Post-Tensioned Slab Failure

CROSS (Confidential Reporting on Structural Safety)

Report Overview

Lessons learned after the end of a slab burst during a cable tensioning operation are shared by a reporter.

Report Content

A reporter has shared some key points from an investigation after the end of a slab burst during a cable tensioning operation. An operative hit by the debris sustained relatively minor injuries, however the consequences could have been much worse. The ‘live’ end of the cable being tensioned moved as the fixing in the concrete failed, exploding the slab in an area around 1.5×1.5m.

There was a range of potential causal factors including over stressing of the cable, concrete strength and structural design. The contractor, the post-tensioning (PT) installer and the PT designer concluded that localised under-strength concrete was used, due to the method adopted on site of grouting the mobile pump line and discharging into the permanent works.

It is thought that this is a rare occurrence, although a similar incident had occurred on a previous project when heavy rain on the day of a pour caused a local weakness in concrete and failure at the end of a cable.

Lessons learned

A. Concrete concerns:The fabric of the slab was destroyed near a tendon ‘block’ arrangement which was only tensioned once the concrete had reached a strength of 25N. Procedures were in place and were used to verify the strength of the concrete from both the concrete frame contractor and PT contractors’ perspectives, thus the concrete should not have failed.

Outcome: grout in pump lines must not be discharged into the slab area and must not form part of the permanent works.

B. Duty of care/informal reservations:The PT contractor had suggested that despite the achieved 25N strength test results, that they have previously verbally informed the concrete frame contractors’ supervision staff of their concerns regarding the concrete. Anecdotal suggestions after an incident is normal, but in case there are serious issues, concerns should be formalised at the time.

Outcome: PT contractor is to be encouraged to properly state their concerns in writing on programme, structure or safety.

C. Bursting concern:Following the incident, the subsequent risk potential was considered, and the robust segregation area advocated in the PT contractor’s risk assessment was implemented and additional coverings as ‘Blast Mats’ added (i.e. plywood or tarpaulin).

Outcome: PT contractor was asked to review their RAMS accordingly.

D. Concrete quality assurance:Concrete frame contractor to revise concrete method statement to include a statement on grout discharge. The PT contractor is to include a statement within their PT method statement to emphasise the importance of good compaction and ensuring homogenous concrete.

Outcome: Quality assurance checks to include ensuring grout in pump lines and heavy rain is not incorporated into the permanent concrete works, especially at the start of the pour.

E. Awareness:Refresher tool-box talks to be conducted for the concrete gang (and on other concrete frame contractor projects) as recommended by the PT contractor.

 

Comments

This incident highlights a strong justification for CROSS reports. It appears that something serious happened without any one party being obviously negligent. Lessons were learned by the parties involved, but disseminating the danger and precautionary measures more widely ought to be highly valuable.

The reporter, and the organisations concerned, are to be complimented on releasing their findings for the benefit of others who might be faced with similar situations