Final 2.0 Flashcards
1
Q
whats the other main component in steel other than iron?
A
Carbon (comes from coal)
2
Q
what do you call steel when theres too much carbon
A
Cast iron (>1.7% carbon) its not used in construction because its too britle, weak in tension so cant be used as replacement of steel.
3
Q
what are the 3 ways to strengthen steel
A
-heat treatement (The formation of additional grain boundaries)
-Alloying (The introduction of interstitial and substitutional atoms)
-strain hardening (The generation and concentration of dislocations)
4
Q
whats the main downside with strengthening steel in connections beside reduced ductility
A
reduced weldability
5
Q
what are the main downsize to using welding alternative
A
bolting is expensive and time consuming
6
Q
what is toughness
A
abilitity to absorbe energy before failure in particular under impact conditions, by deforming plasticaly before fracture
7
Q
name the most practical way to measure toughness
A
Area under the curves (good for structural steel since its ductile)
The area under the stress-strain curve in the uniaxial tension test. This method is not normally used for brittle materials, because it does not represent material behavior under brittle fracture.
8
Q
explain how strain hardening decreases toughness
A
area under the curve decreases under strain hardening (see Q doc for graph)
9
Q
order the following materials from more to least variable
A
1 Timber (natural so macroscopic defects)
2 Concrete (unskilled labor so no control conditions but controlled ingredients)
3 Steel (factory made so very little defects)
10
Q
how do we account for variability in structural design
A
phi factor for concrete (0.6 or 0.65) steel (0.85-0.9) timber other factors/ design adjustement
11
Q
why is phi factor for steel higher
A
less variable
12
Q
What is the maximum permanent strain in the linear elastic region?
what mechanical properties can be calculated in this region?
A
0 when something is elastic
Youngs modulus by taking slope of stress strain
13
Q
what is a charachteristic of steel in the linear elastic region compared to other metals
A
when it yields it stops being linear
proportional limit when it stops being linear = yield pts
14
Q
why is ductility preffered in construction mats like steel?
when does it fail?
why is it used to indicate failure?
A
-Warning before failure
-when it yields
- Starts to collapse
15
Q
what is the physical difference between the uniform plastic deformation and localised plastic deformation
A
see Q doc q 11
16
Q
whats the difference between a ductile vs brittle coupon steel
A
brittle fails at 90 degrees
ductile yields at 45 degrees before failing
17
Q
what is the difference in corosion reaction if the steel is submerged vs exposed
A
submerged steel only undergoes 1st stage of corrosion
18
Q
what reactant is limited for corrosion when submerging steel
A
lack of oxygen
19
Q
what condition exposed or submerged steel leads to expansion due to corrosion and what does this mean to concrete surrounding in each conditions
A
exposed because reaction progresses further.
exposed concrete will get spalling and submerged nothing will happen to it but steel loses its tensil load carrying properties.
20
Q
why is it a bad idea to use 2 grades of steel for bolts and structural member. how does the main issue occur?
how can this concept be used to protect steel inreinforced concrete
A
galvanic corrosion
one of them acts as annod and the other as cathode so difference in electrochemical potential ( ability to react)
sacrificial annod, it will corrode before the cathode. A piece of metal with high electrochemical potential like zinc is used (doesnt expand when it corrodes) so that will be the main piece of corrosion ( site of deterioration because its connected to rebar)
21
Q
why are welded connection worst area for damage to occur?
A
higher stress region leads to more corrosion
crevasse corrosion happens more often @ connection
low fatigue strength @ heat affected zone
22
Q
What are the 3 components of the microstrusture at a weld?
A
-weld metal
-base metal
-heat affected zones
23
Q
what resists tensile loading perpendicular to the grain and parallel to the grain at the microscale
A
-Parallel: strong primary covalent bond between microfibrils (primarly S2 , most microfibrils are parallel to the axis of the cell
- perpendicular : S3 ans S (very weak strength) , walls and weak secondary bonds and lignin (bonds cells together)
24
Q
what are the 3 failure modes of wood in bending? which is the most common and how does it affect the microstructure?
A
compressive, tension and shear
- shear
at the macro, grain will be separated
at the micro the lignin will be shredded of
25
assuming that no other design adjustment are relevant, why is it bad idea to use mean strength of wood when designing a structure?
