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Road
Purpose
The rigid pavement is used where high traffic loading is
expected. The pavement is durable and should last at least
40 years before needing replacement. Some rigid pavements
incorporate a bituminous overlay that can be replaced when
worn out.
 Road
Structure
The road is made up of reinforced concrete able to withstand
wear and loading, the design mix of the concrete is to grade
C40. The road slab is equivalent to wearing, base and
sub-base of a flexible pavement. Unlike a flexible pavement
the this type of pavement does not deflect during traffic
loading. A slip membrane is used to prevent moisture
penetrating into the sub-base during laying and also allows
the slab to move freely when expanding or contracting under
thermal stress. The quality of the concrete is of utmost
importance to ensure that the slab achieves its strength and
durability. An 'air entrainment' agent is used in the mix to
provide workability and increased frost protection. The
trapped air in the concrete is to be maintained in the top
50mm of the slab in the form of very minute bubbles and
should not be confused with an air void. Care during
compaction of the concrete needs to be taken to prevent over
compacting and expelling the air required. The slab
thickness is determined by the traffic loading expected and
can be 150mm for residential areas and 300mm for trunk roads
and motorways, the maximum slab thickness is 320mm for any
situation.
Joints in Concrete
Because of the thermal stresses placed on the road the
concrete is laid in slabs separated by joints designed to
allow expansion and contraction. The slabs are held together
using dowel bars one end being cast into the slab during
laying and the other end de-bonded to allow for movement of
the slab. There are two types of joints used in normal
construction, they are 'traversing' and 'longitudinal'
joints'.
Traversing Joints These
are formed at right angles to the longitudinal line of the
carriageway except at junctions and roundabouts. The joints
are formed during the road construction by means of a
separation filler or a crack inducer laid in the sub-base to
induce a crack forming when the concrete is contracting. A
warping joint is used where reinforcement mesh has not been
used. This type of joint is designed to take up the bending
stresses of un-reinforced concrete. |
Traverse Joint (Expansion)
Traverse Joint
(Contraction)
Traverse Joint
(Contraction)
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Strength of Concrete
The strength of concrete is measured in N/mm˛, this is
measured by a cube test in laboratory conditions. Cube
samples of 150mm square are taken from the mix and allowed
to cure at a controlled temperature of 20 degrees
centigrade. A crushing strength of 31 N/mm˛ at 7 days
should be achieved before the road is open to traffic. The
strength of concrete increases over time.
Workability of Concrete
To be able to lay and compact concrete it must be workable.
This is measured using a slump test on-site or a compacting
factor test in a laboratory. The concrete workability should
be constant throughout the mix. A plasticising agent can be
used in extreme temperatures to maintain workability. Where
a mix is too stiff to handle or compact an equal amount of
water and cement should be added. By adding just water to
the mix to make it wetter will reduce the strength of the
concrete.
Reinforcement
The rigidity of the slab is maintained by the use of
reinforcement mesh, this mesh also prevents the slab from
cracking under load. The mesh used must be free from oil,
dirt and loose rust. Sheets of reinforcement laid must
overlap and be tied together to prevent movement during the
laying and compacting of concrete. There must be a minimum
cover of concrete of 60mm (+/-10mm) for slabs less than 270
thick and 70mm (+/-10mm) for slabs over 270mm thick.
Surface Finish
The surface of the slab must be brushed at right angles to
the direction of traffic by using manual or mechanical
means. This finish provides a textured surface to allow for
skid resistance. The depth of the textured finish should be
0.75mm, this figure can be reduced to 0.65mm prior to
opening to traffic.
Reinstatement of Rigid
Roads
The reinstatement of the road must be in accordance with the
Specifications for the Reinstatement of Openings in
Highways.
Main Considerations for Reinstatement
When reinstating the road care must be taken in the
preparation to receive concrete. Any joints must be replaced
as existing along with reinforcement of the same weight.
Reinforcement must be tied with wire to the existing
reinforcement by exposing at least 150mm of it during
preparation. Failure to tie the mesh together will allow the
mesh to rise to the surface during compaction. The edges of
the slab must be cut with a pavement saw to a depth of 30mm,
+ or - 10mm to give a straight, clean and tidy finish. The
remaining depth is trimmed to give a slight slope of about
27° + or - 18° to provide some support to the reinstated
concrete.
The quality of the concrete needs to be checked to ensure
that workability and ultimate strength is maintained.
Compaction should be by mechanical means although for small
reinstatements hand tamping may be carried out. Prior to
laying the concrete the exposed edge should be wetted to
allow the concrete to key with the existing concrete slab.
The concrete must be 'cured' to allow for even drying and
strength. The curing method used will depend on weather
conditions at the time of laying. In the summer months it is
preferred to wet the concrete after the initial set to slow
the drying process or the concrete will dry to quickly and
form cracks. In winter or during cold spells it is covered
to create a pocket of trapped air which is kept warm by the
chemical reaction of the concrete during setting and prevent
frost damage.
Where the trench is less than 300mm from an existing edge,
ironwork, joint, etc., the trench should be trimmed back to
include the feature in the reinstatement.
Dowel bars used in replacing any joints must in accordance
with the specifications.
There should be no significant trip along the interface
between the existing road and the reinstatement.
A textured finish, as existing is to be applied to the
reinstatement to maintain skid resistance.
Any bituminous overlay is to be replaced as existing.
The road should not be opened to traffic loading until the
crushing strength of 25 N/mm˛ has been achieved.
This list
is not exhaustive.
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