Pre-Stressed Steel: Part One

Abstract:

Are you interested in learning more about prestressing steel strand? Contact us today to secure an expert consultation!

Pre-stressing raises both the quality and the resistance to tension and compression characteristics of the steel; the technique actually manages to create a state of co-action in which the tensions and deformations are opposed to those induced by the loads which will subsequently act upon the structure. It also raises the resistance to tension of reinforced concrete which is, in fact, negligible.

Pre-Stressed Steel: Part One

Introduction

Although seemingly recent, pre-stressed steel is a material whose origins date back a long way. The adoption of the technique of pre-stressing is attributed to Paxton, who in 1851 utilized this technique for the realization of the Crystal Palace, unaware of the great discovery he had made.

Koenen was the first to propose pre-stressing steel bars. He suggested doing this in 1907, before applying concrete, in order to avoid the formation of cracks and thus stumbled across the innovation of reinforced concrete (R.C.). Unfortunately however, his attempts failed because at that time the phenomena of fluage and shrinkage were unknown. In fact, the real “father” of pre-stressing is Eugène Freyssinet, who in 1928 defined pre-stressing as a technique which consists in subjecting a material, in his case reinforced concrete, to loads which produce stresses opposed to those in operation, through the use of cables which have first been laid in the stressed mass.

The reasons which gave rise to this material may be found in the mechanical characteristics of concrete which, in fact, shows great ability to absorb forces of compression but a low resistance to tension which is allowed to be absorbed by the metallic reinforcement. The latter, however, under the effect of tension tends to lengthen and, on account of the phenomenon of bonding, pulls the concrete along with it.

Consequently, if the stresses of tension are high, the concrete will crack. The cracks do not destabilize the structure but could lead to possible further deformation and expose the reinforcement to the danger of oxidization which in turn produces a reduction of its own resistance. It can be deducted that R.C. can tolerate loads up until the cracking limit.

Unlike R.C., steel is a material which has high resistance both to tension and to compression. As a consequence, by making a comparison between pre-stressed steel and reinforced concrete, we can immediately note that in the first place, this technique further raises both the quality and the resistance to tension and compression characteristics of the steel; the technique actually manages to create a state of co-action in which the tensions and deformations are opposed to those induced by the loads which will subsequently act upon the structure. In the second place it raises the resistance to tension of reinforced concrete which is, in fact, negligible.

Forms of Pre-stressing Steel

The development of pre-stressed concrete was influenced by the invention of high strength steel. It is an alloy of iron, carbon, manganese and optional materials. In addition to pre-stressing steel, conventional non-pre-stressed reinforcement is used for flexural capacity (optional), shear capacity, temperature and shrinkage requirements.

Wires. A pre-stressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different types of wires are as follows:
1) Plain wire: No indentations on the surface.
2) Indented wire: There are circular or elliptical indentations on the surface.

Strands. A few wires are spun together in a helical form to form a pre-stressing strand. The different types of strands are as follows:
1) Two-wire strand: Two wires are spun together to form the strand.
2) Three-wire strand: Three wires are spun together to form the strand.
3) Seven-wire strand: In this type of strand, six wires are spun around a central wire. The central wire is larger than the other wires.

Tendons. A group of strands or wires are placed together to form a pre-stressing tendon. The tendons are used in post-tensioned members. The following figure shows the cross section of a typical tendon. The strands are placed in a duct which may be filled with grout after the post-tensioning operation is completed (Figure 1).

Figure 1: Cross-Section of a typical tendon

Cables. A group of tendons form a pre-stressing cable. The cables are used in bridges.

Bars. A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire. Bars are available in the following sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm.

Figure 2 shows the different forms of pre-stressing steel.

Figure 2: Forms of reinforcing and pre-stressing steel

Types of Pre-stressing Steel

The steel is treated to achieve the desired properties. The following are the treatment processes:

• Cold working (cold drawing) is being done by rolling the bars through a series of dyes. It re-aligns the crystals and increases the strength.
• Stress relieving is being done by heating the strand to about 350°C and cooling slowly. This reduces the plastic deformation of the steel after the onset of yielding.
• Strain tempering for low relaxation is being done by heating the strand to about 350°C while it is under tension. This also improves the stress-strain behavior of the steel by reducing the plastic deformation after the onset of yielding. In addition, the relaxation is reduced.

Properties of Pre-stressing Steel

The steel in pre-stressed applications has to be of good quality. It requires the following attributes:
1) High strength
2) Adequate ductility
3) Bendability, which is required at the harping points and near the anchorage
4) High bond, required for pre-tensioned members
5) Low relaxation to reduce losses
6) Minimum corrosion.

The tensile strength of pre-stressing steel is given in terms of the characteristic tensile strength (fpk). The characteristic strength is defined as the ultimate tensile strength of the coupon specimens below which not more than 5% of the test results are expected to fall.

Figure 3(a): Test set-up

Figure 3(b): Testing of tensile strength of pre-stressing strand

The minimum tensile strengths for different types of wires as specified by the standard codes are given.

Table 1: Cold Drawn Stress-Relieved Wires (IS: 1785 Part 1). The proof stress should not be less than 85% of the specified tensile strength.

