CIVE40007 Materials Flashcards
(263 cards)
1
Q
What percentage of wood used in the UK is produced here?
A
~32%
2
Q
Which wood type is mainly used in construction?
Hardwood ? Softwood ?
A
Softwood (Caniferous)
3
Q
Advantages of wood in construction?
A
- 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
4
Q
Disadvantages of wood in construction ?
A
- 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
5
Q
Is wood a sustainable material ?
A
- 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
6
Q
How can wood be used sustainably in construction?
A
- 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
7
Q
What is the FSC?
Wood
A
non-governmental organisation dedicated to promoting responsible management of the world’s forests, to combat both illegal, unethical and environmentally damaging logging.
8
Q
Give the composition of dry wood
% weight, state, origin & function
A
- 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)
9
Q
Explain the structure of glucose
Wood
A
(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
10
Q
What is the difference between hemicellulose & cellulose?
A
Hemicellulose is similar to cellulose but with different sugar monomers
11
Q
Describe Lignin
A
- Lignin is a massive random polymer of phenylpropane alcohol
- Non-biodegradable part of wood
12
Q
Describe process of wood drying
A
either kiln or air drying
as cut, wood ~85% moisture content
13
Q
What is the difference between heartwood and sapwood ?
A
- Colour (sometimes)
– Durability
– Permeability
– But not strength
Sapwood outer layer (in from bark)
Heartwood inner core
14
Q
Describe the microstructure of wood ?
A
- multi-component
- hygroscopic
- anisotropic
- inhomogeneous
- discontinuous
- inelastic
- fibrous
- porous
- biodegradable
- renewable
15
Q
Is wood a heterogenius structure ?
A
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
16
Q
Is wood an inelastic or elastic material ?
A
Inelastic
Loading and unloading curves do not correspond – viscoelasticity
Due to lignin which is an amorphous polymer
17
Q
What are the attaks on wood that reduce durability?
A
fungal decay; dry & wet rot
bio-deteriation by micro-organisms / insects
18
Q
How does wood strength vary with moisture content?
A
higher moisture content, lower compressive strength
(since water weakens inter-fibre bonding)
19
Q
What are plastics ?
A
- organic materials
- carbon based
- derived from finite crude oil resources
- monomers -> polymerisation -> polymers
20
Q
When & what was the first synthetic plastic ?
A
Bakelite in 1907
21
Q
What bonding forms polymers?
A
Covelant bonding (sharing electrons)
22
Q
what is electro-negativity ? what is the rough electro negativity of the elements that form polymers ?
A
electron attrating potential of an element
C, H, N, O, P, S close to 2.5
23
Q
What is the source of most plastics ?
A
crude oil (hydrocarbons & non-hydrocarbons)
24
Q
What are the two classifications of polymers ?
A
thermoplastics & thermosetting plastics
the behavioral difference due to their molecular structure / microstructure
25
Describe thermoplastics
- moulded & remoulded repeatedly under applied heat
- can be reheated multiple times
- held together by weak intermolecular bonding
- straight chain & branched chain
26
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**
27
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
28
which type of polymer is bakelite ?
thermosetting
29
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
30
Describe polystyrene
- styrene polymerises to form polystyrene
- generally atactic (random arrangement)
- rather than syndiotactic / crystalline
31
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
32
Give structural properties of polymers
- low compressive strength
- low stiffness
- high toughness
- low density
- durable
- flexible
33
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
34
What are plastic processing methods ?
- extrusion
- injection moulding
- compression moulding
35
what is the rough plastic production globally ?
300 million tonnes per year, about 1/4 attributed to construction
36
what is the main plastic used in building & construction
PVC (poly vinyl chloride)
- for pipes & ducts
- for insulation
- for windows & flooring
37
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
38
What is bitumen ?
produced from crude oil during fractional distillation process
- viscous
- consists of polycyclic aromatic hydrocarbons
- heaviest oil fraction ( highest boiling point)
39
why is bitumen used ?
excellent waterproofing & adhesive properties
(binder for asphalt road surfacing)
40
describe the structure & chemical composition of bitumen ?
- complex
- ~85% C, ~10% H, S, O, N
- main chemical groups : asphaltenes & maltenes
41
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
42
how do properties of alkanes change with number of carbon atoms ?
higher number carbon atoms = higher boiling point
hence giving basis for oil refining
43
what are the different groups of hydrocarbons ?
alkanes, aromatics, cycloalkanes, alkenes, alkynes
44
what determines molecular weight of polymer ?
conditions of polymerisation
45
are polymers crystalline or amorphous ?
can be either, often have semi-crystalline regions where they exhibit both
varying properties
46
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
47
advantages of composite / engineered wood products ?
- more consistent behaviour
- redistribution/removal of defects
- reduced variability
- combine desirable properties of wood and other materials
48
advantages of glulam ?
