Aggregate structure characterisation of asphalt mixtures using two-dimensional image analysis

Abstract

In current practice of mixture design, volumetric properties such as voids and binder content along with mechanical properties such as modulus or rutting resistance are used as the main quality indicators. Visualisation is an important tool that has not been widely used in asphalt mixtures. As part of the Reunion Internationale des Laboratoires et Experts des Materiaux activities, the aggregate structure has been identified as a possible important mixture characteristic in need of measuring and quantifying. This paper is a report on part of this effort. Software for processing and analysing two-dimensional images of asphalt concrete mixtures to provide information about the aggregate structure within a mix was developed. Images with accompanying volumetrics and gradation information can be processed with the software and a virtual sieve analysis of aggregates within the image is performed to verify a match with known measured gradations. Once images were successfully processed, analysis is performed to determine the number of contact points between aggregates as well as radial distribution and orientation of each aggregate. Segregation of aggregates within each specimen was also determined. Mixtures with a broad range of variables were compacted in the laboratory, using a number of compaction methods of various countries. In addition, mixtures with various nominal maximum aggregate sizes, aggregate type (limestone or gravel) and design ESALs (E-3 or E-10) were compacted in the US gyratory compactor, using two pressures (600 and 300 kPa) and two temperature levels (120°C and 60°C). Results indicate that the aggregate structure is affected by compaction methods and conditions although volumetrics are very similar. The results show that a fresh look at evaluating the aggregate structure within mixtures is required.

Keywords:

• aggregate structure
• imaging
• asphalt mixtures
• compaction
• rutting

Acknowledgements

The authors thank the many volunteer hours of the RILEM Task Group 2 members, specifically those from the University of California Pavement Research Center, Asian Institute of Technology, Technische Universität Braunschweig, Michigan Technological University and University of Wisconsin-Madison. A special thanks to Mr Carl Johnson for his contributions and evaluation of the early iterations of software development.