27110 - Wood - Mahidolmucc.mahidol.ac.th/~egpcp/Handout271/27110 - Wood.pdf · – Conversion of...

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C O N S T R U C T I O N M A T E R I A L ST I M B E R

© 2010 Praveen Chompreda, Mahidol University 2

O U T L I N E

• Trees– Components of Tree Trunk– Wood Cells– Hardwood vs. Softwood

• Physical Properties– Wood Defects– Moisture & Shrinkage– Density & Specific Gravity

• Lumbering– Conversion of Timber– Seasoning

• Mechanical Properties – Flexural, Compressive, Tensile, and

Shear Strengths– Factors Affecting Strength

• Deteriorations & Preventions– Mechanical, Physical, Chemical, and

Microorganisms deteriorations– Wood Protection Products– Fire Protection

• Wood Products– Plywood– Particle Board– Hard Board

Leonardo da Vinci timber footbridge, Norway2001, 40 m main spanSource: www.makingthemodernworld.org.uk

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T R E E SComponents of Tree TrunkWood CellsTree GrowthHardwood vs. Softwood

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C O M P O N E N T S O F T R E E T R U N K

Bark

เปลือกไม้

Cambium

แนวแม่เซลล์

Pith

ใจ Xylem

เนือ้ไม้

Phloem

เปลือกชัน้ใน

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T R E E T R U N K – B A R K & P H L O E M

• Outer Bark (เปลอืกไม)้ is a protective layer consisting of dead cells

• Can be very thick or quite thin depending on wood species

• Phloem or Inner Bark (เปลอืกชัน้ใน) is a thin layer of living cells used for transporting sugars to other parts of the tree

• Cambium (แนวแมเ่ซลล)์ is a thin layer separating the Phloem (Inner Bark) from the Xylem

Barkเปลือกไม้

Cambiumแนวแม่เซลล์

Pithใจ Xylem


Phloemเปลือกชัน้ใน

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T R E E T R U N K – X Y L E M & P I T H

• Xylem can be further categorized into Sapwood (กระพีไ้ม้) and Heartwood (แก่นไม้)

• Sapwood is a lighter color layer used for transporting water and foods

• Heartwood is the inner part of Xylem acting as a reservoir for wastes and is darker in color

• Pith is the soft thin-walled cells at the center of the trunk

Barkเปลือกไม้

Cambiumแนวแม่เซลล์

Pithใจ Xylem


Phloemเปลือกชัน้ใน

SapwoodHeartwood

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W O O D C E L L S

• Wood Cell consists of 4 major components– Cellulose – Long chain of glucose molecules, supply wood strength– Hemicellulose – Shorter chain of glucose and other sugar molecules,

providing framework for cellulose– Lignin – Large molecule that acts as binder to cellulose and

hemicellulose chains and make them rigid– Extractives – Other chemicals presented in small amounts and

responsible for color, odor, taste, and decay resistance of different woods

Cedar Redwood

Different Extractives

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T R E E G R O W T H

• Trees grow both in height and diameter– Height growth is through elongation of trunks only at

the tip– Diameter growth is through the cambium, which

produces Xylem towards the center of the trunk and Phloem towards the bark

• In temperate regions, trees grow mostly in early spring season– Springwood (Earlywood) has lighter color large thin-

walled cells– Summerwood (Latewood) has darker color small

thick-walled cells– The different color of springwood and summer wood

creates Annual Rings (วงปี)• In tropical regions (including Thailand), trees grow

continuously throughout the year. Thus, the annual ring is not well-defined

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C O M P O N E N T S O F T R E E T R U N K

Source: Illston and Domone (2001) 10

H A R D W O O D V E R S U S S O F T W O O D

• Hardwoods are woods from broad-leaved trees such as Oak, Poplar, Maple, Ash, Walnut, Cherry

• Softwoods are woods from coniferous trees such as Pine, Douglas Fir, Cedar

The word Hard and Soft is not indicative of its hardness property !!!

UNITED STATES

THAILAND (มอก. 421- 2525)

• ไมเ้นือ้แข็ง (Hard Wood) คอืไมท้ีม่ีความตา้นทานแรงดดัโคง้สงูสดุเกนิ 100 MPa ในสภาพทีเ่ป็นไมแ้หง้และมีความทนทานตามธรรมชาตเิกนิ 6 ปี

• ไมใ้บกวา้ง (Hardwoods) คอืไม ้ประเภทใบกวา้งทีม่เีมล็ดอยูใ่นรังไข ่มีใบเลีย้งคู ่และมทีอ่สง่นํ้าในเนือ้ไม ้ขยายใหญเ่ป็นพเิศษ

