Anti-corrosion

Wood decay is caused by infestation of wood-rotting fungi. The hydrolase produced by such fungi decomposes cellulose, lignin and cell contents in wood cell walls for nutrients, which gradually reduces wood strength until it completely loses bearing capacity.

The growth of wood-rotting fungi requires three simultaneous conditions: wood moisture content above 18%, temperature between 2℃ and 35℃, and oxygen supply. Blocking any one condition can prevent decay. An ancient Chinese saying mentions that wood at the bottom of a well remains intact for thousands of years, which proves wood stays free from rot in oxygen-deficient water. Wooden structures are widely used in daily life, so temperature and oxygen cannot be isolated. The only feasible way is to keep wood moisture content below 18% to maintain a dry state and resist fungal erosion. Therefore, all parts of wooden structures, especially key support joints, must be well ventilated so that they can dry in time even after getting damp.

Accordingly, structural anti-corrosion measures shall be prioritized in wooden structure design. For wooden roofs fitted with thermal insulation ceilings, the ceiling shall be installed under the lower chord of roof trusses with a reserved gap, to keep the entire truss in a uniform temperature environment. If the thermal insulation layer is laid above the lower chord, the support joints of roof trusses have to be enclosed in walls to form sealed spaces for stable temperature effect. However, slight eave leakage will dampen these enclosed joints, which are difficult to dry quickly. Once wood maintains moisture content over 18% for a period of time, fungi will multiply and release excessive moisture to wet surrounding wood, forming a vicious cycle and accelerating decay spread. In the past, many roof truss support joints were severely damaged in this way, even leading to overall roof collapse.

Wooden poles and piles buried underground tend to decay at the soil layer junction due to dampness and oxygen. Deeply buried parts stay intact for lack of oxygen, while upper parts above ground remain sound owing to low moisture content below 18%. Hence, wooden components exposed to frequent or intermittent dampness, as well as beam ends and wooden blocks sealed inside walls, must be treated with preservatives to inhibit fungal reproduction.

Preservatives are formulated with toxic chemicals, divided into water-soluble, oil-soluble, oily and paste types. Mixed creosote oil, a kind of oily preservative blended with coal tar creosote and coal tar, is ideal for damp-prone wood components. It is insoluble in water and features long service life. Pitch is often mistaken for preservative due to its black viscous appearance similar to creosote oil, yet it only provides waterproof effect without anti-corrosion function. Coating unseasoned wood with pitch will hinder natural air drying and cause adverse results.

Timber of different tree species varies in decay resistance due to different cell inclusions. Masson pine and birch are low decay-resistant species. For the same tree species, sapwood rots more easily than heartwood, so timber with high sapwood proportion has poorer anti-rot performance. All such timber used for wooden construction needs preservative treatment.

Insect Prevention

The main wood-boring insects are termites and beetles, among which termites cause more extensive and serious damage.

Termites are social insects living in hidden colonies. There are more than 2,000 species worldwide and nearly 100 species in China, mainly distributed in warm and humid areas along the Yangtze River Basin and southern China. They feed mainly on wood and depend heavily on water. Being photophobic, they move outside nests through soil tunnels and prefer to build nests near wooden facilities and water sources. Thus wooden components in dark humid areas such as kitchens and bathrooms are most vulnerable to termite damage.

Common wood-damaging beetles in China include house longhorn beetles, powderpost beetles and auger beetles. Longhorn beetles feed on wood fibers; their larvae bore tunnels inside wood, pupate at tunnel ends and fly out through oval holes after eclosion, mainly damaging broad-leaved timber such as casuarina. Powderpost and auger beetles feed on starch and sugar inside wood, mainly harming sapwood of broad-leaved trees. Adult beetles lay eggs in wood pores, so timber with large pores like oak, hickory and black locust suffer the most severe infestation. Larvae hollow out wood into powder, leaving only thin outer shells covered with wormholes and powdery insect frass.

Beetles mainly infest dry low-moisture wood, while termites do more harm to damp wood. Structural moisture-proof treatment that isolates wood from water sources can reduce termite damage, yet it is only an auxiliary method. Wooden structures and products in insect-prone areas must be treated with insecticides.

Precious timber such as nanmu, red sandalwood and teak have strong termite resistance; Chinese fir, cryptomeria and camphor wood also have certain defensive effects. However, most tree species are susceptible to termite erosion, especially masson pine. All wooden products made of vulnerable timber need insect-proof treatment.

To ensure the durability of wooden buildings, multi-functional agents with both anti-corrosion and insect-proof effects are widely adopted globally. Boron-phenol mixture is a water-soluble compound mixed with boric acid, borax and sodium pentachlorophenate. Wood components can be soaked in its aqueous solution. Absorbing 4.5 to 6 kilograms of dry agent per cubic meter of wood can achieve ideal anti-corrosion and insect-proof effects. Since this agent is easy to leach out when exposed to water, it is only suitable for dry wood parts. Oil-soluble pentachlorophenol and lindane mixture are applied to damp-prone wooden components.

Fire Resistance

Fire protection for wooden structures focuses on testing their fire resistance limit and taking targeted measures to improve fire endurance in accordance with building fire protection standards. The fire resistance limit refers to the duration for a wooden component to withstand simulated fire temperature of 700~1000℃ before losing its original function, especially load-bearing capacity. For example, a 5cm thick glued wood door panel has a fire resistance limit of 1 hour; a 17×17cm wooden beam under 10MPa stress fails after 40 minutes; a 15×15cm wooden column with height of 3.5 meters under 4MPa stress collapses in 25 minutes, while a 29×29cm wooden column under 6MPa stress can last 50 minutes. It proves wooden components possess good fire resistance, especially those with large cross-sections. Wood is composed of hollow cells with low thermal conductivity. Meanwhile, charcoal layer formed on burning wood surface delivers excellent heat insulation and slows down thermal decomposition of wood.

Wooden components catch fire within the first 2 minutes in fire conditions, with a carbonization rate of about 0.8 millimeters per minute in the next 8 minutes. The rate drops to 0.6 millimeters per minute afterwards due to charcoal formation. Different tree species have different carbonization rates. Apart from laboratory tests, fire resistance limit can also be calculated based on known carbonization data.

For unprotected wooden components, solid timber with larger cross-section is recommended to boost fire resistance. Tests show that glued laminated timber has similar fire performance with solid wood of the same size, so large-size glued laminated wooden structures are conducive to fire prevention. There are two main ways to improve fire resistance. The first is to add plaster or gypsum board cladding. For instance, a 30×30cm wooden column coated with 2.5cm wire mesh plaster can reach a fire resistance limit of 1.5 hours. The second is to inject fire retardants or apply fireproof paint. Acrylic latex fireproof paint decomposes into phosphoric acid at 100~200℃ to dehydrate and carbonize wood and reduce flammable gas generation. It expands and foams at around 250℃ to form honeycomb thermal insulation layers, which stop small fires from spreading and realize self-extinguishment away from open flame.