Wearing course of the Road is an important layer among
difficult crusts provided in Road construction. Hot asphaltic concrete wearing
course has proved to be a must standard surfacing for a heavy duty roads and airfields.
ASPHALTIC CONCRETE
Hot Mix Asphaltic Concrete is a carefully proportioned
mixture of coarse aggregate, fine aggregate and mineral filler coated with
bitumen. Both the aggregate and bitumen are heated prior to mixing. Heating and
mixing operations are performed in a plate named as a Hot Mix Plant.
Thickness of the
Asphalt depends upon :
- The traffic intensity and
- The quality of the base course.
Materials Required :
- Coarse aggregate : It may consist of Crushed Stone Slag or graver retaining on the ASTM Sieve No. 8 (I.S.S No. 240)
- Fine aggregate : It may consist of Natural Suitable Sand, Crushed Stone Screenings or mixture of two passing through ASTM Sieve No. 8 and retained on ASTM Sieve No. 200 (I.S.S. No. 8)
- Filler : It may consist of finely ground particles of Cement Limestone, hydrated lime, or other fine mineral dust having hot less than 65% passing ATSM Sieve No. 220 (I.S.S. No. 8)
Straight run bitumen with particle ranging from 30 to 100 can
be used. All the mineral ingredients listed above are heated in the Hot Mix
plant at a temperature range of 155 C – 163 C and the bitumen a temperature 150
C – 177 C and mixed together and Hot Mix is immediately conveyed to the road
site where it is to be laid. At road site, this hot mix is spread to the required
thickness with a self propelled mechanical paver. The temperature of the mix at
the time of laying must be in the range of 121 C – 163 C. Immediately after
spreading of mix by paver it is thoroughly compacted with a 8 to 10 tonne
roller. Traffic is allowed after completion of rolling and when the mix has
cooled down to the surrounding temperature.
Merits of Asphaltic
Concrete :
- A resilent, waterproof, dust free, anti skiding and smooth riding surface.
- A heavy duty surface capable of withstanding the highest stresses imposed on it.
- In offers greatest resistance to sheer stresses.
- Prevents the overstressing of underlaying construction layers.
- Durability of withstand effect of traffic and whether.
- Flexibility to prevent cracking due to deflection or fatigue.
- Stability to withstand deformation under traffic.
