A. Bill Palmer, president of Complete Construction Consultants in Lyons, Colo., responds: If your pumping contractor is planning on increasing the amount of water in the concrete mix to increase its “pumpability,” you have reason to be concerned, since this would lower the cured concrete’s compressive strength. But most specifications prohibit adding water at the job site, and this isn’t the best approach for producing a pumpable mix anyway. In fact, a wet, high-slump mix often won’t pump as well as a stiffer one because the coarse aggregate will separate from the mix and clog the pump hose. Generally speaking, the maximum slump for a pumped mix is about 6 inches.
The ideal concrete mix for pumping typically contains air-entraining agents, a little extra sand (compared with a standard concrete mix), often some fly ash, and a well-graded aggregate blend. To make the concrete easier to pump and place as it comes out of the hose, many batch plants also add water-reducing admixtures — known as...
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