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Questions and Answers
Questions and Answers
Which of the following is NOT a consideration for minimizing creep and relaxation in prestressed concrete design?
Which of the following is NOT a consideration for minimizing creep and relaxation in prestressed concrete design?
- Use high-strength steel
- Avoid overloading
- Protect the member from moisture
- Maximize stress concentrations (correct)
Corrosion of prestressing steel can be caused by chloride ions.
Corrosion of prestressing steel can be caused by chloride ions.
True (A)
What role does temperature play in the behaviors of creep and relaxation in concrete?
What role does temperature play in the behaviors of creep and relaxation in concrete?
Temperature affects the rate of creep and relaxation in prestressed concrete.
Creep is the time-dependent increase in __________ under constant stress.
Creep is the time-dependent increase in __________ under constant stress.
What is a consequence of high temperatures on prestressed concrete structures?
What is a consequence of high temperatures on prestressed concrete structures?
Match the following types of corrosion with their causes:
Match the following types of corrosion with their causes:
Fire can damage prestressed concrete despite its fire resistance.
Fire can damage prestressed concrete despite its fire resistance.
Increased risk of __________ occurs at low temperatures due to the effects on steel.
Increased risk of __________ occurs at low temperatures due to the effects on steel.
What is one of the primary consequences of corrosion of prestressing steel?
What is one of the primary consequences of corrosion of prestressing steel?
Concrete cracking can help prevent the penetration of corrosive agents.
Concrete cracking can help prevent the penetration of corrosive agents.
What is the effect of a high water-to-cement ratio on concrete during the curing process?
What is the effect of a high water-to-cement ratio on concrete during the curing process?
______ is the increase in strain in concrete that occurs under sustained loading.
______ is the increase in strain in concrete that occurs under sustained loading.
Which of the following factors does NOT affect the shrinkage and creep of concrete?
Which of the following factors does NOT affect the shrinkage and creep of concrete?
Match the following terms with their definitions:
Match the following terms with their definitions:
Fracture of the prestressing steel can lead to structural collapse.
Fracture of the prestressing steel can lead to structural collapse.
What determines how much a concrete structure will deform under load?
What determines how much a concrete structure will deform under load?
What is the endurance limit of low-carbon steel relative to its ultimate tensile strength?
What is the endurance limit of low-carbon steel relative to its ultimate tensile strength?
Creep is a phenomenon that occurs when a material is subjected to a load for a very short period.
Creep is a phenomenon that occurs when a material is subjected to a load for a very short period.
What term describes the breakdown of a material's structure due to repeated cycles of stress?
What term describes the breakdown of a material's structure due to repeated cycles of stress?
The endurance limit of _______ steel can vary depending on the alloying elements and heat treatment.
The endurance limit of _______ steel can vary depending on the alloying elements and heat treatment.
Which type of steel has a higher endurance limit than low-carbon steel?
Which type of steel has a higher endurance limit than low-carbon steel?
The endurance limit of cast iron is generally higher than that of steel.
The endurance limit of cast iron is generally higher than that of steel.
1 MPa is equivalent to ______ psi.
1 MPa is equivalent to ______ psi.
Match the types of materials with their endurance limits:
Match the types of materials with their endurance limits:
Which of the following helps minimize shrinkage and creep in concrete?
Which of the following helps minimize shrinkage and creep in concrete?
Theoretical models for predicting shrinkage and creep are based on experimental data.
Theoretical models for predicting shrinkage and creep are based on experimental data.
What is one method to reduce movements in concrete structures due to temperature changes?
What is one method to reduce movements in concrete structures due to temperature changes?
Durability is the ability of a material to resist ______ or other destructive influences.
Durability is the ability of a material to resist ______ or other destructive influences.
Match the following methods with their purpose:
Match the following methods with their purpose:
Which factor does NOT affect the durability of concrete?
Which factor does NOT affect the durability of concrete?
Concrete contracts when it is heated.
Concrete contracts when it is heated.
Name one empirical model used for predicting shrinkage and creep.
Name one empirical model used for predicting shrinkage and creep.
Which type of admixture is used to reduce the amount of water needed in a concrete mix?
Which type of admixture is used to reduce the amount of water needed in a concrete mix?
Corrosion inhibitors are used to enhance the permeability of concrete.
Corrosion inhibitors are used to enhance the permeability of concrete.
Name two applications of structural lightweight concrete.
Name two applications of structural lightweight concrete.
____ admixtures introduce tiny air bubbles into the concrete mix.
____ admixtures introduce tiny air bubbles into the concrete mix.
Match the following types of admixtures with their functions:
Match the following types of admixtures with their functions:
Which admixture type improves strength and durability by reacting with cement?
