Factors affecting bitumen–aggregate adhesion(continue)Print
Physio-mechanical adsorption of bitumen into the aggregate depends on several factors, including the total volume of permeable pore space, the size of the pore openings, and the viscosity and surface ension (surface energy) of the bitumen (Thelen, 1958). The presence of a fine microstructure of pores, voids and micro-cracks can bring about an enormous increase in the adsorptive surface available to the bitumen. It has also been shown (Plancher et al., 1977; Scott, 1978) that fractions of the bitumen are strongly adsorbed in the aggregate surface to a depth of approximately 180A ° (18 × 10−9 m). It has been suggested that the good mechanical bond achieved on a rough aggregate can be more important than the aggregate mineralogy in maintaining bitumen–aggregate adhesion (Lee and Nicholas, 1957). The properties of the bitumen are also important in the acquisition and subsequent retention of the bitumen–aggregate bond. In particular, the viscosity of the bitumen during coating and in service, polarity and constitution all influence the adhesion characteristics. However, it is the nature of the aggregate that is, by far, the most dominant factor influencing bitumen–aggregate adhesion. Table 19.1 summarises the main factors that influence bitumen–aggregate adhesion. It is considered that approximately 80% of these factors are controllable during production and construction. Disbonding mechanisms in asphalts Many studies have been carried out to determine the mechanism of bitumen disbonding in asphalts (Asphalt Institute, 1981; Hughes et al., 1960; Taylor and Khosla, 1983). There are two main methods by which the bitumen– aggregate system may fail: adhesive and cohesive mechanisms. If the aggregate is clean and dry and the mixture is effectively impermeable, the mode of failure will be cohesive. However, in the presence of water, the failure mode will almost certainly be due to a loss of adhesion caused by stripping of the bitumen from the aggregate surface. Several mechanisms of disbonding are possible, and these are discussed below. The description above is an oversimplification, and the loss of adhesion and the effects of moisture damage are often described as a mixture of adhesive and cohesive damage. The situation is further complicated by scalar effects, in which cohesion of a bitumen–filler mastic is described, whereas such a mastic is a function itself of adhesive strength and physical factors such as bitumen absorption by the filler.
Displacement theory relates to the thermodynamic equilibrium of the threephase bitumen–aggregate–water system. If water is introduced at a bitumen–aggregate interface, consideration of the surface energies that are involved shows that the bitumen will retract along the surface of the aggregate.
Detachment occurs when a thin film of water or dust separates the bitumen and aggregate, with no obvious break in the surface of the bitumen film being apparent. Although the bitumen film completely encapsulates the aggregate particle, no adhesive bond exists, and the bitumen can easily be peeled from the aggregate surface. This process may be reversible (i.e. if the water is removed, the bitumen may re-adhere to the aggregate). A second mechanism of disbonding must occur to allow the ingress of water between the bitumen and the aggregate.