anticorrosive

Wood decay is the result of wood rot fungi. The hydrolases in wood rot fungi can decompose the cellulose, lignin and cell contents that make up the wood cell wall as nutrients, so that the strength of wood gradually decreases until all the bearing capacity is lost.

The growth of wood rot fungus must meet the following three conditions at the same time: the moisture content of wood is higher than 18%; The temperature is in the range of 2 ~ 35 ℃; There is an oxygen supply. If one of them can be removed, decay can be prevented. There is an old saying in China that "the wood at the bottom of the well will not rot for thousands of years", which means that the wood will not rot due to lack of oxygen in the water. Wood structure is inseparable from human life. Temperature and oxygen cannot be eliminated. The moisture content of wood can only be controlled within 18%, even if it is dry to prevent the erosion of wood rot fungi. Therefore, it is required that all parts of the wooden structure, especially the key parts such as support nodes, should be in good ventilation conditions, and can be dried in time even if they are affected with moisture. Therefore, when designing the wooden structure, we should first consider the structural anti-corrosion measures of the structure: for example, if the wooden roof with temperature isolation ceiling is set, the ceiling must be hung under the lower chord of the wooden roof truss, and the bottom surface of the lower chord shall be kept at a certain distance from the temperature isolation layer, so that the whole roof truss is located in the same temperature field. If the thermal insulation layer is placed on the lower chord of the wooden roof truss, the support nodes of the roof truss have to be built in the wall to form a closed space, so as to ensure the good effect of the positive temperature field under the thermal insulation layer. However, when the eaves leak slightly, the bearing joints can be soaked. Because they are in a closed state, it is difficult to dry in a short time. As long as the water content rate of wood is higher than 18% for a certain time, wood rot fungi can grow, and wood rot fungi will discharge several times the water originally used to maintain growth in the process of reproduction, wet the adjacent wood, produce a vicious circle and spread decay. In the past, many support joints of wooden roof truss have been seriously decayed and damaged, and even caused the collapse of the whole roof.

The wooden poles or piles buried in the soil are soaked by the water in the soil and supplied with oxygen in the next section on the surface of the soil layer, so they are rotten. The reason why the part buried deep in the soil does not rot is lack of oxygen. The reason why the higher part above the surface is not rotten is water shortage (i.e. the water content is less than 18%). Therefore, wood structures or components that are often or intermittently damp, as well as wood beam ends or wood bricks that have to be closed in the wall, must be treated with preservatives to prevent the propagation and growth of wood rot fungi.

Preservatives are prepared by chemicals with certain toxicity, including water solubility, oil solubility, Oils and pastes. For wood members that are often affected by moisture, the mixed anticorrosive oil belonging to oil preservative, also known as anthracene oil, should be used. It is prepared from coal creosote (i.e. wood anticorrosive oil) and coal tar. It is not easy to lose in case of water and has a long effect. Asphalt is black and viscous in appearance, similar to anthracene oil, and is often misused as a preservative. However, asphalt can only be waterproof but not anti-corrosion. Coating asphalt on non dried wood is counterproductive and hinders the air drying of wood.

Due to the different contents of cells, the decay resistance of wood of different tree species is also different. Pinus massoniana and birch belong to tree species with poor corrosion resistance. For the wood of the same tree species, sapwood is more perishable than heartwood, so the tree species with a large proportion of sapwood have poor corrosion resistance. When the wood of these species is used to make wood structures, they shall be treated with preservatives.

Insect control

The insects that eat wood mainly include termites and beetles. Termites are more harmful than beetles.

Termites are a kind of insects that live in groups. There are more than 2000 species in the world and nearly 100 species in China, mainly distributed in the Yangtze River Basin and warm and humid areas in the south. Termites use wood as their main food and cannot live without water, and they are photophobic. They feed outside the nest and travel in the ant road built of soil. Therefore, nests are often built where there are wood components or wood products and close to the water source. Therefore, wooden components in dark and humid parts such as kitchen and bathroom are most vulnerable to termite erosion.

