Contd. from Part 3 …..
( https://civilconstructionresourcez.wordpress.com/2012/01/14/roads-with-flexible-pavement-part-3/ )
In this post the various quality test requirements on bituminous courses or the asphalt portion of pavement have been highlighted.
Like cement concrete mixes, the bituminous mixes or the the bituminous concrete used as pavement seal too need to be designed for economy as well as for best quality results. The mix design, which is also called the Job Mix Formula, for the bituminous base course is different from that of the bituminous wearing course even though the design procedure is the same. This is because certain characteristics such as the size of aggregates and some quality requirements for both the courses may be different. Here too trial mixes are conducted to arrive at the final mix proportions. Detailed procedure for asphalt mix design has not been discussed as it is a lengthy one and exact steps are not available in the notes I keep.
The salient quality requirements for coarse aggregates to be used in the bituminous base course are: Fineness Index (maximum 15%), 10% fines value (140 KN min.), Bitumen stripping (minimum 95%), Aggr. impact value (max. 30%), Water absorption (max. 2%), Mg sulphate soundness (max. 12%).
The binder content, i.e. bitumen content, in the bituminous base course is kept between 3 to 5% while designing the mix. Mineral filler, if needed to be mixed with coarse aggrgates of the bituminous base course, is usuall crusher run rock dust of suitable quality. The combined aggregates for bituminous base course including mineral filler when needed needs to fall in either of the two classes, A and B, of AASHTO particle size distribution and also should have a minimum Sand Equivalent of 45.
Bitumen to be used is defined as a petroleum derivative of penetration grade 60 to 70 conforming to AASHTO M20. The Marshall Test Procedure is often used to determine the optimum % of bitumen for the mixture.
Salient physical properties for bituminous base course as per AASHTO-T245 are: Stability (min. 12KN), Flow (2 to 4 mm), Voids in total mix (4 to 6%), Voids filled with binder (55 to 70%), Voids in aggregates (min. 14%), Residual voids in total mix after 500 blows (3%), Index of retained strength as per AASHTO-T165 (75%).
Some more important details would be discussed in Part 5 of this series.
( https://civilconstructionresourcez.wordpress.com/2012/01/18/roads-with-flexible-pavement-part-5 )