Concrete Durability Flashcards
Can concrete last forever?
Under ideal conditions yes. If submerged in calcium bath and doesn’t try out
What are the transport mechanisms that cause concrete deterioration?
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
Know the difference between porosity and permeability and the drawings
How do porosity, w/c ratio, and transport mechanism relate?
High porosity = high chance of transport and connection between holes
Low wc = less interconnection
What is wicking?
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
What is leaching?
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
What is efflorescence?
Manifestation of leaching. When water evaporates and leaves behind calcium. Sign that there’s a problem with water flow
Relates to imbibes
What are hard water and soft water?
Hard water (groundwater lakes and rivers): contain minerals and not detrimental to concrete
Soft water (rain snow and ice): no minerals
Describe alkali silica reaction
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
Why doesn’t sand contribute to ASR?
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
What is pessimum amount?
Max amount of expansion due to a certain amount of reactive silica
Increased particle size causes decreased ASR
What are all the things that can affect asr? NOT WATER AND TEMP
Particle size, crystallininry of silica, ratio of silica to alkali.
How to prevent ASR?
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
How to detect ASR?
Add uranyl acetate and that makes makes deterioration visual under UV lights
ASR expansion happens at the surface and that causes popouts
How are dams affected by ASR?
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
What is alkali carbonate reaction (ACR) ?
Slower than ASR. Also an expansive reaction involving carbonate rocks (dolomitic)
What are carbonate rocks susceptible to?
Fine grained dolomite (small crystals)
Calcite
Interstitial clay
Dispersed in clay matrix
What is sulphate attack?
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
What are the steps of sulphate attack?
- Sulphates enter concrete from outside source
- Reacts with CH to produce gypsum
- Gypsum reacts with monosulphoaluminate to form ettringite
2 and 3 are expansive
What are ettringites and why are they bad?
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
How to do you prevent sulphate attack?
With less C3A (but we need C3A to decrease clinkering temp).
What is effect of seawater and it’s relation to sulphate attack?
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
What is internal attack of sulphate?
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.
What is physical sulphate attack?
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
What is acid attack?
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
What is the word to describe freeze thaw damage?
Attrition. Cyclical, adds up over time.
What contributes to freeze-thaw damage? And define hydraulic pressure
- Water expansion
- 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).
What is entrained air?
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
What is solar heating?
Freeze at night, thaw during day. Lower portion of concrete is always ice so more expansion. Pressure gradient due to litvan’s model
How does heat and fire affect concrete structure?
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
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.
What is surface spalling?
Damage from aggregate expansion due to heat
What is #1 deterioration cause of concrete?
Corrosion of reinforcement. Electrochemical reaction where concrete protects steel from more corrosion.
What is pore solution? What is passivation film?
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
What are the reasons of loss of steel corrosion protection?
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)
Describe corrosion as electrochemical process
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
What is anode and cathode?
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
What are effects of corrosion?
Reduction of cross-sectional area at the anode. Spalling or cracking of concrete due to expansion stresses created by rust formation.
What are the different methods to prevent corrosion?
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
What kind of cracking occurs due to corrosion?
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
Protection / Repair of corrosion
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
What are the three types of surface wear?
Abrasion
Erosion
Cavitation
What is abrasion?
Solid objects move along or rub against the concrete surface
Vehicular traffic and mechanical devices
What is erosion?
Solid particles in fluids cause abrasion. Debris impacting rubbing rolling and grinding
What is cavitation?
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.
Sea water is the worst in all these attacks
