Concrete part 2 Flashcards
How do we calculate compressive strength?
fcu = Fu/A where: fcu- compressive strength at 28 days Fu- Force at failure at 28 days A- cross-sectional area of the cylinder
What shape can we also measure compressive strength on other than a cylinder?
Cubic samples when the load is applied the 45-degree cracks will interact with each other causing a higher strength.
What does strength development depend on? (6)
1) Cement composition
2) Water/cement ratio
3) Curing humidity
4) Curing temperature
5) Type of aggregates
6) Compaction
What is the modulus of elasticity for ordinary concrete for PC?
Ec = 9.1fcu^0.33
What is the elastic modulus Ec for 30MPa concrete?
9.1 x 30^0.33 = 26GPa
What are the main degradation mechanisms of concrete?
1) Drying shrinkage
2) Freeze-thaw
3) Creep
4) Fatigue
5) Sulphur attack
6) Chloride attack
7) Carbonation
8) ASR alkali-silica reaction
What is the cause of drying shrinkage?
During hydration, water is consumed through self-desiccation that causes shrinkage.
What is the cause of freeze thaw and sulphur attack?
Pressure from crystals forming and expanding in the pores.
What is freeze-thaw?
Water turning into ice.
What is a sulphate attack?
Monosulfoaluminate turning into ettringite.
What can free-thaw cause?
Fatigue due to its cyclic nature.
How can we avoid freez-thaw?
Introduce additional porosity with air-entraining admixtures.
How can we avoid sulphur attacks?
Use low-aluminium cement.
What is the cause of ASR?
Pressure from a phase expanding inside the pores.
What phase happens during ASR?
The gel formed by alkali ions that are usually present in the cement and react with amorphous aggregates.
How can we avoid ASR?
Use aggregates low in amorphous silica and use low alkali type of cement (more expensive).
What is creep?
Stress and strain in concrete increase progressively overtime for years.
What is the cause of creep?
Under stress, water layers in the nanostructure of C-S-H flow viscously.
What is the effect of creep?
Excessive deformations can ultimately cause cracks and failure.
What does creep depend on?
1) At what age the load/strain is applied
2) Humidity
3) Elastic modulus
4) Temperature
What do both chloride attacks and carbonation induce?
Corrosion of the reinforcement of steel bars.
What is a chloride attack?
The surface of steel reinforcement in concrete, soon after mixing, reacts with the solution and form a protective layer of iron oxide.
What is carbonation?
CO2 from the atmosphere dissolves in the solution of concrete and reacts with CH to form calcium carbonate. Calcium carbonate reduces pH of the solution. At pH<9 corrosion starts.
How well used is concrete?
The most widely used artificial material on earth with more than 3.5x10^9 tons produced per year.
