Case study: Crack propagation in situ vs. service life prediction

In 2006/2008 a test track with several test fields on a district road in Germany was realised to investigate the influence of different layer thicknesses on the performance of the whole asphalt pavement structure in terms of service life time. One of the test fields was undersized and realised with only two asphalt layers instead of regular three asphalt layers. This was done to provoke an early destruction of the track. The goal was to investigate the influence of a reduced asphalt pavement thickness on service life in just a couple of years and to compare the field data with the predicted service life for the same pavement, especially taking into account fatigue cracks. This case study made it possible to validate and evaluate the applied prediction method, which is currently the only known method of calculating expected service life in terms of years. In 2019 fatigue cracks along the road-way were analysed by cores drilled out from the crack areas. Some cores showed cracks running completely through the entire asphalt construction. Other cores showed cracks that occurred only in the wearing course. To understand the correlation between crack type or crack propagation and stress and strain conditions within the test track the cores, respectively cracks, were secondly analysed by numerical investigations using the finite element method. Therefore the temperature and frequency dependent mechanical performance of the asphalt materials in the different pavement layers was investigated by appropriate laboratory testing methods. The outcomes of these tests were used along with local traffic and temperature dependent conditions as input data for numerical investigations and design life calculations to determine the damage accumulated during service time. Finally the observations in situ were compared with the results of the numerical analyses of the stress and strain conditions in pavement and of the service life predictions.