Prestressed Concrete Design CE 414 Lesson 2

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.

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.