Freeze-thaw damage evolution characteristics of foamed concrete based on three-dimensional laser scanning

Freeze-thaw cycle is an important factor affecting the durability of concrete structures. Studies have shown that freeze-thaw has a great influence on the macroscopic mechanical behavior and microscopic pore structure of foam concrete. Surface damage is the most intuitive manifestation of freeze-thaw damage. At present, it is mainly evaluated by empirical methods, lacking quantitative evaluation methods and indicators. At present, the research on the structural surface morphology based on three-dimensional point cloud data is mostly concentrated in the field of geotechnical engineering, and less involved in building materials such as foam concrete. Therefore, it is of great significance to quantitatively evaluate the surface damage degree of foam concrete by three-dimensional scanning, explore the relationship between morphological parameters and macroscopic mechanical properties, and understand the evolution characteristics of freeze-thaw damage of foam concrete.

According to JG / T266-2011 ' foam concrete ', foam concrete with different densities was prepared. Three-dimensional structured light scanning test, uniaxial compression test and relative dynamic elastic modulus test were carried out on foam concrete with densities of 600 kg / cm3 and 800 kg / cm3 in freeze-thaw environment. Three-dimensional point cloud data were processed by Geomagic Studio and Cloud Compare software. The surface morphology and damage characteristics of foamed concrete under freeze-thaw environment were quantitatively analyzed based on the parameters of slope root mean square Z2, structural coefficient SF, roughness profile coefficient Rp and three-dimensional roughness coefficient R3p.

①With the increase of freeze-thaw cycles, the mass loss rate of foamed concrete increases gradually. The mass loss rates of F600 specimens after 20,40 and 60 freeze-thaw cycles are 4.39 %, 6.41 % and 9.46 %, respectively, while those of F800 specimens are 3.05 %, 4.94 % and 7.32 %. ②Freeze-thaw cycles will lead to a decrease in the compressive strength of foam concrete. Among them, the strength change of 600 kg / m3 specimen showed a certain linear characteristic, which gradually decreased from 1.16 MPa to 0.71 MPa, with a decrease of 39.05 %. The compressive strength of 800 kg / m3 specimen decreased slightly at the beginning of freeze-thaw cycle, which was 6.80 %. After 20 freeze-thaw cycles, the decrease of compressive strength began to increase, and the decrease of compressive strength was 37.80 % after 60 freeze-thaw cycles. ③The relative dynamic elastic modulus of foamed concrete decreases gradually with the increase of freeze-thaw times. After 20 freeze-thaw cycles, the relative dynamic elastic modulus of 600 kg / m3 and 800 kg / m3 specimens remained at about 90 %, while the decrease rate of relative dynamic elastic modulus in the first two cycles ( 20 and 40 cycles ) was significantly lower than that in 60 freeze-thaw cycles. ④Before freezing and thawing ( initial stage ), the surface of SF600-1 test block is relatively flat and the fluctuation is small. After 20 freeze-thaw cycles, the surface damage process entered the erosion stage, and the surface of SF600-1 foam concrete showed obvious damage such as erosion and spalling. Large pieces of spalling appeared in the middle of the surface of the test block. The transverse length of the spalling area was 45.55 mm, the maximum spalling depth was 6.16 mm, and the maximum height difference of the contour line was 5.53 mm higher than that before freezing and thawing. After 40 freeze-thaw cycles, the damage process of foamed concrete enters an accelerated stage. The range of spalling area increased significantly, and the lateral length reached 64.16 mm, which was 40.85 % higher than that of 20 freeze-thaw cycles. After 60 freeze-thaw cycles, the damage entered the final stage, the edge of the test block began to appear large volume spalling, and the lateral length of the spalling area increased to 75.70 mm. ⑤The root mean square Z2 of the surface contour slope of F600 foam concrete after 0,20,40 and 60 freeze-thaw cycles were 6.74,13.05,16.68 and 20.62, respectively, and the numerical standard deviations were 0.28,1.17,1.68 and 1.42, respectively. The root mean square Z2 of the surface contour slope of F800 foam concrete under different freeze-thaw cycles is 6.16,9.75,12.10 and 18.78, and the numerical standard deviations are 0.33,1.72,2.28 and 2.19, respectively. Before freezing and thawing, the Z2 value deviation of F600 and F800 foam concrete was 8.56 %, which increased to 25.28 % and 27.46 % after 20 and 40 freeze-thaw cycles, and decreased to 8.94 % after 60 freeze-thaw cycles.Conclusion: ①Freeze-thaw is an important factor in the deterioration of foam concrete. After 20,40 and 60 freeze-thaw cycles, the mass loss rates of 600kg / m3 specimens were 4.39 %, 6.41 % and 9.46 %, respectively, and the relative dynamic elastic modulus were 91.01 %, 86.82 % and 67.11 %, respectively. The mass loss rate of the 800kg / m3 specimen is 3.05 %, 4.94 % and 7.32 %, and the relative dynamic elastic modulus is 91.51 %, 87.78 % and 71.98 %, respectively. ②The surface damage of foamed concrete develops in stages, which can be roughly divided into initial stage, erosion stage, acceleration stage and final failure stage. The failure process shows the characteristics of peeling off layer by layer and gradually developing from the middle to both sides. The two-dimensional contour curve of the specimen surface obtained by three-dimensional structured light scanning can effectively characterize the freeze-thaw damage evolution process of foamed concrete. ③The surface morphology parameters of foamed concrete gradually increase with the increase of freeze-thaw cycles. In the erosion stage and the acceleration stage, the surface damage degree of low-density foamed concrete is greater. After 20 freeze-thaw cycles, the Z2 value of 600 kg / m3 specimen increases by 93.77 %, and the 800 kg / m3 specimen is 58.33 %. ④Among the morphological parameters of foamed concrete, the correlation between the root mean square of slope and the retention rate of compressive strength is the highest, and there is a linear relationship. The gray correlation coefficients under the two densities are 0.662 and 0.741, respectively. It is recommended to characterize the surface morphology of foamed concrete with the root mean square of slope. The relationship between the morphological characteristic parameters of 800 kg / m3 foamed concrete and the compressive strength of the specimens is closer. The grey correlation degree of surface morphology parameters is greater than that of 600 kg / m3 foamed concrete. In addition, the same morphological parameter variation causes greater strength loss on high-density foamed concrete.