Formwork Removal Times

[kaisersoze]

Is there a summary guide for the minimum time after which the forms have to remain in place? The contractor asked me the other day if he could remove the forms for grade beam after 24 hrs, the specs say 7 days which seems a bit excessive. Personally I think 3 days should be OK.

Job Location: Greater Chicago Area

Thanks,

Kaiser

 

[JAE]

I believe there is a section in ACI 318 on this topic (away from desk right now).

The issue is on the contractor to show that early removal (with lower fc values) is OK with respect to the structure holding its own weight.

With ACI, the forming removal is usually followed by re-shores. I think it is in the early chapters somewhere - 4, 5, or 6.

 

[kaisersoze]

Thanks JAE. I also found this previous thread to be useful:

http://www.eng-tips.com/viewthread.cfm?qid=249983&page=5

Kaiser

 

[DTGT2002]

In your case, 318-08 indicates removal of formwork is acceptable when said removal will not damage the concrete, as you are removing a vertical form from concrete cast on grade. Futher, the commentary suggests 12 hours is reasonable.

Other ACI documents, guides to water retain structures and guides to architectural concrete (as I recall), offer more performance based recommendations, with a concrete strength around 1000psi required such that the tensile strength should be adequate to overcome the adhesion of the form.



I find this section of 318 quite challenging, as it presents directions to the contractor within the design code - these must be repeated in the specifications to be taken on by the contractor. Further, this section is often read to imply the contractor needs to provide an alternate structural analysis and model of the building. Using reasonable means of structural analysis to design a shoring system and understanding the strength of the building in concstruction are no doubt the responsibility of the contractor, but attempting to recreate the analysis of the building is dangerous at best and a waste of funds at least.

Refer to section 2 of ACI 301 for further details.


Does your project refer to 301 or do the specifications incorporate similar content?



Daniel

 

[csd72]

The greener the concrete is when you remove the formwork the greater the creep deflection

 

[RHTPE]

Haven't checked in to the forum in a while. Early form removal can also exascerbate water loss from the concrete, which can affect curing. Might be wise to consider application of a curing compound.

Ralph
Structures Consulting
Northeast USA

 

[henri2]

RHIPE you are correct. If forms are removed early, then the exposed concrete must be cured. In this case if forms are removed after 1 day, then curing must continue for an additional 6 days to comply with the code requirement of 7 days minimum --Ref:ACI 318 sec 5.10.1 or 5.11.1 depending on edition.