Q.: I have a problem with concrete segregation that appears to be related to the coarse aggregate. When we use crushed stone from one source the concrete segregates in the chute of the mixer truck. When we use stone from another source we get no segregation. Both aggregates meet the ASTM C 33 grading specifications. I would like to find some way to use the stone that causes segregation because it costs less and it is closer to the plant. The concrete is air-entrained and we use the same sand in both mixes.

A.: You do not say anything about mix proportions. Possibly you are using the same ratio of coarse to fine aggregate for both coarse aggregates. Even though both aggregates fall within the ASTM C 33 limits there may be a considerable difference between their two gradings. Very likely the two aggregates also differ somewhat in angularity. Either or both of these would mean that each one has its own best ratio of stone to sand. We suspect that the mix which you say segregates is a harsh mix that contains too little sand.

Increasing the sand content may not turn out to be the only way or the best way to improve the mix. Possibly you could adjust the grading of the problem aggregate so that it more nearly resembles that of the better aggregate. If you could find some third source of aggregate, perhaps an aggregate of an intermediate size, you might be able to add a small amount of it in place of some of the problem aggregate and thereby improve the total grading. Experimenting this way with laboratory size trial mixes might lead to an easy and inexpensive solution.