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Concrete > Durability > Flashcards

Durability Flashcards

1
Q

Fresh state of concrete

A
  • lasts 2-4 hours
  • setting locks defects in microstructure
  • curing prevents water escaping so reaction can take place
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2
Q

microstructure influenced by:

A
  • capillary porosity determined by w/c ratio and curing
  • bleeding and settlement causing voids and channels
  • segregation of very wet mixes
  • poor compaction = excessive voids
  • poor placing and handling = segregation
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3
Q

Factors controlling hardening:

A
  • dispersion of cement grains within paste
  • temperature of concrete (low temp = slow dev. but high strength)
  • quality of moisture curing during first few days
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4
Q

Low W/CM

A
  • low capillary porosity
  • small pores poorly connected
  • low permeability
  • high strength
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5
Q

High W/CM

A
  • high capillary porosity
  • large pores well connected
  • high permeability
  • low strength
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6
Q

Hardened State

A
  • heterogenous material due to micro and macro variations

- durability controlled by quality of cover concrete

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7
Q

ITZ

A

Interfacial Transition Zone

  • high proportion of calcium hydroxide and ettringite
  • high porosity compared to bulk paste
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8
Q

Reasons for Failure

A
  • structural collapse
  • foundation failure
  • accidents
  • inadequate durability
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9
Q

Intrinsic Durability

A
  • macro defects: cracking, compaction voids, delaminations
  • meso defects: capillary porosity, bleed lens and channels
  • micro defects: ITZ, gel pores
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10
Q

entrained air bubbles

A
  • deliberate
  • good for freeze thaw - allows for expansion
  • can be used to reduce material quantity
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11
Q

entrapped air void

A

want to AVOID

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12
Q

service life predictions

A
  • expected to be maintenance free for 50 years (100 for bridges)
  • in reality, exposed to more severe environments often need repair in 15-25 years
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13
Q

pore solution or external water/solution

A
  • corrosion
  • alkali aggregate reaction
  • sulfate attack
  • acid attack
  • frost attack/salt scaling
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14
Q

high permeability

A
  • easy for chloride ion to get to steel

- capillary pores connected

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15
Q

porosity of concrete

A
  • compaction pores
  • entrained air
  • capillary pores
  • gel pores
    important in concrete since it is a brittle material
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16
Q

compaction pores

A
  • 1-10 mm

- affects strength and durability

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17
Q

entrained air

A
  1. 2 - 0.3 mm

- affects workability and strength

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18
Q

capillary pores

A

10nm - 100micron

- affects strength and durability

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19
Q

gel pores

A

2-10 nanometres

- affects shrinkage and creep

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20
Q

concrete microstructure has three major types of defects

A
  • macro defects: major cracking in material
  • meso defects: capillary porosity
  • micro defects: interfacial transition zone (ITZ)
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21
Q

enhanced microstructure produced by the following

A
  • reduced cracking: better site practice and good design
  • reduced porosity: mix design, use of SCM
  • better ITZ: SCM
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22
Q

beneficial effects of SCM

A
  • micro-filler effect
  • increased CSH
  • wall effect
  • pore blocking
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23
Q

Pozzolanic effect

A
  • hydration of calcium silicates in Portland cement produces calcium hydroxide (weak and porous)
  • reaction of silica fume with calcium hydroxide produces increased quantity of cementitious material
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24
Q

ASR

A

Alkali-Silica Reaction

  • reaction between alkalis and siliceous rocks
  • products may cause abnormal expansion and cracking
  • affects all types of structures
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25
Q

alkalis

A
  • sodium

- potassium

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26
Q

siliceous rocks or minerals

A
  • opaline chert
  • strained quartz
  • acidic volcanic glass
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27
Q

ASR mechanism

A
  • if silica reactive it may be “attacked” by OH- and then Na+ and K+
  • forms alkali-silica gel composed of Na, K, and Si
  • gel absorbs water from surrounding cement paste and expands
  • causes internal stresses and eventually leads to cracking
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28
Q

Requirements for ASR

A
  • reactive silica
  • sufficient alkali
  • sufficient moisture
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29
Q

Alkalis in portland cement

A
  • represent a small proportion of the cement

- most end up in pore solution and associated OH sufficient to produce a pH in range of 13.2 to 14.0

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30
Q

Solutions to alkali in cement

A
  • can add microsilica/silica fume (SCMs) to react with OH-

- change aggregate (can be difficult)

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31
Q

Effect of relative Humidity on ASR

A

little significant expansion if relative humidity maintained below 80%

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32
Q

thickness of ITZ

A

20-40 microns

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33
Q

four states of corrosion possible for RC

A
  • passive state (steel embedded in uncontaminated corrosion)
  • pitting corrosion (chloride-induced)
  • general corrosion (carbonation-induced)
  • active, low corrosion (saturated concrete)
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34
Q

need following conditions for corrosion:

