Maintaining the target for entrained air in fresh concrete is like walking on a tightrope over a canyon on a windy day. You have the goal to arrive at the other side. And you know the path you have to take. But it's a difficult task keeping all the variables in balance, and the consequences of failure are bad no matter which side you fall on.
Monitoring entrained air is an operational problem common to all producers. Almost all exposed concrete in moderate and cold climates is specified to have entrained air. Concrete with entrained air is far more durable in freeze-thaw conditions.
There's little disagreement about how much air should be entrained. Specifiers and producers look to Table 1.1 in ACI 201-1 for guidance. The document outlines the recommended air content for frost-resistant concrete. In general, most specifiers request 6% of total volume to be air bubbles.
Maintaining procedures to meet this target is the balancing act. If air volume falls short of the specification significantly, deterioration may be faster, leading to bad concrete and expensive replacement. On the flip side, since entrained air lowers strength, too much air can affect final compressive strength.
Each percentage point of air reduces concrete strength by an estimated 5%. So air content elevated by only two percentage points can cut the strength of a 4000 psi mix by 400 psi. This could cause the sample to miss the strength specification.
Controlling air content is more than an admixture dosing issue. Field conditions, ambient temperature changes, and delivery times can make it difficult to consistently hit the target.
A QC program that executes tight control of air content helps build customers' confidence in their suppliers. As more project engineers and managers turn to performance specifications, producers will need to demonstrate that they can predict and control critical properties like air volume.
Fortunately, help is on the way. First, the admixture industry is responding by offering more stable air entraining admixtures (AEAs). Second, many suppliers of supplementary cements, often the causes of some variances, are adopting tighter production techniques to provide a more consistent mix ingredient.
Monitoring air content is costly and time-consuming, says Terry Murphy of Meyer Material Co. in Illinois. “If my air content didn't vary, I could eliminate half of my QC department's workload,” he says. QC managers across the country echo this.
Several factors affect fresh concrete's air content. A technical bulletin from admixture supplier W.R. Grace (TB0200) lists 27 items that influence the air entrained in a mix. What's even more challenging is while a QC technician may know that a particular factor will tend to raise or lower the air content, the technician does not know the amount of change.