You can refer to standard EN 206-1:2000 for the classification of curing rates depending on strength ratios after 2 and 28 days. The standard (available in full as pdf on the internet if you search for it) gives lots of information and references all the standards you will need. Somewhere there should be the information you are looking for.

Jing,

General requirement for the age of concrete for striking form work is 7 days.

Contractors would normally look for solutions where striking can be executed earlier, say at 3 days after the pour. This is when their rates of pouring are high, i.e. in multi-story flat slab construction.

Early striking can be achieved by using concrete additives.

Some research have been carried out by BRE, Concrete Society, etall and I have attached relevant reports for you.

It may also be beneficial to look through the refs below.

http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=346

http://www.ecocem.ie/technical,strength.htm

Regards,

Ruslan

Listing requirements of each standard:

AASHTO: 3 days

CALTRANS: 3 days, for temperatures above 15ºC

ACI 318 (Building Code), ACI 301 (Standard specification for Structural Concrete): 7 days, for temperatures above 10ºC. Or 3 days with high early strength concrete.

ACI 308 (Standard practice for curing concrete): 3 day with tipe III cement; 7 days with tipe I cement; 14 day with tipo II cement; 7 days with pavement; temperatures above 5ºC.

ACI 330R: 3 days for lightweight trafic, 7 days for heavyweight trafic. Temperatures above 4ºC.

This info plus the excellent info posted by Ruslan Koutlukaev, I thought that is complete the answer.

It depends on the load it has to bear and type of construction.