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CROSS Safety Report

Failure of precast concrete cover slab to circular shaft

Report ID: 505 Published: 1 April 2015 Region: CROSS-UK

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Overview

A reporter describes how several precast concrete cover slab units collapsed shortly after being installed.

Key Learning Outcomes

For precast manufacturers:

  • Quality control and competent supervision in the precast facility can help to ensure that the precast elements are built in accordance with the design

  • Independent supervision on site by the design team or a third-party inspector can reduce the risk of any unauthorised changes from occurring

For construction professionals:

  • It is standard practice for the principal contractor to carry out a pre-pour inspection to check that the subcontractor has fixed the reinforcement in accordance with the drawings, cover is correct etc. prior to placing concrete

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An incident recently occurred on a project involving the construction of a 12.5m internal diameter x 12.5m deep storage shaft. The reporter says that the cover slab for the storage shaft was designed as an interlocking arrangement of seven precast concrete slabs with half joints between the individual units. Once the precast concrete slabs had been lifted into position above the shaft a layer of in-situ concrete was to be placed over them to form the finished composite cover slab.

Collapse of precast units

The cover slab was a bespoke design prepared by a specialist supplier who also manufactured the precast concrete slabs off site at their production facility. When the precast concrete slabs arrived on site, they were lifted into place. However, approximately 10 minutes after the last slab had been placed, five of the seven slabs collapsed into the shaft. Fortunately, no one was on the cover slab at the time of the collapse, and no one was injured as a result of the incident.

The investigation into the incident looked at a number of possible causes including design errors and low concrete strength. However, it very quickly became clear from discussions with the specialist supplier and destructive examination of the failed precast concrete slabs on site, that the reinforcement in the half joints between the slabs had not been installed as per the design.

It very quickly became clear from discussions with the specialist supplier and destructive examination of the failed precast concrete slabs on site, that the reinforcement in the half joints between the slabs had not been installed as per the design.

Pre-pour inspections

Reinforcement had the correct concrete cover, but the bars were not the correct shape, diameter or spacing. The reasons for this were attributed to the fact that the units were manufactured over a holiday period when the usual steel fixers were unavailable. When bespoke concrete elements are constructed on site it is standard practice for the principal contractor to carry out a pre-pour inspection to check that the subcontractor has fixed the reinforcement in accordance with the drawings, cover is correct etc. prior to placing concrete.

Reliance on third party accreditation and certification

It is the reporter’s opinion that when procuring bespoke precast concrete units manufactured off-site by a specialist supplier a risk assessment should be carried out to determine what level of inspection of the manufacturing process should be carried out by the purchaser.  The reporter continues that third party accreditation and certification of the supplier’s quality assurance (QA) procedures should not be relied on as giving sufficient confidence that the manufacture of individual units has been carried out correctly.

The reporter’s firm has subsequently carried out a thorough review of in-house processes covering testing and inspection and the way in which these deal with off-site manufacture.

Expert Panel Comments

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Reputable precast concrete suppliers will have recognised QA processes to avoid such occurrences. Good practice may suggest that precast units should be checked on delivery although in this case the problems of incorrect shape, diameter or spacing might well not have been detected. Treatment of off-site manufactured bespoke products is somewhere between that for on-site construction and that for off-site manufacture of mass produced products.

What level of checking is required?

For construction on site a contractor would probably check everything, and for mass produced products a contractor would probably check little or nothing - relying on third party product certification. The level of checking for bespoke items made off-site should be determined from a risk assessment process which evaluates likelihood and consequences of a non-compliant product. This might include the contractor making checks at the precast works.

The level of checking for bespoke items made off-site should be determined from a risk assessment process which evaluates likelihood and consequences of a non-compliant product. This might include the contractor making checks at the precast works.

Clearly whatever procedures should have been in place did not operate on this occasion, with potentially fatal consequences. It is reassuring that a thorough in-house review by the contractor took place to avoid a recurrence. A key question in any safety assessment is: ‘Do you know that what you thought was being constructed was actually constructed?’

Historical problems with half joints

Another point is that half joints have a long history of problems partly because they are shallow, and the numerical capacity tends to be sensitive to small errors in cover and the positioning of reinforcement. One of the most costly forms of failure is when a mass produced element has been wrongly designed or manufactured so that the defect is repeated multiple times. Car manufacturers sometimes have this problem resulting in mass recalls of vehicles.

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