The concrete mix design must be correctly proportioned so that the concrete will flow easily and uniformly through the pipe-line. The pipe diameter should be 3.5 to 4 times greater than the maximum aggregate size in the concrete. This helps ensure that the pipeline will not become blocked. Another factor in mix design is the consitency (slump or flow test) of the concrete. A higher consistency will allow the concrete to move more readily within the pump and pipe-line. However, an excessively high slump can cause the concrete to segregate resulting in plugs of coarse dry material in the pump or pipe-line, the paste beign squeezed out.
What makes concrete pumpable? Pumpable concrete must be capable of being pushed under pressure through a pipe-line as a cylinder, separated from the pipe wall by a lubricating layer of mortar (water, cement, and sand). A concrete mix must be such that the concrete can pass through reducers in the pipeline system, and can go around bends in the line. In order to obtain this type of pumpability, the mix must be dense, cohesive, and have a sufficient paste and mortar fraction to minimise voidage. The mortar volume required depends on the line size, efficiency of concrete pump, and pressure available for pumping the concrete.
The Good Practice Guide for the Safe Use of Concrete Pumpsí can be downloaded free of charge from the CPA website at www.cpa.uk.net. The guidance supersedes old code of practice which was first released in 2004.