Concrete: The Strength the Code Requires, and the State That Doubles It

US concrete is specified by compressive strength and exposure, not by a UK-style mix ratio. Here is the free code's minimum table, the air-entrainment footnotes people skip, and the state amendment that catches everyone out.

How US concrete is specified

By 28-day compressive strength in psi, plus an exposure condition. Not by a 1:2:4 style mix ratio. If you were trained on mix ratios, this is the translation you need and it is the whole reason for this card.
The paperwork calls it specified compressive strength, f'c. The yard calls it a 3,000 psi mix. Same thing.
Around 70 percent of the 28-day strength at 7 days is a scheduling rule of thumb, not a code figure. Do not strike or load off it without the actual break results.
f'c is a MINIMUM you specify, not a prediction of what your mix will make. A supplier designs the mix to exceed it, because the acceptance criteria are statistical.

IRC Table R402.2, minimum psi at 28 days (negligible / moderate / severe weathering)

Basement walls, foundations, other concrete not exposed to weather	2,500 / 2,500 / 2,500
Basement slabs and interior slabs on grade, except garage floor slabs	2,500 / 2,500 / 2,500
Basement walls, foundation walls, exterior walls, other vertical concrete exposed to weather	2,500 / 3,000 / 3,000
Porches, carport slabs, steps exposed to weather, garage floor slabs	2,500 / 3,000 / 3,500
Which column is yours	Weathering potential is assigned by the map and table at IRC Table R301.2. It is a location fact, not a judgment call, and it does not follow state lines.

The footnotes, which is where the mistakes are

Moderate or severe weathering	The concrete SHALL be air entrained. Total air content not less than 5 percent and not more than 7 percent by volume.
The one people miss	Concrete in the UNEXPOSED locations that is subject to freezing and thawing DURING CONSTRUCTION must be air entrained as well. A November pour is caught by that even though its finished position never sees weather.
Steel-troweled garage floors	Air may be reduced to not less than 3 percent if f'c is raised to not less than 4,000 psi. That is the code writing down the trade-off, because power-troweling air-entrained concrete is what causes surface delamination.
Deicing salt exposure	R402.2 caps fly ash, other pozzolans, silica fume, slag and blended cement as a percentage of total cementitious material for garage slabs and exterior flatwork exposed to deicing chemicals.

THE FINDING: the model table is not the answer, your state's amendment is

Minnesota amends this table outright. Minn. R. 1309.0402 adds a row the model IRC does not have at all, FOOTINGS, at 5,000 psi in negligible, moderate AND severe weathering.
The model code has no footings row. Footings fall under "basement walls, foundations, and other concrete not exposed to weather", at 2,500 psi. So Minnesota's figure is DOUBLE the model minimum for the same concrete.
Minnesota's own footnote lets you back down to 2,500 psi with an approved water or vapor resistant admixture, and the 5,000 does not apply to deck and porch post footings or to floating slabs.
A contractor working from the model IRC, or from any national summary of it, orders a 2,500 psi footing in Minnesota and is 2,500 psi light against a specified figure. Nothing in the model table hints that a state has moved it.
So the question is not only WHICH EDITION your jurisdiction adopted. It is what your state changed on the way. Check your state's own amendments to IRC Chapter 4 before you order, not just the edition number.

Where ACI 318 comes in, and where it does not

You do not need to buy it for this	The minimum strengths above are in the free adopting code. ACI is where the engineered detail lives, not where the residential minimum lives.
What the IRC points at	Materials must comply with ACI 318 Chapters 19 and 20 or ACI 332, and the deicing-exposure cementitious limits come from ACI 318 Section 19.3.3.4.
Exposure categories and classes	ACI 318 Table 19.3.1.1. The requirements by class are a different table, 19.3.2.1. A citation to "Table 19.3.3" for exposure classes is wrong and it circulates widely.
Exposure class F3	Present in ACI 318-14. Anyone telling you it is new in a recent edition is wrong, and a reader who believes it concludes his older-edition project is not covered by it.
The psi values by exposure class	NOT PRINTED HERE, on purpose. ACI 318 is paywalled, we have not read it, and the version of that table circulating in secondary summaries did not match its own cited sources when we checked. Get it from the standard or from your engineer, not from a card.

Table R402.2 is residential model code and binds you only through the edition your jurisdiction adopted. Minnesota is used above because its amendment is published and checkable; it is an example, not the only state that has moved these figures. Commercial and multifamily work goes to the IBC and to a structural specification, not to this table. Nothing here covers mix design, admixtures, reinforcement, curing, cold or hot weather working, or acceptance testing and cylinder breaks, all of which decide whether the concrete you actually poured meets the figure you specified.

Sources: iccsafe.org · revisor.mn.gov · concrete.org

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