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CROSS Safety Report

Collapse of a hollow core unit

Report ID: 258 Published: 1 October 2012 Region: CROSS-UK

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This refers to the collapse of one modest span precast hollow core concrete unit and the excessive deflection of others on a building which was under construction.

Key Learning Outcomes

For precast manufacturers and suppliers:

  • Quality assurance and competent supervision in the precast yard can help to ensure that precast elements are constructed in accordance with the design

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This refers to the collapse of one modest span precast hollow core concrete unit and the excessive deflection of others on a building which was under construction. The collapse occurred when the only load was self-weight and fortunately the falling debris missed workers below and there were no injuries. The units, which contained embedded polystyrene, were composed of bottom pre-stressed panels with a top layer of concrete placed in the works a few hours later.

The usual method of manufacture, says the reporter, would be to use normal concrete, with the two pours being separated by a suitable interval and vibrated with a poker to give interlock between the bottom slab and the connecting ribs from the top sections. In this case self-compacting concrete was used with no vibration between the bottom and top pours.

The result was an almost smooth surface at the horizontal joints and it was here that delamination occurred on site as a 50mm thick pre-stressed slab tried to support over 200mm of dead weight from the top part of the sandwich. Other panels were removed from site because of defects unrelated to the delamination such as cracking and excessive camber or no camber. Some were removed which showed signs of shear failure cracking.

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This is similar to report 342 (GRP panels as permanent shuttering) in that supplied components did not meet expectations. Proper quality control is vital at all stages and here there seems to have been a lack of appreciation of the need to provide a shear key between the two concrete elements.

Quality assurance is not just a form-filling exercise and to be successful depends upon having adequately trained and vigilant supervisors throughout the process. Health and Safety Executive files contain many cases where collapses of shuttering under load have led to deaths and serious injuries.

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