Compressive strength degradation of engineered bamboo subjected to fungal attack

Glue laminated bamboo (glubam) is a type of engineered bamboo material developed for applications in building structures and interiors. This paper focuses on the fungal (Aspergillus niger) colonization from 14 to 56 days in thick-and thin-strip glubam board with investigation of physical, mechanical (compression), and microcosmic properties. Two-degree of carbonization treatment was employed to improve the antifungal property of the thick-strip glubam. After 56 days of infection, the deep-degree carbonized thick-strip glubam presents better anti-mold properties than medium and non-carbonized specimens. For thin-strip glubam, both parallel and perpendicular to the main bamboo �ber direction were considered. The longitudinal thin-strip glubam retains decent compressive properties, while the transverse specimens stay a stable compressive strength along all fungal tests. The paper reports the experimental values of mass loss, color changes, compressive strengths, modulus of elasticity in compression, and microstructure observations from optical and SEM microscopy at different fungal exposure timespans.


Introduction
In today's push towards a carbon neutral society, bamboo is gaining a wide interest as potential green material for structural applications, due to its properties of fast growth, lightweight, and low-carbon footprint [1][2][3] .As an important type of non-wood forest product, many signi cant industrial applications, in reconstituted forms like laminated panels, have been invented beyond traditional uses of the original round culms.The author's group de ned and developed an engineered bamboo for structural purposes called glued laminated bamboo or glubam, or a bamboo-based glued laminated timber [4][5][6] .The latent capability in the construction area is veri ed 4,[6][7][8][9] .
In nature, moso bamboo (Phyllostachys pubescens) suffers from biologically induced deterioration by fungi such as Aspergillus niger (A.niger), Penicillium citrinum (P.citrinum), and Trichoderma viride (T.virde) [10][11][12] .The mold resistance of glubam is determined by the main ingredient, moso bamboo, which might be more sensitive to fungi than wood-based materials [12][13][14] , given that bamboo is composed of more sugar, starch, protein, and other substances.To determine the degree of fungal induced degradation, mass loss is a typical indicator and results from the degradation of cellulose, hemicellulose, and lignin in bamboo 15 .Xu et al. 16 investigated the chemical structure and microstructure of bamboo, degraded by white-rot and brown-rot fungal for 12 weeks.Several observation methods were used including fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which explained the fungal biodeterioration on moso bamboo speci cally.Meng et al. 17 fabricated a composite based on phenol-formaldehyde resin and bamboo ber for engineered bamboo-related materilas.The biological resistance of the bamboo-based ber reinforced composites was carried out under both white-rot and brown-rot fungi impacts.Several morphological observation methods were employed to analyze the degradation of specimens including uorescence microscope, SEM with energy-dispersive X-ray, and TEM.Kumar et al. 18 analyzed the resistance of bamboo scrimber against white-rot and brown-rot fungi.Whiterot fungi led to a higher average weight loss of specimens than brown-rot fungi.Optical micrographs were demonstrated to show the penetration details of fungal hyphae from the surface into the bamboo scrimber.