-Half of the wood would fail at lower strength than designed
- 5% exculsion limit
- only 5% of the specimen that are loaded will fail at the designed stress
26
why do very thick members of wood/timber survive fires explain how the mechanism works
-forma a layer of char on the outside -char is not flammable & is an insulator and when the fire reaches pyrolise zone T is lowered
27
explain the methode of applying fire retardant on wood in a new & old structure and explain the difference
-new can use pressure impregrenation (1,3cm) : 1 place wood in vaccum , 2 dip in fire retardant and 3 release vacuum
- existing paint a surface coating (2,3mm). painting it on and will be absorbed capilary suction
28
what are the environmental and economic benefits of wood
and why cant wood compete with concrete?
-renewable ressource & its cheap
- volume demand is just way too high
29
what are the 2 main limitation to using structural timbers in construction and how do you get around it
size limitation
defects (can limit allowable stress)
can avoid this by using composites ( can make whatever size and limit the defects)
30
explain why its more practical to curve glue lam or paralum than timber members
stress increase the further you are from centre/neutral axis so longer the member the higher stress, the smaller the member the lower the stress for the same size see q 24
31
what is common between mold and bacteria
both harmful to humans and both dont significantly damage wood
32
explain how and why dry rot fungi is a misnomer
wood as to be saturated for funghi to exist
can take water from source that can be 2-3 m away
33
what protects a structure from outside environement?
what are 2 main ways they can be degraded
and how does this affect wood susceptibility to bacterial and fungal attack
-building enveloppe (walls, sheeting, roofing)
1 is holes for dryer ducts , 2 is balcony railing (hole pierced through the wall and can degrade) for 1 and 2 this is whitout proper sealing (usually not done)
- higher moisture will come into the structure
34
why is it that if you break a small hardwood cube it has a higher strength than concrete but a big beam strength woould be lower?
wood has no macroscopic defects in the wood, timber beams on the other hand, has defects so it reduces the strength
35
go see sl 15 and 16
u
36
why does not all wood float, and why does fully saturated wood sink?
wood p=1.5, air p = 0 so if the density of the wood is lower than 1 it floats.
saturated sink since p water = 1 and solid = 1.5 and anything between 1 and 1.5 will sink
37
in wood which part absorbs water first, wich releases water first cell wall or cell cavity
cell wall absorb the water first but release last
38
what is fiber saturation pts
moisture content at wich cell wall are fully saturated
39
when below saturation pts what happens to wood in terms of durability, strength , mechanically
stronger ( should be able to explain so look it up)
durability ( more durable when dry because fiber saturation pts, fungus can grow)
40
explain why counting growth rings is a good but not foolproof method of telling a trees age
good approximation because form usually once a ear but not foolproof because of different moisture available. Drought = 0 rings but more rain = more rings. tropical regions also dont have different sunlight & precipitation every season
41
what is greenwood & why is it a bad idea to build with Greenwood (freshly cut)
a piece of greenwood will dry out naturally so shrinkage. if it set in construction, stuff will move & change place or cause stress in wood because it cant move
42
final q go see q last slide
p
43
what is a polymer
family of synthetic materials made up of repetition off high weight molecules in the form of a flexible chain
44
whats the structure of polymers
repeating small unit called monomer is bonded by covalent bonds, then undergo process of polymerization where monomers link together to form larger polymers.
45
whats the different types of polymerization?
addition : either homogeneous type (A+A+A..) or copolymer type (A+B+A+B) or condensation :
polymerization between two monomers with the expulsion of a simple by-product like water, hydrogen, chloride etc
46
what are the types of polymers and their strength and weakness
linear and branched chain polymers are flexible and weak in nature due to secondasry Van der Waals force wioch hold monomers chain together
cross linked and network polymers are more rigid and hence stronger here the chains are bonded with covelent bonds rather than weak vand Der waal
47
what are some geneal properties of polymers?
lightweight
corrosion resistant
low in strength
not suitable for high temps (toxic fumes)
relatively inexpensive
readily formed into a variety of shapes
some are transparent
electrical insulators
low coef of friction
some can convert light to electricity
48
what are the 3 groups of polymers
thermoplastics
thermosets
elastomers
49
define thermoplastic
branched chain of polymers wich are usually obtained by addition polymerization. these can be softened hardened and resoftened by appluication of heat. this method is used to reshape and make tyhem recyclable.
50
what are some characteristic of thermoplastic polymers
- Anisotropic
- Soften and melt upon heating
- Ductile
- High fracture energy
- More easily repaired than thermosets - Good environmental resistance
- High melt viscosities
- No chemistry during fabrication
- Potential for recycling
- Indefinite shelf life
51
define thermoset polymers
belong to three dimensional cross linked or network polymers, so stronger and harder than linearly structured thermoplastics. usually products of condensation polymerization and will undego setting and hardneing on heating and cooling but once set they cant be reproduced unlike thermoplastic due to loss of part of a molecule.