- structural properties
- dimensional stability
- large sizes
- reduced material wastage
- less material variability
- aesthetic
- utilisation of waste material
49
describe glulam ?
usually european whitewood + PRF adhesive
homogenous or mixed
usually ~ 45 mm deep laminates
length up to 45m
50
describe LVL
| wood
laminated veneer lumber :
- bonding veneers ~3mm thick
- ~26m long
- uses: roof/floor beams, flange of I-joists, bridge decking
51
describe PSL
| wood
parallel strand lumber
- cutting peeled veneers into long strands, coat with glue, combine using heat & pressure
52
Describe I-joists
| wood
-strong, stiff
-straight
-light
-long
-dimensionally stable
-cost effective
-easy to handle
-reduced use of timber material
-quality assured
53
When was the first use of cast iron in construction ?
| metal
Iron Bridge, Coalbrookdale (England)
in 1779 (during industrial revolution)
54
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
55
which category of metals is largely used in construction ?
alloys
56
what is an alloy ?
metallic substance composed of 2 > different elements (either metals or non-metallic elements)
57
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)
58
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 )
59
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
60
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)
61
which crystalline form is the natural form of iron ?
bcc body centered cubic
62
implications of grain boundries on crystalline metallic structure ?
fractures may occur at intersection between adjacent crystalline regions with different orientations
intergranular fracture
63
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)
64
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
65
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
66
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).
67
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)
68
describe the different types of carbon steel
- hypoeutectoid - < 0.8 wt %
- eutectoid - 0.8 wt % level of C
- hypereutectoid - > 0.8 wt %
69
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
70
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
71
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
72
describe steel composition at eutectoid point
entirely pearlite
73
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 )
74
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
75
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
76
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)
77
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
78
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
79
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
80
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.
81
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
82
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
83
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
84
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)
85
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
86
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
87
describe the corrosion of iron
Oxidizing iron supplies electrons, to reduce oxygen
from the air : Fe -> Fe*2+* + *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:
Fe*2+* + 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)
88
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
89
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
90
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
91
concrete ?
composite material : coarse aggregate particles (stones/gravels) + fine aggregate particles (sand) embedded in a binding medium (mixture of Portland cement and water)
92
concrete paste ?
cement + admixtures + water = 'Binder'
93
concrete mortar ?
concrete paste (cement, admixtures & water) + fine aggregate
94
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
95
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
96
what materials were primarily used in early-age cement ?
Lime (from limestone) (CaCO3)
burnt gypsum (CaSO4)
97
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
98
when was Portland Cement first widely used ?
1824
99
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
100
non hydraulic cement ?
- limes
- gypsum cements & plasters
(cannot harden in prescence of water)
101
production of ordinary Portland
cement ?
1. ~ 75 % limestone/chalk + ~ 25 % clay/shale - crushed & blended
2. heated to ~1450degC - calcination
3. forms clinker (silicates & aluminates)
4. ground to powder
5. add ~4 % gypsum - adjust setting properties etc.
102
what forms exist of cement ?
ready-mix
pre-cast
retail (small-scale)
103
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
104
examples of SCMs ?
limestone (+ gypsum), Granulated blast
furnace slag, Silica fume, Coal fly ash, Calcined clay, Agricultural residue ashes (e.g. rice husk ash)
105
main components of portland cement ?
1. C3S (alite) : strength
2. C2S (belite) : long-term strength
3. C3A (aluminate) : early strength
4. C4AF (ferrite) : contributes to colour (white)
106
hydration of Portland cement ?
```
```approximate reactions : belite (C3S) & alite (C2S) react with water forming calcium hydroxide, calcium silicate hydrate (C-S-H) & ettringite
- other hydrate products formed depending on initial chemical composition
107
rate of hydration of Portland cement ?
consists of setting and hardening ?
rate affected by :
- temperature
- water-cement ratio
- presence of additives
- fineness of the cement particles
108
setting of portland cement ?
cement paste changes from a plastic or fluid state to an unworkable solid state
2 x phases : initial setting (plastic workable paste after immediate addition of water) and final setting (Stiff unworkable paste)
109
hardening of portland cement ?
Rigid solid gaining strength with time
time scale of weeks to months
- hydrated compounds continue to develop and interlock, forming a dense and strong matrix
110
capillary porosity ?
| cement
capillary pores formed by excess water from hydration, which is evaporated leaving air voids (0.05-10μm diameter)
- can affect the durability and performance of concrete structures (freeze-thaw cycles or chemical attack)
111
effect of water to cement ratio
(w/c) on microstructure ?
affects the porosity, strength, and durability of the cement paste
- higher w/c ratios leading to more hydrated products and a higher porosity in the cement paste
- high w/c = disorganised sparse microstructure
- low w/c = more compact
111
effect of water to cement ratio
(w/c) on microstructure ?
affects the porosity, strength, and durability of the cement paste
- higher w/c ratios leading to more hydrated products and a higher porosity in the cement paste
- high w/c = disorganised sparse microstructure
- low w/c = more compact
112
alternatives to Portland cement ?
alternative cement binders
addition of alkali-activated materials to increase reactivity of SCMs with water = faster setting process
113
aggregate ?
| concrete
a granular material used in construction. May be natural, manufactured or recycled
- major influences on workability, density, strength, dimension stability, durability, etc.
114
sources of aggregate ?
1. primary aggregates (**natural**, ~90% used)
- crushed rock
- sand & gravel
2. secondary aggregates (**manufactured, artificial**)
- industrial by-products
3. recycled aggregates
- construction & demolition waste
4. marine aggregates
- chloride & shell content (affects** workability, limit to <10% **)
must undergo removal of overburden, processing (screening, washing, blending, stockpiling) & quality control
115
Why add aggregates to concrete?
influences :
- workability, water demand and mix design
- mix design and density
- mechanical properties
- bond with cement paste
- long term durability
- resists volume changes due to
wetting/drying, freezing/thawing
116
effect of shape and surface texture of aggregate particles on Portland Cement ?
shape : angular / irregularity can increase strength, require more cement/water (higher cost)
surface texture : affects bond between the aggregate and cement paste, changing surface area, effecting bond strength
grading : affects packing density -> durability & strength
avoid flaky & elongated particles
116
effect of shape and surface texture of aggregate particles on Portland Cement ?
shape : angular / irregularity can increase strength, require more cement/water (higher cost)
surface texture : affects bond between the aggregate and cement paste, changing surface area, effecting bond strength
grading : affects packing density -> durability, stability & strength
**avoid flaky & elongated particles**
117
examples of deterious substances in concrete aggregates ?
