Study On Mechanical Properties And Interface Characteristics Of Urea-Formaldehyde Microcapsules Mbedded In Cement-Based Materials

Nowadays,concrete material is one of the most used construction materials in construction engineering.However,as a heterogeneous porous brittle material,the concrete structure is prone to local cracking or damage.How to reduce and repair cracks has become one of the urgent problems in the construction industry.The emergence of self-healing cement-based materials has provided the construction industry with efficient,energy-saving,and sustainable development methods of restoration.This paper studies the mechanical properties and interface characteristics of ureaformaldehyde(UF)microcapsule self-healing concrete based on the principle of bionics,and intends to analyze and discuss the interaction and influence between the UF microcapsule and cement matrix.The main research contents and conclusions are as follows:(1)Research on the surface morphology and mechanical properties of UF microcapsules: The microcapsules were obtained by SEM analysis as spherical materials with rough surfaces and unevenly distributed particles.Four groups of microcapsule samples treated with different environments were designed for nanoindentation testing,and it was found that the microcapsules treated with simulated pore solution(PH = 13)had the best mechanical properties and showed different deformation and recovery characteristics.(2)Research on the surface morphology and mechanical properties of cement-based composite material samples: A cement-based composite material sample containing only cement paste and UF microcapsules was designed.Based on the SEM image,3D contour scanning,nanoindentation test and nanoscratch test results,it is found that the mechanical properties of the cement matrix are far superior to microcapsules.The mechanical properties of the cement matrix close to the microcapsules have declined,and there is a mechanical property transition zone with a width of about 10-15μm between them,which the mechanical properties are between the cement matrix and the microcapsules.The mechanical properties of microcapsules have a position effect,which is shown as the mechanical properties close to the cement matrix have been enhanced.(3)Discussion on the interface transition zone of cement-based composite samples: Based on nano-scratch test,SEM & EDS spectrum analysis,combined with existing research,it is speculated that there may be an interface transition zone between the microcapsule and the cement matrix.(4)Research on the influence of the cement matrix on the mechanical properties of the microcapsules: Based on the nanoindentation test results at a depth of 5000 nm,it was found that the cement matrix’s restraining effect on the microcapsules was affected by the characteristic size value.After reaching the critical characteristic size value,the closer the indentation point is to the cement matrix or the deeper the indentation depth,the more obvious the influence of the characteristic size.(5)Cement-based composite material FEM simulation and geometric theory calculation analysis: A FEM model was established to simulate the nanoindentation test of microcapsules in cement-based composites.The results show that the cohesive behavior is more suitable with the state of the microcapsules in the cement matrix.The change of the elastic modulus of the microcapsules only affects the magnitude of the stress,and the change of the position of the indentation point will affect the trend of the curve.The simulation results and test results have some errors in the curve and indentation hardness.The geometric model was established,and the instantaneous scratch depth of the microcapsules under ideal conditions was calculated based on the results of the finite element simulation.The calculation results are similar to the test results.