Durability Flashcards
Why is it very important to consider deterioration when designing structures?
Severe premature deterioration can occur if durability is not adequatly considered
What can concrete deterioration lead to?
Safety, financial and environmental concerns
What are the two definitions of durability?
“ability to resist weathering, chemical attack, abrasion or any forms of deterioration, when exposed to its intended environment”
“ability to perform its intended funtion, retain its form, required strength and servicibility, throughout its expected design life”
What is the definition of design life?
The minimum period a structure is expected to perform without significant loss of utility and without excessive maintenance
What are the transport properties (penetrability)?
- Permeation
- Diffussion
- Absorption
- Wick action
What is the definition of permeation?
Flow induced by a pressure gradient
What is the definition of diffusion?
Flow induced by a concentration gradient
What is the definition of absorption?
flow induced by capillary action into unsaturated concrete
What is the definition of wick action?
Flow induced by combination of permeability, diffusion and absorption in structures exposed to water on one side, and drying on the other
What chemical is involved in all types of major degredation?
Water
What controls he rate of deterioration?
The movement of water and dissolved aggressive agents by transport mechanisms
Why is transport particularly relevant to conrete
It is highly porous and contains many cracks and porous interfaces
Contains steel, which can be corroded from inside the concrete
Long service lives are expected
What are some deterioration mechanisms?
- Reinfocement corrosion: carbonation-induced, chloride-induced
- Freeze/ thaw
- Sulphate attack
- Alkali aggregate reaction
- salt scaling
- leaching
- acid attack
- delayed ettringite formation
What causes corrosion in metals?
Reacting with oxygen and moisture to go back to their orginal oxide form
Why is metal corrosion such an important issue?
It presents a serious safety concern
It can generate a huge cost to fix
Do acids or alklais protect or corrode metals?
Alkalis = protection Acids = corrosion
Why does concrete help to reduce corrosion of metal reinforcements?
It is strongly alkalinic
Provides protection and regenerates as long as alkaline environment is maintained
What helps protect reinforcements in concrete?
An iron oxide layer formed by the alkali (passive layer)
How do you increase the concentration of the alkalis in conrete?
Drying
What decreases the concentration of alkalis in concrete
Wetting, leaching and carbonation
What is the pH in concrete?
13.5
What breaks down the passive layer?
Carbonation and chloride ingress
At what pH does steel start to depassivate?
<11
What factors other than a reduced pH are needed for corrosion?
The presence of oxygen and water
What causes carbonation?
The diffusion of CO2 from atmosphere and dissolves into the pore solution
What is the product of carbonation in concrete?
Carbonic acid
What is the produce of the reaction between carbonic acid and the hydration products?
Calcium Hydroxide - CH which precipitates calcium carbonate
What is formed when C-S-H is carbonated?
CaCO3 and silica gel
How do we detect and measure carbonation depth?
Phenolphthtalein solution - pH indicator goes from colourless to purple @pH<9.5
Colourless indicates the area of the carbonated concrete
What is the equation for the depth of carbonation?
D = K*sqrt(t) K = mm/sqrt(year)
Where does carbonation occur from?
The exposed concrete surface
Does what happens to the rate of carbonation over time and why?
Slows down as CO2 has to move through the carbonated zone
What slows down the rate of corrosion?
Dense, higher alkalinity, dry (no reaction), fully saturated
At what point does corrosion start?
when the carbonation front reaches the steel surface
What is the pH reduced to during carbonation?
~8.5
Where does chlorine tend to come from during chloride induced corrosion?
Sea water or de-icing salts
Why is chloride-induced corrosion particularly problematic?
It doesn’t require a drop in pH
What happens during chloride-induced corrosion?
Chloride destroys the passive layer.
Forms ferrous chloride at the intermediate phase
CHloride is regenerated to form deep pitting corrosion
What role do anodes and cathodes play in chloride-induced corrosion?
Fe2+ ions at the anode dissolve into solution
Electrons flow through rebar into the cathod and combine with water oxygen to form OH- ions
Pore solution acts as an electrolyte
What kind of chemical reaction is chloride-induced corrosion?
Electro-chemical
What is the service life of a concrete structure defined as?
The time taken for corrosion to initate plus the time taken to reach a critical limit state
What are types of critical limits in concrete?
- cracking
- spalling
- delamination of the cover
- strucutral collapse
What are some of the consequences of corrosion damage?
- reduced area of steel
- reduced load carrying capacity
- cracking and loss of bond between rebar and concrete
- increases risk of ingress of aggressive agents
What is frost action?
Water expanding on freezing causes hydraulic pressure on pore walls, repetitive freezing and thrawing damage
What are some damage types from frost action?
- Progressive damage
- spalling
- disintegration
How much does capillary pore water expand on freezing?
~9%
What surfaces are most vulnerable to frost action?
Horizontal surfaces, roads, roof slabs.
Especially prone in places where de-icing salts are used
How do you make concrete more frost resistant?
- air entrainment
- using dense concrete and protecting it from water ingress
- increase amount of aggregate
What is conventional sulphate attack?
Sulphate ions are transported in water and react with cement paste to form ettringite and gypsum
Sulphate attack initially increases strength, so why is it dangerous?
The more product that is created the more expansion, the higher the risk of severe cracking
What type of sulphate is particularly aggressive and why?
Magnesium sulphate - targets C-S-H and softens cement paste
What is Thaumasite?
When carbonates from aggregate particles changes the reaction between C-S-H and sulphate to rpoduce thaumasite, replacing C-S-H and causing expansion
Why is thaumasite degredation so dangerous?
It casues the bond between aggregate and cement to decrease and the concrete to soften and eventually disintegrate
What is an alkali-aggregate reaction?
When alklais from the pore solution and reactive aggregates form a gel that can absorb water
Why is an alkali aggregate reaction dangerous?
Gel expands as it takes on water - gel is confined and internal pressure leads to cracking in the aggregate and paste
What is the most common form of alkali-aggregate reaction
Alkali silica reaction
the other common form is an alkali carbonate reaction
What % of Na2O should be avoided when used with reactive aggregates?
> 0.6% will cause significant expansion and then damage
What can be done to reduce the chances of alkali-aggregate reactions?
Use low alkali cement
What is the range of crack widths from alkali aggregate reactions?
0.1-10mm (similair to sulphate attack, freezing/thaw or plastic shrinkage)
What is the identifying feauture of alkali aggregate reactions?
Colourless jelly like exudation on the cracks