Considering the similarity of engineered bamboo-or wood-based materials, the fungal impacts on such materials remain an uncertain issue.Various fungal show different tendencies, resulting in improvement or reduction of the mechanical performances.Liu et al. 19 found that the toughness of bamboo based highdensity polyethylene composites increased with the incubation times of the white-rot fungus, Trametes versicolor.The typical fungus can reduce the rigidity of raw materials, but enhance the pore volumes, leading to a better interlock between bamboo and plastic.Feng et al. 20 gured out that the fugal degradation caused a lower modulus of rupture, tensile strength, and impact strength, but a higher modulus of elasticity on the wood plastic composites.Clemons et al. 21con rmed the decrease in exural strength of wood lled polyethylene composites after the fungal attack.However, the main reason for the reduction was still undetermined because the process of moisture absorption happened during the corrosion process.
With the concern of biological effects of fungal, several treatment methods protect bio-based materials, such as chemical modi cation by fungicides 22,23 or TiO 2 lms 24 , physical modi cation by exterior wood 25 or polymethylsilsesquioxane/Cu-containing nanoparticles xerogel coating 26 , oil treatment 14,[27][28][29] , and heat treatment [30][31][32][33] .Among them, thermal treatment becomes an eco-friendly and effective way to balance fungal durability and mechanical properties of engineered bamboo-based materials.Yang et al veri ed the fungal resistance improvement after saturated vapor thermal treatment, at 190 ℃ for 10 hours, on bamboo scrimbe 33 .They assessed the infected area of the original bamboo scrimber and heat-treated bamboo scrimber with four types of mold and observed once a week for 28 days.The method of carbonization was con rmed to improve the mold-proof behavior of bamboo scrimber without any other treatments.Ogutuga et al. 34 pointed out that during the manufacture of laminated bamboo panels, both thicker laminate and thermal treatment can protect laminated bamboo from brown-rot attack.Shangguan et al. 35 studied the physical and mechanical properties of bamboo scrimber after heat treatment, illustrating that the compressive strength reached a peak when the bamboo was carbonized at 170 ℃.
They gured out that carbonization could solidify the inside adhesives, leading to to enhancement of the physical, mechanical and chemical properties of the bamboo scrimber.Heat treatment can not only help bio-based materials against fungal attacks but also strengthen the stiffness.However, the heat treatment on bamboo-based materials might be strongly adverse.Li et al. 36 found that the bending strength of bamboo bundle curtains decreased by half at 200℃.There can be a reasonable temperature to balance the mechanical properties and anti-fungal behavior.Brito et al. 37 found that the optimal carbonized condition of heat-treated glued laminated bamboo, based on the adhesive of resorcinol-formaldehyde (RF), should be 160 ℃ and lasted for one hour to increase the dimensional stability and better performances of axial compressive and bending strengths, compared to other conditions.Schmidt et al prepared a novel African highland engineered bamboo scrimber was manufactured and tested the soft rot fungi resistance and stiffness after 3 h heat treatment at temperatures of 160, 180 and 200 ℃ heat treatment for three hour 38 .The durability of the bamboo scrimber was maintained as rst class after the fungal attack.Compared to the untreated specimens, carbonization performed a higher level of fungal resistance and better mechanical performances based on bending tests.One thing that must be pointed out is that the fungal decay procedure would be postponed by the thermal progress, but never terminated.
The durability assessment and the carbonization effectiveness for fungal resistance of glubam are still limited.This study focuses on the A. niger deteriorated effects on glubam, which are mainly manifested in the physical and mechanical properties of density, mass loss, changes of surface colors and microstructures, and compressive performances such as compressive strength and modulus.The materials used in this research were non-, medium-, and deep-carbonized thick-strip glubam at two directions, including longitudinal and transverse onces, thin-strip glubam.All specimens were investigated by optical microscope and scanning electron microscopy (SEM), and the compressive properties was explained by quasi-static compression tests.