• ไมเ้นือ้ออ่น (Soft Wood) คอืไมท้ีม่ีความตา้นทานแรงดดัโคง้สงูสดุตํา่กวา่ 60 MPa ในสภาพทีเ่ป็นไมแ้หง้และมีความทนทานตามธรรมชาตไิมเ่กนิ 2 ปี

• ไมใ้บแคบ (Softwoods) คอืไมป้ระเภทใบแคบทีม่เีมล็ดอยูน่อกรังไข ่และไมม่ีทอ่สง่นํ้าในเนือ้ไมข้ยายใหญเ่ป็นพเิศษ

PC1

Slide 10

PC1 ไมส้น?Praveen Chompreda, 19/11/2005

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• Softwood (in the US sense), has almost uniform cell structures and most of the cells (about 90%) are aligned in the longitudinal direction. The cells that align transversely are called rays.

Source: Illston and Domone (2001)

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• Hardwood (in the US sense) has nonuniform structure. It has vessels (or pores) structure for conduction. This is not found in softwood.

• Also, only 80-95% of hardwood cells are in the longitudinal direction.


Vessel

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• วศิวกรรมสถานแหง่ประเทศไทยฯ แบง่ไมอ้อกเป็น 5 ประเภท ตามกําลงัรับแรงของไม ้

ไม้เนือ้อ่อนมากกะท้อน จาํปา-ป่า จกินม ยม-หอม ยางขาว สองสลึง

ไม้เนือ้อ่อนกราด กระเจา กะบาก ตะปูน-ขาว พะยอม ยางแดง สัก อนิทนิล

ไม้เนือ้แขง็ปานกลางกว้าว ตะเคียนทอง ตะเคียนหมู ตะแบก ตาเสือ นนทรี พลวง มะค่าแต้

ไม้เนือ้แขง็กันเกรา แดง ตะคร้อไข่ ตะคร้อหนาม เตง็ ประดู่ มะค่าโมง ยมหนิ รัง เลียงมัน หลุมพอ สัก-ขีค้วาย เคี่ยม

ไม้เนือ้แขง็มากกระพีเ้ขาควาย เขล็ง ตนีนก บุนนาค

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P H Y S I C A L P R O P E R T I E SWood DefectsMoisture & ShrinkageDensity & Specific Gravity

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W O O D D E F E C T S

• The lesser the defects, the more desirable is the wood

• Natural defects occur by the nature of the wood such as knot, irregular grain, and some types of shakes

• Artificial defects are caused by mishandling of the timber or incorrect seasoning techniques

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M O I S T U R E & S H R I N K A G E

– The weight and volume of wood change as the moisture condition of the wood changes

– Moisture Content is calculated as:

– Wd = weight of oven-dry wood, Wm = moist weight

– Water may be in cell walls (called Imbibed Water) or cell cavities (called Free Water)

– The point when cell walls is saturated and no free water is called Fiber Saturation Point

– Shrinkage occurs when moisture content drops below fiber saturation point

m d

d

W -WMC = ×100%

W

17


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• Moisture content in the wood changes with the ambient relative humidity until it reaches the equilibrium condition

• Equilibrium moisture content is the moisture content at which wood neither gains nor loses moisture to air at a given relative humidity and temperature

• For the same relative humidity, the equilibrium moisture content decreases as the temperature increases

Source: Wilcox et al. (1991)

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M O I S T U R E & S H R I N K A G E• Another way of plotting moisture content v.s. relative humidity v.s. temperature


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– Tangential Shrinkage > Radial Shrinkage > Longitudinal Shrinkage (~0)


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• Shrinkage varies greatly among different species of woods

• Tangential shrinkage is significantly greaterthan Radial shrinkage for most woods

• Different in rate of shrinkage in two direction may result in warping and/or shake

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Source: Forest Products Laboratory (1990) 23



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D E N S I T Y & S P E C I F I C G R A V I T Y

• Density & Specific Gravity– Density is calculated as:

– Specific gravity is calculated as:

• Density of water is 1 g/cm3. Therefore, density of wood in g/cm3 is numerically identical to its specific gravity

Oven-Dry WeightDensity =

Volume at 12% MC

Density of Wood

Specific Gravity = Density of Water at 4 C

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D E N S I T Y

• Density = M/V• Mass (M) can be obtained by

weighing the specimen in air• Volume (V) cannot be accurately

computed by measuring the dimensions of the specimen

• Obtain the volume by weighing the specimen in water

air water

wood

W -WV =

g

Wood Specimen

Wair = Mg B = wVwood g

T = Wwater

Water

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D E N S I T Y & S P E C I F I C G R A V I T Y

• Density & Specific Gravity– Wood substance (cell wall

material) has specific gravity of about 1.5 regardless of species

– Density or Specific Gravity of wood is an indication of the amount of wood substancepresented in the piece

