Application of plateau value to predict fatigue life

Fatigue resistance of asphalt mixtures represents an important parameter for pavement design. This fatigue resistance is calculated through laboratory tests which require some time depending on the strain level applied to the specimen. For very low strain levels, identical to the one installed in the pavement, one test may last more than one week depending on the testing frequency. The time needed for the development of the fatigue law may last long-er. Recent developments introduced an energy approach, based on the ratio of dissipated energy change, which leads to a plateau value that requires short testing to predict the specimen failure. This paper presents the implementation of this approach to evaluate the fatigue resistance of Portuguese asphalt mixtures. Three asphalt mixtures were studied. The first mixture was tested using two strain levels and three specimens for strain level. The second mixture was tested using three strain levels and three specimens per strain level, whereas the last mixture was tested with 18 specimens, 6 per strain level. The approach seems to have potential to be used for fast evalu-ation of the fatigue response of asphalt mixtures

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 rela-tionship. To know these two constants, at least two fatigue tests are needed, performed at differ-ent strain levels. k1 and k2 can be correlated and, in this case, the relationship between the fati-gue 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 dif-ferent types of aggregate gradations

Analysis of the variation in the stiffness modulus through four-point bending tests

Pavement layers are characterized by basic material properties, among which the stiffness modulus and the Poisson ratio assume a preponderant importance in linear elastic anal-ysis. The stiffness modulus is usually assessed through samples extracted from a pavement or prepared in laboratory. The number of specimens used to calculate the stiffness modulus of an asphalt mixture plays an important role in the precision of pavement design. Thus, this paper presents a study to evaluate the number of tests to assess the stiffness modulus of asphalt mix-tures through the four-point bending technique. The results obtained from five different asphalt mixtures were used to evaluate the stiffness dispersion to define the number of tests. A separate study conducted to define the variation of the stiffness modulus allowed to ensure a certain pre-cision in pavement design. This analysis was made for several pavement structures in which the asphalt layer thickness and the subgrade stiffness varied among the samples