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09/10/2011
48352/S2011/L10/RS 1
Construction Materials
Wood and Composites
Lecture 10
Dr Rijun Shrestha
09/10/2011 48352 L10 Spring Semester 2011 1
Contents Wood Numerical problems Break Wood (contd.) Composites
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Wood and Composites (Part 1)
Wood
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Application of wood Internal / External
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Application of wood (contd.) Decorative / Structural
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Application of wood (contd.) Domestic Large-Span structures
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Application of wood (contd.) Commercial Bridges
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Advantages Light weight Hi h t th t i ht ti High strength to weight ratio Aesthetic value Good insulation characteristic Environmental benefits Naturally produced material - renewable Untreated wood – completely biodegradable Less energy to produce compared to steel,
concrete, aluminium, plastics. Stores carbon
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Wood used in different forms
Sawn Timber6
Engineered Wood Products (EWPs) Plywood Laminated Veneer Lumber (LVL) Glue Laminated Timber (Glulam) Cross Laminated Timber (CLT) Particle board
5
Oriented Strand Board (OSB), etc
1 23
4
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Structure of wood - cell Plant cell – composed to three main chemicals
Cellulose Cellulose network of molecules fibrous
Lignin a gel type substance - “woody” property Bonds various cells together
Hemicellulose
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Hemicellulose cross linking - binds cellulose into the cell
Spirally wound fibres
Straightfibres
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Structure of wood – cell structure
fib res
v e sse ls
e arly w o o d
ra y s
ra y s
ra y s
ce lls
h a rd w o o d
y
la te w o o d
so ftw o o d
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Vessels only in hardwoods– distinguishing structural feature between hardwood and softwood
Hardwood – birch, maple, oak - flowering Softwood – pine, spruce, fir – non-flowering
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Structure of wood – cell structure
Wood cells formed between b k d dbark and wood Inner side – new cells
added to Xylem (woody tissue transporting nutrients ad water)
Outer side – new cells
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added to Phloem (bark like tissue transporting food materials)
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Structure of wood Sapwood and Heartwood
H t d HeartwoodProvides structural supportCells become blocked with depositsDifficult to impregnate with preservative
SapwoodYounger outermost wood
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Conducts water and stores foodMore susceptible to fungal and insect attack due to
presence of starchesEasy to impregnate with preservatives
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Structure of wood Earlywood and Latewood EarlywoodProduced in flush of growth “springwood”Large diameter, short length, thinner wall
fibres
Latewood
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LatewoodProduced later in growing season
“summerwood”Better strength characteristics
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Mechanical properties
Different properties in three different directionsp p Orthotropic Longitudinal, Transverse, andRadial
Longitudinal
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Radial
Tangential
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Strong parallel to grain & Stiff parallel to grain
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Weak perpendicular to grain
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Mechanical properties AS 4063.1 – 2010 Modulus of Rupture Modulus of Elasticity
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Mechanical properties (2) Tensile strength parallel to the grain Tensile strength perpendicular to the grain
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Mechanical properties (3) Compressive strength parallel to the grain Compressive strength perpendicular to the
grain
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Mechanical properties (4) Shear strength parallel to the grain Shear strength perpendicular to the grain
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Mechanical properties (5)
Cleavage strength – resistance to force acting perp. to grain and tending to split a member
Impact strength – energy needed to break a specimen
Hardness – resistance against wear and marking
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g
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Factors affecting mechanical properties
Species Position in tree Age of tree Climatic conditions Density Brittleheart Rate of growth Late wood
Temperature Load duration Defects Moisture content
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Late wood
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Factors affecting mechanical properties (1)
DensityDenser species – better mechanical propertiesDenser species - hard to dryDensity within a species - affected by factors
such as growth defects
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Factors affecting mechanical properties (2)
Brittleheart d th h t f th t wood near the heart of the tree - core high compressive forces during early growth stages
low impact or shock resistance
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Factors affecting mechanical properties (3)
Rate of growthSpecies with medium rate of growth have
better strength characteristics compared to slow and fast growth material
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Factors affecting mechanical properties (4)
Percentage of latewood thicker walled cells in wood formed in late