– Typical Range of Specific Gravity of Thai Woods: 0.5-1.2

ไม้ Specific Gravityแดง 1.05

ตะเคียนทอง 0.76ตะเคียนหนู 0.87

เตง็ 1.07ประดู่ 0.83

มะค่าแต้ 0.99มะค่าโมง 0.85มะฮอกกะนี 0.66

ยาง 0.69รัง 1.11สัก 0.63

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L U M B E R I N GConversion of TimberSeasoning

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L O G G I N G• Use band saw or chain saw to cut

the tree• After the tree is felled, the

branches and leaves are removed and the logs are cut to length

• The logs are then transported to a saw mill to be cut into wood

Lumberjacks circa 1910Modern Harvester MachineSource: Wikipedia Source: Wikipedia

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L O G G I N G

Log transport by river

Logs are cut to length before transport

Log transport by road

Source: Wikipedia

Source: Wikipedia

Source: Wikipedia

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L O G G I N G

Logs waiting to be cut at sawmill

Small-scale portable sawmill

Finished product!

Source: Wikipedia

Source: Wikipedia

Source: Wikipedia

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C O N V E R S I O N O F T I M B E R

• There are two broad ways of cutting timber into planks

• Plainsawing – is a cut tangent to the annual rings exposing tangent surface(obtaining flat-grained lumber)

• Quatersawing – is a cut in the radialdirection from the center of the log exposing radial surface (obtaining edge-grained lumber)

• Which method is to be used depends on:

– Strength desired (radial cut plank is stronger, stiffer, and less likely to warp)

– Wood grain (some wood exhibit beautiful grain when cut radially)

– Shape and defects of the timber– Costs (radial cut is more difficult and

generates more waste)

Source: Forest Products Laboratory (1990)

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• In wood such as oak, a desirable feature called “medullary ray” is exposed when cut radially

Source: Wikipedia


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• Experience is needed to select the most economic way to cut the timber• Different parts of the section may be suitable for different applications• Wood defect also plays an important role in the selection


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Modern sawmill uses computer to calculate the most economic pattern and has laser-guided blades

Source: Wikipedia

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S E A S O N I NG O F T I M B E R

• The objective of seasoning (การอบผึง่ไม)้ is to reduce the moisture inside the “green” wood to equilibrium moisture content– If unseasoned wood is used, shrinkage will occur– Natural drying of wood through evaporation can take years– Even if seasoned, wood can still shrink or swell under changes in

humidity – There is no need to completely eliminate the moisture inside the wood

(i.e. to oven-dry) because wood will reabsorb the ambient humidity

• Two main types– Air Seasoning– Kiln (Artificial) Seasoning

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• Air Seasoning – May take about 1 year for 1

inch-thick wood and much longer for thicker wood

– Control the rate of drying by changing the spacing between pieces of wood

Source: Wikipedia

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• Kiln Seasoning– Kiln is a chamber which has fans to blow heated air and jets to introduce steams– Steam is needed to control the rate of drying at the surface

• prevent the “case hardening” in which the surface is dried and shrunk while the core is still wet

• Prevent splitting from rapid drying

– Kiln seasoning is much faster than air seasoning – usually takes about 2-10 weeks depending on the wood species and thickness

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M E C H A N I C A L P R O P E R T I E SFlexural StrengthCompressive StrengthTensile StrengthShearing StrengthOther PropertiesFactors Affecting Strength

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M E C H A N I C A L P R O P E R T I E S

• Tensile Strength– Parallel to grain– Perpendicular to grain

• Compressive Strength– Parallel to grain– Perpendicular to grain– Oblique to grain

• Flexural Strength (Modulus of Rupture)

• Shearing Strength– Parallel to grain– Perpendicular to grain

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T E N S I L E S T R E N G T H

• Tensile strength parallel to grain is much larger (in the order of 50:1) than tensile strength perpendicular to grain

• Example of tension parallel to grain is a truss; however, wood rarely fails by direct tension

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T E N S I L E S T R E N G T H

• Formula for Tensile Strength:

• For the tensile strength perpendicular to grain, wood is stronger when the failure plane is in tangential surface than the radial surface – average value is usually reported

>

t

T =

A

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C O M P R E S S I V E S T R E N G T H

• Compressive strength parallel to grain is higher (in the order of 10:1) than the compressive strength perpendicular to grain

• Wood compressed perpendicularto grain never “fails” even at large deformation often used as support or bearing plate

• For compression perpendicular to grain, wood is stronger when load is applied in the tangentialdirection than the radial -- average value is usually reported

>

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C O M P R E S S I V E S T R E N G T H

• Wood under compression parallel to grain, such as columns, is often limited by the buckling strength, rather than the compression strength of the wood

