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

Concrete Construction, Dec, 2002 by Ken Hover

Air-entrained concrete is made by adding a detergent (an air-entraining admixture) to stabilize air bubbles trapped during mixing, In fresh concrete, the air bubbles reduce the water demand of the concrete and make the mix stickier, which helps to reduce segregation and reduces bleeding when air content is around 3%. If the air content is higher, the increased stickiness makes the concrete more difficult to finish, and the bubbles can lead to blisters and delaminations on steel-troweled surfaces. As the concrete hardens, the cement paste sets around these bubbles, leaving bubble-shaped voids in the hardened concrete.

A typical cubic yard of air-entrained concrete has 5 to 15 billion of these microscopic bubbles with a combined volume of 4% to 8% of the total concrete volume. If for some reason you had to paint the inside surface of all of these air voids, you would need enough paint to cover 10,000 to 20,000 square feet in a single cubic yard of concrete!

As air content increases, paste content, aggregate content, or both have to decrease, and since paste, stone, and sand are all stronger and denser than air, a 1% increase in air content lowers compressive strength by 200 to 300 psi, and density drops by about 1%.

Furthermore, the bubbles in the fresh concrete are formed, trapped, and lost in the mixing process, and the surviving bubbles are then broken, squeezed, expanded, shaken, and floated during placing, vibration, and finishing. Consequently the final air content in the hardened concrete can be notoriously variable, difficult to predict, and tough to measure reliably. So why would anybody invite these potential difficulties by intentionally putting an air-entraining admixture in the concrete?

Why do we need air?

Air is sometimes added for water reduction, segregation resistance, and reduced bleeding. But you can more confidently achieve these same benefits with fewer finishing problems and without the risk of strength loss by adjusting the mix proportions and the combined aggregate grading, and by using normal, mid-range, or high-range water reducers (superplasticizers). To justify the disadvantages sometimes associated with entrained air, you need a compelling reason to use it. That compelling reason is that air is one of the keys to making concrete resistant to damage from freezing and thawing of absorbed water.

The first key to frost resistance is to select a durable aggregate; air entrainment cannot help when the aggregate is cracked, porous, absorbent, or weak. When aggregate popouts are the problem, changing the aggregate--not adding more air--may be the solution.

The second key is to reduce the concrete's ability to absorb water while providing sufficient resistance to the pressure caused by expansion as internal water freezes. This is done in accordance with the requirements of the ACI 318 Building Code by specifying a minimum [f'.sub.c] of 4500 psi and limiting the water-cement or water-cementitious materials ratio of the concrete to 0.45 whenever the concrete will be exposed to de-icing chemicals or to freezing and thawing while moist.

The third key to frost resistance is to whip billions of air bubbles into the concrete so that when the temperature drops below freezing, the expanding ice (and the unfrozen water pushed ahead of it) can squeeze into empty air voids. As long as the distance that ice and water have to travel to get to the nearest air void is no more than 1/100 inch, most concretes can tolerate the freezing pressures. All else being equal, the shorter the travel distance ("the spacing factor"), the lower the freezing pressure; so if 1/100 inch is good for most concretes, 8/1000 inch is better.

To space the bubbles close enough to control the freezing pressure requires a large number of microscopically small bubbles. Unfortunately the air bubbles in air-entrained concrete vary from the size of cement grains to the size of coarse aggregate particles, with an average radius in the range of 3 to 6 thousandths of an inch (3 to 6 mils).

To know if the bubbles are close enough in the hardened paste, you have to know how many bubbles you have. To do that you need to know their total volume and their average size. Volume we know, more or less, from standard air tests. Size we know only from microscopic surveys of hardened concrete or from nonstandard tests of fresh concrete. In some cases the specifier will require that tests be performed to evaluate the average size of the air voids (using the ASTM C457 test method). Most of the time, though, we just control the mix on the basis of total air volume, with the amount required based on experience, and with fingers crossed that the average bubble size will be small enough.

The total volume of air voids in the hardened concrete should be about 18% to 20% of the volume of the paste. When the sand content is about equal to the paste content, the total volume of air should therefore be about 9% of the total mortar volume, and this is the basis for the air contents required in Table 4.2.1 of the ACI Building Code. The table shows the required air content for the severity of exposure and nominal maximum aggregate size, reflecting the fact that mixes with larger coarse aggregate particles usually require less water, cement, and sand, and therefore have a lower mortar and air content. (Smaller aggregates are not inherently less durable than larger aggregates.) These code requirements are minimum values and would not normally be reduced in construction specifications for projects in moderate or severe freeze/thaw environments.