Air Entrainment
Deliberately entraining a system of microscopic air bubbles is the standard way US concrete resists damage from repeated freezing and thawing. This page covers the F1/F2/F3 exposure categories, how the target air content is set, and its strength trade-off.
F1, F2, F3 — the freeze-thaw exposure scale
ACI 318 assigns every element one freeze-thaw exposure category on the F scale, ranging from F0 (not applicable) up through F1 (moderate exposure to freezing and thawing, with limited water saturation), F2 (concrete in continuous contact with moisture prior to freezing, without exposure to deicing chemicals), to F3 (concrete exposed to freezing and thawing cycles that will be in continuous contact with moisture and exposed to deicing chemicals) — the most severe category. See Exposure Categories & Durability for the full four-scale system these categories belong to.
How target air content is set
For elements assigned an F1, F2, or F3 category, a target air content is set based on the combination of the exposure category's severity and the concrete's nominal maximum aggregate size (NMAS) — in general, a smaller NMAS calls for a higher target air content, and more severe exposure categories call for higher target air content, at a given aggregate size. This is a concept and a relationship, not a fixed number: the exact target percentages are set out in the current ACI 318 edition and should be read from that table rather than assumed. The calculator's airEntrained and targetAir fields are exactly where this decision is entered.
ASTM C260 admixtures
The bubble system that provides freeze-thaw protection is created with an air-entraining admixture, covered by ASTM C260, the standard specification for air-entraining admixtures for concrete. These admixtures stabilize a network of microscopic, closely spaced, spherical air voids in the fresh concrete, distinct from the larger, irregular entrapped air that occurs in any mixed concrete without deliberate entrainment and does not provide the same protection.
The strength trade-off
Entrained air content trades off against compressive strength: each additional percentage point of entrained air generally reduces strength somewhat, all else equal. This is one reason air-entrained mixes typically use a slightly lower w/cm than an equivalent non-air-entrained mix at the same strength target — to offset some of the strength reduction from the added air while still gaining the freeze-thaw protection. The mix design balances hitting the durability-driven target air content against the strength the project requires.
Frequently asked questions
What is the difference between entrained and entrapped air?
Entrained air is a system of microscopic, deliberately introduced bubbles created with an air-entraining admixture, sized and spaced to resist freeze-thaw damage. Entrapped air is the larger, irregular, unintentional air that ends up in any concrete from mixing and placing, and it does not provide the same freeze-thaw protection.
Does more air always mean better freeze-thaw protection?
Only up to the target set for the exposure and aggregate size — air content is a trade-off against strength, since more entrained air generally reduces compressive strength. The target air content aims for enough protection without giving up more strength than needed.
What does ASTM C260 cover?
ASTM C260 is the standard specification for air-entraining admixtures used in concrete — the chemical additives that create and stabilize the microscopic air bubble system.