What Strength Concrete to Order, and What Else to Tell the Plant
Code sets minimum psi by weathering exposure and air content separately; here is how to read both, and what ASTM C94 expects a ready-mix order to state.
Use the Concrete Calculator — work out how many cubic yards or bags the pour needs once you know which mix to order.
The psi comes from the weather column, and sometimes the seismic category
The first figure on most ready-mix orders is f'c, the specified compressive strength, and on a house built under the 2024 International Residential Code the floor for it is set in Table R402.2. The table's columns are not uses of the slab; they are weathering potential. Footnote (a) fixes the test age at 28 days. These are minimums. The footnote letters in the cells are covered in the air section.
Unexposed concrete carries 2,500 psi in all three weathering columns, and the biggest jump is on the last row: exposed porches, carport slabs, steps and garage floors go from 2,500 psi in a negligible area to 3,500 psi in a severe one, a rise of 1,000 psi. Section R404.1.3.3.1 requires concrete foundation walls built to the IRC's prescriptive wall provisions to reach 3,000 psi at 28 days in Seismic Design Categories D0, D1 and D2, against 2,500 psi in categories A to C. For quantities rather than mix, use the concrete calculator.
| Type or location (shortened) | Negligible | Moderate | Severe |
|---|---|---|---|
| Basement walls, foundations, other concrete not exposed to weather | 2,500 psi | 2,500 psi | 2,500 psi (c) |
| Basement slabs and interior slabs on grade, except garage floors | 2,500 psi | 2,500 psi | 2,500 psi (c) |
| Basement, foundation and exterior walls and other vertical concrete exposed to weather | 2,500 psi | 3,000 psi (d) | 3,000 psi (d) |
| Porches, carport slabs and steps exposed to weather; garage floor slabs | 2,500 psi | 3,000 psi (d, e, f) | 3,500 psi (d, e, f) |
Your column is a local answer, not a forecast
The model code leaves the weathering entry in Table R301.2 blank for the local jurisdiction to complete, and footnote (a) to that table says it is filled in as negligible, moderate or severe using Figure R301.2(1), the weathering probability map for concrete. The notes to that map describe severe as heavy snowfall plus long spells without natural thawing, so that deicing salt is used widely, and warn that the boundary lines are approximate and local conditions can be better or worse than the region suggests.
The IRC is also a model code that states and cities adopt on their own schedule, often with amendments. The International Code Council announced its 2027 International Codes, including a new IRC, on 24 August 2026; nothing here has been checked against them. So ask the building department two things: which weathering index applies at this address, and which code edition is being enforced. Chapter 44 of the 2024 IRC ties its concrete rules to ACI 318-19 and ACI 332-20, and R402.2 itself points to both, so a later ACI edition does not govern your permit unless your adopted code names it.
Air content is its own line on the order
Footnote (d) requires air-entrained concrete with total air of 5 to 7 percent by volume, and it sits on the moderate and severe cells of the two exposed rows. A 3,500 psi porch mix with no entrained air does not comply. Footnote (c) adds a construction-stage rule for the severe column: unexposed concrete subject to freezing and thawing during construction must be air-entrained to footnote (d), so a basement slab left open to a winter sky can count.
Footnote (f) is the one place the IRC table links the two numbers. A garage floor given a steel-troweled finish may drop to not less than 3 percent air, but only if f'c is raised to at least 4,000 psi. ASTM C94/C94M-24a has a similar rule in 8.1: its default air targets may be cut by one percentage point where the specified strength is 5,000 psi or more. Both are fixed permissions, not a way to convert air into psi. The same edition's Note 7 warns that specifying more air than its table may reduce strength without adding durability.
R402.2, which footnote (e) points to, also limits fly ash, other pozzolans, silica fume, slag and blended cements in garage floor slabs and in exterior porches, carport slabs and steps that will see deicing chemicals, by citing the percentages in Section 19.3.3.4 of ACI 318 rather than printing them.
What ASTM C94 expects the order to say
The IRC's foundation-wall provisions require mixing and delivery to comply with ASTM C94/C94M or ASTM C685 (R404.1.3.3.2), and C94 lists what an order must state. The 2024 IRC's Chapter 44 references C94/C94M-21b, ASTM's catalogue lists C94/C94M-26c as the active edition, last updated 12 August 2026, and the C94 section numbers below come from C94/C94M-24a, the edition consulted for this page. Check the numbering against whichever edition your contract names.
