Estimation of penetration requirements for prime coat

Abstract

Prime coat application is a necessary activity in road constructions. The main function of the prime coat is to seal capillary voids in the base course surface to prevent migration of moisture to the base course and underneath layers. However it is not carried out up to the required standards. The prime penetration into the road base is the most important consideration. In the local context, the application of prime coat has been a common practice in road construction projects though there is no specification for penetration requirements. This is an issue of concern as there should be a basis for selecting the best type of prime coat in terms of the penetration and penetration requirement for respective type of processed bitumen as well. Commonly used types of processed bitumen for prime application in Sri Lanka are MC-30 (Medium Curing cut back bitumen) and CSS-1 (Cationic Slow Setting). The experimental research was done using both types of bitumen and the Aggregate Base Course (ABC) specified in the Standard Specifications for Construction and Maintenance of Roads and Bridges [2nd Edition – June 2009] specification as the road base. Specimens were casted in AASHTO specified CBR moulds at selected degree of compaction and was sprayed at selected rates under typical defined Sri Lankan conditions. Penetration requirement and required curing time were selected for each degree of compaction and rate of application, and then the penetration requirements for respective required curing time were plotted against the base compaction and rate of application. There is no significant difference of required penetration for both type of bitumen but MC-30 has shown a relatively higher penetration for few time intervals to make it arguably the best to select in terms of the penetration into the base among the two types of bitumen tested. The penetration requirements for MC-30 and CSS-1 with the time are presented and can be used to make decisions on the best type of bitumen to use in terms of penetration and required penetration for a selected type of bitumen with curing time.