土木工程材料 課件 Chapter 8 Wood

Materials in Civil

andConstructionEngineering土木工程材料Chapter

8WoodIntroductionProperties

of

Wood8.3

Applications

of

Wood

in

Architecture8.4

Corrosion

and

ProtectionContent8.1Introduction8.1 IntroductionWood,

because

of

its

availability,

relatively

lowcost,

ease

of

use,anddurability,

continues

tobean

importantcivil

engineering

material.

Woodisused

extensively

forbuildings,bridges,

utility

poles,floors,roofs,

trusses,

andpiles.

Civilengineering

applications

include

both

natural

woodand

engineered

wood

products,suchas

laminates,plywood,

andstrand

board.Woodisused

as

architectural

anddecoration

material

for

its

severaladvantages

as

follows:

high

specificstrength

(lightweight

andhighstrength),

high

frost

resistance,

chemical

resistance,

low

thermalconductivity,

easyprocessing,

easy

components

connection.

Andfor

surethewoodalso

hasfollowing

disadvantages:

unevenstructure,

natural

flaw,

highmoisture

absorption,

whichleadtolarger

sizechange,

easier

warping,

cracking,burninganderosion.Butifwe

can

use

reasonably,

theabovedisadvantages

can

beovercome.Trees

are

a

kindof

natural

resources

withslowgrowth.However,

there

isagreat

demand

fortrees

invariousindustries.

In

China,

theforest

coverage

rateis

about16.55%,whilethe

world

average

forest

coverage

rateis31.7%.

Forcivil

engineering,

itis

importanttounderstand

theproperties

of

woodcorrectly

andmakerational

andeconomical

use

ofwoodresources.8.1 Introduction8.1.1 ClassificationAccordingto

treespecies,

trees

aremainly

classified

into

two

species:

conifer

and

broadleaf.ConiferThe

main

characteristics

of

conifers

are

as

follows:

The

leaves

are

slime

and

long

and

needle-like,

the

trunksare

straight

and

tall,

and

ligneous

tissue

is

soft,

liable

to

process.

Of

superior

strength,

apparent

density

is

low,

andshrinkage

deformationis

low.

The

most

of

them

used

in

architecture,

mainly

for

doorsand

windows,

decoration

orbearing

component.

The

common

species

are

pine,juniper,

cypress,

etc.Broad-leaved

TreeThe

main

characteristics

of

broad-leaved

trees

are

as

follows:

Leaves

are

broad

and

shape

in

sheets,

so

mostof

which

are

hardwood.

The

straight

parts

of

the

trunks

are

short,

and

ligneous

tissue

is

hard,

noteasy

to

process.The

apparent

density

is

high,

and

the

shrinkage

deformation

is

high,

which

is

easy

to

crack

or

warp.

This

kind

oftrees

are

used

for

minor

load-carrying

member

in

interior

decoration

or

veneer.

The

common

species

are

birch,elm,

etc.8.1 Introduction8.1.2 StructureThe

performance

of

wood

derives

from

the

structure

of

the

wood.

Civil

and

construction

engineers

need

tounderstand

the

way

the

tree

grows

and

the

anisotropic

nature

of

wood

in

order

to

properly

design

and

constructwoodstructures.

Thestructureof

woodcan

be

classified

into

macrostructure

and

microstructure.1.

TheMacrostructure

of

WoodThe

wood

structures

that

can

be

seen

by

eyes

or

through

magnifying

glass

are

called

the

macro

structure

ofwood.In

order

to

observe

closely,

the

trunksare

cut

into

threedifferent

sections:Tangential

section:thesection

thatparallelswith

the

trunk

axis

andtangent

withthe

annualring.Radial

section:

thesection

thatpasses

thetrunkaxis;Transverse

section:the

section

thatisvertical

againstthetrunkaxis;8.1 IntroductionThe

wood

is

made

up

of

bark,

xylem

and

pith.

Bark

is

mainly

used

for

burning

except

certain

species

of

trees(cork

oak,

and

yellow

pineapple

tree)

whose

bark

can

be

used

to

make

heat-proof

materials.

