Equipment Type

Balancing HMA Production And Placement

Provided By Volvo Construction Equipment, Shippensburg, Pa. To achieve the best results and most profitable operation on a paving project, contractors must balance a complex series of events and equipment choices during hot mix asphalt (HMA) production and especially in the critical compaction phase.

October 01, 2007

Provided By Volvo Construction Equipment, Shippensburg, Pa.

To achieve the best results and most profitable operation on a paving project, contractors must balance a complex series of events and equipment choices during hot mix asphalt (HMA) production and especially in the critical compaction phase.

Production

The size of the project determines whether a contractor will rely on a top-end stationary drum mix facility that produces up to 700 tons per hour of HMA or a portable drum/batch plant that produces 30 tons to about 100 tons per hour of HMA.

Delivery

Transport vehicles, some designed specifically to haul HMA, play a critical role in temperature control and segregation. The longer a mix is stored in a silo and the longer the delivery time to the paver, the greater the temperature loss. Once the type and capacity of the haul vehicles is matched to the paver's capacity, the delivery of HMA from the plant to the paving project requires consistency timed with the speed of the paving train so that the mix is placed at uniform temperature.

Placement: Laydown

Placement of HMA is a two-part process. Laydown speed depends upon the paver operator's skill in three primary areas. The first focuses the operator's attention forward to the amount of HMA being delivered to the paver. The second focuses downward at the width and thickness of the pavement panel being laid. The third focus is behind, to see if the compactor train is keeping up with the paver.

This delicate balance in production and placement must be maintained throughout paving. If the delivery of mix to the paver is interrupted, the paver must slow down or at worst, stop. If the paver outruns the compactor train, the entire operation is at risk of failing to achieve target density and/or pavement smoothness.

Placement: Compaction

The second step, compaction, must be completed within a finite time period, an interval referred to as time available for compaction (TAC) and varies based on the environment, mix properties and mix temperature. By the time the breakdown compactor begins the compaction process, the mix may have cooled 10 percent to 15 percent from the point of production.

Most compactor manufacturers provide charts to show the expected TAC on a pavement panel placed at a certain temperature over a base of a certain temperature and measured thickness. 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 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 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.

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 spacing means the compactor can roll at a maximum speed of 208 feet per minute or 2.4 miles 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 Ingersoll Rand have developed 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 and still achieve pavement surface smoothness. The higher vibration frequency provides a 50-percent faster speed to keep up with faster, more productive pavers.

Balance

The best paving projects find balance between placement and production, between mix delivery and production, and careful control of the paving speed and compaction of the mix at consistent temperatures.

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