A
  • reactive metal
  • oxidising agent
  • moisture
  • electrolyte that allows easy ionic movement
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35
Q

oxidation of iron

A
  • oxidises and hydrates to form a range of ferrous and ferric compounds
  • these products may occupy up to 6 times the original volume of the metallic iron
  • significant expansion pressures cause cracking in concrete
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36
Q

Chloride-induced corrosion

A
  • presence of salt and water creates the right conditions for rapid corrosion rates - generates pits and expansive rust
  • minimum concentration of chlorides required to disrupt passive oxide film on embedded reinforcement
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37
Q

Carbonation-induced corrosion

A
  • occurs in relatively dry environments and corrosion rates moderate
  • usually aesthetic damage
  • rates of more than 1mm/year only occur in very poor quality concrete
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38
Q

parameters influencing corrosion damage

A
  • geometry of structural elements
  • cover depth
  • moisture condition of concrete
  • age of structure
  • presence of cracking
  • service stresses in concrete
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39
Q

Chloride-induced damage

A
  • deep pitting of reinforcement
  • significant cracking and spalling
  • eventually affects structural integrity
  • costly to repair if ignored until damage is obvious
40
Q

carbonation-induced damage

A
  • general corrosion with little pitting
  • minor cracking and rust staining
  • mostly affects aesthetics, not integrity
  • can be repaired relatively cheaply provided not left too late
41
Q

Designing for durability, need to consider:

A
  • structural interactions
  • environmental conditions
  • quality of construction
  • level of durability required
42
Q

most common durability design errors

A
  • underestimating environmental exposure
  • inadequate cover
  • reluctance to use SCM’s in concrete
  • no allowance for cracking of concrete during service
  • lack of enforcement of curing specifications
  • complete lack of supervision
43
Q

Visual evidence of corrosion

A
  • rust stains
  • cracking along reinforcing
  • spalling of cover
44
Q

visual evidence of ASR

A
  • expansive map cracking

- restrained cracks following reinforcing

45
Q

Visual evidence of shrinkage

A
  • defined cracks after 3-6 months drying

- excessive displacemetn

46
Q

Visual evidence of chemical damage

A
  • leaching of concrete surface
  • salt deposits
  • spalling expansion
47
Q

Visual evidence of fire damage

A
  • surface discolouration
  • softening
  • micro-cracking
48
Q

Visual evidence of structural damage

A
  • major cracking in areas of high stress
  • localised crushing
  • deflections
49
Q

Delaminations

A

internal cracks or voids that run parallel to the concrete surface - therefore hard to detect
- particularly unwanted on bridge decks as defect is aligned with direction of principle stress

50
Q

Delamination survey

A
  • chain drag
  • acoustic methods
  • hammer
51
Q

types of delamination

A
  • poor finishing (bleed water rises and forms a lens)
  • corrosion induced (severe exposure, closely spaced reinforcing)
  • ASR expansion-induced (heavily stressed, long-span bridge decks)
52
Q

Cover survey

A
  • locates position and depth of reinforcing steel in concrete
  • cover to reinforcing one of the most important factors controlling durability
  • reduction of cover from 50mm to 30mm may reduce the time to corrosion from 50 to 10 years
53
Q

Chloride testing

A
  • determines % chloride per mass concrete at particular depth
    <0.4% = low risk
    0.4-1.0% = moderate risk
    >1% = high risk
54
Q

carbonation depth

A
  • carbonation occurs under relatively dry conditions so carbon dioxide can diffuse
  • carbonation-induced corrosion occurs at low covers, exposed to fluctuating moisture levels
55
Q

Rebar Potentials

A
  • measures thermodynamic risk of corrosion
  • cannot evaluate rate
  • mostly used for chloride-induced corrosion of decks
  • use in combination with other tests
56
Q

Resistivity Testing

A
  • concrete resistivity controls the rate at which steel corrodes in concrete once favourable conditions for corrosion exist
  • high resistivity = low travel from anode to cathode = low rate of corrosion
57
Q

factors influencing resistivity

A
  • moisture content
  • permeability
  • ionic concentration
58
Q

Corrosion Rate

A
  • only reliable method of measuring corrosion activity in concrete
  • fluctuate widely depending on environment - need to take more than one measurement
  • testing is time consuming
  • requires experience/knowledge
  • results are definitive
59
Q

Cover survey reliability affected by:

A
  • reinforcement of deep covers (usually reliable to within 5mm, but less reliable for particularly deep sections)
  • areas of closely spaced bars
  • measuring different bar sizes and types
  • interference from magnetic material
60
Q

Rebar Potential limitations

A
  • interpretation of plots take experience
  • delaminations may disrupt potential fields
  • effect of environment and cover depth
  • does not measure corrosion rate
61
Q

Limitations of resistivity

A
  • affected by carbonation and wetting
  • avoid surface conductive layers
  • do not measure in vicinity of steel
  • unstable reading on dry concrete
  • a complimentary test
62
Q

