Passing the Torch

By: Dale Decker

In 1993, when the Strategic Highway Research Program (SHRP) ended, the HMA industry was confronted with a "burn the ships" approach to Superpave implementation. With the number of holes in the system in 1993, sinking may have been a more likely scenario at that time.

But due to the hard work and perseverance of a large number of industry, agency and academic professionals, the holes in the Superpave system are being plugged and before you know it, the HMA industry will truly be ready to "burn the ships" and move fully forward to Superpave technology, but don't start the fires quite yet.

The Superpave implementation process is going well. Yes, there have been some problems but, yes, there have been many projects that have been very successful. Perhaps we shouldn't be too surprised at either of these situations. The process is moving at an evolutionary, rather than revolutionary, pace with therefore a higher probability of success. Appropriate changes have been and are being made to recognize the experience of local industry and agencies. The HMA industry is quickly moving towards the Federal Highway Administration (FHWA) goal of Superpave implementation in 2000.

Having said all that, let's talk about some of the elements of change that are taking place in the Superpave process. This discussion will be presented in the following categories: materials and mix design, plant operations, placement and compaction operations, field management, performance testing, and training. The objective is to address the current status in each of these subjects.

Materials and mix design

There are three sub-categories in this topic: aggregates, binders, and mix design. A few observations will be made for each.

Aggregates

The Superpave aggregate consensus standards are specifications for high quality materials for use in HMA and should therefore have a positive effect on HMA performance. Many agencies across the country already have requirements for aggregates similar to the Superpave recommendations.

States have adopted specifications for aggregates based on local experience and will require a specific test in cases where the results help them differentiate materials that have been used successfully from those that have not worked well. Agencies must continue to evaluate materials on a local level with an understanding of historical performance to determine the applicability of the Superpave recommendations for their specific materials.

Several new aggregate criteria are included in the Superpave specifications. It should be noted that the Superpave aggregate requirements are on the blend of aggregates used in the mix, not on the individual aggregate components.

Rather than grading bands, control points are used on specific sieve sizes to establish the aggregate gradation blend. In addition, a restricted zone is placed around the maximum density line as a limitation on the amount of rounded, natural sand in the mix. While there is no argument that an excessive amount of rounded natural sand may cause mixture instability, many HMA producers believe that the restricted zone is not the best way to control fine aggregates.

The type of sand (manufactured versus natural; rounded versus angular) is a critical element that may not be fully addressed by the restricted zone. Thus, some materials passing through the zone may perform satisfactorily.

Although AASHTO has changed the restricted zone from a requirement to a guideline, some specifiers still do not allow the aggregate blend to pass through the zone. NAPA recommends that past experience with materials rather than an arbitrary zone of restriction should prevail.

Another new aggregate test is the fine aggregate angularity (FAA) test that is designed to ensure a high degree of fine aggregate internal friction and rutting resistance. (Coarse aggregate angularity [fractured faces] has been a part of many specifications for years and also is a part of Superpave requirements.) The FAA was originally developed to evaluate natural aggregates. The test determines the percentage of air voids present in uncompacted aggregates less than 2.36 mm in size (a higher void content indicates more fractured faces). The FAA test procedure appears to be appropriate but the requirements for the test results may need to be adjusted based on local experience with materials. In addition, there is a difference in definition of particle size between ASTM and AASHTO that needs to be resolved.

The last of the new aggregate tests for Superpave is the flat and elongated particles test. A version of this test has been used in the past but with different definitions. The Superpave procedures specify a percentage by weight of coarse aggregate particles that have a maximum to minimum dimension of greater than five.

Based on comments from aggregate producers from across the country, very few aggregates do not meet the flat and elongated requirement. The 5:1 requirement may be too liberal in order to meet volumetric requirements for the mixture. Discussion is under way to possibly change the ratio to 3:1 with an appropriate modification to the percentage allowed. This change is currently being balloted by AASHTO.

Overall, the aggregate specifications are a move in the right direction to improve performance of HMA. Each of the issues discussed here can be resolved. Research projects are currently under way to address each of the issues. The projects will be completed prior to the 2000 target date for Superpave implementation.

The ultimate goal, of course, is to have a test on the final Hot-Mix Asphalt, which can be used to help the producer select the best materials for the mixture. Such a performance test would eliminate the need to test each individual component of the HMA mixture.