- **organic impurities **(interfere with hydration, setting & hardening)
- **fine materials** (affects workability & aggregate-paste bond)
- **weak/unsound particles** (disintegrate/develop harmful reactions)
- **gypsum**
- **chlorides** (accelerate reinforcement corrosion)
- **reactive silica & carbonate** (react with alkalis in pore solution)
118
effects of porosity of aggregate particles in concrete
** Absorption capacity ** -(Highly absorbent aggregates can absorb water from the cement paste, leading to a reduction in workability and an increase in drying shrinkage. This can also result in a weaker concrete, as the water that is absorbed by the aggregate is not available for hydration of the cement paste)
**Permeability** of resulting concrete
119
Interfacial transition zone (ITZ) ?
| concrete
paste region surrounding aggregate particles
- pn average lower cement content and higher porosity compared to bulk paste
- due to inefficient packing of cement grains on large aggregate
- Weak region, structurally inferior
120
admixtures ?
Material (other than cement, additions, water and aggregates) that is added to a paste, mortar or concrete, during the mixing process to **modify the fresh and/or hardened state properties**
- generally water-based liquids
- ≤ 5% by weight of cement
121
main categories of admixtures
1. superplasticisers
2. air entraining agents
3. accelerators
4. retarders
122
superplasticisers
| admixtures in concrete
(increase its workability without the need for additional water)
- **lower surface tension** - helps disperse particles (workability)
- Consists of anionic polar group joined to long hydrocarbon chain that is polar & hydrophilic (makes cement **hydrophilic**)
- Cement particles become deflocculated
and well-dispersed (more uniformly dispersed)
thus improves hydration (& early age strength)
123
air-entraining agents
| admixtures in concrete
- consists of anionic polar group joined by a long hydrocarbon chain that is nonpolar & hydrophobic
- Covers air bubbles formed during mixing with a sheath of air-entraining molecules repelling one another – stabilising effect **think soap**
- Entrained air voids remain empty, i.e. not filled with hydration products
- prevent water in voids (improved freeze-thaw resistance & durability)
- improve workability
124
Accelerators
| admixtures in concrete
* Promote dissolution of cement compounds
* **Accelerate setting and hardening**
* Higher rate of heat release (exothermic)
* used in cold weather or limited time scale, repaires
125
retarders ?
| concrete admixtures
* Impedes the dissolution of cement compound
* Extend setting time and inhibit hardening
* used in hot weather & conditions where prolonged durability is required
126
early age of concrete
| different stages
1. Batching
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Finishing
7. Curing
8. Formwork removal (6-7 MPa)
127
Fresh concrete must have the ability to ?
EASILY MIXED
EASILY TRANSPORTED
EASILY PLACED IN POSITION
EASILY COMPACTED
EASILY FINISHED
without segregation or bleeding & with available equipment
128
batching & mixing processes ?
| earlly age concrete
- measure ingredients (**mass**)
- check aggregate moisture content
- coat aggregate surface with cement paste to produce fresh concrete of uniform composition, undisturbed by subsequent operations
- Wet mixing time ~ 1 to 3 min (depends on mixer type, size & speed )
- prolnged mixing
- placed within 1.5hrs
129
transporting & placing processes ?
| early age concree
* **Methods**: direct from truck,
conveyor belts, pumps, skips,
wheelbarrows.
* **Aim**: place concrete as near as
possible to final its position, mix
remains cohesive, no
segregation.
* Place in uniform layers, compact
each layer before placing next.
* Each subsequent layer placed
whilst underlying layer still
plasti
130
compacting ?
| early age concrete
- minimise vol. trapped air (from 5-20% in fresh concrete)
- has strength, density, permeability & durability effects
- Affects bond with rebar &
corrosion initiation
131
methods of compacting ?
| early age concrete
- Manual ramming
& tamping
- Vbrators
(poker, formwork, beam):
* Vibration ‘fluidise’
concrete, reduces internal
friction, allows particle
packing and expels
entrapped air
132
methods of compacting ?
| early age concrete
- Manual ramming
& tamping
- Vbrators
(poker, formwork, beam):
* Vibration ‘fluidise’
concrete, reduces internal
friction, allows particle
packing and expels
entrapped air
133
finishing concrete techniques ?
* Screeding: strike-off excess with sawing
action across surface with straight edge
* Darby / bull float: smooth down high
spots, embed large aggregates & fill small
hollows left after screeding
* Floating / trowelling: with flat wood or
metal blades to compact surface,
brings paste to surface & remove
remaining imperfections.
* Brooming: with rake / broom if skid
resistance is required.
Aim : produce flate, dence & level surface
134
slump test ?
| early age concrete
most common test :
- Δ height between slump cone and highest point of slumped concrete (mm)
- measures consistency/batch variation
disadvantages :
Heavily operator dependent does not measure compactibility
135
flow table test ?
| early age concrete
Flow (mm) is the average spread of concrete = (A + B)/2
indicates mix cohesiveness & segregation
for high workability mixtures :
400-500mm high workability
500-650mm very high
136
roller - compacted concrete ?
