CIVE40007 Materials Flashcards
What percentage of wood used in the UK is produced here?
~32%
Which wood type is mainly used in construction?
Hardwood ? Softwood ?
Softwood (Caniferous)
Advantages of wood in construction?
- Excellent combination of physical properties
- High compressive and tensile strength
- Relatively low density
- Readily available
- Relatively low cost
- Good thermal properties
- Good durability under certain conditions
- Predictable fire behaviour
- Sustainable material if harvested from a sustainable forest
- Compatible with other engineering materials
- Aesthetically pleasing
Disadvantages of wood in construction ?
- Many different types with widely different properties
- Certain level of variability in performance
- Properties vary in different directions
- Wood often contains inherent flaws
- Significant waste generated from each tree
- Durability can be poor under partially wet conditions (in soil)
- Attacked by certain insects, bacteria and fungi
- Transport costs – forests are often not near markets
- Need to dry before use
- Dimensional stability
- Fire performance
Is wood a sustainable material ?
- Only if sustainable foresting methods are used
- only renewable construction material
- Low embodied energy consumption
- Low in-use energy consumption
due to low thermal conductivity - 1 m^3 wood stores about 1 tonne of CO2
- organic, non-toxic
- Over 90% of all wood consumed in Europe is sourced from European forests
- Forests act as huge carbon sinks
How can wood be used sustainably in construction?
- Sustainable forest – using recognised harvesting principles and crop rotation techniques.
- Low embodied energy - since it is significantly less resource intensive during production from a raw material to a usable construction material
- excellent insulating material
- good energy efficiency
- reduces the ‘energy footprint’ of a building
What is the FSC?
Wood
non-governmental organisation dedicated to promoting responsible management of the world’s forests, to combat both illegal, unethical and environmentally damaging logging.
Give the composition of dry wood
% weight, state, origin & function
- Cellulose - 50% - Crystalline state - from Glucose - acts as microfibre
- Lignin - 25%- amorphous - Phenyl-propane - Matrix
- Hemicellulose and pectin - 20% - Semi-crystalline - from Galactose Mannose Xylose - acts as matrix
- Extractives - 5% - monomeric - Terpenes, Phenolics - Toxicity
Therefore, ~70% weight carbohydrate (cellulose & hemicellulose)
Explain the structure of glucose
Wood
(50% of the dry weight of the earth’s biomass is in the form of glucose polymers)
- ring structure
- β linkages in cellulose to form polysaccharides - strong to form microfibrils
- cellulose composed of glucose units
- (C6H10O5)n
- crystalline polymer of glucose
What is the difference between hemicellulose & cellulose?
Hemicellulose is similar to cellulose but with different sugar monomers
Describe Lignin
- Lignin is a massive random polymer of phenylpropane alcohol
- Non-biodegradable part of wood
Describe process of wood drying
either kiln or air drying
as cut, wood ~85% moisture content
What is the difference between heartwood and sapwood ?
- Colour (sometimes)
– Durability
– Permeability
– But not strength
Sapwood outer layer (in from bark)
Heartwood inner core
Describe the microstructure of wood ?
- multi-component
- hygroscopic
- anisotropic
- inhomogeneous
- discontinuous
- inelastic
- fibrous
- porous
- biodegradable
- renewable
Is wood a heterogenius structure ?
No, very different properties across and along the grain
Along grain : high compressive & tensile strength, weak shear
Across grain : weak compressive & tensile strength, strong shear
Is wood an inelastic or elastic material ?
Inelastic
Loading and unloading curves do not correspond – viscoelasticity
Due to lignin which is an amorphous polymer
What are the attaks on wood that reduce durability?
fungal decay; dry & wet rot
bio-deteriation by micro-organisms / insects
How does wood strength vary with moisture content?
higher moisture content, lower compressive strength
(since water weakens inter-fibre bonding)
What are plastics ?
- organic materials
- carbon based
- derived from finite crude oil resources
- monomers -> polymerisation -> polymers
When & what was the first synthetic plastic ?
Bakelite in 1907
What bonding forms polymers?
