Objective This paper investigates the effects of natural aging on treatability of a Chinese fir beam in a sub-regional manner to provide a scientific basis for preservation of wooden heritage buildings as well as reuse of old timber.

Method Taking a modern Chinese fir wood as a control, a naturally aging Chinese fir beam in service for about 100 years was divided into three parts, namely sapwood, outer heartwood and inner heartwood, along the radial direction from outside to inside, and the treatability of each part was determined by vacuum impregnation. The mechanism of treatability change in each part was further analyzed from the chemical compounds, microstructure and pore structure.

Result The number and proportion of pores with larger pore diameters in each part of aging Chinese fir were obviously higher, which improved connectivity between internal pores of wood, and resulted in significantly higher treatability in each part of aging Chinese fir than in modern Chinese fir. In cross-section, the penetration depth of preservatives in each part increased markedly, with the retention in sapwood, outer heartwood and inner heartwood being 2.98, 3.17 and 6.94 times of modern Chinese fir, respectively. The chemical compounds in aging Chinese fir showed that the sapwood was degraded mainly by lignin and hemicellulose, with a higher degree of lignin degradation, the outer heartwood was degraded mainly by hemicellulose, and the inner heartwood had no obvious degradation of its chemical composition. The microstructure showed that the tracheids closer to the interior in radial direction of aging Chinese fir beam were compressed to a higher degree, which might result from the long-term radial loading. It largely explained the phenomenon that the retention multiple of aging Chinese fir to modern Chinese fir raised from sapwood to the inner heartwood.

Conclusion The degree and mechanism of aging varies obviously in different parts of radial direction of aging Chinese fir beam, which lead to the treatability increase in each part. A long-term loading has large effect on internal microstructure of an aging beam.