Everyone in the concrete industry has received the dreaded call about low strength on a job. Typically, high water content, high air content, or low standards from the testing company is the primary cause of the problem.

But there are cases where these issues do not completely explain the low-strength results. In the early- to mid-1990s more low-strength issues started to appear that couldn't be traced back to the usual suspects.

It was about this time that air entraining agents went from being predominately vinsol resin to the newer synthetic agents. It has been well-documented that the newer generation of air entraining agents produces a smaller, more stable bubble. This generally increases the durability of the concrete by increasing its freeze/thaw resistance.

However, there have been several cases in these low-strength investigations where air voids have clustered around the coarse aggregate. The departments of transportation in New Jersey, Delaware, South Dakota, Minnesota, Virginia, New York, Michigan, and Ohio have all reported claims of significant strength loss due to air voids clustered around the coarse aggregate

Since this issue is difficult, if not impossible, to diagnose while concrete is in the plastic state, it is vital to understand and minimize the issue in the mix design phase. Knowledge of the concrete materials and mix design will lead to eliminating the issue.

The main effect of air void clustering around the coarse aggregate is a significant reduction in compressive strength. Since clustering is usually only noticed when the compressive strengths are below design strength at 28 days, the concrete is likely in service at that time.

The methods used to test air content in the field do not indicate if the air voids have clustered around the coarse aggregate. These tests will give a result of the total air content. There is currently a test that can give a total air content reading and the spacing and size of the bubbles.

The test is conducted on the paste segment of the concrete. Since there is no aggregate in the sample that the test is conducted on, it would be impossible to detect the clustering issue. Although this provides much more valuable data than the standard air test, it still does not indicate if the air voids are clustered around the coarse aggregate. It is also an extremely expensive and rarely used test.

It is in the hardened concrete samples that the issue can be diagnosed. Unfortunately, the concrete is in place at this point and any remediation is likely to be extremely expensive. Even in the hardened state, it can be difficult to diagnose the air void clustering.

It typically takes 28 days before there is an indication that the concrete's strength might be less than adequate. After this, typically the in situ compressive strength of the concrete is tested to ensure that the compressive strength tests were conducted correctly.

If these results are lower than the design strength, then usually a sample of the concrete is taken and submitted for a petrographic examination. This can take two months before the cause of the low strength is determined. That is why it is crucial that the cause and possible remedies of air void clustering be understood and minimized in the design phase.

Air voids as the cause

All of the data from the literature and the testing do not provide an absolute cause for the air voids clustering around the coarse aggregate. But there is a considerable amount of information and data that indicate the size of the air voids is likely the underlying issue.

Synthetic air entraining agents form smaller air voids. Higher air contents appreciably increasing the potential for the air voids to cluster around the coarse aggregate could be directly related to this phenomenon. As the air content increases, the average size of the air voids tends to decrease. Even the varied results seen with the different levels of fine aggregate can be explained by the size of the air voids. Crushed aggregate can increase the amount of smaller air voids in a system.

According to PCA research, retempering the concrete leads to an increased potential for air voids clustering around the coarse aggregate. The severity of the air void clustering was increased dramatically when the concrete was retempered with the synthetic air entraining agents. The amount of strength loss was dependent on the type of air entraining agent, but it ranged from 8 percent to 22 percent. It was also noted that the specific surface area increased as the concrete was retempered in most cases. Once again, the finer air void system could be the root cause leading to the increase in clustering and decreasing strength.

The same PCA study found that the crushed limestone aggregate mixes lost 22 percent of their strength due to air void clustering. The same mix with a siliceous aggregate only lost 8 percent of its strength. There could be many reasons for this, but it may also relate back to the size of the air void. The study states “crushed or angular aggregate aid in retention of smaller size entrained air bubbles, resulting in a decrease in spacing factor and increase in specific surface of the air voids.”

One test studied the effect of limestone on the formation of air voids around the coarse aggregate. To summarize, the percentage of coarse aggregate was varied in the mixes. They were tested to see if this variable affects the degree of air void clustering. The percentages of fine aggregate in the mixes were 45 percent, 40 percent, and 35 percent.

When the air content was at a normal level, around 6 percent, the concrete with 40 percent fine aggregate had the best compressive strength. When the air was increased to around 8 percent, the 40 percent mix had the lowest compressive strength. Another intriguing aspect of these tests was that the 45 percent fine aggregate mix always came in second place. The reason for these swings could stem from the effect coarse aggregate has on the air void size and the degree of clustering.