Increased air, decreased strength
As the air content increases, there is almost always some decrease in strength. Typically, this strength loss is about 4 percent for every 1 percent increase in total air content. There was a difference of about 3 percent air between the higher air mixes and normal air mixes. This should yield a decrease in strength of about 12 percent.
However, the actual decline in strength was more than 25 percent. These higher air content concrete samples produced significantly more visual signs of air void clustering around the coarse aggregate. The plastic air results for both mixes, high and normal air content, were within what most specifications would allow. But a 25 percent decrease in strength would be catastrophic.
The issue of having air voids that cluster around the coarse aggregate happens predominately in high-strength or high cementitious content mixes. These are typically utilized on structural elements. If the strength of the concrete is not adequate, the expense of repairing or removing the concrete can be colossal. Combine this with the difficulty identifying if the air voids are clustering around the coarse aggregate in the plastic concrete, and the problem may seem insurmountable. There are some precautions that can be taken to minimize the exposure to this issue.
The fact that good concrete testing procedures will minimize the chance of air voids clustering around the coarse aggregate may be self-evident, but it is also accurate. Even with the best equipment and materials, there are many issues that can affect the air content.
For example, a change in concrete temperature of about 20 degrees F can change the air content by more than 1½ percent according to the PCA's Design and Control of Concrete Mixtures. This could easily push a well-designed mix at 80 degrees F into a mix that is prone to having air voids clustering around the coarse aggregate at 60 degrees F. The importance of checking the air content often during the day where dramatic temperature swings are expected cannot be overstated. As was noted, a 1½ pre-cent change in air content can dramatically increase the potential for air void clustering and hence, low strengths.Proportioning issues
Proper proportioning of concrete mixtures is another obvious issue in minimizing potential quality problems. Testing showed that the degree of air void clustering varied with the percentage of fine aggregate.
The amount, type, and size of the coarse aggregate can affect the size of the air void system, and the size of air void system appears to be related to the amount of air void clustering around the coarse aggregate. For these reasons, any high cementitious mixes (more than 564 pounds per cubic yard) should be tested with varied fine aggregate contents.
Most producers have a level of fine aggregate that they currently use. Varying this by 5 percent above and below this level should minimize the potential for air void clustering around the coarse aggregate. This could be done in combination with a test that simulates retempering of the concrete.
Varying the fine aggregate content and testing the effects of retempering should provide data on whether the mix is prone to air void clustering around the coarse aggregate. While this testing does not guarantee that the mix will never have air void clustering, it provides some level of confidence and knowledge of the mixes' behavior.
The strength decrease from air voids clustering around the coarse aggregate caused by retempering was almost 20 percent in some cases, according to the PCA research. Retempering the concrete is an issue that is out of the control of most producers. But there are some means that can be taken to reduce the occurrence of retempering.
Certain admixtures extend the period of time concrete remains workable. These can reduce the amount of retempering that is done in the field. Different cements have different periods of slump retention, whether used alone or in combination with admixtures or supplementary cementitious materials.
It is vital that a concrete producer have a thorough understanding of how their materials interact. This can aid in reducing the amount of retempering and can reduce the amount of air void clustering around the coarse aggregate.
Another tool producers can use is a 20X magnifying loop. Breaking concrete cylinders exposes the aggregate. Usually, the aggregate is not fractured when the strengths are low and the break occurs along the paste/aggregate interface.
When viewing the exposed aggregate under 20X magnification, air voids will be visible if they've clustered there. The figure on the left shows how this clustering appears. This method is not a definitive determination of whether air voids clustering around the coarse aggregate is the main problem. But if “foam” is viewed on the coarse aggregate particles, further investigation and testing to verifiy if the air void clustering is the main issue is warranted.
As more research becomes available and the issue becomes more fully understood, further recommendations may come to light. Good quality control and rigorous testing are the best defense against many issues. Air voids clustering around the coarse aggregate is no exception.
Christopher Kennedy is technical services manager at St. Mary's Cement. Email firstname.lastname@example.org.