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Maderas. Ciencia y tecnología

On-line version ISSN 0718-221X

Maderas, Cienc. tecnol. vol.21 no.3 Concepción July 2019

http://dx.doi.org/10.4067/S0718-221X2019005000306 

ARTICULOS

Effect of brown rot degradation on mass loss and compressive strength of Chinese poplar (Populus simonii)

Huadong Xu1 

Yanan Di1 

Jed Cappellazzi2 

Jeffrey J. Morrell3 

1College of Engineering & Technology, Northeast Forestry University, Harbin, China

2Department of Wood Science & Engineering, Oregon State University. Oregon, USA

3National Centre for Timber Durability and Design Life, University of the Sunshine Coast, Queensland, Australia

Abstract:

The wood of poplar species is generally perceived as susceptible to decay, however, poplar is still widely used as columns in traditional Chinese buildings. Understanding how decay affects the compressive properties of this material will help engineers better assess wood condition during routine inspection and maintenance. The effects of decay on compressive properties of Chinese poplar were explored using a brown rot decay fungus (Gloeophyllum trabeum). Changes in compression strength were fairly linear and more closely correlated with mass loss (R 2= 0,75). The results suggest that residual compressive strength could be roughly predicted using wood density as a surrogate measure.

Keywords: Biodeterioration; decay fungi; Gloeophyllum trabeum; mechanical properties; Populus simonii.

Introduction

Decay fungi can have profound effects on wood properties at early stages of decay. Mass loss as low as 1 to 3% can result in bending strength loss approaching 60 to 80% (Wilcox 1978, Yang et al. 2006). Brown rot decay fungi tend to be associated with more rapid loss of wood strength properties than white rot fungi, due to their ability to randomly cleave cellulose chains far in advance of fungal growth (Zabel and Morrell 1992). Bending and tensile strength are among the properties commonly used to assess the effects of fungal attack on wood properties; but wood is used in numerous loading situations where other properties are important (Jin et al. 1988, Curling et al. 2002, Ge et al. 2016, Liese and Stamer 1934 (as cited in Brische et al 2008), Malda et al. 2015, Winandy and Morrell 1993, Witomski et al. 2016).

Timbers are often used in columns where compression and bending properties are both important (Forest Products Laboratory 2010). Unlike tension or bending strength, compressive strength tends to be more closely related to density and should therefore be more closely correlated with mass loss (Forest Products Laboratory 2010). This would render the decay strategy differences of white and brown rot fungi less important. However, the tendency of brown rot fungi to depolymerize cellulose may have other, more subtle effects on compressive strength (Jin et al. 1988).

Poplars are commonly planted in North China where their fast growth and ability to withstand harsh conditions make them an attractive species for reforestation efforts. Poplars are generally perceived to be non-durable, but they have a long history of use as columns in traditional Chinese structures where they are exposed out of soil contact and generally protected from wetting. However, decay does occur in these structures and it is important to understand how fungi affect column properties.

The goal of this project was to evaluate the effects of a brown rot fungus on compressive strength parallel to the grain of Chinese poplar (Populus simonii).

Materials and methods

Small clear beams of Chinese poplar (Populus simonii, Carrière) specimens were cut from a 1 m-long green log section. The log was cut 1,3m above the ground from a 30-year-old poplar tree harvested from Dailing Forest in the Grand Khingan Mountains located in northeast China. Twenty one samples of air-dried P. simonii were cut to 20 by 20 by 50 mm long. The samples were oven dried (104 °C) and weighed (nearest 0,001 g) before being sterilized by heating at 121 °C for 90 minutes.

Decay chambers were prepared by adding 15 g of Chinese poplar sawdust and 8,5 g of corn meal to 150 g of clean river sand (AQSIQ SAC 2009a). The mixture was placed into a 500 ml Erlenmeyer flask. One hundred ml of a mixture containing 9,4% malt extract and 1 g of unrefined cane sugar were added to the sand mixture. Four pieces of poplar (20 by 20 by 5 mm long), each with a small hole drilled on the surface were placed onto the sand surface to serve as feeder strips for the test fungus. The flasks were sealed with tight fitting cotton plugs before being sterilized by heating at 121 °C for 90 minutes and allowed to cool. The plugs limited the risk of contamination but allowed for some air exchange.

The test fungus (Gloeophyllum trabeum (Pers.ex. Fr.) Murr. (Isolate # 5,98 Northeast Forestry University, Harbin , China) was grown on 1,5% potato dextrose agar until it completely covered the media surface. Small discs were cut from the edge of the actively growing culture and placed into the holes drilled into the poplar feeder strips. The flasks were covered and incubated at 25 °C and 70% relative humidity until the fungus covered the feeder strip surfaces. The test blocks were placed onto the feeder strips (cross section face down) and the flasks were incubated at 25 °C and 70% RH for 15 to 90 days.

