Roof Slab for underground structure 1

[AminSaalaar]

How soon can the formwork for the roof of an underground vault be removed and how soon can loading be allowed on top of it? This structure is an underground vault with dimenstion of 12'x12'x12' with thickness of 10" for all elements w/ 35MPa concrete.

Thanks

 

[JedClampett]

This is usually expressed as a percentage of the design strength rather than a time. Typically 80% of design compressive strength needs to be achieved before the shoring is removed. Before applying a heavy loading, like soil, I would require 100% of the design strength.

 

[AminSaalaar]

Thanks. Although I know it's ideal to reach the 100% design compressive strength before backfilling/loading but sometimes it's difficult to leave a pit open in the middle of street over a month just for the curing period.

 

[JedClampett]

If you need to fill the excavation, I would recommend a high early strength mix. You can reach full strength in a matter of hours. It's very common in the road industry.
In any case, it's unlikely you would have to wait a month, even with normal concrete.

 

[msquared48]

You can always remove the major shoring and reinstall temporary shoring that could be taken out of a manhole later, if one is available.

Mike McCann
MMC Engineering

 

[rapt]

If you are applying full loading you theoretically need full design strength. But the main problem will be with deflections. Creep deflection will be far higher with early loading and concrete modulus does not necessarily increase as quickly as strength with high early strength concrete, so that is also a worry for deflections.

And if you are putting in soil and compacting it, I assume the loading is going to be close to the full design load from the compacting process!

 

[AminSaalaar]

Thank you all. all of the comments are helpful

 

[EliteConcrete]

Nice topics and helpful comments. Thanks for sharing.

Elite Concrete Restoration, Inc.

http://www.eliteconcreterestoration.com/

On Time, On Spec, On Budget

 

[fearghusq]

could you not remove formwork and use temporary propping within the vault to allow back filling to begin. Again it depends on your access, but if you have room to remove formwork within the vault @ 80% strength (likely 7-10 days if proper curing achieved) demould/strike formwork and prop.
When full strength is reached (as determined from samples) remove temporary propping within.