• Formula for compressive strength

where is the critical stress where buckling occurs. Within the proportional limit (linear elastic behavior), this can be calculated using Euler’s formula. depends on the elastic modulus (E), length (L), and cross-sectional dimensions (r)

2

critical 2

C E =

A (L/r)

critical

c critical

C =

A

critical

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B E N D I N G S T R E N G T H

• Wood under bending usually fails on compression side before the tension side (unless the grain is not parallel with the longitudinal direction of the member)

• Maximum bending stress is called modulus of rupture

• This is really not the stress in the top or bottom wood fiber because the compressive stress is nonlinear near the maximum strength

• Comp // < MOR < Tens //

M cMOR =

I

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B E N D I N G S T R E N G T H

• Bending Failure Modes

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S H E A R I N G S T R E N G T H

• Shear strength parallel to grain (either longitudinal or rolling shear) is much smaller than the shear strength perpendicular to grain (vertical shear)

• Shear failure perpendicular to grain rarely occurs as it often precedes by bending failure or compression failure at load point

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S H E A R I N G S T R E N G T H

• Shear Strength Formula:

• Shear strength parallel to grain is critical in designing bolt holes

V =

A

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O T H E R P R O P E R T I E S

• Modulus of Elasticity– Slope of the linear portion of the

compression stress-strain curve within the elastic range

• Hardness Test– Measure the hardness of the

surface by pushing a metal ball on the surface to a certain depth and measure the force

E =

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O T H E R P R O P E R T I E S

• Cleavage Test– Measure the potential that the wood can be tear along the grain

• Nail Withdrawal Test– Measure the ability of wood to hold a nail

• Impact and Toughness Test– Using a drop mass or pendulum to apply impact force to a simply-supported

beam specimen– Toughness is the work required to fail the specimen which, together with the

strength, can indicates how ductile the wood is

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F A C T O R S A F F E C T I N G S T R E N G T H

• The strength (compressive, tensile, or shear) and stiffness (modulus of elasticity) of wood is generally affected by the followings:– The grain angle relative to loading

direction (therefore, wood is said to be an Anisotropic material)


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– Knots (Defects): knots cause the distortion of grain, thus reduce the strength capacity. The sizes, distributions, and locations of knots are the key factors. The strength of a real structural-size timber is lower than the strength of small clear test specimens due to the presence of knots.


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– Density (or Specific Gravity): The higher the density, the greater the strength. The density is affected by the species of the wood and, for each specie, the rate of growth. The latewood is stronger and denser than the earlywood due to the thicker cell walls.


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Source: Illston and Domone (2001) 55


– Moisture Content: The general trend is that the strength decreases as the moisture content increases. However, different strength properties are affected at various degrees.


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– Temperature: The strength decreases as the temperature increases. The effect is greater at high moisture content.


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D E T E R I O R A T I O N S & P R E V E N T I O N SMechanicalPhysicalChemicalMicroorganismsInsectsPreventionsWood Protection Products

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D E T E R I O R A T I O N O F W O O D

• Deterioration of wood may be divided into 5 categories– Mechanical– Physical– Chemical– Microorganisms– Insects

• Different species of wood has different resistant to deteriorations due to different in cell structures and extractives presented. Sapwood part is less durable than the heartwood because it has no extractives.

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– Floors subjected to heavy equipments rolled on them

– Wood in desert was blown by sand– Structural member at loose joint rub against

each other when subjected to vibration– Creep of wood under sustained load causes

loss of strength

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– Ultraviolet exposure (especially at high elevations) - causes the breakdown of lignin, which holds the wood cell together

– Repeated cycles of wetting and drying –causes shrinkage failures and warping

– High heat (in the order of 200 °C) or lower heat (at > 120 °C or so) but for a long period of time.

Damage due to UV Rays

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– Wood is quite resistant to acids but can be destroyed by alkalis due to the dissolution of lignin and hemicellulose

– Contacts with alkaline vapors in petroleum or chemical plants

– In some woods, iron in rusting fasteners can cause iron-catalyzed hydrolysis that destroy wood

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– Fungi• Mold • Stain• Soft Rot Fungi• Decay Fungi

– Bacteria

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M I C R O O R G A N I S M D A M A G E S

Soft Rot (Alligator Skin Pattern & Soft)

White Rot from Decay FungiBrown Rot from Decay Fungi (Cubic cracks)

Mold Fungi

Source: Wikipedia

Source: Wikipedia Source: Wikipedia

Source: Wikipedia

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– Termite– Beetles– Wood borers– Etc…

Source: UNEP (2000)

Source: UNEP (2000)

Source: UNEP (2000)

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T E R M I T E S

• There are more than 2600 species of termite but they can be broadly classified into 4 main types:– Dampwood Termite – Drywood Termite– Subterranean Termite– Arboreal/ Mound Builders Termite

