Concrete Durability

Question 1

Can concrete last forever?

Answer 1

Under ideal conditions yes. If submerged in calcium bath and doesn’t try out

Question 2

What are the transport mechanisms that cause concrete deterioration?

Answer 2

Absorption, permeation, and diffusion. Something bad needs to go in
Absorption : surface tension bring liquid into unsaturated porous solids
Permeation: pressure gradient cause movement
Diffusion: concentration gradient, motion of molecules or ions. Majority of damage to marine structures

Question 3

Know the difference between porosity and permeability and the drawings

Question 4

How do porosity, w/c ratio, and transport mechanism relate?

Answer 4

High porosity = high chance of transport and connection between holes
Low wc = less interconnection

Question 5

What is wicking?

Answer 5

Water is partially submerged. The portion in contact with air has water evaporation and that causes the concrete to absorb more water than if it were fully submerged

Question 6

What is leaching?

Answer 6

When CH is dissolved in water in concrete.
Rate of leaching depends upon amount of dissolved salts in the water and the temp of water

Question 7

What is efflorescence?

Answer 7

Manifestation of leaching. When water evaporates and leaves behind calcium. Sign that there’s a problem with water flow
Relates to imbibes

Question 8

What are hard water and soft water?

Answer 8

Hard water (groundwater lakes and rivers): contain minerals and not detrimental to concrete
Soft water (rain snow and ice): no minerals

Question 9

Describe alkali silica reaction

Answer 9

Chemical reaction between soluble alkalis contained in the hcp and the silica in the aggregates. Water becomes alkaline from hcp and reacts w silica in aggregates and this creates amorphous alkali silicate gel
Causes expansion and the gel fills the ITZ. Makes ITZ stronger at first and then when ITZ is full has no place to expand anymore and that’s the problem

Question 10

Why doesn’t sand contribute to ASR?

Answer 10

It is 100% silica but it’s highly crystalline so not at all reactive
Opposite of solicits glass with is amorphous and reacts easily. Quebec has a lot of reactive aggregates

Question 11

What is pessimum amount?

Answer 11

Max amount of expansion due to a certain amount of reactive silica

Question 12

Increased particle size causes decreased ASR

Question 13

What are all the things that can affect asr? NOT WATER AND TEMP

Answer 13

Particle size, crystallininry of silica, ratio of silica to alkali.

Question 14

How to prevent ASR?

Answer 14

Blast furnace prevents it and also fly ash and slag is less efficient but cheaper. SCMs!
Low alkali cement means less reaction and less expansion

Question 15

How to detect ASR?

Answer 15

Add uranyl acetate and that makes makes deterioration visual under UV lights
ASR expansion happens at the surface and that causes popouts

Question 16

How are dams affected by ASR?

Answer 16

The expansion wants to go into empty spaces ie the holes where the turbines are and that’s bad because the turbines start scratching up against the concrete

Question 17

What is alkali carbonate reaction (ACR) ?

Answer 17

Slower than ASR. Also an expansive reaction involving carbonate rocks (dolomitic)

Question 18

What are carbonate rocks susceptible to?

Answer 18

Fine grained dolomite (small crystals)
Calcite
Interstitial clay
Dispersed in clay matrix

Question 19

What is sulphate attack?

Answer 19

Sulphate is brought in by water. This sulphate reacts with CH to form gypsum and that gypsum reacts with monosulphoaluminate which forms ettringites and that causes deterioration
Also, lower wc means less water means more dense means less sulphate attack

Chemical reaction between sulphate ions and components of hcp. Causes expansion and cracking and softening and disintegration of paste

Question 20

What are the steps of sulphate attack?

Answer 20

1. Sulphates enter concrete from outside source
2. Reacts with CH to produce gypsum
3. Gypsum reacts with monosulphoaluminate to form ettringite
   2 and 3 are expansive

Question 21

What are ettringites and why are they bad?

Answer 21

They’re crystals formed form the reaction of gypsum with monosulphoaluminate (found in hcp). They are bad because they are needle shaped which causes crack initiation and propagation

Question 22

How to do you prevent sulphate attack?

Answer 22

With less C3A (but we need C3A to decrease clinkering temp).

Question 23

What is effect of seawater and it’s relation to sulphate attack?

Answer 23

Seawater is really bad in terms of sulphate but it also has things that reduce the horrible exposure (chlorides and magnesium hydroxide) so it’s goes from awful exposure to medium

Question 24

What is internal attack of sulphate?

Answer 24

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.

Question 25

What is physical sulphate attack?

Answer 25

When sulphate gets in, at 32.5° it changes shape and expands and then contracts when below this temp which allows more sulphate to get it and it’s cyclical and bad

Question 26

What is acid attack?

Answer 26

An outside-in attack. Virtually instantaneous. As acid eats away at the surface, leaves insoluble salts that create a barrier that protect against the acid. Acid + basic (concrete is alkaline) = salt. That’s why you need constant downpour of acid for it to actually be dangerous

Question 27

What is the word to describe freeze thaw damage?

