Q.: When our ready mix trucks discharge concrete, drivers sometimes report seeing balls of nearly dry material coming down the chute. When broken open, the balls appear to be mainly cement and sand with some coarse aggregate particles in them. What causes this to happen and how do we prevent it?
A.: Richard Meininger of the National Ready Mixed Concrete Association says these balls are usually caused by a dry pack of cement and sand in the hub end of the mixer. The material stays there during the mixing cycle and doesn't break loose until the truck starts to discharge. The clumps get rounded into a spherical shape by the tumbling action in the drum as they move to the discharge end.
Because this is caused by getting too much fine material packed into the hub end of the mixing drum, the solution is to get some water and coarse aggregate into the hub end first. When charging the mixer, first batch part of the coarse aggregate and water. This will help prevent dry pack and should solve your problem.