Chemical and non chemical durability of concrete Flashcards
2 ways of protecting concrete form chloride
1-dense binder- low water to cement ratio to lower permeability
2- pozzolanic reactions- good durability
whats the effects of chloride
steel rusts, expands and concrete cracks
name the two chloride penetration tests
1- ponding test
2- rapid chloride permeability
explain the ponding test
concrete cylinder or slab and place a pool of chloride on top
after several months measure how far its travelled through it
use this to calc the diffusion coefficient
N.B:electricity can be used to speed up the chloride movement-migration coefficient
pros and cons of the ponding test
=: very accurate
-: very slow
describe the rapid chloride permeability test
apply a voltage and measure the current passed through the specimen in 6 hrs- measures permeability- the more charge that passes through the higher the chloride permeability
cons of the rapid chloride permeability test
cant be used to compare materials with different chemistries because the charged that passes through depends on factors eg dissolved ions in pore solution moving under the applied electrical field
whats the hybrid method used to measure chloride penetration
use chemical acceleration migration but split the sample and measure the actual depth at the end
whats an internal sulphate attack
excess sulphate within concrete causing accumulation of damage
what is external sulphate attack
sulphate from the environment entering causing chemical changes in cement paste
what conversion happens in sulphate attacks
Afm is converted to Aft causing expansion of binder
what does MgS04 do to concrete
it removes the calcium from C-S-H to form soft low strength phases
how do you make cements resistant to sulphate
1- use slag blends or low C3A clinker (then favours C-S-H formation over Afm)
2- high slag content reduces permeability in the long term
testing for sulphate resistance
1-immerse concrete in sulphate rich solution, measure specimen length regularly
2- testing for conversion of Afm to Aft phases
What is a Thaumasite sulphate attack
happens in cool wet conditions when carbonate and sulphate are present
-C-S-H is converted to thaumasite and becomes mush
how is the non chemical durability of concrete assessed ( what happens to effect the durability
-cement constains small quantities of alkalis in the pore solution upon hydration, if the aggregate contains reactive components it can be attacked by the pore solution
Explain what happens in an alkali aggregate reaction
reactive aggregate reacts with alkali pore solution
- makes a white silicate gel which expands
- cracks the paste allowing moisture to enter cracking it even more
- map cracking
2 testing of alkali aggregates
1- quick test-
2- concrete prism test
3-carbonation
explain the quick test for alkali aggregates
placing mortar bars in Na(OH), 80 degrees, measure expansion at 16days.
due to high alkalinity theres no influence from the alkali in the cement, more of a test for the aggregate reactivity
explain the consrete prism test
- extra alkali added to cement
- stored at 38 degrees for 1 year
- if not failed after one year wait another and if not failed still it passes
- very long
explain carbonation testing
- used to measure the alkalinity of the concrete
- C02 consumes poprtlandite whose job it is to hold Ph high
- want to measure the depth of C02 penetration in to concrete
- phenolphthalein indicates ph change: pink for high Ph, colourless for below 9
explain what freeze thaw is
water enters cracks, freezes and expands
salt makes the volume change more dramatic
how to test for freeze thaw
freeze thaw cycle it repeatedly, +4/-18 degrees every 4 hrs
measure elastic modulus, dimensions and mass
what is air entrainment
used to protect from freeze thaw
put bubbles<1mm into the concrete
they give the water space to expand into when freezing
