Lesson 8 - Cement & Concrete Flashcards
What is Strength in Concrete?
Strength is related to the stress required to cause fracture.
Why is strength the property generally specified in concrete design?
- Because compared to most other properties, testing of strength is relatively easy.
- Many properties of concrete are directly related to strength and can be deduced from the strength data
What is the Interface Transition Zone (ITZ) in Concrete?
- The weakest link of the chain, considered the strength-limiting phase in concrete.
- Structure of the ITZ, have a great influence on the stiffness or elastic modulus of concrete.
- Existence of the ITZ is the primary reason that concrete is more permeable than the corresponding paste.
What are the factors compressive strength of concrete?
- Water to cement ratio
- Air Entrainment
- Cement type
- Aggregate
- Mixing water
- Admixtures
- Curing conditions
- Testing parameters
What are the type of admixtures added to concrete and what do they do?
Accelerating admixtures:
- Increase rate of hydration
- Provide an increased early strength
Retarding admixtures:
- Decrease the early strength of concrete due to the delay in setting time
Water reducing admixtures:
- Do not have any effect on the hydration of the cement
- When used to improve workability, no significant change in strength should be expected
Mineral admixtures:
- Silica fume is generally used to increase strength
- Fly ash and blast furnace slag may reduce the initial strength but may increase long term strength.
How do the curing conditions affect compressive strength?
- Time
- Humidity
- Temperature
How do the testing parameters affect compressive strength?
- Specimen parameters
- Loading conditions
Types of deformation in concrete?
- Load induced: elasticity and creep
- Moisture induced: shrinkage and swelling
- Temperature induced: expansion and contraction
What is secant modulus?
- Often used in design instead of elastic modulus.
- Is a static modulus since it is determined friends experimental stress-strain relation on a test cylinder.
- It decreases with an increase in stress
Factors affecting modulus of elasticity?
Modulus increases with:
- Increasing age
- Increasing concrete strength
- Increasing aggregate modulus
- Increasing loading rate
- Decreasing water to cement ratio
What is shrinkage?
The driving force for water movement is environmental conditions causing diffusion of water outward. (i.e. water is being lost)
What is creep?
The driving force is stress, which causes water to move from one location to the other from within the concrete (i.e. no water is being lost)
Plastic Shrinkage
- Loss of water from fresh concrete (early volume change) while it is still in a plastic state (8 hrs)
- If not prevented, can cause cracking
- Most common situation is surface cracking due to evaporation of water from the surface
- Loss of water exceeds the water by bleeding
How to minimise plastic shrinkage cracks?
Most effective method of controlling plastic shrinkage is through controlling the evaporation rate at the surface and temperature of concrete.
Could be done by:
- placing concrete when ambient temp. is low (early morning or late night)
- providing sun shades our wind breaks to control surface temps
What is dying shrinkage?
- Occurs after concrete has set and hardened
- Takes place in first few months
- Caused by withdrawal of water from concrete stored in unsaturated air
- Part of this shrinkage is recovered on immersion of concrete in water (reversible shrinkage)
- Is also irreversible shrinkage
What is autogenous shrinkage?
- Special case of dying shrinkage
- Withdrawal of water from capillary pores by the hydration process causes shrinkage.
- Dimension change of cement paste, mortar, or concrete caused by chemical shrinkage.
- Low w/c ratio = higher autogenous shrinkage
Is carbonation shrinkage reversible?
No
What are contraction joints?
- A way to regulate the location of the cracking
- Joins define where the cracking will form, rather than allowing for random crack formation.
What is the difference between between the factors that influence shrinkage compared to creep.
There isn’t any, all the factors that influence shrinkage influence creep in a similar way.
Effects of creep?
Effects:
- Strain
- Deflection
- Stress distribution
Varies with the type of structure:
- RC columns = Transfer of force to steel bars
- Eccentrically loaded column = Creep buckling
- Beams = Excessive deflection
- Prestressed concrete = Loss of prestressed
Effects:
- Strain
- Deflection
- Stress distribution
Varies with the type of structure:
- RC columns = Transfer of force to steel bars
- Eccentrically loaded column = Creep buckling
- Beams = Excessive deflection
- Prestressed concrete = Loss of prestress
