TradesHow this works
Concrete strength is set by two things: what the concrete has to carry, and what the weather does to it. This tool takes the greater of the strength your application needs and the ACI minimum for your freeze exposure, then adds the air entrainment and water-cement limit the exposure calls for. In a cold climate the exposure usually wins, which is why a plain footing can still need a 4500 psi air-entrained mix.
Specify the strength to your ready-mix supplier as f′c at 28 days. When air is required, order a step up so the air-related strength loss still lands you on target. Confirm the details against your local code.
Common questions
What PSI concrete do I need?
It depends on both the job and the climate. Footings and interior slabs are usually 3000 psi, driveways and exterior flatwork 4000, and structural work 4000 to 5000. But if the concrete freezes and sees deicing salts, the exposure rules push it to at least 4500 psi with air entrainment no matter what the application would be alone.
Why does my slab need air entrainment?
Tiny air bubbles give freezing water room to expand without cracking the concrete, so any concrete exposed to freeze-thaw needs 5 to 7 percent entrained air. It is the single most important thing for exterior flatwork in a cold climate, and it slightly improves workability too.
Does air entrainment lower strength?
Yes, by roughly 4 to 5 percent per 1 percent of air. That is why, when air is required, you order the design strength a step higher so the tested strength still lands on your target. The durability gain is well worth the small strength trade.
Sources: ACI 318 Exposure Category F (freeze-thaw / deicing) minimum strength, air, and w/c; ACI 332 residential and NRMCA/industry strength-by-application norms (ACI). Engine version 1.0.0. Risk tier 1.