52
characteristic of thermosetting polymers
Isotropic
- Do not melt upon heating
- Decreased stiffness at high temperatures - Brittle (though yield in compression)
- Low fracture energy
- Sensitive to damage
- Environmentally stable
- Low viscosity before setting
- Limited shelf life
53
define elastomers or rubbers
linear polymer structure with some cross-linking between molecules, thus providing the elastic properties. they have. very low elastic modulus so they have the ability to deform under low loads without permante change in shape. go see slide 9 in pp for stress strain curve
54
characteristics of elastomers /rubbers
Elasticity (can stretch 9-10 times their original dimension and shape is completely recovered upon removal of applies stress.
Flexibility (almost as flexible as cloth)
Toughness (due to high deformability)
Impermeability to water and air
Resistance to corrosion
Plasticity and ease of molding
55
define silicones
special type of elastomer, inorganic polymers so no carbon (Si-O-Si). can be produced from low viscous fluids or from rigid cross-linked chains. These polymers are characterized by high dimensional and thermal stability, chemical inertness, low toxicity, and good electrical and anti-adhesive properties.
56
what is polymer functionality?
another way to classify different polymers. its defined as the number of sites at wich the new molecules can be attached to the mer.
57
what are the three ways a polymers properties change as it increases in size
1: increased melting pts
2: increased strength
3: increased stiffness
58
what are 2 characteristic that dont vary greatly from one polymer to another
thermal conductivity and specific heat
59
why do solid polymers have very low permeability
because they dont have interconnected poresso it makes an excellent material for transmitting measurable amount of gases vapors and liquids
60
what causes polymer to lower their durability
environemental degradation (causes physical and chemical changes at microstructural level) like photo-oxydation or thermal oxidation this leads to embrittlement or the breaking down of chains into oxidized fragments that are water soluble and easily washed out
61
what are syntehtci polymers resistant to
most of them are resistant to bacterial or fungal attacks
62
what are the different types of additives for polymers polymers
Plasticizers improve flow, reduce brittleness, and tend to lower glass transition temperature in amorphous polymers.
Fillers are added to reduce cost.
Reinforcement agents improve mechanical properties.
Stabilizers improve heat resistance, UV, oxidation.
63
what is amorphous polymers
ressembles glass,
The polymer chain mobility decreases with a decrease in
temperature.
Below a certain temperature (glass transition temperature, Tg), the polymer becomes a rigid solid.
At temperatures above Tg, polymers behave as viscous solids (elastomers).
When the temperature is below Tg, the polymer is hard and brittle.
64
what is crystalline polymers
linear polymers can adopt a structure similar to that of the regular lattice arrangement of metallic or ionic crystals.
These polymers are known as crystalline polymers.
The crystalline structure also includes some amorphous regions.
Polymers are crystalline when neighboring chains become aligned.
These crystal-like regions are called micelle.
65
what is foamed polymers
are low density polymers. They can be rigid (e.g. styrofoam), or flexible/elastic (e.g. mattresses of polyurethane foam).
One method of manufacture is to pass steam over plastic beads to produce the foam structure in a die.
66
what is foamed in-place insulation
is a two component liquid resin combined either within or immediately outside of a spray nozzle and deposited in place.
The plastic hardens when it comes into contact with a cold surface and gets immediately air-cured.
Foamed in-place polyethylene has limited resistance to, and must be protected from, abrasion, flame, and ultraviolet light.
67
what are the two types of composite materials
particular and fiber reinforced
68
what are the primary components of fiber reinforced composites
1. Matrix – the bulk material surrounding the fibers
2. Fibers – reinforcement for the matrix
3. Interface – contact area between matrix and fibers
4. Additives – other constituents introduced to produce a desired effect
69
whats the purpose of the matrix in composites
- maintain shape
- hold reinforcement phase together
- transfer tensile stress to fibers
- transfer tensile stress between fibers
- support fibers in compression
- protect reinforcement from environment - conduct/resist electricity
- reduce mass
70
whats the purpose of the fiber in composites
- provide strength
- increase stiffness
- improve energy absorption
71
whats the interface purpose for composite
allow stress transfer between matrix and fibre
If the interfacial contact region is a distinctly separate phase, it is normally called the interphase (e.g. coupling agents).
72
whats the purpuse of additives for a composites
Plasticizer – Lower glass transition temperature Lubricant – Improve mixing, lower friction
Stabilizer – Improve resistance to heat, UV, oxidation Fire Retardant – Inhibit burning (toxic)
Filler – Improve mechanical properties, reduce cost
73
what are the three important characteristic a strong fiber must posses
1. Small Diameter
Tensile strength increases as fiber diameter decreases.