EXPERIMENTAL PROGRAM
The fungal attack tests were carried out with A. niger for glubam specimens.The quasi-static compressive strength and modulus of elasticity of glubam boards with deferent degrees of fungal attack were examined using universal loading testing machine.

Preparation of engineered bamboo specimens
The preparation of glubam, contained a two-step lamination process 1,7,9 .The rst process is to integrate the bamboo strips into the large size and standardized board, through a hot-pressure process, under ~5 MPa pressure and 150 ℃ temperature, which owns a typical plate size of about 2000 to 2500 mm in length (or longer), and 600 to 1200 mm in width.The second process is pressure lamination of the elements cut from the boards at room temperature.The essential ingredients of glubam were 3 to 6 years old moso bamboo (Phyllostachys pubescens), urea formaldehyde adhesive (for thick-strip glubam) and or phenol formaldehyde adhesive (for thin-strip glubam).The second process is to produce structural components using engineered bamboo boards by a cold-press lamination process with two-component epoxy or phenol adhesives.In this study, the specimens were made by cutting the engineered boards.The thick-strip laminated glubam board is made by pressing several layers of 5~8 mm thick and 200 mm wide bamboo strips, while the thin-strip laminated glubam sheets are made by laminating ~2 mm thick bamboo strip mats to have a total thickness of about 10 to 35 mm.The thin strips are netted and dried to reduce their water content below 10 ± 2%.All the strips are saturated in adhesives for about 2 to 3 minutes, and consequent by drying again to control the moisture content in a range of 16 ± 2%.The two types of glubam specimens are illustrated in Fig. 1.The thick-strip glubam shown in Fig. 1 (a), nominated as G1, had six layers aligned in parallel to bamboo ber with a total thickness of 30 mm.The thin-strip glubam designated as G2, contains 15 layers of orthometric thin bamboo mats, in which the longitudinaltransverse ratio of bamboo ber is 4:1, shown in Fig. 1 (b).All the specimens owned the same geometric dimensions of 30 mm × 30 mm × 45 mm.The direction of parallel-to-grain bamboo bers was also the long side of G1 specimens.
Three degrees of carbonized conditions were used to produce G1 specimens.Based on the different carbonized temperatures, G1 specimens can be categorized as non-carbonized G1 (G1 n ), mediumcarbonized G1 (G1 m ), and deep-carbonized G1 (G1 d ).The medium degree of steam heat treatment on G1 materials was at 175 ℃ for 90 mins and the deep one was 185 ℃ for 90 mins.As for G2 specimens, there was no extra carbonized process conducted but the orthometric cutting directions were considered amongst G2 materials, shown in Fig. 1. (b).When the main bamboo ber direction was longitudinal as the long side of specimens, the G2 specimens can be de ned as G2 l , whilst, the transverse specimens can be de ned as G2 t .The physical details of all specimens can be illustrated in Table 1.
Before consequent fungal attacks, all oven-dry specimens were sterilized at 121 ℃ for 27 mins, under the same moisture content environment.From the controlled group without fungal attack, the average moisture content was 3.29% (SD = 0.08) for G1 n , 3.15% (SD = 0.36) for G1 m , 2.73% (SD = 0.26) for G1 d , 3.63% (SD = 0.37) for G2 l , and 3.56% (SD = 0.41) for G2 t .Due to the bio clean requirement of the experiments, the sterile program was taken as the conserving program to control the moisture contents of all the specimens.The testing matrix is shown in Table 2.A total of 120 specimens were applied for fungal observation and mechanical tests.Each case involved 24 specimens, 12 of which for fungal impacts, and another 12 for the compression tests.For thick-strip glubam, the compression tests were not conducted for the compression tests along the transverse orientation, since the bamboo strips are con gured only in the longitudinal direction 39 , which is the main direction for its application.

Preparation of potato dextrose agar medium (PDA)
According to GB/T 13942.1 40 and ASTM G21 41 standards, fungal cultures were maintained on potato dextrose agar medium (PDA; Solarbio, China).23 g agar was mixed with 500 ml ultra-distilled water and heat treated until completely melted.The solution was sterilized at 115 ℃ for 20 mins.The details are shown in Fig. 2 (a).

Preparation of river sand sawdust substrate
The cultivated container was a medical stainless steel disinfection box with 30 cm long, 20 cm wide, and 5 cm deep.According to GB/T 13942.1, one part of the river sand sawdust substrate was constituted by 150 g clean river sand with 20 to 30 as the mesh number, 15 g wood sawdust, 1 g brown sugar, and 8.5 g corn our 40 .The other part of the substrate was made up of a solution of dextrose (4.31 g) and 600 ml of ultradistilled water.A total of 12 feeder bamboo strips were cut to a size of 50 mm × 20 mm × 2 mm for each culture glubam specimen, which was located on the surface of the substrate.Both parts were sterilized at 121 ℃ for 27 mins, which can be illustrated in Fig. 2 (a).

