Crumb Rubber–Bitumen Interactions: Cold-Stage Optical Microscopy

The problem of waste rubber tires is a global issue. One method of reusing tires is to process them into crumb-sized particles for use in flexible pavement applications by incorporating crumb rubber (known as crumb rubber modifier, CRM) into asphalt mixes. An understanding of the interaction between rubber and bitumen is therefore of vital importance. A review of the existing techniques and equipment used for the investigation of the microstructure of polymer-modified bitumens has revealed the high cost of the equipment and complexity of the techniques, and that it is not possible to employ such methods in the case of crumb-rubber-modified bitumens because of the sample size limitation. In the present work a new technique to obtain images of microstructures has been developed for use as an alternative standard procedure. Cold-stage optical microscopy uniquely provides a new method of observing the physical characteristics of crumb rubber particles in bituminous binders and can be useful when the particle shapes and binder properties are correlated. A 100-pen binder from Kuwait was modified with crumb rubber from truck tires of 600 μm nominal maximum size (30 mesh) at concentrations of 3% and 10% by weight of the binder. Data obtained from storage stability tests and extensive particle swelling (growth in size) studies were used to assess and illustrate the applicability and ability of the technique to obtain reproducible and clear images for any combination of crumb rubber type, size, content, bitumen type, mixing time, and temperature.