• Protection strategies and extermination vary among types

Queen

KingWorkers

Soldier

Source: UNEP (2000)

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T E R M I T E S

• Dampwood Termite– Most of the species like very damp wood – Their colony is often small and the damage is usually slow (decay fungi may be

more of a problem in the same environmental condition)

Source: UNEP (2000)Source: UNEP (2000)

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T E R M I T E S

• Drywood Termite– Most species attack dry wood and can live in a low moisture environment– Difficult to detect as it lives inside the wood– Sign of drywood termite is its fecal pellets near their nest

Source: UNEP (2000) Source: UNEP (2000)

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T E R M I T E S

• Subterranean Termite– Live in soil underground or in moist condition, sometimes live in a tree– They travel to their food source through tunnels in wood or shelter tubes

(which made up of soil, bits of wood, and their fecal material)

Source: Wikipedia

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T E R M I T E S

• Arboreal/ Mound Builders Termites– Usually build their nest on trees, fences, under roof etc…– Reach the building through aboveground tunnel

Source: Wikipedia Source: Wikipedia

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P R E V E N T I O N S O F D E T E R I O R A T I O N

• Key to the prevention of wood deteriorations:

W A T E R ! ! !

• Microorganisms, especially fungi, need water to live• Subterranean termite and many insects like moist condition• Moisture causes shrinkage and swelling• Water dilutes wood cell structure in the presence of some

chemicals (such as alkali)• Water washes away wood protection agents

They call all happen at the same time!

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P R E V E N T I O N S O F D E T E R I O R A T I O N

• Wood structure should be designed such that water will not accumulate anywhere (termite, fungi, & moisture from soil)

• Avoid direct ground contact with wooden parts (termite & moisture from soil)

• Foundation and attic should be dry and well ventilated ((insects and microorganisms)

• Select the right kind of wood for the job (durability)• Apply wood protection and/or finishing chemicals (insects and

microorganisms)• Building should be designed such that inspection can be done easily• Provide physical barrier and/or screens between building and surrounding

area (insects especially termite)

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W O O D P R E S E R V A T I V E S

• Wood preservatives provide protections from Insects/ Microorganisms

• Ways to apply wood preservatives– At lumber manufacturing plants

• Pressurizing• Dip, Hot/Cold Soaking

– At construction sites• Dip• Brush, Spray

• Types• Oilborne Preservative• Water Soluble Preservative

• Leechable Type• Fixed Type

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Pressure Chamber

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• Oilborne Preservatives– Utilizes oil to carry preservatives into the wood– Examples:

• Coal-Tar Creosote/ Petroleum Creosote– Heavy black-brown liquid produced by condensing vapors

from heated carbon-rich sources, such as coal or wood. It is sometimes mixed with tar oils and petroleum oils.

– Advantages: Very effective, Very toxic to insects & fungi, not water-soluble, good absorption to wood, inexpensive

– Disadvantages: Strong odor, flammable, impart a dark color to the wood, wood is unpaintable, may cause irritation when contacted with skin. Long-term exposure may lead to skin cancer

– Mostly used for railroad ties, bridge timbers, piling, and utility poles

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Creosote Treated Wood

Source: Wikipedia

Source: Wikipedia

Source: Wikipedia

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• Pentachlorophenol (C6HCl5O)– White organic solid with needle-like crystals and a phenolic

odor– Advantages: Toxic to insects & fungi, not water-soluble– Disadvantages: Environmental damage issues – if only low level

is contaminated in the water sources, it may cause damages to liver or kidney, cancer, damage to nervous system; strong odor; flammable; impart a dark color to the wood; wood is unpaintable; may cause irritation when contacted with skin

– Mostly used for railroad ties, bridge timbers, piling, and utility poles

• Copper Naphthenate– Advantages: Moderately effective– Disadvantages: Can induce corrosion in metals such as

fasteners and hinges

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• Water-Soluble “Leachable” Preservatives– Dissolved in water but also can be washed away so

they are not suitable for exterior applications– Examples:

• Zinc Chloride, Sodium Fluoride– Advantages: Inexpensive, help paint attach

to wood better, odorless, not flammable, easy to handle and transport (white powder to be dissolved in hot water)

Zinc Chloride

• Borates– Naturally occurring minerals that exist in small amounts in rock,

soil, water and all living things. – Advantages: Effective fungicide/ insecticide, low toxic to human, low

environmental impact, odorless, colorless, non corrosive to metals

Source: Wikipedia

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• Water Soluble “Fixed” Preservatives– Dissolved in water but fixed on wood once applied– Examples:

• Copper Chrome Arsenate (CCA) – most popular• Ammoniacal Copper Arsenite (ACA)• Ammoniacal Copper Zinc Arsenite (ACZA)• Acid Copper Chromate (ACC)• Copper Chrome Boron (CCB)• Fluor Chrome Arsenate Phenol (FCAP)• Copper Azole

– Advantages: Will not be leached out by water, does not change color of wood at low levels, odorless, wood can be painted

– Disadvantages: Effectiveness varies among wood species, should not be used where it is in contact with food or water

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• Many of the wood preservatives has environmental and health issues

– European Union ban the non-commercial use of creosote in 2003– Pentachlorophenol general public purchase in the USA was restricted in 1980s– CCA use in USA is currently restricted by Environmental Protection Agency

(EPA) to only certified applicator. No residential wood treated with CCA was sold since 2003.

