Durability Flashcards
What is durability defined as?
The ability to resist weathering, chemical attack, abrasion or any forms of deterioration when exposed to the environment
What is the design life?
The minimum period a structure is expected to perform without significant loss of utility and without excessive maintenance
What are the four methods of water ingress
permeation
diffusion
absorption
wick action
describe permeation
flow induced by pressure gradient
describe diffusion
flow induced by concentration gradient
describe absorption
flow induced by capillary action into unsaturated concrete
describe wick action
flow induced by a combination of permeation, diffusion, absorption. Occurs when one side of concrete is dry while the other is wet
What do we mean by transport?
The movement of “something” through hardened concrete
Why is transport relevant to concrete?
It is a highly porous material
What do all deterioration mechanisms involve?
water and the ingress of other aggressive species
Metals originate in an [] form, corrosion is merely nature returning refined metals to their natural state
oxide form
Why doesn’t steel rebar corrode rapidly?
metals corrode in acid
concrete is an alkaline solution, so is therefore protective
A passive iron oxide layer forms around rebar, and protects so long as the solution is alkaline
When drying, concrete pore solution is:
concrentrated
when wetted, concrete pore solution is:
diluted, leaching and carbonation occur
The passive iron oxide layer is destroyed either by
carbonation
or chloride attack
steel depassivates when
pH of solution is less than 11
and chloride threshold level is reached near the steel surface
steel corrodes if what two molecules are present?
O2 and H2O
How does carbonation of concrete begin
CO2 from atmosphere dissolves in pore solution to form carbonic acid
Carbonic acid reacts with [] to form [] during carbonation
reacts with hydration products
to form calcium carbonate
What does carbonation result in?
depletion of pH
How is the depth of carbonation measured?
phenolphthalein test.
Involves spraying a purple solution into a concrete cross section that loses colour with acidity.
How do cracks affect carbonation depth?
cracks become a site of carbonation, allowing it to continue deeper into the concrete
What equation is used to predict carbonation depth
D=K*sqrt(t)
D is depth
K is carbonation coefficient (mm/year^0.5)
t is exposure time in years
How does chloride-induced corrosion compare to carbonation?
More severe
How does chloride–induced corrosion occur?
chloride attacks passive iron oxide layer
activates steel, forms an anode
corrosion occurs if water and oxygen are available
chloride is not consumed
What are two sources of chloride corrosive agents?
sea water
de-icing salts
reinforcement corrosion is an [] reaction
electro chemical - relies on the movement of e-
What happens in reinforcement corrosion? (basically rusting process)
Iron ions at anode dissolve into solution
Electrons through to cathode, combine with H2O and O2 to form OH- ions
pore solution acts as an electrolyte to complete circuit
What are the effects of corrosion damage?
reduces area of steel
reduces load carrying capacity (loss of strength)
loss of bond between rebar and concrete
gateway for aggressive agent ingress
Total chloride content required to initiate corrosion is []
0.4% wt cement
What is spalling?
When concrete breaks off into smaller peices
How does frost damage occur?
water enters concrete voids
ice crystals push pore walls, damage cement paste
free thaw cycles induce cumulative damage
What are the most vulnerable surfaces to frost damage?
horizontal surfaces
road, roof, slabs subjected to wetting and wherever de-ccing is used
How do you make concrete frost resistant?
Use air-entrainment admixtures
How does sulphate enter groundwater?
decay of organic matter or from industrial/agriculture activities
In simple terms, what is the reaction that governs sulphate attack of concrete?
sulphates + cement paste = ettringite + gypsum
How does sulphate attack work?
sulphate in groundwater penetrates concrete, reacts with cement paste.
If the sulphate content exceeds []% weight of cement, it forms excessive [] and []
4%
excessive gypsum and ettringite
What are the big three hydration products and what are their characteristics?
calcium hydroxide - stacked hexagonal shapes
ettringite - fibrous, needle like structure
calcium silicate hydrate - fine grain sponge looking structure
Seeing as sulphate attack leads to the formation of new hydration products, it initially leads to an increase in . . . but soon results in . . .
strength and density
further expansion, leading to cracking
What is the Thaumasite form of sulphate attack?
it softens concrete, weakening it
How does thaumasite sulphate attack occur?
reaction between CSH and carbonate aggregate and sulphate produces thausamite
This replaces CSH, expanding the concrete and softening it
What is the basic premise of alkali-aggregate reaction?
Alkalis + reactive silica/carbonate + water
=
alkali silicate gel (concrete expansion)
what is the most common form of alkali-aggregate reaction?
alkali-silica reaction
what expansion due to ASR % is concrete expected to fail at?
0.3 - 0.4 %
What are the consequences of ASR?
leaching of colourless gel
facilitates ingress of other aggressive agents
map-cracking
Why is ASR particularly harmful?
There is no reversing the propagation, you may need to remove and replace parts of the structure once it has started
