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Soham Ortiz
Field Engineer
Asked a question last year

Why is propping required for long structures once the formwork is removed?

Why is propping required for long structures once the formwork is removed?

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Seth Morgan
Construction Superintendent

The formworks are removed after the concrete has reached a strength at least twice its removal stress. But the members should be supported with the help of props. The reason is because of creep due to the Interfacial transition zone (ITZ).

The interfacial transition zone is formed in the concrete between the aggregates and the mortar. This zone comprises of high Water cement ratio and hence is the weakest part of the concrete. Due to the high WC ratio, the hydration reaction in the ITZ produces a porous framework in the early stages. This porous network will be filled in subsequently by stronger gels such as CSH gel as the hydration reaction proceeds. But for this to take place, sufficient time is needed. As time increases, the ITZ becomes stronger.

This is why the creep coefficient reduces as we delay the age of loading. When the props are moved, the self-weight of the structure acts on it, which is equivalent to loading the structure.

We know that the creep coefficient when loaded at

Seven days – 2.2

28 days – 1.6

One year – 1.1

Due to the high creep coefficient, the structure may become susceptible to sagging when the props are removed soon.

Theodore Ray
Planning and development surveyor

After concreting work, the time at which striking of formworks should not be too long, otherwise it would affect the colour of structures.

For long-span concrete structures when they have attained a sufficient amount of strength to support their self-weight as well as creep and deflection maker in these structures and cropping is not provided after the removal of formwork.

Hence, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.

Propping for to provision of false for support slabs and pains during their game in cotton strength after concreting.

Thanks You.

Flenn Hale
Construction Manager

Cast concrete does not reach full strength after approximately 28 days. After 4 to 5 days of concrete curing time, a contractor can then continue their work, say the next floor above, after propping up the structure. Propping is done to prevent damage to the concrete integrity during curing.

Its very important to use props after removing the formwork for supporiting the concrete stucture for some time.

Specially the in concrete bridges, the props are used after removing the formwork.

Props provide false support to the structure.  

When the sufficient strength is attained for the long span concrete structures to support their self weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Hence repropping is carried out after removing formwork and thus these props should not be allowed to stand too long because creep loads may overstress them.