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

Concrete Construction, Dec, 2002 by Ken Hover

But, is it a good idea to increase these values in specifications to either achieve more freeze/thaw protection or to compensate for the normal and inevitable losses in air volume that will result from placing, vibrating, and finishing the concrete? This is an important and complicated question. To answer it we must be realistic about our inability to predict precisely the requirements for freeze/thaw durability for a specific concrete in a specific environment. Exactly how many voids are needed, what size, and how far apart they are depend on several things:

* Number of freeze/thaw cycles the concrete has to endure

* Rate of freezing and duration of below-freezing temperature

* Porosity and permeability of the paste

* Volume of the paste

* Degree of saturation

* Presence of de-icing salts

* Geometry of the concrete member, and

* Tensile and compressive strength of the concrete

Usually we don't know all of these factors in detail, but we also lack a reliable way to take them all into account. The experience-based values in the table cannot account for the specifics of all mixes, materials, and environments. This means that meeting the code requirements generally leads to durable concrete, but not meeting the same requirements does not always mean that the concrete will lack durability.

Furthermore, the question of compensating for air losses from the handling of the concrete is critical but not necessarily new. What is new is increased attention to testing air at the point of placement, and what we've found is that dropping, shaking, squeezing, and vibrating concrete normally results in a loss of air volume. One of life's little compensations, however, is that the larger, and therefore less effective, air bubbles are most likely to be lost first, leaving the smaller and more effective air bubbles in the concrete.

What makes this a balancing act is that durability suffers when air content is too low, and strength and finishability can suffer when air content is too high. (Increasing air by 1% has about the same strength-reducing potential as adding 2 gallons of water per yard. Compensating for this requires a cement addition of around 35 to 45 pounds per yard.) Some specifiers solve this problem by giving priority to freeze/thaw durability, with far less emphasis on strength. High air contents are the rule for these specifiers, and high strength acceptance criteria and steel trowel finishes are the exceptions.

Despite these complications, the code requirements are somewhat conservative. The tolerance of [ or -] 1 1/2%, along with the option of a 1% reduction when [f'.sub.c] exceeds 5000 psi, is evidence of that conservatism. (The Portland Cement Association has recently reconfirmed the reasonableness of these provisions.) More good news is that the table values are based on 9% air content in mortar for mixtures with water and cement contents not reduced by water reducers, pozzolans, or blended aggregates. When paste volume is decreased, so is the volume of air needed for frost resistance, and this can give us a bit of playing room when it comes to air requirements for modern mixes.