**Zero-slump** concrete
* Compacted using **vibrating roller** (asphalt equipment)
* Fresh mix must be able to support a
roller while being compacted: dry
enough to prevent sinking of the roller,
but wet enough to allow distribution of
the ingredients during mixing and
compaction.
136
roller - compacted concrete ?
**Zero-slump** concrete
* Compacted using **vibrating roller** (asphalt equipment)
* Fresh mix must be able to support a
roller while being compacted: dry
enough to prevent sinking of the roller,
but wet enough to allow distribution of
the ingredients during mixing and
compaction.
137
examples of concrete finishing innovations ?
- Pumping concrete
- Self compacting concrete
- Sprayed concrete / shotcrete
- Underwater concreting
- Additive manufacturing
all dependant on **rheology** (flow properties ; workability, consistency, and stability of the concrete mix)
138
examples of concrete finishing innovations ?
- Pumping concrete
- Self compacting concrete
- Sprayed concrete / shotcrete
- Underwater concreting
- Additive manufacturing
all dependant on **rheology** (flow properties ; workability, consistency, and stability of the concrete mix)
139
concrete rheology ?
flow properties :
- water content
- cement
- aggregate size grading
- aggregate-cement ratio
- aggregate porosity & absorption
- admixtures
- time
- temp
-
140
concrete curing ?
* Aim: keep concrete as
nearly saturated as possible
- maintain suitable **humidity** & **temp** post-casting
- ensure proper hydration of the cementitious materials and development of the desired strength and durability properties
## Footnote
For satisfactory strength development, not necessary for all cement to hydrate
(rarely achieved in practice)
141
methods of curing ?
1. impermeable sheets
2. steam curing / autoclaving
3. addition of curing compounds
142
potential problems from early age concrete stages ?
* Poor compaction
* Segregation
* Bleeding
* Plastic settlement & shrinkage
143
potential problems from early age concrete stages ?
* Poor compaction
* Segregation
* Bleeding
* Plastic settlement & shrinkage
144
effects of poor compaction
| early age concrete
* Reduces strength
* Reduces bond between concrete & rebar
* Increases transport of aggressive agents
* Visual blemishes
| Strength decrease by 5-6% for every 1% vol. air
145
effects of segregation
| early age concrete
- separation of constitutents (non uniform distribution)
- caused by density & particle size variations, or poor handling
146
Plastic settlement cracks
caused by excessive settlement and bleeding, restrained by large obstructions such as reinforcement bars, large aggregate, change of concrete depth
147
effects of bleeding ?
| early age concrete
- rising of mix water to the top surface
- caused by inability of solids to hold mix water when heavy particles settle
effects :
- gradient in w/c ratio
- laitance on surface
- reduce bond strength between rebar and concrete
- reduce plastic shrinkage cracking
148
bleeding control measures ?
| early age concrete
* Increase cement fineness &
content, reduce water content
* Increase proportion of fine aggregate
* Air entrainment
* Use accelerator, rapid hardening cement
* Reduce rate of evaporation
(early curing)
149
segregation control measures ?
| early age concrete
- improve aggregate grading
- increase fines content
- care in transporting, placing and compacting
- air entrainment
150
plastic settlement cracks control measures ?
- reduce bleeding and segregation
- use air entraining admixture
- adopt re-vibration
151
plastic shrinkage cracks ?
| early age concrete
Caused by rapid loss of water by
evaporation or absorption
- plastic shrinkage due to drying & contracting induces tensile stress, cracks when exceeds
tensile strength
152
control measures of plastic shrinkage cracking ?
reduce rate of surface evaporation by early curing
windbreaks,
sunshades,
cool aggregates
mixing water
Moisten subgrade and formwork
153
how to test compressive strength of concrete ?
casting and curing a set of standard (150 mm) concrete cubes for a period of 28 days, after which the cubes are tested in a hydraulic press to determine their compressive strength
154
why is concrete cube testing > cyclinder ?
- cube test is generally considered to be more reliable due to its larger size
- end restraining effect occurs in cyclindrical
155
Direct tensile test (axial) ?
| concrete properties
- axially applied loads, directly
- secondary stresses usually induced by holding device
- No standardised test exists
156
Splitting Tension test ?
| concrete mechanical properties
applying a compressive load to a cylindrical concrete specimen and measuring the tensile stress
overestimates axial tensile strength (by 10-15%)
* Measured tensile strength increases with
drying
157
Flexural strength test ?
| concrete mechanical properties
loading a concrete beam in a two-point bending configuration and measuring the maximum tensile stress that develops on the bottom surface of the beam
Overestimates axial tensile strength by
~50-100%
158
Flexural strength test ?
| concrete mechanical properties
loading a concrete beam in a two-point bending configuration and measuring the maximum tensile stress that develops on the bottom surface of the beam
Overestimates axial tensile strength by
~50-100%
159
relationship between compressive & tensile strength?
As compressive strength
increases, tensile strength
increases but at decreasing
rate
* Ratio of tensile-compressive
strength ~ 0.1
160
Modulus of Elasticity ?
| concrete
* Measure of stiffness
* Slope of the stress-strain curve
* Used to calculate elastic deflection &
stresses induced by volume changes
161
Modulus of Elasticity ?
| concrete
* Measure of stiffness
* Slope of the stress-strain curve
* Used to calculate elastic deflection &
stresses induced by volume changes
162
what additional information can be obstained from a stress-strain curve of concrete ?
