Both methods work on the basis that the full construction process is com-
pleted following one pass over the prepared foundation of the pavement. After
the usual curing period the slab can then be subjected to normal traffic
loadings.
Equally good results are possible with both types of paving machine. The slip-
form paver has certain advantages/disadvantages associated with it:
A higher output is achievable as less machinery is involved
It will tend to be less expensive as labour costs will be lower due to the
increased level of automation.
But:
Edge slump may occur just after the concrete has left the paver
Greater stockpiles of raw materials such as cement, steel mesh and aggre-
gate are needed in advance of the operation in order to ensure continued
output from the paving train
The contractor operating it may be more vulnerable to weather conditions
A minor quality control failure can cause the entire system to come to a
sudden stop.
7.7.5 Curing and skid resistance
Concrete curing is an essential step in achieving a good quality finished product.
It requires that both the temperature and moisture content of the mix be main-
tained so that it can continue to gain strength with time. If moisture is lost due
to exposure to sunlight and wind, shrinkage cracks will develop. Such problems
due to moisture loss can be avoided if the surface of the concrete is kept moist
for at least seven days. This is usually achieved by mechanically spraying the fin-
ished surface, with exposure to rain avoided with the use of a travelling tentage
as indicated in Fig. 7.19.
Immediately prior to the curing process, the surface should be textured in
order to give it adequate wet-road skidding resistance. It is extremely important
to get the texture to the correct level of quality at the time of construction
as potential difficulties may arise with subsequent surface maintenance dur-
ing its design life. Good skid resistance requires sufficient microtexture and
macrotexture.
Macrotexture permits most of the rainwater caught between the tyres and the
surface of the highway to drain rapidly and depends on grooves being devel-
oped on the surface of the mix in order to ‘texture’ it. Microtexture, on the other
hand, depends on the use of fine aggregate within the mix. It must have
abrasion-resistance properties such that the particles of sand stand proud of the
matrix of the hardened cement paste while subject to traffic loading, therefore
allowing it to penetrate the remaining film of water and maintain tyre contact
Highway Pavement Materials and Design 227
