Tough Shipment

Feb. 10, 2005

Hot-mix asphalt (HMA) has been used for many decades in heavy-duty applications like port terminals. Port facilities pose a unique set of challenges. The pavements have to stand up to heavy static loads. Also, the large, slow-moving tires of the specialized freight handlers can exact a high toll from pavements.

Hot-mix asphalt (HMA) has been used for many decades in heavy-duty applications like port terminals. Port facilities pose a unique set of challenges. The pavements have to stand up to heavy static loads. Also, the large, slow-moving tires of the specialized freight handlers can exact a high toll from pavements.

The key to success lies with developing an appropriate pavement and mix design for the high stresses in these areas. It is critical that there be sufficient thickness and stiffness in the structural layer so that there will be little or no bending at the bottom of the asphalt layer. Controlling moisture in the underlying layers is essential to the pavement’s performance, therefore the drainage systems need to be conservatively designed for long-term effectiveness. When these items are addressed, permanent deformation in the structural layer is not a problem.

Muscular aggregate

To control potential rutting in the HMA lift itself, special procedures must be used. The mix designer has to recognize and address the special needs of this type of application. The goal is to get the aggregate to carry the bulk of the heavy load with a sufficient amount and stiffness of binder to provide the desired service life.

The first step is to specify a high-quality aggregate, which will provide a good stone skeleton. Normally, a larger nominal maximum size aggregate is used in these mixes compared with those in highway construction. Gradations both above and below the maximum density line have been used successfully, however, since the development of Superpave, most designs are on the coarse side of the gradation curve.

High levels of compactive effort are used in the lab design to ensure that stone-on-stone contact is achieved. In addition, a test for voids in coarse aggregate is used to validate that the stone skeleton is adequate. Air voids of 4% are generally specified at optimum binder content and a minimum voids in mineral aggregate is required based on the aggregate’s nominal maximum size. Once the mix design is established, a moisture sensitivity test is performed to determine whether any anti-strip additive is needed. Special consideration is given to the selection of the grade of asphalt binder.

The material must resist low-temperature cracking in the environment of the project. Also, the high-temperature stiffness needs to be adjusted for both the higher loading on the base mix and the higher shear stresses on the surface mix caused by the large, slow-moving tires of the specialized freight handlers.

With a careful volumetric mix design, all the special considerations posed by this application should be addressed and the HMA pavement should perform for its intended service life.

In recent years, a number of designers have been using laboratory performance tests on the compacted mix to validate their designs. This can help ensure that the mix will not only have the required stiffness to meet the structural needs but also provide the shear resistance on the surface to avoid damage from the slow tire loading.

A robust quality-control program must be followed during construction to ensure that what was designed is actually built. While the performance of the HMA is sensitive to all of the factors addressed in the mix design, it is particularly critical that appropriate in-situ density be achieved. Recent research has shown that the minimum lift thickness should be three to four times the nominal maximum size of the aggregate used in the mix.

Port preference

The Port Authority of New York and New Jersey (PANYNJ) has used HMA almost exclusively in paving at both the Port Newark and Port Elizabeth terminals since the late 1940s. As these facilities have grown, often at faster rates than anticipated, the pavement designers have been able to add structural capacity with only minor disruption of service. Even though the original design was for 20 years, in many cases additional thickness had to be added in less than 14 years to meet the ever-increasing and unforeseen structural needs of the pavement.

Since all ports handle containerized freight, their designs and layouts are dependent on the equipment used to facilitate the movement of the containers. Specialized container handling equipment, such as top loaders and straddle carriers, can impart more than twice the loading of a typical tractor-trailer used for highway transport. In these areas, the HMA pavement is normally 10 to 16 in. thick, over 6 to 12 in. of stone base.

In virtually all PANYNJ facilities constructed since 1993, the top 3 in. of HMA was constructed with polymer-modified asphalt (PMA) binder to resist the high shear forces at the surface. As a result of these relatively thick pavement sections, traditional fatigue cracking (e.g., from the bottom up) has not been manifested.

Bill Wheaton of Moffatt & Nichol’s Baltimore office said their firm has been designing port facilities for many years all over the U.S. In addition to the PANYNJ, they have worked on facilities in Maryland, Virginia, California and Washington. In their latest work, they are using a multilayer elastic pavement design model. This can provide for a more accurate characterization of all the materials in the various layers of the pavement structure as well as a more realistic response to the high stress loadings. The result provides a reliable pavement cross section at the minimum cost.

This methodology was used in the Phase 2 design of the Port Elizabeth Yard Improvements project. By employing a PG 82-22 (PMA) in the surface mix and a PG 70-22 binder in the base mix, the designers were able to provide an adequate structural stiffness with a thinner section of HMA. Since the PANYNJ handles about 22 million tons of cargo annually through these ports, it is estimated that this pavement will need to carry up to 1,500 tractor-trailers per day. When Phase 3 is completed, over 440,000 tons of HMA will be in place and this new generation of pavement will service the port into the future.

Wheaton said that in addition to the “ease of surface maintenance” the “greater flexibility for construction stages” which “... allowed a large portion of the terminal to be kept in operation at any time” led to the terminal operator’s selection of HMA for this project.

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