Which admixture type improves strength and durability by reacting with cement?
Structural lightweight concrete is primarily used for its heavy weight.
Structural lightweight concrete is primarily used for its heavy weight.
A type of admixture that reduces the setting time of concrete is called a ____ admixture.
A type of admixture that reduces the setting time of concrete is called a ____ admixture.
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Study Notes
Study Notes
Prestressed Concrete Materials Overview
- Focus on steel and concrete as core materials in prestressed concrete design.
- Important factors include resistance to fatigue, creep, relaxation, temperature effects, corrosion, shrinkage, and durability.
Steel in Prestressed Concrete
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Fatigue Resistance
- Fatigue occurs due to repeated stress cycles, causing material breakdown and potential fracture.
- Low-carbon steel endurance limit is approximately 0.4 times its ultimate tensile strength (UTS).
- Alloy steel generally offers a higher endurance limit; variability depends on alloying and heat treatment.
- Cast iron has a lower endurance limit influenced by type and casting process.
-
Key factors affecting fatigue performance include:
- Type of steel reinforcement
- Stress levels
- Number of load cycles
- Environmental conditions
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Strategies to enhance fatigue resistance:
- Use high-quality materials.
- Design to minimize stress concentrations.
- Avoid overloading.
- Protect against corrosion.
Creep and Relaxation
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Creep: Time-dependent strain increase under constant stress.
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Relaxation: Time-dependent stress decrease under constant strain.
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Influencing factors:
- Stress levels
- Temperature and moisture content
-
Recommendations to minimize creep/relaxation:
- Utilize high-strength steel.
- Design with minimal stress concentrations.
- Implement moisture protection.
Temperature Effects on Materials
-
High Temperature
- Reduces steel’s yield strength, impacting load capacity.
- Prestressed concrete has good fire resistance but can sustain damage, depending on fire severity and structural design.
-
Low Temperature
- Increases steel stress relaxation and delamination risks.
Corrosion of Prestressing Steel
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Common corrosion sources:
- Chloride ions (seawater, deicing salts)
- Carbonation (reaction with CO2)
- Hydrogen sulfide (from industrial emissions, sewage)
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Types of corrosion effects:
- Stress corrosion cracking occurs under high stress and chloride presence.
- Hydrogen embrittlement due to exposure to high hydrogen levels.
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Consequences of corrosion:
- Loss of strength, reducing structure load capacity and failure risk.
- Concrete cracks allowing infiltration of corrosive agents.
- Potential fracture leading to structural failure.
Concrete Properties and Challenges
- Concrete Cracking: Facilitates water and corrosive ingress, accelerating steel corrosion.
- Strength and modulus of elasticity are crucial:
- Strength defines maximum load capacity.
- Modulus relates to deformation under load.
Shrinkage and Creep in Concrete
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Shrinkage: Volume reduction during drying.
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Creep: Increased strain under sustained loading.
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Influencing factors on shrinkage and creep:
- Water-to-cement ratio
- Age of concrete
- Moisture content
- Temperature
- Sustained loading conditions
-
Methods to reduce shrinkage and creep:
- Low water-to-cement ratio use.
- Admixture incorporation.
- Proper curing techniques.
- Structural design considerations.
Temperature-Induced Movement in Concrete
- Movements include expansion and contraction with temperature shifts and resultant shear and bending stresses.
- Mitigation strategies:
- Choose materials with low thermal expansion coefficients.
- Implement thermal breaks.
- Design to minimize thermal stress risks.
- Use control joints.
Durability Factors
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Durability is the ability to withstand environmental and destructive influences.
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Key factors affecting durability:
- Water exposure
- Abrasion resistance
- Chlorides and carbonation effects
- Sulphate susceptibility
- Freeze-thaw cycles
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Enhancements to durability:
- Low water-to-cement ratio.
- Effective curing and protective measures against environmental challenges.
Admixtures in Concrete
- Admixtures enhance properties of concrete during mixing.
- Common types:
- Water-reducing: Decrease water required for workability.
- Air-entraining: Introduce air bubbles for increased workability and durability.
- Cementitious: React with cement to improve strength and durability.
- Retarding and accelerating: Alter setting times.
- Waterproofing: Reduce permeability.
- Corrosion inhibitors: Protect steel reinforcement.
Structural Lightweight Concrete
- Valued for applications requiring reduced weight, improved thermal insulation, and enhanced workability.
- Applications include building structures, industrial uses, and civil engineering projects.
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