* **Tangent modulus**: slope of line drawn
tangent to curve at any point
* **Secant modulus**: slope of line drawn
from origin to point corresponding to
0.4 ultimate stress (fcm)
* **Chord modulus**: Slope of line drawn
from 50µε to point corresponding to
0.4 ultimate stress (fcm)
163
strength of concrete is a function of ?
* Strength of aggregate
* Strength of paste
* Strength of aggregate-paste
interface (ITZ)
all function of porosity
strength inversley proportional to porosity
164
role of Calcium silicate hydrate (C-S-H) in cement ?
Main **hydration product** in Portland cement
binder & source of concrete strength
- Complex structure with variable composition
165
states of water in hardened cement paste?
1. Capillary pore water ('free')
2. Adsorbed water (onto surfaces of hydration products)
3. Interlayer water (hydrogen bonding between C-S-H sheets)
166
capillary action
Movement of liquid up a narrow tube against gravity. Attraction of water molecules to the tube wall is stronger than the attraction between water molecules. Adhesion force pulls water molecules up
| due to adhesion , cohesion & surface tension
167
causes of y movement of water within
the microstructure of cement paste
* Swelling
* Autogenous shrinkage
* Drying shrinkage
* Creep
168
swelling in cement paste ?
Absorption of water by C-S-H
- ↑ pore pressure & forces C-S-H gel apart
| cement paste typically
1300 με after 100 days 2000 με & after 1000 days
168
swelling in cement paste ?
Absorption of water by C-S-H
- ↑ pore pressure & forces C-S-H gel apart
| cement paste typically
1300 με after 100 days 2000 με & after 1000 days
169
autogenous shrinkage ?
sealed conditions (no external movement of moisture)
self-induced volume reduction
- ongoing chemical reaction between cement and water during the early stages of concrete hydration
169
autogenous shrinkage ?
sealed conditions (no external movement of moisture)
self-induced volume reduction
- ongoing chemical reaction between cement and water during the early stages of concrete hydration
170
Drying shrinkage
| concrete
evaporation of water from concrete causing shrinkage
occurs primarily in capillary pores
typically after complete drying :
* Cement paste ~ 4000 με
* Concrete ~ 200-1200 με
* Shrinkage is partially reversible*
causes **cracking** if restrained
171
creep
| concrete
material deforms slowly over time under a constant load or stress
can occur for low loads
causes cracking, deflection, and reduced load-carrying capacity
eventually, if remained constant a decreased rate of deformation is observed - **relaxation**
172
explain creep
| concrete
in hydrated cement paste : **internal movement of
adsorbed water and interlayer water to empty capillary pores**
* Sliding and re-arrangement of
the C-S-H sheets
* Microcracking at the ITZ also
contributes
* Can occur at constant humidity
creeps (no drying involved)
* If concrete dries while under
load, shrinkage and creep
occur simultaneously
* Drying increases the magnitude
of creep
172
explain creep
| concrete
in hydrated cement paste : **internal movement of
adsorbed water and interlayer water to empty capillary pores**
* Sliding and re-arrangement of
the C-S-H sheets
* Microcracking at the ITZ also
contributes
* Can occur at constant humidity
creeps (no drying involved)
* If concrete dries while under
load, shrinkage and creep
occur simultaneously
* Drying increases the magnitude
of creep
173
positives/negatives of creep
Positive :
* Reduces stress
concentrations induced by
shrinkage, thermal movement
etc.
* Reduces risk of microcracking
Negative
* Excessive deflection
* Serviceability problems
(especially in high-rise
buildings and long-span
bridges)
* Loss of prestress in
prestressed concrete
174
Heat evolution during Portland cement hydration ?
1 - Initial dissolution/induction
2 - Induction/dormant period
3 & 4 - Nucleation & growth
5 - Diffusion limited reactions
hydration is exothermic, greater cement content = greater temp rise
175
thermal movement in concrete ?
= ΔT * α ( coefficient of thermal expansion)
casues **cracking**
176
mass concrete ?
concrete elements or structures that are relatively large in size and volume
-heat of hydration generated during the curing process can cause significant temperature differentials within the concrete
causing :
1. expansion
2. thermal shrinkage
3. cracking
177
how to Mitigating thermal cracks in mass concrete ?
* Use less cement
* Use low heat cement
* Use blended cements, with SCMs
* Avoid cement with high specific surface
* Use well-graded aggregates with
larger max. size
* Use superplasticizers or air
entraining agents
* Use aggregates with low coefficient of thermal expansion
Place concrete at low ambient
temperature (night time)
* Precooling of fresh concrete
* Use cold water or chipped ice as part of
mixing water
178
transport properties of concrete
penetrability - significant effects on durability
1. permeation
2. diffusion
3. absorption
4. wick action
179
permeation
flow induced by pressure
gradient
180
diffusion
flow induced by concentration
gradient
181
absorption
flow induced by capillary action
into unsaturated concrete
182
wick action
flow induced by combination of
permeation, diffusion & absorption in structures exposed to water on one
side and drying on the opposite side
183
wick action
flow induced by combination of
permeation, diffusion & absorption in structures exposed to water on one
side and drying on the opposite side
184
reinforcement corrosion
corrosion of steel that is used to reinforce concrete
185
reinforcement corrosion
corrosion of steel that is used to reinforce concrete
186
why doesn’t
steel embedded in concrete
corrode rapidly?
