tutorial 3: fatigue, creep and cement chemistry Flashcards
when and how does fatigue occur
- materials contain microscopic defects which may experience increased localized stresses
- if local stresses too high, cracks form and propagate with cyclic loading of high stress levels
- when crack propagates beyond member capacity resistance sudden failure occurs
what type of structure does fatigue effect
microscopic
how does crack initiation start
localized material resistance is not sufficient to withstand localized load
where does crack initiation occur
near surface flaws, discontinuities or corners
how are ‘beach’ patterns created
- repeated cyclic loading will gradually widen crack and will create beach patterns
during crack propagation, what type of deformations are happening? What happens to the Area?
- deformations are permanent since energy is lost in propagating the crack
- resisting area is reduced until brittle sudden failure occurs
what is a fatigue limit?
- certain number of cycles at a given stress level the element can be subjected to before failure occurs
- stresses below the fatigue limit, number of cycles may be infinite without fatigue failure (for alloys)
what is creep
- long-term plastic deformation under sustained loads below the yield poing
what causes creep
- caused by constant loads for long periods of time
how does temperature affect creep
- more creep at higher temperatures (more energy for plastic deformtion)
how does the weight of the load affect creep
- more creep at higher loads (more energy for deformation)
what type of process is creep failure
deformation-based, time-dependent process
what is gradual failure
- long term strain twist structure beyond design restraints and cause structure to eventually deform beyond material resistance
what are the three concrete phases
- hydrated cement past (HCP)
- interfacial transition zone (ITZ)
- aggregates
which raw materials are necessary for cement production
- source of calcium
- silicate
- aluminum
- sulphates
- iron
what are the most common cement components
oxides (CaO, SiO2, H2O, Al2O3, SO3)
abbreviation for tricalcium silicate and what’s its use
C3S - early strength
Abbreviation for dicalcium silicate and what’s its use
C2S - ultimate strength
abbreviation for tricalcium aluminate and what’s its use
C3A - flux, fast set, sulfate reactive
abbreviation for tetracalcium aluminioferrite and what’s its use
C4AF - flux
abbreviation for gypsum and what’s its use
CSH2 - prevents fast set
what are the most common hydration products
- CSH (calcium silicate hydrate)
- CH (calcium hydroxide)
- ettringite
- calcium monosulphoaluminate
- tetracalcium aluminate hydrate
- calcium aluminoferrite hydrate
what products are created by C3S
CSH + 3CH + HEAT
what are the products of C2S
CSH + CH + heat
C2S vs C3S
C2S : responsible for ultimate strength gaine (beyond one week), decreased reactivity
C3S: increased reactivity, more C3S = more early stregnth gain
what is flux
reduces clinkering temperature during the firing process of cement manufacturing
C3A hydration reaction
C3A + gypsum +4H -> ettringite + heat
- realeased a lot of heat bc C3A is extremely exothermic
- flux
- ettringite reaected into calcium monosulphoaluminate (slows down reaction)
what happens if C3A is unmoderated
will cause a flash set (too much crystal growth without enough plastic time)
gypsum hydration reaction
C3A + gypsum + H -> Ettringite and heat
- slows down set time
- ettingite is insoluble - formation during early hydration slows down water contact with remaining C3A particles
C4AF hydration
C4AF + H + CH -> C6AFH12 + heat
- hydrates early, contributes little to overall strength
- mostly aesthetic changes
- flux
what is the microstructure of CSH and what does that mean..
- dense, amorphous crystalline structure
- interstitial pores too dense for water flow
- responsible for impermeability
CH microstructure
- thin, hexagonal plates
- higher surface- area-to-volume ratio
- more reactive
- leaching during efflorescence
ettringite microstructure
- long, thin spiky crystalline growth
- predominantly unidirectional growth
what are the different stages of concrete during hydration
- dormant period
- initial set
- curing
what happens during the dormant period
- before set point, crystals still suspended in mix water
- workable state
- adding more water at this point changes W/C ratio
what happens during the initial set
- crystalline growth sufficient to form matrix
- water between crystals being continually used in hydration reactions
- initial set occurs when crystals are no longer able to move past each other
- additional water doesnt affect W/C after this point and matrix is formed
what happens during the curing processs
- concrerte will continue to hydrate if there is water present
- additional water doesnt affect W/C - contributes to continuing hydration
- hydration stops below relative humidity of 85%
when does hydration stop
- below relative humidity of 85%
explain why concrete is weaker at the interfacial transition zone( transition zone between bulk cement paste and aggregate)
- wall effect: as the crystals approach an aggregate (Wall) the packing becomes less dense and thus there is less material to support loads at the ITZ and thus there is reduced strength at this region
what are the two actions that can be done to cement to increase early strength and become cement type HE?
1: cement chemistry change: Addition of C3S
- C3S reaction forms relatively more CH
- C3S has increased reactivity compared to C2S
- faster reaction = faster strength gain
2: more preactically: finer cement grind
- higher surface area allows the grains to react faster
- faster hydration means faster early strength gain
why do large test specimens normally have a lower strength than smaller ones made from the same material? how does this apply to flexural testing when considering the differences between the 3 point bending test and the 4 point bending test
- size effect: larger volume exposed to maximum stress gives greater probability of a critical flaw being present that can initiate failure
- 3 point bending test: only the center plane is exposed to maximum stress
- 4 point bending test: the middle third of the beam is exposed to maximum stress
- 4 point strength < 3 point strength
