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EFFECTIVE MAINTENANCE FOR SIBERIAN ROADS SUMMARY: Proper maintenance procedures that prevent water ingress, and retard
low temperature or reflective cracking can extend pavement life
to allow achievement of design life. 1. INTRODUCTION Water ingress in a pavement is a primary reason for premature failure. The pavement becoming wet will reduce its stiffness. This occurs for several reasons:
These are design and drainage issues and must be addressed not
only in the design stage but also during pavement life. Effective treatments must seal and waterproof the surface and prevent water ingress while providing sufficient flexibility and tensile strength to resist thermal cracking due to low temperatures. Such surfaces must also retard reflective cracking. Thus maintenance is as important as construction and design in any program activity. 2. MATERIALS To resist cracking requires a material that can effectively dissipate strains as they arise. In reflective cracking this will be strains created by stress concentration points at existing cracks in the cracked surface. Thermal cracking is created by thermal stresses. The pavement surface contracts leading to a crack initiation when the tensile strength of the binder is exceeded by the compressive stresses. Hence material must allow high levels of extension without fracture. To resist fatigue cracking where deflections are high due to softened bases requires the ability to stretch and recover as well as the ability to blunt cracks as they form. It is known (2) that binder content in mixes in thin layers is related to fatigue life and so any binder which has high viscosity and so can be added to mixtures at higher than usual levels, and not cause bleeding, will improve fatigue resistance. If the binder is capable of maintaining these properties over the service temperature range and over time, then it will extend the pavement life significantly. Polymer modification has been used to extend pavement life (3). However polymer modification is often expensive. Crumb tyre rubber is a better alternative; in binders it is called bitumen (asphalt) rubber. The use of this material to improve the rheological properties of asphalt is long standing and well documented (4-7). Bitumen (Asphalt) Rubber is made by blending granulated tyre rubber
with bitumen and additives. The compatibility of the bitumen with
the rubber is a key issue for the performance of the binder in application.
It is affected by the grading of the rubber, the time and temperature
of mixing and the level of shear in mixing Figure1 shows approximate
grading that is ideal, but even rubber buffings up to 2mm may be
used if reaction times are longer. (8)  Figure 1 Typical Grading of Crumb Rubber (8) The compatibility of bitumen / polymer systems may be defined in several ways. It may be in terms of the achievement of a particular morphology, i.e. the structural arrangement of the polymer particles, chains or groups within the bitumen matrix. It may be in terms of thermodynamic stability, i.e. whether the conformation of the polymer particles or chains are in a low energy state, i.e. whether there is a driving force to increase entropy. It may be in terms of practical storage stability, i.e. will it separate on standing. Or it may be based on whether a given property or set of properties are achieved and can be maintained for a suitable period of time (that is until the material has been applied). A reaction is claimed to occur (6) in which the bitumen and the
rubber particle interact to form a gel coated particle (figure 2).
This is analogous to the process of swelling that occurs in polymer
asphalt systems. (9). This creates a matrix of rubber and bitumen
2.1 Properties Of Bitumen (Asphalt) Rubber Binders The properties of all polymer-modified systems are dependent on the morphology and are optimized when the materials are compatible- that is give finer distributions. (11)- see figure 3. The results of this can also be shown by comparing compatible and incompatible systems using the SHRP PG grading system. Compatible systems have a wider temperature performance range as is shown in table 1.
Table 1: Effect on SHRP PG grading (12)
The major effects of using crumb rubber are to increase viscosity and to improve thermal susceptibility. This is shown in Figure 4 and softening and Fraas point as shown in figure 5. This means that the binders are less susceptible to crack under low temperatures or deform under high temperatures.
Properties in membranes and reduction in reflective cracking rely on they ability to reduce stress in the overlay. This is shown in figure 7. In reduction of thermal stress requires an increase in tensile strength and elongation at break. This is shown in figure 8.
2.2 Properties in Seals, Hotmix and Microsurfacing 2.2a Membrane Seals Seals depend mainly on the binder properties as mentioned above; in SAM seals the requirement of stone retention is added to the requirements. This is shown for Vialit testing at a range of low temperatures in figure 9. AR clearly improved Vialit stone retention compared to non-modified and polymer modified systems.
2.2b Hotmix In hotmix applications AR can be used as an open graded or gap graded material. It has high fatigue strength compared to standard binders and hence longer life. It may be applied at about 50% of the thickness of a conventional binder mix (4) and maintain fatigue life. Figure 10 shows the increase in fatigue life of mixes when made with AR.
Low temperature properties too are superior as is shown in the TRST results for various binders in figure 11. It shows that AR significantly improves the low temperature cracking resistance. This is partly die to the improved binder tensile strength but also the 1-2% increase in binder content that is possible in such mixes.
High temperature mix properties are similarly enhanced as shown in for wheel tracking at 60C testing. See figure 12. AR has excellent rut resistance.
2.2b Microsurfacing This is used mostly in combination with chip seals as a "Cape" seal two-layer system. This combines the excellent crack resistance of AR with the smooth hardwearing microsurfacing. The use of AR binders is possible with emulsions and can improve the abrasion resistance of microsurfacing significantly as shown in figure wet track abrasion testing. Crumb rubber can either be added as dispersion, (RG-1), as dry into the pugmill via the machine fines feeder, or directly emulsified. (10)
3.METHODS The following methods are very successful. Sound Pavements
4. CONCLUSION Asphalt rubber has been used and tested in many countries and regions including USA, Canada, Poland, South America, Australia, Portugal and South Africa, it has been successful in very cold climates (7,14,15) where temperature extremes are as shown in figure 14.  Figure 14 Canadian temperature Cold extremes (7) AR will improve low temperature and high temperature performance and can be used in membrane seals, hot mix or in combination with other maintenance treatments such as microsurfacing. 5. REFERENCES
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