**high alkaline environment** causes reaction with the surface of the steel to form a thin layer of oxide that acts as a barrier against further corrosion
187
concrete pore solution ?
liquid that fills the voids and capillaries within hardened concrete
concentrated by drying & diluted by wetting, leaching and carbonation
187
concrete pore solution ?
liquid that fills the voids and capillaries within hardened concrete
concentrated by drying & diluted by wetting, leaching and carbonation
188
mechanism of reinforcement corrosion ?
1. passive layer destroyed by carbonation or chloride attack
2. steel depassivates
3. steel corrodes if O2 and H2O are present
189
carbonation-induced corrosion ?
carbon dioxide in the air dissolves in pore solution to form carbonic acid & reacts with the calcium hydroxide (hydration product) in concrete, producing calcium carbonate
- pH ↓ (more acidic)
- pH of carbonated pore solution ~ 8.5
190
measurement of carbonation depth ?
| concrete durability
Phenolphthalein Test :
solution of phenolphthalein indicator is applied to the surface of the concrete, which changes color from **pink to colorless when the pH drops < 9**. This method is quick and easy to perform, but it only provides an indication of the surface depth of carbonation
| determine the depth of carbonation in concrete
191
Effect of cracks on carbonation depth ?
| concrete durability
cracks create pathways for carbon dioxide to penetrate deeper into the material, accelerating the carbonation process
192
predicting the time to corrosion from the carbonation depth ?
Depth of carbonation : *D = K sqrt (t)*
K = carbonation coefficient (mm/year0.5)
t = exposure time in years
K is influenced by the exposure environment & properties of concrete:
* Close to zero if pores are completely dry or fully saturated
* Maximum for RH between 50 & 70%
* Pore structure, w/c ratio, curing
* Type of cement used (alkali content)
* Increases with temperature
193
chloride-induced corrosion ?
Chloride attacks passive layer, acts as catalyst and activates steel to form anode
- if H2O & O2 are available = corrosion
- chloride not consumed
- sources of chloride are internal or external
prevention : Total chloride content in concrete should be **no more than 0.4%** by weight of cement (EN 1992-1: 2004)
194
sources of chloride for chloride-induced errosion ?
1.** sea water** - ~ 3.5% soluble salts by mass, Tidal, spray & splash zone
2. **de-icing salts** - (NaCl, CaCl2), treat road surfaces & bridge decks to
melt snow and ice
195
Mechanism of reinforcement corrosion ?
| concrete
* Fe2+ ions at the anode dissolves into solution (oxidation)
* Electrons flow through rebar into the cathode and combine with H2O and O2 to form OH- ions
* Pore solution acts as an electrolyte to complete circuit
*** Electro-chemical reaction**
* OH- travel via pore solution and combine with ferrous ions
Once initiated, corrosion rate is controlled by:
* Electrical resistivity
* Availability of oxygen and moisture
196
effects of reinforcement corrosion damage ?
| concrete
* Reduces area of steel
* Reduces load carrying capacity
* Cracking & loss of bond between rebar
and concrete
* Increases ingress of aggressive agents
* Spalling and delamination
197
Predicting time to corrosion from chloride profile ?
Fick’s second law of diffusion
(Total chloride content required to initiate corrosion is often taken as 0.4% wt. cement)
198
service life = ?
initiation + propagation
limit states :
(1) Corrosion initiation
(2) Cracking of cover
(3) Spalling/delamination
(4) Structural collapse
199
frost damage ? & mitigation techniques ?
| concrete
Water expands ~ 9% vol. on freezing
- Cumulative effect of repeated freezing-thawing
cycles = progressive expansion, cracking,
spalling and disintegration
mitigation:
* Protect concrete from moisture
* Use dense cement paste
* Use frost resistant concrete
* Increase aggregate fraction
200
sulphate attack ?
| concrete
- sources : groundwater from decay of organic matter/indutrial/agricultural activity
- formation of ettringite & gypsum by reaction with sulphate & cement paste
- can lead to cracking, spalling, and structural damage
- Sulfate content > 4% by weight of cement
201
Thaumasite form sulfate attack ?
| concrete
(TSA)
- exposed to a combination of **sulfate** and **carbon dioxide** in the presence of **moisture**
- Low temperatures (< 15°C)
- * Cements with low C3A (< 5%) and SCMs
(such as GGBS) offers good resistance.
202
Alkali-aggregate reactions ?
Alkali-Silica Reaction (ASR) : most common
- Reaction between alkalis from cement (alkali hydroxides)
and reactive silica from aggregates
- produces an expansive gel
consequences :
* Map-cracking
* Leaching of colourless gel
* Affects appearance and serviceability
* Facilitates ingress of other aggressive
agents
203
ceramics ?
inorganic, non-metallic materials
- crystalline, with some glassy phase
204
key properties of ceramics ?
High compressive strength
Tensile strength is relatively low
Low toughness/brittle materials
– no significant mechanisms to stop cracks propagating
Relatively high hardness (some are very hard)
Good thermal insulation
Good electrical insulation
Durability tends to be very good under environmental conditions
Relatively low cost, despite need for thermal processing
205
stone ?
- durable/long-lasting
- lowest energy and carbon dioxide emissions
- requires cheap and skilled labour
206
brick ?
- higher energy costs for manufacture
- low application cost
- relatively low skill
207
ceramic manufacturing ?
- Clays are moulded in a plastic state and then fired (sintered or ‘burnt’)
- Consist of a glassy phase which melts and “glues” together a complex
polycrystalline multiphase body - liquid phase sintering
- Clays: complex hydrated aluminosilicates
208
basic ceramic raw material ?
