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Prestressed Concrete
Materials for Prestressing (1) Prestressing steel
Instructor:
Dr. Sawsan Alkhawaldeh
Department of Civil Engineering
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Non-Prestressing steel
• Plain and deformed wires and bars used for RC.
• The most important properties are: Young’s modulus 𝐸𝑐, Yield strength 𝑓𝑦, Ultimate strength
𝑓𝑢, grade and diameter.
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Properties of standard reinforcing bars (Non-Prestressing steel)
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Typical stress-strain curve of nonprestressing (reinforcing) steel
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Prestressing steel
For prestressed concrete, high strength steel is used to provide high elongation to maintain the steel stress after the prestress loss. Ideally, it should also:
• Remain elastic up to relatively high stress.
• Show sufficient ductility before failure.
• Have good bonding properties, low relaxation, good resistance to corrosion.
• Be economical and easy to handle.
Prestressing reinforcement can be in the form of single wires, strand composed of several wires twisted to form single element, and high strength bars.
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Types of prestressing steel
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Seven-wire standard strand properties
The most commonly used is the 0.5 in diameter strand with minimum tensile strength of 270 ksi (1860 Mpa)
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Most common prestressing steel types
Prestressing reinforcements can be classified as:
• Stress-relieved and low relaxation wires.
• Stress-relieved and low relaxation strands, standard or compacted.
• High tensile steel bars, they must be cold drawn to raise their yield strength and stress relived to increase the ductility.
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Stress-strain curve comparison
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Mechanical properties of Prestressing steel
The main characteristics of prestressing steel are:
• Yield strength (𝑓𝑝𝑦), it’s not well defined and
determine according to a strain criteria.
• Modulus of elasticity (𝐸𝑝𝑠), It’s independent of
the strength and calculated as the slope of the linear elastic portion of the curve.
• Minimum required tensile strength or breaking stress (𝑓𝑝𝑢), the most commonly used strands
are those with an 𝑓𝑝𝑢 value of 1860 MPa.
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Mechanical properties of prestressing steel
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Minimum properties of Prestressing Tendons
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Relaxation of prestressing steel
• It is a loss of prestress when the wires and strands are subjected to essentially constant strain.
• It is identical to creep in concrete, except that creep is a change in strain while steel relaxation is a loss in steel stress.
• Relaxation is expressed as:
∆𝑓𝑅= 𝑓𝑝𝑖log 𝑡
10
𝑓𝑝𝑖
𝑓𝑝𝑦− 0.55 for stress-relieved wires and strands
∆𝑓𝑅= 𝑓𝑝𝑖log 𝑡
45
𝑓𝑝𝑖
𝑓𝑝𝑦− 0.55 for low-relaxation wires and strands
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Relaxation loss vs. time
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Relaxation affecting factors
Steel relaxation is dependent on the type and grade of steel and affected by the initial stress and the temperature.
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Effect of temperature
• The mechanical properties of prestressing steel; ultimate strength, yield stress and elastic modulus; are not affected when ambient temperature varies over a reasonable range.
• Extreme temperature conditions can seriously affect these properties; a sharp decrease in temperature might lead to improvements in strength and modulus, but deterioration in ductility and impact resistance.
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Effect of corrosion
• The magnitude and consequences of corrosion in prestressing steel are much more sever than in reinforcing steel due to that high-grade is more susceptible to corrosion and the diameter of prestressing tendons are relatively small.
• Even a small uniform corrosive layer or corroded spot can reduce the cross sectional area, induce stress concentrations, and eventually lead to premature failure.