Prestressed Concrete Design CE 414 Lesson 2

Questions and Answers

What occurs when a material is subjected to repeated cycles of stress or strain?

Creep

Fracture

Fatigue (correct)

Yielding

Which material has an endurance limit of about 0.4 times its ultimate tensile strength?

Low-carbon steel (correct)

High-carbon steel

Cast iron

Alloy steel

What term describes material deformation under a constant load over time?

Creep (correct)

Ductility

Fatigue

Elasticity

Which type of steel generally has a higher endurance limit than low-carbon steel?

Alloy steel Signup and view all the answers

What factor can influence the endurance limit of cast iron?

Type of casting process Signup and view all the answers

What is the equivalent of 1 MPa in psi?

145 psi Signup and view all the answers

Which of the following factors does NOT influence the endurance limit of steel?

Soil type Signup and view all the answers

Which material typically has a lower endurance limit than steel?

Cast iron Signup and view all the answers

What is creep in prestressed concrete?

An increase in strain over time under constant stress Signup and view all the answers

What design consideration helps minimize the effects of creep?

Minimizing stress concentrations Signup and view all the answers

What is a primary effect of high temperature on prestressed concrete?

Loss of steel strength Signup and view all the answers

What is the consequence of low temperatures on prestressed concrete?

Increased steel stress relaxation Signup and view all the answers

Which of the following is a source of chloride ions that contributes to corrosion?

Seawater Signup and view all the answers

What process occurs when prestressed steel is subjected to high levels of hydrogen?

Hydrogen embrittlement Signup and view all the answers

What is one effective strategy to protect prestressed concrete from corrosion?

Using high-quality materials Signup and view all the answers

Which factor does NOT affect creep and relaxation in prestressed concrete?

Material density Signup and view all the answers

What is one consequence of corrosion in prestressing steel?

Formation of cracks in the concrete Signup and view all the answers

How does strength relate to a structure's load capacity?

It determines the maximum load the structure can withstand Signup and view all the answers

What is shrinkage in concrete?

Reduction in volume as it dries and hardens Signup and view all the answers

Which factor contributes to the increased shrinkage and creep in concrete?

Increased age of the concrete Signup and view all the answers

What happens to concrete under sustained loading?

It shrinks and creeps more Signup and view all the answers

What is creep in the context of concrete?

Increase in strain under sustained loading Signup and view all the answers

How does high temperature affect concrete?

It increases shrinkage and creep Signup and view all the answers

What typically accelerates corrosion of prestressing steel in concrete?

Water and corrosive agents penetrating cracks Signup and view all the answers

Which of the following methods can help reduce shrinkage and creep in concrete?

Using admixtures Signup and view all the answers

What is the primary focus of theoretical models used to predict shrinkage and creep?

Understanding physical mechanisms Signup and view all the answers

What can cause temperature movements in concrete structures?

Thermal expansion and contraction Signup and view all the answers

Which of the following is a way to improve the durability of concrete?

Curing the concrete properly Signup and view all the answers

What type of model combines both theoretical and empirical approaches to predict shrinkage and creep?

Hybrid model Signup and view all the answers

Which option can effectively minimize thermal movements in concrete structures?

Incorporating a thermal break Signup and view all the answers

Which factor does NOT affect the durability of concrete?

Surface color Signup and view all the answers

Using a low water-to-cement ratio can help reduce which of the following in concrete?

Shrinkage and creep Signup and view all the answers

What is the primary function of water-reducing admixtures in concrete?

To reduce the amount of water needed for a workable mix. Signup and view all the answers

Which type of admixture is designed to slow down the setting time of concrete?

Retarding admixtures Signup and view all the answers

What is an air-entraining admixture primarily used for?

To introduce tiny air bubbles into the concrete mix. Signup and view all the answers

Which of the following is NOT a common application of structural lightweight concrete?

Tunnel construction Signup and view all the answers

Which admixture can help reduce the permeability of concrete?

Waterproofing admixtures Signup and view all the answers

What is a key benefit of using structural lightweight concrete?

Lower weight for easier handling. Signup and view all the answers

Cementitious admixtures are added to concrete to primarily achieve which benefit?

