A synthesis of Asphalt Binders Specification, Evaluation and Applicability of Performance Based Specifications in Retrospective of Pakistan Climatic Conditions

Abstract

Almost the entire road network of Pakistan consists on flexible pavement. Flexible pavement undergoes different kinds of distresses, which affects its serviceability. Although there are many causes of road distresses but temperature variation and uncontrolled load contributes the most. Temperature directly affects the behaviour of Asphalt binder which is the main constituent of flexible pavement. Currently Pakistan uses penetration grading system for asphalt binder specifications. . Penetration is an empirical measure of the consistency that is used as an empirical indicator of the rutting and fatigue susceptibility of asphalt binder, and is not related to pavement performance. The primary objective of this study is to analyze worldwide conventional and modern grading systems of asphalt binder and recommend suitable grading system as well as suitable asphalt binder, keeping in view the climatic conditions of Pakistan. To accomplish the objectives review of related literature has been done including analyses of 20 to 30 years temperature data of seventy meteorological stations. Based on the temperature variations, the country is divided into different climatic regions. After analysis of conventional binder grading systems, Performance grading system is critically analyzed for Pakistan, in retrospect of climatic zones of the country. Conventional and PG binder grade tests results for all six binders used in Pakistan have been presented and their suitability in context of our climate is highlighted. PG grading test results of locally available/used binders suggests their corresponding PG grades and likely climatic condition for application. For the application of PG grading system pavement temperature is pre-requisite. SHRP and LTPP models have been utilized to calculate site specific pavement temperature from air temperature data. It is observed that our climatic conditions warrants use of PG 70-10 almost in 4 out of 5 regions, which covers almost 75% of the area, certain areas with extreme temperature requires PG 76-10. In order to minimize the adverse effect of excessive uncontrolled loading on our roads, one grade stiffer binders are proposed on national highways. But currently none of the two local refineries produce PG 70-10 binder (which is most suitable for Pakistan), thus it should be a concern for the highway agencies. In addition to that brief summary of binder modifiers is also included in this study to give an overview of modern techniques utilized to modify binders to withstand rutting distress in flexible pavement. Existing binders available locally include Polymer modified asphalt binder produced by Attock refinery (ARL-PMB) corresponds to harder PG 76-16 while ARL-60/70 (PG 58-22) or NRL-60/70 (PG 64-22) produced at Attock and National refineries respectively are softer compared to the PG 70-10. Harder grade is more prone to cracking, whereas softer grade of more prone to rutting. However the most used binder grades in Pakistan are ARL-60/70 and NRL-60/70, which corresponds to PG 58-xx and PG 64-xx respectively. Consequently, the current construction practices which utilize ARL-60/70 or NRL-60/70 may be prone to excessive rutting