Abstract:
Highway engineers agree that removal of water from the base and upper portion of sub-grade deserves high priority, if good performance is to be achieved for a flexible pavement. Water enters the dense graded bases through cracks, gets trapped and adversely affects the pavement structure and performance under loads. Therefore, designers started using open-graded, highly permeable layers combined with subsurface drainage system, to provide the required drainage capabilities. However, a highly permeable layer can result in construction and rutting problems due to its low stability. One of the solutions is, stabilize the permeable layers with 2 % to 3 % asphalt cement to provide necessary stability for construction and minimize future rutting under heavy traffic.
The concept of permeable bases in pavement system has not so far been used in Pakistan. A study was carried out at MCE in 1995, which recommended further improvement in the stability of permeable base course. This study is therefore, aimed at enhancing the stability by using a harder asphalt i.e. penetration grade 40/50, and for comparison purpose asphalt penetration grade 60/70, used in previous study has also been included. Marshall method has been used to determine the stability with asphalt contents of 2, 2.5 and 3 %, while cement has been added as a filler ranging 1.5, 2, 2.5 %, and 0 % as control. The coefficient of permeability has been determined by constant head method. The results thus obtained from stability and permeability tests have also been compared with NHA gradation B.
An increase of 6 to 23 % in stability has been achieved by using asphalt penetration grade 40/50. The use of cement upto 2% has resulted in increasing the stability by 16 to 24% and potential to stripping has also been reduced. The use of two different grades of asphalt has not shown any significant effect on permeability. Future studies have been recommended to study the suitability of permeable base using stone dust to reduce the cost, and also aggregate from Nowshera source, which has shown much better results in another parallel study.
