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CIVE 202 construction materials > TUT 05 Reinforced concrete durability > Flashcards

TUT 05 Reinforced concrete durability Flashcards

1
Q

how does transport in concrete work

A
  • most deterioration processes require aggressive agents to enter the concrete
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2
Q

why is water the source of many types of durability concerns

A
  • it directly attacks or facilitates the ingress of harmful ions into the concrete
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3
Q

what are the three different water transport mechanisms

A
  • absorption
  • diffusion
  • permeation
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4
Q

what is absorption

A
  • movement of water driven by surface tension in unsaturated capillary pores
  • flow from unsaturated to saturated regions
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5
Q

what is diffusion

A
  • random movement of individual liquid (ions) or gas particules driven by a concentration gradient
  • flow from high to low concentration
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6
Q

what is permeation

A
  • movement of gases or liquids driven by a pressure gradient
  • high pressure to low pressure
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7
Q

what is the difference between porosity and permeability

A
  • porosity is the proportion of voids in a material
  • permeability is the ability of a fluid such as water to flow in a material
  • if there are pores in a material, even up to 99% porosity, if these pores are not interconnected or connecting to the surface then there will be no permeability because water can’t flow between them
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8
Q

what are the different physical attacks (FATESW)

A
  • Freeze-thaw
  • abrasion/Wear
  • temperature change
  • erosion and cavitation
  • salt scaling
  • wet-dry
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9
Q

what type of damage does freeze-thaw cause

A

damage through attrition
- damage induced by internal tensile stresses which are a direct result of repetitive cycles of freezing and thawing

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10
Q

how does freeze-thaw cause damage

A
  • expansion during water to ice phase change
  • hydraulic pressure from differential pore freezing
  • solar heating
  • litvan’s model
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11
Q

explain more in detail the hydraulic pressure process in the freeze thaw

A
  • freezing begins in large cavities and progresses to successively smaller ones due to effect of pore pressure
  • effect or pore pressure: produces hydrostatic pressure as the expansion forces unfrozen water ahead of the freezing front
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12
Q

what is solar heating’s effect on freeze-thaw

A
  • radiation causes surface thaw, water freezes during the night
  • produces thermal gradient in concrete which induces stresses
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13
Q

what is litvan’s model and its effect in freeze-thaw cycle

A
  • vapour pressure gradient between high pressure water and low pressure ice surface (how fish survive during winter)`
  • ice lenses forcing pop out
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14
Q

what are the macroscopic effects of freeze-thaw

A
  • progressive mass loss from surface
  • pop outs
  • crackings/ spalling
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15
Q

how to protect against freeze-thaw

A
  • air entrainment: provides space for water to expand and freeze into
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16
Q

what is salt scaling

A
  • when concrete pore water contains chlorides or dissolved salts
  • the salts crystallize right below the surface, chipping off concrete at the surface
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17
Q

how does wet-dry cause damage

A
  • damage due to constrained expansion:
  • volume changes when water is absorbed/ released
  • damage is cyclic
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18
Q

how to prevent wet- dry damage

A
  • low W/C ratio , lowers permeability
  • put a moisture barrier
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19
Q

why does thermal extremes (fires) affect concrete

A
  • concrete has a low thermal conductance for construction material and is therfore less susceptible to fire damage
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20
Q

why does concrete have a low thermal conductance?

A
  • heat is consumed in the evaporation of water (evaporation in concrete can cause shrinkage and cracking)
  • heat is consumed in the decomposition of hydration products (partially reverses hydration reaction)
  • some aggregates decompose and consume heat
  • decomposed material has even lower thermal conductivity
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21
Q

what does a low thermal conductance mean

A
  • allows core (reinforcement) to remain at lower temperatures and will be relatively unaffected
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22
Q

how does hot/cold damage concrete

A
  • differential temperatures through-out bulk concrete can create stresses that may crack or damage concrete
  • larger problems on mass concrete during early hydration
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23
Q

how to prevent hot/ cold damage in concrete during early hydration

A
  • use low heat of hydration cement to minimize damage
24
Q

examples of physical abrasion

A
  • car tires, snowplows, traffic, construction loads
25
Q

what is erosion in concrete

A

solid particles suspended in water repeatedly rubbing against concrete

26
Q

what is cavitation and how does it affect concrete

A
  • due to fast water flow around irregular geometry
  • tiny bubbles form when water lifts from surface
  • popping bubbles create shockwaves that damage concrete
27
Q

what are the different chemical attacks to concrete (SALCA)

A
  • Sulphate attack
  • alkali-silica reaction
  • leaching/efflorescence
  • acid attack
  • carbonation
28
Q

how does alkali-silica reaction (ASR) damage concrete

A
  • pore water becomes alkaline due to hydration
  • active aggregates react with alkaline pore water solution to produce amorphous silicate gel
  • amorphous silicate gel imbibes pore water and expands a lot
  • gel expands into surrounding hydrated cement paste (HCP) causing volumetric expansion damage
29
Q

what deos alkali silica reaction require to react

A

specific kinds of active silicate aggregates

30
Q

what are the alkali-silica reaction macroscopic effects

A
  • map cracking
  • mass loss at surface
  • volumetric expansion
31
Q

how to prevent alkali-silica reaction

A
  • use non-reactive aggregates!
  • minimize alkalis available in solution
  • lower concrete permeability
  • use SCMs like fly ash to react with the silicates
32
Q

how does sulphate attack cause damage

A
  • reaction between sulphate ions and certain components of the hydrated cement paste (HCP)
  • damage may include expansion and cracking of the concrete and softening and disintegration of the paste
33
Q

what is the sulphate attack reaction (3 steps)

