CHEN Yuan-zhao, LI Zhen-xia, FENG Ji-bing, GUO Teng-teng, YAN Hao-bo, WANG Chao-hui, FANG Chen-ze
Basic measures such as optimizing the gradation and vibration compaction were adopted for solving the problems of cracking, low bearing capacity, and short service life of cement in a full-depth cold-recycled base based on the excellent modification functions of nano-materials. Simultaneously, a small amount of nano-materials was added to a cement-stabilized cold recycled base material to improve its road performance. First, powder nanomaterials (nano-SiO2, graphene nanosheets (GNPs), and hydrophilic graphene oxide nano-sheet GO) were dispersed and pretreated. Second, the gradation was optimized and the nano-materials were incorporated into the recycled mixture. The mechanical effects of the contents of nano-SiO2, GNPs, and GO on the cement-stabilized cold-recycled mixture were studied. The drying shrinkage resistance, temperature shrinkage resistance, and frost resistance of the modified cement-stabilized cold recycled mixtures were compared and analyzed. Finally, the pore structure and microstructure of the cement-stabilized cold recycled mixture before and after modification were analyzed by micro-techniques, such as industrial computed tomography (CT) three-dimensional scanning and scanning electron microscopy (SEM) morphology characterization, and the mechanism of the effects of nano materials on the water-stabilized cold recycled base material were revealed from the meso-scale to micro-scale. The results show that improving the gradation composition of the mixture and adopting a skeleton-dense structure, can increase the density of the mixture and significantly improve its unconfined compressive strength. The unconfined compressive strength of the water-stabilized cold recycled mixture after optimizing the mix ratio exceeds 3.5 MPa. By adding 3% nano-SiO2, the residual compressive strength of the specimen after freeze-thaw cycles increased by 33.8%, and the frost resistance coefficient reached 84.41%. When the content of GO is 0.1%, the residual compressive strength of the specimen after freeze-thaw cycles reached a peak of 5.23 MPa, the strength increased by 48.65%, the frost resistance coefficient reached 86.02%, and the frost resistance ability was significantly improved. Nanoparticles can increase the drying shrinkage deformation of the water-stabilized cold-recycled base material, which has an adverse effect on the crack resistance of the base. However, the degree of effect varies greatly owing to the different types of nanomaterials and development stages. Nanomaterials can promote the hydration of cement hydrates and refine the pore structures of mixtures. Nanomaterials can also improve the interface compactness and bonding strength of ITZ structures, such as aggregate-mortar and old asphalt agglomerate-mortar. The incorporation of GO nanosheets had a significant effect on the microscopic pore structure of the water-stabilized cold recycled material, which further refined the larger capillary pores and densified the cement paste, thereby blocking the connections between the capillary pores. Nano-SiO2 can fill small-sized micropores. GO and GNPs materials can fill large voids using their own sheet structures. The staggered three-dimensional structures of GO and GNPs can effectively play a supporting and pulling role when the mixture is subjected to an internal force, and the effects of reinforcement and crack resistance are produced by their ultrahigh mechanical strength.
