Comparative Analysis of Freeze–Thaw Effects on the Parallel-to-Grain Compressive Properties of Bamboo and Chinese Fir

Abstract

To evaluate the application potential of bamboo in cold regions, this study systematically compared the differences in the effects of freeze–thaw cycles on the longitudinal compressive properties of moso bamboo (Phyllostachys edulis) and Chinese fir (Cunninghamia lanceolata). By subjecting the materials to 0, 5, and 10 standard freeze–thaw cycles, the evolution patterns were analyzed from three aspects: mechanical properties, failure modes, and apparent color. The results show that bamboo exhibits significantly superior freeze–thaw resistance: after 10 cycles, bamboo retained 95.4% of its compressive strength (decreasing from 50.2 MPa to 47.9 MPa), whereas the strength of Chinese fir decreased by 14.2% (from 46.7 MPa to 40.0 MPa). The elastic modulus of bamboo remained stable, while that of Chinese fir decreased by 30.86%. Load–displacement curves revealed that bamboo displayed a ductile plateau after failure, whereas Chinese fir exhibited a linear drop-off. Analysis of failure modes further highlighted the intrinsic differences between the materials: bamboo primarily underwent progressive buckling of fiber bundles, forming typical accordion-like folds; Chinese fir mainly showed brittle failures such as end crushing and longitudinal splitting. Color characterization indicated that the lightness index L of the bamboo outer skin (bamboo green) decreased by 26.1%, while the chromaticity indices a (red) and b* (yellow) increased significantly, showing the most notable changes; the color of Chinese fir and the bamboo inner skin (bamboo yellow) remained relatively stable. This study demonstrates that natural bamboo outperforms Chinese fir in terms of frost resistance, toughness, and strength retention in the short term. The findings provide important experimental evidence and design references for promoting the application of bamboo in engineering projects in cold regions.

1. Introduction

Against the backdrop of the growing global demand for sustainable and renewable materials in the construction industry, bio-based composites have emerged as a significant research focus due to their environmental friendliness and carbon sequestration potential [1]. Bamboo, especially natural bamboo known for its rapid growth and high strength-to-weight ratio, is regarded as a highly promising green building material [2]. Although engineered bamboo products (such as laminated bamboo and bamboo scrimber) manufactured through gluing and reconstitution processes have partly addressed the variability of natural bamboo and have demonstrated excellent mechanical performance in load-bearing components like beams and columns [3,4,5,6], their long-term durability—particularly performance degradation under coupled complex environmental conditions—remains a key scientific challenge limiting their large-scale engineering application [7].

Existing research on bamboo durability has primarily focused on individual or dominant environmental factors. For example, studies have employed outdoor exposure [8], boiling tests [9], and hygrothermal cycling [10] to investigate the effects of UV radiation or hygrothermal aging on its properties. These studies indicate that aging leads to degradation of the main chemical components of bamboo (cellulose, hemicellulose, and lignin), resulting in significant reductions in strength and stiffness [11,12,13]. The failure mechanisms involve fiber fracture, matrix cracking, and interface delamination, among others [2,14,15,16]. However, freeze–thaw cycling—critical for building structures in cold regions—induces damage through the repeated freeze-expansion of internal moisture, generating mechanical stresses that cause microcrack initiation and propagation [17]. This mechanism is distinctly different from photo- or thermal aging. Yet, research in this area remains relatively scarce. Current literature predominantly focuses on UV or hygrothermal aging of engineered bamboo [18,19], while studies on the performance evolution of natural round bamboo under freeze–thaw conditions are notably insufficient. Moreover, there is a lack of systematic comparative analysis with traditional building materials widely used in cold regions [20,21,22]. Such comparisons are essential for understanding how differences in material microstructure (e.g., bamboo’s fiber bundle structure versus wood’s tracheid structure) influence resistance mechanisms to freeze–thaw damage.

