Q.: The 4-inch line from a pump plugged up recently. It turned out that there were some hard lumps of concrete in the mix and a number of these had collected together to plug the line. We had to clean out the pump hopper and discard part of the load. Could these lumps have formed enroute to the job?

A.: This problem has been experienced when the truck mixer has not been properly cleaned after one or more previous loads. When something happens during mixing to break the hardened concrete loose from the vanes the lumps get distributed through the fresh mix. Sometimes when a large piece breaks loose it helps break additional pieces loose. Careless cleaning of the truck mixer can lead to trouble.