Pith

is

in

the

centralpart

of

the

trunk,

whose

texture

is

loose

and

fragile,

and

is

liable

to

be

corrupted

or

eaten

by

insect

worms.

So

thebest

part

for

use

isthe

xylem

of

trunk.In

the

xylem,

the

darker

part

near

the

pith

is

called

duramen,

while

the

light

part

outside

is

called

albumum.The

duramen

contains

little

water

so

that

it

is

not

liable

to

reshape,

and

it

has

high

corrosion

resistance.

While

thealburnum

containsmore

water

that

it

iseasy

to

deform

and

has

worse

corrosion

resistancethan

duramen.On

the

transverse

section

many

centric

circles

can

be

seen,

which

are

called

annual

rings.

Of

them

the

part

indark

color

and

lie

close

are

grown

in

summer,

called

summerwood.

And

the

converse

part

is

grown

in

spring,called

springwood.

The

more

summerwood

wood

has,

the

better

the

wood

is.

The

more

intense

and

evener

annualrings

thewood

has,thebetter

quality

thewoodhas.8.1 Introduction2.

TheMicrostructure

of

WoodThe

wood

structures

that

can

be

seen

through

microscope

are

called

microstructure

of

wood.

Wood

iscomposed

of

numerous

tubular

cells.

The

long

dimension

of

the

majority

of

cells

is

parallel

to

the

tree's

trunk.However,afew

cells,

in

localized

bundles,

growradially,

from

thecenter

to

theoutsideof

the

trunk.Every

cell

can

be

classified

into

two

parts:

cell

wall

and

the

lumen.

The

cell

wall

is

composed

of

fibrils.

Thelongitudinal

combination

is

firmer

than

the

transverse

combination.So

the

cell

wall

is

of

high

strength

inlengthways,

but

of

low

strength

in

transverse.

There

are

very

little

spaces

among

the

fibrils

composed

of

cell

wall,which

enables

thematerial

to

absorb

or

leak

water.The

structure

of

the

cell

determines

the

physical

characteristics

of

wood.

For

example:

the

wood

with

thick

cellwalland

smalllumen

isintense

and

hard,and

its

bulk

specificgravity

ishigh

and

it

is

of

high

strength.8.2Properties

of

Wood8.2 Propertiesof

Wood8.2.1

Physical

Properties1.MoistureContentof

WoodThe

moisture

of

wood

is

measured

in

the

percentage

of

water

content,

which

is

the

percentage

of

the

mass

ofwater

to

the

massof

dry

wood.The

Water

in

WoodThe

water

in

the

wood

can

be

classified

into

the

free

water

that

lies

in

intercellular

space

and

the

absorbedwater

that

lies

inside

the

cell

wall.

The

newly-cut

wood

is

green

wood.

There

is

a

plenty

of

free

water

andabsorbed

water

in

it.

And

the

percentage

of

water

content

ranges

from

70

%

to

140%.

When

wood

becomes

dry,the

free

water

is

the

first

to

evaporate,

but

at

this

time

the

size

and

mechanical

property

of

wood

are

notinfluenced.

When

the

free

water

finishes

evaporating,

the

absorbed

water

begins

to

evaporate.

The

process

ofabsorbed

water

evaporating

is

slow,

and

duringit

the

bulkand

the

strength

changeregularly.Fiber

Saturation

PointThe

status

when

there

is

no

free

water

in

wood,

but

the

cell

walls

are

saturated

with

absorbed

water,

is

calledthefiber

saturation

point.

In

general,

thefiber

saturation

pointof

wood

isfrom

25%

to

35%.8.2 Propertiesof

Wood(3)

Equilibrium

Water

ContentThe

status

that

the

percentage

of

water

content

of

wood

keeps

balance

with

the

surrounding

moisture

is

calledequilibrium

water

content.

