Damage-acoustic emission characterization of basalt fiber foam concrete under freeze-thaw environment

To explore the changing rules of the physical and mechanical properties of BFRFC under different freeze-thaw cycles and different fiber contents, Uniaxial Compression-AE tests were carried out to analyze the dynamic change characteristics of the acoustic emission parameters of BFRFC under different freeze-thaw cycles. Based on the cumulative number of acoustic emission events and stress-strain curves, combined with the basic theory of damage mechanics, a compressive damage constitutive model of BFRFC under freeze-thaw cycles was established, and the antifreeze performance of BFRFC under different fiber contents was quantitatively evaluated, in order to provide technical support for the healthy service of BFRFC in freeze-thaw environments.

With the help of acoustic emission monitoring equipment, the acoustic emission characteristics of BFRFC during uniaxial compression with different freeze-thaw cycles and different fiber content were collected. The decrease in mechanical properties of BFRFC after freeze-thaw cycles was analyzed from the perspective of stress-strain and energy release, and the characteristics of the whole process of uniaxial compression of BFRFC were studied. At the same time, with the help of damage mechanics constitutive model, the compressive damage constitutive model of BFRFC under freeze-thaw environment was established based on acoustic emission and uniaxial compression test parameters, and the damage of BFRFC under different freeze-thaw cycles was quantitatively analyzed.

Through the uniaxial compression-AE joint test, various characteristic parameters such as uniaxial compression stress-strain characteristics and acoustic emission characteristics of BFRFC under different freeze-thaw cycles and different fiber content were obtained. The results show:(1) The stress-strain relationship curve of BFRFC during uniaxial compression shows obvious stages, which are mainly divided into dense stage, elastic stage, yield stage and platform stage. The acoustic emission characteristics show three stages: contact period, steep increase period and slow increase period. The acoustic emission characteristics of BFRFC correspond to each stage of uniaxial compression.(2) As the number of freeze-thaw cycles increases, the bearing capacity of BFRFC decreases. The freeze-thaw cycle increases the proportion of pores and strengthens the buffering effect, thereby reducing the brittleness of the sample, resulting in a gradual decrease in the decrease in bearing capacity. Taking the A10-0.45% sample that has experienced 20, 40, 60 and 80 freeze-thaw cycles as an example, the bearing capacity decreases before and after the yield stage are 35.71%, 29.08%, 26.67% and 18.53% respectively.(3) As the basalt fiber content increases, the cumulative ringing value of the sample first decreases and then increases, and the compressive strength of the sample increases significantly. For the A06 specimen, compared with the fiber-free foam concrete, the peak strength of the specimens with fiber content of 0.15%, 0.30% and 0.45% increased by 26.8%, 73.8% and 108.18%; for the A10 specimen, the corresponding increase are 9.5%, 29.62% and 73.78%. As the fiber content increases, the peak strain increase of the A06 sample is higher than that of the A10 sample, and the post-peak curve decreases more gently, showing obvious ductile damage characteristics. The fibers located on the cracking path can slow down the development rate of cracks, thus significantly Increase the peak strength of the sample, however, too high a dosage will cause fiber agglomeration and aggravate internal damage.(4) The pressure damage constitutive model of samples with various amounts of A06 and A10 under different freeze-thaw cycles was established. The fitted correlation coefficient R ranged from 0.921 to 0.994, which has good correlation and can effectively simulate freeze-thaw. Damage characteristics of BFRFC under cyclic effects: BFRFC damage evolves slowly in the initial stage of compression. When the relative peak stress reaches 0.7, the damage development of BFRFC accelerates; the increase in fiber content can significantly reduce the initial freeze-thaw damage variable value of the specimen. This effectively improves the frost resistance of the material; the frost resistance of 0.3% and 0.45% BFRFC is close to and better.Conclusions： The mechanical properties of BFRFC are closely related to density, fiber content, and freeze-thaw cycles. Basalt fiber has a good toughening effect, and low-density BFRFC has higher toughness. The freeze-thaw cycle increases the porosity and strengthens the buffering effect, thereby reducing the brittleness of the sample, resulting in a gradual decrease in the bearing capacity. The acoustic emission characteristics of its uniaxial compression process record the damage and destruction events inside the sample. The results show that there are a large number of acoustic emission events in the elastic and yield stages of the sample, indicating that more deformation and internal damage occurred in the above stages. With the increase of the number of freeze-thaw cycles, the bearing capacity of BFRFC decreases and the activity of the acoustic emission signal weakens. Freeze-thaw erosion leads to a decrease in the elastic modulus of BFRFC, a decrease in the proportion of cement matrix, an increase in crack development, and the signal generated by deformation is difficult to capture. The addition of basalt fiber can slow down the loss of peak strength to a certain extent and improve the ability to resist freeze-thaw erosion. At high density, the samples with 0.30% and 0.45% content have agglomeration. Insufficient fiber agglomeration and stirring will affect the uniform distribution of the fiber, thereby reducing its bridging and crack resistance, resulting in an increase in the peak strength loss rate of 0.45% BFRFC after freeze-thaw cycles.