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W O O D F I N I S H E S

• Wood finishes provide protection from the environment

– Water-repellent preservative is a non-film-forming liquid consisting of wax/resin in solvents and sometimes with fungicide

– Stain is a non-film-forming liquid consisting of wax in solvents, some pigments, and usually with preservatives. Pigments not only dye the wood color but also help preventing photodegradation of wood cells.

– Penetrating Oil is a non-film-forming finish often used in furniture to protect wood from staining and hot dishes

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Various Colors of Wood Stain

Furniture with Penetrating Oil Finish

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– Film-forming clear finishes (Lacquer, Varnishes, Shellac, Polyurethane)form a thin film on the surface which repel water. They are not suitable for exterior finish because light can pass through and damage wood as well as the finishing itself, which eventually causes the film to peel-off.

– Paint (Oil-based or Water-based (latex) paint) contains color pigments, binder (hold the pigment and provide glossy finish), and solvent. It provides film finish and also prevent photodegradation of wood. Oil-based paints can penetrate wood better than the water-based types and may be used as a primer

• They may be used in combination with each other

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F I R E P R O T E C T I O N O F W O O DBurning of WoodFire Retardants

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B U R N I N G O F W O O D

• There are 3 processes involved when wood is exposed to elevated temperature1. Drying of Wood

• The water inside the wood (in cavities and cell walls) boils up at 100 °C and releasing steam

• This process consumes a lot of heat energy and helps delay the next step• No loss of mass in wood – wood reabsorbs moisture when cooled

2. Thermal Degradation (Pyrolysis)• Occurs at the temperature over 70 °C and ends at about 480 °C • No oxygen is needed in the process• There is a loss in mass and strength – wood constituents decompose to

gases (such as carbon monoxide, formic acid, acetic acid, and methanol) and charcoal

• Charcoal produced resists further burning (charcoal has only 30-50% of thermal conductivity of normal wood)

• Slow pyrolysis occurs at low temperature, Rapid pyrolysis occurs at much higher temperature (150-250 °C)

• Strength loss of wood is much slower than steel or aluminum

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3. Ignition• Pyrolysis product reacts with oxygen (exothermic reaction) and progress to

combustion and burning of fire. Fire becomes self-supporting.• Ignition occurs only under favorable conditions:

– Gases from pyrolysis must be combustible– Gases and oxygen must be in appropriate concentration– Must have sufficient heat

• Large section of wood burns slowly because oxygen cannot reach the inside• Spontaneous Ignition (no pilot fire) occurs over 430 °C but may occurs over

long period of slow pyrolysis at low temperature• Induced Ignition occurs by pilot fire raise the temperature at contact point

to ignition level• Combustibility of wood depends on the species of wood

– Flammable extractives that may present– Density (affect thermal diffusivity)

• Spreading of fire is a series of ignition (one area is acting as a pilot fire for the adjacent area)

88

I M P R O V I N G F I R E C H A R A C T E R I S T I C S

• Retarding pyrolysis (prevent strength loss)– Minimize heat of combustion– Insulate wood from heat of fire– Promoting charcoal formation

• Reduce flame spread or surface flammability– Minimize or dilute combustible gas produced– Blanket surface with non-combustible gas to prevent reaction with oxygen

• Prevent afterglow of charcoal– Blanket surface with non-combustible gas

• Reduce formation of smoke– Most people are killed by smoke inhalation, not the heat of fire– However, most of the smoke are produced from building contents that burn before

the wood structure

89

F I R E R E T A R D A N T S

• Currently, there is no perfect fire retardant for wood– Poor retention capacity– Costs– Many reduce only flame spread but do not reduce pyrolysis– Many attract moisture into the wood (raise the equilibrium MC)– Corrosion of metals

• Examples are: – Ammonium phosphate– Ammonium sulfate– Ammonium chloride– Borax (sodium tetraborate) – Zinc chloride

• Fire retardants are usually pressure-treated at the manufacturing plant with wood preservatives

