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Q. A concrete supplier for one of my projects wants to provide lightweight aggregate concrete with 6% air content to meet the specified unit weight. I typically don't allow air entrainment in floor slab mixtures to help minimize the risk of blistering from early finishing. What requirements or guidance on this topic is included in ACI committee documents or other resources?
A. This has been a controversial issue in recent years. While lightweight concrete is a useful material for meeting fire-resistance ratings, an air content of 6% is probably not necessary to meet the required equilibrium density, which is often misunderstood in relation to the fresh unit weight measured at placement. In any case, concrete with a target air content of 6% should not be finished by hard troweling.
ACI documents deliver a somewhat mixed message on air entrainment, with conflicts generally due to differing requirements for protecting concrete from freezing damage and providing a flat surface finish. For example, both Section 4.4.1 of the ACI 318-111 Code and Section 4.2.2.4 of the ACI 301-102 specification require concrete to be air-entrained under certain exposure conditions. However, the Optional Requirements Checklist for Section 4.2.2.4 of ACI 301-10 informs the specifier that "air entrainment should not be used in flatwork to receive a hard steel-troweled finish." Section 7 of ACI 301-10 specifically requires air entrainment for lightweight concrete exposed to weather but does not require air entrainment for all lightweight concrete and does not dictate the type of finishes that can be applied to exterior versus interior lightweight concrete. Section 11 of ACI 301-10, however, states that: "Concrete for slabs to receive a hard trowel finish shall not contain an air-entraining admixture or have a total air content greater than 3%."
The concern is not hard-troweling lightweight concrete but hard-troweling air-entrained concrete. Entrained air slows the bleeding rate of concrete slabs and increases the risk of trapping bleedwater and coalesced air beneath a densified surface. In most cases, only one of the two requirements (air entrainment or troweled finish) applies. Exterior concrete exposed to freezing and thawing, for example, is normally given a broom textured finish for slip/ skid resistance, and hard-troweled finishes are typically limited to interior concrete floors that will not be subjected to large...