Despite the wealth of knowledge on engineered bamboo, a notable gap exists in the systematic evaluation of natural round bamboo’s resilience to specific climatic challenges, especially in cold regions. Freeze–thaw cycles, characterized by repeated freezing and melting of water within the material’s microstructure, can induce significant mechanical stress, leading to cracking, spalling, and ultimately, a loss of structural integrity. This degradation mechanism is fundamentally different from the UV and hydrothermal aging previously studied. Understanding the freeze–thaw resistance is paramount for the reliable design and application of bamboo in temperate and alpine climates.

To address this research gap, this study systematically investigates the effects of freeze–thaw cycles on the mechanical properties of natural bamboo. Freeze–thaw cycling experiments were designed (−20 °C freezing/20 °C thawing, with 0, 5, and 10 cycles) to evaluate the freeze–thaw durability of natural Moso bamboo (non-engineered bamboo), using Chinese fir as a control. Following the national standard (GB/T 1927.11 [23]), changes in longitudinal compressive strength and modulus of elasticity for both materials were measured before and after freeze–thaw exposure. The evolution of failure modes was documented in detail, and material surface color changes were quantitatively characterized using the CIE Lab color system. The findings are expected to provide crucial experimental data and theoretical foundations for accurately evaluating the suitability of bamboo in cold regions, offering important references for the safety design and durability maintenance of bamboo structures in such climates.

2. Materials and Methods

2.1. Materials and Specimen Preparation

This study used natural bamboo and Chinese fir as the research materials. The bamboo specimens were obtained from 4-year-old moso bamboo (Phyllostachys edulis), with the nodal regions excluded during sampling to ensure material uniformity by using only internodal sections. The Chinese fir specimens were selected from defect-free, straight-grained sound wood. All specimens were processed into rectangular samples measuring 20 mm (radial) × 20 mm (tangential) × 30 mm (longitudinal) in accordance with relevant standards, as shown in Figure 1.

Figure 1. Dimensions of test specimens for Chinese fir and bamboo.

To investigate the effect of freeze–thaw cycles on material properties, three levels of freeze–thaw cycles were established: 0 cycles (i.e., no freeze–thaw treatment, serving as the control group), 5 cycles, and 10 cycles. Six replicate specimens were prepared for each experimental condition to ensure data validity and statistical reliability. The specimen labeling convention was as follows: based on the number of freeze–thaw cycles, bamboo specimens were labeled as B00, B05, and B10 (where “B” denotes bamboo, and “00”, “05”, “10” correspond to 0, 5, and 10 freeze–thaw cycles, respectively); Chinese fir specimens were correspondingly labeled as CF00, CF05, and CF10 (where “CF” denotes Chinese fir). This labeling system allows clear and accurate identification and classification of specimens during subsequent testing and data processing.

Prior to testing, the initial moisture content of each specimen type was measured. The average initial moisture content was 13.6% for bamboo and 10.2% for Chinese fir.

2.2. Accelerated Aging: Freeze–Thaw Cycles

This freeze–thaw cycle test was conducted with reference to relevant standards for concrete and wood materials. As shown in Figure 2, the entire process consisted of three stages: pre-conditioning, freeze–thaw cycling, and post-test conditioning. First, to standardize the initial state of the specimens, all specimens were pre-conditioned at a temperature of 20 ± 2 °C and a relative humidity of 65 ± 5% for 14 days to allow their moisture content to reach equilibrium. Subsequently, the freeze–thaw cycling stage began. Each cycle consisted of 12 h of freezing at −20 ± 2 °C, followed by 12 h of thawing in a constant-temperature water bath at 20 ± 2 °C. This 24-h period constituted one complete cycle, with the respective specimen groups undergoing 0, 5, or 10 cycles. After the cycling was completed, to eliminate the influence of moisture content variations on the mechanical test results, all specimens were again placed in an environment of 20 ± 2 °C and 65 ± 5% relative humidity for 14 days. This conditioning ensured that subsequent performance tests accurately reflected the effects of freeze–thaw aging rather than the influence of moisture changes.

Figure 2. Process of freeze–thaw cycles.

