How can we eliminate the cement balls that we’ve recently discovered in our concrete? This problem seemed to start after we recently installed a new batch controller on our transit-mix plant. Before that, we had operated the plant manually.
We’ve always monitored slump and mix consistency closely. Also, since the problem has started, we have been very diligent in checking that our drivers rotate their drums for the required 100 revolutions on each load. Thinking that our mixer fins were causing the problem, we brought every driver in on the last rainy day to clean out every mixer. Then the mechanics measured every fin to be sure that they were within manufacturers’ tolerances. Yet we still have this problem.
Also, we often notice color variations when we produce integrally colored concrete. But even with uncolored concrete, customers report that some areas of the hardened concrete are a darker gray than others.
Do you think that this is an equipment failure, a driver problem, or some batching problem?When this question was recently posted on the Aggregate Research Inc. Industry Forum (www.aggregateresearch.com/forum), it drew some very practical solutions to a common problem.
From your description, the cement balling and concrete discoloration don’t appear to be the result of a mechanical or driver problem.
Most respondents suggested that the cause had to do with the batching sequence. They recommend that you turn your attention to the timing of the discharge of ingredients into the mixer.
The key to eliminating cement balling is keeping the cement from contacting the mix water directly in the drum. Cement has a high affinity for water. So upon contact with water, the dry cement powder acts like a sponge and globs up. The balls form as these globs roll against the inside drum surface.
The key to prevention is to program a material-loading sequence that deposits the cement and the coarse aggregate into the drum at the same time. Whatever your plant feed limitations are, you must avoid placing the cement in a pool of water in the drum. You can do this by adjusting the loading rates of all of the materials.
Check your cement loading rate. If the discharge rate is set too fast, it can cause the cement to ball up. Sometimes the batching technician in older operations thinks that the plant’s initial setup is constrained by an older and slower cement delivery system and will program the sequence to start up the auger or blower earlier in the batching process.
Also, you might want to check at what point the aggregate and cement come together in the drum. One producer reported that installing an extension "boot" to the cement outlet can keep cement and coarse and fine aggregate separate. His plant operator has found that this separation is even more important if the sand is wet.