Mold infection and cultivation experiment
According to the GB/T 18261 42 , A. niger was selected and inoculated on the PDA under sterile conditions and cultivated in the incubator at 28 ± 2 ℃, 75 ± 2 % relative humidity (RH) for 7 to 10 days 40 .The sterile puncher with a diameter of 4 mm was collected to drill 9 fungus blocks to control the total fungus amount.The fungus blocks should be buried 5 mm below the surface, shown in Fig. 2 (b).Once the surface of the river sand sawdust substrate was covered with fungus, which took another 7 to 10 days to grow, the sterilized glubam specimens were placed on the top of feeder bamboo strips and the cultivation happened after the stainless container lidded.To concentrate more on the fungal incubation from glubam panel surface, the cutting faces were warped in plastic lm.The specimens and A. niger were cultivated in an incubator at 28 ± 2 ℃, 80 ± 2 % RH for 56 days.All infected operations were carried out within a productprotective horizontal laminar-ow clean bench (Heal Force Ltd., Shanghai).The effect of infection time was also conducted and 6 specimens from each case were selected to test both fugal in uences (3 specimens) and the compressive properties (the other 3 specimens) after being infected for 14, 28, 42, and 56 days, respectively.At the end of each period, fugal was removed from the glubam surface by alcoholwet wipes.The specimens were weighed again to calculate the mass loss (ML, %) according to Eq. ( 1), where, M 1 is the matter before incubation (g) and M 2 is the matter after incubation (g).

Quasi-static compression tests
Compression tests in the z-axis (main bamboo ber direction) were performed as shown in Fig. 3, according to ASTM D 143 43 .A universal testing machine with 100 kN capacity was employed for quasistatic compression tests.The loading was applied continuously throughout the test at the same rate of 2 mm/min to measure the compression strength.The modulus of elasticity in compression was analyzed by 2-cycle loading, all G1 and G2 l specimens were loaded from 4000N to 8000N, whilst G2 t specimens were loaded from 2000N to 4000N, based on the ultimate compressive loads of the specimens 44 .The following Eq.( 2) 45 and Eq.(3) 44 are applied to assess the compressive behaviors, where,   is parallel to grain compressive strength in MPa;   is the maximum load value in N; b is the width of the specimen in mm; t is the thickness of specimen in mm;   is the modulus of elasticity;  is the range of the loadings from the upper and lower limits, and  is the increment of the corresponding strains.To collect the strain values during the compressive test, two orthogonal strains were used on each side of the specimens, which indicates that a total of 8 strain gauges recorded the data.

Optical Microscopy
Amongst three specimens for fungal observations from each type of glubam specimens at each period, two blocks were picked to cut along (45 mm × 30mm × 10 mm) and perpendicular (30 mm × 30mm × 10 mm-15 mm) to the long side direction separately.A diamond wire cutting machine (STX-202A, Shenyang Kejing Auto-instrument Co., Ltd.) was used to slice up the specimens.After drying the sections naturally, optical microscopy photographs were taken with the Nikon microscope and camera (LV150NA; Nikon) to examine the decay of the longitudinal and transverse slices.

Scanning Electron Microscopy
The control and fungi-exposed specimens were cut into 5 mm × 5mm × 2 mm (width × length × thickness) slices from the infected surface of each case under various time spans by diamond wire cutting machine.Slices were cleaned with alcohol-wet wipes and dehydrated in a dryer at 60 ℃ for 30 mis.After the pretreatment, the slices were carefully mounted on SEM stubs.A Carl Zeiss SEM micrograph machine (GeminiSEM300; Germany) was used for electron microscopy observation.