Enhance strength and durability. Signup and view all the answers

Which type of admixture protects steel reinforcement in concrete?

Corrosion inhibitors Signup and view all the answers

Study Notes

Steel Properties in Prestressed Concrete

Fatigue Resistance: Defined as the breakdown of material under repeated cycles of stress leading to fracture.
Creep: A time-dependent deformation of materials under sustained load, potentially causing sudden fractures.
Endurance Limits:
- Low-carbon steel: About 0.4 times its ultimate tensile strength (UTS).
- Alloy steel: Generally has a higher endurance limit than low-carbon steel, varying with alloying elements and heat treatment.
- Cast iron: Typically shows a lower endurance limit than steel, influenced by type and casting process.

Design Considerations for Fatigue and Creep

Factors Affecting Fatigue: Type of steel, stress level, load cycles, stress concentrations, and environmental conditions.
Resistance Strategies:
- Use of high-quality materials to enhance durability.
- Designing structures to minimize stress concentrations.
- Avoiding overload scenarios and protecting from corrosion.

Creep and Relaxation

Definitions:
- Creep: Increasing strain under constant stress over time.
- Relaxation: Decrease in stress under constant strain over time.
Contributing Factors: Stress level, temperature, and moisture content.
Minimization Techniques: High-strength steel, careful design to avoid excess stress, and moisture protection.

Effects of Temperature on Prestressed Concrete

High Temperature: Causes loss of yield strength and can damage structures despite good fire resistance.
Low Temperature: Leads to increased stress relaxation and risk of delamination.

Corrosion Issues in Prestressing Steel

Corrosive Influences:
- Chloride ions from seawater and deicing salts.
- Carbonation from reactions with carbon dioxide.
- Hydrogen sulfide from industrial sources.
Damage Mechanisms:
- Stress corrosion cracking from high stress and chloride ions exposure.
- Hydrogen embrittlement from hydrogen exposure.
- Potential loss of strength, concrete cracking, and catastrophic structural failure.

Concrete Properties in Prestressed Structures

Common Failure Modes: Concrete cracking allowing corrosive infiltration.
Strength vs. Modulus of Elasticity: Strength indicates load capacity; modulus of elasticity denotes deformation under load.

Shrinkage and Creep in Concrete

Definitions:
- Shrinkage: Reduction in concrete volume as it dries.
- Creep: Increase in strain under sustained loading.
Influencing Factors:
- Water-to-cement ratio, age of concrete, moisture content, temperature, and sustained loading conditions.
Reduction Strategies:
- Low water-to-cement ratio, use of admixtures, proper curing, and minimizing structural design impacts.

Temperature Effects on Structural Movements

Common Movements:
- Expansion and contraction with heat and cold.
- Shear stresses and bending moments resulting from temperature fluctuations.
Mitigation Techniques: Use low thermal expansion concrete, thermal breaks, structure design to minimize stress, and control joints.

Durability of Concrete

Definition: Durability reflects a material's resistance to weathering and destructive forces.
Durability Threats: Water, abrasion, chlorides, carbonation, sulfates, and freeze-thaw cycles.
Enhancement Methods: Utilize low water-to-cement ratios, admixtures, proper curing techniques, and environmental protection.

Admixtures in Concrete

Purpose: Materials added to improve concrete properties during mixing.
Types of Admixtures:
- Water-reducing: Decreases water needed for workability.
- Air-entraining: Introduces air bubbles to enhance properties.
- Cementitious: React with cement to enhance strength and durability.
- Retarding and accelerating: Adjust setting times as required.
- Waterproofing and corrosion inhibitors: Improve impermeability and protect steel reinforcement.

Structural Lightweight Concrete

Definition and Use: A versatile material ideal for applications requiring reduced weight, better thermal insulation, and improved workability.
Common Applications: Building structures, industrial, and civil engineering projects.

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Description

This quiz focuses on Prestressed Concrete Materials covered in CE 414. Understanding these materials is crucial for effective design and implementation in civil engineering projects. Prepare to test your knowledge on the specifics of prestressed concrete and its applications.