2.3. Color Characterization

Bamboo and wood are bio-based materials with a natural feel. Their unique colors and textures provide sufficient visual stimuli, which are closely related to human visual psychology. Wood color is one of the physical quantities representing the surface visual characteristics of wood and is also a sensitive indicator of its natural aging. The color change depends on the chemical composition of the wood and the degree of natural aging. Color evaluation is often based on the CIE standard colorimetric system (Figure 3), which primarily includes the lightness index (L*), the red–green chromaticity index (a*), and the yellow-blue chromaticity index (b*), to analyze the patterns of color change in wood after natural aging. In the CIE (L*a*b*) system, L* is the lightness index, representing the brightness coordinate of an object’s color in an approximately uniform three-dimensional space; a* and b* are the chromaticity indices. Color measurement was performed using a handheld grating spectrophotometer. The lightness, hue, and saturation of the test specimens were measured. The CIE system exhibits perceptual uniformity along all axes in its three-dimensional color space. It is categorized into scales for lightness index and chromaticity indices.

Figure 3. CIE L*a*b* color space diagrams.

For the natural bamboo specimens, as they are untreated and naturally consist of two distinct surfaces—the bamboo rind (outer green layer) and the bamboo culm (inner yellow layer)—color characterization tests were conducted separately on both the rind and culm surfaces. Twelve random measurement points were tested on each surface, and the average value was taken as the representative value. For the Chinese fir specimens, measurements were taken on the side surface of the specimens. Similarly, twelve random measurement points were set, and the average value was selected as the final representative value.

2.4. Mechanical Testing

In accordance with the specifications of the Chinese National Standards GB/T 1927.11-2022 “Physical and mechanical test methods for small clear wood specimens—Part 11: Determination of compressive strength parallel to grain” and GB/T 15777-2017 “Method for Determining the Modulus of Elasticity in Compression Parallel to Grain of Wood” [24], the standard processed specimens were tested to determine their compressive strength and modulus of elasticity parallel to the grain. As shown in Figure 4, this test utilized an electronic universal testing machine with a capacity of 2 tons. The loading rate was set at 1 mm/min, applying a load uniformly and continuously along the specimen’s longitudinal grain direction until failure occurred. The ultimate compressive strength parallel to the grain was calculated based on the maximum load recorded at the moment of specimen failure.

Figure 4. Compression test parallel to grain.

For this experimental study, the focus is on the ultimate longitudinal compressive load (P_max), ultimate compressive strength (f_max), and elastic modulus (E) during the elastic stage of the specimens. The ultimate load and strength can be read directly from the testing machine. For the determination of the modulus of elasticity, an extensometer with a 10 mm gauge length was used to accurately measure the deformation of the specimen within the elastic stage. During the testing process, the extensometer was carefully removed when the load reached approximately 60% of the estimated maximum load to ensure the safety of the instrument. The modulus of elasticity in compression parallel to the grain was calculated according to the standard specifications. This involved selecting the linear segment of the stress–strain curve between the upper limit of 0.4 times the maximum load (0.4 f_max) and the lower limit of 0.2 times the maximum load (0.2 f_max). The final value was determined by the ratio of the stress increment to the strain increment within this selected range.

3. Results and Discussion

3.1. Typical Failure Modes

For Chinese fir, as shown in Figure 4, the typical characteristics of failure modes mainly involve the buckling, bending, or shear fracture of fibers due to instability. Buckling Failure (Figure 5a): Some specimens exhibit distinct “washboard-like” folds. This is a typical failure mode under longitudinal compression. When the pressure reaches its limit, the cell walls of the wood buckle longitudinally due to instability, which macroscopically appears as fiber folding. This indicates that the material has good toughness and does not fracture suddenly. End Crushing and Splitting (Figure 5b): The ends of some specimens are crushed, accompanied by longitudinal cracks. This occurs because the friction between the specimen ends and the pressure machine plates constrains the transverse expansion of the wood, leading to internal tensile stresses that cause splitting. Shear Failure (Figure 5c): Some specimens (such as the one in the lower right corner) exhibit fracture surfaces at approximately 45-degree angles. This results from shear stresses along the grain direction under longitudinal compression exceeding the transverse tensile strength or longitudinal shear strength, leading to shear slip along wood rays or weak planes.