A
  1. Sulphates enter concrete via pore water solution and diffusion/absorption/permeation
  2. Sulphates react with CH to produce gypsum
  3. Gypsum then reacts with monosulphoaluminate in HCP to produce ettringite
34
Q

what is leaching in concrete

A

dissolution of components of cement paste (CH) in pore water solution

35
Q

what is efflorescence in concrete

A

when water reaches surface, it evaporates and leaves residue on surface
- requires water without many ions already dissolved in it

36
Q

what are the macroscopic effects of leaching and efflorescence

A
  • weakening of the cement paste (removes CH)
  • staining on surface
37
Q

how to prevent leaching and efflorescence

A
  • minimize water flow through structure
38
Q

what is hard water

A
  • ground water, lakes and rivers
  • contains dissolved minerals
  • not detrimental to concrete
39
Q

what is soft water

A
  • rain, melting snow and ice
  • does not contain calcium ions or other minerals
  • readily dissolves calcium containing products
  • the rate fo leaching depends on the amounts of dissolved salts in the water and the temperature of the water
40
Q

how does acid attack affect concrete

A
  • direct dissolution of cement matrix in acid (usually on an issue with exposure to acid for extended periods of time)
41
Q

what are the macroscopic effects of acid attack on concrete

A
  • weakening or destruction of paste
42
Q

how deos carbonation damage concrete

A
  • infiltration of CO2 into concrete (as gas or ion)
  • progresses from surface layer inwards
  • (lowers pH or pore solution) : CO2 molecules react with CH to form calcium carbonate (lowers pH or pore solution)
43
Q

macroscopic effects of carbonation on concrete

A
  • increases corrosion rate of reinforcing steel (deprivation)
  • carbonates can strengthen matrix
44
Q

what is an electrochemical attack on concrete

A

corrosion of reinforcing steel

45
Q

how does corrosion of reinforcing steel work

A
  • electrochemical process that requires formation of a cathode and an anode and an electrical current between them
  • chemical reaction between water, oxygen and metallic iron that produces rust
  • rust volume larger than original metal’s volume
46
Q

how does concrete protect against electrochemical attack? physically

A
  • concrete restricts the entering of the basic components required to start corrosion (Water and oxygen)
47
Q

how does concrete lose its ability to protect agains electrochemical physical attack

A
  • insufficient cover
  • concrete with poor transport properties
  • damage to concrete (cracking , spalling, scaling)
48
Q

how does concrete protect against electrochemical attack? chemically

A
  • concrete pore solution has a high PH which leads to the formation of a protective iron oxide film around the steel bar called the passivation film at pH>10
49
Q

how does concrete lose its ability to protect againstelectrochemical chemical attack

A
  • passivation film is lost because of lowering of the pore ph solution (carbonation or acid attack)
  • penetration of chlorides into the concrete
50
Q

what are the macroscopic effects of electrochemical attack (reinforcement corroding)

A
  • volumetric expansion of embedded steel (induces tension cracks, spalling, delamination which all increases permeability)
  • reduction of cross-sectional area at anode (structural level issues due to under-reindorcement)
51
Q

how to prevent agains electrochemical attack

A
  • maintain high ph environment around reinforcement (more OH- ions in pore solution minimize kinematics)
  • lower permeability of concrete (decreases chloride ingress)
52
Q

exam questions: expain the consequences of reinforcement corroding

A
  • expansion of rebar cracks concrete and thus increases permeability
  • steel’s cross-sectional area is reduced which reduces steel’s load carrying capacity
53
Q

exam question: what is the difference between leaching and efflorescence

A
  • leaching is the dissolution of calcium hydroxide into soft water
  • efflorescence is the physical deposition of crystals on the surface of concrete, leaving white strains
54
Q

exam question: explain the process of damage in sulphate attack

A
  • sulphates enter concrete via pore water solution and diffusion,absorption/permeation
  • sulpahtes react with CH to produce gypsum
  • gypsum reacts with monosulphoaluminate in HCP to produce ettringite
  • reactions are expansive and restrained expansion causes damage through cracking
55
Q

explain the process of damage in alkali-silica reaction

A
  • alkalis in pore solution react with silica from aggregates to form amorphous silicate gel
  • amorphous silicate gel imbibes (absorbs) pore water and greatly expands
  • once pores are filled by expansion, expansion becomes restrained and cracks occur
56
Q

Concrete chemically protects embedded reinforcing steel from corrosion through the formation of a passivation film. Describe two ways in which this protection can be lost.

A
  • penetration of chlorides - will attack and break down the passivation film once a sufficient concentration accumulates at the steel surface
  • carbonation (co2 reaction with calcium hydroxide) - leads to a drop in pH of the pore solution. Passivation film breaks down as pH approaches 11
57
Q

list and describe two ways in which concrete protects embedded reinforcing steel from corrosion

A

chemical: passivation film
physical: prevents reactive agents from entering

CIVE 202 construction materials (10 decks)

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