In

order

to

avoid

deformation

and

splits

of

wooden

products

caused

by

the

change

ofmoisture

of

wood,

the

wood

must

be

dried

until

the

percentage

of

water

content

reaches

the

equilibrium

watercontent.

In

the

north

area

of

China,

the

equilibrium

water

content

is

about

12%,

while

in

the

south

area

thebalanced

percentage

of

water

content

is

15%-20%.

The

kiln-dried

wood's

percentage

of

water

content

is

4%-12%.2.Dry

Shrinking

and

WetSwellingWhen

absorbed

water

content

in

cell

walls

changes,

the

deformation

of

wood

may

arise,

which

is

wetswelling

and

dry

shrinking.

During

the

process

that

wood

is

dried

fromdamp

status

to

cellar

saturation

point,the

size

of

wood

remains

still

but

mass

decreases.

Only

when

wood

remains

being

dried

until

the

absorbedwater

in

cell

wall

begins

to

evaporate,

do

the

wood

begin

to

shrink.

And

when

the

absorbed

water

in

woodbeginsto

grow,the

wood

will

startto

expand.8.2 Propertiesof

WoodBecauseshrinkingdirection.the structureof woodis not even, thetotheand swellingalso variedfrom directionThe shrinkage value is the smallest indirection

of

long

grain,

and

bigger

in

the

radial

direction,and

the

most

in

the

chord

direction.

When

wood

becomesdry,

the

size

and

the

shape

of

section

may

change

alot.The

shrinkage

effect

makes

a

great

difference

to

theusage

of

wood.

It

may

cause

the

wood

split

or

warp,

evenmake

the

structure

of

wood

loosen

or

heave.

The

mostfundamental

measure

to

avoid

these

negative

impact

is

todry

the

wood

before

processing

to

keep

the

moisturecontent

of

the

wood

in

balance

with

the

humidity

aroundthe

wood

parts.Physical

PropertiesMoistureContentof

WoodDryShrinkingand

Wet

Swelling8.2 Propertiesof

Wood8.2.2 Strength

and

AffectingFactors1.

Thestrength

ofwoodStrength

properties

of

wood

vary

to

a

large

extent,

depending

on

the

orientation

of

grain

relative

to

the

directionof

force.

According

to

the

ways

that

wood

bears

force,

the

strength

of

wood

can

be

classified

into

tensile

strength,compression

strength,

bending

strength

and

sharing

strength.

And

the

tensile

strength,

compression

strength

andsharing

strength

also

vary

with

the

parallel

grain

(the

direction

of

force

parallels

with

the

fiber

direction)

andtransverse

grain

(the

direction

of

force

is

vertical

against

the

fiber

direction).

The

parallel

grain

strength

is

quitedifferent

from

the

transverse

grain

strength.

According

to

the

Table

8.1,

you

can

see

how

to

make

good

use

of

allspeciesof

wood

on

thebasis

of

their

strengthsseparately.Table

8.1

The

relationships

between

strengths

of

woodCompressionstrengthTensileStrengthBendingstrengthShear

Strengthparallel

graintransversegrainparallel

graintransversegrainparallel

graintransversegrain11/10-1/32-31/20-1/33/2-21/7-1/31/2-18.2 Propertiesof

WoodIn

Chinese

construction

projects,

the

physical

and

mechanical

properties

of

the

commonly

used

woods

are

shown

inTable

8.2.Table

8.2

The

physical

and

mechanical

properties

of

commonlyusedwoodName

of

WoodPlace

ofProductionAir

DryDensity/(kg/m3)Parallel

GrainCompressionStrength/MPaParallel

GrainTensile

Strength/MPaBendingStrength/MPaParallel

Grain

ShearStrength/MPaRadial

planeChord

planeConiferous

wood:Cedar

woodHunan37138.877.263.84.24.9Sichuan41639.183.568.46.05.9RedpineNortheast44032.898.165.36.36.9Masson

pineAnhui53341.999.080.77.37.1Dahurian

larchNortheast64155.7129.9109.48.56.8Picea-jezoensisNortheast45142.4100.975.16.26.8Weeping

cypressHubei60054.3117.1100.59.611.1Broad-leavedwoodToothed

oakNortheast76655.6155.1124.111.812.9German

oakAnhui93052.1155.4128.615.918.0FraxinusmandshuricaNortheast68652.5138.1118.611.310.5PoplarShanxi48642.1107.079.69.57.38.2 Propertiesof

WoodIn

accordance

with

Wood

physical

performance

mechanics

test

methods,

the

spotless

standard

specimens

are

usedto

test

wood

strength.

