Q. Plans for an infill project call for a poured stem-wall foundation, but the urban site has limited access for a regular concrete truck. A pumper truck would be ideal, but I’m concerned that the pumped mix won’t provide the required 4,000-psi concrete. Is this a legitimate concern?
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 superplasticizers — to their concrete. But you don’t have to spell the mix out to your ready-mix supplier, since most suppliers have experience with concrete for pumping. Just be prepared to tell him the size of your pumping contractor’s pump and diameter of his hose, how far and how high the concrete will be pumped, the total quantity of concrete needed, and the placement rate.
To avoid delays that can lead to the concrete stiffening in the hose, be sure to coordinate timing between the pumping contractor and the ready-mix producer so there’s a steady supply of concrete. Also, some grout slurry should be available before starting to pump so it can be run through the hose to slick the line. Have a plan for dealing with the concrete remaining in the hose and pump hopper when you’re done. The best solution is often to dump it back into a ready-mix truck.
Finally, make sure your crew knows proper safety procedures for concrete pumping, including the hand signals needed for communication between the placing crew and the pump operator. You can find the signals on the American Concrete Pumping Association’s Web site (concretepumpers.com), in the downloadable guide “Checklist for Pumping Ready Mixed Concrete.”
