Chemical and non chemical durability of concrete Flashcards Preview

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Flashcards in Chemical and non chemical durability of concrete Deck (24)
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1
Q

2 ways of protecting concrete form chloride

A

1-dense binder- low water to cement ratio to lower permeability
2- pozzolanic reactions- good durability

2
Q

whats the effects of chloride

A

steel rusts, expands and concrete cracks

3
Q

name the two chloride penetration tests

A

1- ponding test

2- rapid chloride permeability

4
Q

explain the ponding test

A

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

5
Q

pros and cons of the ponding test

A

=: very accurate

-: very slow

6
Q

describe the rapid chloride permeability test

A

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

7
Q

cons of the rapid chloride permeability test

A

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

8
Q

whats the hybrid method used to measure chloride penetration

A

use chemical acceleration migration but split the sample and measure the actual depth at the end

9
Q

whats an internal sulphate attack

A

excess sulphate within concrete causing accumulation of damage

10
Q

what is external sulphate attack

A

sulphate from the environment entering causing chemical changes in cement paste

11
Q

what conversion happens in sulphate attacks

A

Afm is converted to Aft causing expansion of binder

12
Q

what does MgS04 do to concrete

A

it removes the calcium from C-S-H to form soft low strength phases

13
Q

how do you make cements resistant to sulphate

A

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

14
Q

testing for sulphate resistance

A

1-immerse concrete in sulphate rich solution, measure specimen length regularly
2- testing for conversion of Afm to Aft phases

15
Q

What is a Thaumasite sulphate attack

A

happens in cool wet conditions when carbonate and sulphate are present
-C-S-H is converted to thaumasite and becomes mush

16
Q

how is the non chemical durability of concrete assessed ( what happens to effect the durability

A

-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

17
Q

Explain what happens in an alkali aggregate reaction

A

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
18
Q

2 testing of alkali aggregates

A

1- quick test-
2- concrete prism test
3-carbonation

19
Q

explain the quick test for alkali aggregates

A

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

20
Q

explain the consrete prism test

A
  • 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
21
Q

explain carbonation testing

A
  • 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
22
Q

explain what freeze thaw is

A

water enters cracks, freezes and expands

salt makes the volume change more dramatic

23
Q

how to test for freeze thaw

A

freeze thaw cycle it repeatedly, +4/-18 degrees every 4 hrs

measure elastic modulus, dimensions and mass

24
Q

what is air entrainment

A

used to protect from freeze thaw
put bubbles<1mm into the concrete
they give the water space to expand into when freezing