Mass loss
After different fungal exposure tim, the mass loss and the average value for each glubam specimen are shown in Fig. 4. The error bars in Fig. 4 represent the standard deviations based on six specimens.It should be pointed out that the specimens from G1 n and G1 d exhibit a slight increase in weight after a 14day infection.The high temperature and humidity for the cultivation of A. niger weight led to an increasement of the weights.During the rst 14-day infection, the weights of specimens were in uenced by the growth of fungus, which consumed the nutrition from both the substrate and specimens.The weight gained from fungus and the lost from the nutrition of bamboo resulted in uncertain changes amongst specimens.Considering the carbonized effects on glubam, compared with the mass loss of G1 n , the fungal resistance of G1 m was limited, whilst specimens of G1 d demonstrated effective protection from the fungal attack.From the 14 to 28 days of fungal cultivation, G1 n and G1 m groups lost weight signi cantly and then decreased slowly after 28 days.As the exposed time goes by, the average mass loss of G2 specimens remained similar from 14 to 56 days, but the standard deviations showed higher uctuation than G1 materials.Although there is no extra mold-proof method taken place in G2 specimens, the bamboo mats were covered by adhesive to maintain the stability of thin-strip glubam.At the end of fungal decay, the average mass loss of four types of glubam, except G2 l , became roughly the same, which showed a 5.36% weight loss.As for the bamboo-based materials, the carbonization or arrangement of bamboo strips had a minor in uence on the mass loss when fungi (A.niger) exposure continued long enough.

Color changes
Fungal deterioration can cause permanent discoloration on the glubam surface and interior, as shown in Compared with G1 specimens, the surface color of G2 specimens was less obvious and discrete.

Compressive behavior
The experimental results from compressive tests after fungal attacks are shown in Table 3 for the compressive strength and modulus of elasticity.Comparisons among different glubam materials and different fungal attack time can be conducted.
Fig. 6 illustrates the typical failure modes of thick-and thin-stirp glubam materials.The delamination was noticed for all types during the tests, which leads to splitting of the bamboo strip layers.Due to the onedirection attack, the fungal contact surface of glubam specimens exacerbated the property of asymmetry, which leads to eccentric compression.The type of wedge splits was also noted in both top and bottom of specimens.The out-of-plane ber buckling failure occurred in G1 specimens, whereas the in-plane ber buckling failure was carried out in G2 l materials.For G2 t specimens, the cracks extended porous defects without adhesive.The temporal in uences on G1 and G2 specimens were limited from 14 to 56 days of infection but compared with the control group without fungal tests, more cracks after failure would be found.In addition, the carbonized pretreatment appears to have a slight in uence on the failure modes, which led to more signi cant splits.
The compressive strengths can be assessed by Eq. ( 2), and shown in Fig. 7. Compared with the results of control group, the mold impacted on thick-and thin-strip glubam obviously.G1 and G2 l specimens have a strength degradation after the rst 14-day fungal tests, reaching 38.54% (G1 n ), 42.09% (G1 m ), 45.03% (G1 d ), and 25.47% (G2 l ), respectively, whilst, G2 t only degraded by 20.20%.Notably, the compression strengths of G1 m , G1 d , G2 t specimens showed a slight increase as time went by.Considering the growth of A. niger, the specimens experienced the larger numbers of fungus after longer time, indicating that less nutrition from glubam can feed them.After the turning point of compressive strengths, such as 14-day infection for G1 d and 28-day infection for G2 l , some A. niger lost activities, and deep-carbonization strengthen thick-strip glubam by diminution of the content of hemicellulose, pentose and other chemical materials, which would restrict the fungal development.It is suspected that the growth of A. niger lled in the porous bamboo fabric structures resulted in the increased compression strength of G2l from 28 days to 56 days of infection.The heat treatment played a crucial role in the mold-proof characteristic from the view of compression strength results, in which the G1 d remained as much as possible compressive resistance after 56-day fungal deterioration.
Fig. 8 shows the modulus of elasticity in compression and averages evaluated in glubam, based on the calculation of Eq.3.Besides G1 n , the nal remained modulus of elasticity presented similar, which were 8.68 GPa (G1 m ), 8.69 GPa (G1 d ), 10.14 GPa (G2 l ), and 8.35 GPa (G2 t ), respectively.The fungal attack modulus of elasticity in compression on G1 n specimens was demonstrated destructively.The modulus of elasticity in compression of medium carbonized thick-strip glubam remained stable during the against of A. niger, indicating that the timespan effect on G1 m was limited.The compression property of G1 d after 14day exposure presented an irregular behavior, which increased at 28 and 42 days then and decreased rapidly.Compared to the untreated specimens, medium and deep carbonized pretreatments provided protection from fungal deterioration.
As for thin-strip glubam, the average modulus of elasticity in compression performance of G2 l showed uncertain tendencies from the infection start, caused by the huge variations amongst three replicates in each group.The quality control of longitudinal thin-strip glubam should be considered more in further works.Taking the medians of G2 l specimens into account, the peak arrived at a group of 42-day attacks, which means G2 l specimens would survive more values of compression modulus to resist a longer time of fungal deterioration.The modulus of elasticity in compression of G2 t increased after 28 days, similar to the performances of compression strengths but showing more signi cance by the modulus.
With the application of a total of eight stain gauges glued on the side faces of each specimen, Poisson's ratio can be calculated from Eq. ( 4).It is notable that the existing uncertainty to assess the Poisson's ratio of anisotropic materials, glubam.According to Eq. ( 4), a simpli ed experimental test was carried out.
Where  is the Poisson's ratio,   is the transverse strain, and  is the axial Due to the fungal attack pattern, one side was exposed, which indicates that the after-corrosion specimens suffered from eccentric compression.To simplify the calculation, the data from two orthometric strain gauges on each side averaged, listed in Table 4. Fungal degradation had minor impact on the Poisson's ratios of ve groups.However, a higher degree of carbonization led to a large value of Poisson's ratio.

