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Tips for Balancing Production Rates in Hot Mix Operations

Mon February 06, 2006 - Northeast Edition
CEG



Proper handling of trucks is vital in achieving a successful paving project. A well-run trucking operation will require good communications and driver training. It can result in improved traffic flow, fewer accidents, shorter waiting times at the plant and at the paving site.

Often it seems that truck drivers are the last to know what is planned for the job site, but the first to be blamed for problems. Communication is the key. Drivers should be included in preconstruction meetings, if possible, or at least be given detailed directions and maps.

Include details such as:

• What they will be hauling?

• What time they are expected to be at the facility?

• What is the estimated cycle time?

• How many loads can they expect per day?

• What is the scheduled tonnage for the day?

• Where are the access arrangements for the job site?

• Where they are to wait, if needed?

• Where can they clean out their truck on the job?

Training

Truck drivers normally haul a variety of products including rock, gravel, sand, construction debris, and hot mix asphalt (HMA). Each product has unique requirements for hauling. Until drivers are trained about the particulars of hauling HMA, they will not be able to function as a real member of your paving team. If they don’t understand why cleaning the bed is important, how stopping short of the paver will prevent a mark in the mat, or why it is important for them to maintain their spacing, they will not be able to do their job properly. So, give them the proper training — it will make your job that much easier.

NAPA offers two training aids to help train drivers. The first is an easy-to-read booklet that summarizes proper techniques, titled Truck Driving Techniques (TAS-1). The second is a short video about a real truck driver hauling HMA, entitled Hauling HMA (TAS-19).

Types and capacities of haul trucks vary widely around the country. Types include end dump, bottom dump, and horizontal discharge. State laws limit capacity based on the number of axles or gross vehicle weight.

When all the haul trucks have the same capacity, the trucks can haul in any order, but need to maintain their position in the hauling train once it has been established.

When haul trucks have varying capacities, it will be necessary to evenly distribute the different sizes into the entire truck fleet to ensure a uniform delivery rate.

For example, a fleet has 10 trucks available, five hold 15 tons each, and five hold 21 tons each. If all the smaller trucks stay together and the larger trucks stay together, a contractor would experience an uneven rate of delivery. This would cause problems at the site, which could affect the quality of the mat. Evenly spacing out the different capacities will even out the delivery rate.

Cycle Time

To adequately schedule the haul trucks, an accurate cycle time is critical.

Cycle time includes the time it takes for:

• Pre-loadout preparation (time to spray the truck bed, waiting in line).

• Loadout time (time under the silo).

• Time to get delivery ticket and trap the load.

• Haul time to the job site. This time should be determined during the same time periods a contractor is scheduled to be paving. A drive that takes 20 minutes at some parts of the day may take 90 minutes during others. Will the trucks be caught in rush hour? Are there special truck route requirements?

• Waiting on site to dump.

• Dumping the load into the paver and on-site clean up.

• Return haul to the plant.

All of these factors combined equal the cycle time for the trucks.

Site Access and Start Up

Is the site easily accessible? Are there restrictions on construction traffic? If a contractor knows there are special procedures to access the site, make sure the dispatcher knows and thoroughly explains the steps to each driver. Again, a site map showing the paving site is a cheap investment to ensure the truck drivers know how to get to the site on time.

On jobs with little planning, more mix than needed is ordered at unrealistic times, only to sit on the job site because the paving crew wasn’t ready. The best procedure is to schedule loads based on intended rate and to keep realistic expectations based on experience.

Remember to stagger the first load time to allow for the joint construction and start up. Normally, a short delay between the first couple of loads and the scheduled delivery rate is adequate to allow for a clean start up. If full rate is requested at start up, someone else may not get the tonnage they need, the trucks will bunch up, and you will spend all day trying to get them (and keep them) separated.

Paving

Often thought of as the key to productivity, the paver is actually no more important than any of the other elements when it comes to balancing production. However, paving operations must be set up properly and maintained during construction.

In the past, the speed of the paver was left to the paver operators, who had little input into the project planning and scheduling. Their job was to steer the paver, empty the trucks, and put down HMA. The speed of the paver is an integral part of the whole balancing act and when properly set up and maintained, will produce a uniform, high-quality mat. Paver operators must understand their responsibility of running the paver at a uniform rate.

Smoothness, Density, and Pay Adjustments

For many years, untrained paver operators took it upon themselves to run the paver as fast as necessary to empty the truck and send them on their way. This hurry up and wait mentality often resulted in a paver outrunning the rollers and sitting on the mat, waiting for the next round of trucks while the mix in the hopper cooled off. The net result of an overall inconsistent construction process includes potential end-of-load segregation problems, transverse bumps, and uneven mat densities.

As specifications change from a method-type to an end-result type, pay adjustments for mat quality are being introduced. Pay adjustments on mat smoothness and density have increased the paving team’s attention to the details that produce a quality pavement.

One main method to increase smoothness and decrease density variations is to keep the paver and rollers moving as consistently as possible. If the paver has to stop (and it will), keep the head of material uniform in front of the screed. The best way to ensure this consistency is to keep the HMA level in the paver hopper from dipping below the conveyor flow gates.

Start Up

Poor scheduling often results in several trucks waiting on a job site while the paving crew is still preparing the joint in the past, there was a big push to empty the waiting trucks. Don’t do it! Pushing up the paver speed now will later force the paver to sit while waiting for the next round. A stopped paver with an unbalanced screed will affect mat quality, which may affect pavement. The best solution is ordering a few trucks and placing a short hold, then requesting the full production rate.

By sticking with a predetermined paver speed, a contractor will still empty the trucks at a given rate, and also will space the trucks back out. Don’t fall into the habit of speeding up the paver to eliminate a small backup of trucks. This will only cause a backup at the facility and further problems later.

High Production

High production jobs, like mainline paving and base construction, take a great deal of coordination to run properly. Therefore, achieving a balance of operations should be the No. 1 concern. High volumes of material can mean lower unit cost and higher resulting profits if the material produced is high quality. In general, as long as all elements can maintain the pace, maintaining a balance should be fairly simple.

On some projects, a material transfer vehicle may be used. In this case, the paver normally doesn’t have to stop between loads and can run consistently. Using a material transfer device also involves coordination, and you might be able to place more tonnage since paver downtime is decreased.

Low/Erratic Production

Where tonnage requirements vary during the day, achieving and maintaining a balance of operations is much more difficult.

Patching, leveling, handwork, tapers, and placing driveways are far less productive than placing mainline paving. Here, experience is the key, especially when the haul distance is great.

Obviously, production rates will be lower, and sometimes a contractor will have to live with a backup of trucks or no trucks at all. Only experience will tell what production rate will be during these operations. Frequent communication with the HMA facility and truck drivers is the key to ensuring a smooth operation.

Commercial paving is normally a slower and more erratic operation than mainline roadway paving because the paving crew has to work in and around obstacles like parking bays, islands, light poles, etc.

Compaction

The final step in construction is the compaction process, which is a key element in the final quality and payment for the product. Compaction affects both smoothness and density, which are typically used for acceptance of the product. Details on proper roller techniques are presented in NAPA’s publication Roller Operations for Quality (IS-121).

For a more complete treatment of this topic, order Balancing Production Rates in Hot Mix Asphalt Operations (IS-120). This publication also contains forms to assist in calculating mix delivery production rate, paving production rate, and roller production rate.

For more information, call 888/468-6499 or visit www.hotmix.org.

(This story appears courtesy of “HMAT” magazine)