Note:- New BS EN 12591
Wearing Course = Surface Course; Base Course = Binder Course;
Roadbase = Base
Road Purpose
The composite road is used where
the nature of the sub-grade has low load bearing capacity. Cement
Bound Material (CBM) is used in the roadbase to provide extra
strength to the road.
 Road
Structure and Design
The road is made up with CBM3 or
lean mix concrete as a road base in place of granular sub-base
material to provide strength and rigidity during traffic loading.
The load placed on the road is transferred across a greater plane
than that of a standard flexible road. This type of road should
not be confused with a rigid road having a bituminous overlay.
Sub-Base
The sub-base is designed to assist in the load spread over the
sub-grade. It also allows drainage into the sub-grade and is often
used during construction as a temporary running surface and is the
main platform on which to build the road. The sub-base is also
used to make up the thickness of the pavement to at least 450mm to
give frost protection to the sub-grade in frost susceptible soils.
Sub-base is always made up with unbound granular materials.
Reinstatement of Composite Roads
The 'Specification for the Reinstatement of Openings in Highways'
is used when designing the reinstatement and deciding the
materials to be used. The specification also gives the permitted
tolerances and intervention limits on all reinstatements.
Main Considerations for Reinstatement
When reinstating the road there must be great efforts to see that
the workmanship and quality are of the highest standard. This will
ensure that the reinstatement will stand up to the expected
loading and prevent further visits to the site thus more expense.
Time should be allowed for to ensure quality is maintained.
The CBM must be reinstated as existing to the same levels using
CBM3 and the basecourse and wearing course must also match the
existing with a minimum of 40mm of wearing course.
Some roads which feature a CBM roadbase may also include
reinforcement. Where this is found to be the case the authority
must be consulted prior to final reinstatement.
Foamed concrete is used with the permission of the authority. If
used, there must be operational safety measures taken to avoid
people stepping onto the mix until it has set. Loading would not
be allowed until the foamed mix has reached a minimum crushing
strength defined in the specifications as 4N/mm˛ at 28 days up to
a maximum of 14N/mm˛ at 28 days.
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