90

W O O D P R O D U C T SPlywoodParticle BoardHard Board

91

W O O D P R O D U C T S

• Besides lumber, following products are also derived from wood:– Plywood– Particle Board– Hardboard– Paper– Resin Products (turpentine, tannins, volatile oils, dyes)– Sugar– Plastic– Etc…

92

P L Y W O O D

• Plywood is a crossbanded assembly of wood (veneer) layers joined with adhesive

• Crossbanding = arrangement of wood fiber in alternating direction

• Layers may have different thicknesses, or of different species

• More layers are needed to achieve thick plywood

• The number of layers is usually the same on each side of the core(center layer). Therefore, there are odd number of layers

Source: Wilcox et al. (1991) 93

P L Y W O O D

• Properties– Tensile strength of plywood is between that of solid wood in the

transverse direction (small) and longitudinal direction (large)– Shrinkage of plywood is between that of solid wood in the fiber

direction (very small) and solid wood across the grain (high shrinkage)– Warping may occur due to different moisture condition of face and

back or due to differences in species, thickness, and natural variability on both sides

– There are interior and exterior grades, depending on the adhesive used

94

P A R T I C L E B O A R D

• Particle Board is obtained by bonding fragments of wood with glue (binder/ resin) and compressing under pressure to form a sheet (Extruded or Mat-Form)

• The higher the pressure, the stronger the board (particle board may have 20-30% greater in density than wood it made up of)

• Glue only coats fractions of the particle surface

• Particle board may have layers containing different particle size, or have particle size gradually changes from the middle to the surface (the core will have cheap coarse particles)

Source: Wilcox et al. (1991) 95

P A R T I C L E B O A R D

• Mat-Formed Particle Board swell/shrink in plane slightly more than plywood but less than lumber in tangential/radial directions

• Mat-Formed Particle Board swell in thickness much more than timber and plywood. This is because the particles that were compressed during the manufacturing try to “spring back” moisture protection is important, especially around the edges

• Extruded Particle Board swell very little in thickness but much more in plane considerable warping may occur

• Particle board often used as a core with veneered surface such as cabinets, countertops

96

H A R D B O A R D

• Hardboard is made up of wood pulp (lower-grade than those used for paper making) compressed under heat

• No glue is used. The heat and pressure activates lignin inside the pulp to act as a binder

• Examples of applications: perforated board (for ventilation or sound absorbing), interior siding panels, picture frame backing

97

S T A N D A R D S F O R T I M B E R P R O D U C T S

• มอก 421-2530 ไมแ้ปรรปู: ขอ้กําหนดทั่วไป– กําหนดประเภท ขนาด การแปรรปู การแบง่ชัน้คณุภาพไม ้สําหรับไมท้ีไ่ดจ้ากการแปรรปูดว้ยเครือ่งจักร

• มอก 424-2530 ไมแ้ปรรปูสําหรับงานกอ่สรา้งทั่วไป– สําหรับไมท้ีต่อ้งนําไปใชใ้นการรับแรงอดัหรอืแรงดงึ เชน่ทําคาน เสา จันทัน แป

• มอก 178-2538 แผน่ไมอ้ดั– กําหนดประเภท ขนาด ชัน้คณุภาพไม ้สว่นประกอบ การผลติ และคณุสมบตัทิี่ตอ้งการ สําหรับไมอ้ดัทีท่ําจากแผน่ไมบ้างตัง้แต ่3 ชัน้ขึน้ไป

98

S T A N D A R D F O R S A W N T I M B E R

• มอก 421-2530– ไมท้ีใ่ชแ้ปรรปูแบง่เป็น 2 ชนดิ คอืไมส้กั และไมก้ระยาเลย (ไมอ้ืน่ๆนอกจากไมส้กั)– ความหนามาตรฐาน 12 16 19 22 25 32 38 44 50 63 75 88 100 113 125 138

150 200 มลิลเิมตร– ความกวา้งมาตรฐาน 25 38 50 63 75 88 100 113 125 150 175 200 225 250

275 300 350 400 มลิลเิมตร– ความยาวมาตรฐาน ไมส้กั เริม่ที ่0.30 เมตร และเพิม่ชว่งละ 0.15 เมตร

ไมก้ระยาเลย เริม่ที ่0.30 เมตร และเพิม่ชว่งละ 0.30 เมตร– เรยีกขนาดตาม ความหนา x ความกวา้ง x ความยาว– การแปรรปูไมจ้ะมกีารเลือ่ยเผือ่ขนาดทัง้ความหนาและความกวา้ง ตัง้แต ่1.5 มลิลเิมตรสําหรับไมส้กัและไมก้ระยาเลยขนาดเล็ก ถงึ 15 มลิลเิมตรสําหรับไม ้กระยาเลยขนาดใหญ่