2. High Aspect Ratio
Composites
Fibre Characteristics
Surface area to volume ratio increases/decreases with aspect ratio.
3. High Flexibility
Permits a variety of techniques to be used for making composites (e.g. weaving, filament winding). Typical for a material having a high E and small diameter.
74
why do Fiber-reinforced thermosets do not usually fail in a brittle manner
because they give a warning before failure
75
how do you first detect micro cracking
change in slope in the stress strain curve
76
Stiff and elastic reinforcing fibres are used for what?
help reduce creep in polymeric materials.
77
what are some advantages to fiber reinforced polymers
-Tailored mechanical & physical properties
- High specific strength
- High specific stiffness
- Low density
- Can be formed to complex shapes
- Corrosion resistance
- Chemical resistance*
- Electrical resistance*
- Electromagnetic neutrality
- Low heat conductance
- Low cost*
- Compatible with most other materials*
- Can be colored
78
what are some disadvantage of fiber reinforced polymers
Brittle at low temperatures
- Properties degrade at high temperatures* - Moisture sensitive
- Poor impact resistance*
79
what is asphalte made of
its a viscuous cementitious material mostly made of bits or high molecular weight hydrocarbons
80
brielfy describe asphalt cement
viscous, cementitious, high molecular weight hydrocarbons. can be found naturally in lakes, rocks but also made artifically as by product of crude oil treatement. its used in roads, roofing and weather resistant elements. its a thermo plastic mat, doesnt have sufficient flow properties for casting, must be placed and consolidated using static or vibratoru compaction
81
what components in asphalte have a large effect on physicial anr rheological properties
O,N,S and trace metal
82
descibre age hardeneing for asphalt
Asphalt cement is subjected to heating for variable time periods at a wide range of temperatures during mixing, curing and service life
* Substantial changes occur in the structure and composition of the asphalt molecules during these processes
* Due to volatilization of light hydrocarbon fractions and oxidation by reacting with the oxygen in the environment
* These changes in turn cause the asphalt cement to harden or become less ductile
83
what are the rheology properties
-Viscosity is the fundamental materials property relating the rate of shear strain in a fluid to the applied shear stress (resistance to flow)
* Consistency is an empirical measure of the resistance offered by a fluid to continuous deformation when subjected to a shearing stress
Rheological properties are highly temperature dependent
84
how do you most commonly measure asphalt sonsistency and hoe do you use the result
most commonly measured using the empirical penetration resistance test
* Measured in terms of depth (in units of 0.1 mm) that a standard needle penetrates a sample under standard conditions of loading, time and temperature
* Used to identify an asphalt as to its hardness or grade
* Used to measure changes in hardness due to heating or changes in temperature.
85
whats the flash point test
Determines temperature to which an asphalt can be safely heated in the presence of an open flame
86
whats the ductility test for ashapl cement
Measures the distance a standard asphalt cement sample will stretch without breaking under standard testing conditions (5 cm/min at 25o C)
High Ductility -- High temperature susceptibility Low Ductility -- Poor adhesive property
87
what is ashplat cement stiffness dependant on
Dependent on temperature and rate or duration of loading or stress application
– At low temperatures, or short load durations, behaviour is elastic
– At high temperatures, or long load durations, behaviour is plastic
* Typical field behaviour lies somewhere between these two extremes
88
what does a change in stifness of ashpalt cement with temperature associated with?
change in viscosity
the greater the rate of change of viscocity with temparature, the more temperature susceptible the asphalt cement becomes (i.e. the more liable it is to fracture or crack at low temperature or to deform excessively at high temperature)
89
descibre the importance of asphalt cement tensil strength and on what it depends
* Important under conditions which promote elastic behaviour and brittle fracture.
