Forming Method,mechanical Properties And Economic Evaluation Of Cold Recycled Mixture Using Foamed Asphalt

Asphalt pavements in China have now entered the repair and maintenance phase.In this phase,70～90 million tons of Reclaimed Asphalt Pavement(RAP)will be generated every year.The recycling of asphalt pavement has become a hot topic of asphalt pavement research in recent years.Hot recycled asphalt pavement is the more common and traditional method for engineering applications,but the method requires heating of aggregates and asphalt,which causes environmental pollution by emitting large amounts of greenhouse gases.This is contrary to the original intention of protecting the environment and saving resources.Foam asphalt cold recycling is a recycling technology produced at room temperature without heating the aggregate and recycling RAP material.This method reduces greenhouse gas emissions,and its good environmental attributes have made it popular in recent years.However,the practical application of this mixture indoor forming method is found to be incompatible with the field compaction process,and scholars rarely study the performance decay and economics of this material during use in the seasonal freezing zone.In this study,the effect of "hot secondary compaction" on the cold recycled foam asphalt mixture during paving was analyzed by coring on-site and monitoring the temperature at different depths of the pavement.The volumetric and mechanical properties of the indoor specimens were compared with the corers to determine the indoor secondary forming method and the curing conditions.In order to determine the decay behavior of the foam asphalt cold recycled mixture in the seasonal freezing zone,the specimens were formed by the secondary forming method and subjected to different numbers of freeze-thaw cycles.Finally,the economics and environmental friendliness of the cold recycled foam asphalt mixes during the life cycle were analyzed by using the whole life cycle cost analysis method.The main conclusions are as follows.In practice,the foam asphalt cold recycled mix is subjected to "hot secondary compaction" from the hot mix,which softens the asphalt in the mix and the aged asphalt on the RAP surface,enhancing the bonding effect,reducing the void ratio of the mix,and improving the splitting strength.By comparison,the secondary large Marshall compaction(130+20 times)can ensure the full improvement of the asphalt bond while avoiding the destruction of the bonding interface between the aggregate and the asphalt mastic,which is a more recommended indoor forming method.After the temperature test of the pavement in the field and the performance comparison of the specimens,the maintenance temperature of 60℃ is more consistent with the field conditions.In the indirect tensile strength test,the main cracking area of the foam asphalt cold recycled mix was within 4 cm of the center in the horizontal direction.In the indirect tensile fatigue test,the mixture received less stress,resulting in a larger crack extension,with the main cracking area being within 6 cm of the center in the horizontal direction.The change of the horizontal strain characteristic cloud of the main cracking area was analyzed,and the fracture energy was more characterized the crack resistance of the cold recycled foam asphalt mixture,and the fracture energy had a good correlation with the void ratio,with the increase of the number of freeze-thaw,the void ratio increased,the fracture energy decreased,and the change was gradually reduced.The crack resistance index can better characterize the crack expansion rate,which is related to the pore angularity,and the pore angularity decreases with the increase of freeze-thawing times,and the crack expansion rate gradually becomes slower.The exponential model can better predict and reflect the decay condition of the crack resistance and fatigue life of cold recycled foam asphalt with freeze-thaw action.This study analyzes the field forming mechanism of foam asphalt mixtures,proposes an indoor forming method consistent with the field compaction effect,evaluates its mechanical properties under freeze-thaw conditions and its environmental and economic performance during the whole life cycle,and provides theoretical support and empirical reference for its use in the monsoon zone.