The effect of fungal attack on wood properties was assessed at 15-day intervals by removing eight test samples that were oven dried and weighed as previously described. The difference between initial and final weight loss was used to calculate mass loss. The top 20 mm of each block was cut off and the remainder was conditioned to constant weight at 23 °C and 65% RH (to an approximate moisture content of 12%). The remainder of each block was used for microscopic examination.

Compression parallel to the grain was evaluated following procedures described in Chinese Standard GB/T 1935-2009 (AQSIQ SAC 2009b). Briefly, the specimen was placed on a CMT-6305 Testing machine (SUNS Company, Zhuhai, China) and load was applied to the cross section at a rate of 10 mm/minute. Load and displacement were continuously recorded and the test continued until the specimens reached the yield limit. Compressive strength was then calculated using Equation 1:

(1)

Where: σw is the compressive strength parallel to grain at w % of moisture content in the sample, MPa; P max is the maximum load, N; b is specimen width, mm and t is specimen depth, mm.

The compressive strength data were then adjusted using Equation 2:

(2)

Where σ12 is the compressive strength parallel to grain at 12% of moisture content in specimen, MPa; W is actual moisture content of specimens, %.

The remaining 20 mm long section exposed to the decay fungus was used to examine fungal colonization. Small cubes were cut from the inner end of the specimens, dried through a graded alcohol series and finally soaked in pentene. The pentene was allowed to evaporate and the dry specimen was sputter coated with gold palladium. The specimens were examined using a Quanta 200 Electron microscope at an accelerating voltage of 10,0kV. A minimum of 5 fields were examined for each specimen. This examination was not quantitative; it was only intended to determine where the fungus was most prevalent in the wood cells.

Results and discussion

Mass loss averaged 2,14% after 15 days of fungal exposure and steadily increased with incubation time to an average of 44% mass loss after 90 days (Table 1).

Table 1: Effect of exposure to G. trabeum on mass loss and compressive strength parallel to the grain of Chinese poplar specimensa 

aValues represent means of 8 specimens per time point, while figures in parentheses represent one standard deviation

The rate of decay was initially slow, increased markedly between 30 and 45 days and then slowed. This pattern is consistent with previous tests where mass loss is limited while the fungus grows through the substrate, consuming readily available sugars, and then becomes more substantial once the fungus begins to actively degrade the wood polymers (Bari et al. 2017, Li et al. 2018). The mass losses found after 90 days were also consistent with the classification of Chinese poplar as non-resistant to fungal attack according to ASTM D2017-2001 (ASTM 2001).

Compressive strength did not change significantly between 0 and 15 days of fungal exposure. While these results suggest that the fungus had little effect on properties, previous results clearly illustrate the tendency of this fungus to cause sharp reductions in flexural properties (Yang et al. 2006). Compressive strength decreased by nearly 18% at the 30 day assessment, indicating the fungus had begun to induce more substantial effects on wood properties. Compressive strength declined to only 24% of its original value after 90 days of exposure.

Previous studies have shown that brown rot attack of wood induces near exponential losses in bending and tensile strength and that these effects occur early in the decay process when fungal mass losses remain relatively minor (Ge et al. 2016, Wilcox 1978). Compressive strength losses appear to deviate from this trend with losses in compressive strength following a near linear relationship with either mass loss or time (Figure 1).

Figure 1: Relationship between mass loss and compressive strength of Chinese poplar blocks exposed to G. trabeum in a decay test for up to 90 days. 

Figure 2: Examples of the degree of fungal colonization of Chinese poplar blocks exposed to G. trabeum for 0 to 90 days. 

Fungal colonization, as assessed using SEM, followed similar trends to those observed for mass loss. Fungal hyphae were scattered in fibers in blocks exposed to the test fungus for only 15 days and then became increasingly common over the next 75 days (Figure 2). Hyphae were abundant in the vessels, especially after 75 and 90 days of exposure. The results were consistent with the degree of both mass and compressive strength losses.

Conclusions

Exposure of Chinese poplar specimens to G. trabeum was associated with steady, closely correlated declines in both mass and compressive strength. The results suggest compressive strength losses can be more easily assessed by changes in mass than fungal associated effects other wood properties.

Acknowledgments

This work was financially supported by the Fundamental Research Funds for the Central Universities of China (2572018BL08), Research and development project of application technology in Harbin (2017RAQXJ078), and China Scholarship Council funding (201706605016). There is no conflict of interest between authors.

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Received: June 26, 2018; Accepted: December 22, 2018

Corresponding author: jmorrell@usc.edu.au

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