Contd. from Part 2
( https://civilconstructionresourcez.wordpress.com/2012/01/13/roads-with-flexible-pavement-part-2-2/ ) …..
For sub-grade, tests like Modified Proctor Density, Optimum Moisture Content (OMC) etc. are important pre-construction tests. The key quality parameters to be remembered for sub-grade are: CBR value should not be less than 40% at in situ density and moisture content. Minimum in situ dry density not to be less than 95% of Modified Proctor Density at a OMC /- 2%.
When the sub-grade is to be prepared by filling rather than by cutting, selected earth-fill materials are usually used. These fill materials are laid in compacted layer as per specifications. Some of the important tests conducted on selected fill materials are: Sieve analysis, Atterberg limits, CBR, Modified Proctor test, Sulphate and Chloride content, Organic matters etc. The liquid limit and the plasticity index (PI) for selected fill should not exceed 35% and 10 respectively.
For grannular base materials the ratio of material passing through 75 micron sieve to that of 425 micron sieve should not exceed 0.66. Also, material passing 425 micron sieve needs to have minimum liquid limit of 25% and PI needs to be less than 6. Whenever the above conditions are not fulfilled suitable blending becomes necessary.
Screening is necessary to determine whether blended materials fall in the designated gradation. Same is true for aggregates base course as well. A centrally located mixing plant is quite useful for blending purpose.
The conforming materials are spread over sub-grade for compaction only after the sub-grade section is fully ready to receive the same and written approval is obtained for proceding with it. It is to be noted that tack and prime coats, which are usually made up of cut-back bitumen or bituminous emulsion, are applied on top surface of the completed sub-grade before commencing pavement work.
Contd. in Part 4 ….
( https://civilconstructionresourcez.wordpress.com/2012/01/14/roads-with-flexible-pavement-part-4/ )