Covelant bonding (sharing electrons)
what is electro-negativity ? what is the rough electro negativity of the elements that form polymers ?
electron attrating potential of an element
C, H, N, O, P, S close to 2.5
What is the source of most plastics ?
crude oil (hydrocarbons & non-hydrocarbons)
What are the two classifications of polymers ?
thermoplastics & thermosetting plastics
the behavioral difference due to their molecular structure / microstructure
Describe thermoplastics
- moulded & remoulded repeatedly under applied heat
- can be reheated multiple times
- held together by weak intermolecular bonding
- straight chain & branched chain
Describe thermosetting plastics
- set when first cooled ( undergoes a chemical reaction which locks monomer chains )
- cannot be reprocessed through reheating
- subsequent heating destroys plastics
- cross-linked with covelant bonding
- or network covelant bonding
- strong covelant bonds
what are the effects of side chains on polymer properties ?
lots of side chains (Low density polymers formed under high temp & pressure), leads to weaker polymers with lower melting points
few side chains (high density polymers, formed under low temp & pressure), leads to stronger polymers with higher melting points
which type of polymer is bakelite ?
thermosetting
what is a copolymer ? what are the common types ?
- prepared from more than one monomer
- random, alternating, block & graft polymers
- combinations of basic polymers for desired properties
Describe polystyrene
- styrene polymerises to form polystyrene
- generally atactic (random arrangement)
- rather than syndiotactic / crystalline
what are two common polymerisation methods ?
addition - repeated chain addition reactions between monomers with double carbon bond
condensation - reaction between two different monomers causing removal of a small molecule, usually water
Give structural properties of polymers
- low compressive strength
- low stiffness
- high toughness
- low density
- durable
- flexible
give chemical properties of polymers
- combustible
- low melting point
- high molecular weight
- variable molecule size
- softens at low temps
- low thermal conductivity
- eletrical insulator
- low permeability
What are plastic processing methods ?
- extrusion
- injection moulding
- compression moulding
what is the rough plastic production globally ?
300 million tonnes per year, about 1/4 attributed to construction
what is the main plastic used in building & construction
PVC (poly vinyl chloride)
- for pipes & ducts
- for insulation
- for windows & flooring
Applications of plastics in construction industry ?
HDPE ( high density polyethylene) - used in landfill composite liner systems - used in pipes -
LDPE ( low density polyethylene ) landfill cover
What is bitumen ?
produced from crude oil during fractional distillation process
- viscous
- consists of polycyclic aromatic hydrocarbons
- heaviest oil fraction ( highest boiling point)
why is bitumen used ?
excellent waterproofing & adhesive properties
(binder for asphalt road surfacing)
describe the structure & chemical composition of bitumen ?
- complex
- ~85% C, ~10% H, S, O, N
- main chemical groups : asphaltenes & maltenes
Describe the role of bitumen in asphalt ?
critical ratio of filling of bitumen in asphalt
- ideal : bitumen almost completely fills voids, both stone skeleton & bitumen contribute to properties, compaction essential for impermeability
how do properties of alkanes change with number of carbon atoms ?
higher number carbon atoms = higher boiling point
hence giving basis for oil refining
what are the different groups of hydrocarbons ?
alkanes, aromatics, cycloalkanes, alkenes, alkynes
what determines molecular weight of polymer ?
conditions of polymerisation
are polymers crystalline or amorphous ?
can be either, often have semi-crystalline regions where they exhibit both
varying properties
Give example of wood composites ?
- glulam
- osb - oriented strand-board
- lvl - laminated veneer lumber
- psl - parallel strand lumber
- lsl - laminated strand lumber
- pre-fabricated I joists
advantages of composite / engineered wood products ?
- more consistent behaviour
- redistribution/removal of defects
- reduced variability
- combine desirable properties of wood and other materials
advantages of glulam ?
- structural properties
- dimensional stability
- large sizes
- reduced material wastage
- less material variability
- aesthetic
- utilisation of waste material
describe glulam ?
usually european whitewood + PRF adhesive
homogenous or mixed
usually ~ 45 mm deep laminates
length up to 45m
describe LVL
wood
laminated veneer lumber :
- bonding veneers ~3mm thick
- ~26m long
- uses: roof/floor beams, flange of I-joists, bridge decking
describe PSL
wood
parallel strand lumber
- cutting peeled veneers into long strands, coat with glue, combine using heat & pressure
Describe I-joists
wood
-strong, stiff
-straight
-light
-long
-dimensionally stable
-cost effective
-easy to handle
-reduced use of timber material
-quality assured
When was the first use of cast iron in construction ?
