Durability of Bituminous Structures
Durability is the ability to survive and continue to give an acceptable performance. In the case of roads, it is necessary that the structure should survive for the specified design life, although it is accepted that not all aspects of performance can be sustained for this duration without some restorative maintenance. The design guide for UK roads suggests a design life of 40 years (Highways Agency, 2006), although this can usually only be achieved in stages, as shown in Fig. 1.
The durability of a flexible road structure depends on the durability of the materials from which it is constructed, in particular the bituminous materials. Bituminous materials may deteriorate in a number of ways. The bitumen itself will harden with exposure to oxygen and through temperature effects, the aggregate may not be of sufficient quality so that some individual particles may break down, or there may be loss of adhesion between the bitumen and aggregate particles.
These forms of deterioration are caused by weathering (moisture) and the action of traffic. These agents act at the road surface, which is particularly vulnerable. However, deterioration can also occur in the body of the material and this is controlled by the void content and permeability of the material.
Ageing of Bitumen
The ageing or hardening of bitumen is an inevitable result of exposure of bitumen to the atmosphere. The rate of hardening will depend on the conditions and the nature of the bitumen. There are two main processes that occur: oxidation and loss of volatiles.
In the oxidation process, oxygen molecules from the air combine with the aromatics and resins to form asphaltenes. Thus there is an increase in the polar, high molecular weight fraction at the expense of the lower molecular weight components. This results in an increase in the viscosity of the bitumen. Also, the bitumen becomes unstable owing to the discontinuity that develops between the saturates and the rest of the components.
This instability causes a lack of cohesion within the bitumen, which may lead to cracking. The rate of oxidation is highly dependent on temperature, and is rapid at the high temperatures used for mixing and laying bituminous materials.
Loss of Volatiles
Loss of volatiles will occur if there is a substantial proportion of low molecular weight components in the bitumen and if the bitumen is subjected to high temperatures. However, for penetration grade bitumens the loss of volatiles once the material has been laid is relatively small.
The hardening of bitumen results in a lowering of penetration, an increase in softening point and an increase in penetration index. Therefore an excessive amount of hardening will cause the material to become brittle at low temperatures and vulnerable to cracking.
A convenient way of representing the ageing of bitumen is by means of an ageing index, calculated as the ratio of the viscosity of the aged bitumen to that of the original bitumen.
In practice, the ageing of bitumen is most marked during the mixing process because of the high temperatures involved in a batch or drum asphalt production plant. For example, the penetration value of a 50 pen-bitumen will fall to between 30 and 40 depending on the duration of the mixing and the temperature used; subsequent high-temperature storage will cause further ageing.
Thus the penetration value could be reduced by as much as a half. Ageing of bitumen on the road is generally a much slower process. This is because the temperatures are much lower and the availability of oxygen is restricted by the void content and permeability of the mixture. In more open-textured (porous) asphalt mixtures with a large volume of interconnected voids, air can readily permeate the material, allowing oxidation to occur.
However, in dense mixtures with high binder contents, such as hot rolled asphalts and stone mastic asphalts, the permeability is low and there will be very little movement of air through the material. In both cases, ageing will be more rapid at the surface than in the bulk of the material because there is a continual availability of oxygen and the surface will reach higher temperatures. Figure 2 shows the ageing index of bitumen after mixing, storage, transport, paving and subsequent service.
Bitumen Ageing Tests
Tests related to the ageing of bitumen can be broadly divided into tests performed on neat bitumen and tests performed on asphalt mixtures. Most laboratory ageing of bitumen utilises thin film oven ageing to age the bitumen in an accelerated manner. Typically, these tests are used to simulate the relative hardening that occurs during the mixing and laying process (‘short-term ageing’).
To include ‘long-term hardening’ in the field, thin film oven ageing is typically combined with pressure oxidative ageing. The most commonly used short-term ageing test is the rolling thin film oven test (RTFOT), standardised in BS EN 12607-1.
The RTFOT involves rotating eight glass bottles each containing 35 g of bitumen in a vertically rotating shelf, while blowing hot air into each sample bottle at its lowest travel position. During the test, the bitumen flows continuously around the inner surface of each container in relatively thin films of 1.25 mm at a temperature of 163°C for 75 minutes.
