A double acting hydraulic jack with a A double acting hydraulic jack - PDF document

Prestressed Concrete Hashemite University The Hashem ite University Departm ent of Civil Engineering Lecture Lecture 2 2. .1 1 Methods Methods of of Prestressing Prestressing Dr. Hazim Dwairi Dr Hazim Dwairi Dr Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Advantages of Prestressing Advantages of Prestressing • Section remains uncracked under service loads Section remains uncracked under service loads � Reduction of steel corrosion (increase durability) � Reduction of steel corrosion (increase durability) Reduction of steel corrosion (increase durability) Reduction of steel corrosion (increase durability) � Full section is utilized (Higher moment of inertia, Full section is utilized (Higher moment of inertia, higher stiffness, Less deformations. higher stiffness, Less deformations. � Increase in shear capacity Increase in shear capacity � Suitable for use in pressure vessels, liquid retaining Suitable for use in pressure vessels, liquid retaining structures structures � Improved performance (resilience) under dynamic Improved performance (resilience) under dynamic and fatigue loading. and fatigue loading. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 1

Prestressed Concrete Hashemite University Advantages of Prestressing Advantages of Prestressing • High span High span- -to to- -depth ratios depth ratios � Larger spans possible with prestressing (bridges � Larger spans possible with prestressing (bridges, Larger spans possible with prestressing (bridges, Larger spans possible with prestressing (bridges buildings with large column- buildings with large column -free spaces) free spaces) � Typical values of span Typical values of span- -to to- -depth ratios in slabs are depth ratios in slabs are given below. given below. � For the same span, less depth compared to RC For the same span, less depth compared to RC member. member. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Advantages of Prestressing Advantages of Prestressing • Suitable for precast construction Suitable for precast construction � Rapid construction � Rapid construction Rapid construction Rapid construction � Better quality control Better quality control � Reduced maintenance Reduced maintenance � Suitable for repetitive construction Suitable for repetitive construction � Multiple use of formwork Multiple use of formwork � Availability of standard shapes. � Availability of standard shapes Availability of standard shapes Availability of standard shapes. • The following figure shows the common types of The following figure shows the common types of precast sections. precast sections. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 2

Prestressed Concrete Hashemite University Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Disadvantages of Prestressing Disadvantages of Prestressing • Prestressing needs skilled technology. Hence, it Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete. is not as common as reinforced concrete. is not as common as reinforced concrete is not as common as reinforced concrete • The use of high strength materials is costly. The use of high strength materials is costly. • There is additional cost in auxiliary equipments. There is additional cost in auxiliary equipments. • There is need for quality control and inspection. There is need for quality control and inspection. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 3

Prestressed Concrete Hashemite University Types of Prestressing Types of Prestressing • External or internal prestressing External or internal prestressing � This classification is based on the location of the � This classification is based on the location of the This classification is based on the location of the This classification is based on the location of the prestressing tendon with respect to the concrete prestressing tendon with respect to the concrete section. section. • Pre Pre- -tensioning or post tensioning or post- -tensioning tensioning � This is the most important classification and is based This is the most important classification and is based on the sequence of casting the concrete and applying on the sequence of casting the concrete and applying tension to the tendons. tension to the tendons. • Linear or circular prestressing Linear or circular prestressing � This classification is based on the shape of the This classification is based on the shape of the member prestressed. member prestressed. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Types of Prestressing Types of Prestressing • Full, limited or partial prestressing Full, limited or partial prestressing � Based on the amount of prestressing force, three � Based on the amount of prestressing force three Based on the amount of prestressing force, three Based on the amount of prestressing force three types of prestressing are defined. types of prestressing are defined. • Uniaxial, biaxial or multi Uniaxial, biaxial or multi- -axial prestressing axial prestressing � As the names suggest, the classification is based on As the names suggest, the classification is based on the directions of prestressing a member. the directions of prestressing a member. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 4

Prestressed Concrete Hashemite University Methods of Prestressing Methods of Prestressing • Mechanical jacking of tendons, very popular Mechanical jacking of tendons, very popular • Th • Thermal prestressing by application of electric Th Thermal prestressing by application of electric l l t t i i b b li li ti ti f f l l t i t i heat. heat. • Pre Pre- -bending high strength steel beam and bending high strength steel beam and encasing its tensile flange with concrete. encasing its tensile flange with concrete. • Chemical prestressing by means of expansive Chemical prestressing by means of expansive cement which expands chemically after setting cement which expands chemically after setting t t hi h hi h d d h h i i ll ll ft ft tti tti and during hardening, known as self stressing. and during hardening, known as self stressing. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Form s of Prestressing Steel Form s of Prestressing Steel • Wires: Wires: � Prestressing wire is a single unit made of steel � Prestressing wire is a single unit made of steel. Prestressing wire is a single unit made of steel Prestressing wire is a single unit made of steel. • Strands Strands: : � Two, three or seven wires are wound to form a Two, three or seven wires are wound to form a prestressing strand. prestressing strand. • Tendon Tendon: : � A group of strands or wires are wound to form a A group of strands or wires are wound to form a prestressing tendon prestressing tendon ز t t i i t t d d ز • Cable Cable: : � A group of tendons form a prestressing cable. A group of tendons form a prestressing cable. • High High- -strength Bars. strength Bars. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 5

Prestressed Concrete Hashemite University Pretensioning Pretensioning • Steel tensioned before casting the concrete. Steel tensioned before casting the concrete. Strands are tensioned concrete is cast around Strands are tensioned concrete is cast around Strands are tensioned, concrete is cast around Strands are tensioned, concrete is cast around the strands, then the strands released when the strands, then the strands released when concrete attains required strength. Prestressing concrete attains required strength. Prestressing force introduced into concrete by bond force introduced into concrete by bond � Anchoring of tendons against the end abutments Anchoring of tendons against the end abutments � Placing of jacks � Placing of jacks Placing of jacks Placing of jacks � Applying tension to the tendons Applying tension to the tendons � Casting of concrete Casting of concrete � Cutting of the tendons. Cutting of the tendons. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Pretensioning Pretensioning Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Methods of Prestressing Methods of Prestressing Dr. Hazim Dwairi 6