Bonded vs unbonded post-tensioning strand
The difference is not only corrosion protection. It changes how force is transferred to the concrete, how the tendon is grouted, and what you must order alongside the strand.
Bonded strand bonds to the concrete after grouting; unbonded strand stays free to move along its whole length.
In a bonded system, the strand runs through a duct that is grouted after stressing, so the strand and the concrete act together and the force is transferred along the tendon. In an unbonded system, the strand is coated with grease and sheathed in polyethylene, so it remains free to move and the force is transferred only at the anchorages. The choice affects corrosion protection, the consequences of a single tendon failure, and the accessories you must order with the strand.
The mechanical difference
Both systems apply compression to the concrete. The difference is where that compression is delivered and how the tendon behaves over the life of the structure.
Corrosion protection
Both systems protect the strand, but by different means and with different inspection implications.
What changes on site
The field work differs enough that the two systems are rarely interchangeable on a project without a design decision.
What you must order alongside the strand
This is where procurement mistakes happen. The strand alone does not make a system.
Which system to specify
The choice is a design decision, driven by structural behaviour, exposure conditions, fire requirements and the consequences of tendon failure. For slabs, unbonded systems are common; for bridges and heavily loaded elements, bonded systems are often preferred because bonded tendons have redundancy and better fire performance. The specification must come from the engineer, and the strand, anchorage, duct and equipment must then be sourced as a compatible package.
Common questions on system choice
What is the main difference?
Bonded strand runs through a duct that is grouted after stressing, so the strand bonds to the concrete along its length and transfers force continuously. Unbonded strand is grease-coated and polyethylene-sheathed, remains free to move, and transfers force only at the anchorages.
Which is better for corrosion protection?
Both protect the strand, but differently. A bonded system relies on the grout and duct, so grouting quality and duct integrity are critical. An unbonded system relies on factory-applied grease and sheath, which is consistent but must be inspected and repaired for damage on site.
Can I use the same strand for both?
No. Unbonded strand is supplied with the grease coating and polyethylene sheath already applied at the factory. For a bonded system you order bare strand plus corrugated duct and grout; for an unbonded system you order the sheathed strand and protective end covers.
Need this confirmed for your order?
Send the standard, diameter and quantity with your inquiry and the relevant document set will be confirmed before shipment.
Contact our engineersOrdering a post-tensioning system?
Send the system type, strand diameter and anchor type — we will confirm the complete package and its compatibility.