Answer 27

Attrition. Cyclical, adds up over time.

Question 28

What contributes to freeze-thaw damage? And define hydraulic pressure

Answer 28

1. Water expansion
2. Hydraulic pressure: big pores freeze first and cause pressure gradient small pores have higher pressure therefore lower freezing point therefore they are less frozen and so water flow into there because it’s lower pressure and that causes cracking. Function of distance to void boundary, resistance to flow and rate of freezing
   Worst freeze thaw when concrete is in -8° environment (freezing point of concrete).

Question 29

What is entrained air?

Answer 29

All bubbles have same size so no pressure gradients and less crack formation. Freeze thaw worse in tidal zone because high tide thaw and submersion and low tide freeze

Question 30

What is solar heating?

Answer 30

Freeze at night, thaw during day. Lower portion of concrete is always ice so more expansion. Pressure gradient due to litvan’s model

Question 31

How does heat and fire affect concrete structure?

Answer 31

Concrete is resistant to heat, steel isn’t, so concrete protects steel.
Low thermal conductivity, heat is consumed by evaporation of water and some aggregates decompose and consume heat

Question 32

Water loss and shrinkage cause strength reduction and tensile
stresses, resulting in cracking and spalling.
Still, concrete is much less susceptible than steel, which conducts
heat rapidly, causing severe loss of structural integrity in beams
and columns.
Core of element, including reinforcement, will remain at lower
temperatures and be relatively unaffected at low exposure
temperatures or times.

Question 33

What is surface spalling?

Answer 33

Damage from aggregate expansion due to heat

Question 34

What is #1 deterioration cause of concrete?

Answer 34

Corrosion of reinforcement. Electrochemical reaction where concrete protects steel from more corrosion.

Question 35

What is pore solution? What is passivation film?

Answer 35

The alkaline solution found in the pores of hardened concrete. It’s high pH and creates passivation film around steel caused from concrete and steel interacting. Passivation film is a protective iron oxide film

Question 36

What are the reasons of loss of steel corrosion protection?

Answer 36

Physical: Good transport of things that cause corrosion, failure to protect against chloride sources. Chemical: chlorides cause corrosion because they destroy the passivation layer. CO2 (carbonic acid) exposure causes pH to drop and that ruins the passivation layer (depassivation at 11 ph)

Question 37

Describe corrosion as electrochemical process

Answer 37

Corrosion requires and cathode. These form on steel surface (electron flow). To avoid corrosion, we get a sacrificial material that becomes anode and that causes less corrosion on steel. Corrosion occurs at ANODE

Question 38

What is anode and cathode?

Answer 38

Anode loses electrons and loses iron ions and that iron builds up at the cathode leaving the anode exposed and corroded and cathode consumes electrons which create OH that rust with the iron

Question 39

What are effects of corrosion?

Answer 39

Reduction of cross-sectional area at the anode. Spalling or cracking of concrete due to expansion stresses created by rust formation.

Question 40

What are the different methods to prevent corrosion?

Answer 40

Sufficient cover: put more concrete around it - steel further from surface so further from sources of damage

“Improved” transport: make it harder for things to get in

Corrosion inhibitors: adding things in

Corrosion resistant reinforcement:
Ex: galvanized steel (zinc around it), stainless steel, epoxy coated steel, fibre reinforced polymers

Question 41

What kind of cracking occurs due to corrosion?

Answer 41

Usually crack to nearest surface

Spalling if at the corner

Delamination: often have rebar in planes - cracks connect to each other along that plane (interior so you can’t see it until it’s too late) - after cracking, steel no longer perfectly bonded so cannot transmit stresses

Question 42

Protection / Repair of corrosion

Answer 42

Remove, clean, replace: take off concrete, fix corrosion, cover again; problem - suddenly expose to low pH and chlorides so damage accelerated

Corrosive resistant reinforcement: fully replace the steel

Corrosion inhibitors: add them to either new or existing concrete

Sacrificial anode: add an anode outside of steel that is more reactive and will deteriorate instead - expensive so can use other stuff and add current to help

Cancellation current: complicated calculation to counter current and stop corrosion

Protective overlays: add sealant on surface to prevent things getting in - ofter where it deteriorates so don’t last

Question 43

What are the three types of surface wear?

Answer 43

Abrasion
Erosion
Cavitation

Question 44

What is abrasion?

Answer 44

Solid objects move along or rub against the concrete surface
Vehicular traffic and mechanical devices

Question 45

What is erosion?

Answer 45

Solid particles in fluids cause abrasion. Debris impacting rubbing rolling and grinding

Question 46

What is cavitation?

Answer 46

Cavitation damage occurs on concrete surface when
discontinuity or irregularities is encountered in the path of high
velocity water flow. This discontinuity or irregularity in the flow
path cause the water to lift off the flow surface, creating
negative pressure zones and resulting bubbles of water vapor.
The bubbles collapsing is what causes deterioration and surface wear. Causes shock wear and that causes tensile stress. Exponential - more surface wear means worse change in geometry means worse cavitation.

Question 47

Sea water is the worst in all these attacks