Figure 5. Typical failure mode of Chinese fir. (a) Buckling Failure, (b) End Crushing and Splitting Failure, (c) Shear Failure.

As can be seen from Figure 6, all three typical failure modes occurred under each test condition. However, with the increase in the number of freeze–thaw cycles, the occurrence of (Figure 5b) End Crushing and Splitting Failure and (Figure 5c) Shear Failure increased, indicating that the lateral tensile capacity of the Chinese fir specimens was compromised.

Figure 6. Failure mode of Chinese fir.

For bamboo, as shown in Figure 7, the primary failure modes are as follows: Buckling Failure (Figure 7a). This is the most typical ductile failure mode of bamboo under longitudinal compression. The bamboo is compressed into distinct “wrinkles” or an “accordion” shape. This occurs primarily when the longitudinal compressive load reaches its limit, causing the thin-walled cell walls within the bamboo to first undergo elastic instability, leading to the collapse of the cells. This failure process is progressive and absorbs a significant amount of energy. The better the ductility of the bamboo, the more pronounced and dense the wrinkles become. This is considered a ductile failure. Splitting Failure (Figure 7b). During compression, longitudinal cracking occurs simultaneously. This is mainly due to the end constraint effect: friction between the compression plates of the testing machine and the ends of the specimen restricts the transverse expansion of the bamboo under compression (Poisson effect). This constraint generates transverse tensile stress within the specimen. Since bamboo has weak transverse (radial or tangential) tensile strength, longitudinal splitting results. Shear Failure (Figure 7c). The failure in most specimens is not a single mode but a composite one. For instance, a specimen may initially undergo local crushing, forming wrinkles, followed by stress redistribution that leads to shear fracture in another part.

Figure 7. Typical failure mode of Bamboo. (a) Buckling Failure, (b) Splitting Failure, (c) Shear Failure.

Figure 8 presents all the failure modes of the bamboo specimens. As can be seen from the figure, compared with the Chinese fir specimens, the failure mode of bamboo was predominantly Buckling Failure, while the other two failure modes—End Crushing and Splitting Failure and Shear Failure—were observed much less frequently.

Figure 8. Failure mode of Bamboo.

3.2. Color Change

As shown in Figure 9, changes in the color of all materials were observed after freeze–thaw cycles, specifically manifested as a numerical decrease in lightness (L* value). The initial lightness varied among the materials: Chinese fir (CF) had the highest initial L value (71.23), followed by bamboo culm (BC) (70.67), while bamboo rind (BR) showed the lowest initial L value (52.22).

Figure 9. L* of bamboo and Chinese fir with different cycle times.

After 10 freeze–thaw cycles, the L value of Chinese fir decreased from 71.23 to 63.62, a reduction of 10.69%; the L value of bamboo culm decreased from 70.67 to 63.45, a reduction of 10.22%. After only 5 cycles, its L value dropped from 52.22 to 38.60, a reduction of 26.07%. As the number of cycles increased, the differences in lightness among the materials showed a decreasing trend numerically. Overall, based on the data, bamboo rind appeared more responsive to freeze–thaw cycles, while the change in L value for Chinese fir was relatively small.

As shown in Figure 10, the a* value, representing the red–green chromaticity, exhibited an upward trend across all materials after freeze–thaw cycles, which corresponds to a color shift toward red in the observed data. The extent of this change differed among the materials. Bamboo rind (BR) showed the largest numerical increase, with its a* value rising from 9.20 to 10.61, an increase of 15.27%. Chinese fir (CF) followed, increasing from 7.90 to 8.80, or 11.38%. In comparison, bamboo culm (BC) displayed the smallest numerical change, with its a* value increasing only from 6.23 to 6.39, a rise of 2.65%. Throughout the testing, the initial color differences were maintained in the data: BR retained the highest a* value (most reddish), followed by CF, while BC remained the least reddish.

Figure 10. a* of bamboo and Chinese fir with different cycle times.