The vertical circular carriage rotates at a rate of 15 revolutions/minute and the air flow is set at a rate of 4000 ml/minute. The method ensures that all the bitumen is exposed to heat and air and the continuous movement ensures that no skin develops to protect the bitumen.
The conditions in the test are not identical to those found in practice, but experience has shown that the amount of hardening in the RTFOT correlates reasonably well with that observed in a conventional batch mixer. Long-term ageing of bitumen can be achieved using the pressure ageing vessel (PAV), which was developed to simulate the in-service oxidative ageing of bitumen in the field.
The method involves hardening of bitumen in the RTFOT followed by oxidation of the residue in a pressurised ageing vessel. The PAV procedure (AASHTO R28-06) entails ageing 50 g of bitumen in a 140 mm diameter pan (approximately 3.2 mm binder film thickness) within the heated vessel, pressurised with air to 2.07 MPa for 20 hours at temperatures between 90 and 110°C.
Permeability is an important parameter of a bituminous mixture because it controls the extent to which both air and water can migrate into the material. The significance of exposure to air (ageing) was described in the previous article.
Water may also bring about deterioration by causing the bitumen to strip from the aggregate particles (adhesive failure), or causing weakening (damage) of the bitumen and bitumen/ filler/fines mastic (cohesive failure).
Measurement and Voids Analysis
The measurement of permeability is, in essence, a simple task, achieved by applying a fluid under pressure to one side of a specimen of a bituminous mixture and measuring the resulting flow of fluid at the opposite side. Both air and water have been used as the permeating fluid.
Table 1 presents typical ranges of permeability for three common types of asphalt mixture (Chen et al., 2004).
Factors Affecting Permeability
The permeability of a bituminous mixture depends on a large number of factors. Of particular importance are the quantity of voids, the distribution of void size and the continuity of the voids.
Figure 3 shows how permeability varies with total voids in the mixture for a range of typical asphalt concrete, hot rolled asphalt, stone mastic asphalt and porous asphalt mixtures. It can be seen that there are significant differences in air void size, distribution and connectivity, and therefore permeability, in mixtures with the same proportion of air voids.
Figure 3 shows that the relationship between permeability and air voids is in all cases exponential. The voids are also affected by the nature of the aggregate. The shape, texture and grading of the particles will govern the packing and hence void content at a particular bitumen content. The amount of compactive effort employed is also important.
The quality of the adhesion of a bitumen to an aggregate is dependent on a complex assemblage of variables. Table 2 identifies a number of factors that have an influence on the adhesion performance of bituminous mixtures.
Although some of these relate to the ambient conditions and aspects of the mixture as a whole, the principal factors are the nature of the aggregate and, to a lesser extent, the bitumen.
The Nature of the Aggregate
The mineralogical and physical nature of the aggregate particles has an important bearing on adhesion, the adhesive capacity being a function of chemical composition, shape and structure, residual valence, surface energy and the surface area presented to the bitumen.
Generalisations about the effect of mineralogy are difficult because the effects of grain size, shape and texture are also important. However, in general, the more siliceous aggregates such as granites, rhyolites, quartzites, siliceous gravel and cherts tend to be more prone to moisture-related adhesive failures.
The facts that good performance with these materials has also been reported, and that failures in supposedly good rock types such as limestones and basic igneous rocks have occurred, emphasise the complexity of the various material interactions.
Therefore caution should be exercised when attempting to make generalisations on the adhesion performance of aggregates of different or even similar mineralogy. The surface character of each individual aggregate type is important, particularly in relation to the presence of a residual valence or surface charges. Aggregates with unbalanced surface charges possess a surface energy, which can be attributed to a number of factors including broken co-ordination bonds in the crystal lattice, the polar nature of minerals, and the presence of adsorbed ions.
Such surface energy will enhance the adhesive bond if the aggregate surface is coated with a liquid of opposite polarity. Absorption of bitumen into the aggregate depends on several factors, including the total volume of permeable pore space and the size of the pore openings. The presence of a fine microstructure of pores, voids and microcracks can bring about an enormous increase in the absorptive surface available to the bitumen.
