Chemical and non chemical durability of concrete

Question 1

2 ways of protecting concrete form chloride

Answer 1

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

Question 2

whats the effects of chloride

Answer 2

steel rusts, expands and concrete cracks

Question 3

name the two chloride penetration tests

Answer 3

1- ponding test

2- rapid chloride permeability

Question 4

explain the ponding test

Answer 4

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

Question 5

pros and cons of the ponding test

Answer 5

=: very accurate

-: very slow

Question 6

describe the rapid chloride permeability test

Answer 6

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

Question 7

cons of the rapid chloride permeability test

Answer 7

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

Question 8

whats the hybrid method used to measure chloride penetration

Answer 8

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

Question 9

whats an internal sulphate attack

Answer 9

excess sulphate within concrete causing accumulation of damage

Question 10

what is external sulphate attack

Answer 10

sulphate from the environment entering causing chemical changes in cement paste

Question 11

what conversion happens in sulphate attacks

Answer 11

Afm is converted to Aft causing expansion of binder

Question 12

what does MgS04 do to concrete

Answer 12

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

Question 13

how do you make cements resistant to sulphate

Answer 13

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

Question 14

testing for sulphate resistance

Answer 14

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

Question 15

What is a Thaumasite sulphate attack

Answer 15

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

Question 16

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

Answer 16

-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

Question 17

Explain what happens in an alkali aggregate reaction

Answer 17

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

Question 18

2 testing of alkali aggregates

Answer 18

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

Question 19

explain the quick test for alkali aggregates

Answer 19

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

Question 20

explain the consrete prism test

Answer 20

• 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

Question 21

explain carbonation testing

Answer 21

• 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

Question 22

explain what freeze thaw is

Answer 22

water enters cracks, freezes and expands

salt makes the volume change more dramatic

Question 23

how to test for freeze thaw

Answer 23

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

measure elastic modulus, dimensions and mass

Question 24

what is air entrainment

Answer 24

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