And on those tough jobs, the technician can find it difficult to compensate for multiple factors at the same time. Even the most experienced technician can find it almost impossible to be on target on every load when the amount of air content changing from each batch is unpredictable.
Worse, some of these factors causing a change in air content are decided at the last moment in the field. Since the most important air content reading is the last one, a producer can't assume that if fresh concrete leaves the plant in spec, it will be placed in spec.The causes
Most QC managers agree there are six main factors influencing air content: carbon in fly ash, mixing time, concrete temperature, loss from pump placement, adding water in the field, and interaction from other ingredients and admixtures. These factors interfere with the ability of conventional AEAs to create and maintain air bubbles in concrete. Conventional AEAs are all surfactants. They include soaps and detergents.
AEAs react readily with the water, the agitation, and the cement. The AEA is inserted directly into a mixer with the batch water. As the drum agitates, the agents create a foam made up of tiny bubbles. These bubbles disperse themselves throughout the mix because the surfactants also have an attraction for the cement.
In mixing, air content typically increases for a few minutes as the agitation froths the AEA. Then, air content falls as mixing time increases. Hot weather can cause the foam to disperse, speeding up the loss of air content. Pumping concrete compresses air, destroying bubbles and reducing air content. Adding water in the field creates new bubbles, raising air content. Some admixtures and high-alkali cements interact with AEAs, destroying bubbles and reducing air. All of these effects vary.
Carbon absorbs surfactants, leaving less AEA to make bubbles. While the batch operators can add more AEA to compensate, there's always the question of how much attaches to the carbon, and how much remains free to produce bubbles. How different fly ashes vary in the amount of reactive carbon is another challenge.
With tighter supply, producers have noticed that the carbon problem is growing worse. As the supply of low-carbon ash lessens, producers are trying to find ways to incorporate more high- and variable-carbon material. “We had 16 loads of fresh concrete rejected because we unexpectedly received a load of fly ash with very different carbon contents from what we had been receiving, and it changed our air content sharply,” says Jack Gibbons of Prairie Material in Bridgeview, Ill.Stable air entrainer
Recently, a new type of AEA called Miracon has been released that is reported to be more stable than conventional AEAs. Instead of a surfactant, this admixture is formulated with polymers that are chemically inert.
Charles Welker of Miracon Technologies Inc. explains, “Typically, mixes designed with our polymer air entrainment are unaffected by carbon levels or by high-alkalinity cements, and are considered stable when used with all other common admixtures. Producers can use nearly any fly ash and the effect on the air content can be virtually zero.”
Tests and field experience indicate that polymer air entrainment is also less affected by mechanical action such as mixing, the compression of pumping, or outside temperature. “The amount of air you put in at the plant is the amount you get in the field, with minimal variation,” says Welker. “Even adding water has shown no effect.”