Durability of concrete Flashcards
What is durability?
- Ability to resist weathering, chemical attack, abrasion or any forms of deterioration (chemical, physical or mechanical)
- Ability to perform its intended function and retain its form
What is the design life of bridges?
- 120 yrs
What is the design life of halls of residence?
- 50 yrs
What is involved in all major degradation?
Water
What are the drivers for durability?
- Safety
- Socio-economy
- Sustainability
What does deterioration involve?
- Deterioration involves physical, chemical or mechanical actions
What can degradation be caused by?
Degradation can be caused by external (environment) or internal agents
What are examples of transport properties?
- Absorption
- Permeation
- Diffusion
- Wick action
What do the transport properties determine?
Determine the durability of concrete and of steel embedded within it, as well as the effectiveness of structures
What can be transported in hardened concrete?
- Water
- Vapour
- Liquids
- Dissolved ions
- Gases
What are some deterioration mechanisms?
- Reinforcement corrosion
- Freeze/thaw
- Sulphate attack
- Alkali-aggregate reaction
- Salt scaling
- Leaching
- Acid attack
- Delayed ettringite formation
What do metals have a tendency to do?
Natural tendency to revert (corrode) to original state when exposed to environment
When does corrosion occur?
Corrosion occurs when pH is low
Why does steel in concrete not corrode rapidly?
- Metals protected from corrosion in an alkaline solution
- Concrete pore solution is highly alkaline
What is the dense and impenetrable film called which protects steel from corrosion
Passive iron oxide layer
How can pore solution change?
- Concentrated by drying
- Diluted by wetting, leaching and carbonation
How can passive layer break down?
By carbonation or chloride attack
When does steel corrode?
Steel corrodes if O2 and H2O are present
At what condition does steel depassivates?
- pH of pore solution < 11
- Critical chloride threshold level is reached near the steel surface
How is calcium carbonate formed from concrete?
Carbonic acid reacts with hydration products
How can you check for carbonation depth?
- Phenolphthalein test
What happens when ph>10 in concrete?
Solution turns purple
How can you calculate depth of carbonation?
D=k√t
What is the carbonation coefficient k influenced by?
- Influenced by properties of concrete
- Increases with temp
What are the negative effects of carbonation?
- Reduces pH to ~ 8.5
- Depassivates steel, corrodes if moisture &oxygen is present
How does chloride induced corrosion work?
- Chloride attacks passive layer, acts as catalyst and activates steel to form anode
- Corrosion occurs if water and oxygen are available
- Chloride is not consumed
What are the sources of chloride?
- Internal - cement
- External - diffusion, sea water, de-icing salts
What damage can corrosion cause?
- Reduces area of steel
- Reduces load carrying capacity
- Cracking & loss of bond between rebar and concrete
- Increases ingress of aggressive agents
- Spalling and delamination
What is an effective way from protecting concrete against freeze-thawing?
Air entrainment is an effective way of protecting concrete from freezing and thawing damage. It is achieved by adding a surface active agent in very small dosages to the concrete mixture.
What are the effects of freeze-thawing?
expansion, cracking, spalling and disintegration
How can you make concrete more frost resistant?
- Protect concrete from moisture
- Use dense cement paste
- Use frost resistant concrete
- Increase aggregate fraction