* Dependent on its viscoelastic and physical characteristics, rate of loading, and temperature
* Reaches a maximum of 2-4 MPa at low temperatures and decreases as load duration and temperature increase
90
how do you grade asphalt cement
By Viscosity:
AC-5 400 – 600 Poises AC-10 800 - 1200 Poises AC-40 3200 – 4800 Poises
By Consistency:
85/100 Pen 40/50 Pen 60/70 Pen
91
what are the required properties needed in asphalt cement
* Low temperature susceptibility
* Good adhesion and cohesion
* Good durability – low rate of hardening
* Proper viscosity for high and low service temperatures (to avoid rutting and shrinkage cracks)
92
what is liquid asphalt and what are the two types
The viscosity of an asphalt cement can be reduced by combining it with a liquid solvent of lower viscosity
After placement the solvent evaporates, leaving only the asphalt cement, resulting in a gradual increase in viscosity
There are two basic types of liquid asphalts: - Cutback Asphalts
- Emulsified Asphalts
93
what is cutback asphalt
Cutback asphalts are produced by blending an asphalt cement with a hydrocarbon solvent
There are three types, differing in the rate at which the material cures or hardens
slow curing ( diesel fuel solvent in 0-50% concentration)
medium curing (MC) (kerosene 15-45%)
rapid curing (rc) (gasoline 15-45)
94
what is emulsified asphalts
Emulsified asphalts are produced by breaking asphalt cement into very fine droplets and dispersing them in a mixture of water and a surface-acting emulsifying agent
Typical proportions: 55-75% asphalt cement 0.5-3.0% emulsifying agent
Coalesces when asphalt cement content reaches 80%
Rapid Setting (RS) Medium Setting (MS) Slow Setting (SS)
95
what is emulsified asphalt main characteristic
Can be anionic, cationic or nonionic, depending to the type of ionic charge which is induced on the droplets.
* Anionic (negatively charged): more compatible with positive surface charges such as limestone
Emulsifying agent – fatty acids
* Cationic (positively charged): more compatible with negative surface charges such as siliceous aggregates
Emulsifying agent – fatty amines
96
what is the bond formation and strength between asphalt cement and aggregate dependant on?
physical and chemical properties of both components
97
what are the failure of the ashpalt cement - aggregate bond a result of
abnormal loading condition and or stress cpncenztrartion
displacement or detachment of the binder from the aggregate surface by water (stripping)
98
where is asphalt concrete primarly used
as the surface layer in a flexible pavemrnt structurea
99
what is the difference between open graded and dense graded asphalt concrete
open : gap graded aggregate and excess asphalt cement, asphalt cement acs as the matrix and the aggregate as filler, viscouous in nature
dense: well graded aggregate and sufficient asphalt cement, aggregate acts as a continousous phase due to pts to pts contact with asphalt as the filler, act as solid
100
what is asphalt concrete stiffness dependant on
asphalt cement content and concentration and geo properties of the aggregates
101
what leads to longitudinal cracking
fatigue failure along the wheel path, at later stage perpendiculat cracking as well
102
what is rutting
most common form of deformation, wheel path channelized due to densification or lateral movement of the mix. its affected by cement content, filler content and shape of aggregates particles. its formed due to stresss along th wheel path causing tensil stress outwards
103
what is shoving & slipping
caused by shear flow of mixture or slippage between layers. it manifests as an abrupt wave while corrugatiion takes form of ripples or ridges. slippage occurs because of poor bond between hot mix asphalt and underlying layer. shear flow may be result of unstable mixture
104
what is joint reflection cracking
cracks in the flexible overlay of a rigid pavement occuring directly over the underlying rigid pavement joint. movement of the rigid slab beneath the asphalt concrete because of thermal and moisture changes generally not load initiated byt load can hasten deterioratuon
105
what is responsibkle for high permeability and increased durability in asphalt concrete
high asphalt cement content and dense aggregates gradations combiened with good compactuon
106
when is asphalt concrete durability at a maximum
when aggregates are covered completly by asphalt cement and no air voids are left to facilitate permeability
107
how can you reduce the breaking of adhesive bonds between aggregate and asphalt cement through the action of water
reduce void content of asphalt concrete
avoid using the use of know hydrophilic aggregates
washing aggregates to remove coatings
pre coating aggregates with bitumen or diluent
use higher temperatiure in mixing phase to drive off water and reduce asphalt viscosity
108
what controls the expansion or contraction in asphalt concrete
effect of asphalt binder
109
what is contraction cracking
consequence of thermal contraction or shrinkage of an asphalt concrete under freezing conditions. occurs when shrinkage is restrained, tensile stresses develop that can exceed the tensile strength of the mixture. starts at surface and progresses down through layers
110
what readily dissolbes asphalt cement
petroleum products
111
whats a pothole
structural/ physical degradation mechanism, water penetrartes into subgrade below asphalt pavement , it forms water pockets and freezes when temp drop leading to expansion thus cracking the asphalt. vehicle loads cars localized failure over voids
112
what is the most common additivein asphalt
ASA, (anti stripping agent), enhanced the bonding stability between aggregate asphalt interface by forming water resistant complex
113
how do you properly add ASA
apply directly to aggre surface through pre treatmeent of aggregates.
114
