Time of Loading - Creep

[Drapes]

As part of the assessment for creep, the age of the concrete when it is "first loaded" is subjective as it depends on what "load" is actually considered.

For example, in multi-storey construction, once the concrete slab has set, could you not argue that the slab would “feel” some load almost straight away, irrespective on whether the slab is supported on formwork or backpropped over several floors. In this instance, the slab is in effect supported by a series of springs which represent the backpropped levels, but the slab will still deflect and be loaded up to some extent straight away.

Is there any general consensus or guidance on this?

 

[JAE]

I guess I've always viewed creep over a larger time span...so if the "initial loading" is considered being right after re-shoring, or after removal of all shores, or after occupancy - in the grand scheme of things I'm not sure that matters all that much when you are looking at 5 years plus time span.

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[BridgeSmith]

You should be able to superimpose the creep values for the initial loading and that for the increased loading at a greater age. You calculate the creep for the initial load, based on the stresses it produced, and add to it the creep based on the additional load and the change in stress it produces, using the time factors for when the additional load is applied.

 

[rapt]

Drapes,

The amount of creep is dependent on the compressive stress at any time.

Yes, there might be some very small stresses induced while still propped. But their effect will be nominal and are normally ignored.

For every load event on s slab, there will be a change in overall creep, no matter how large the event and for how long it goes.

Common sense is normally applied and averages used. E.G. An assumed Permanent loading is used to account for LL effects on creep. It is called Quasi Permanent Live Load in Eurocode for a reason, because it is an assumed average effect.

 

[Drapes]

Thank you all for your comments, very helpful.

So if we assume backpropping will be maintained to a slab over at least three floors for say 28 days, will it be reasonable to take the initial load application as 28 days?

 

[rapt]

Yes, for the SW loading and anything that has been added in that time. Fora normal lightly loaded building (office/retail/parking or domestic with lightweight partitions), that would be close to the permanent loading which Creep is normally based on.

For something with higher loading added later, more than 1stage of loading would be required.