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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.
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 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 , 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 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.
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
The steel is treated to achieve the desired properties. The following are the treatment processes:
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
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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: Part 1). The proof stress should not be less than 85% of the specified tensile strength.
Table2: As-Drawn wire (IS: Part 2). The proof stress should not be less than 75% of the specified tensile strength.
Table 3: Indented wire (IS: ). 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: 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: - provides the following guidelines which can be used in absence of test data.
Table 4: Modulus of elasticity (IS: - )
Do you know what are the types of PC strands? As a PC strand manufacturer, we share this article with you.
Bonded prestressing strand is a stranded steel cable composed of multiple high-strength steel wires that have been stress relieved (stabilized). It is suitable for prestressed concrete or similar applications. The main characteristics of bonded prestressing strands are high strength and good relaxation, in addition to being straighter when unfolded. The common tensile strength grade is MPa, and there are also strength grades such as , , , , MPa. The yield strength of this steel is also higher.
Unbonded prestressing strand is made of ordinary prestressing strand coated with anticorrosive grease or paraffin wax and then wrapped with high-strength polyethylene. Bondless prestressing strands are mainly used in post-tensioning prestressing systems, the difference between them and bonded prestressing strands is that the prestressing does not bond with the surrounding concrete. During its working period, the prestressing tendon is always allowed to slide longitudinally relative to the surrounding concrete. The prestressing force is transmitted to the concrete exclusively by the anchorage. During construction, the unbonded tendons can be laid directly into the formwork like ordinary reinforcement. The concrete is then poured and the anchorage is tensioned after the concrete has reached its design strength, without the need for pre-drilled holes, grouting, etc.
Epoxy-coated filled strand has an average epoxy coating thickness of 0.6~0.8mm and is tested for continuity of coating before production. It is ensured that there are no more than 2 pinholes per 30m when the product leaves the factory. In addition to the excellent anti-corrosion properties, the epoxy resin coating has sufficient toughness. In the process of steel strand stretching, the epoxy resin coating extends synchronously with the steel wire without brittle cracking. When the strand is broken at the limit, the epoxy resin coating is consistent with the strand fracture. Compared with the PC strand, its ultimate strength is increased. The strand is completely wrapped with epoxy resin so that the strand forms a solid whole. Therefore, when the strand is tensioned with a clip anchor, the material properties of each wire are fully utilized. The overall strength of the strand can be increased by 2% to 3%.
It uses special surface treatment technology and a high-pressure electrostatic spraying method. It forms a uniform epoxy resin anti-corrosion layer with a certain thickness on the steel substrate. Make the physical and chemical properties of the product stable. It has the characteristics of heavy anti-corrosion, acid resistance, alkali resistance, salt resistance, temperature resistance, insulation, etc. The products are used in bridges, docks, airports, major buildings, sewage plants, especially on marine buildings. It can improve the service life by more than 50 years and is a new type of anti-corrosion steel for construction.
Huayongxin provides PC Strand Bonded and PC Strand Unbonded. If you are interested in our products, please feel free to contact us.
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