What Are Tendon Truncations?

A tendon truncation is a specialist construction process where a new epoxy-based anchorage is formed within an existing post-tensioning tendon. The anchorage zone is engineered and strengthened to achieve performance comparable to a proprietary cast-in-place anchorage system.

Truncations are most commonly required when:
• Existing slabs are altered or cut to create new openings or structural changes
• Damage has occurred to an existing anchorage
• Design modifications require tendons to be safely terminated at new locations

In accordance with the Australian Standard, “Anchorages for tendons shall be capable of developing the tendon to at least the minimum tensile strength.” The Post-Tensioning Institute of Australia (PTIA) also promotes the use of tendon truncations whenever tendons are to be preserved.

Why Truncations Matter

Post-tensioning systems contain significant stored energy. If inadequately grouted, this energy remains within the strand, and any damage can cause the uncontrolled release of that stored force. Even when tendons are adequately grouted, poorly executed truncations can compromise structural performance and reduce section design capacity.

People are naturally cautious about visible hazards, but in post-tensioned structures, the greatest risks are often hidden within the concrete. That’s why truncations should never be treated as demolition activities — they are structural operations that directly affect the behaviour of the building’s post-tensioning system.

Bond and Load Transfer

A fully grouted tendon in a smooth galvanised duct should never be considered an anchorage in itself. Bond relationships between the strand, grout, duct, and concrete typically serve only to transfer incremental stresses during strength events, linking residual tendon stresses to those at yield (proof stress).

Without a properly engineered anchorage or truncation, if a tendon were cut, the bond would be required to transfer the entire load at yield — often two to three times greater than what would be required had an anchorage or truncation be used.

Slabtec’s Proven Approach

At Slabtec, we have developed and proven tendon truncation procedures that have been independently load-tested to verify anchorage performance. Our expertise ensures anchorage forces are transferred safely and reliably, maintaining the structure’s integrity while allowing flexibility for modifications.

Our advice to stakeholders typically includes:
• Where truncations will be required
• Recommended methodologies
• Structural considerations for engineers during temporary stages
• Safe and unsafe areas for truncations
• Alternatives where conditions are unfavourable

The Truncation Process on Site

Excavating concrete around stressed tendons is a high-risk operation that must be carefully managed to prevent tendon damage and ensure safety.

Assumptions should never be made that:
• Tendons are fully grouted, or
• They are located exactly as shown on design drawings or unverified scan reports.

Ungrouted tendons are not uncommon and must be identified early. The condition of the grout should be assessed by experienced operatives during excavation, as this directly affects how truncations are performed and whether remedial grouting is required before any slab demolition proceeds.

Slabtec provides documented QA reporting for the as-built tendon condition as part of every project, ensuring full transparency for the Design Engineer’s review.

Best Practice

To maintain safety and structural performance, Slabtec recommends the following principles:

• Perform truncations when penetrating a slab, as “a grouted tendon is not in itself an anchor.”
• Perform truncations only by professionals who understand post-tensioning systems and can recognise potential issues such as ungrouted, unstressed, or unsafe locations.
• Engage specialist contractors who can remediate potential issues and ensure that anchorage forces are safely and predictably transferred.

Key Takeaway

Truncations are not demolition tasks — they are structural engineering operations that demand expertise, precision, and control. Engaging qualified structural reuse specialists ensures forces are safely transferred, risks are mitigated, and the structure’s long-term performance is preserved.

TECHNICAL INSIGHT

When Truncations Go Wrong: Why Experience Matters

“Truncations are not demolition tasks — they are structural operations that directly affect a building’s post-tensioning system.”

Background

During recent strengthening works at a major shopping centre, Slabtec were contracted to complete tendon truncations on a beam as part of a broader structural strengthening package.

Soon after initiating works, concerns were raised by the on-site team regarding the potential impact of the truncations on the beam’s structural integrity. Although these were initially dismissed, Slabtec’s Director conducted further inspections and immediately halted all works pending review.

Following escalation to the project’s structural engineers, Slabtec’s assessment was confirmed — the truncations posed a potential risk to the beam, and mitigation measures were implemented to restore full structural capacity.

Tendon Truncations

A tendon truncation involves creating a new epoxy-based anchorage within an existing post-tensioning tendon. The anchorage zone must be strengthened to perform comparably to a proprietary cast-in-place system.

Truncations are commonly required when:
• Existing slabs are modified to create new openings or structural changes
• Damage occurs to an existing anchorage
• Sections of tendons need to be safely terminated due to design alterations

Each truncation must safely transfer anchorage forces without compromising the structure’s strength or serviceability.

Why Specialist Expertise is Essential

Truncations directly modify the behaviour of post-tensioned elements. When not properly engineered, they can lead to:
• Loss of load-carrying capacity in beams or slabs
• Damage to adjacent tendons and anchorages
• Reduced confidence in the structural adequacy of the system
• Costly remediation and program delays

At Slabtec, we have developed and proven tendon truncation procedures that have been independently load-tested to verify anchorage performance. Our approach ensures that forces are safely and reliably transferred, maintaining the integrity of the structure while allowing flexibility for slab modifications.

Lessons from this project

This project highlights the risks associated with treating truncations as demolition activities rather than as structural engineering operations.

The decision to involve qualified post-tensioning specialists prevented potential damage, safeguarded the structure, and ultimately saved the project significant time and cost.

Truncations should be priced and managed separately from demolition works.

Engaging a specialist ensures that the process is properly engineered, controlled, and documented — protecting both the structure and the project outcome.