As shown in Figure 11, after freeze–thaw cycles, the three materials showed different patterns of variation in the color parameter b, which reflects changes in the yellow-blue dimension (an increase in positive value indicates a shift toward yellow). In terms of the observed data, Bamboo Rind (BR) exhibited the largest numerical change, with its b value increasing from 30.88 to 36.26 after 10 cycles, a relative increase of 17.41%, indicating a noticeable shift toward yellow. Chinese Fir (CF) also showed an increase in b* value, from 29.43 to 31.46, a rise of 6.89%, suggesting a degree of yellowing. In contrast, Bamboo Culm (BC) displayed the smallest overall numerical change, following a non-linear trajectory: its b* value first decreased from 27.41 to 26.22 after 5 cycles, reflecting a temporary shift toward blue, before rising to 27.73 after 10 cycles. The net increase was 1.17%, indicating relatively minor change in b* value for this material under the tested conditions.

Figure 11. b* of bamboo and Chinese fir with different cycle times.

3.3. Mechanical Properties

Figure 12 and Figure 13 show the compressive load–displacement curves and the average curves of bamboo and Chinese fir under different numbers of freeze–thaw cycles, respectively. As can be seen from Figure 12, bamboo initially exhibits an elastic stage under compression. When the load approaches approximately 80% of the peak load, the curvature of the curve begins to decrease, entering a “softening” stage. After the peak, the curve drops slightly and then enters a relatively long plateau stage. Notably, as the number of freeze–thaw cycles increases, the rate of decline after the peak load becomes more pronounced.

Figure 12. Compression load–displacement curve of bamboo. (a) B00, (b) B05, (c) B10.

Figure 13. Compression load–displacement curve of Chinese Fir. (a) CF00, (b) CF05, (c) CF10.

Figure 13 shows the compressive load–displacement curves of Chinese fir under different numbers of freeze–thaw cycles. Similar to bamboo, Chinese fir exhibits an elastic stage followed by a softening stage before reaching the peak load. However, after the peak point, the load–displacement curve of Chinese fir shows a continuous downward trend, without a plateau region similar to that observed in bamboo.

Figure 14 presents the average load–displacement curves of bamboo and Chinese fir under various working conditions. From the curve shapes, it can be observed that after different numbers of freeze–thaw cycles, the ascending branches of the bamboo curves show little difference and are nearly coincident. However, after passing the peak load, the specimens subjected to 10 freeze–thaw cycles begin to decline first, followed by those with 5 cycles, while the reference group (0 cycles) exhibits the most delayed descent. This trend is also reflected in the displacement corresponding to the peak load (ΔB00 = 1.74 mm, ΔB05 = 1.54 mm, ΔB10 = 1.27 mm). Furthermore, the steepness of the descending portion of the curves increases with the number of freeze–thaw cycles, in the following order: 10 cycles > 5 cycles > 0 cycles.

Figure 14. Average load–displacement curve in compression test.

For Chinese fir specimens, the overall trend of the load–displacement curves is similar to that of bamboo, but noticeable differences appear even in the initial loading stage: the initial slope of the curves decreases as the number of freeze–thaw cycles increases, i.e., CF00 > CF05 > CF10. Unlike bamboo, the displacement corresponding to the peak load shows a positive correlation with the number of freeze–thaw cycles (Δ_CF00 = 1.27 mm, Δ_CF05 = 1.46 mm, Δ_CF10 = 1.60 mm). After reaching the peak load, the slope of the descending portion also gradually increases with more freeze–thaw cycles, indicating that freeze–thaw action exacerbates the brittle characteristics of Chinese fir during the failure stage.

In summary, the primary influence of freeze–thaw cycles on both materials is manifested in the post-peak mechanical behavior. Moreover, as the number of freeze–thaw cycles increases, the load-bearing capacity of the materials declines more rapidly during failure, and brittle behavior becomes more pronounced.

As shown in Figure 15 and Table 1, freeze–thaw cycles were observed to exert a clear negative effect on the compressive strength parallel to the grain of both bamboo and Chinese fir. Specifically, the ultimate compressive load and compressive strength of both materials exhibited an overall decreasing trend as the number of freeze–thaw cycles increased. Throughout all stages of freeze–thaw exposure, the absolute strength of bamboo remained consistently higher than that of Chinese fir

Figure 15. Ultimate load (strength) between bamboo and Chinese fir under different cycles.

