Experience has shown that gradation of both fine and coarse aggregate is often the most important factor in determining the pumpability of a concrete mixture, although other mix design characteristics also contribute. If gradation is poor, the excess of some aggregate sizes or efficiency of others will produce voids in the mix that permit the water to escape from it under pumping pressure. In most of the cases in which mixes cannot be pumped it will be found that there is something wrong with the mix itself. The problem may be gap grading in the sand, nonuniform aggregate preparation at the batch plant, inadequate mixing, too much slump or too little, aggregates that are too absorptive or perhaps too large for the pump or pipeline.
For pumpable mixes, the maximum size of coarse aggregate should be limited to 33 percent of the minimum opening in either the pump or the pipeline. For well-rounded aggregates this maximum size can be increased to 40 percent. Hence, a one and one-half inch gravel aggregate could be used in a pumping system with four inch openings. The properties of fine aggregate (sand) are more important in the design of pumpable concrete mixes than the properties of coarse aggregate. When combined with the cement and water, fine aggregate provides the fluid (mortar) which conveys the large solids (coarse aggregate) in suspension, thus making a mix pumpable. The fine aggregate should meet gradation requirements of ASTM C33 as closely as possible to the middle of the grading range.
Presently, there is no recognized testing method or equipment available for laboratory determination of pumpability. Accordingly, a full scale field trial is advisable using actual job conditions anticipated for batching and truck mixing, pump and operator selections, and pipeline and hose layouts of the exact height and distance contemplated . Such a test will involve time, effort and money, but it will furnish evidence of job dependability and produce innovations for efficiency and economy that will benefit the contractor, pump operator, and concrete supplier.