99

S T A N D A R D F O R S T R U C T U R A L T I M B E R

• มอก 424-2530 แบง่ไมอ้อกเป็น 3 ชัน้คณุภาพ– ชัน้ 80 คอืไมท้ีม่คีวามตา้นทานแรงอดัและแรงดงึไมน่อ้ยกวา่รอ้ยละ 80 ของไมช้นดิเดยีวกนัทีป่ราศจากตําหน ิและมขีนาดของตําหนติา่งๆไม่เกนิขนาดทีก่ําหนดไวใ้นคณุลกัษณะทีต่อ้งการในมาตรฐาน (เชน่ไมท้ี่รับแรงดงึ ตอ้งมตีาไมเ่กนิ 1/8 ของความกวา้งหรอืความหนาทีม่ตีา เป็นตน้)

– ชัน้ 67 คอืไมท้ีม่คีวามตา้นทานแรงอดัและแรงดงึไมน่อ้ยกวา่รอ้ยละ 67 ของไมช้นดิเดยีวกนัทีป่ราศจากตําหน ิและมขีนาดของตําหนติา่งๆไม่เกนิขนาดทีก่ําหนดไวใ้นคณุลกัษณะทีต่อ้งการในมาตรฐาน

– ชัน้ 50 คอืไมท้ีม่คีวามตา้นทานแรงอดัและแรงดงึไมน่อ้ยกวา่รอ้ยละ 50 ของไมช้นดิเดยีวกนัทีป่ราศจากตําหน ิและมขีนาดของตําหนติา่งๆไม่เกนิขนาดทีก่ําหนดไวใ้นคณุลกัษณะทีต่อ้งการในมาตรฐาน

80

67

50

100

S T A N D A R D F O R P L Y W O O D

• มอก 178-2538 แบง่ไมอ้ดัออกเป็น 3 ประเภท– ภายนอก - ใชก้าวทีท่นทานตอ่ลมฟ้าอากาศ– ภายใน - ใชก้าวทีท่นพอสมควร– ชัว่คราว – ใชก้าวทีม่คีวามทนทานจํากดั เหมาะกบังานชัว่คราว

• แบง่ชัน้คณุภาพไดเ้ป็น 4 ชัน้ โดยแตล่ะชัน้จะกําหนดขนาดใหญท่ีส่ดุของตําหนทิีจ่ะมไีด ้– ชัน้คณุภาพ 1 (เกรด I) เหมาะสําหรับงานทีต่อ้งการแสดงผวิหนา้ไม ้– ชัน้คณุภาพ 2 (เกรด II) เหมาะสําหรับงานทีค่วรทาสทีับผวิหนา้ไม ้– ชัน้คณุภาพ 3 (เกรด III) เหมาะสําหรับงานทีต่อ้งทาสทีับผวิหนา้ไม ้หรอืที่ๆ ไม่อาจเห็นผวิหนา้ได ้

– ชัน้คณุภาพ 3 (เกรด III) เหมาะสําหรับงานทีผ่วิหนา้ไมไ้มม่คีวามสําคญั

• ความหนา 2 3 4 6 10 12 15 และ 20 มลิลเิมตร

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R E F E R E N C E S

• ASTM (1994), Standard Methods of Testing Small Clear Specimens of Timber, D 143-94, West Conshohocken, PA

• ASTM (1994), Standard Test Methods for Specific Gravity of Wood and Wood-Based Materials, D 2395-94, West Conshohocken, PA

• Forest Products Laboratory (1990), Wood Engineering Handbook, 2nd Edition, Prentice-Hall, New Jersey.

• Microsoft Corporation (1999), Encarta Encyclopedia, Richmond, WA.• Smith, R. C., Andres, C. K. (1989), Materials for Construction, 4th Edition, McGraw-

Hill, New York, 401 pp.• Watson, D. A. (1986), Construction Materials and Processes, 3rd Edition, McGraw-Hill,

New York, 486 pp.• Wilcox, W. W., Botsai, E. E., Kubler, H. (1991), Wood as a Building Material: A Guide

for Designers and Builders, John Wiley & Sons, New York, 215 pp.• UNEP, FAO, Global IPM Facility Expert Group (2000), Finding Alternatives to

Persistent Organic Pollutants (POPs) for Termite Management, http://www.chem. unep.ch/pops/termites/termite_fulldocument.pdf, 47 pp.

• http://www.wikipedia.org• http://www.ccaresearch.org

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R E C A P

• Trees (components of tree, wood cells, growth of wood, hardwood & softwood)

• Physical characteristic (defects, moisture & shrinkage, density & specific gravity)

• Timbering (conversion of timber, seasoning)• Mechanical Properties (bending strength, tensile strength, compressive

strength, shear strength)• Deterioration of wood (fungi, termites, etc)• Wood protections• Other wood products (plywood, particle board, hardboard)

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