Mihai Chelmus · Construction Testing, Investigation & Remediation Specialist Email: mihai.chelmus@swantest.co.uk |  Tel: +44 734 228 1981 Construction structural testing, load verification and on-site assessment for buildings, temporary works and safety-critical elements across London.

High-value residential towers across London are increasingly being reconfigured long after completion. Penthouse owners are removing slabs, inserting staircases, cutting lift voids and reworking layouts inside buildings that were never designed to be altered. Many of these structures are built using post-tensioned reinforced concrete. This construction method introduces a completely different level of structural risk when intrusive works are required.

In 2026, cutting a post-tensioned slab without verified tendon mapping, inspection and stabilisation is no longer considered engineering. It is uncontrolled structural exposure. This field case study documents a live investigation and slab modification project carried out inside a £150 million penthouse development at Levels 10 and 11 of a central London tower. The works demonstrate how post-tensioned concrete can be safely modified using specialist scanning, controlled breakout, tendon inspection and structural stabilisation techniques.

 

Project Context

The project involved a luxury penthouse spanning Levels 10 and 11 of a high-rise residential tower in London. The property owner required a new structural opening to be formed within the Level 11 floor slab as part of an internal reconfiguration. The slab was a reinforced concrete flat slab incorporating pre-tensioned tendons. The original structural design did not allow for slab penetrations in the proposed location.

Given the value of the asset and the risks associated with post-tensioned construction, a full structural investigation and controlled modification strategy was required before any cutting works could be undertaken.

Structural Risk

Post-tensioned slabs rely on high-strength steel tendons stressed under load to provide structural capacity. These tendons are typically located within the slab depth and are not visible once the structure is complete. Accidental damage to a live tendon can result in sudden release of stored energy, local slab failure, progressive cracking or catastrophic structural compromise.

Typical risks associated with post-tensioned slab modification include:

• Uncontrolled tendon rupture
• Loss of pre-stress
• Localised slab failure
• Crack propagation
• Long-term durability reduction
• Insurance and warranty invalidation

For high-value residential developments, these risks carry both structural and financial consequences.

Investigation Methodology

Swantest was appointed as specialist investigation contractor to design and deliver a full post-tensioned slab investigation and modification strategy. The works were led by Mihai Chelmus, Operations Manager at Swantest, overseeing site execution and risk control.

The investigation methodology was designed to achieve four primary objectives:

• Accurately locate all post-tensioned tendons within the proposed opening zone
• Verify tendon condition prior to cutting
• Provide structural stabilisation prior to slab penetration
• Enable safe cutting of the slab without loss of structural performance

Tendon Scanning

A full slab scan was undertaken using Proceq GP8800 high-resolution GPR scanning equipment.

The scanning works mapped:

• Tendon alignment
• Tendon spacing
• Tendon depth profile
• Reinforcement layout
• Structural zones of influence

The scanning allowed the design team to define the precise geometry of the proposed opening and adjust its position to avoid direct tendon conflicts. All tendon routes were marked directly onto the slab to create a physical cutting exclusion zone.

Controlled Breakout

Once the opening location had been fixed, controlled breakout zones were formed at four strategic locations around the proposed opening.

These breakout zones were designed to:

• Expose tendons for inspection
• Verify tendon condition
• Allow structural stabilisation to be installed
• Provide confirmation of tendon alignment

Breakout was undertaken manually using controlled hand tools to minimise vibration transmission into the slab. Vibration levels were continuously monitored using hand-mounted vibration monitors to ensure safe working thresholds were maintained throughout the operation. No mechanical saw cutting was permitted at this stage.

Tendon Inspection

Following breakout, the post-tensioned strands were fully exposed in four inspection windows.

Each tendon group was inspected for:

• Corrosion
• Mechanical damage
• Grout presence
• Sheath condition
• Anchorage alignment

The inspection confirmed that the tendons were in good condition, with no evidence of corrosion, strand deterioration or grout voiding. This verification was critical in confirming that the slab retained its original design capacity prior to modification.

Structural Stabilisation

Once tendon condition had been confirmed, high-strength structural grout was installed into each breakout zone.

This served two functions:

• Localised strengthening of the slab around the opening zone
• Load redistribution prior to cutting works

The grout installation created a reinforced load path around the opening perimeter, enabling the slab to be safely cut without inducing stress concentrations around the tendon zones. Only once this stabilisation had achieved full strength was permission granted for slab cutting works to proceed.

Outcome

The slab opening was successfully formed without damage to any tendons and without any loss of structural performance. No cracking was observed beyond the breakout zones and no loss of pre-stress was recorded. The penthouse structural layout was modified in line with the architectural design without compromising the building’s structural integrity.

Industry Lessons

This project demonstrates a growing reality across London’s high-end residential market. Buildings are being altered long after completion. Many of these assets were never designed for post-construction modification. Post-tensioned slabs are now common in towers delivered between 2005 and 2022. Any modification of these structures without specialist investigation represents a latent structural risk.

The engineering lesson is clear:

• Post-tensioned slabs must be treated as live structural systems.
• They require verification before intervention.
• They demand controlled methodology.
• They cannot be guessed.

As London’s residential market continues to mature, post-construction structural modification will become increasingly common. Specialist investigation will become a baseline requirement rather than a project luxury.