What is Prestressed Concrete?
Why use prestressed concrete?
The ultimate tensile strain value of concrete is only 0.0001～0.00015mm. To ensure that the concrete does not crack, the stress of the steel bar can only reach 20～30N/mm2; even if cracks are allowed, when the crack width is limited to 0.2～0.3mm, the stress of the steel bar can only reach 150-250N/mm2, so that the strength of high-strength steel cannot be fully utilized.
The basic principle of prestressed concrete?
Before the component bears the external load, prestress is applied to the concrete in the tension zone of the component to offset the tensile stress generated by the external load during the use, which delays the appearance of concrete cracks and limits the development of cracks. Thereby improving the crack resistance and rigidity of the component.
Classification of prestressed concrete
1. The sequence between tensioned steel bars and concrete pouring:
Pre-tensioning method: cohesive force transfer
Post-tensioning method: anchor transfer
(According to the bonding state of the prestressed tendons, it can be divided into: bonded and unbonded)
2. Different construction methods are divided into:
Precast prestressed concrete
Cast-in-place prestressed concrete
Laminated prestressed concrete
Prestressed tendons and prestressed equipment
1. Prestressed tendons
Main categories: steel strand, steel wire, and heat-treated steel bar.
1) Steel strand
Generally, it is made of three-strand, seven-strand Φ3, Φ4, or Φ5 high-strength steel wire twisted into a spiral with a reaming plate, and then made by low-temperature tempering. Diameter 8.6 mm～15.2 mm; 1570, 1720 and 1860 MPa; elastic modulus 1.95×105N/mm.
2) Steel wire
The use of stress-relieving straightening and tempering steel wire can eliminate the residual stress generated in the cold drawing of the steel wire, increase the proportional limit, yield strength, and elastic modulus of the steel wire, and improve plasticity. The steel wire is divided into smooth, indented, and spiral rib steel wire according to its shape. Diameter 4mm～9mm; 1570, 1670 and 1770MPa; elastic modulus 2.05×105N/mm2.
3) Heat-treated steel bars
For some hot-rolled IV steel bars (such as 40Si2Mn, 48 Si2Mn, 45 Si2Cr, etc.), high-strength hard steel without significant flow range formed by quenching and tempering. It is formed after quenching using the waste heat of hot-rolled steel bars and then tempering at medium temperature.
Diameter 6mm, 8.2mm, 10mm; 1470MPa; Modulus of elasticity 2.0 × 105N/mm2.
2. Prestressed anchorage
1) Stressing anchorage: Anchored at the end of the component, it is integrated with the component to bear the force, and the one that is no longer removed is called the anchor. It is mostly used for post-tensioning.
2) Dead-end anchors: extrusion anchors and H-type embossed anchors. The extrusion anchor can be installed in the concrete or outside the structure. It is suitable for both bonded and unbonded steel strands. H-type embossed anchors are only used when the fixed end has a large space and sufficient bonding length, and the cost is the lowest.
3) Steel wire anchorage
4) Grips: grips and temporarily fixes the prestressed tendons during the forming process of tensioned steel bars and concrete. After the concrete reaches a certain strength, it is removed and reused as a grip. It is mostly used for pre-tensioning.
3. Tensioning equipment
1) Hydraulic tension jack
Multi-strand center hole jack: commonly used YDC jack, 650-6000KN; suitable for tensioning group anchor, DM type head anchor, etc.
Tie rod jacks: DM type screw end rod anchors, JLM precision-rolled screw steel anchors, LZM cold cast anchors, etc. can be stretched.
Cone anchor jack: only used for wire bundles with steel cone anchors.
2) Electric oil pump