- clay minerals (rich in alumina, silica & water)
- wet clays (plastic) - able to be formed & moulded
- harden when fired/sintered
- fine minerals (flakes, fibres)
- may contain iron oxide, silica and rock fragments
- six categories :
brick clay
bentonite
common clay
fire clay
Fuller’s earth
kaolin
209
clay structure ?
arranged in **layers** (layer silicates)
- consisting of sheets of silicon and oxygen atoms that are bonded to other atoms such as aluminum, magnesium, or iron
- layers held together by weak electrical charges to form **platelets**
210
ceramic sintering process ?
ceramic powder : size range 0.5 - 5.0 µm
- ceramic powder mixed with binder & shaped
- sintering temp : from 850°C for tiles to >1650°C for engineering
ceramics
- liquid phase added : low melting point
- liquid draws the solid together by viscous flow, driven by capillary pressure
- (Liquid phase may cool to a glass - poor high temperature properties
Or crystallise - improved high temperature properties)
211
ceramic microstructure ?
consists of :
Crystalline phases
Amorphous (glassy) phase
Porosity
- non-mobile dislocations + pores & surface flaws (stress concentrators) + no stress relieving mechanism = high hardness, low toughness (brittle), low tensile & high compressive strength
- ceramics are full of flaws
212
Griffith's Equation ?
explains why surface defects and pores reduce the strength of ceramics & Indicates how flaw size determines strength
Tensile fracture stress σF is controlled by defects present either from fabrication or from surface damage :
σF = K*Ic* / α sqrt( pi * a )
K*Ic* = Fracture toughness
α – geometrical factor (~1)
a – size of biggest crack under stress
213
effect of sintering temperature on ceramic properties ?
generally as sintering temp. ↑ ,
density, strength, and hardness ↑
porosity and ductility ↓
214
ceramic processing technologies ?
slurry formation, moisture optimisation & spray drying
powder pressing :
- hot
- uniaxial
- isostatic
extrustion : Ceramic may be mixed with organic binders
Used for bricks and pipes (continuous & consistent shaping)
slip casting : Uses a ceramic slurry (slip)
Porous gypsum moulds
215
ceramic production stages ?
granular powder → powder processing → compacted green body → sintering → ceramic component
216
Adobe bricks ?
(clay/mudblocks) - unfired
- one of the oldest and most widely used
- sun-dried blocks
217
brick manufacturing ?
clay extraction → moulding (pressed at high pressure) → extrusion (high rates, continuous) & wire cutting → drying → sintering (~900-1100degC)
218
brick efflorescence ?
Harmless soluble materials leach out : composed of sodium, potassium and magnesium sulphates
- Salts in brickwork are dissolved by water
- As wall dries the salt solution becomes more concentrated
- They deposit on the surface as white discolouration
219
iron staining ?
| bricks
(certain wire cut bricks and those from clays with high iron content)
- Under certain conditions iron salts migrate to the surface
- They oxidise to produce a brown stain
- saturation of immature bricks
220
lime staining ?
| bricks
Free lime present in mortar leaches out and leaves lime staining
- Occurs when work left without being covered
- Exposure to rain triggers the process
- White deposit forms where water removes lime from cement
221
factors affecting durability of brickwork ?
inherently **durable**
Destructive agents affecting masonry:
* Water
* Frost
* Temperature change
Repeated **freeze/thaw cycles** do most damage
**No simple correlation** between frost resistance and strength/water absorption
222
key properties of glass ?
Transparent
High stiffness (Young’s modulus)
Brittle (low toughness, associated with catastrophic failure)
Reasonable strength in compression
Relatively low strength in tension but highly variable
Hard, but relatively easily damaged
Normal glass fractures to give sharp surfaces
Excellent corrosion resistance
Low leaching of contaminants
223
glass microstructural characteristic ?
- non-crystalline, amorphous material
- glass lacks a long-range ordered structure
- atoms in glass are random and disordered
- inorganic product of fusion (melting) which has cooled to a rigid condition without crystallization
224
glass composition ?
**SiO4 tetrahedron**
(most commercial glasses)
Si - O bond is covalent (sharing electrons) - very strong
225
formation of glass ?
- complete melting of the raw materials
- no clear melting temp.
- Specific volume does not have an abrupt transition at a fixed temperature
- Retains amorphous structure of a liquid phase
- No regular arrangement of atoms and
no dislocations
226
viscosity vs temp for glass
viscosity ↑ as temp ↓
(Increased viscosity inhibits ability to form a crystalline solid)
lower viscosity - soda-lime glass
227
alkali addition to glass ?
Addition of alkali oxide breaks up structure by lowering melting temp
As alkali is added, ionic non-bridging oxygen atoms are formed
e.g. Alkali Silicate Glasses **M2O + SiO2**
227
alkali addition to glass ?
Addition of alkali oxide breaks up structure by lowering melting temp
As alkali is added, ionic non-bridging oxygen atoms are formed
e.g. Alkali Silicate Glasses **M2O + SiO2**
228
raw materials in glass production
silica sand (glass sand), felspathic sand (alumina sand), sodium carbonate (soda ash), calcium carbonate (limestone), magnesium carbonate (dolomite), felsparm nepheline syenite, aplite, sandspar
229
commercial glass composition ?
70% - 74% SiO2 silicon dioxide - silica
12% - 16% Na2O sodium oxide - soda
5% - 11% CaO calcium oxide - lime
1% - 3% MgO magnesium oxide
1% - 3% Al2O3 aluminium oxide
| soda lime silica glass
230
types of glass components ?
