Steel in Structural Design

Steel

• Steel is a common material used in prestressed concrete structures.
• Steel has good tensile strength, but can be susceptible to fatigue and creep.
• Fatigue is the breakdown of material due to repetitive stress cycles.
• Creep is the gradual deformation of material under constant stress over time.

Fatigue

• Fatigue is a major consideration in structural design.
• The endurance limit of steel is the stress level below which material can withstand indefinite load cycles without failure.
• Low-carbon steel has an endurance limit around 40% of its ultimate tensile strength.
• Alloy steel has a higher endurance limit compared to low-carbon steel.
• Cast iron has a lower endurance limit compared to steel.
• Factors influencing fatigue resistance include type of steel, stress level, load cycles, stress concentrations, and environment.
• To mitigate fatigue, use high-quality materials, minimize stress concentrations, avoid overloading, and protect from corrosion.

Creep & Relaxation

• Creep is the time-dependent deformation of a material under constant stress.
• Relaxation is the time-dependent decrease in stress under constant strain.
• Factors influencing creep and relaxation include stress level, temperature, and moisture content.
• To minimize creep and relaxation, use high-strength steel, reduce stress concentrations, avoid overloading, and protect from moisture.

Effects of High Temperature

• High temperatures can reduce the strength of steel by lowering its yield strength.
• Prestressed concrete structures have some fire resistance, but the steel can still be damaged by fire.
• Damage severity depends on the fire intensity and structural design.

Effects of Low Temperature

• Low temperatures can increase steel stress relaxation.
• Low temperatures can also increase the risk of delamination in concrete structures.

Corrosion of Prestressed Steel

• Prestressed steel is vulnerable to corrosion caused by chloride ions from sources like seawater and deicing salts.
• Carbonation, the reaction of carbon dioxide with concrete's calcium hydroxide, can also lead to corrosion.
• Hydrogen sulfide from industrial emissions, sewage treatment, and decaying organic matter can contribute to corrosion.
• Stress corrosion cracking occurs when high stress and chloride ions are present.
• Hydrogen embrittlement happens when prestressed steel is exposed to high hydrogen levels.

Effects of Corrosion

• Corrosion can reduce steel strength, impairing the structure's load-carrying capacity and increasing failure risk.
• Corrosion can cause cracks in concrete, allowing water and corrosive elements to further accelerate corrosion.
• Corrosion can lead to steel fracture, potentially causing structural collapse.

Concrete

• Concrete is a primary component of prestressed concrete structures.
• Concrete cracking can introduce pathways for water and corrosive agents, accelerating steel corrosion.
• Concrete's strength determines its load capacity, while its modulus of elasticity controls its deformation under load.

Shrinkage & Creep

• Shrinkage is the volumetric reduction of concrete as it dries and hardens.
• Creep is the increase in strain of concrete under sustained load.
• Factors influencing shrinkage and creep include:
  • Concrete type: Higher water-to-cement ratio increases shrinkage and creep.
  • Age: Shrinkage and creep increase with concrete's age.
  • Moisture: Dry concrete experiences more shrinkage and creep than wet concrete.
  • Temperature: High temperatures lead to more shrinkage and creep.
  • Loading conditions: Concrete under sustained load exhibits increased shrinkage and creep.

Reducing Shrinkage & Creep

• Reduce water-to-cement ratio.
• Use admixtures.
• Implement proper concrete curing.
• Design the structure to minimize shrinkage and creep.

Predicting Shrinkage & Creep

• Theoretical models are based on the understanding of shrinkage and creep mechanisms.
• Empirical models rely on experimental data.
• Hybrid models combine theoretical and empirical approaches.

Temperature

• Temperature changes cause movement in concrete structures.
• Expansion occurs when concrete heats up, and contraction happens when it cools.
• Temperature variations can induce shear stresses and bending moments in concrete structures.

Reducing Temperature-Induced Movements

• Use concrete with a low coefficient of thermal expansion.
• Incorporate thermal breaks.
• Design the structure to minimize thermal stresses.
• Utilize control joints.

Durability

• Durability is a material's resistance to weathering and destructive forces.
• Factors affecting concrete durability include:
  • Water exposure
  • Chlorides
  • Carbonation
  • Sulphates
  • Freeze-thaw cycles
  • Abrasion

Enhancing Concrete Durability

• Use a low water-to-cement ratio.
• Employ admixtures.
• Implement proper concrete curing.
• Protect the concrete from the environment.

Admixtures

• Admixtures are materials added to concrete during mixing to improve its properties.
• Common admixture types include:
  • Water-reducing admixtures: Enhance workability by reducing water requirements.
  • Air-entraining admixtures: Introduce air bubbles into the concrete mix.
  • Cementitious admixtures: React with cement to improve strength and durability.
  • Retarding admixtures: Slow down concrete's setting time.
  • Accelerating admixtures: Speed up concrete's setting time.
  • Waterproofing admixtures: Reduce concrete's permeability.
  • Corrosion inhibitors: Protect steel reinforcement from corrosion.

Structural Lightweight Concrete

• Structural lightweight concrete is a versatile material with applications in buildings, industrial facilities, and civil engineering projects.
• It's advantageous where weight, thermal insulation, or workability are crucial considerations.
• Applications include:
  • Building structures
  • Industrial applications
  • Civil engineering projects