metal
Iron Bridge, Coalbrookdale (England)
in 1779 (during industrial revolution)
Describe molecular bonding in metals?
metal ions in a lattice structure bond by force of attraction between free electrons and metallic cations (metallic bonding)
giant structure where electrons in outermost layer of metal atoms are free to move
hence high melting & boiling point
which category of metals is largely used in construction ?
alloys
what is an alloy ?
metallic substance composed of 2 > different elements (either metals or non-metallic elements)
structure of alloys ?
metallic crystalline structure
- Microstructure determined by
processing techniques used and
characterised by the size and shape
of the grains of different phases,
and their orientation and distribution
(alloys with same chemical composition can have differing properties due to microstructure differences)
structure of alloys ?
- metallic crystalline structure similar to pure metals with other elements in the metallic lattice
- introduction of other elements into metallic lattice reduces ability for layers to slide ( hence stronger than pure metals )
what are the three types of crystal structures for metals & alloys ?
- body-centered cubic (bcc)
- face centered cubic (fcc)
- hexagonal close packed (hcp)
fcc & bcc are more spatially efficient & more commmon
types of imperfections in crystals ?
metals
vacancies
dislocations (extra line of atoms in structure
interstitial atom (introduce stresses)
substitutional atoms (introduce stresses)
grain boundaries (interface between adjacent crystalline regions with different orientations)
which crystalline form is the natural form of iron ?
bcc body centered cubic
implications of grain boundries on crystalline metallic structure ?
fractures may occur at intersection between adjacent crystalline regions with different orientations
intergranular fracture
implications of dislocations in crystal structures ?
metals
- responsible for plastic deformation of metals
- Stress required to plastically deform a crystal is much less than the stress
calculated from considering a defect-free crystal structure - Dislocation motion determines yield stress - permanent deformation
- Under relatively low shear stresses the dislocation moves along in the
direction of the imposed stress - Yield stress can be increased by creating obstacles to dislocation motion
(where exra plane of atoms inserted into crystal structure)
describe steel
alloy of iron & carbon containing less than 2% carbon and
1% manganese and small amounts of silicon, phosphorus, sulphur and oxygen.
more than 3,500 different grades of steel, with varying properties
recent advancements
what are strengthening mechanisms for metals ?
- control of grain size
(grain boundaries as barriers to dislocation motion
Reducing grain size therefore increases yield stress) - work hardening
(strain field due to dislocations - work hardening - Plastic deformation gives increased dislocation density leading to
increased interaction and higher strength.
Dislocations, increase in density during plastic flow and those moving on
intersecting slip planes tangle and pile up.
This means that an ever increasing shear stress is required for deformation,
increasing the yield stress.) - solid solution strengthening
stress field due to introduced interstitial atoms introduced, interacting with surrounding strain field thus inhibiting dislocation motion) hence higher yield strength - precipitation strengthening/hardening
(find distribution of second phase particles with associated strain field, makes it harder for dislocation motion, higher yield stress) pin & lock dislocations
what is a eutectoid reaction
three-phase reaction by which, on cooling, a solid transforms into two other solid phases at the same time. If the bottom of a single-phase solid field closes (and provided the adjacent two-phase fields are solid also), it does so with a eutectoid point.
Fe-C system, there is a eutectoid point at 0.8wt% C and 723°C.
The phase just above the eutectoid is austenite or gamma (γ).
describe the metallurgy of iron
iron is allotropic/polymorphic - exhibits different crystal structures at different temperatures
most importantly bcc to fcc at 912 degC
(body centered cubic to face centered cubic)
describe the different types of carbon steel
- hypoeutectoid - < 0.8 wt %
- eutectoid - 0.8 wt % level of C
- hypereutectoid - > 0.8 wt %
describe hypoeutectoid steel
- Fe + ~ 0.4 wt % carbon
- formation of ferrite grains at boundaries as austenite is cooled ( in (α + γ) region of the phase diagram)
- transformation of remaining austentite to ferrite & cementite
- Fine lamellar structure called pearlite - (Fineness of the pearlite depends on cooling conditions)
- analogous to a metal-ceramic nano-composite material
- Ferrite is relatively soft and ductile while the cementite is hard and brittle
what is pearlite
alternating layers of ferrite and cementite formed simultaneously from remaining austentite at 723 degC
Fineness of the pearlite depends on cooling conditions
analogous to a metal-ceramic nano-composite material
Fine lamellar structure
describe hypereutectoid steel
- > 0,8 wt % carbon
- Fe3C forms at austenite grain
boundaries - Continuous brittle phase and therefore
- the steel is brittle
- Hypereutectoid steels can have
improved properties by heat treatment
microstructure consisting of
cementite surrounding pearlite - Cementite precipitates at austenite grain boundaries,
with remaining austenite transformed into pearlite
describe steel composition at eutectoid point
entirely pearlite
what are symbols for ferrite, austenite & cementite ?
