If you refer to the ACI Code 2008 Index, page 460, look under Concrete and you will see the minimum strength requirements at different sections.
See 1.1.1, 5.1.1, 19.3.1, 21.1.4.2, 22.2.3
Nonetheless, from 5.1.1, it states:
Concrete shall be proportioned to provide an average compressive strength, f'cr, as prescribed in 5.3.2 and shall satisfy the durability criteria of Chapter 4. Concrete shall be produced to minimize the frequency of strength tests below f'c, as prescribed in 5.6.3.3. For concrete designed and constructed in accordance with the Code, f'c shall be not less than 2500 psi.
I usually have footings designed for 3000 psi though.
What you can try to do is to go to Google books and do a search on ACI 318. They have ACI 318-05 listed there. Because it's a limited preview, you will most likely have to pick your pages carefully.
This means:
1)go to books.google.com
2)search for "aci 318"
3)the search results will give you link to the limited preview of ACI 318-05
4)click on that link
5)on the upper right hand corner, it says "search in this book"
6)type in 5.6.5
7)results will show page 67
8)click on that
9)use the left and right arrows on the top to go to page 68
I've looked over page 68 and 69 of the ACI 318-05 and ACI 318-08 in the Code column for 5.6.5. The difference is pretty much in the wording of 5.6.5.3
ACI 5.6.5.3 states:
Cores shall be obtained, moisture conditioned by storage in watertight bags or containers, transported to the laboratory, and tested in accordance with ASTM C42. Cores shall be tested no earlier than 48 hours and not later than 7 days after the coring unless approved by the licensed design professional. The specifier of tests referenced in ASTM C42 shall be the licensed design professional.
As always, we'll need to reference these codes often, so it's best to buy a copy of the ACI 318-08. Some structural websites or companies will often post the "Significant changes" to ACI 318-08 online, so that those who have the ACI 318-05 can adjust accordingly with their engineering judgment.
Why do you want to spec a low strength concrete? The current applicable codes do not rference the '08 documents, but will in a year or so.
If you are looking at something that is easily removable there are other types of concrete.
The material cost savings are negigile between 2500 psi and 3000 psi and will probably get the same concrete if you just specify a minimum strength. The cost to place and finish is the same, so any savings is negligible and reduces and cushion in the design.
If it for residebtial construction, 2500 psi is a common minimum and may not require any reinforcement since a narrow strip footing rarely has a need for reinforcement. The footing is only to privide a clean, level surface to build off of.
