The surface deterioration of the pavement provides a measure of the damage caused by traffic, environmental conditions and aging of the materials that constitute the wearing course. The type and cost of maintenance operations required for a road section is significantly influenced by the type, extent and severity of the defects present in the pavement (1). It is recognized that in reality, the set of indicators that characterize the state of the surface does not evolve in isolation, but through an interaction between them, other elements and the previous state of the set. It has been shown that the progress of cracking and rutting are related: at the beginning of the service life, an initial rutting occurs, the growth rate of which decreases with the increase in the number of cycles. Once cracking begins to be evident, the modulus of the asphalt layers falls, which causes an increase in the stresses that accelerate the rutting process, together with the possible entry of water into the structure depending on the maintenance tasks (2 ). Figure 1 outlines the deterioration behavior of the pavement considering both evolutionary periods.

Figure 1. Evolution of the deterioration of a pavement structure. Taken from Rational Design Models: Failure Criteria (2)

In today's pavements, the action of repeated loads is the most notable cause of deterioration. As previously mentioned, the growth in the volumes of cargo transported and the increase in the allowable weight per axle increase the probability that the pavement will experience fatigue and cumulative deformation failures (3).

1 Fatigue cracks

The National Highway Directorate of Uruguay defines fatigue cracks as failure lines mainly caused by stresses and / or lack of bearing capacity of the pavement (1).

The triggering of fatigue cracks is mainly attributed to tensile stresses in the lower part of the asphalt layer due to the bending of said layer due to the repeated passage of vehicles. This cracking starts and progresses through the asphalt phase and/or at the asphalt-aggregate interface and gradually propagates to the surface (bottom-up cracks) (4). They begin to show up as small longitudinal fissures in the tracks branching out, to later form a closed mesh (colloquially called crocodile skin). At that point, the failure is declared severe, eventually causing material detachment (1).

The fatigue process affects the asphalt layers, progressively reducing their effective work modules, which causes a redistribution of the induced stresses throughout the structure. This stress variation is dependent on the contribution of these asphalt layers to the overall stiffness of the structure. It may then happen that structures that have greater relative rigidity of the asphalt layers with respect to the structure as a whole, result in a decrease in useful life (2).

The fatigue failure criterion relates the allowable number of load repetitions to the tensile strain, until the condition of the pavement is considered sufficiently deteriorated to establish the end of its life. Fatigue laws are generally elaborated from laboratory tests and calibrated in the field (5).

Folder fatigue failures have historically been combated in two ways. On the one hand, an attempt has been made to give the folder such a thickness that the stress applied by traffic generates small deformations that do not produce the possibility of cracking or cumulative deformation. A greater thickness reduces the magnitude of the tensile stresses in the lower fiber of the asphalt layer and makes it more difficult for cracks to propagate, since they have to travel a longer distance to reach the surface (6). This approach is the most intuitive and simple to solve, but also the most expensive (7). On the other hand, the tensions in the asphalt layer can be reduced by supporting the folder on other layers that are sufficiently resistant and not very deformable. In these cases, it is important to compare the cost of each of the reinforcement options and study that the behavior of these layers does not affect the performance of the folder. For example, a cemented base will substantially improve its resistance capacity but will eventually generate shrinkage cracks that will be transmitted to the asphalt layer.

2 Permanent deformation

By permanent deformation phenomenon, also known as rutting, it is understood the alteration of the level of the tread layer due to subsidence along the treads (1) that brings about a lack of safety and comfort of the users who walk on the pavement.

Permanent deformations in asphalt mix layers are caused in a specific way or in combination by a set of factors. In the first place, the exposure of the pavement to high temperatures directly affects the viscoelastic properties of the asphalt present in the asphalt mixture causing it to flow under lower loads and it is generally evident early, even during the first months of summer. Other climatic factors such as thermal gradient and reflectivity of the pavement affect the severity of rutting to a greater or lesser extent (8).

On the other hand, traffic gives rise to cyclical loads, where in each cycle some work is done to deform the pavement surface as a combination of densification and shear deformation. Densification implies a decrease in the volume of the material, while shear deformation involves a plastic flow of the material with or without changes in volume (8). The factors that intervene in this behavior are the magnitude of the load, the inflation pressure of the tires and the speed of movement of the vehicles (9).

There are also other factors that directly contribute to rutting related to the composition of the asphalt mix, such as the low void content, high percentage of asphalt content, the use of an inappropriate asphalt and the use of uncrushed aggregates (10). Finally, there are factors related to the geometric characteristics of the route such as the width of the lane, which influences the transversal distribution of the vehicles, and the longitudinal slope that affects the distribution of the load transmitted by the tires to the pavement (9).

Bibliography

1. DNV. Pavement evaluation instructions. Montevideo : s.n., 2000.
2. Giovanon, Oscar. Rational design models: Failure criteria. Rosario : s.n., 2001.
3. Rico Rodríguez, Alfonso, Téllez Gutiérrez, Rodolfo and Garnica Anguas, Paul. Flexible pavements: Problems, design methodologies and trends. Querétaro: Mexican Institute of Transportation, 1998.
4. Safaei, Farinaz, Castorena, Cassie and Kim, Richard. Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling. North Carolina : Mechanics of Time-Dependent Materials, 2016. Vol. 20.
5. Monteros, Carlos Javier Vasquez. Damage equivalence factors in flexible pavements: analysis for typical conditions in Argentina. Buenos Aires: s.n., 2016.
6. Ogundipe, Olumbide. Mechanical Behaviour of Stress Absorbing Membrane Interlayers. United Kingdom: University of Nottingham, 2012.
7. Gaspar, Matheus, and others. Highly Modified Asphalt Binder for Asphalt Crack Relief Mix. 2017, Transportation Research Record: Journal of the Transportation Research Board, págs. 110–117.
8. Morea, Francisco. Analysis of the rutting behavior of different mixtures in loaded wheel tests according to BS 598-110 and CEN 12697-22. Antigua Guatemala: XVII Ibero-Latin American Asphalt Congress, 2013.
9. Martucci, José Luis and Pastorini, Magdalena. Rehabilitation of rutted pavements. Montevideo: VII Congress of the Uruguayan Highway, 2009.
10. Nikolaides, Athanassios. Highway Engineering: Pavements, Materials and Control of Quality. EUA : Taylor & Francis Group, 2015.