Adhesion of bitumen and moisture damage in asphaltsPrint
Adhesion of bitumen and moisture damage in asphalts
The primary function of bitumen in an asphalt is to act as an adhesive. Bitumen is required either to bind aggregate particles together or to provide a bond between particles and an existing surface. When used in an asphalt, bitumen binds together the aggregate, sand and filler that comprise the mixture. In a pavement, the bitumen needs sufficient cohesive and adhesive strength to ensure minimum loss of aggregate under the shear stresses induced by traffic. Although the incidence of premature failure attributed to adhesion is relatively rare, failures, when they occur, may involve considerable expense. The need to ensure adhesion between the aggregate and the bitumen is, therefore, very important. The adhesion of bitumen to most types of dry and clean aggregate presents few problems. However, aggregate is easily wetted by water, the presence of which can result in unexpected problems. These may occur at any time during the life of an asphalt, from the initial coating of the aggregate during the mixing process to its time in service, when it has to maintain an adequate bond between the bitumen and aggregate under traffic conditions. Damage to asphalts in the presence of water is a complex phenomenon and has been the subject of research over many decades. Although it can be described and quantified to an extent using thermodynamic first principles, in practice moisture damage is a complex phenomenon and is commonly assessed using simulative tests in the laboratory. Measurement of adhesion is notoriously difficult to achieve in a reproducible manner, and many test methods have been employed to measure the combined forces of adhesion and cohesion, often referred to as ‘adhesive strength’. Several laboratory procedures have been developed to assess the dry strength of asphalts and their adhesive strength in the presence of water, typically termed ‘moisture (induced) damage’. The aim of this chapter is to introduce the fundamentals of adhesion and cohesion, draw attention to aggregate–bitumen adhesion and the ways in which it can be assessed, and how it may be possible to reduce the possibility of premature failures in service resulting from a lack of adhesion. Factors affecting bitumen–aggregate adhesion In practice, many factors (in addition to thermodynamics) influence the bitumen–aggregate bond; the importance of aggregate mineralogical composition has been recognised for many years (Saal, 1933; Winterkorn, 1936). Failure of the aggregate–bitumen bond is commonly referred to as ‘stripping’. One of the main factors is the type of aggregate. This has a considerable influence on bitumen adhesion due to differences in the degree of affinity for bitumen. The vast majority of aggregates are classified as ‘hydrophilic’ (water loving) or ‘oleophobic’ (oil hating). Aggregates with high silicon oxide content (e.g. quartz and granite (i.e. acidic rocks)) are generally more difficult to coat with bitumen than are basic rocks such as basalt and limestone. The majority of adhesive failures have been associated with siliceous aggregates such as granites, rhyolites, quartzites, cherts, etc. The fact that satisfactory performance is achieved with these same aggregates and that failures occur using aggregates that have good resistance to stripping (e.g. limestone) emphasises the complexity of bitumen–aggregate adhesion, and raises the possibility that other factors may play a role in the failure. Other factors affecting the initial adhesion and subsequent bond are the +surface texture of the aggregate, the presence of dust on the aggregate and, to a lesser extent, the degree of acidity of the water in contact with the interface. It is generally agreed that rougher aggregate surfaces have better adhesion characteristics. However, a balance is required between wetting of the aggregate (smooth surfaces being more easily wetted) and rougher surfaces, which hold the bitumen more tenaciously once wetting has been achieved.