Analytical and laser scanning techniques to determine shape properties of aggregates used in pavements

Abstract

Pavement layers are constructed using a combination of materials, of which rock aggregates constitute a larger proportion. Current understanding is that the performance of pavements is dependent on the aggregate shape properties which include form, angularity and surface texture. However, direct and accurate measurements of aggregate shape properties remain a challenge. The current standard test methods used to evaluate aggregate shape properties cannot measure these properties accurately. Among the reasons contributing to the difficulties in the determination of aggregate shape properties is irregular shapes of aggregate particles. Therefore, current research efforts focus on developing accurate, reliable and innovative techniques for evaluation of aggregate shape properties. The work presented in this dissertation contributes to the current innovative research at the Council for Scientific and Industrial Research (CSIR) in South Africa, to automate the measurement of aggregate shape properties. The CSIR’s present research is aimed at improving pavement performance through better materials characterisation, using laser scanning and advanced modelling techniques. The objective of this study was to investigate improved techniques for the determination of aggregate shape properties using analytical and laser scanning techniques. A three-dimensional (3-D) laser scanning device was used for scanning six types of aggregate samples commonly used for construction of pavements in South Africa. The laser scan data were processed to reconstruct 3-D models of the aggregate particles. The models were further analysed to determine the shape properties of the aggregates. Two analysis approaches were used in this study. The first approach used the aggregate’s physical properties (surface area, volume and orthogonal dimensions) measured by using laser scanning technique to compute three different indices to describe the form of aggregates. The computed indices were the sphericity computed by using surface area and volume of an aggregate particle, the sphericity computed by using orthogonal dimensions of an aggregate particle, and the flat and elongated ratio computed by using longest and smallest dimensions of an aggregate particle. The second approach employed a spherical harmonic analysis technique to analyse the aggregate laser scan data to determine aggregate form, angularity and surface texture indices. A MATLABTM code was developed for analysis of laser scan data, using the spherical harmonic analysis technique. The analyses contained in this dissertation indicate that the laser-based aggregate shape indices were able to describe the shape properties of the aggregates studied. Furthermore, good correlations were observed between the spherical harmonic form indices and the form indices determined by using the aggregate’s physical properties. This shows that aggregate laser scanning is a versatile technique for the determination of various indices to describe aggregate shape properties. Further validation of the laser-based technique was achieved by correlating the laser-based aggregate form indices with the results from two current standard tests; the flakiness index and the flat and elongated particles ratio tests. The laser-based form indices correlated linearly with both, the flakiness index and the flat and elongated particles ratio test results. The observed correlations provide an indication of the validity of laser-based aggregate shape indices. It is concluded that the laser based scanning technique could be employed for direct and accurate determination of aggregate shape properties.