Table 1. Comparison of test indexes of compressive mechanical properties parallel to grain.

Number of Cycles	Bamboo			Chinese Fir
	Pmax (N)	fmax (MPa)	E (GPa)	Pmax (N)	fmax (MPa)	E (GPa)
0	20,765.7	51.9	6.3	18,710.5	46.8	9.6
	19,801.7	49.5	11.6	18,922.0	47.3	11.2
	20,299.9	50.7	6.4	18,246.0	45.6	12.4
	20,592.1	51.5	7.9	18,928.3	47.3	8.9
	19,148.9	47.9	8.4	18,752.2	46.9	9.6
	19,766.3	49.4	10.5	18,480.7	46.2	8.9
Ave	20,062.4	50.2	8.5	18,673.3	46.7	10.1
CV	3.01%	2.99%	25.31%	1.43%	1.44%	13.92%
5	19,738.7	49.3	9.6	16,997.8	42.5	6.2
	19,568.8	48.9	5.4	16,616.2	41.5	6.2
	21,310.7	53.3	10.2	18,469.8	46.2	8.2
	20,982.2	52.5	9.8	18,461.5	46.2	8.4
	20,979.5	52.4	10.6	17,925.1	44.8	7.7
	20,433.6	51.1	8.2	16,530.9	41.3	6.5
Ave	20,502.2	51.3	9.0	17,500.2	43.8	7.2
CV	3.50%	3.54%	21.50%	5.12%	5.19%	15.57%
10	20,383.1	51.0	10.0	15,777.0	39.4	6.3
	19,993.0	50.0	9.4	15,882.2	39.7	6.2
	18,980.7	47.5	10.0	16,564.6	41.4	7.5
	17,646.4	44.1	8.6	16,125.5	40.3	7.2
	19,292.3	48.2	9.1	16,033.1	40.1	7.0
	18,602.3	46.5	9.0	15,737.0	39.3	7.7
Ave	19,149.6	47.9	9.4	16,019.9	40.0	7.0
CV	5.13%	5.17%	6.04%	1.91%	1.93%	9.81%

Regarding compressive strength, the average ultimate load of bamboo was 20,062.4 N at 0 cycles, corresponding to a strength of 50.2 MPa. After 5 freeze–thaw cycles, the strength slightly increased to 51.3 MPa (load: 20,502.2 N). This minor increase might be attributed to material variability, and the performance can be generally considered stable during this initial phase. However, after 10 cycles, the strength markedly decreased to 47.9 MPa (load: 19,149.6 N), representing a decrease of approximately 6.6% compared to the initial strength. This indicates that bamboo possesses a certain resistance to freeze–thaw damage initially, but with increasing cycles, its internal structure gradually deteriorates, leading to a significant reduction in load-bearing capacity.

In contrast, Chinese fir demonstrated greater sensitivity to freeze–thaw cycles. Its initial strength was 46.7 MPa (load: 18,673.3 N). After 5 cycles, the strength decreased to 43.8 MPa (load: 17,500.2 N), a reduction of about 6.82%. After 10 cycles, the strength declined further to 40.0 MPa (load: 16,019.9 N), marking an additional 8.46% drop compared to the strength after 5 cycles. From 0 to 10 freeze–thaw cycles, the total strength reduction for Chinese fir reached 16.5%. This shows that its performance begins to decline from the early stages and continues progressively, with a greater overall reduction than observed in bamboo. Specifically, after 10 cycles, bamboo retained 95.6% of its initial strength, whereas Chinese fir retained only 85.8%, highlighting bamboo’s superior resistance to freeze–thaw damage.

As shown in Figure 16 and Table 1, in terms of elastic modulus, bamboo exhibited a unique response pattern: its average elastic modulus increased from 8.52 GPa at 0 cycles to 9.47 GPa after 5 cycles, an increase of approximately 11.2%. From 5 to 10 cycles, it slightly decreased to 9.35 GPa, with a minimal change of only 1.3%, indicating that its performance had entered a stable phase. Furthermore, the CV of bamboo’s elastic modulus gradually decreased with progressive freeze–thaw cycles (from 13.92% to 9.81%), suggesting reduced variability among specimens and a trend towards more uniform and stable structural behavior.

