Flexible pavements will transmit wheel load
stresses to the lower layers by grain-to-grain transfer through the points of
contact in the granular structure (see Figure 19:1). The wheel load acting on
the pavement will be distributed to a wider area, and the stress decreases with
the depth. Taking advantage of this stress distribution characteristic,
flexible pavements normally has many layers. Hence, the design of flexible
pavement uses the concept of layered system. Based on this, flexible pavement
may be constructed in a number of layers and the top layer has to be of best
quality to sustain maximum compressive stress, in addition to wear and tear.
The lower layers will experience lesser magnitude of stress and low quality
material can be used. Flexible pavements are constructed using bituminous
materials. These can be either in the form of surface treatments (such as
bituminous surface treatments generally found on low volume roads) or, asphalt
concrete surface courses (generally used on high volume roads such as national
highways). Flexible pavement layers reflect the deformation of the lower layers
on to the surface layer (e.g., if there is any undulation in sub-grade then it
will be transferred to the surface layer). In the case of flexible pavement, the
design is based on overall performance of flexible pavement, and the stresses
produced should be kept well below the allowable stresses of each pavement
of Flexible Pavements
The following types of
construction have been
used in flexible
layered flexible pavement,
depth asphalt pavement, and
asphalt mat (CRAM).
Conventional flexible pavements are layered
systems with high quality expensive materials are placed in the top where
stresses are high, and low quality cheap materials are placed in lower layers.
Full - depth asphalt pavements are constructed by
placing bituminous layers directly on the soil sub-grade. This is more suitable
when there is high traffic and local materials are not available.
Contained rock asphalt mats are constructed by
placing dense/open graded aggregate layers in between two asphalt layers.
Modified dense graded asphalt concrete is placed above the sub-grade will
significantly reduce the vertical compressive strain on soil sub-grade and
protect from surface water.