Binder

The Performance Graded (PG) binder specification proposed in Superpave provides a better system for grading asphalt binders than has been used in the past. The asphalt binder will be tested to provide an analysis of both high and low temperature behavior of the material.

Many agencies have already adopted the PG grading system. In some cases, the asphalt product is unchanged, simply a new "name." In other cases, dramatically different products are required.

The SHRP research was based on neat or unmodified asphalt cements. Because many PG grades will require modification, further definition of test requirements is needed to describe modified materials. Work is currently under way through the National Cooperative Highway Research Program (NCHRP) to accomplish this clarification. This project is scheduled for completion in 1999. Until the project is completed, contractors and agencies must continue to rely on binder manufacturers for recommendations on selection criteria.

Another issue important for the contractor is selection of mixing and compaction temperature. With neat binders, use of Superpave recommendations is appropriate. However, with high stiffness, modified binders, recommendations from the manufacturer will again be necessary.

Cost effectiveness of modified binders must be evaluated by the specifying agency. While improvement in laboratory test results can be shown, this improvement may not translate into improved performance on the roadway, which is necessary to justify the additional cost of the product. Likewise, not all HMA pavements need to have the highest grade of binder in order to perform satisfactorily.

Mix design

Superpave recommends major changes in the mix design process. The uses of gyratory compaction in conjunction with volumetric property controls are the cornerstones for the process. The use of gyratory compaction for laboratory and field production of HMA mix specimens is a proven technology that is repeatable, consistent and quiet. Use of this technology will improve the HMA mix design process.

NCHRP has undertaken studies to address some issues related to mix design. One is to convert well-performing mixtures to appropriate gyratory compaction levels. Another is to refine the recommendations for the number of gyrations to make them more accurate for all materials under all traffic conditions. These projects will be completed in 1999.

NAPA believes that one change in philosophy regarding Superpave should be made: Superpave should be considered a mix design process, not a specific type of mix. As a mix design process, other mix types such as stone matrix asphalt (SMA) and large stone mixes (LSM) can be included in the system. This philosophy would broaden the Superpave process to include all hot-mix asphalt types. Conceptually, then, a series of mix types could all be classified as Superpave. This would ensure that practitioners would have only one design process in their laboratory.

Superpave describes several different design levels based on traffic volumes. Engineering judgment is necessary to select appropriate mix types for a specific pavement application. The issue of cost effectiveness must also be a significant consideration in the decision-making process.

Plant operations

The HMA Industry is beginning to develop a wealth of information on the production of Superpave-designed mixes. Several hundred projects have been placed nationwide in 1997. A report describing construction guidelines is to be published by NAPA in early 1998. The report was based on a workshop of agency and industry experts and is available from NAPA.

Plant operations that need to be considered include stockpiling aggregates, feeding aggregates to the plant, storing binders, drying aggregates, mixing aggregates and binder, and storing the completed HMA. While coarser mixes may change the look of the mix, the handling of stockpiles and cold feeds is essentially the same as for conventional HMA. Accurate blending of various aggregates including RAP is vital because the Superpave aggregate specifications apply to the blended material as a whole.

Binder storage at the plant will only change to the extent required for stiffer binders. The PG binders may require plant equipment capable of dealing with high-stiffness materials. In order to avoid contamination, some producers may find it necessary to install separate tankage at the plant for each binder type.

Some mid-range PG grades will be modified from one supplier but, from another supplier, may not be modified, based on the selection of crude oil used by the manufacturer. This is only of importance to the contractor if the two products are mixed in a tank (or pipe). An incompatibility could occur. The best advice is to consult with your binder supplier prior to any such blending.

Along these same lines, storage stability may be an issue for some modified products. Once again the binder supplier should be able to provide this information.

Drying and mixing of materials are the same process for Superpave and conventional materials. However, some changes have been observed in how the process operates. Superpave mixes tend to be a bit more coarse than mixes to which most contractors are accustomed. Coarser aggregates may change drying efficiency. Furthermore, depending on the rock geology, more aggregate breakdown may occur and there will probably be more wear on the plant. Breakdown of the aggregate can significantly affect the volumetric properties of the resulting mixture.

Increased stiffness of the binder will require increased mixing temperature in order to get adequate particle coating. Some HMA facilities may have to adjust production rates in order to heat the aggregates to a higher temperature. A change in production rate can also affect volumetric properties of the mixture, due to aggregate breakdown, moisture in the mixture, and hardening of the asphalt binder. In most cases, however, these effects would not occur.

