Abstract: To meet the mechanical performance requirements of foam concrete as the filling material for the reserved deformation layer of tunnel surrounding rock support, a novel deformable filling material called micro carbon nanotubes reinforced fly ash foam concrete (CNTAFC) was developed. The preparation parameters of CNTAFC were optimized by establishing a compressive strength prediction model using the Box-Behnken Design response surface methodology based on single factor analysis. Furthermore, the load deformation characteristics of CNTAFC specimens were investigated through a combination of uniaxial compression tests and digital speckle correlation measurement (DSCM). Results demonstrate the reasonability and effectiveness of the CNTAFC compressive strength regression model. With a CNT (carbon nanotube) content of 0.13%, a bone-cement ratio of 24.75%, and a fly ash content of 70.57%, the predicted 28-day compressive strength is 5.936 MPa, with an error of 2.54%. The CNTAFC sample exhibits improved peak strength and enhanced ductility. The average post-peak strength exceeds 60% of the peak strength, and the post-peak strain range accounts for over 30% of the ultimate strain. These characteristics satisfy the performance requirements for pressure relief and energy absorption in surrounding rock. The localized initiation of CNTAFC deformation can be distinguished under strain and stress conditions, of which the strain condition is more appropriate for determining deformation localization. When deformation localization is activated, the CNTAFC strength approaches its peak strength, and the Poisson's ratio increases significantly.

Key words: foamed concrete, carbon nanotubes, fly ash, preparation, deformation properties, response surface method, compressive strength