In little more than five years the pumping of concrete has been jet propelled from relatively crude pioneer beginnings into a sophisticated construction practice respected for its ability to telescope both pumping distance and time. The pumps, carriers, the limes and booms are proliferating in number, power, and capacity. A man who understands the basic economics of pumping has a wide choice of sizes, capacities, features, and reach. But what about the concrete itself? Does a contractor considering pumping have to be nervous about the ingredients, or can he adopt the new method with confidence, since he already knows how to distinguish good concrete from bad? The rule of experience is: maximum rock size should not exceed 40 percent of minimum clearance in the system. How does this work out in practice? It means maximum 1 inch rock for the conventional 3 inch hose, maximum 1 and one-half inch for 4 inch hose and pipe. For the cement, working range is 4 and one-half sacks minimum; 5 and one-half is desirable, and 6 or more may required to overcome aggregate deficiencies. Mixes up to 7 and one-half sacks are not uncommon. High early, expansive, quick-set and special products mixes have been handled satisfactorily. Some pumps also handle mixes with less than 5 sacks for back-fill. Some operating tips for better pumping: depending on mix and temperature, concrete can be held in the lines up to 1 hour and pumping resumed; good housekeeping and cleaning with proper attention to the lubricant and routine maintenance assure continuous, trouble-free placer operation; coating the outside of the machine with form oil makes it easy to keep clean; foreign material such as wood, rags, tramp iron, and clay balls tend to interfere with pumping so be alert and use a hopper screen; finally, good grouting or slicking of the line is necessary.