafficked) surface of asphalt wearing courses to
increase the skid resistance.
Other bituminous mixtures that the petrographer
should be aware of include mastic asphalt, sand carpet,
porous asphalt, stone mastic asphalt (SMA), and grouted
macadam. Mastic asphalt is dense, voidless asphalt that
is rich in hard bitumen and filler. The filler used is coarser
than that used in road asphalt. The mastic may be
extended with coarse aggregate depending on end-use. It
is impermeable and used to surface large flat roofs,
multistorey car park decks, and bridge decks. Sand carpet
is similar to mastic asphalt in that it is bitumen rich, but
contains less filler, the remainder of the mix being made
up with fine aggregate. Unlike mastic asphalt, it contains
no coarse aggregate. It is used in several specialized
applications such as on bridge decks where it is used to
protect underlying waterproofing polymer membranes
(sand carpet is sometimes coloured red when used for this
application).
Porous asphalt comprises uniformly graded (open-
graded) aggregate with low bitumen content and an open
texture (20% large interconnected air voids). It is free
draining and is used on airfield pavements to prevent
aquaplaning and it is increasingly used on roads as it
reduces the rolling noise of traffic.
Stone mastic asphalt (SMA) comprises gap-graded
aggregate with voids almost completely filled by a rich
mastic of bitumen/fine aggregate/filler. Cellulose fibres
or polymer modified bitumen may also be incorporated.
The material was designed in Germany to resist studded
snow tyres and SMA is now being used in the UK and
North America as it has good resistance to rutting and
high durability.
Grouted macadam is laid using a two-stage process
and comprises open-graded asphalt topping with a void
content of 20-25%, which is later flooded with
cementitious mortar. The resulting composite has a high
resistance to point loading and chemical/fuel spillage. It
is being increasingly used at container parks, docks,
airport aprons, and garage forecourts.
The common causes of premature deterioration in
bituminous mixture surfacings are related to loss of
structural strength and wear, disintegration, or loss of
surface characteristics. Currently under-utilized, micro-
scopy can provide valuable information regarding the
causes and magnitude of road construction material
deterioration.
The principal applications of petrographic
examination to investigation of bituminous mixtures are:
• Determining the number material types and layers
present (and their thickness).
• Identifying aggregates and filler used.
• Determination of the air void structure.
• Identifying the causes and extent of defects and
deterioration.
In the absence of a specific standard procedure for
petrographic examination of bituminous mixtures, the
guidance of EN 12407 (British Standards Institution,
2007) or ASTM C856 (ASTM International, 2004) may be
adapted. An initial visual and low-power microscopical
examination is conducted to determine the number of
layers, their thickness, and to look for any macroscopic
evidence of deterioration. Coarse aggregate and
chippings exposed at the upper surface of roads can be
examined for evidence of damage. Observation of slices
cut perpendicular to the road surface is particularly
useful for studying the properties of coarse aggregate.
Details of the aggregate size, shape, and distribution can
be accurately determined using image analysis
techniques (Schlangen, 1999). Slices that have been
impregnated with fluorescent resin are useful for
observing the air void structure (Eriksen, 1999). High-
power microscopical examination of thin sections is used
to identify the surface texture, geological type, and
potential durability of aggregates and fillers used and to
assess the effectiveness of the bitumen coating.
Figures
364
and
365
show macadam from a roadbase,
comprising well graded crushed dacite aggregate with a
low content of bitumen binder that barely coats the
aggregate and leaves air voids between aggregate
particles. Figure
366
shows hot-rolled asphalt wearing
course comprising single-sized crushed rock aggregate
with a relatively high bitumen content, leaving no voids.
Calcium carbonate dust filler is clearly seen within the
