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873 Propping of post-tensioned slabs

Report ID: 873

Published: Newsletter 58 - April 2020

Report Overview

An infill slab strip was not poured in time, resulting in overloading of the slab below during a concrete pour.

Report Content

When casting in-situ concrete slabs, they are typically propped and supported by the two floors below. For a concrete framed building a reporter was involved with, this was how the contractor constructed the floors. The slab was post-tensioned, and the contractor had left strips out in the slab to allow it to be post-tensioned. Once the slab had been tensioned, the small infill slab strips were to be cast.

During the construction at one level, the infill slab strip at the level below had not yet been poured. Therefore, the weight of the slab being cast, and the weight of the floor below was all being taken solely by the floor two levels below. Fortunately, this slab was designed for high superimposed and finishes loads. If this had not been the case, the slab could have failed, says the reporter.

Lessons learnt are that the contractor needs to ensure that post-tensioning strips are always cast and have achieved the required design strength if they are to be used to prop the slabs above.


The broader lesson is the wider one of the interactions between design and construction. In reality, the two phases are intimately linked, and it is not at all uncommon for critical design cases to occur during the construction phase. It is therefore vital that the design and construction teams co-operate to assure that designs can be safely built and thereafter be safe in service. Once a safe method of construction has been designed, it is essential that this method of construction is followed.

In this case, it was fortunate that the lower slab was strong enough to withstand the wrong sequence - this would not always be the case.


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Propping of post-tensioned slabs

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