Highway Engineering

Highway Pavement Materials and Design 219

Hopper receiving

mix

Control thickness of

layer

Paver

Augers spreading

mix evenly over

entire width

Screedboard and

vibrotamper

Material for final
compaction

Prepared surface

Material

delivered by

truck / dumper

Figure 7.14Operational features of a paving machine.

A paver (Fig. 7.14) is used for the actual placing of the bituminous material.

It ensures a uniform rate of spread of correctly mixed material. The truck/trailer

tips the mixture into a hopper located at the front of the paver. The mix is then

fed towards the far end of the machine where it is spread and agitated in order

to provide an even spread of the material over the entire width being paved. The

oscillating/vibrating screed and vibrotamper delivers the mix at the required ele-

vation and cross-section and uses a tamping mechanism to initiate the com-

paction process.

Compaction of the bituminous mix

When the initial placing of the mix is complete it must be rolled while still hot.

Minimum temperatures vary from 75°C to 90°C depending on the stiffness of

the binder. This process is completed using either pneumatic tyre or steel wheel

rollers. The tyre pressures for pneumatic rollers vary from 276 kPa to 620 kPa,

while the steel wheel rollers vary from 8 to 18 t. If the latter are vibratory rather

than static, 50 vibrations per second will be imparted. The rolling is carried out

in a longitudinal direction, generally commencing at the edge of the new surface

and progressing towards the centre. (If the road is superelevated, rolling com-

mences on the low side and progresses towards the highest point.)

It is important that, on completion of the compacting process, the surface of

the pavement is sufficiently regular. Regularity in the transverse direction is mea-

sured using a simple 3-metre long straight edge. Deviations measured under the

straight edge should in no circumstances exceed 3 mm.