Air in concrete: how come and how much? Part 1 of a two-part series

Concrete Construction, Dec, 2002 by Ken Hover

To explore this in more detail, remember that when it comes to air voids and frost protection, what really matters is how far apart the air voids are, and this depends on the number, size, and total volume of the bubbles relative to the paste volume--not to the concrete volume. What matters most are these characteristics in the hardened concrete--not in the fresh concrete. The graph on page 60 shows the combined effect of bubble size and paste content required to achieve a spacing factor of 0.008 inch in a mixture with a water-cement ratio of 0.45. As water content is reduced (such as by water reducers or by improved aggregate grading) and as bubble size decreases, the required total air content decreases as well.

Let's take the example of a mixture with a 3/4-inch coarse aggregate in a severe environment. For these conditions the 318 Code requires a maximum w-c of 0.45 and a minimum air content of 6%. ACI 211.1 (Mix Proportioning) suggests a water requirement of about 305 pounds of water per cubic yard for a 3- to 4-inch slump. If, by using admixtures and aggregate blending, we were able to reduce this water content by 10% to 275 lb/CY, we could develop adequate freeze/thaw durability at an air content in the hardened concrete of 5% if the average air-void radius were 5 mils. We could achieve frost resistance at 4% hardened air if the average bubble size were about 4 1/2 mils, as, is fully possible with modern air-entraining admixtures. As long as the 4% air that remained in the hardened concrete was fine enough, this concrete could sustain a 2% air loss during handling, from 6% at the truck, and not jeopardize durability. If the air content had been increased to 8% in anticipation of a 2% loss, the strength could have dropped by 500 psi or more.

Air is almost always lost in the processes of placing, vibrating, and finishing, and for many years the ACI 318 requirements have been generally satisfactory when air has been measured at the truck only. Therefore, it is apparent that using these as required values at the chute prior to experiencing a 1 1/2% to 2% loss of the larger air bubbles has generally worked. It may be necessary, however, to target higher values when anticipated losses are greater or when construction operations selectively remove the finer bubbles. Getting a handle on these situations will be discussed next month.

Table 4.2.1--Total air content for
frost-resistant concrete *

Nominal maximum      Air content, as a percentage of
aggregate size,          total concrete volume.
  in inches.
                   Severe exposure    Moderate exposure

      3/8                7.5                6
      1/2                7                  5.5
      3/4                6                  5
       1                 6                  4.5
     1 1/2               5.5                4.5
       2                 5                  4
       3                 4.5                3.5

* From ACI 318-02 Building Code for Structural Concrete, Section 4.2.1