The two most common tests of new concrete include the strength of a sample of concrete tested usually at 28 days and testing the air content of the fresh concrete on-site as the construction is progressing. But which one of these tests is more important? Which one should we pay more attention to?
If one of these tests is on the “marginal” side, is the concrete still acceptable?
This article helps explain the importance of these tests and provides information with which to decide if your concrete is still OK if these tests indicate strength or air content are not within expected levels.
First, let’s talk about strength. Concrete is a versatile, strong material that is easily used for both vertical (structural) and horizontal (paving) applications. But strength is a main concern primarily in structural applications such as walls, bridges, parking structures, and buildings where a minimum strength is necessary to ensure that the structural calculations performed by the engineers are valid and that the structure can carry the expected loads without cracking the concrete.
But what really matters is the strength of the concrete structure itself. And those concrete samples tested back at the lab are not a great indicator of the actual concrete strength in the element under consideration. The most common strength test utilizes a cylindrical or beam-shaped sample of concrete taken from the fresh concrete as it is being placed, and that sample is then taken back to the lab usually within a day or two, and cured in laboratory conditions until the time of the test (often 28 days).
If the structure is in a different temperature or moisture condition than the laboratory, then the strength test is only an indication of the potential strength of the concrete mixture, not an actual strength in the structure itself. If the actual strength of the structure is needed, other test methods such as maturity sensors, field-cured specimens, or in-place strength measurements should be used to determine strength for form or shoring removal, post-tensioning, or opening to traffic needs.
For pavements, floors, and slabs on grade, strength is a much lesser concern. A possible low-strength concrete mixture will not result in catastrophic failure of the slab but would instead result in marks or surface depressions from any early applied loads, and very rarely, at worst, a crack. But in most cases, the low strength is often a result of something other than bad concrete. In many cases, suspected low strength is usually the result of improper curing temperature of the samples at early ages.
In the summer months, concrete samples that are stored up to the allowable 48 hours in the field in a cooler with a closed lid, without any water or ice to help absorb the generated heat, will often reach 120°F to 140°F or more (versus the required ambient range of 60°F to 80°F). This will bake the test samples and will result in lower concrete strengths when tested. During winter construction, test samples that are not properly protected and allowed to freeze, the result will be the same – low concrete strengths.
Now let’s dig into air content. Exterior concrete mixtures that are exposed to the elements need to have purposefully entrained microscopic air bubbles which act as miniature pressure relief valves inside the concrete that allow freezing and thawing to occur without damaging the concrete. Most specifications require air to comprise approximately 5% to 8% of the volume of the concrete in order to be considered freeze-thaw resistant.
The total air content is measured in a sample of fresh concrete in the field, as the concrete is being placed. As long as the total air content (entrapped plus entrained) is within the specified range, the concrete is assumed to be durable. This holds true for most concrete mixtures and air-entraining admixtures, but the main concern is the size and spacing of the microscopic air bubbles throughout the cement paste. Smaller bubbles spaced evenly throughout the microstructure are better than larger bubbles spaced further apart. The size and spacing of air bubbles are not directly measured but are usually adequate for durability if the air content is within the specified range.
As you can guess, there is a delicate balance between strength and air content. The higher the air content, the less concrete there is in a given volume, which results in lower strength. The rule of thumb for the relationship between air and strength says that for every 1% of air content over the design value, the concrete strength will decrease approximately 3% to 5%. So most concrete producers usually don’t try to target the higher end of the range unless there are other concerns or considerations such as loss of air during transport, pumping, or consolidation of the concrete.
MDOT specifications differ slightly from general industry standards and require concrete air contents in the range of 5.5% to 8.5% to achieve full pay for the concrete item. However, an air content at the higher end of that range (say, 7.5% to 8.5%) does NOT mean that the concrete is of lesser quality than the same mix with an air content of 6.0% or 6.5%. Higher air content actually translates to better durability, because there are more air bubbles to withstand freeze-thaw action.
Strength is generally not an issue for pavements and slabs on grade, since other specification requirements result in actual concrete strengths that are much higher than the minimum strengths used for design purposes. So targeting the higher end of the air content range is good practice, and will not sacrifice too much strength and will actually result in longer-lasting concrete. As long as the air content is anywhere within the specified range, and the minimum strength is achieved, you can expect the concrete to be both durable and strong enough to last as long as expected.
In some applications such as parking structures, strength and air may be equally important. However, minimum strengths are typically very easily achieved, which makes air content and its associated durability more critical. In general, the answer to the question posed in the title is that air content is usually more important than strength for durability and longevity of the concrete, as long as minimum strengths are achieved.