growing season denser and stronger than the ones formed in
the early growing season
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Factors affecting mechanical properties (5)
Position in tree Timber from bottom logs are sightly denser Therefore, stronger
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Factors affecting mechanical properties (6)
TemperatureHigher temperature lowers the strength
propertiesRelated to moisture content Temperature change affects the relative
humidity - affects the moisture content Generally – reversible
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Generally reversible Prolonged exposure above 90 degrees C,
however, irreversible
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Factors affecting mechanical properties (7)
Duration of load Ti b d l d ti l d Timber creeps under long duration load Incremental deformation under constant
load Amount and rate of creep depends upon Moisture migrationAmbient conditions (temperature, RH)
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Member size Creep deformation in green timber is more
significant than seasoned timber under constant humidity
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Factors affecting mechanical properties (8)
Defects K t Knots Grain distortion Decay Insect attack
Discussed later
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Moisture Content
in wood water ofWeight CM E d %
Strength normally increases as wood dries Modulus of rupture and compressive
strength parallel to grain - increase by 70-100% at 12% m c
woodof dry weightoven . CM Expressed as %
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100% at 12% m.c. However, M.C has reverse effect on impact
resistance. Checks, splits and honeycombing
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Moisture in Wood Cells100% Growing
tree
Unseasoned timber
free water
25% bound waterPartially
removed bound waterSeasoned
timber15%
fibre saturationPartially seasoned
timber
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Moisture content and Shrinkage Moisture in two forms
Bound water cell walls Bound water – cell walls Free water – cell cavities
Fibre saturation point Level of moisture content when the cell walls are fully
saturated and there is not moisture in the cell cavities Varies between 21 and 32% 30% for engineering calculations
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g g Shrinkage/swelling below FSP
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Shrinkage and Swelling Change in moisture content beyond FSP has no effect
on shrinkage and swellingon shrinkage and swelling Only when the moisture content is below FSP, wood
shrinks/swells with change in moisture content Different rates in longitudinal, radial and tangential
direction
67
89
g ra
te %
Tangential
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01
234
56
0 5 10 15 20 25 30
Moisture Content %
Shr
inka
ge/s
wel
ling
Longitudinal
Radial
Tangential
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Numerical ProblemA wood sample weighing 412.5g was weighed and then oven dried at 103C till a constant weight was reached If the moisture at 103C till a constant weight was reached. If the moisture content was calculated to be 22.75%, find the weight of the oven dried sample.Answer 336.05 g
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Numerical ProblemA 300 mm wide radiata pine plank is cut out such that the width is
in the tangential direction of the annual rings Calculate the in the tangential direction of the annual rings. Calculate the width of the plank if the moisture content changes from 35% to 15%.
The tangential shrinkage/swelling rate for radiata pine is known to be 9% when its moisture content is varied from fibre saturation point to oven dried state. The fibre saturation point for radiate pine is 27%.
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(Answer = 288 mm)
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Production Sawing Seasoning Surfacing Grading Preservative treatment
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Production (1) Sawing Different types of saws can be used Circular Frame Band
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Production (2) Seasoning-process to remove water from
dwood Kiln drying Air drying Other – Chemical, microwave
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Reason for drying Dimensional stability in service condition Strength Drying facilitates preservative treatment Prevention from decaying and insects Reduced weight – easier transportation
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Production (3) Surfacing To achieve plain wood surface Better results when done after seasoning
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Production (4) Grading Visual stress grading Machine stress grading Proof grading
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Production (5) Preservative treatment Water-borne preservatives Light organic solvent-borne preservatives Oil-borne preservatives
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Defects in wood (1) Knots – remnants of branches captured by
i t kgrowing trunk
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Defects in wood (2) Grain distortion
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Defects in wood (3) Shakes, Checks and splits
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Defects in wood (4) Bark pocket
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Defects in wood (5) WarpBowSpring/CrookCup Twist
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Other defects (6) Wane/want – missing pieces from cross-section Reaction wood Pitch/resin pocket
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Resin pocket Reaction wood
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Defects in wood (7) Organisms that degrade wood
Fungal growth Fungal growth Oxygen Temperature Food Moisture
Insects – beetles, termites, borers Bacteria
Prolonged contact with soil
