The dependence of linear viscoelasticity limits of cold-recycled mixtures on time of curing and compaction method

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Cold-recycled mixtures are currently among the most widely used and investigated methods that enable recycling of old pavement structures in an environmentally friendly manner. Upon milling, the old pavement structure – whose gradation can be improved with addition of virgin aggregate – is mixed and compacted at ambient temperature. The main binding agents are bituminous emulsion and cement. Due to their dual binding behaviour, cold-recycled mixtures present various problems and challenges in terms of correct testing. One of such challenges is the testing of stiffness modulus of cold-recycled mixtures. Apart from dependence of modulus on test temperature and time of curing, recent research proved its highly nonlinear behaviour, much wider than that of classical asphalt mixtures. The paper presents the results of research of linear viscoelasticity limits based on testing conducted in Simple Performance Tester (SPT) device for one cold-recycled mixture recipe, using samples prepared and compacted in different manner. One mixture was prepared in laboratory conditions from materials obtained from the field section. Second mixture was prepared on the basis of field-mixed materials (material prepared and mixed in field, compacted in laboratory). Additionally, cores obtained from the field at 28 and 180 days after compaction were tested. All mixtures were tested at the temperature of 20 °C. Tests were performed 7, 14, 28, 42, 90 and 180 days after compaction. The controlled strain mode was chosen, with strain ranging from 20 to 200 μstrain. Stiffness modulus and phase angle were measured. The test showed that the linear viscoelasticity limits for stiffness modulus testing changed with the time that had elapsed since the compaction for all specimens – the initially low values (of 45–55 μstrain) increased with time and reached constant values (of 75–95 μstrain), with different levels depending on the specimen preparation method.