CONCRETO & Construções | 57
passenger cars and trucks.
With the Close Proximity Method
(CPX, ISO/CD 11819-2) it is chiefly
the tyre-pavement noise that is mea-
sured by attaching a microphone
close to the test tyre. These tyres
are usually mounted on an acous-
tically insulated trailer. The type of
test tyre that is used will obviously
play an important role in determining
the results.
In the United States, the On
Board Sound Intensity (OBSI) meth-
od is often used (Ref. 5), which is
somewhat comparable to the CPX.
Instead of sound pressure, however,
sound intensity is measured by us-
ing two microphones which are bet-
ter able to isolate the tyre-pavement
noise for separate measurement.
Since CPX and OBSI measure-
ments take place very close to the
tyre, they will result in high values of
noise levels. Obviously, those are
not the noise levels to which humans
are being exposed in real life. This
should be kept in mind when read-
ing the case studies, further on in
this paper.
3. BASIC RULES FOR
A QUIET PAVEMENT
The following factors have an in-
fluence on the tyre-pavement noise:
u
good smoothness of the road
with the absence of bumps or ir-
regularities in the surface (which
is also called “megatexture”);
u
a homogenous but non-sys-
tematic distribution of small ag-
gregates, up to 10mm on the
surface (which is considered as
“macrotexture”). The air is able
to escape between the gaps in
the aggregate. Important note:
a perfectly flat surface is not
low-noise;
u
it is better to have a negative tex-
ture than a positive one (see Fig-
ure 4);
u
porosity: a maximum content
of empty space (up to 20% or
more), which enables sound to
be absorbed, insofar as the pores
remain open at the surface;
u
limited stiffness of the surface
layer.
In optimising a quiet pavement
surface, if possible, various fac-
tors should be taken into account
at once. Moreover, no concessions
should be made in the durability of
the pavement. In addition, a solu-
tion with long-term noise perfor-
mance should be preferred to sur-
faces deteriorating after few years.
Similar to wearing surfaces of po-
rous asphalt, it is possible to build
surfaces of porous (or very open)
concrete. By leaving out the sand
component in the composition, ac-
cessible gaps are created between
the coarse aggregates. In order to
achieve better adhesion between
the stones, polymers may be added.
Very open concrete is comparable
to very open asphalt: there is great
potential for noise reduction and
water-spray is eliminated. However,
there are also downsides: there is a
risk of clogging and the surface is
u
Figure 4
Positive vs. negative texture
u
Figure 5
Coarse concrete 0/40 or 0/32, left, based on porphyry aggregates,
and right, based on crushed gravel
u
Figure 6
Two slipform pavers laying a
double-layered jointed plain
concrete pavement on the A1,
near Vienna
