Concrete -Durability Flashcards
What is durability?
Ability to exist for a long time without significant damage
What are the categories of deterioration mechanisms?
- Physical attack
- Chemical attack
- Electrochemical attack
What are water transport mechanisms for deterioration?
- Diffusion
- Absorption
- Permeation
What is diffusion?
- Movement of liquid or gas driven by a concentration gradient
- High concentration -> low concentration
What is permeation?
- Movement of liquid or gas driven by a pressure gradient
- High pressure -> low pressure
What is absorption?
- Movement of water driven by surface tension
- Unsaturated -> saturated
What are the types of physical attacks?
- Freeze-thaw
- Abrasion and wear
- Cavitation and erosion
- Temperature changes (heat and fire)
What is freeze-thaw?
- Damage induced by repetitive cycles of freezing and thawing of water
- Water expands from freezing
- Large pores freeze first because large pores freeze at a high temperature
- Smaller pores freeze later at a lower temperature
- Hydraulic pressure is pushing water into smaller and smaller pores which cracks the concrete
What is abrasion and wear?
Physical rubbing between surfaces
What is cavitation and erosion?
- Tiny bubbles created when water lifts from surface
- Popping bubbles creates a shockwave and damages concrete
- Damage due to physical particles suspended in water rubbing against concrete
What happens to temperature changes (heat and fire)?
- Concrete is a bad thermal conductor
- It is therefore resistant to fire damage
What are the types of chemical attacks?
- Alkali-silica reaction (ASR)
- Sulphate attack
- Leaching/efflorescence
- Acid attack
- Carbonation
What is ASR?
- Chemical reaction between alkali in paste and silica in aggregates
- Cracks form inside aggregates
- On a macro scale, map cracking
What are factors that affect ASR?
- Nature of reactive silica
- Amount of reactive silica
- Particle size of reactive material
- Amount of alkalis available
- Amount of moisture available
How do you prevent ASR?
- Avoid reactive aggregates!
- Limit amount of alkalis in cement paste
- Use SCMs in cement paste
- Lower concrete permeability
What is a sulphate attack?
Chemical reaction between sulphate ions and hydrated cement paste
What are the effects of a sulphate attack?
- Expansion and cracking of concrete
- Softening and disintegration of paste
What are the forms of a sulphate attack?
- External sulphate attack (groundwater or river water)
- Physical sulphate attack
- Thaumasite
- Internal sulphate attack
- Waste / Sewage
Describe the stages of an external sulphate attack?
- Sulphates are dissolved in water and enter the concrete from an outside source
- Sulphate reacts with CH to create gypsum (CSH2)
- Gypsum reacts with monosulphoaluminate in the hcp to create ettringite
Step 2 and 3 are expansive which is why concrete cracks
How do you prevent a sulphate attack?
- Lower C3A content to lower expansion
- Low w/c ratio
- Low permeability concrete
What happens in an internal sulphate attack (delayed ettringite formation)?
- If concrete is hardened at high temperatures, ettringite is destroyed and sulphates are absorbed into CSH
- Once concrete is cooled, sulphates react with CH to form monosulphoaluminate and then to form etttringite
What is leaching?
Dissolution of calcium hydroxide of cement paste in pore water
What is efflorescence?
When pore water reaches surface, it evaporates and leaves white stains from calcium hydroxide
How to prevent leaching?
- Minimize transport properties (low w/c ratio, SCMs)
- Minimize calcium hydroxide content (SCMs)
What is the difference between hard water and soft water for leaching?
- Hard water from lakes, rivers, ground water contains dissolved minerals and is not detrimental to concrete
- Soft water like rain, snow and ice does not contain dissolved minerals and is detrimental to concrete
What is an acid attack?
Chemical reaction between surface of concrete and acid solution
What is a carbonation attack?
- Infiltration of CO2 into concrete
- CO2 reacts with CH to create calcium carbonate
- Lowers pH of pore solution
What are the effects of a carbonation attack?
- Lower pH of water pore which increases corrosion rate of reinforcing steel (removal of passivation film)
What is an electrochemical attack?
- Affects the reinforcing steel by increasing its volume
- Creates tensile stresses in the concrete
- Concrete is very weak to tensile stress but strong in compressive stress
How does concrete protect the reinforcing steel?
- Physically: Prevents sulphates, water, chlorides, oxygen to come into contact with steel
- Chemically: pore solution is very high in pH which creates an iron oxide film around steel
- This film is called a passivation film
How does concrete stop protecting reinforcing steel?
- Loss of physical protection: insufficient concrete to cover, cracking in concrete, concrete with poor transport properties
- Loss of chemical protection: passivation film is lost because of lower pH of pore solution (carbonation or acid attack), penetration of chlorides
Explain the process of damage in alkali-silica reaction
- Alkalis in pore solution react with silica from aggregates to form
amorphous silicate gel - Amorphous silicate gel absorbs pore water and greatly expands
- Once pores are filled by expansion, expansion becomes
restrained and cracks occur
Explain the consequences of reinforcement corroding
- Expansion of rebar cracks concrete & thus increases permeability
- Steel’s cross-sectional area is reduced, which reduces steel’s load
carrying capacity