This depends on the petrographic characteristics of the aggregate as well as its quality and state of weathering. It is generally accepted that rougher surfaces exhibit a greater degree of adhesion. A balance is, however, required between the attainment of good wettability of the aggregate (smooth surfaces being more easily wetted), and a rougher surface that holds the binder more securely once wetting has been achieved. The presence of a rough surface texture can mask the effects of mineralogy.
The Nature of the Bitumen
The important characteristics of bitumen affecting its adhesion to aggregate are its viscosity and surface tension, and its polarity. The viscosity and surface tension will govern the extent to which bitumen is absorbed into the pores at the surface of the aggregate particles.
Both these properties change with temperature, and mixing of aggregate and bitumen is always done at high temperature (up to 180°C for 40/60 pen bitumen) in order that the bitumen coats the aggregate surface readily. Bitumen will also chemically adsorb on to aggregate surfaces.
Strongly adsorbed bitumen fractions have been identified at the bitumen–aggregate interface, forming a band on the order of 180 Å thick. Ketones, dicarboxylic anhydrides, carboxylic acids, sulphoxides and nitrogen-bearing components have been found in this layer (Ensley, 1975).
The strongly adsorbed components have been found to have sites capable of hydrogen bonding to the aggregate, though in the presence of water the available bonds prefer the more active water. Migration of some bitumen components to the interface is inferred and therefore a dependence on binder composition, mixing temperature and viscosity.
Figure 4 illustrates the process, with molecules of bitumen at the surface aligned in the direction of polarity of the substrate (aggregate), usually a negative surface. The zone of orientation of bitumen molecules extends for a thickness of several thousand molecules.
Mechanisms for Loss of Adhesion
Breakdown of the bond between bitumen and aggregate, known as stripping, may occur for a number of reasons. However, the principal agencies are the action of traffic and moisture, and these often act in combination. The effect of moisture is significant since it causes loss of adhesion in a number of ways (Caro et al., 2008).
A number of mechanisms have been postulated for loss of adhesion, most of which involve the action of water. These are described below and each may occur depending on the circumstances.
Displacement: This occurs when the bitumen retracts from its initial equilibrium position as a result of contact with moisture.
Figure 5 illustrates the process in terms of an aggregate particle embedded in a bituminous film (Majidzadeh and Brovold, 1968). Point A represents the equilibrium contact position when the system is dry. The presence of moisture will cause the equilibrium point to shift to B, leaving the aggregate particle effectively displaced to the surface of the bitumen. The positions of points A and B will depend on the type of bitumen and its viscosity.
Detachment: This occurs when the bitumen and aggregate are separated by a thin film of water or dust, though no obvious break in the bitumen film may be apparent. Although the bituminous film coats the aggregate particle, no adhesive bond exists and the bitumen can be cleanly peeled from the surface.
Film rupture: This occurs when the bitumen fully coats the aggregate but where the bitumen film thins, usually at the sharp edges of the aggregate particles (Fig. 6).
Blistering and pitting: If the temperature of the bitumen at the surface of a road rises, its viscosity falls. This reduced viscosity allows the bitumen to creep up the surface of any water droplets that fall on the surface, and it may eventually form a blister (Fig. 7a). With further heating the blister can expand and cause the bitumen film to rupture, leaving a pit (Fig. 7b).
Spontaneous emulsification: Water and bitumen have the capacity to form an emulsion with water as the continuous phase. The emulsion formed has the same (negative) charge as the aggregate surface and is thus repelled. The formation of the emulsion depends on the type of bitumen, and is assisted by the presence of finely divided particulate material such as clay materials, and the action of traffic.
Hydraulic scouring: This is due principally to the action of vehicle tyres on a wet road surface. Water can be pressed into small cavities in the bitumen film in front of the tyre and, on passing, the action of the tyre sucks up this water. Thus a compression–tension cycle that can cause debonding is invoked.
Pore pressure: This mechanism is most important in open or poorly compacted mixtures. Water can become trapped in these mixtures as densification takes place by trafficking. Subsequent trafficking acts on this trapped water and high pore pressures can result. This generates channels at the interface between bitumen and aggregate (Fig. 8) and eventually leads to debonding.
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