Q.: If a truck mixer remains broken down for an extended period of time and the drum cannot be rotated, what is the best thing to do?
A.: We know of no good way to handle the situation if the drum cannot be rotated. The main concern is how to save the drum. Most companies make it their basic strategy to do what they can to get the drum turning again. Drivers should be instructed that if the drum will not turn they must notify the plant immediately so that all necessary help can be recruited quickly.
For the worst case--that the truck is overturned--vigilant ready-mix companies try to be prepared by keeping track of where they can promptly mobilize a crane or other lifting rig, and they become familiar enough with each truck so that they understand the proper lifting points. In the process of lifting, some concrete is likely to spill out. It may be desirable to try to empty the drum as much as possible before lifting by getting a local fire department to flush it with fire hoses, but this depends on whether both concrete and water can be made to drain from the opening or if the access plate is at a low point and can be removed for better drainage.
If the truck is upright and the drum is hydraulically driven, a simple solution is to bring up another truck and connect to its hydraulic system. This works if it is the hydraulic pump or line and not the hydraulic motor on the disabled truck that has failed. If there is a failed hydraulic motor, mechanics should be brought to the scene promptly with a replacement motor that has been kept in readiness for just such an emergency.
One way to get a mechanically driven mixer drum turning is to put a rubber belt around it and the drum of another mixer truck which has been drawn alongside. This belt might be a length of conveyor belt.
If the drum can only be turned temporarily, the bolted-on plate, or hatch, on the side of the drum should be removed and the drum turned until the resulting opening points downward. Meantime a local fire department should have been called to flush out the drum with fire hoses. Cleaning up the gutter or site is easier than removing hardened concrete from the drum.
It has been suggested that one could try to inject carbon dioxide from a gas cylinder, using a long probe reaching to the bottom, and moving the probe from time to time to reach all portions of the mix. The purpose would be to produce noncementitious products, the way the carbon dioxide from unvented heaters produces soft, low-quality floor surfaces. However, we feel that because of the many cementitious compounds present in the mix, all in different stages of hydration, the result would be quite unpredictable; one possible result could be accelerated hardening.
Similarly, cooling with liquid nitrogen also seems impractical because of the tremendous quantities that would be required to lower the temperature enough to delay hardening for any substantial period.
Finally, all efforts to turn the drum and discharge the load sometimes do fail and the truck eventually gets back to the yard filled with hardened concrete. In such a case the access panel can be removed from the drum and the mass broken up with a hydraulic splitter. One truck containing 5 1/2 cubic yards was cleaned out in 5 days by this operation (see "Splitter Breaks Up a Hard Problem", Concrete Construction, February 1982, page 208). Alternatively, the concrete might be broken up with a chemical splitting agent (as described in "Nonexplosive Demolition of Concrete and Rock," April issue, page 366).