The pump normally used in the large-line systems consists of a receiving hopper, two valves, a piston and a cylinder. The hopper ususally includes remixing blades to maintain the consistency of the concrete and to keep it moving. Pumping proceeds on a two-stroke action. The suction stroke of the piston is arranged to coincide with the opening of the inlet value from the hopper and the closing of the outlet valve which controls the flow of the mix into the transport line (pipe or hose). This suction stroke, assisted by gravity, draws the concrete into the cylinder. On the forward stroke the opposite occurs (the inlet valve closes and the outlet valve opens) to enable a slug of concrete, approximately equal to the displacement of the piston, to be forced along the pipeline. The cycle of the engine keeps the pipeline completely filled at all times so that there is a continuous rate of discharge at the form. As it moves along the pipeline the concrete takes the shape of a plastic cylinder. It is forced along the pipeline on a film of mortar that is self-troweled to the surface of the pipe around its full periphery by the slug of concrete itself. The principle of pumping concrete is based entirely on this self-lubricating factor. The transport line should always be laid out carefully since the layout can have a major effect on the efficiency of the installation. Obviously, the most efficient set-up would be the straighest possible pipeline between the pump and the formwork. Bends should be kept at at minimum and should be desingned with the smallest radius poosible. At the end of the line the pipe shold be located high enough above the form so the mix can be distributed easily but not so high as to permit a free fall that would lead to segregation. With some systems, the discharge line frequently terminates in a hand-held placement hose. The fear of blockages and the loss of tme and concrete that a blockage will cause are probably the biggest objections most contractors hold against pumping. Theoretically, blockage should not occur if the proper mix is used and the equipment is well maintained. But field practices don't always respond to theory and if the line does become blocked, the cause probable will lie: with improper priming, with the mix, with the operator, or with the equipment, usually the transport line.