Increasing the strength of destroyed wood of wooden architecture monuments by surface modification

. The Anglican Church in Arkhangelsk built in 1833 represents a wooden architecture monument. The article describes the strengthening of partially destroyed samples of the Anglican Church wood by surface modification. The first layer of the sandwich coating is nitrilotrimethylphosphonic acid, which forms covalent bonds with the substrate, partially strengthening the wood. The second layer is an epoxy resin solution, which forms covalent bonds with the coating of the first layer, with hydroxyl groups of the first layer involved in the curing of the second layer as well. A two-layer surface coating is formed, while the strength of the wood increases by 2 – 2.5 times, water absorption decreases by 3 times, and mass loss in combustion is no more than 9% according to GOST 27484-87. The monument preservation increases.


Materials and results
In 2017, Princess Anne visited the Anglican Church during the celebration of the 75th anniversary of the first Arctic convoy.
According to the results of the survey of the Anglican Church in 2016, the decay of wood along the entire perimeter of the building, the presence of cracks, and the partial destruction of wood were identified. The survey was conducted by LLC Arkhstroyekspertiza.
To study the strengthening of the destroyed wood, samples of the Anglican Church sole timber were taken (Figure 1). For the purpose of various tests, samples of the Anglican Church sole timber were sawn to the required dimensions in accordance with GOST 16483.11-72 and GOST 16483.3-84. When sawing, the straightness of the grain along the length parallel to its arbitrary faces was preserved where possible. The sizes of the samples depending on the test type are shown in Table 1. Water absorption 10x10х5 Infrared spectroscopy The strength tests were carried out in accordance with GOST 16483.10-73 and GOST 16483.11-72 in the laboratory of Moscow State University of Civil Engineering using the Instron 3382 machine. The results are presented in Table 2.  Table 3 shows the results of determination of conventional ultimate strength in compression parallel to the grain in accordance with GOST 16483.3-84    The study of modified samples of partially destroyed wood by infrared spectrometry (IR spectrometry) allow revealing the formation of new covalent bonds of the substrate with a modifier, which causes a significant increase in the strength of the samples. Figure 3 shows graphs of Fourier IR spectroscopy of the untreated sample of the Anglican Church sole timber, as well of the samples treated with NTP 40% and MPA 40%. [9][10][11][12]

Discussions
Water absorption of the samples of the Anglican Church wood, modified with a sandwich coating. Wood is a heterocapillary system with capillary spaces of different sizes. The ability of wood to absorb moisture and retain it in the pores and capillaries is called water absorption. Water absorption of the wood significantly affects the durability, since surface hydrolytic destruction of the wood surface occurs in the presence of moisture. In the presence of biodegradators this process is significantly accelerated, which represents one of the main causes of wood destruction. Reducing the water absorption of historical wood is of great importance for the durability of wood structures and the preservation of historical monuments. The influence of modifiers on the water absorption of the samples of the Anglican Church wood was studied. Dependence of water absorption of the wood samples on the modifier type, and on the dwell time, is reflected by the corresponding graphs ( Figure 4). To determine the efficiency of the tested impregnating fire retardant agents, it was also necessary to determine the fire hazard of the partially destroyed wood. For this purpose needle flame tests in accordance with GOST 27484-86 were carried out. Initially, the samples were weighed. [13][14][15][16][17][18] Then, observations of the samples during exposure to fire for two minutes were performed. At the end of the test, the samples were weighed again, and the mass loss to be used for evaluation of the fire-proof efficiency group was calculated. The results are presented in Table 5. Under conditions of variable humidity, the wood of monuments is destroyed much faster. The destruction is accelerated by microorganisms, which are catalysts for hydrolytic destruction. With moistening and subsequent drying, cracks form, which reduce strength. Percentage characteristics of wood are determined by the content of cellulose. The destruction degree of a wood is characterized by the change in its cellulose content.
Based on the data given in Tables 2 and 3, the samples surface modified with sandwich coatings containing 40% of NTP and 20% of epoxy adhesive have the best strength characteristics. The quantitative increase in strength is determined by all parameters, see Table 4.
When analyzing the IR spectra of the untreated partially destroyed wood and the wood modified with NTP 40% and MPA 40%, new bands appear in the absorption region of the characteristic frequencies of 2285 cm -1 and 928 cm -1 . This indicates the formation of new covalent bonds of modifiers with the substrate.
When modifying the partially destroyed wood with boric acid, NTP and MPA modifiers, the water absorption decreases by 10%, 30% and 40% relative to the untreated wood. In a layer-by-layer treatment with water repellents and flame retardants, water absorption decreases by half, and the results insignificantly differ depending on the type of water repellent used, by 2-3% on average. The samples of the Anglican Church wood covered with methylphosphonic acid 40% + EP adhesive 20% and NTP + Akvidur TT 20% have the lowest water absorption. Thus, in a layer-by-layer modification of wood with fire retardants and water repellents, a stable decrease in water absorption takes place.
The coatings tested have good fire-retardant properties, taking the wood to the first fireproof efficiency group -not more than 9% of the mass loss when tested in accordance with GOST 27484-86.

Conclusions
The restoration of the lost strength of a partially destroyed wood is possible with a surface modification by reactions with phosphorus-containing compounds and application of a second polymer layer, for example a 20% epoxy layer. The internal phosphorus-containing layer must enter into absorption chemical interactions with the epoxy layer of the sandwich coating. The developed sandwich coatings increase the wood strength 2 -2.5 times, and water absorption decreases by 2.53 times. The treated wood joins the first fire-proof efficiency group, and also it gets antiseptic properties. The obtained results allow increasing the durability of wooden structures and contributing to the preservation of wooden