Published on 6/27/2018
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Lets talk about concrete truck mixers
How mixers work, how to clean them and how to save them when disaster strikes
Concrete transit mixers, or concrete truck mixers, are beautiful things. When they are clean and working that is. They keep fresh concrete alive, often for hours, before the concrete is placed by the awaiting concreters. Yet mixers can become dirty, and over time need constant cleaning and maintaining. They can fail as well. And when a concrete truck mixer fails, concrete hardens in your mixer barrel, and you may as well kiss your drum goodbye. And concrete mixer drums do not come cheap...
1. How do concrete truck mixers keep fresh concrete alive?
Cement is a binding material. It has sulfates, aluminates, silica and some extra materials. When you add water a chemical reaction takes place and it is an exothermic reaction. This water consumes itself to form the binder of the cement and then the concrete sets. This setting time depends on the proportion of the contents of the material that are mentioned above. It can vary from 30 minutes to some hours.
When you rotate the drum what generally happens is, this reactions is prolonged, it cannot take place so quickly, the reason being the particles are no more stable to carry out the reaction. You can see this effect when you mix the cement. You add some percentage of water and mix it. Sometime later what you will see that, if you don't mix it anymore it dries a bit but when you start disturbing it again, you can see that water is bleeding out from the cement paste or concrete mixture. Also engineers use slightly higher amount of water while mixing the paste so that it do not set quickly.
In case of the concrete trucks, the paste is formed by adding the water when it leaves the concrete plant, then in the way it is kept on rolling to avoid the concrete from setting. Also we add some additives to increase the setting time of concrete. You know what, when a truck is jammed in a traffic and if the driver feels that the cement will set if he doesn't do anything. He simply adds sugar in the mixture and rotates the drum. This will increase the setting time. Sugar is the most common admixture used for increasing the setting time.
Source : quora.com
Many transit mixer drivers carry some sugar with them in the
event of a brakdown to keep the fresh concrete
alive before it sets and hardens in their drum
2. How do you clean and maintain the Mixer?
Once the last of the concrete trickles out of the drum or dribbles out of the pump, the truck isn't empty. A small amount of concrete always remains inside the truck, especially in drum mixers. You can wash the truck out, run water through the drum while it rotates and evenpressure wash the inside. But some concrete is always left. And it dries. The next trip a little more is left. And it dries. Over time, this becomes a problem, reducing the volume of the mixer and its capacity.
The mixer should be cleaned daily when it is in continuous operation or following each period of use if it is in operation less than a day. If the outside of the mixer is kept coated with oil, the cleaning process can be speeded up. The outside of the mixer should be washed with a hose, and all accumulated concrete should be knocked off. If the blades of the mixer become worn or coated with hardened concrete, the mixing action will be less efficient. Badly worn blades should be replaced. Hardened concrete should not be allowed to accumulate in the mixer drum. The mixer drum must be cleaned out whenever it is necessary to shut down for more than 1 1/2 hours. Place a volume of coarse aggregate in the drum equal to one-half of the capacity of the mixer and allow it to revolve for about 5 minutes. Discharge the aggregate and flush out the drum with water. Do not pound the discharge chute, drum shell, or the skip to remove aggregate or hardened concrete. Concrete will readily adhere to the dents and bumps created. For complete instructions on the operation, adjustment, and maintenance of the mixer, study the manufacturer’s manual.
All gears, chains, and rollers of mixers should be properly guarded. All moving parts should be cleaned and properly serviced to permit safe performance of the equipment. When the mixer drum is being cleaned, the switches must be open, the throttles closed, and the control mechanism locked in the OFF position. The area around the mixer must be kept clear.
Several companies offer pre-treatments, chemical and acid baths and high-pressure washes for drums and interiors. But eventually, the concrete will build to a point where the only viable alternative is mechanical removal. Many smaller companies handle this on their own, sending in a worker with a jackhammer to chip out the hardened concrete whilst dust protection equipment must be issued to the crew engaged in performing the work to prevent cement and sand from being blown into their eyes and faces.
Source : howstuffworks.com
Hardened concrete is difficult to remove from in and around mixer barrels
3. How do you save the drum of a concrete truck mixer?
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?
Electrical faults such as remote control failures, hydraulic pump failures, hydraulic oil leaks, power failures and concrete truck breakdowns in general can lead to a mixer stopping from turning. 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.
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.
Source : concreteconstruction.net
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