Optical microscopy
The optical micrographs of ve groups are exhibited in Fig. 9, in which the specimens from each group with various infection timespans were sliced by a diamond wire cutting machine along both laminated direction (x-z plane) and transverse direction (x-y plane) in Fig. 9 (a) and (b), respectively.Considering the dimensions of slices, a 5× lens was used to look into typical surfaces.The color differences from various carbonized degrees and various timespans of G1 specimens are di cult to be distinguished.However, increasing black stains by fungal infection is signi cant with a longer time of G1 groups in Fig. 9 (a), whilst, it is dramatically observed from the rst 14 days to the end of infection of G1 n in Fig. 9. (b).
Compared with G1n and G1 m , transverse optical micrographs, the fungal affected areas are limited after 56-days of exposure.
As for G2 specimens, the porous manufacturing structure led to huge space for A. niger to live.For example, in Fig. 9 (b), G2 l -56d captured the prospect white or yellow hypha, which would turn to black 46 .
With the characteristic of A. niger, the type of mold was con rmed that it would produce both cellulase and β-xylanase 47 , which indicates that the fungus consumed the nutrition and left holes in specimens, like G1 m -42d.It is worth noticing that the capacity of offering the cellulase enzyme by A. niger decreased the mechanical properties of bio-based materials more.
The details of A. niger impacts on bamboo are shown in Fig. 10, in which both longitudinal and crosssection optical micrographs are considered.It is noticeable that the optical micro photos of cross-sections are more visible than those of radial sections.For bamboo bers, Fig. 10 (a) and (b) illustrate that A. niger can degrade cellulose, where black spots existed in the ber bundles.The inside of G1 m-1 -56d was covered with fungus not only on the matrix but also on the pictured void in Fig. 10 (d).

