Timber Flashcards
The vessel cell walls begin to dry out when the moisture content of the timber
falls below what %?
28%
The vessel cells are made of?
Cellulose
Approximately how much timber is used as construction timber?
60%
Name three engineered timber products
I-beam, Plywood (glues together - glued laminated sections ‘’glulam’’), MDF, Laminated timber, Blockboard, Particleboard, Fibreboard, Wood wool slabs, Shingles, Compressed straw bales, Thatch
What requirements are needed for wood-rotting fungi to grow?
Moisture (at least 20%), oxygen and supply of food
Air drying of timber in the UK can reduce moisture levels but only down to about what %?
20%
During their larval stage, wood-boring beetles typically bore through timber
for a period of?
2 years
Which is the least dense timber?
Balsa (200kg/m3)
The process of cutting tree trunks into planks is called?
Conversion - sawing of the timber. Happens before Seasoning.
Two other processes are peeling (used in making plywood) and slicing (used to make decorative veneers).
The process of allowing timber to dry out to a moisture level similar to its intended surroundings is?
Seasoning - during seasoning (type of drying process) moisture is lost and the moisture content falls to about 28%. Removes moisture but NOT ALL moisture!!
The density of earlywood compared with the density of latewood is?
Lower
Seasoning is the process of?
Removing water from the timber
Moisture movement in timber is maximum in?
The tangential direction
Before rotting fungi can grow, timber must have a moisture content of at least what %?
20%
The effect of the Earth’s forests on the atmosphere is called?
A carbon sink - Timber absorbs CO2 (Carbon dioxide) and breathes out O2 (Oxygen)
It is easier to split timber?
Along the grain - Properties such as stiffness (E), compressive strength and tensile strength depend on directions – whether its parallel or perpendicular to the grain
The main cause of insect attack in UK timber comes from?
Beetles - lay their eggs on timber
Which is the densest of timbers?
Beech (750kg/m3)
Timber is classed into what 2 classes
Hardwood and Softwood - further onto earlywood (porous) and latewood (more dense)
What kind of resource is Timber?
Renewable - but consumption is constantly decreasing due to environmental reasons
Name different uses for Timber
Timber roofs, footbridge, docks, foundations and other concrete technology, kitchens - furniture
Is Timber Anisotropic in, 1, 2 or 3 directions?
3 directions - longitudinally (top-bottom of the trunk), radially (centre - outer bark), circumferentially (direction - around the trunk)
When the tree is growing what is happening to the vessel cells?
They are full of moisture but they DO NOT move (shrink or swell when full of sap)
Vessel Cells provide what to the timber?
Strength and Stiffness - as well as transport water up the tree in the early growing season each year
With further drying of timber what happens to Vessel Cells?
Vessell Walls (Cellulose) begin to dry out and they shrink - Thus it is drying out the vessel cell walls (drying below 28% moisture) which causes the moisture movement
In what directions is the greatest moisture movement in timbers?
Because the vessel cell walls are arranged in rings, the moisture movement will be greatest in the tangential (circumferential) direction
What are some common terms for timber movement caused by drying?
Cupping, Diamonding and Twist
What 2 Seasoning methods are there?
Air seasoning (can reduce to 20% in UK) or kiln seasoning (hot chamber - can reduce as low as 10%). In the UK, hardwood planks are usually air-dried for 18 to 24 months before being kiln-dried.
Name 6 methods of degradation of decay of timbers…
Biological (fungal & insect attack), Chemical, Photochemical, Thermal, Fire and Mechanical
What do beetles do to timber?
The beetles eat the organic material causing loss of mechanical strength. Strength is lost because the cellulose cell walls are cut through, and can be very serious if the wood becomes badly infested.
What proportion of timber is harvested for construction?
At least one-third of all timber harvested goes into construction, the rest goes into paper-production, like fuel or is wasted during the logging process
How is ANISOTROPY related to timber?
Because of the way that timber grows, with most of the vessel cells running along the length of the tree, the timber has different properties in different directions. In other words, the properties are different along the grain from those measured across the grain - this is called ANISOTROPY
We all know that timber can swell in damp weather and shrinks in dry weather, what is this process called?
This swelling and shrinkage is called Moisture Movement. The amount of movement will depend on the species of wood, dense hardwoods such as Teak will show little movement, whereas Yellow Canadian Birch shows high movement. Other species such as English Elm give medium movement effects
How is Plywood produced?
The lamellae produced by peeling are glued together timber sheets. Notice that there is always an odd number of lamellae
How long is beam quality oak measuring 350mmX300mm usually air-dried prior to the sale?
8 years. Whereas Softwood 25mm thick, if air-dried in the Spring, should dry to about 20% moisture content in 1.5 to 3 months. Hardwood 25mm thick, if stacked in the Autumn, will take 7 to 9 months to dry to 20%, and 50mm hardwood will take about 1 year
What is the optimal temperature for grown for fungi?
The optimum temperature for growth varies for different species of fungi but is usually within the range from 20 to 30°C. Little growth takes place below 5°C and fungi are usually killed by prolonged heating to 40°C
Chemical Degradation information…
The acid resistance of impermeable timbers is greater than that of most common metals; iron begins to corrode below pH 5, whereas an attack on wood commences below pH 2, and even at lower pH values, proceeds at a slow rate. In alkaline conditions wood has good resistance up to pH 11
What does Photochemical Degradation look like?
Exposure to sunlight causes the colouration of the heartwood of most timbers to lighten. A few like Rhodesian teak and Douglas fir will darken.
What does Thermal Degradation look like?
There is some uncertainty about what minimum temperature will degrade timber, but it would appear that temperatures as low as 60°C can induce degradation over many years of exposure. The rate of degradation will rise markedly with an increase in temperature and time of exposure; hardwoods appear to be more susceptible to thermal degradation than softwoods
What is the effect of Fire on Timber?
Wood is an organic (hydrocarbon) material and in its dry state it will burn. When the wood is heated to 160 - 200°C, it will pyrolyse and give off flammable gases
What is the effect of Mechanical Degradation on Timber?
After 50 years the load which a piece of timber can withstand is less than half the stress it can carry at the onset of loading. Similarly, there is a reduction in stiffness (Young’s Modulus of Elasticity, E) with the time that shows itself as an increase in deflection with time under constant stress
Timber overstressed in compression in the longitudinal direction forms kink bands and compression creases. Such defects can reduce the tensile strength of the wood by 10 - 15%, but the loss of toughness under impact conditions can be as high as 50%. Mistreatment of wooden ladders and scaffold boards on site has led to injury and loss of life on numerous occasions.
