Why do they Put the Sponge in Concrete Pipe?

Ever wondered why construction workers drop a sponge ball into concrete pipes after finishing a job? See, once the concrete work is wrapped up, there's usually some leftover residue sitting inside the pipes. Over time, that stuff hardens and builds up in layers, which eventually clogs the pipe completely. To prevent this, workers use either water or air pressure to force the ball through the line, pushing out all the junk and leftover cement.