Preparation And Properties Of Nanomaterials Modified Fly Ash-Cement Concrete For Pavement

The degradation of concrete pavement performance is mainly due to the damage of concrete materials,including early shrinkage cracking and fatigue cracking in the service process,which leads to the damage of pavement structure.Due to the pozzolanic activity of fly ash,the performance of concrete prepared partially replace cement with fly ash can develop for a long time,so as to resist the degradation of concrete performance caused by external effects.In addition,the introduction of fiber can reduce the risk of pavement shrinkage cracking and fatigue failure.However,the low pozzolanic reactivity of the low calcium fly ash and the weak interfacial adhesion between fiber and cement matrix are the bottleneck problems that limited access to concrete with long-term service performance by using fly ash and fiber,and the construction of long-life concrete road.Nano-materials are expected to improve the pozzolanic reactivity of fly ash and the interfacial adhesion between fiber and matrix,thus improving the strength and crack resistance of concrete,and the long-term service performance of concrete.On the other hand,the improvement of nano-materials for pavement concrete depends significantly on its dispersion.Therefore,the preparation technology of nano-modified fly ash,nano-modified fiber and nano-modified concrete,and the performance improvement law and mechanism of nano-modified concrete are systematically studied in this dissertation.Firstly,in view of the low reactivity of low calcium fly ash in cement-based materials,resulting in low early strength of concrete and slow development of strength,a preparation method for nano-Ti O₂ modified fly ash cement concrete with high efficiency dispersion and utilization of nano-materials and easy construction is proposed.Firstly,nano-Ti O₂ powder was mixed with fly ash,followed by ball milling process,resulting to the uniform adsorption of nano-Ti O₂ powder on the surface of fly ash.Finally,nano-Ti O₂ modified fly ash cement-based material was prepared without changing the traditional construction technology of concrete.This method can well disperse nano-materials and avoid the construction impact of dispersion process.Almost 100%of nano-Ti O₂ particles are nano particles.The adsorption of Nano-Ti O₂ on fly ash structure can give full play to the role of nano-materials,effectively improve the early surface reaction activity of fly ash and the interface binding force between fly ash and cement matrix.The results show that the flexural strength and compressive strength of fly ash cement-based material modified by nano-Ti O₂ with 20wt%of fly ash as cementitious material are increased by 37.74%and39.11%respectively,indicating its potential value in improving the application of fly ash in pavement concrete.Secondly,in order to solve the problem of weak interface adhesion between PVA fiber and cement matrix,a surface treatment method of in-situ growth of nano-Si O₂at room temperature and normal pressure was proposed based on the characteristics of rich hydroxyl groups on the surface of PVA fiber.After the surface treatment,the surface of PVA can be evenly covered with Si O₂ particles with the size of 38nm after12h in-situ growth,so as to realize the surface properties and morphology modification of the fiber.It was found that there was a strong interaction between the fiber and Si O₂ nanoparticles,and the adsorption can be well maintained even under the disruptive ultrasonication treatment.Si O₂ nanoparticles can effectively improve the surface roughness of the fiber from R_q=2.3nm to R_q=13.3nm,and improve the mechanical binding force between fiber and cement matrix.In addition,Si O₂nanoparticles with high pozzolanic activity can participate in the hydration reaction of cement,effectively increasing the density of the interface between fiber and cement matrix.With 1wt%of modified fiber,the ultimate flexural strength and corresponding deformation of the composite are increased by 187%and 980%respectively.We highlight that the proposed method is efficient,room temperature,suitable for PVA fiber surface modification and large-scale production,which provides technical guarantee for improving the application of fiber in concrete.Thirdly,by adding modified fly ash and modified fiber,nano-modified concrete was prepared without changing the traditional construction technology of concrete.The influence of nano-modification on the mechanical properties of concrete was systematically studied,and the relationship between flexural strength and compressive strength of concrete was analyzed.By correlating the microstructure with composition of concrete,the strengthening mechanism of nano modified concrete is revealed.According to the characteristics of pavement concrete subjected to cyclic loading for a long time,the flexural fatigue properties of nano modified concrete were studied.It is found that the interface reinforcement can effectively improve the fatigue performance of concrete.When the required fatigue period of runway is 104,the fatigue limit of nano modified concrete is increased by 23%.It is found that the introduction of fiber can improve the energy dissipation capacity of concrete,and it can be further improved by enhancing the interfacial adhesion,which shows the value of nano-modification to improve the fatigue performance and prolong the service life of pavement concrete.Finally,the cracking performance of concrete under ring confined shrinkage was studied,and the influence of micro-interface nano-modification on the shrinkage cracking resistance of concrete was evaluated.It is found that the use of nano modified fiber can effectively inhibit the shrinkage of concrete and reduce the risk of cracking,and ensure the strength development and environmental corrosion resistance of concrete.COMSOL finite element simulation further uncover that the spatial and temporal distribution of chloride ion in concrete,based on which the influence of nano-modification on the service life of concrete in chloride environment is evaluated.It is found that the pore structure of cement matrix is improved by adding nano modified fly ash.The chloride diffusion coefficient of concrete is reduced by42%,and the service life of concrete is more than 50%higher than that of ordinary concrete under the same thickness of protective layer.It is found that the reliability of nano-modified concrete is 15%higher than that of ordinary concrete.Micro interface nano-modified long concrete used in reinforced pavement can effectively inhibit cracking,resist chloride ion erosion and comprehensive fatigue stress,therefore is an effective strategy for the development of long-life road concrete.