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Why is so difficult to accurately design a pavement?

September , 8th 2019 | Author: Felipe Cava (@) Read: 617 times

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For a long time, the charts developed through the AASHO Road Test, and something still in force in Brazil through DNER method (1981), led us to a brief and hasty conclusion that pavement structures are easy to design.

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Eventually we calculate the cumulative number of equivalent standard axis (N), find values in charts and we substitute in the inequalities to find the equivalent thicknesss of granular material. Something easy, right? But not quite.

Loads carried by heavy vehicles on highways or in mining, for example, are distributed on the pavement by means of tires. From basic physics we know tire inflation pressure is the ratio between the load carried and the area of load distribution.

It is precisely in these small issues mentioned above that the difficulty of accurately designing a pavement begins.

It’s well known that the standard road axle is composed of a simple axle with dual tires, with a load of 8,2 ton (80kN) and a tire inflation of 80psi (0,56MPa), these values are often used in mechanical analysis as a project criterion or stress and strain analysis.

However, with the advancement of technology, companies are developing increasingly inflatable tires to reduce rolling resistance, improve fuel consumption and extend tire life cycle in heavy vehicles. According to FERNANDES (1994) during AASHO Road test the tire inflation pressure ranged from 75 to 80 psi, but with the arrival of radial tires the pressures today range from 100 to 105 psi in the United States of America, in Europe with the use of extra-wide tires the inflation reaches 140 psi. In Brazil, although there is a lack on data, it can be considered a tire pressure reaching 120psi.

This increase in tire inflation pressure but keeping the same applied load results in a smaller area of loading distribution and reflects a significant increase in stress in the pavement layer, mainly in the bottom of asphalt coating.

Figure 1 illustrates a stress analysis where the same loading of simple axle with dual tires (80kN) and the same pavement structure were considered but changing the tire pressure from 80psi to 100psi.

Figure 1 – Pavement stress analysis changing tire pressure

For the base of asphalt coat, located 50mm from the surface in this case, it is observed that the ranging of tire pressure from 80psi to 100 psi results in the increase of approximately 24% in tensile stress. However, as also verified by FERNANDES (1994) and considering the fourth power law an increase of 24% in the structural answer results in the increase of 136% in pavement deterioration.

In some countries, such as the Guide to Pavement Technology Part 2: Pavement Structural design from Austroads (2017), a tire pressure ranging from 750 to 800kPa (approximately 110psi) is suggested, although it still indicates that available data show a range from 500kPa to 1200kPa.

In other words, the pavement design is much more complex than what is often taught in the classroom or presented by empirical methods. The variation of tire pressure reflects a significant increase in the pavement damage and dramatically reduce structural life cycle. Furthermore, other factors such as overloading on the pavement structure or still the relation between layers adhesion may influence the pavement service life.

 

Written by Felipe Cava and translated for the English version of Prontubeam

https://alemdainercia.wordpress.com/2019/08/13/por-que-e-tao-dificil-dimensionar-pavimentos-com-precisao/

 

References:

 

AUSTROADS. Guide to Pavement Technology Part 2: Pavement Structural Design da AustRoads. Sidney, 2017.

FERNANDES JÚNIOR, J.L. “Investigação dos efeitos das solicitações do tráfego sobre o desempenho de pavimentos”. Escola de Engenharia de São Carlos – EESC USP. São Carlos, 1994.

 

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About the author
Felipe Cava . Civil Engineer | Transportation Infrastructure | Além da Inércia Website
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