Q.: Occasionally one of our mixer trucks breaks down before reaching its destination. Our problem is what to do with the load of fresh central-mixed concrete the mixer is carrying. Frequently the truck will be down for a period of one to three hours. Can commercial retarders or other products be added to the concrete to keep the mix alive and still of good quality until the repairs are completed and the truck is underway again? (Assume that we can still turn the drum while the mixer truck is broken down.)
A.: Saving the concrete, yet not prolonging the set so much that it will disrupt job operations, is something of a neat trick. The time of set could be extended by adding a retarding admixture. But the amount to add would depend on whether or not there is already a chemical admixture in the batch. It would also depend on how long it has been since the concrete was originally mixed (because that will determine to what extent the early hydration reactions have proceeded). Furthermore, the time of year will have an effect, because hydration is faster in hot weather than in cold. You might run a test mix to which, after some selected period of time, you add about half a normal dose of retarder, then determine what the retarder has finally done to the hardening time and the 28-day strength. Then, maintaining the same job conditions, you could make the same measurements with smaller or larger dosages, depending on what seems to be indicated by the results of the first mix.
You must bear in mind that the effect of the admixture will change with any of the changes in conditions already discussed, but also that it is likely to change with any change in cement brand or change in amount of pozzolan. If the subject is of sufficient importance to your operations you could try to determine the proper amounts for each of the material and environmental conditions anticipated and establish a chart for future guidance.