The prediction of fatigue life using the k1-k2 relationship

Fatigue resistance is used in the analysis and design of pavements to predict their life cycle. It is evaluated through time consuming laboratory tests, mainly when performed at very low strain levels. At low strain levels the testing time can last more than one day. Due to the heterogeneity of the material, a large number of samples are tested during days or weeks. The results of fatigue tests are expressed in terms of the number of cycles for the tensile strain level applied. Two constants (k1 and k2) obtained from a statistical analysis take part in this relationship. To know these two constants, at least two fatigue tests are needed, performed at different strain levels. k1 and k2 can be correlated and, in this case, the relationship between the fatigue life and the strain level has only one constant, which can be evaluated using the results obtained by a fatigue test. This paper presents the evaluation of the k1 and k2 relationship for Portuguese mixtures based on the results of 32 different asphalt mixtures composed by four different types of aggregate gradations