Concrete from a pipeline had higher slump than the concrete that went into the pump hopper. We suspect the pump was leaking water into the concrete. Is this common?
This problem may occur when the rubber seal on the piston becomes worn, as happens after pumping about 1,000 cubic yards of concrete. Water from the pump water jackets used to keep the cylinder and piston clear of mortar probably leaked past the seal. If the cylinder liner and piston themselves are worn they will have to be replaced because a new seal will not be adequate. Such replacement is usually necessary after pumping 4,000 to 7,000 cubic yards of concrete.