Self Healing of Asphalt Mixtures

Abstract

Traffic is increasing rapidly in terms of number of vehicles and also in axle loads. In order to maximize the availability of the pavement and to minimize hindrances to traffic because of maintenance works, long life pavements are needed. An asphalt pavement with self repairing capabilities is believed to be very useful to this respect. The self healing phenomenon of asphalt mixtures is known for many years by road engineers. Bituminous materials are expected to repair themselves during hot summers and (long) rest periods. However, the underlying mechanism is not well understood, and a proper way to measure it is not available. Research questions are: what is the self healing phenomenon, how to measure it effectively and efficiently and how to upgrade it if possible. In order to answer these questions, investigations were carried out in this thesis. This research focuses on understanding the self healing mechanism of bituminous materials and the effects of material modifications, by means of testing and modelling. The research started with a critical literature review. Preliminary research was conducted to explore possible self healing modifiers. Novel self healing modifiers like ionomers, supermolecular rubbers and nanoparticles were chosen and investigated. Upon analyzing the change of the material properties and the self healing capability due to modifications, it was observed however, that all the novel modifications used in this research are not quite beneficial for the self healing improvement of bituminous materials. A normal soft bitumen was observed to be the best healer among all the modified bitumens tested. Further research was conducted to assess the self healing capability of bituminous materials in further detail. Three test methods were developed to mimic the self healing phenomenon at different levels being from bitumen level to mixture level. The self healing phenomenon was directly related to a measurable crack. In each of the test methods used, cracks were produced first in a controlled way, and after that the healing process of these cracks was investigated. The test methods covered the following aspects: • A two-piece healing (TPH) test was developed to investigate the self healing behaviour of pure bitumen using the Dynamic Shear Rheometer (DSR). During the TPH test, the healing process was mimicked by pressing two pieces of bitumen together in a parallel-plate system. The development of the complex shear modulus during the closure of the gap width and during healing rest periods was monitored and used as a healing indicator. • A modified direct tension test was developed to assess the self healing capability of bituminous mastics. The cracks were first introduced via mechanical loading, and then healing rest periods were applied. After healing, the specimens were reloaded to determine the recovery of the material strength; this recovery was used as a healing indicator. This test can be used to investigate the self healing capability of an open crack with two total fractured surfaces and to determine the self healing capability of meso cracks. • A beam on elastic foundation test (BOEF) was developed to investigate the self healing phenomenon of asphalt mixtures with a notched asphalt concrete beam fully glued on a low modulus rubber foundation. After a crack is produced by imposing monotonic loading, the BOEF setup allows fully closure of the crack due to the confinement of the rubber foundation. After a healing period, the beams were reloaded and the stiffness recovery and strength recovery were used as indicators of healing. • The results of the various tests showed that the self healing capability of bituminous materials can be ranked successfully at different healing times, temperatures and damage levels. The self healing process of damage in bituminous materials consists of two main phases, namely the crack closure and the strength gain phase. The driving force can be either thermal (temperature) or mechanical (by confinement, pressure). The self healing capability is related to the viscosity of the bitumen, which increases with increasing healing time, temperature and when the crack size is very small. Finite element modelling was done to further investigate the self healing phenomenon in the tests. A smeared type cohesive zone model was used to model healing by defining the stiffness and strength recovery process. In this way, the self healing phenomenon was directly linked to a crack repairing process. Based on the research results, a better understanding of the self healing phenomenon was achieved. This thesis ends by discussing some important aspects of building a durable asphalt pavement with self healing capabilities. The self healing capability of an asphalt mixture should be optimized to obtain pavements with an enhanced durability.