Storage of mixtures designed by Superpave is no different from conventional HMA. The operator must be cognizant of potential for excessive hardening of the binder, draindown, and segregation, all the same issues before Superpave.

Placement and compaction

Hauling of the Superpave-designed HMA to the job and placement through the paver are functions that require good techniques that are widely known in the HMA industry. If modified binders are used, the mixture will tend to be very sticky requiring some additional attention to detail.

Probably the most common problem experienced for Superpave-designed mixtures is field compaction. Some of the problems that have been observed are not related to Superpave but are related to lack of good construction techniques. Some issues relate to lower optimum asphalt content for Superpave-designed mixes, along with higher crushed particle content.

However, there are some real differences in the way coarse-graded Superpave-designed mixtures act during the compaction process. Many contractors report that the mixtures cool more quickly, perhaps requiring additional rollers on the job. Due to a change in definition, the rule-of-thumb for lift thickness to maximum particle size is different for Superpave-designed mixtures. Conventional wisdom was to use a 2:1 ratio for lift thickness to maximum particle size. However, this is now recommended to be 3:1 for Superpave-designed mixtures.

As with all mixes, the contractor must understand mixes and their relationship to compaction. On some Superpave-designed mixtures, a tender zone has been identified in the approximate temperature range of 200¡-240¡ F. In these cases, the mix can be satisfactorily compacted above or below the range but the mixture is very tender within the temperature range. While not true for all mixtures, this tender zone has been observed in several locations. Pneumatic-tired rollers appear to be the only type of compactor that can be used in the zone if it occurs. As more experience is gained with this type of mix, clarification of the causes of the problem is expected. Until then, awareness of the potential of the problem is the watchword.

NAPA publications, Paver Operations for Quality, and Roller Operations for Quality are excellent references for proper paver and roller operations.

Field management

An appropriate field management process is critical for the implementation of Superpave technology. The design system for the pavement is only as good as the process control during construction. If the volumetric control system is not understood, adopting Superpave will not improve pavement performance. Implementation of Superpave and an effective program of field management can and must occur simultaneously if Superpave is to succeed. Specifying agencies and HMA contractors must work together to produce high-performance HMA pavements based on volumetric principles.

Many agencies now require the contractor to be responsible for mix design and process control. This will not change using Superpave. The industry must use Superpave technology to build on existing knowledge of HMA pavements.

The NAPA publications Field Management of Hot Mix Asphalt, and Quality Control for HMA Operations describe sound approaches to process control.

Performance testing

Performance prediction is a very complicated process. It is quite easy to perform a test in the laboratory and develop a set of data on the materials being evaluated. It is much more difficult to use those data to predict the performance of HMA materials in real-world conditions. So many variables exist "on the ground" that it is extremely difficult to exactly predict pavement performance.

The Superpave shear tester (SST) was developed as a tool to predict pavement performance. While the SST may prove valuable as a research tool, it is not expected to be used in a routine mix design process. An economical, easy-to-use testing system for evaluation of mixtures is sorely needed. A variety of devices have been used in various locales around the world. However, there is no clear indication that any of the devices is the only way to go.

A major research project is being funded by FHWA to evaluate the several devices. The most likely scenario from this research is that specific engineering parameters from the test will be identified and not a specific recommendation for a single test procedure (Note: Please contact NAPA toll-free at 888/468-6499 for a white paper on performance testing of HMA).

Will a pavement engineer ever be able to predict performance of HMA materials? Perhaps the work currently under way by FHWA will add greatly to the knowledge base that is required to predict performance. Do we have the test methods and models to predict performance today? No. Will performance prediction be required for routine mix design and process control at the HMA facility? Probably not. Depending on the outcome of the current research projects, appropriate testing equipment may become available that would allow practical evaluation of HMA.

Training

The process of training personnel in the HMA industry is under way but much remains to be done in this effort to reach the estimated 300,000 workers. While the use of volumetric properties for HMA mix design is not a new concept, there are many areas of the country that have never used the process. QC/QA processes must also be a part of the training effort. This training process will not occur overnight. It requires a planned, methodical system for reaching all directly involved in the mix design process. We believe that establishing pilot projects within an agency is an excellent way to allow hands-on experience to occur.