At our precast plant, we put the air-entraining agent on the fine aggregates during batching. I've heard that it should be added with the mixing water. Is our procedure incorrect?
No. Adding an air-entraining agent to the fine aggregate isn't a problem. In fact, if you're using water-reducing admixtures in the concrete, they may have to be batched separately from the air-entraining agent because of potential compatibility problems. Thus, you could add the water reducer to the mix water and the air-entraining agent to the aggregate.