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CIVE 202 > Concrete-Durability > Flashcards

Concrete-Durability Flashcards

1
Q

durable

A

able to exist for a long time without significant deterioration

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2
Q

Durability of PC Concrete

A

“The ability to resist weathering action,
chemical attack, abrasion, or any other process of deterioration.”

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3
Q

How long does concrete last

A

Under Ideal Conditions: Virtually forever.
Under Normal Conditions: Depends on exposure conditions
(i.e. deterioration mechanisms)

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4
Q

General Categories of Deterioration Mechanisms:

A

Chemical Attack
Physical Attack

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5
Q

how does deterioration initiate

A

Generally, surface attack of concrete is an extremely slow deterioration process.
In most cases, aggressive agents must enter the concrete to cause significant damage

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6
Q

three primary transport mechanism to allow penetration of aggressive agents

A

Absorption
Permeation
Diffusion

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7
Q

porosity effect on attack

A

as capillary porosity increases, fraction connected increase, making it more susceptible to deterioration

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8
Q

absorption

A

Transport of liquids into unsaturated porous solids
due to surface tension acting in capillaries

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9
Q

Permeation

A

Movement of gases or liquids through a saturated
porous medium due to a pressure gradient

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10
Q

Diffusion

A

Transfer of mass by random motion of free molecules
or ions in the pore solution due to a concentration gradient

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11
Q

Absorption and Diffusion are affected in a similar manner

A

a denser paste acts to restrict movement

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12
Q

Leaching

A
  • the hydrolysis of cement paste components
    (particularly calcium hydroxide) by water flowing through the
    concrete
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13
Q

define hard water and impact on concrete

A

Hard Water (Groundwater, Lakes, Rivers) contains chlorides, sulfates, bicarbonates of calcium and magnesium. Not detrimental to concrete

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14
Q

define soft water and impact on concrete

A

Soft Water (Rain, Melting Snow & Ice) contains no calcium ions or other minerals. Readily dissolves calcium containing products

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15
Q

rate of leaching depends on

A

the amount of dissolved salts in the water and the temperature of the water

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16
Q

prevention of leaching

A
  • Minimize transport properties (low W/C, SCMs)
  • Minimize calcium hydroxide content of hcp (SCMs)
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17
Q

efflorescence

A

migration of salt to surface

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18
Q

alkali silica reaction

A

chemical reaction between the
soluble alkalis contained in the hcp and certain reactive forms of silica found in the aggregates

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19
Q

factors affecting reaction

A
  • nature of the reactive silica
  • amount of reactive silica
  • particle size of reactive material
  • amount of alkalis available
  • amount of moisture available
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20
Q

pessimum amount

A

max amount of expansion for reactive silica in aggregate

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21
Q

effect of particle size on ASR

A

small particles have higher surface area, so more extensive reaction

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22
Q

prevention of alkali silica reaction

A
  • Identify and avoid reactive aggregates.
  • Limit the amount of alkalis available in the hcp:
    Na2O + 0.65 K2O < 0.60
  • Add an SCM to the concrete mix
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23
Q

how do you test for ASR?

A

UV fluorescence technique

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24
Q

Alkali Carbonate Reaction (ACR)

A
  • Expansive reactions involving carbonate rocks (dolomitic limestone)
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25
Q

Carbonate rocks susceptible to expansive reactions possess the following features (4):

A
  • Very fine grained dolomite (small crystals)
  • Considerable amounts of fine-grained calcite
  • Abundant interstitial clay
  • Dolomite and calcite crystals evenly dispersed in clay matrix
26
Q

Sulphate attack

A
  • A chemical reaction between a sulphate ions
    and certain components of hcp
27
Q

damage during sulphate attack

A

Damage may include expansion and cracking of the concrete, as well as softening and disintegration of the paste

28
Q

primary forms of sulphate attack

A

– External sulphate attack
– Physical sulphate attack
– Thaumasite
– Internal sulphate attack (DEF)
– Waste/Sewage

29
Q

3 step reaction of sulphate attack

A
  1. Sulphates must first enter the concrete, usually from an
    outside source.
  2. Sulphates react with CH to produce gypsum:
  3. The gypsum reacts with the monosulphoaluminate in the hcp to form ettringite
    (2 and 3 are expansive)
30
Q

effect of seawater on sulphate attack

A

though high levels of sulphates are present in
seawater, sulphate attack is mitigated to some extent.
- Magnesium hydroxide chemically protects against sulphate attack.
- Gypsum and ettringite are more soluble in solutions containing chloride ions

31
Q

Internal Sulphate Attack

A

– Delayed Ettringite Formation (DEF)
Curing at elevated temperatures destroys ettringite and the sulphate is absorbed by the C-S-H.
After cooling, the sulphate again becomes available to form ettringite, resulting in expansion and cracking

32
Q

Acid attack

A

a chemical reaction between an external source of
acidic liquid and hcp and, in some cases, aggregates.