The common wood beetles in China are longicorn beetle, longicorn beetle, bark beetle and bark beetle. Longicorn beetles feed on the fiber of wood. The larvae moth into a tunnel in the wood. When they mature, they form pupae at the end of the tunnel. After eclosion, the adults bite an oval hole and fly out. It mainly endangers broad-leaved trees such as Casuarina equisetifolia. Bark beetles and bark beetles feed on the starch and carbohydrate of wood, so they mainly harm the sapwood of broad-leaved trees. Adults like to lay eggs in the tube holes on the wood surface. Therefore, oak, hickory, Robinia pseudoacacia and other tree species with large pipe holes suffer the most. The larvae moth the interior of the wood into powder shape, leaving only a thin shell. The surface is covered with small insect eyes, and there are often powdery moths around it.

Beetles mainly attack dry wood with low moisture content, while termites do more harm to wet wood. Therefore, taking structural moisture-proof measures to separate the wood members from the water source has a certain effect on reducing the harm of termites. There is only one kind of anti termite agent or moisture-proof treatment for wood structures.

Nanmu, red sandalwood, teak and other tree species have strong resistance to termites, and Cunninghamia lanceolata, Cryptomeria lanceolata and camphor also have certain resistance to termites. However, most tree species are vulnerable to termites, such as Pinus massoniana. Therefore, wood structures or wood products made of wood from tree species vulnerable to termites should be treated with insect repellents.

In order to ensure the durability of wood structure, all countries in the world use both anti-corrosion and anti insect agents. For example, the boron phenol mixture prepared with boric acid, borax and sodium pentachlorophenol is a water-soluble agent. Wood components can be soaked in the aqueous solution of the agent. If each cubic meter of wood can absorb 4.5 ~ 6kg of agent (dry agent weight), the purpose of anti-corrosion and insect prevention can be achieved. Because this agent is easy to lose in case of water, it is only suitable for non damp wood components. Oil soluble pentachlorophenol and lindane mixture shall be used for wood components susceptible to moisture.

fire prevention

The fire prevention of wood structure and its components is mainly to determine its fire resistance limit, and take measures to improve the fire resistance limit of wood components according to the requirements of fire resistance rating of buildings. The fire resistance limit of wood members refers to the time from the beginning to the loss of its original function (loss of bearing capacity for load-bearing members) when a member burns in a special furnace according to the flame with simulated fire temperature (700 ~ 1000 ℃). If the door leaf glued with 5 cm thick square wood is used, the fire resistance limit is 1 hour; Section 17 × The stress of 17 cm wooden beam reaches 10 MPa and the fire resistance limit is 40 minutes; Section 15 × 15 cm, 3.5 m high, the wooden column with stress up to 4 MPa was damaged after 25 minutes; The section is 29 × The 29 cm wooden column was stressed to 6 MPa and failed after 50 minutes. It can be seen that wood members have certain fire resistance, especially those with large section. This is because wood is composed of hollow cells with low thermal conductivity. And in the process of wood combustion, a layer of charcoal is formed on the surface, and charcoal also has good thermal insulation performance, which slows down the thermal decomposition of wood.

Under the action of fire, the wood components are on fire for the first 2 minutes, and the carbonization rate in the next 8 minutes is about 0.8 mm per minute. Due to the formation of charcoal layer, the carbonization rate slows down to 0.6 mm per minute. The carbonization rate of different tree species is different. In addition to the test, the fire resistance limit of wood members can also be estimated according to the carbonization rate of different tree species.

For wood members without protective layer, integral wood members with large section size shall be used as far as possible to improve the fire resistance limit. The test shows that the fire resistance of laminated members is similar to that of wood members with integral section. Therefore, the use of laminated wood structure with large section is conducive to fire prevention. There are two ways to improve the fire resistance of wood structure. One is to add plastering layer or gypsum board, such as 30 × The fire resistance limit of 30cm wooden column plus 2.5cm steel wire mesh plastering layer can be increased to 1.5h. On the other hand, it can be impregnated with fire retardant agent or coated with fire retardant paint, such as acrylic emulsion fire retardant paint, which can decompose phosphoric acid at the temperature of 100 ~ 200 ℃, so as to dehydrate and carbonize the wood, reduce the formation of combustible gas, expand and foam at about 250 ℃, and form a honeycomb fire insulation layer, Do not burn a small fire to prevent the expansion of the initial fire. Once it leaves the flame, it can be extinguished by itself.