Corrosion rate applications

A
  • generally used once areas of corrosion activity have been identified
  • requires multiple readings over course of year to develop estimated corrosion rates
63
Q

advantages of corrosion rate measurements

A
  • provide definitive assessment of corrosion
  • particularly useful at start of corrosion process
  • predictions of future damage can be made
64
Q

3 main transport mechanisms

A
  • sorption (capillary suction)
  • permeation (hydraulic conductivity)
  • diffusion
65
Q

rate of ingress dependent on

A
  • microstructure
  • environment
  • service stresses
  • internal reactions
66
Q

Durability Index Tests

A
  • absorption
  • permeation
  • diffusion
67
Q

absorption test

A
  • water sorptivity test

- useful for assessing cover concrete quality as affected by curing

68
Q

permeation test

A
  • oxygen permeability test

- able to assess gaseous permeation/diffusion through concrete - used for carbonation

69
Q

diffusion test

A
  • bulk diffusion or resistivity test

- able to assess resistance of concrete to ionic diffusion of chlorides

70
Q

What is sorptivity?

A
  • measure of unsaturated flow of fluids into concrete
  • assesses capillary forces that result in fluid being drawn into pore structure
  • near surface effect
  • simple to do in lab, but not in field
71
Q

problems with field sorptivity test

A
  • don’t know the saturation levels

- different saturations at different sites

72
Q

review of sorptivity test

A
  • only relevant for near-surface properties
  • sorptivity not constant with time
  • moisture content and sampling conditions important
73
Q

Diffusion

A
  • motion of molecules of water causes molecules to move from regions of high to low concentration
  • equilibrium when molecules in equal concentration in all regions
74
Q

Bulk Diffusion Test

A
  • typical test length 35-40 days

- might extend to 90 days for concretes with lower diffusion rates

75
Q

Review of Bulk Diffusion test

A
  • measures “apparent diffusion” which does not address binding of ions
  • fairly long term test
  • test uses saturated sample to avoid surface absorption
  • fairly complex analysis
76
Q

Rapid Chloride Resistance Tests

A
  • determines non-steady state diffusion coefficient

- good for comparative testing of different concrete mixes

77
Q

permeation through concrete

A
  • rate of mass flow proportional to pressure gradient across concrete
  • resistance to flow given by coefficient of permeability
  • coefficient not intrinsic to the material - affected by test method
  • conditioning of sample very important
78
Q

chlorides at steel cause:

A

cracking

79
Q

carbonation causes:

A

rust staining

80
Q

insufficient cover causes:

A

spalling

81
Q

poor quality concrete causes:

A

delamination

82
Q

Patch repair check list

A
  • locate embedded post-tensioning
  • conduct structural review before removing significant concrete
  • exposed corroded rebar should be undercut to ensure adequate coverage and bond with new concrete
  • full circumference of exposed bar should be cleaned
  • loose rebar should be tied to other secure bars
  • of more than 25% bar cross section lost, structural review
  • edges of patch should be straight and square with surface to ensure maximum integrity of the patch
83
Q

“Anodic Ring” Effect

A
  • formation of new anode adjacent to patch

- patch just delays or moves corrosion

84
Q

Improving performance of patch repairs

A
  • treat surface
  • treat steel rebar
  • install discrete sacrificial anode
  • install galvanic anode
85
Q

options to treat surface

A
  • membranes
  • dense overlay
  • penetrating sealer
86
Q

options to treat steel rebar

A
  • epoxy coating

- zinc painting

87
Q

effect of treating steel rebar

A
  • electrically isolates steel in the patch and prevents becoming cathode
  • reduces risk of incipient anode formation
  • don’t need to protect this part
88
Q

effect of installed galvanic anode

A
  • how far protection extends depends on resistivity of concrete
  • provides cathode protection to steel in vicinity of patch
  • can be counterproductive if have good concrete with high resistivity
89
Q

sacrificial anode

A
  • less noble metal connected to steel reinforcement
  • confers protection through sacrificial corrosion
  • only suitable when resistivity of system low
90
Q

impressed current

A
  • force steel to act as cathode using applied current
  • slotted system
  • titanium anode mesh: add at start and turn on when needed
91
Q

requirements for impressed current

A
  • electrical continuity of reinforcement
  • concrete conductivity
  • no short circuits
  • direct current supply
  • impressed current anode
  • electrolyte
92
Q

electrochemical chloride extraction

A
  • DC power supply between rebar and temporary anode outside of concrete
  • forces Cl- ions away from rebar and to solution at anode
  • only done for brief period of time (weeks)
  • some popularity but mixed results
  • decreased effectiveness of remaining concrete - easier to penetrate
93
Q

FRP wrapping

A
  • works best for circular cross-sections

- prevents chloride penetration

94
Q

NZ aggregates causing ASR

A
  • Waikato River Sands

- Taranaki andesite

95
Q

DEF

A

Delayed Ettringite Formation
- delayed formation of mineral ettringite - a normal product of early cement hydration
- result of high early temperatures (above 70 - 80 degrees)
- water from external source required
-

Concrete (5 decks)

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