What psi should a concrete slab be?

For an outdoor slab, the National Ready Mixed Concrete Association's advice is to order 4,000 psi, matching ACI 332, the code for residential concrete; it drops to 3,500 psi for concrete that won't be continuously moist or get deicing chemicals. That advice is written with freezing in mind, and in Waco the low drops to 32°F or below on 34.1 days in an average year (1991–2020 normals).

Under the International Residential Code, garage floor slabs, porches, carport slabs and weather-exposed steps need 2,500 psi in negligible weathering areas, 3,000 in moderate ones and 3,500 in severe ones, measured at 28 days.