SEM microscopy
After 28 and 56 days of exposure, the SEM images of ve groups of glubam specimens were taken and shown in Fig. 11.The fungus of A. niger colonized on the glubam surface and inside parenchyma cells (matrix) and conductive tissues (voids) 48 , shown in Fig. 11 (a) to (c).The surface of specimens was cleaned by alcohol cleansing wipes but the hypha was distributed everywhere and greew up from holes of G2 l -28d.Compared with G1 n and G1 m specimens, the fungus developed mildly in G1 d , of G2 l and G2 t indicating that the deep carbonization and the orthometric ber manufacture restrained the growth of A. niger.
On the other hand, the colonization of A. niger is unnoticed by the end of the experiment compared with the micrographs of 28-day infected specimens, demonstrated in Fig. 11 (f) to (j).After 56 days the decay was slow due to the lack of nutrition to cultivate fungus.The micro views pointed out another important information that the A. niger was capable of producing cellulase.The appearance of bamboo bers became rough and uneven, and the boundaries of parenchyma cells were blurry.Those are the evidence of damage to the bamboo structures.More transparent cracks and gaps were emerging in Fig. 11 (g), (h), and (j), indicating the failure happened in the longitudinal in the rst period and then transversely.
The longitudinal-section SEM micrographs of ve groups of glubam at the end of fungal tests are shown in Fig. 12.After an 56-day fungal attack, the results indicate that both cavity formation and erosion decay took place, where the cell walls thinned and discrete notches appeared, as shown in Fig. 12 (a).The colonization of fungal hyphae resulted in disorder and unsystematic ber bundles, due to the consumption of matrix rst in Fig. 12 (b).Most cell walls are affected and fragmented.From the longitudinal view of the G1 d specimen, the inner structure of the parenchyma cell in the green circle was noticed.Based on the bidirectional ber manufacture, the surface of the selected G2 l specimen remained smooth and wrapped in the matrix, compared with other groups in Fig. 12 (d).After 56 days advanced stage of decay was observed, the cell wall was eroded, due to the extensive degradation in Fig. 12 (e).

Discussion
Aspergillus niger can cause permanent discoloration on the glubam surface, which leads to the fungus called black mold.According to the color changes, a computational approach was used to qualify the area of infection, based on MATLAB.(2022a).The processing procedure is illustrated in Fig 13, by taking the specimens of G1 m group for example.The original photo of the exposure surface was taken under similar surroundings and uniform light irradiation.All the original photos were converted into gray images, which extracted the most important information about black areas.Based on the gray photo, the grayscale contour can be inducted numerically.
For example, a total of six specimens is demonstrated in Fig. 13, each grayscale contour was aligned and departed into three segments.Small values of the gray scale represent possible bamboo nodes or defects as the dark zone.The large values from the bright zone of grayscales were recognized as the bamboo surface remained the original color.Besides, the surface degradation was gured out in the selected zone.The thresholds between the two sectors would be reconstructed for each group.
In terms of computational programs, the surface degradation can be assessed in Table 5.With the fungal attack continued, G1 n performed a constant increase in the color change areas.The peak ratio of infection to whole surface areas assessed by computer appeared after different fungal colonized timespans.During the rst 14 days of tests, G2 l performed better fungal resistance than other groups, in which the other four groups showed roughly the same area of infection.The limited infected areas of G1 m and G1 d groups can explain the tendency of compressive properties from the discolored point of view, which indicates that the mechanical estimate of antifungal glubam specimens can be veri ed by picture processing.Carbonization appears to be an effective the pretreatment method to protect glubam from A. niger impacts.The large ratio of color changes on transverse thin-strip glubam occured in every fugal experimental stage, due to the poriferous alignment structure offering abundant living spaces for A. niger.
Regression analyses of the relations between surface degradation portions from the image estimation by computer and ultimate strengths by compressive tests can be shown in Fig. 14.The 95% con dence interval of each group is demonstrated by the shadow area.It is noticeable that the compressive strengths of specimens decreased obviously, mainly due to the large area of fungal infection.The decreasing tendency illustrates the possibility of connecting the degradation areas to strengths directly.However, it is troublesome to quantify the strengths by linear equations accurately, with the consideration of the unsatis ed coe cient of determination and 95% con dence interval, shown in Fig. 14.The reliability of the tting curve is limited by the consideration of irregular surfaces, especially for G2 specimens.In further works, promising tting equations could be modi ed by more testing specimens and a better imagination procedure.

