Pavement Design in Road Construction
BY Kiran Biradar
Pavement design is the major component in the road construction. Nearly one-third or one-half of the total cost of construction , so careful consideration should be taken in design of pavement. Type of pavements:
Factors which affect the selection of these pavements
- Initial cost
- Availability of good materials
- Cost of maintenance
- Environmental conditions
- Availability of industrial wastes
- Traffic intensity
Criteria for determining the pavement (flexible)
Thickness is vertical compressive strain comes to the subgrade due to the standard axle laden of magnitude 8.17 kN (8170 kg) , if more than this causes permanent deformation in form of rutting. The maximum rutting can be accepted in village road as 50 mm before maintenance and the analytical evaluation can be done according to IRC:37. For rigid and semi-rigid pavement tensile stress is taken as the design criteria.
As per the IRC:37 design traffic should be 0.1 msa to 2 msa (million standard axles). Weight of commercial vehicle (laden) is considered as 3 tonnes or more. For design traffic we consider the existing traffic and rate of growth. Traffic study should be done as per the IRC:9.
The no. of years to be taken until the major reconstruction. Design life depend upon the environmental conditions, materials used ,maintenance etc. For rural roads design life of 10 years is considered. In low volume roads for the thin bituminous surfacing design life of 5 years is considered.
Computation of design traffic:
- a=no. of commercial vehicles/day for design
- p=no. of commercial vehicles/day at last count
- r=annual growth rate of commercial traffic
- n=no. of years between last count and year of completion of construction
- x=design life in years
- To provide support to the pavement as its foundation.
- Top 30 cm of the cutting or embankment at formation level in rural roads consider as subgrade.
- A minimum of 100% of standard proctor compaction should be attain in subgrade.
- For clayey soil 95% and moisture content of 2% in excess of optimum value.
- Soil below subgrade should be compacted to 97% of standard proctor compaction.
- Conduct on sample which remoulded at OMC and dry density.
- Test should be done per km depend on soil type.
- If CBR less than 2% for 100 mm thickness then minimum CBR of 10% is to be provided to the sub-base for CBR of 2%.
- If CBR more than 15% , no need to provide sub-base.
Selected materials placed on subgrade which is compacted to 98% of IS heavy compaction. Function of sub-base is to distribute the stresses over a wide area of the subgrade imposed by the traffic. Materials:
- CBR of 15%
- Passing through 425 micron IS sieve
- L.L<25 and P.I<6
Waste material such as Fly ash, Iron and steel slag Recycled concrete Municipal waste are also used. When subgrade is silty or clayey soil and annual rainfall of area is more than 1000 mm, a drainage layer of 100 mm then formation width should be provided.
Base course: to with stand high stress concentrations which develop due to traffic under the wearing surface. Different types of base course used are:
- Crusher-run macadam
- Dry lean concrete
- Soft aggregate base course
- Lime-fly ash concrete
Thickness of surface course depend upon the traffic volume and type of material used for it. For gravel roads extra thickness should be provided because of lost in thickness due to the traffic action. Bituminous wearing courses must be made up of good quality aggreagate with aggreagate impact value not exceeding 30 % in order to reduce degradation of aggregates by crushing.