Table2: As-Drawn wire (IS: 1785 Part 2). The proof stress should not be less than 75% of the specified tensile strength.

Table 3: Indented wire (IS: 6003). The proof stress should not be less than 85% of the specified tensile strength.

The minimum tensile strength of high tensile steel bars according to IS:2090 is 980 N/mm2. The proof stress should not be less than 80% of the specified tensile strength.

The stiffness of pre-stressing steel is given by the initial modulus of elasticity. The modulus of elasticity depends on the form of pre-stressing steel (wires or strands or bars). IS:1343 - 1980 provides the following guidelines which can be used in absence of test data.

If you are looking for more details, kindly visit Titanium Carbide Bars.

Additional resources:
Which Is Better Galvalume Or Galvanized: A Comparison of Two Common Metal Coating Options
Ultimate Protection: PE Anticorrosion Wrap Tape Review

Table 4: Modulus of elasticity (IS: 1343 - 1980)

Date Published: Aug-2008

TRY FOR FREE

Search Knowledge Base

Enter a phrase to search for:

Search by

Full text
Keywords

Headings
Abstracts

This article belongs to a series of articles. You can click the links below to read more on this topic.

Finding stress-strain diagrams in the Total Materia database

The Total Materia Extended Range includes a unique collection of stress-strain curves and diagrams for calculations in the plastic range for thousands of metal alloys, heat treatments and working temperatures. Both true and engineering stress curves are given for various strain rates where applicable.

Finding a stress-strain graph in the database is simple and takes only seconds.

Enter the material of interest into the quick search field. You can optionally narrow your search by specifying the country/standard of choice in the designated field and click Search.

After selecting the material of interest to you, click on the Stress-Strain diagrams link to view data for the selected material. The number of available stress-strain diagrams is displayed in brackets next to the link.

Because Total Materia stress-strain curves are neutral across standard specifications, you can review stress-strain diagrams by clicking the appropriate link for any of the subgroups.

Besides the stress-strain curves at different temperatures, stress and strain data are given in a tabular format which is convenient for copying to, for example, a CAE software.

It is also possible to view stress-strain curves and data for other working temperatures.

To do this, simply insert a new temperature into the ‘Enter temperature’ field within the defined range.

After clicking the Calculate button, a new curve is plotted and values in the table now correspond to the temperature you have defined. See example below for 250°C.

For you’re a chance to take a test drive of the Total Materia database, we invite you to join a community of over 150,000 registered users through the Total Materia Free Demo.

The overall reason to pre-stress steel is that it raises both the quality and the resistance to tension and compression characteristics of the steel.

Although apparently recent, pre-stressed steel is a material whose origins date back a long way. The adoption of the technique of pre-stressing is attributed to Paxton, who in 1851 utilised this technique for the realisation of the Crystal Palace, unaware of the great discovery he had made.

Koenen was the first to propose pre-stressing steel bars . He suggested doing this in 1907, before applying concrete, in order to avoid the formation of cracks and thus stumbled across the innovation of reinforced concrete. Unfortunately his efforts failed because at that time the phenomena of fluage and shrinkage were unknown.

In fact, the real “father” of pre-stressing is Eugène Freyssinet, who in 1928 defined it as a technique which consists in subjecting a material, in his case reinforced concrete, to loads which produce stresses opposed to those in operation, through the use of cables which have first been laid in the stressed mass.

The reasons may be found in the mechanical characteristics of concrete which shows great ability to absorb forces of compression but a low resistance to tension which is allowed to be absorbed by the metallic reinforcement. The latter, however, under the effect of tension tends to lengthen and, on account of the phenomenon of bonding, pulls the concrete along with it.

Unlike reinforced concrete, stressed steel is a material which has high resistance both to tension and to compression. As a consequence, by making a comparison between pre-stressed steel and reinforced concrete, we can immediately note that in the first place, this technique further raises both the quality and the resistance to tension and compression characteristics of the steel. In the second place it raises the resistance to tension of reinforced concrete which is, in fact, negligible.

Types of Pre-stressing Steel

The steel is treated to achieve the desired properties. The following are the treatment processes:

• Cold working (cold drawing) is being done by rolling the bars through a series of dyes. It re-aligns the crystals and increases the strength.
• Stress relieving is being done by heating the strand to about 350°C and cooling slowly. This reduces the plastic deformation of the steel after the onset of yielding.
• Strain tempering for low relaxation is being done by heating the strand to about 350°C while it is under tension. This also improves the stress-strain behaviour of the steel by reducing the plastic deformation after the onset of yielding. In addition, the relaxation is reduced.

Properties of Pre-stressing Steel

The steel in pre-stressed applications has to be of good quality. It requires the following attributes:

1) High strength
2) Adequate ductility
3) Bendability, which is required at the harping points and near the anchorage
4) High bond, required for pre-tensioned members
5) Low relaxation to reduce losses
6) Minimum corrosion.

So there you have it.  Hope this is understandable, if not why not download our whitepaper on Stressed Steel from here .

Contact us to discuss your requirements of Titanium Carbide Guide Roller. Our experienced sales team can help you identify the options that best suit your needs.