Figure 16. Elastic modulus between bamboo and Chinese fir under different cycles.

In contrast, the elastic modulus of Chinese fir showed a significant decline: it dropped sharply from 10.10 GPa at 0 cycles to 7.20 GPa after 5 cycles, a decrease of 28.7%. From 5 to 10 cycles, it decreased slightly further to 7.00 GPa, with the rate of decline slowing to 3.1%. Its CV also gradually decreased from an initial 13.91% to 9.81%, reflecting that freeze–thaw cycles led to more consistent performance among specimens, albeit at an overall degraded level.

4. Discussion

It is crucial to acknowledge that the findings presented here primarily reflect the initial and intermediate phases of freeze–thaw damage. Extrapolating these results to predict long-term performance (e.g., after 50 or 100 cycles) would be speculative, as damage accumulation beyond 10 cycles may become non-linear, involving more complex interactions such as the coalescence of microcracks. Therefore, the superior performance of bamboo observed in this study is indicative of its better early-stage durability compared to Chinese fir, but its performance over a full service life requires further long-term investigation.

The damage caused by freeze–thaw cycles primarily originates from the volume expansion of water upon freezing, which generates immense pressure on the cell walls, leading to the initiation and propagation of microcracks. Repeated cycling exacerbates these damages, thereby weakening the load-bearing capacity of the cell walls, which macroscopically manifests as reduced strength.

Bamboo contains inherent microscopic pores and initial defects. During the first few freeze–thaw cycles, the micro-stresses generated by the freezing expansion of water might have induced an “adjustment” effect on the microstructure of bamboo. This could be analogous to the slight strength gain sometimes observed in certain concrete materials during early freeze–thaw exposure. Another possibility is that the bamboo specimens were not in a fully stable state before testing; the initial cycles might have caused internal stress relief or structural stabilization, leading to the observed increase in elastic modulus. In the initial state (0 cycles), significant variability existed among different specimens due to bamboo’s natural heterogeneity (e.g., density, fiber distribution, microstructural differences). The freeze–thaw cycles acted as a “homogenization” or “aging pretreatment” process, applying identical environmental stress to all specimens. Weaker initial parts might have been “strengthened” or “eliminated” (if damaged severely, they would appear as outliers), and all specimens reached a more consistent “stable state” after cycling. This process reduced the differences between specimens within each group, resulting in more concentrated data.

Chinese fir is a porous material that absorbs significant moisture into its pores and cell cavities before freeze–thaw testing. When the temperature drops below freezing, the water within the pores freezes and expands by approximately 9%. This expansion exerts tremendous tensile stress on the wood’s cell walls (particularly the weaker middle lamella), leading to the formation and propagation of microcracks. As the number of freeze–thaw cycles increases, these microcracks continuously connect and expand, severely compromising the skeletal structure responsible for wood’s rigidity, consequently causing a substantial reduction in elastic modulus. The most significant damage often occurs during the initial cycles. This is because the wood structure is most intact initially, with ample internal moisture, making the destructive effect of ice crystal expansion on the sound cell walls most pronounced. This explains the “cliff-like” drop in elastic modulus observed between 0 and 5 cycles. After 5 cycles, major cracks have likely initiated, and the structure has sustained a certain level of damage; subsequent cycles cause relatively less additional damage, leading to a flattening of the performance decline curve.

Bamboo possesses a highly oriented fibrous structure (vascular bundles). These fiber cells have thick walls and a dense structure, providing excellent longitudinal mechanical properties. This structure allows bamboo to better resist damage during the initial stages of freeze–thaw cycling, demonstrating superior durability. Chinese fir, as a softwood, has a relatively simpler cellular structure with thinner cell walls. Consequently, it is more susceptible to damage under freeze–thaw action, leading to earlier and more significant performance degradation.