1. glass forming oxides (dominant component)
2. fluxes to reduce melting temp
3. propery modifiers
4. colouring agents < 1 wt%
5. processing agents (e.g. As-oxide to promote bubble removal) < 1 wt%
231
physical properties of glass microstructure ?
homogenous - no interfaces to scatter light
no potential for dislocations
no crack propogation prevention mechanism
high stress concentrating eggect of surface cracks/defects
observed tensile strength much lower than theoretical
232
effects & mitigation of glass surface flaws ?
surface flaws = stress concentrators (as seen in Griffith's Eqn)
- surface flaws introduced by abrasion with hard materials
- reduction by **polishing** (mechanical, flame, acid etching)
- large flaws on glass surface significantly lower fracture stress (fails at lower stress)
233
static fatigue ?
| glass
(type of failure mechanism)
strength of glass under load decreases with time
- failure occurs at greater stress if rapidly applied
- decrease in stregth accelerated by increased humidity & temp
- due to the **diffusion of water to the crack tip**,
lengthening the 'critical crack length, reducing the ultimate fracture stress
234
Glass manufacturing process ?
Raw materials
Batch formation
Melting
Cooling
Forming
Annealing (heat-treatment)
Quality control
Packaging
234
Glass manufacturing process ?
Raw materials
Batch formation
Melting
Cooling
Forming
Annealing (heat-treatment)
Quality control
Packaging
235
flat glass ?
produced in large, flat sheets or panels, typically for use in architectural, automotive, or other industrial applications
- uses **float process**
236
float process for flat glass production ?
Molten glass, at approximately 1000degC, is poured continuously from a furnace onto a large shallow bath of molten **tin**
- floats & forms a level surface
237
float process for flat glass production ?
Molten glass, at approximately 1000degC, is poured continuously from a furnace onto a large shallow bath of molten **tin**
- floats & forms a level surface
238
float process for flat glass production ?
Molten glass, at approximately 1000degC, is poured continuously from a furnace onto a large shallow bath of molten **tin**
- floats & forms a level surface
239
glass melting ?
temp ~ 1600degC
in a large furnace
- 2,500 tonnes of molten glass
240
annealing/tempering ?
| glass
controlled cooling so that stress is removed
- in an insulated chamber known as a lehr
- temp gradually reduced until it emerges as a cooled ribbon of glass
241
tinted glass ?
added to the raw materials at melting stage
Co and Ni tint glass grey
Ferrous oxide (FeO) produces blue tint
Ferric iron (Fe2O3) produces yellow tint
Combined they give glass a green tint
242
coated (on-line) glass ?
Glass can be coated on-line in the float process as the ribbon of glass is formed
Chemical vapour deposition applies microscopically thin coating
At a temperature of about 600ºC.
Pilkington K Glass™, Pilkington Energy Advantage™ and Pilkington Activ™
thin coating of metal oxide or other materials to** enhance its optical, thermal, or other properties**
243
wired glass ?
Wired glass is made by a rolling process
Steel wire mesh sandwiched between separate ribbons of glass (semi-molten)
Passed through consolidating rollers which may also impress a pattern.
Rough cast surface may be polished to obtain clear transparency.
Uses include fire resistance and safety glazing.
244
laminated glass ?
- bonding two or more layers of glass together
using a special plasticised interlayer
- e.g. polyvinyl butyral (PVB)
- Processed under controlled heat and pressure conditions
- Similar refractive index to the glass – no effects on light transmission
- Absorbs over 99% of ultraviolet rays found in natural sunlight
245
recycled glass in construction ?
glass aggregates
- cement component (microfiller)
- unbound aggregate
- aggregate in bituminous materials
- lightweight glass aggregate
waste encapsulation (for radioactive materials)
246
eutectoid reactions?
- results in formation of pearlite
- causes austentite to transform into pearlite consisting of ferrite & cementite
- describes the phase transformation of one solid into two different solids
247
phase diagrams ?
- can be used to understand the effect of slow cooling on the microstructure of steels
- provide information on the types of phases formed and their compositions during slow cooling
- The composition is on the x-axis and temperature is on the y-axis
- The lever rule provides information on the relative amounts of different phases present
- A tie line can be used to give information on the composition of different phases present
248
pearlite ?
- Can be formed by slowly cooling austenite with a hypoeutectoid composition
- Can be formed by slowly cooling austenite with a hypereutectoid composition
249
Rate of carbonation of concrete is dependant on ?
Temperature
Moisture state of the concrete
Cement type
CO2 content of the air
250
For concrete made with a particular cement type,
higher capillary porosity usually means ?
Reduced compressive strength
Lower E
More rapid chloride penetration when exposed to sea-water
Reduced abrasion resistance
251
pure iron ?
Exists as ferrite (α-iron) with a body centred cubic (BCC) structure below 912 degrees C
B Has low solubility for carbon at temperatures below 912 degrees C
252
rapid cooling / quenching of carbon steel ?
Results in the formation of a very hard but relatively brittle material known as martensite
Increases the strength but reduces the toughness of steel compared to slow cooled materials T
phase diagrams NOT applicable
253
shape factor ?
- measure of efficiency of material usage
- upper limits often due to manufacturing methods used to shape a material
- influence on structural efficiency
254
shape factor ?
- measure of efficiency of material usage
- upper limits often due to manufacturing methods used to shape a material
- influence on structural efficiency
