Balancing Production and Placement of Hot Mix Asphalt

Dale Starry,
Director, Strategic Technologies

Successful asphalt paving projects do not happen by accident. They are the result of careful planning and implementation. From restricting or rerouting traffic flow to making sure the right asphalt equipment is on-site to perform the job, a complex series of events comprises an asphalt paving project, especially in the critical asphalt laydown phase.

Production
To achieve the best results and most profitable operation on an asphalt paving project, contractors must balance the elements of hot mix asphalt (HMA) production and placement. A number of hot mix asphalt plants are available. Top-end stationary drum mix facilities produce up to 700 tons (635 metric tons) per hour of HMA. On the other end of the spectrum, portable drum / batch plants produce 30 to more than 100 tons (90 metric tons) per hour of hot mix asphalt.

HMA Production - Top-end Stationary Drum Mix Facility

Paving projects are rarely adjacent to a hot mix asphalt plant, so transportation of the material is necessary. A wide variety of types and sizes of transport vehicles are available, many specifically designed to haul HMA. Live bottom and moving floor trailers feature the largest haul capacities; dump trailers and trucks have less capacity.
Regardless of the type and capacity of the haul vehicles, the delivery of hot mix asphalt from the plant to the asphalt paving project requires consistency and regularity so that the mix being placed is of uniform temperature. Specified density and smoothness of the finished asphalt pavement can only be achieved when the mix temperature is uniform.

Placement: laydown
Placement of hot mix asphalt is a two-part process. The first portion of placement is laydown, accomplished using an asphalt paver. An asphalt paver has two major components - the tractor and the screed. The tractor is the prime mover that is used for self-propulsion of the asphalt equipment. The screed is the working tool, the equipment that spreads the HMA into asphalt pavement.

Placement of HMA with an Asphalt Paver

The operator must judge three factors when determining paving speed. First, the operator must see how much hot mix asphalt is being delivered to the asphalt paver. Second, the operator must look down at the width and thickness of the asphalt pavement panel being laid.
Third, the operator must check behind the asphalt paver to see if the compactor train is able to keep up with the paver.

This balance between production and placement needs to be maintained throughout asphalt paving. If the delivery of asphalt mix to the paver is interrupted, the asphalt paver needs to stop. Current practice is to rapidly stop and start the paver so the screed level does not fluctuate because of asphalt paving speed or head-of-material in front of the screed. If the paver outruns the compactor train, the entire asphalt paving operation is at risk of failing to achieve target density and/or asphalt pavement smoothness.

Placement: compaction
The second part of the placement process is compaction. The compaction of hot mix asphalt must be completed within a finite time period.
This interval is referred to as time available for compaction (TAC). The TAC varies based on environmental considerations, mix properties and mix temperature.

Charts of Expected TAC on a pavement panel

An asphalt mix cools continually, although at varying rates, from the time it leaves the pugmill or mixing drum. The longer a mix is stored in a silo and the longer the delivery time to the asphalt paver, the greater the temperature loss. Temperature drops as the mix moves through the mobile conveyor, sometimes called a material transfer vehicle, and the paver. After being spread by the asphalt paver's screed into the width and thickness required for the project, the mix continues to cool. By the time the breakdown compactor begins the compaction process, the mix may have cooled 10 to 15 percent from the point of production.
The TAC varies from less than 10 minutes to greater than 20 minutes based only on the difference in temperature of the base.
Obviously, it is more difficult to achieve specified density in 10 minutes than in 20, with all other factors remaining constant.
Compaction speed must match asphalt paving speed for optimum project efficiency. If a vibratory compactor is utilized for breakdown, the rolling speed is controlled by the need to achieve pavement surface smoothness. A vibratory compactor drum impacts the surface of the asphalt pavement each time an unbalanced weight inside the drum spins one complete revolution. Vibratory compactor drums must make between 10 to 14 drum impacts for each foot of travel (33 to 46 drum impacts per meter of travel).

Vibration frequency of the compactor is measured using a vibrating reed tachometer or similar tool. Dividing vibration frequency by the required drum impacts per length of travel determines the rolling speed of the compactor with vibration. For example, a vibratory compactor with 2,500 vibrations per minute frequency that delivers 12 drum impacts per foot (39 impacts per meter) spacing means the compactor can roll at a maximum speed of 208 feet per minute or 2.4 miles per hour (3.85 kilometers per hour). This vibratory compactor cannot increase rolling speed if it fails to keep pace with the paver. To do so would change impact spacing and risk failure to achieve smoothness.
This is why vibratory compactor manufacturers like Volvo Construction Equipment provide high frequency vibrating compactors.
These units have a vibration frequency of 4,000 vibrations per minute or greater. With the same drum impact spacing of 12 impacts per foot, a vibratory compactor with 4,000 vibrations per minute can increase rolling speed to 330 feet per minute or 3.8 miles per hour (6.1 kilometers per hour) and still achieve pavement surface smoothness. The higher vibration frequency provides a 50 percent faster speed to keep up with quickly moving and more productive asphalt pavers.

Balance
What is the key to effective placement of hot mix asphalt? Balance. Balance between placement and production and between mix delivery and production yield the best results. Careful control of the asphalt paving speed enables efficient laydown and compaction of the mix at consistent temperatures. The best asphalt paving projects find balance between each of these elements.