In summary, freeze–thaw cycles reduce the compressive strength parallel to the grain of both bamboo and Chinese fir. Chinese fir exhibits higher sensitivity to these cycles compared to bamboo, with strength declining earlier and more rapidly. Bamboo demonstrates superior freeze–thaw resistance and strength retention ratio. After 10 cycles, its strength advantage over Chinese fir becomes even more pronounced.

Secondly, the comparison of failure modes and the displacements corresponding to the peak load clearly reveals the significant differences in toughness between the two materials. As shown in Figure 17, the failure of Chinese fir is mainly characterized by macroscopic cracks, such as crushing at the ends and longitudinal splitting; after cyclic loading, some macroscopic drying shrinkage cracks even appear at the top of the Chinese fir specimen. In contrast, the failure of bamboo primarily manifests as progressive buckling of fiber bundles, forming typical “accordion-like” folds. This failure mode can absorb a substantial amount of energy and is considered a ductile failure, indicating that bamboo can undergo significant plastic deformation during the failure process without sudden fracture. This property is highly advantageous for structural safety under accidental overload or impact loading. Fundamentally, this difference stems from bamboo’s stronger and more continuous cellulose fiber structure compared to that of Chinese fir.

Figure 17. Transverse cracks in Chinese fir.

Furthermore, color change serves as an important visual indicator of surface aging in materials and provides additional evidence. The color parameters (L, a, b*) of the bamboo green layer changed most significantly, indicating that its surface chemical components (such as chlorophyll and phenolic compounds) are the most sensitive to freeze–thaw cycles. In comparison, Chinese fir and the bamboo yellow layer exhibited relatively better color stability. This suggests that when bamboo is used in architectural aesthetics and may be exposed to freeze–thaw environments, attention must be paid to its appearance changes—particularly the bamboo green surface, which will undergo noticeable visual alterations. This should be fully considered during the initial design stage.

5. Conclusions

This study systematically investigates the axial compressive properties of round bamboo and Chinese fir under freeze–thaw cycles, leading to the following main conclusions:

Different patterns of mechanical property degradation: Freeze–thaw cycles negatively affect the mechanical properties of both materials, but in distinct ways. Chinese fir exhibits a significant synchronous decline in both strength and elastic modulus, consistent with the freeze–thaw damage behavior of traditional porous materials. In contrast, round bamboo demonstrated excellent strength retention, with a strength retention rate as high as 95.6% after 10 cycles, and its elastic modulus showed an overall increase, rising from 8.5 GPa to 9.4 GPa after 10 cycles, despite minor fluctuations during initial cycles. Overall, in the short term, round bamboo exhibits superior freeze–thaw durability compared to Chinese fir.

Failure modes reveal material toughness: Analysis of failure morphology indicates that Chinese fir failure is primarily characterized by macroscopic cracks, reflecting brittle behavior. Round bamboo, however, mainly fails through progressive buckling of fiber bundles—a ductile failure mode capable of absorbing substantial energy. This highlights its inherent superior toughness and suggests better safety reserves under unexpected loads.

Variation in performance dispersion: As freeze–thaw cycles progress, the dispersion of mechanical test data for both materials increases, revealing the heterogeneous nature of freeze–thaw damage. Interestingly, the standard deviation of round bamboo’s elastic modulus decreases with increasing cycles, implying that the freeze–thaw process may have a homogenizing effect on its microstructure.

Differential sensitivity in apparent color change: Color characterization results show that the bamboo green layer is most sensitive to freeze–thaw cycles, with a sharp decrease in lightness (L* value). The color of Chinese fir and the bamboo yellow layer remains relatively stable. This provides a reference for evaluating the appearance durability of materials used in cold regions.

The findings of this study indicate that round bamboo is a more promising structural material than Chinese fir for civil engineering in cold regions, particularly in applications requiring resistance to initial freeze–thaw damage. However, the results are based on a limited number of cycles. Engineers should be cautious when extrapolating these findings to long-term performance (e.g., decades of service) without further validation. It is recommended that future research and relevant design codes consider the effects of more severe freeze–thaw exposure.