- Ferrite is α ( with BCC structure ) & δ
- Austenite is γ (with FCC structure)
- Fe3C is cementite ( 6.67 % Carbpn )
what are the common steels used
Many steels in general engineering use are essentially binary alloys of iron and
carbon (C), often with less than 0.8 wt.% C
what other elements are added to steel
- Carbon steels also contain some manganese (~0.45-0.9 %), phosphorus (0.025-0.060 %) and sulphur (0.030-0.050 %)
- Four main groups are recognised, each group meeting specific engineering
product service requirements e.g. high cold formability, strength, wear resistance - Properties can be enhanced and controlled through heat treatments
- properties of steels within these groups dictate the fabrication
route/method by which the particular product can be created e.g. cold rolling,
hot forging, etc
what happens during rapid cooling
- phase diagram becomes invalid and metastable phases may form
- crystal lattice tried to switch from fcc (austentite) to bcc (ferrite)
- excells carbon → distorted body-centered lattice → martensite
- very hard, but very brittle (similar to ceramics)
describe microstructure and properties of martensitic steel
- many interfaces
- heavily dislocated
- high & strongly varying local stresses
- high resistance to dislocation motion
- hard and brittle
- can be tempered to produce optimum steel microstructure
how can the properties of martensitic steel be improved
- heat treatment ( tempering) of martensite at 200-600degC allows C atoms to diffuse out of martensite
- Fe3C present as uniform distribution of fine, round precipitates leadinding to high strength and toughness
- quenched and tempered steels
- properties dependant on tempering temperature
properties of stainless steel ?
> 11 wt % Cr with Ni & Mn also present
- Cr → Cr203 film → corrosion & oxidation protection
- mostly austenitic → non-magnetic
- ferritic & martensitic stainless steels possible → increases range of mechanical properties
composite material
how is stainless steel corrosion & oxidation resistant ?
- protective coating of
passive chromium rich oxide film due to Chromium oxidisation
extremely thin, this invisible inert film is tightly - bonded to the metal and extremely protective in a wide range
of corrosive media.
describe the blast furnace in steel production
- iron ore + coke + limestone added at top
- air blown at bottom
- oxygen in air reacts with hot coke = carbon monoxide (oxidation)
- this gas changes iron oxide → iron (Reduction)
- liquid iron collects at bottom covered by a layer of molten slag
- iron tapped off & solidifies
- molten slag run off seperately
- Molten Fe (iron) from the blast furnace has 4 - 4.5 wt % C and other impurities’
REDOX reaction
what is a basic oxygen converter (BOC)
steel
reduces carbon content of iron to required level from ~4 - 1.5 wt% carbon
- cylindrical vessel
- oxygen blown through, reacts with carbon to form carbon monoxide (90%) and CO2
- reduces to 0-1.5 wt % carbon
what is an electric arc furnace (EAF)
alternative to blast furnace:
- charged with scrap (recycled) steel or iron from blast furnace
- contains electrodes
- current passed through to form an arc
- O2 blown into the steel. Lime and fluorspar (CaF2) are added to form slag
- furnace is tilted to remove slag floating on the surface
- Molten steel is poured (tapped) into a ladle for secondary steelmaking
- EAF typically makes 150 tonnes in around 90 minutes
economic impact of corrosion ?