In -24a, Section 6, Ordering Information, says that where no project specification governs, the purchaser's order includes, among other items, the designated size or sizes of coarse aggregate (6.1.1), the slump wanted at discharge (6.1.2), the total air content at discharge for concrete facing freeze-thaw cycles (6.1.4), and which of Options A, B or C sets the proportions (6.1.5). Under Option A (6.4) the producer takes full responsibility for proportioning and the purchaser states the required compressive strength, tested at 28 days unless the order says otherwise. Under Option B (6.5) the purchaser takes responsibility and states the cement content, the maximum allowable water content and any admixtures. Under Option C (6.6) the producer proportions the mix, and the purchaser states the required strength and a minimum cement content.
For a homeowner Option A is the practical choice, since B asks you to design the mix. For foundation walls built under R404.1.3 the IRC adds its own limits. Nominal maximum aggregate size may not exceed one-fifth of the narrowest distance between forms or three-fourths of the clear gap between bars, or between a bar and the form (R404.1.3.3.3, with an approved exception for removable forms), so an 8-inch-thick wall caps it at 1.6 inches before the bar rule applies. Slump may not exceed 6 inches in removable forms without approval and must exceed 6 inches in stay-in-place forms, measured by ASTM C143 (R404.1.3.3.4).
Put together, an order for an exposed porch slab in a severe area reads: 3,500 psi at 28 days, 5 to 7 percent air, a slump agreed with the finisher, a coarse aggregate size the plant stocks, Option A, the yardage and, if the slab will see deicing chemicals, the R402.2 limit on fly ash, slag and the other materials it lists.
A bag's strength figure is not an order
The typical-properties table on QUIKRETE's data sheet for Concrete Mix No. 1101, revised October 2022, gives 2,500 psi at 7 days and 4,000 psi at 28 days by ASTM C39 and a 2 to 3 inch slump by ASTM C143. Those are the maker's typical test results, not an f'c a supplier has contracted to meet, and the sheet gives no air content value. That is not proof the product lacks air, but the sheet alone does not show the 5 to 7 percent footnote (d) requires on an exposed porch in a moderate or severe area. Ask the manufacturer in writing before using bags there.
Sources
- 2024 International Residential Code, Chapter 4 (read in the UpCodes rendering of the GSA Residential Code 2024, which adopts the 2024 IRC without amendments): Section R402.2 and Table R402.2 Minimum Specified Compressive Strength of Concrete, footnotes a-f (28-day age; 2,500/3,000/3,500 psi by weathering potential; 5-7% air; 3% air with 4,000 psi for steel-troweled garage floors; footnote e pointing to R402.2; SCM cap referred to ACI 318 Section 19.3.3.4); R404.1.3.3.1 (3,000 psi in SDC D0-D2, 2,500 psi in A-C); R404.1.3.3.2 (ASTM C94 or C685); R404.1.3.3.3 (nominal maximum aggregate size); R404.1.3.3.4 (slump limits, ASTM C143)
- 2024 International Residential Code, Chapter 3 (same rendering): Table R301.2 (left blank in the model code for the local jurisdiction) and its footnote a (weathering column filled in as negligible, moderate or severe from Figure R301.2(1)); Figure R301.2(1) Weathering Probability Map for Concrete, notes (lines approximate; definition of severe)
- 2024 International Residential Code, Chapter 44 Referenced Standards (same rendering): ASTM C94/C94M-21b (referenced by R404.1.3.3.2, R608.5.1.2); ACI 318-19 and ACI 332-20 (both referenced by R402.2)
- ASTM C94/C94M-24a, Standard Specification for Ready-Mixed Concrete, preview text on the iTeh Standards catalogue page: Section 6 Ordering Information (6.1, 6.1.1, 6.1.2, 6.1.4, 6.1.5, 6.4 Option A, 6.5 Option B, 6.6 Option C), 8.1 (one-point air reduction at 5,000 psi or more), Note 7 (air above Table 1 may reduce strength)
- ASTM International store, catalogue record for ASTM C94/C94M-26c: status Active, last updated August 12, 2026 (full text not read)
- International Code Council news release, 24 August 2026, on the 2027 International Codes (names the IRC among them)
- QUIKRETE Concrete Mix, Product No. 1101 data sheet, revised October 2022: Table 1 Typical Physical Properties (slump 2-3 in, ASTM C143; 2,500 psi at 7 days and 4,000 psi at 28 days, ASTM C39); no air content value on the sheet
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