Experimental Research On Decay Of Timber Component

Timber decay,contributing to the deterioration of material properties,stiffness and mechanical properties of timber components,is the key factor that affects the durability of timber structures.At present,most researches have focused their attention on material level of timber decay and mainly adopted the weight loss rate as a measurement indicator.However,to estimate the residual bearing capacity and remaining life of decayed timber components in structures,decay depth is of greater significance in measuring decay degree,especially at the initial decay stage.Therefore,quantifiable detection index reflecting the degraded mechanical properties of timber components and its time-dependent law need to be explored.It is also crucial in the evaluation of durability of existing timber structures.To investigate the time-dependent law of timber decay,a combination of the artificial accelerated decay test and non-destructive testing technique in a constant temperature and humidity environment is employed and an 84-day accelerated decay test is conducted.The test procedure consists of fungi selection,medium preparation,incubator modification,preparation before inoculation,inoculation,cultivation,and decay.The samples are periodically tested every 14 days,including visual inspection of decay grades,measurement of weight loss rate,mechanical property test and non-destructive testing using resistograph.Through the detailed analysis of the impedance curve,an analytical method suitable for determining the impedance curve of slightly surface-decayed samples and a criterion for judging the critical point of decayed area and healthy area were proposed.The impedance ratio,ratio of the mean impedance value of decayed area to that of healthy area,was used to characterize the changes of decay depth after decay,achieving ideal results.The test results show that the impedance degradation rate increases with decay time in the form of power functions.The correlation coefficient is particularly large and its increase rate gradually slows down.In addition,the degradation rate of the three mechanical properties(including flexural modulus,bending strength,and compression strength parallel to grain)exhibits a significant increase with decay time in the form of power functions and the increase rate gradually slows down with time.Moreover,flexural modulus has the highest degradation rate,which reaches about 20% at 84 days of decay.The degradation rate of bending strength and compression strength parallel to grain is slightly lower,and they have exhibited similar increase rate,reaching 15% and 14% respectively at 84 days of decay.The variation trend of weight loss rate with decay time is not prominent,only 1.60% at 84 days of decay.The correlations among weight loss rate,mechanical property and impedance ratio were analyzed.The results show that the degradation rate of mechanical properties exhibits a noteworthy linear relationship with the degradation rate of weight loss rate and impedance ratio.The correlation with the degradation rate of impedance ratio is very high,of which the index of flexural modulus degradation is about 0.72.The index of bending strength and compression strength parallel to grain is very close to each other,which is 0.54 and 0.52 of the degradation rate of impedance ratio respectively.The correlation between degradation rate of mechanical properties and weight loss rate is slightly lower.Hence,mechanical property and impedance ratio of timber components are more reliable indicators of decay degree.Decay depth is difficult to be detected in practical engineering and the distribution of decay is very random along the length and cross-section of timber components.Given this information,the concept of equivalent decay depth is proposed,and it can be calculated through detectable indexes,such as mechanical properties,impedance ratio and weight loss rate.Equivalent decay depth derived from degradation rate of mechanical property shows a significant increase of power function with the decay time.The equivalent decay depth obtained from compression strength parallel to grain is much larger than that from flexural modulus and bending strength and it reaches 2.83 mm,1.38 mm and 1.54 mm respectively at 84 days of decay.The equivalent decay depth derived from impedance ratio reaches 2.54 mm at 84 days of decay.The regularity of the equivalent decay depth derived from weight loss rate is not obvious,and its variation trend with decay time is not significant.Through the comparison of multiple detection indexes,a time-varying model of timber decay depth under constant temperature and humidity was preliminarily established.And the most reliable evaluation index was figured out.Meanwhile,based on literature review,the variation trend of timber decay depth with time under natural environment was discussed.