Freeze-thaw resistance of basalt fiber reinforced foam concrete

Basalt fiber reinforced foam concrete (BFRFC), as a lightweight and environmentally friendly composite building material, has attracted much attention for its excellent mechanical and thermal insulation properties. At present, the damage pattern and cracking characteristics of BFRFC freeze-thaw deterioration are not clear, and there is a lack of analysis on the macroscopic property indexes that can reflect the freeze-thaw damage of the material, such as the water absorption rate, the mass loss rate, and the relative kinetic elastic modulus, so it is necessary to further carry out a complete and systematic investigation based on the effects of density, fiber mixing and the number of freeze-thaw cycles on the durability performance of BFRFC.

Uniaxial compression-acoustic emission tests were conducted on BFRFC specimens with different densities and basalt fiber dosages under different freeze-thaw conditions. The stress-strain curves and energy release characteristics of BFRFC under freeze-thaw conditions were measured with the help of MTS Landmark 370 hydraulic servo testing machine and Sensor Highway III all-weather structural health monitoring system. Based on the distribution pattern of the acoustic emission RA-AF values, the b-value change curve and three macroscopic freezing performance indexes (water absorption, mass loss rate and relative dynamic elastic modulus), the damage characteristics of freeze-thaw deterioration and the changing law of freezing performance of BFRFC were investigated.

The analysis of BFRFC single pumping compression properties, acoustic emission energy release characteristics, and three macroscopic freezing resistance performance indicators (water absorption, mass loss rate, and relative kinetic elastic modulus) under freeze-thaw conditions are shown:(1) The stress-strain relationship curve during uniaxial compression of BFRFC has obvious stages, which can be divided into contact stage, elastic stage, yield stage and platform stage. Under the influence of freezing and thawing, the overall strength of the specimen decreases; and the bridging effect of basalt fibers can effectively reduce the freezing and swelling damage of the material, as well as delay the development of cracks, improve the peak bearing capacity of the specimen, inhibit the loss of peak strength, and play the role of toughening and reinforcing. In addition, the mechanical properties of high-density grade BFRFC are better than those of low-density grade.(2) The freezing action degraded the internal structure of the material, so that the matrix will be the first to slip and crack along the internal weak surface, resulting in an increase in the percentage of shear damage. With the increase of basalt fiber dosage and material density, the percentage of tensile damage increased, in which the percentage of tensile damage increased by 28.0%, 29.8%, 28.9% and 32.4% for 0, 25, 50 and 75 freeze-thaw cycles, respectively, for the 0.45% dosage 1000 kg/m density specimen compared to the 0.15% dosage 600 kg/m density specimen.(3) The acoustic emission b-value change curve can be roughly divided into three stages, corresponding to the elastic stage, yield stage and plateau stage during uniaxial compression: small-scale rupture dominates the material damage in stage I; small-scale and large-scale crack extension alternately dominate in stage II; and large-scale rupture surfaces are formed inside the material in stage III. Freezing and thawing environments can aggravate the internal damage of BFRFC, forcing its internal large-scale rupture phenomenon to occur in advance. The enhancement of fiber doping and material density can effectively inhibit the development of large-scale cracks.(4) With the increase of the number of freeze-thaw cycles, the water absorption and mass loss rate of BFRFC increased, and the relative dynamic elastic modulus decreased. As the density of the specimen increases, the water absorption of BFRFC decreases, the mass loss rate decreases, and the relative kinetic elastic modulus slightly increases. With the increase of fiber doping, the water absorption of BFRFC decreases, the mass loss rate decreases, and the relative dynamic elastic modulus increases. Under 75 freeze-thaw cycles, the mass loss rate and relative kinetic elastic modulus of 1000 kg/m density specimen with 0.45% doping were below 3% and above 70%, respectively, which showed good anti-freezing characteristics.Conclusions: Combined with the analysis results of BFRFC stress-strain relationship curve, acoustic emission RA-AF value point distribution, b-value change curve and macroscopic indexes reflecting freeze-thaw damage, it can be seen that higher density and higher fiber mixing of BFRFC has better frost resistance and durability, and it can better maintain the structural integrity and better resistance to deterioration under the freezing and thawing environment. Overall, the reinforcing effect of basalt fibers effectively compensated for the lack of the original foam concrete's ability to resist freeze-thaw deterioration under extreme conditions. The higher fiber mixing and density level can make it show a broader application prospect in the waterproof protection layer of bridge and tunnel projects, or the surface lining of key parts of hydraulic buildings in cold areas, such as the upstream face of dams, spillways and culvert drainage structures that are frequently affected by freezing and thawing.