I. Product Introduction
1. Definition and Structure
Unbonded Steel Strand is a special steel strand used in prestressed concrete structures. Its core feature is that the steel strand is not directly bonded to the concrete. The specific structure is as follows:
Core Steel Strand: Usually made of 7 high-strength steel wires, made of high-carbon steel, with a tensile strength grade of 1860MPa, with high strength and low relaxation performance.
Wrapping layer: The outer layer of the steel strand is wrapped with a layer of high-density polyethylene (HDPE) sheath, and the sheath is filled with anti-corrosion lubricating grease to form a completely isolated "unbonded" state.
2. Key Features
Unbonded performance: The sheath is separated from the concrete, and the steel strand can slide relative to the concrete when subjected to force, avoiding the stress transfer failure problem caused by concrete cracking in traditional bonded steel strands.
Anti-corrosion ability: HDPE sheath and grease layer can effectively isolate moisture, oxygen and corrosive media, significantly improving the durability of steel strands, especially suitable for humid and corrosive environments (such as marine engineering and chemical construction).
Flexible construction: No need to reserve channels or grouting, the construction process is simplified, and it can be directly laid in the formwork. After the concrete is poured, a prestressed system can be formed, shortening the construction period.
Uniform stress distribution: Due to the sliding characteristics, the stress distribution of steel strands under load is more uniform, reducing local stress concentration and improving the crack resistance and ductility of the structure.
3. Comparison with bonded steel strands
Comparison dimensions Unbonded steel strands Bonded steel strands (such as smooth PC steel strands)
Bonding state The steel strands are not bonded to the concrete and can slide The steel strands are bonded to the concrete through grouting
Anti-corrosion measures Self-contained HDPE sheath + grease, strong anti-corrosion Need to rely on grouting of the channel for anti-corrosion, and the quality of grouting affects durability
Construction process No need for channel reservation and grouting, simple construction Need to reserve channels, thread bundles, grouting, and complex processes
Stress transfer Stress is transferred by anchors at both ends and can slide in the middle Stress is transferred by bonding force, and concrete cracking may affect stress distribution
Applicable scenarios Large-span, crack-prone structures or projects with high anti-corrosion requirements Conventional prestressed concrete structures
II. Application areas
1. Construction projects
Large-span floor slabs and roofs: For large-span buildings such as shopping malls, gymnasiums, and airport terminals, unbonded steel strands can reduce beam height, increase space utilization, and avoid concrete cracking affecting appearance.
High-rise building conversion layer: Bearing huge loads in the structural conversion layer, using its uniform stress distribution characteristics to improve the crack resistance and safety of the conversion layer.
Post-tensioning prestressed components: Especially suitable for components that cannot be grouted or the grouting quality is difficult to ensure (such as prefabricated assembled components).
2. Bridge engineering
External prestressed bridge: When used as an external cable, the unbonded steel strand is easy to maintain and replace later, and is suitable for reinforcing old bridges or building new external prestressed bridges.
Marine environment bridge: The anti-corrosion performance of the HDPE sheath can resist seawater erosion and extend the service life of the bridge (such as bridges in coastal areas and cross-sea bridges).
3. Special structures
Nuclear power plant containment: It needs to resist radiation and environmental corrosion for a long time, and the high durability of the unbonded steel strand can meet safety requirements.
Liquid storage structure (water tank, oil tank): Avoid leakage caused by concrete cracking and ensure the sealing and safety of the liquid storage structure.
Permafrost area engineering: In permafrost areas with frequent temperature changes, the sliding characteristics of the unbonded steel strand can reduce structural damage caused by frost heave force.
4. Reinforcement and renovation projects
Reinforcement of existing structures: Prestressed reinforcement of old buildings by laying unbonded steel strands, without destroying the original structure, and convenient construction.
Seismic reinforcement: Use its ductility and stress redistribution ability to improve the structure's ability to resist collapse under earthquakes.
III. Construction points
Laying requirements: Fix the steel strands according to the designed position to avoid damage to the sheath and ensure smooth sliding during tensioning.
Tensoring process: Use two-end or one-end tensioning, and the tensioning sequence must meet the design requirements to avoid uneven stress.
Anchor treatment: After tensioning, the anchor needs to be sealed and anti-corrosion treated (such as applying anti-corrosion grease and wrapping the sealing sleeve) to prevent the anchor from rusting.
Quality control: Focus on checking the integrity of the sheath, the amount of grease filled, and the quality of the anchor installation to ensure non-bonding performance and durability.
IV. Advantages summary
Strong durability: The dual anti-corrosion system (sheath + grease) is suitable for harsh environments.
Efficient construction: Eliminate the grouting link, shorten the construction period, and reduce construction costs.
Easy maintenance: The external unbonded steel strand can be replaced individually, reducing the difficulty of later maintenance.
Excellent structural performance: Uniform stress distribution, good crack resistance, suitable for projects with high requirements for deformation and crack control.
Unbonded
Prestressed Steel strands have demonstrated significant technical advantages in modern prestressed concrete structures through unique structural design, especially in terms of durability, construction convenience and structural adaptability, and have become the preferred material for large-span and complex environment projects.
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