Objectives This paper investigated the effect of natural aging on the 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 natural aged 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 the outside to the 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 the connectivity between the internal pores of the wood, and resulted in significantly higher treatability in each part of aged Chinese fir than that in modern Chinese fir. In cross-section, the penetration depth of preservatives in each part increased markedly, with the retention in the 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 the radial direction of the 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 raised from the sapwood to the inner heartwood.

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