Fatigue of asphalts

Layers of pavements under the effect of axial loads are subjected to repeated tension. The magnitude of the tensile strain developed in pavements under a certain load mainly depends on the stiffness of the layers. For flexible pavements and after detailed measurements in situ, it was found (Pell 1967) that the magnitude of the tensile strain ranges between 30 and 200 microstrain, for a standard axle load (8 tonnes). This tensile strain generates conditions for fatigue of asphalt layers to occur, which appears as cracking. 

Pavement cracking, owing to the fatigue of asphalt layers, is one of the main modes of failure. 

Asphalt resistance to fatigue depends on its composition and properties of constituent materials. Thus, each type of asphalt possesses different fatigue behaviour. This behaviour should be determined in order to design the pavement so that it does not crack prematurely. 

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Epps, J. A., Monismith, C. L., Fatigue of Asphalt Concrete Mixtures—... 

Pell P.S. 1967. Fatigue of asphalt pavement mixes. Proceedings of 2nd International Conference on the Structural Design of Asphalt Pavement. Ann Arbor, MI University of Michigan. 

Local depressions are caused by local inadequate compaction of the unbound layers or failure of the subgrade. When accompanied by cracks, fatigue of asphalt layers has also... 

The fatigue life of well designed blow sand Thermopave mixes (e.g., over 50% finer than No. 100 mesh) is slightly below that of asphalt concrete. The fatigue life of mixes in which coarser sands are used, however, is comparable with that of asphalt concrete. 

This paper present the result of laboratory tests carried out to study the relative performance of waste tire rubber in asphalt pavement. The result demonstrates that waste tire rubber reinforcement is the most advantageous pavement reinforcement technique for improvement of asphalt fatigue life. 

Medani. T.O., Molenaar. A.A.A., Estimation of Fatigue Characteristics of Asphalt Mixes Using Simple Tests . 

The ductility of asphalt is a measure of the flexibility of the asphalt and is expressed as the distance in centimeters that a standard briquette can be elongated before breaking (ASTM D-113, IP 32). Ductility is a combination of flow properties and reflects homogeneity, cohesion, and shear susceptibility it is an indication of fatigue life and cracking. 

Baoshan Huang, Xiang Shu, Gang Zuo.2013. Using notched semi circular bending fatigue test to characterize fracture resistance of asphalt mixtures. Engineering Fracture Mechanics, (109) 78—88. 

The improvement of asphalt s fatigue life delays the development of the pavement s fatigue cracking. 

However, it does not improve the elastic behaviour of the asphalt (bituminous mixture). Thus, asphalts with thermoplastic polymer-modified bitumen present good resistance to deformation at high ambient temperatures, but generally their fatigue cracking resistance is similar to or even less than that of asphalts with unmodified bitumen. 

The mechanical behaviour of asphalts is determined by resistance to tensile forces, known as resistance to fatigue cracking, and resistance to compressive forces, known as resistance to permanent deformation. The first is expressed by the relationship between tensile strain and number of loadings, known as the fatigue equation. The second is expressed by the relationship between compressive strain and number of loadings, known as the creep equation. 

If fatigue lives (number of loading cycles) are plotted in terms of constant strain, results from different stiffness of asphalt, owing to temperature and time of loading, coincide in one straight line as shown in Figure 7.28. Thus, the criterion for failure is strain (temperature... 

Figure 7.27 Typical fatigue lines of asphalt at different temperatures.

The European standard related to resistance to fatigue of hot mix asphalt, CEN EN 12697-24 (2012), specifies five alternative test methods for characterising the fatigue of bituminous mixtures. Four of the tests are bending direct tensile tests and one is an indirect tensile test. In detail, the five tests methods specified by CEN EN 12697-24 (2012) are as follows ... 

The fatigue test is required by CEN EN 13108-1 (2008) to be used for the mix design of asphalt concrete using the fundamental approach. 

Some laboratories and organisations have developed various methods/techniques (equations or nomograms) from which the fatigue performance of asphalt can be predicted more easily with reasonable accuracy. The predictive methods are based on asphalt stiffness, bitumen properties (penetration index, volume of bitumen or softening point) or air voids. 

A method for predicting the fatigue life of asphalts by the use of a nomograph has been developed by Bonnaure et al. (1980). The required data are percentage volume of bitumen, V, penetration index of the bitumen (PI), stiffness modulus of the mixture, d initial strain... 

Bonnaure E, A. Gravois, and J. Udron. 1980. A new method for predicting the fatigue life of bituminous mixes. Proceedings of the Association of Asphalt Paving Technologists, Vol. 49, p. 499. 

Himeno K., T. Watanabe, and T. Maruyama. 1987. Estimation of fatigue life of pavement. 6th International Conference on Structural Design of Asphalt Pavements. Ann Arboi MI. 

Rowe G.M. 1993. Performance of asphalt mixtures in the trapezoidal fatigue test. Proceedings of Association of Asphalt Paving Technologist, Vol. 62. Seattle, WA Association of Asphalt Paving Technologists. 

Van Dijk W. and W. Visser. 1977. The energy approach to fatigue for pavement design. Proceedings of Association of Asphalt Paving Technologists, Vol. 46, pp. 1-40. 

The design criteria or distress modes adopted by most analytical and semi-analytical methodologies for flexible pavement design are as follows (a) fatigue of treated layers, that is, the asphalt layer or the hydraulically bound layer not to crack under the influence of traffic and (b) deformation of the subgrade, that is, the subgrade to be able to sustain the traffic without excessive deformation. 

Since the advent of asphalt paving blocks in 1824, efforts to Improve the properties of asphalt surfacing materials have centered on polymeric additives. In the 1840 s patents describing the modification of bitumen with gutta percha or natural rubber appeared and as each new polymer was developed, its potential interaction with asphalt was evaluated. Since the addition of polymers increases the cost of the corresponding polymer modified asphalt cement (PMAC), the new material must increase the durability of the mixture and meet all climatic requirements without compromise. Indeed, addition of appropriate polymers to asphalt increases fatigue cracking resistance, reduces the extent of permanent... 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

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