State of the art of tribological tests for bituminous binders

Kinetic friction is a physical phenomenon which originates when two or more bodies are in contact and in relative motion, and causes energy consumption and wear. Lubricants are widely used in many fields to reduce kinetic friction and their behaviour is usually characterized through appropriate tribological tests. In fact, the science of tribology (from the Greek word “tribo” that means to rub and the Latin word “logia” that means study) investigates interactions between surfaces in relative motion. In the field of road materials, during asphalt mixing and compaction, bitumen acts similarly to lubricants, reducing friction between aggregates, and its lubricating properties significantly affect the energy required. According to recent studies, some Warm Mix Asphalt additives are able to reduce production and compaction temperatures (and therefore energy consumption) of asphalt mixtures by potentially improving the lubricating behaviour of the binder. Thus, tribological tests have recently been introduced in the investigation of bituminous binders to characterize their lubricating properties. This paper aims at providing the state of the art of tribological tests currently employed for the study of bituminous binders, as well as useful suggestions for improving these procedures. Since the introduction of such tests in the field of road materials is quite recent, an overview on tribology and tribological tests on common lubricants is presented, with the aim to highlight the main aspects to take into account when applying the tribological characterization of bituminous binders.