58 | CONCRETO & Construções
prone to ravelling (aggregates com-
ing loose from the surface).
4. LOW-NOISE EXPOSED
AGGREGATE CONCRETE
The technique of aggregate ex-
posure is today the most frequently
used surface finishing method on
concrete motorways in Europe. It
offers a comfortable surface com-
bining a good skid resistance with a
low rolling noise.
In the late 1970s, in Belgium,
surface finishing by chemically ex-
posed aggregate concrete was in-
troduced. The aim was above all to
achieve skid resistance, but tyre-
pavement interaction noise was not
yet a criterion. The concrete compo-
sition consisted of large aggregates
(32 to 40 mm) which were exposed
on the surface. These were highly
noisy pavements and are no longer
applied today.
In Austria, the fine exposed ag-
gregate concrete was optimised as
the surface layer of a double-layer
jointed concrete pavement. The bot-
tom layer (15 to 20 cm thick) may
contain larger stones up to 31.5 mm,
while for the top layer, (5cm thick)
only durable polish-resistant stones
with a maximum aggregate size of 6
to 11 mm are used. Since the top
layer mixture consists exclusively of
small aggregates, these will emerge
densely distributed on the surface
after compaction of the concrete
and washing out of the surface.
The first applications on Austrian
highways date back to 1990. Since
then, a significant part of the high-
way network has been constructed
in this way.
In Germany, for many years, the
concrete road surfaces were finished
with a dragged burlap cloth. This
produces a very shallow texture that
is effective in terms of tyre-pavement
noise but can lead to problems con-
cerning skid resistance. That is why,
for the past few years, the technique
of double-layer exposed-aggregate
concrete has been applied.
In Belgium, a first step in improv-
ing the exposed aggregate concrete
was the reduction of the maximum
aggregate size from 31.5 to 20 mm.
Moreover, the proportion of fine
aggregate, measuring from 4 to
6.3mm (or 8mm), was increased to
a minimum of 20% (or 25%) of the
mixture of sand and gravel. In this
way, the larger stones sink and the
smaller ones rise during the vibration
of the concrete. After exposing the
aggregates by brushing, the smaller
aggregate will lie on the surface and
form the proper macro texture for a
quiet concrete pavement. The same
technique can also be applied for a
maximum aggregate size of 14 mm.
By further optimising the com-
position (high content of fine ag-
gregate) and a better application
technique (modern slipform pavers),
it has been possible to further re-
duce the tyre-pavement noise of a
u
Figure 7
Grading curve of a concrete mix with a Dmax of 20 mm and a higher dose
of small aggregates (4 to 8mm) for a low-noise single layered
exposed-aggregate concrete pavement
u
Figure 8
E17 De Pinte – Kruishoutem: single-layer exposed-aggregate
concrete 0/20
Noise levels measured with CPX at 80 km/h: 98.5 dB(A) for a truck
tyre and 99.1 dB(A) for a passenger car tyre
