Lesson 1-3 Flashcards
Forms of Pre-stressing Steel
Tendon
Wires
Strands
Cable
Bars
a stretched element used in a concrete member to impart pre-stress to the concrete.
Tendon
a single unit made of steel.
Wires
two, three or seven wires are wound
Strands
a group of strands
Cable
Methods of Prestressing
Pre-tensioning
Post-tensioning
The tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through a bond over the transmission length near the ends.
Pre-tensioning
when a material is subjected to repeated cycles of stress or strain and its structure breaks down and ultimately leads to fracture
Fatigue
when a material is subjected to a load for a very long time it may continue to deform until a sudden fracture occurs.
Creep
Types of Steel
Low-carbon Steel
Alloy Steel
Cast Iron
5 Factors that affect fatigue
Type of steel reinforcement
Stress level
Number of load cycles
Presence of stress concentrations
Environment
the time-dependent increase in strain under constant stress
Creep
the time-dependent decrease in stress under constant strain
Relaxation
Effect of High Temperature
Loss of strength
Fire resistance
Corrosion of Prestressing Steel
Chloride Ions
Carbonation
Hydrogen Sulfide
Stress corrosion cracking
Hydrogen Embrittlement
can come from a variety of sources, including seawater, deicing salts
Chloride Ions
reaction of carbon dioxide with the calcium hydroxide in concrete
Carbonation
can come from a variety of sources, including industrial emissions, sewage treatment plants, and decaying organic matter.
Hydrogen Sulfide
prestressed steel is subjected to high levels of stress and chloride ions
Stress corrosion cracking
prestressed steel is subjected to high levels of hydrogen
Hydrogen Embrittlement
Effects of Corrosion
Loss of strength
Cracks in the concrete
Fracture of the steel
cracks can allow water and other corrosive agents to penetrate the concrete and further accelerate the corrosion of the steel
Concrete cracking
the reduction in volume of concrete that occurs when it dries and hardens
Shrinkage
the increase in strain in concrete that occurs under sustained loading
Creep
Factors affecting shrinkage and creep
Type
Age
Moisture content
Temperature
Loading conditions
Methods to predict shrinkage and creep
Theoretical models
Empirical models
Hybrid models
based on the understanding of the physical mechanisms of shrinkage and creep
Theoretical models
based on experimental data
Empirical models
combination of theoretical and empirical approaches
Hybrid models
Movements in concrete structures
Expansion
Shear
Bending
the ability of a material to resist weathering or other destructive influences
Durability
materials that are added to concrete during mixing to improve its properties
Admixtures
Common types of admixtures
Water-reducing admixtures
Air-entraining admixtures
Cementitious admixtures
Retarding admixtures
Accelerating admixtures
Waterproofing admixtures
Corrosion inhibitors
reduce the amount of water required to produce a workable concrete mix
Water-reducing admixtures
introduce tiny air bubbles into the concrete mix
Air-entraining admixtures
react with the cement in the concrete mix to improve its strength and durability
Cementitious admixtures
slow down the setting time of the concrete mix
Retarding admixtures
speed up the setting time of the concrete mix
Accelerating admixtures
reduce the permeability of the concrete mix
Waterproofing admixtures
protect the steel reinforcement in concrete from corrosion
Corrosion inhibitors
Effects of Vibration
Low-intensity vibration
Moderate-intensity vibration
High-intensity vibration
the ability of a material to resist sudden forces without breaking or deforming
Impact resistance
a material will fail in shear when the shear stress reaches a critical value
Mohr-Coulomb failure criterion
the ability of a material of resist forces that tend to cause two parts of the material to slide past each other
Shear resistance
Types of Cracks
Hairline cracks
Wide cracks
Delamination
are small, narrow cracks that are typically less than 0.1 mm wide
Hairline cracks
more than 0.1 mm wide
Wide cracks
the separation of the concrete from the reinforcement
Delamination
4 Common causes of crack
Loading
Shrinkage
Corrosion of reinforcement
Defects in the concrete
the deformation of reinforced concrete beams can be controlled by using a variety of techniques, such as using high-strength concrete, using closely spaced reinforcement, and using stirrups
Crack Control
the deflection of reinforced concrete beams can be controlled by using a variety of techniques, such as using a deeper beam, using a smaller load, and using prestressing
Deflection Control
Method of analyzing deformation
Elastic Theory
Plastic Theory
Finite Element Method
assumes that the concrete and steel in the beam behave elastically
Elastic Theory
assumes that the concrete in the beam behaves plastically after it reaches its yield strength
Plastic Theory
a more sophisticated method that can account for the non-linear behavior of concrete and steel
Finite Element Method
Factors of affecting deflection
Load-carrying capacity
Deflection resistance
Crack control
Durability