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Prolonged contact with soil Marine organisms
Marine boring organisms
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Wood and Composites (Part 2)
Composites
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What are Composites? Two or more materials combined Property is a function of constituents Superior properties compared to constituentsStiffnessStrengthDensityCorrosion resistance
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Corrosion resistance Fatigue life Insulation and thermal resistance
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Composites Constituent materials have different properties Alloys – not compositesSimilar properties for constituents
Examples – Fibre Reinforced Polymers (FRP)ConcreteReinforced Concrete
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Reinforced Concrete Engineered Wood Products
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Composites - examples Fibre Reinforced Polymers (FRP) Fib Fibres Carbon Glass Aramid Nylon Silicone
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Polymer Epoxy Polyurethanes
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Composites - examples Concrete Cement Aggregate Sand Admixtures
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Composites - examples Reinforced Concrete Concrete Reinforcement Bars
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Composites - examples Engineered Wood Products Timber Glue Reinforcement
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Application – FRP for strengthening
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Application - FRP for Strengthening
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Strengthening of a reinforced masonry archSource: International Institute for FRP in Construction
FRP Photo Competition '05:http://www.iifc-hq.org/photocompetition05/
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Application - FRP Reinforcement
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Laying Aslan 100 GFRP Rebar: Sierrita de la Cruz Creek Bridge near Amarillo Texas USA Spreading concrete over a grid of
carbon fibre reinforcing barsSource: International Institute for FRP in ConstructionFRP Photo Competition '05:http://www.iifc-hq.org/photocompetition05/
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Application - All FRP
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The Johnson County, Kentucky Swinging Bridge, USA, is 128 m long. It is the
longest FRP bridge superstructure in the world.
FRP cable stayed bridge inJiansu Province, China
Source: International Institute for FRP in ConstructionFRP Photo Competition '05:http://www.iifc-hq.org/photocompetition05/
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Application - All FRP
h l k d ( k
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The Clear Creek Bridge (Kentucky, USA) is 18.3 m (60 ft) long and is the first bridge to use hybrid carbon/glass
FRP pultruded beams.
First Highway-rated All-Composite Bridge in Missouri,
USA (span = 9m)Source: International Institute for FRP in ConstructionFRP Photo Competition '05:http://www.iifc-hq.org/photocompetition05/
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Application - All FRP
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60-foot RStandard(tm) Modular Composite Utility Pole
FRP handrails
Source: International Institute for FRP in ConstructionFRP Photo Competition '05:http://www.iifc-hq.org/photocompetition05/
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Phases Continuous – Matrix Metal Polymer
Discrete – Fibres, Particles
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Matrix Metal High strength Abrasion resistance High operating temperature Corrosion Weight
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Matrix Polymer Low cost Low weight Corrosion resistant Sensitive to temperature UV light Low elastic modulus
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Low elastic modulus
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Composites - Classification Microscopic – fibres, particles m Fibre Reinforced Particle Reinforced
Macroscopic – larger size constituents e.g. aggregate, rebars
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Microscopic composites Two phaseContinuous – matrix (polymer, metal)Dispersed – fibres/particles
Property governed by distributed phaseShapeSizeDistribution
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DistributionOrientation
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Fibre reinforced composites Fibres – dispersed phase Fibres main load carrying components Fibres – main load carrying components Matrix – binds fibres in place Fibre types
Carbon Aramid Glass Nylon
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Nylon Silicone
Superior properties due to fewer internal defects in fibres
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Important parameters Fibre volume Type of fibre Type of resin Fibre orientation Quality control during manufacturing
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Tensile strength
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Tension test set-up Tensile failure of plate
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Particle reinforced composites Particles dispersed through matrix Particle size 0.01 to 0.1 micronMatrix – main load carrying components Particles prevents dislocation of matrix
Particle size > 0.1 micron Load shared by particles and matrix Particles act as fillers
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Particles act as fillers
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Macroscopic composites Concrete
Cement pastep aggregate
Reinforced concrete Cement paste Aggregate Reinforcement bars
EWP Glue Wood
A h lt t
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Asphalt concrete Bitumen Aggregate Filler
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