Q: Presently, our specifications for exterior flatwork call for a compressive strength of 3500 psi with 4% (±1%) entrained air. I've been told that some municipal agencies require up to 9% entrained air for exterior concrete exposed to freeze-thaw conditions. How much entrained air should we specify for sidewalks, patios, curb and gutter, and other exterior flatwork?

A.: The proper amount of entrained air to resist the effects of freeze-thaw is dependent on such factors as aggregate size and freeze-thaw exposure. Guidelines and recommendations for the amount of air required can be found in ACI 301,"Specifications for Structural Concrete for Buildings," and ACI 318, "Building Code Requirements for Reinforced Concrete." These guidelines can be incorporated by reference when writing specifications. The table summarizes and compares the suggested air contents. ACI 301 requirements apply only to concrete subject to destructive exposure (see definition in box) and are stated in a range without a tolerance. Some specifications allow a 1% reduction in air content when compressive strengths are greater than 5000 psi, because higher-strength concretes have greater frost resistance.ACI 332,"Guide to Residential Cast-in-Place Concrete Construction," suggests air contents similar to ACI 318. ACI 332 includes a map of the United States to assist in determining exposure levels.For more information about air contents for frost-resistant concrete, see the article "Specifying Air-entrained Concrete" (Concrete Construction, May 1993, pp. 361-367).

Total Air Content for Frost-resistant Concrete

Nominal Maximum Aggregate Size (inches) .3755-964

Air Content (percent) 6.753 .5-6 .55

ACI 3187 .551 .52 .5-5 .5

ACI 3016-1064 .53

Moderate Exposure .54-85.5 3 .5

Severe Exposure 5.513-64 .5

Destructive Exposure 74.52 1.5-4 .5

Note:

Total Air Content for Frost-resistant Concrete
Nominal Maximum Aggregate Size (inches) .375

5-9

6 4
Air Content (percent) 6 .75 3 .5-6 .5 5
ACI 318 7 .5 5 1 .5 2 .5-5 .5
ACI 301 6-10 6 4 .5 3
Moderate Exposure .5 4-8 5.5 3 .5
Severe Exposure 5.5 1 3-6 4 .5
Destructive Exposure 7 4.5 2 1.5-4 .5

The requirements of ACI 318 match the recommended total air content values of ACI 210.2R, ACI 211.1, ACI 345, and ASTM C 94. The required or recommended air contents depend on aggregate size and freeze-thaw exposure conditions (described below). ACI 318 specifies a tolerance of ±1 1/2% on total air content. ACI states a range and doesn't indicate a further tolerance.

Moderate exposure

-Conditions in a climate where freezing is expected, but where concrete will not be continually exposed to moisture or free water for long periods before freezing and will not be exposed to deicing agents or other aggresive chemicals. Examples include slabs that are not in contact with wet soil and will not receive direct applications of deicing agents.

Severe exposure

-Conditions in which concrete is exposed to deicing chemicals or may become highly saturated by continued contact with moisture or free water before freezing. Examples include pavements, curbs, gutters, and sidewalks.

Destructive exposure

-Conditions in which concrete is exposed to freezing and thawing, severe weathering, and deicing chemicals. (Similar to severe exposure conditions.)