33
Q

Acid Attack Sequence

A

Attack is normally limited to surface of concrete only. Progresses inward.
Dissolution of compounds soluble in the given acid takes place virtually instantaneously.
In most cases, this reaction forms insoluble calcium salts which build up and protect the concrete from further attack

34
Q

Freezing and Thawing

A

Damage is induced by internal tensile stresses which are a direct result of repetitive cycles of freezing and thawing.
Freeze/thaw damage is through attrition - one cycle does very little damage, it takes many cycles before the damage adds up to significant levels

35
Q

4 contributing factors

A
  1. Expansion of water
  2. Hydraulic pressure
  3. Solar heating
  4. Litvan’s model
36
Q

Expansion of water

A

Just before freezing, volume increases 9%

37
Q

Hydraulic pressure

A

All of the water in concrete does not freeze at the same time, but follows a gradual process as freezing begins in the larger cavities and progresses to successively smaller ones due to the effect of pore pressure.
This produces a hydraulic pressure as the expansion forces unfrozen water ahead of the freezing front.
Magnitude of hydraulic pressure is a function of:
- Concrete’s resistance to flow
- Distance to void boundary
- Rate of freezing

38
Q

Solar heating

A

Two-directional freezing at surface due to daily thawing from incident solar radiation

39
Q

Litvan’s Model

A

A vapor pressure gradient is created between surface ice and super-cooled pore water. Induces movement of water toward surface.
A dense, impermeable surface layer will restrict this movement and potentially cause mechanical failure

40
Q

scaling

A

removal from surface

41
Q

heat/fire relationship with concrete

A

Low rate of heat penetration due to:
- Low thermal conductivity.
- Heat is consumed by evaporation of water.
- Heat is consumed in decomposition of hydration products.
- Some aggregates also decompose and consume heat.
- Decomposed material has even lower thermal conductivity

PRODUCTS REINFORCING STEEL BEAMS

42
Q

corrosion of reinforcement

A

An electrochemical attack mechanism affecting the reinforcing steel which results in a volume increase, thus inducing tensile stresses in the concrete.
Structural concrete requires steel reinforcement to carry the applied tensile stresses.
Concrete is normally capable of providing excellent protection to the steel and prevent it from corroding

43
Q

how does concrete prevent corrosion in steel bars

A

Physically: the concrete restricts ingress of the basic components required to initiate corrosion (water, oxygen, chlorides)
Chemically: the pore solution in concrete typically has a very high pH, which leads to the formation of a protective iron oxide film around the steel bar

44
Q

passivation film

A

protective iron oxide film around bar caused by high pH in concrete

45
Q

Primary physical reasons for loss of protection (4)

A
  • Insufficient cover over reinforcement.
  • Concrete with poor transport properties.
  • Failure to protect concrete from chloride sources.
  • Damage to concrete (cracking, spalling, scaling)
46
Q

Primary chemical reasons for loss of protection (2)

A
  • Penetration of chlorides into concrete. Passivation layer is destroyed when chloride ion content reaches 0.2 – 0.4% in region adjacent to steel.
  • Carbonation (due to CO2 exposure) of concrete leads to a reduction in pH. Depassivation occurs as pH approaches 11
47
Q

why is corrosion an electrochemical process

A

– it requires the formation of a cathode and an anode, with an electrical current flowing between them

48
Q

what is the anode in corrosion of reinforcement

A

iron metallic atoms are oxidized to fe 2+ ions, which dissolve into the surrounding solution, producing electrons

49
Q

what is the cathode in corrosion of reinforcement?

A

electrons are consumed and OH- ions formed. water and oxygen required for this to occur

50
Q

deleterious effects of corrosion of steel

A
  1. Reduction of the crosssectional area of the steel at
    the anode
  2. Spalling or cracking of the concrete due to the expansion stresses created by rust formation
51
Q

prevention in design (8)

A
  • Sufficient Cover
  • Improved Transport Properties
  • Corrosion Inhibitors
  • Corrosion Resistant Reinforcement
  • Galvanized Steel
  • Stainless Steel
  • Epoxy Coated Steel
  • Fibre Reinforced Plastics
52
Q

Protection/Repair of Existing Structure (9)

A
  • Remove, Clean, Replace
  • Corrosive Resistant Reinforcement
  • Corrosion Inhibitors
  • Cathodic Protection
  • Sacrificial Anode
  • Cancellation Current
  • Protective Overlay
  • Waterproof Membrane
  • Watertight Concrete Overlay
53
Q

surface wear

A
  • Progressive mass loss from a concrete surface
    due to repetitive attrition cycles
54
Q

Surface wear is divided into three primary mechanisms:

A
  • Abrasion
  • Erosion
  • Cavitation
55
Q

Abrasion

A

Refers to dry attrition as another solid objects moves
along or rubs against the concrete surface.
Primarily relates to vehicular traffic or mechanical devices but can also occur in walls of silos or bins

56
Q

Erosion

A

Wear caused by the abrasive action of solid particles
suspended in fluids.
Caused by the physical action of debris impacting, rubbing, rolling, and grinding against the concrete surface.
Common on canal linings, spillways and pipes for water or sewage transport

57
Q

cavitation

A

– Loss of mass caused by the formation of vapor
bubbles and their subsequent collapse due to sudden changes of direction in rapidly flowing water

58
Q

how does cavitation work and what does it require

A

Requires:
- Rapid water flow (exceeding 12 m/s)
- Surface irregularities

At irregularities, water flow separates from concrete surface creating zone of lowered vapour pressure causing bubbles to form.

As bubbles move downstream to regions of normal pressure they collapse violently, creating a shock wave.

Shock wave can induce high tensile stresses in concrete if this occurs near the concrete surface

59
Q

deleterious effects of sea water

A

-Leaching - Constant exposure to seawater and/or flow
-AAR - Alkalis in seawater (if reactive aggregate is present)
-Sulphate Attack - chemical reaction + crystallization (W/D)
-Acid Attack - High CO2 contents possible (pH < 7.5)
-Freeze/Thaw - Accentuated in tidal zone
-Corrosion - High Cl- content
-Surface Wear - flow, waves, sediment, floating objects

60
Q

destruction of concrete in sea water- shape

A

makes an almost hour glass shape

CIVE 202 (12 decks)

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