Prestressed Concrete Materials PDF

BICOL UNIVERSITY COLLEGE OF ENGINEERING LEGAZPI CITY CE 414 PRESTRESSED CONCRETE DESIGN...

BICOL UNIVERSITY COLLEGE OF ENGINEERING LEGAZPI CITY CE 414 PRESTRESSED CONCRETE DESIGN LESSON 2: PRESTRESSED CONCRETE MATERIALS PREPARED BY: HANA MYKA C. GATON, MSCE FACULTY HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN  STEEL HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE Fatigue - when a material is subjected to repeated cycles of stress or strain and its structure breaks down and ultimately leads to fracture Creep - when a material is subjected to a load for a very long time it may continue to deform until a sudden fracture occurs HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE 1 MPa = 10 6 Pa = 1 N/mm 2 = 145.0 psi (lbf/in 2 ) HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE Low-carbon steel Has an endurance limit of about 0.4 times its ultimate tensile strength (UTS). Alloy steel Has a higher endurance limit than low-carbon steel. The endurance limit of alloy steel can vary depending on the alloying elements and the heat treatment. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE Cast iron Has a lower endurance limit than steel. The endurance limit of cast iron can vary depending on the type of cast iron and the casting process. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE Type of steel reinforcement Stress level Number of load cycles Presence of stress concentrations Environment HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN RESISTANCE TO FATIGUE Use high-quality materials Design the member to minimize stress concentrations Avoid overloading Protect the member from corrosion HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CREEP & RELAXATION Creep is the time-dependent increase in strain under constant stress. Relaxation is the time-dependent decrease in stress under constant strain. Stress level Temperature Moisture content HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CREEP & RELAXATION Design considerations to minimize creep and relaxation: Use high-strength steel Design the member to minimize stress concentrations Avoid overloading Protect the member from moisture HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN EFFECTS OF HIGH TEMPERATURE 1. Loss of strength The steel loses strength due to the reduction of its yield strength. 2. Fire resistance Prestressed concrete structures have good fire resistance, but they can still be damaged by fire. The steel may lose strength and may corrode. The extent of the damage will depend on the severity of the fire and the design of the structure. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN EFFECTS OF LOW TEMPERATURE Increased steel stress relaxation Increased risk of delamination HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CORROSION OF PRESTRESSING STEEL Chloride ions - can come from a variety of sources, including seawater, deicing salts Carbonation - reaction of carbon dioxide with the calcium hydroxide in concrete Hydrogen sulfide - can come from a variety of sources, including industrial emissions, sewage treatment plants, and decaying organic matter. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CORROSION OF PRESTRESSING STEEL Stress corrosion cracking - prestressed steel is subjected to high levels of stress and chloride ions Hydrogen embrittlement - prestressed steel is subjected to high levels of hydrogen HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CORROSION OF PRESTRESSING STEEL 1. Loss of strength This can reduce the load-carrying capacity of the structure and increase the risk of failure. 2. Cracks in the concrete These cracks can allow water and other corrosive agents to penetrate the concrete and further accelerate the corrosion of the steel. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CORROSION OF PRESTRESSING STEEL 3. Fracture of the steel This can cause the structure to collapse. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN  CONCRETE HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN CONCRETE Failure in prestressed concrete structures: Concrete cracking Cracks can allow water and other corrosive agents to penetrate the concrete and further accelerate the corrosion of the steel. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN Relation between strength and modulus of elasticity Strength determines the maximum load that a structure can withstand Modulus of elasticity determines how much the structure will deform under load. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP Shrinkage is the reduction in volume of concrete that occurs when it dries and hardens. Creep is the increase in strain in concrete that occurs under sustained loading. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP Type : Concrete with a high water-to-cement ratio will shrink and creep more than concrete with a low water-to- cement ratio. Age : Shrinkage and creep increase with the age of the concrete. Moisture content : Concrete that is dry will shrink and creep more than concrete that is wet. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP Temperature: Concrete that is exposed to high temperatures will shrink and creep more than concrete that is exposed to low temperatures. Loading conditions: Concrete that is subjected to sustained loading will shrink and creep more than concrete that is not subjected to sustained loading. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP Ways to reduce the amount of shrinkage and creep 1. Using a low water-to-cement ratio 2. Using admixtures 3. Curing the concrete properly 4. Designing the structure to minimize shrinkage and creep HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN SHRINKAGE & CREEP Methods used to predict shrinkage and creep 1. Theoretical models: Theoretical models are based on the understanding of the physical mechanisms of shrinkage and creep. 2. Empirical models: Empirical models are based on experimental data. 3. Hybrid models: Hybrid models combine theoretical and empirical approaches. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN TEMPERATURE Movements in concrete structures 1. Expansion: Concrete expands when it is heated and contracts when it is cooled. 2. Shear: Temperature changes can cause shear stresses in concrete structures. 3. Bending: Temperature changes can cause bending moments in concrete structures. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN TEMPERATURE Ways to reduce the amount of movements in concrete structures Using a concrete with a low coefficient of thermal expansion Using a thermal break Designing the structure to minimize thermal stresses Using a control joint HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN DURABILITY Durability is the ability of a material to resist weathering or other destructive influences. Factors that can affect the durability of concrete Water Abrasion Chlorides Carbonation Sulphates Freeze-thaw cycles HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN DURABILITY Things that can be done to improve the durability of concrete Using a low water-to-cement ratio Using admixtures Curing the concrete properly Protecting the concrete from the environment HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN ADMIXTURES Admixtures are materials that are added to concrete during mixing to improve its properties. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN ADMIXTURES Common types of admixtures: 1. Water-reducing admixtures: These admixtures reduce the amount of water required to produce a workable concrete mix. 2. Air-entraining admixtures: These admixtures introduce tiny air bubbles into the concrete mix. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN ADMIXTURES 3. Cementitious admixtures: These admixtures react with the cement in the concrete mix to improve its strength and durability. 4. Retarding admixtures: These admixtures slow down the setting time of the concrete mix. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN ADMIXTURES 5. Accelerating admixtures: These admixtures speed up the setting time of the concrete mix. 6. Waterproofing admixtures: These admixtures reduce the permeability of the concrete mix. 7. Corrosion inhibitors: These admixtures protect the steel reinforcement in concrete from corrosion. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN STRUCTURAL LIGHTWEIGHT CONCRETE Structural lightweight concrete is a versatile material that can be used in a variety of applications. It is a good choice for applications where weight, thermal insulation, or workability are important considerations. HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN STRUCTURAL LIGHTWEIGHT CONCRETE Applications 1. Building structures 2. Industrial applications 3. Civil engineering applications HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN  THANK YOU FOR LISTENING HANA MYKA C. GATON, MSCE FACULTY CE 414 – PRESTRESSED CONCRETE DESIGN