Damage Analysis And Study On Micro Deterioration And Protection Of Material Properties Of Yingxian Wooden Pagoda

As a typical representative of Chinese civilization,Chinese ancient wooden architectures have made outstanding creations and contributions in art,culture,architecture,structure,materials,and construction techniques.The ancient wooden architectures that survived the world have different degree of damages,and the deterioration of wood and its inducing other damages have made the protection of ancient wooden architectures being the focus of attention.Wood in ancient architectures often manifests as micro-structure degradation under environmental action and mechanical damage under load.Although extensive researches have been conducted on the mechanical and physical properties of ancient wooden architectures,it is rarely interpreted from a microscopic point of view.Macroscopic damage is often caused by the attenuation of the wood cell wall structure.In-depth research on the microscopic scale is the basic requirement for its deterioration analysis.Understand and master the deteriorated mechanism of the components materials of ancient wooden architectures and propose corresponding protection measures can provide a theoretical basis for the long-term preservation and subsequent repair decision of ancient wooden architectures,which have practical engineering guidance significance.In this dissertaion,Yingxian Wooden Pagoda in Shanxi Province was taken as the research object.First,through sorting out multiple measured charts over the years and conducting field investigations,the development and causes of residual deformation of each floor and the whole structure were obtained,and the key indicators that need to be repaired under the functional requirements of the pagoda structure were put forward,and the relationship between the damage characteristics of the transversely compressed structure of the first and second floor and the deterioration of the material properties were were analyzed as well.Secondly,using a variety of modern characterization and analysis techniques at the microscopic scale,the decay and deteriorated mechanism in the cell wall structure on the surface of components were studied,and the restoration of the pagoda based on residual deformation and material deterioration were discussed.Finally,on the basis of the obtained deteriorated mechanism and flame-retardant requirements,an in-situ polymerization method was adopted to construct a wood protective coating with integrated functions of anti-ultraviolet,water resistance,and flame retardant.The main research results are as follows:(1)By analyzing the vertical and horizontal displacements of the columns,floors,and the whole structure of Yingxian wooden pagoda,it was found that the nine floors of the wooden pagoda were inclined to the northeast horizontally,and the displacement angle reached 1/80.0.As the columns from heads to feet on third and the fifth floors were tilted to the northeast and northwest with the inter-story displacement angles of 1/16.0 and 1/25.0 respectively,these were the key indicators that need to be repaired under the functional requirements of the wooden pagoda structure.This inter-story displacement angle of the columns from heads to feet on the third floor far exceeded the large earthquake requirements of modern concrete frame structures and steel structures.Based on the anti-seismic mechanism of the"rocking column"of such ancient wooden structural architectures,the safety margin of the pagoda swinging in the direction of the residual inclination was greatly decreased,and the seismic capacity was significantly reduced.The cross-grain compression residual deformation of the beam ends,the Gong roots,and the small Dous on the bottom two floors of the pagoda were large,and micro-cracks were common,and consequently the corresponding load-bearing and deformation capabilities were significantly reduced.(2)Scanning electron microscopy(SEM),elemental analysis(EA),attenuated total reflection fourier transform infrared spectrometry(ATR FT-IR),and X-ray diffraction(XRD)were employed to characterize the surface morphology,element content,main chemical component content,and cellulose crystallinity index of the aged wood samples that were taken from the surface of the components at different positions in Yingxian Wooden Pagoda.The results show that the cell wall of aged wood were generally collapsed,twisted,and cracked by marginal pits,showing obvious signs of decay.The content of lignin and hemicellulose decreased while the crystallinity of cellulose increased.The main degradation process was the preferential degradation of hemicellulose,and the subsequent degradation of lignin,and there was also degradation in the amorphous region of cellulose.Among all the samples,the samples taken from the outer floor and the higher level were more severely degraded;The degradation degree of the outer column samples were higher than that of the inner column samples,and the preservation condition of beams were better than that of the columns on the same floor.(3)Scanning electron microscopy-X-ray energy spectrometry(SEM-EDS)and XRD were used to analyze a large number of unknown particles attached to the wood cell wall,combined with EA,synchrotron radiation(XANES,XRF),pH analysis(pH),ATR FT-IR and X-ray photoelectron energy spectrum technology(XPS)to carry out qualitative and quantitative analysis of invasive elements in wood,wood pH and structural changes of main chemical components.The results indicated that a large number of unknown particles attached to the cell walls of aged wood were hydrated crystalline salt crystals formed by invasive inorganic elements such as sulfur,calcium,sodium,and chlorine.There were a lot of free inorganic ions existed in aged wood.The sharp increase in the content of sulfur and nitrogen in elemental analysis and the accumulation of high concentrations of sulfate indicated that environmental factors such as acid rain and the atmosphere were the main sources of these invasive inorganic elements.The deteriorated mechanism of the aged wood in Yingxian wooden Pagoda was that a large amount of accumulated inorganic salts caused mechanical damage to wood cell structure;the increase in acidity indicated that the acid-producing substances in acid rain meeting water to form hydrogen ions,leading to the hydrolysis of the holocellulose;the characteristic peak of lignin in aged wood is missing,the destroyed structure of lignin methoxy and the increase in the content of carbon-oxygen bonds indicated that the lignin were oxidatively degraded,which was mainly caused by the combined effects of ultraviolet rays and oxygen.(4)According to the damage characteristics of the components and the deteriorated mechanism of the microstructure,it was pointed out that the dual effects of material deterioration and load made the direct pressure-bearing parts of the transverse-grained pressure-bearing components severely damaged and densely spaced,which came from the bottom two floors of the pagoda;if unloaded,the gaps in these areas would generally expand.Multiple gaps caused the wood in these areas to have a huge surface area in contact with the atmospheric environment,which would accelerate the deterioration of these areas.(5)Based on the obtained deteriorated mechanism and flame retardace requirements,by adopting the in-situ polymerization method,the ammonium polyphosphate(APP)was as the core material.The titania(TiO₂)nanoparticles modified by the silane coupling agent KH-550,and melamine resin(MF)were used as the shell material to coat APP.The MF-TiO₂@APP microcapsules were formed,and MF-TiO₂@APP/WPU wood protective coating was prepared.The test results shown that when the core-shell mass ratio of MF-TiO₂@APP was 3:2 and the addition amount of TiO₂ nanoparticles was 2.5%,the MF-TiO₂@APP microcapsules had a suitable particle size,and water resistance and UV resistance significantly were improved as well.When the addition amount of MF-TiO₂@APP microcapsules was 18%and the coating thickness was 180?m,compared with the WPU,its thermal decomposition carbon residue yield of the protective coating after preparation were increased from 9.62%to 31.62%,formed a stable residual carbon strcture covering the surface of the substrate;its peak heat release rate(pk-HRR₂)and total heat release(THR)value were reduced to 113.5 k W/m² and61 MJ/m²,respectively,which increased the thermal hazard;compared with APP/WPU,it showed a reduced average UV transmittance by 42%;it meeted the 72-hour water resistance test standard.The prepared protective coating achieved the purpose of water resistance,UV resistance and flame retardant multi-effect protection of wood.