In

experiment,

there

are

different

damaging

conditions

when

wood

suffers

from

differentexternal

forces.

Parallel

grain

compression

damage

is

caused

by

loss

of

stability

of

cell

walls,

not

fiber

breakage;Transverse

grain

compression

damage

is

caused

by

significant

deformation

after

compression;

Parallel

grain

tensiledamageiscaused

by

tearamong

fibersand

then

tensile

failure.Due

to

the

differences

of

force

on

wood

fiber

direction，the

status

when

wood

is

sheared

can

be

classified

intoparallel

grain

shear,

transverse

grain

shear

and

transverse

grain

cutting.

Parallel

grain

shear

damage

is

caused

bylongitudinal

displacement

and

transverse

grain

tension

resulting

from

tear

among

the

bonding

of

fibers;

Transversegrain

shear

damage

is

caused

by

the

tear

among

transverse

bonding

of

fibers

in

shear

plane;

Transverse

grain

cuttingdamageiscaused

by

fibers

being

cutand

thestrength

at

present

isabout3

to

4

times

than

that

of

parallel

grain

shear.8.2 Propertiesof

Wood2.Factors

Affecting

the

WoodStrengthBesides

its

own

structure,

the

strength

of

wood

is

also

determined

by

such

factors

as

the

percentage

of

woodmoisture,

the

defects

(knots,

irregular

grain,

splits,

decay

rot

and

worm

rot),

the

duration

of

outside

force

andtemperature.(1)

Water

Content

of

WoodStrength

of

the

wood

is

greatly

affected

by

water

content.

When

the

wood

contains

less

water

than

the

saturationpoint,

the

percentage

of

moisture

reduces,

and

the

absorbed

water

becomes

less

and

less,

so

that

the

strength

of

woodrises.

To

the

contrary,

the

absorbed

water

increases

and

the

cell

walls

expand,

then

the

structure

loosens

and

thestrength

of

wood

lowers.

When

the

percentage

of

moisture

exceeds

the

fiber

saturation

point,

only

free

water

ischanging,

and

thestrength

of

woodremainsstill.8.2 Propertiesof

WoodThe

national

standard

Method

of

Testing

in

Compressive

Strength

Paralled

to

Grain

of

Wood

(GB

1935-2009)provides

standard

strength

value

when

moisture

content

is

12%.

The

strength

of

other

moisture

content

shall

beconverted

by

thefollowing

formula:

12

W

[1

(W

12)]In

theformula:

12

the

strength

of

12%moisture

content

(MPa)；

W

the

strength

of

W%

moisture

content

(MPa)；W

moisture

content

(%)；α

the

coefficient

of

moisture

content,

when

the

water

content

is9%-15%,

the

numeral

values

aredeterminedaccording

to

Table

8.3.Table

8.3

Moisture

correction

factorStrengthTypeCompression

StrengthTensileStrength

Parallel

to

GrainBendingStrengthSharing

TensileStrength

Parallel

toGrainαParallelgrainTransversegrainBroadleaf

woodConiferwood0.050.0450.01500.040.038.2 Propertiesof

WoodEnvironment

TemperatureTemperature

has

direct

influence

on

the

wood

strength.

The

experiment

shows

that

when

the

temperature

rises

from25

C

to

50

C

,

the

wood

compression

strength

will

be

reduced

by

20%-40%

and

the

wood

sharing

strength

will

bereduced

by

12%-20%

because

the

collide

among

wood

fibers

is

softened.

