RE-SHORING CONCRETE SLAB AND BEAM

[ajk1]

We are reviewing a project where the contractor proposes to remove the forms and the associated shores for 9 bays (and fly the forms to the next level) before installing the re-shores. We expect that this might mean that at least some of the the slab and beams(i.e. those at the start of the form removal cycle)will have no re-shores for more than a day.

Question:

Is there any publication (ACI or other?)that gives a recommended maximum time (hours?) that a reinforced concrete one-way slab (and its supporting beams) can be unshored after the shores are removed? I am going to look at ACI SP4 now but we don't have the latest edition in the office. Of course the concrete must have adequate strength (75% of the specified 35 MPa strength) before any forms forms/shores are removed, and we will see to that. The question is more directed at increased deflection caused by early age creep.

 

[ishvaaag]

http://www.gamcoform.com/dropshore/documents/ACI Reshore.pdf

Incomplete quote

http://books.google.es/books?id=m_8...&resnum=3&ved=0CC8Q6AEwAg#v=onepage&q&f=false

Incomplete quote

http://books.google.es/books?id=PJg...&resnum=4&ved=0CDIQ6AEwAw#v=onepage&q&f=false

There being on sale intelligent formwork systems where some of the props can be held in place holding the tributary load where everything else can be removed, this question should appear less and less.

 

[ajk1]

Thnak you Ishvaaag for this very useful and complete information. It is very much appreciated.

 

[Ron]

Seventh Ed. of Formwork for Concrete (M.K. Hurd), ACI Committe 347, does not address creep and reshoring that I could find quickly. Only addresses strength relative to concrete curing state.

 

[ajk1]

Ron - thanks for taking the time to look at this for me. This information is useful. Much appreciated.

Ishvaag - thanks again to you too. However, I was not clear in my original post, that these are flying forms. My apologies for omitting this crucial piece of information. I don't think there are any flying form systems that would have posts left in place because they would block the path for removal of the form assembly. Right?

 

[fredPE]

I have done a fair amount of reshoring. The reshores are to take the loads from construction operations of the structure above. So as long as you don't apply wet concrete loads from the next pour, and don't put a ton of weight on the curing slabs, you don't necessarily need reshoring to be put in place.

Shore removal is a separate issue. Your concrete slab has to have attained sufficient strength to be self supporting, or they can't take the shores out. Backshoring is for cases where you are worried about deflections in your slab. If you remove shores before full concrete strength is achieved, your dead load deflection will be greater than assumed in your design calculations because your E is smaller when the self weight loads are applied. Plus, your modulus of rupture is smaller, so you may end up with more cracking, making your I smaller as well.

 

[ishvaaag]

... I have not seen such flying forms ... those external could be made in tridentine form to leave behind prop guides, or if wide enough, 2 or 3 rows per span could do the same trick with retained propped guides.

 

[DTGT2002]

It is the recommended practice of ACI Committee 347 (Formwork for Concrete) that all slabs requiring reshoring should be reshored by the end of the working day in which they are stripped.


ACI 347-04 is the most current version of the Guide to Formwork for Concrete which is also included in the back of SP-4.

With that said, the slab likely has adequate strength to carry the loads of the system itself up above (likely 7-15psf - just a guess) plus the associated nominal construction activities.

The concern is obviously the early age concrete cracking and deflecting - thus the recommendation to reshore by the end of the day.


I am confused as to the inability to install reshoring. I work as the engineer for a large concrete formwork contractor, and with a flying table system, the reshoring is usually a fairly simple installation.

That said, if installing 100% of the reshoring would be a challenge, perhaps an initial installation of 50% of the final reshores required to support construction could be installed such that some means is in place to limit early deflections? A design that accounts for the contruction loads occuring above and including the form system weight might be a reasonable approach.

Most specifications speak to reshoring at the time of concrete placement - at least in my mind. They also speak to never applying more load than the structure is designed for, but don't always speak to the early age concrete and concerns about reshoring.

A similar situation occurs at the roof of the concrete structure - when shoring is complete, the shoring would be stripped and removed without reshoring upon attainment of the specified f'c value, short of specifications or construction documents indicating otherwise.


In summary, the shoring has to be removed and is best to be removed from multiple bays at one time to ensure the structure is taking on it "final" deflected shape such that it may carry its own load. As such, there is always a time in a typical reshored structure where the slab must be unshored to facilitate material movement.

It is the best practice to reshore any slab in mutli level reshored construction by the end of the working day. I would defend that it is reasonable to have have less than 100% of the reshoring required for the next level placement be considered reshored for the purposes of an initial reshored state by the end of the day.

Hope it helps,


Daniel Toon

 

[ajk1]

Thank you Daniel Toon. You have provided the answer that we were looking for, namely that ACI 347-04 says the slab should be reshored by the end of the day.

Our concern about leaving it longer than the end of the day is that there might be "additional" deflection.

There are 9 "spans" of flying forms to remove per "bay", so we are not sure that the contractor can remove all of these flying forms and intall all of the reshored in the 9 spans, in one day. He cannot start installing reshores until all 9 spans of flying formwork are removed because the reshores would be in the way of pulling out the flying forms.

It is currently under construction, so we will see how much he can do in a day.

Thanks to you we can now bring to the contractor's attention ACI 347-04 (although it is not referenced in the contract documents). Thanks again. Much appreciated.