CROSS Safety Report
Post-tensioned slab failure during tendon stressing operations
This report is over 2 years old
A reporter disseminates the lessons learned after the end of a slab burst while stressing the tendons in a post-tensioned concrete slab.
Key Learning Outcomes
For construction professionals:
Be aware that large stressing forces and stored energy in the concrete are generated during the post-tensioning process
Post-tensioning operations should be carried out by people who have the relevant training and are competent to do so
If concrete ‘blow-out’ is a concern it should be raised on site and the contractor’s risk assessment and method statement (RAMS), required under CDM 2015, can be updated to address the concerns
Regular toolbox talks with work crews are a good way of highlighting any risks associated with work activities
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
A reporter has shared some key points from an investigation after the end of a slab burst during a cable tensioning operation (Figure 1). 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 1.5x1.5m.
Factors that can lead to concrete blow-out in PT slabs
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. This was due to the method adopted on site of grouting the mobile pump line and discharging into the permanent works.
The contractor, the post-tensioning (PT) installer and the PT designer concluded that localised under-strength concrete was used
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. The following are some lessons that can be learned.
Concrete strength 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.
Duty of care/informal reservations
The PT contractor had suggested that despite the achieved 25N strength test results, that they had 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.
Reducing the risk of concrete ‘blow-out’
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 risk assessments and method statements (RAMS) accordingly.
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.
Raising awareness of risks on site
Refresher toolbox talks to be conducted for the concrete gang (and on other concrete frame contractor projects) as recommended by the PT contractor.
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In the photo showing the damaged anchor location, I can see no evidence of anti-bursting reinforcement at the anchorage location. I would expect something in the order of 12mm bars at 250, links or U-bars. It may be there but just not evident on the photo?
Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
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.