In

addition,

if

the

wood

is

in

hot

and

drysurrounding,

it

may

become

fragile.

During

the

processing

of

wood,

boiling

method

is

often

employed

to

reduce

itsstrengthcontemporarily

to

meet

the

needs

of

processing

(such

as

the

productionof

plywood).The

Duration

of

Outer

ForceThe

limit

strength

of

wood

standsfor

the

capability

of

standing

the

outer

force

in

a

shorttime.The

limit

that

the

woodcan

stand

in

a

long

run

is

the

rupturestrength

of

wood.

Because

plastic-flow

deformation

will

occur

towood,the

strengthof

wood

will

be

reduced

with

the

lasting

of

loading

time,

and

the

rupture

strength

of

wood

may

be

only

50%-60%

of

thelimitstrength

of

wood.DefectsThe

wood

strength

is

judged

by

the

samples

without

defects.

In

fact,

during

the

growing,

cutting

and

processingprocess

of

wood,

there

may

be

such

defects

as

knots,

splits

and

worm

rot.

These

defects

make

the

wood

uneven,

anddestroy

wood

structures;

all

these

influences

may

reduce

the

strength

of

wood,

especially

the

tensile

strength

and

thebending

strength.Besides

the

factors

above,

the

species

of

trees,

growing

surroundings,

the

age

of

trees,

and

different

parts

of

treesall

influence

the

woodstrength.8.3Applications

of

Wood inArchitecture8.3 Applicationsof

Woodin

ArchitectureDuring

the

construction

process,

the

wood

should

be

used

rationally

according

to

the

species,

the

grade

and

thestructure.

And

we

should

also

try

to

avoid

using

thebig

ones

for

fractionand

thegood

ones

for

trifles.8.3.1 Species

andSpecifications

of

WoodThe

wood

used

in

architecture

can

be

classified

into

primitive

streak,

log,

sawn

timber

and

crosstie

accordingto

its

usageand

status

of

processing.Primitive

streak

means

the

wood

without

bark,

root

and

treetop.

And

usually

it

is

not

processed

into

certainlength

or

diameter

by

certain

size.

Primitive

streak

is

often

used

as

scaffold,

architecture

material

and

furniture.Log

means

the

wood

without

bark,

root

and

treetop.

And

usually

it

is

processed

into

certain

length

or

diameteraccording

to

certain

size.

Log

is

often

used

as

frame,

purlin

or

rafter,

etc.

Furthermore,

it

can

also

be

used

as

piletimber,pole,mine

timber,

etc.

When

processed,

it

can

also

be

made

into

plywood,

ship

modelandmachine

model.8.3 Applicationsof

Woodin

ArchitectureSawn

timber

means

timber,

which

has

been

processed

and

sawn.

The

timberwhose

width

is

three

(ormore

than

three)times

of

its

thickness

is

called

plate.

While

the

timber

whose

width

is

less

than

three

times

of

its

thickness

is

calledsquare

log.Sawn

timberis

often

usedin

architecture,

bridge,

furniture,

ship,

automobile

or

pocking

box.Crosstie

means

the

timber

processed

according

to

the

section

and

length

of

sleeper.

Crosstie

is

often

used

in

railwayconstruction.8.3.2 Engineering

ApplicationsWood

has

the

following

advantages:

high

specific

strength

(light

weight

and

high

strength),

high

frost

resistance,chemical

resistance,

low

thermal

conductivity,

easy

processing,

easy

components

connection.

As

wood

is

flammable,special

attention

should

be

paid

to

wood

processing.

The

properties

ofwood

decide

the

utility

scope

ofthe

wood.In

structure,

wood

is

mainly

used

as

frame

and

roof,

but

it

is

less

used

in

modern

buildings.

Many

ancient

buildingshave

a

high

level

oftechnology

and

art

unique

styles.Because

woods

have

beautiful

patterns

and

special

gloss,

after

surface

processing,

wood

is

used

in

building

interiorsand

decorations

commonly.In

order

to

improvethe

utilization

rate

of

wood,

the

cornerand

debris