A drop in the bucket.
By Alex Morales
You have just produced a high-quality concrete batch and you are absolutely certain that you know the exact water-cementitious ratio of the mix. It is properly mixed and being discharged from the mixer, ready for transport to be placed in formwork. This moment is critical – do you know where this batch is going? How’s it getting there?
Let’s assume the following:
- The concrete is discharged from the mixer onto a conveyor belt
- The conveyor belt transports the fresh concrete to a bucket
- An overhead crane lifts and transports the bucket to the form
Albeit simplified, this scenario is useful in highlighting three important pieces of equipment used in the typical precast concrete production facility – conveyor belts, concrete buckets and overhead cranes – and you should be familiar with their characteristics and maintencance requirements.
Conveyor belts have been termed “the lifeline of concrete plants.” The term likens conveyor belts to arteries and veins in the body that carry blood to important organs to sustain life. The analogy is appropriate because without the ability to effectively transport concrete around the plant, a precast concrete business cannot thrive.
So if you’re using a conveyor belt system to transport fresh concrete, you need to ensure you understand it and care for it properly. A conveyor system is made up of various components including belts, idlers, pulleys and drives.
It is best if the entire system is capable of starting under full loads. Of course the system is not designed to start under a full load each time it is used, but in the event of a power outage or a sudden work stoppage, you will need to restart the system while it is loaded and it should be designed to handle this additional stress.
The belts typically get the most attention, since they are the most visible part of the system. Many purchase used belts, and these must be properly sized. As always, follow the manufacturer’s recommendations for use and maintenance of conveyor belts; however, if installing used conveyor belts, it often is difficult to know for sure who the manufacturer is, so take extra time to ensure you identify the original manufacturer. Your supplier of used equipment can be helpful in determining this information.
There are two types of splices used to piece together a conveyor belt: mechanical and vulcanized. Mechanical splices are quick, easy and inexpensive. However, they need to be filed smooth and may lack sufficient strength. Vulcanized splices are more difficult and expensive, but they are very strong and can have a long life. A system can function properly if a mechanical splice is used on the belt, of course, but vulcanized splices are preferable. Check with your manufacturer to ensure the proper type of splice.
Two important considerations for belts used to transport fresh concrete are the “edge distance” and the “angle of surcharge,” and you should take both of these into consideration when determining the correct size for your operation. The edge distance is the amount of space from the edge of the fresh concrete on the belt to the edge of the belt itself. The minimum edge distance is calculated according to this formula:
The angle of surcharge is the angle formed by the pile of fresh concrete on a moving belt with the horizontal. For concrete slumps of approximately 2 to 6 inches, the angle of surcharge should be between 0 and 10 degrees. The angle of surcharge is influenced by various factors, including aggregate type, the water/cement ratio of the concrete batch being transported and any additives (mineral or chemical admixtures) used. You should work closely with the conveyor manufacturer in order to determine proper belt sizing to ensure that your conveyor system can handle your concrete batches effectively.
Wipers are part of the belt system and are typically located directly behind the discharge pulley. Wipers direct mortar for proper discharge and must be kept clean. Do not underestimate the importance of clean wipers – much like the wipers on your vehicle, the wipers on your conveyor system are often neglected until they are not functioning properly.
Maintaining a clean conveyor system is important to ensure its smooth operation. However, never clean or adjust any part of the system while the system is operating. Use lockout/tagout (LOTO) procedures to disconnect the system from its power source prior to any maintenance. Never water-wash the belts. Light tapping followed by wire brushing and oiling is typically sufficient, or follow the manufacturer’s recommended procedure.
One final consideration for a conveyor system is the speed at which it runs. The speed of the belt, expressed in feet per minute (fpm), is based on the size of the belt and the angle of surcharge as expressed in the chart below.
For instance, a belt size of 16 inches moving at a speed of 500 fpm and carrying fresh concrete at an angle of surcharge of 0 degrees can convey 115 yards of concrete per hour. This charge should be used as a guide, but final determination of belt speed should be confirmed with the manufacturer:
Concrete buckets are an important part of the concrete placement process. They are typically made of metal (usually steel) and are transported with the aid of an overhead crane that suspends the bucket over formwork for final placement.
As with conveyor systems, it is important to keep the buckets clean, free from concrete buildup – and free of water. If you water-wash these buckets, ensure that the buckets are completely dry before putting them back into service, since you should at all costs prevent the potential tampering of the water/cementitious ratio of the concrete batch.
Never allow concrete to harden within or on the bucket. This not only impedes the natural effect of gravity on the concrete batch during discharge, but it also affects the overall weight of the bucket. For safety reasons, unnecessary additional weight should be eliminated from loads that will be suspended.
Yes, buckets may be moved with cranes, forklifts, pallet lifts and rollers to the casting locations. While these are popular means of moving concrete to forms, it is labor-intensive and time-consuming. Although not part of the scenario listed at the beginning of this article, traveling buckets are an alternative.
Traveling or flying buckets are automatic devices that go between the mixer and drop locations via rail systems. While they can run without direct supervision, their flexibility is generally limited to the predetermined locations. Also, the rails can limit building use and access. When designing a plant, all of these things should be taken into consideration.
Overhead cranes provide efficient movement of equipment and raw materials in precast concrete facilities. However, an overhead crane also represents a high risk for personal injury, and operators of overhead cranes should be properly trained on the correct use of this equipment. Operators must use good judgment and common sense when using an overhead crane to carry a full concrete bucket to the formwork.
Overhead cranes are typically operated by handheld pendulum controls that move loads laterally (front and back), longitudinally (side to side) and vertically. The essential parts are the crane bridge, the trolley and the hoist hook assembly. The bridge is supported by an end truss and travels the length of the bay on rails. The trolley travels from side to side in the bay and carries the hoist hook assembly.
Obviously, although the system is stationary, the overhead crane is capable of carrying loads in three-dimensional movements within the bay. Manufacturers of overhead cranes recommend varying degrees of inspections, which should be followed unless more stringent requirements are specified by OSHA or some other local safety governing body.