~ 3.5% of GDP in developed countries through direct (replacement, preventative measures, corrosion resistance) & indirect (plant shutdown, loss of product, loss of efficiency, contamination)
what is corrosion
an electrochemical effect
- a redox electron transfer reaction
- Metal atoms are oxidised - form positive ions and give up electrons (the site of this is the anode, anodic reaction)
- Electrons formed are transferred to another chemical species
This is a reduction reaction (cathodic reaction) – gaining electrons
oxidation occurs at the anode
reduction occurs at the cathode
why does iron so readily rust
Iron with oxygen in wet conditions - energetically favourable to form rust
- iron found as iron oxide in iron ore, To free the iron from the oxide we have to supply energy in a blast furnace, where redox reactions occur
- extracted iron tends to reform to oxide in electrochemical processes
- Energetically favourable for Fe to revert to the oxide
describe the corrosion of iron
Oxidizing iron supplies electrons, to reduce oxygen
from the air : Fe -> Fe2+ + 2e-
electrons more through iron to outside of droplet : O2 + 2H20 + 4e- -> 4OH-
In the droplet, the hydroxide ions can move to react with the iron(II) ions
moving from the oxidation region. Iron(II) hydroxide is precipitated:
Fe2+ + 2OH-
→ Fe(OH)2
Rust is then quickly produced by the oxidation of the precipitate:
4Fe(OH)2
(s) + O2
(g) → 2Fe2O3 . H2O(s) + 2H2O(l)
what is the significance of the galvanic series
corrosion
Galvanic series gives the relative reactivity of common materials in seawater
increasingly inert = cathodic
increasingly active = anodic
therefore place with a metal lower in the galvanic series (incresingly active/anodic) to protect
how to avoid corrosion ?
- material choice
- physical barrier (e.g. paint - difficult to ensure coating will last in structures)
- Galvanic protection and galvanizing (use galvanic couple, more reactive metal)
- Cathodic protection - supply electrons exernally (electrical circuit), forces reverse of oxidation, makes metal cathodic
advantages of galvinising
- corrosion resistance (zinc weathers slowly, sacrificial protection to exposed areas)
- coating toughness (bonded metallurgically)
- lowest lifetime cost (high relative initial cost)
- long life (often >40 years)
- ease of inspection
concrete ?
composite material : coarse aggregate particles (stones/gravels) + fine aggregate particles (sand) embedded in a binding medium (mixture of Portland cement and water)
concrete paste ?
cement + admixtures + water = ‘Binder’
concrete mortar ?
concrete paste (cement, admixtures & water) + fine aggregate
key advantages of concrete ?
-
Raw materials widely distributed, readily available and cheap (available in Earth’s crust)
-Mix proportions and ingredients can be varied to produce different properties (workability, strength, stiffness, density, toughness) - Strong and stiff in compression
- Can be cast in-situ or prefabricated and
assembled on site
key disadvantages of concrete ?
- Weak in tension
- Cracks when subjected to tensile stresses
- Long-term deformations: creep and shrinkage
- porous
- Not widely recycled
- Cement and concrete industry is a massive CO2 emitter
what materials were primarily used in early-age cement ?
Lime (from limestone) (CaCO3)
burnt gypsum (CaSO4)
lime reactions ?
- calcination (heating) of natural calcium carbonate (limestone) @ ~ 700-900degC
- Quicklime produced mixed with water to form portlandite (Ca(OH)2, hydrated or slaked lime)
- hardens slowly by reacting with absorbed CO2 from atmosphere
when was Portland Cement first widely used ?
1824
hydraulic cement ?
by reacting with water to form hydrated calcium aluminate (hydration) phases that set into a rock-like mass
- portland cement
- calcium aluminate cements
- pozzolanic cements
- Hydrophobic cements
- Natural cements
- Expanding cements
non hydraulic cement ?
- limes
- gypsum cements & plasters
(cannot harden in prescence of water)
production of ordinary Portland
cement ?
- ~ 75 % limestone/chalk + ~ 25 % clay/shale - crushed & blended
- heated to ~1450degC - calcination
- forms clinker (silicates & aluminates)
- ground to powder
- add ~4 % gypsum - adjust setting properties etc.
what forms exist of cement ?
ready-mix
pre-cast
retail (small-scale)
what is a SCM in cement production ?
supplementary cementitious materials
- any reactive nonclinkered solid material used in cement (exclusing admixtures)
- addition occurs either at cement plant or at concrete batching plant
examples of SCMs ?
limestone (+ gypsum), Granulated blast
furnace slag, Silica fume, Coal fly ash, Calcined clay, Agricultural residue ashes (e.g. rice husk ash)
main components of portland cement ?
- C3S (alite) : strength
- C2S (belite) : long-term strength
- C3A (aluminate) : early strength
- C4AF (ferrite) : contributes to colour (white)