Conclusions
In this study, the effect of fungal (Aspergillus niger) degradation on the compressive performances of glubam was studied, in which the heat-treated thick-strip and orthometric thin-strip glubam were considered.A series of physical, mechanical and microcosmic observations were carried out for ve types of glubam, including non-carbonized thick-strip glubam (G1 n ), medium-carbonized thick-strip glubam (G1 m ), deep-carbonized thick-strip glubam (G1 d ), longitudinal thin-strip glubam (G2 l ), and transverse thinstrip glubam (G2 t ).The main conclusions can be drawn as follows: 1.The mass loss of each type of glubam material was uncertain.Specimens from thick-strip glubam performed an analogous tendency in mass loss, where the mass lost rapidly from 14 to 28 days of infection and arrived at the peak after 42 days.The mass loss of G2 l became irregular from the beginning to the end.On the contract, the mass loss of G2 t remained stable as time went by.
2. Aspergillus niger was veri ed to cause surface color changes in every group of glubam materials.The heat treatment protected thick-strip gluban from dramatic discoloration, compared G1 m and G1 d to G1 n specimens.The surface of thin-strip glubam remained the most original surface color and discrete speckles appeared rather than continuous black colonies in G1 specimens.A consequent computational method by MATLAB (2022a) was applied to assess the infected areas quantitatively.
3. The compressive behaviors of glubam are conducted, including failure modes, compressive strength and modulus of elasticity in compression.The thermal treatment in uences the failure modes of thick-strip glubanm nitely, whilst the transverse glubam showed more cracks than longitudinal specimens.
4. As for compressive strength, due to the activity of fungus, the rst 14-day fungal attack all specimens extremely.The deep degree of carbonization helped thick-strip glubam survive from serious destruction of compressive strength.A slight increase emerged after 28 days of infection like G1 d and G2 l , due to the growth of fungus ful lled the original drawbacks of glubam materials.Compared to G2 t specimens, G2 l kept a higher level of compressive strength.However, the merit of G2 t material is that the effects of A. niger on compressive strength are slight.
5. The optical micrographs of longitudinal and transverse sections from character glubam slices were conducted.The existence of A. niger was more likely to be detected in the transverse graphs than the longitudinal ones from the rst 14 days to the end of fungal tests.From megascopic optical observations, fungus generated cellulase enzymes that expanded in bamboo bers rather than parenchyma cells.
. The morphological characteristics of glubam decay by A. niger were investigated using SEM.It is remarkable the fungal hyphae grow up in bamboo matrix and bers, come out of the defects of glubam, and adjoined on vessels.Both cavity formation and erosion decay happened.

Declarations
Author's contributions         Compression strength of glubam related to time.
Modulus of compression of glubam related to timespans.

Fig 5 .
Fig 5. Signi cant color changes were observed on specimens infected by A. niger after 14, 28, 42, and 56 days.With the prolonged exposure time of the fungus, increasing areas of fungal infection were seen by color changes.Remarkably, the discolored places appeared near the bamboo nodes more likely among G1 specimens when the infected surface layer of bamboo contained bamboo strips with nodes.A larger and darker color change area was found on the surface of G1 n , compared with G1 m and G1 d .The discoloration on the surface of G1 d specimens was lighter, whereas the splits by degumming occurred more frequently.

Figures
Figures

Figure 2 Preparation
Figure 2

Figure 4 Time
Figure 4

Figure 9 Optical
Figure 9

Table 4 .
CQC conceived and designed the overall study and helped to analyze the results and draft the manuscript.SJZ carried on the fungal experimental work.YBHK carried on the mechanical experimental work.TJ helped with the fungal experimental work.WWH supported the fungal experimental work and reviewed the paper.YTH helped with the fungal experimental work.DWZ supported the fungal experimental work and reviewed the paper.YX conceived, designed and coordinated the overall study, helped to analyze the results, and revied the paper.All authors read and approved the nal manuscript.Average Poisson's ratio of glubam.

Table 5 .
Surface color degradation area portions.