EAPA Superpave from the European Perspective

The move into the next century and the expected growth in transportation will require an efficient and high-performing infrastructure. Safety and high-traffic flow is important, and any restrictions due to reconstruction and maintenance must be kept at a minimum. For road transportation, this means high-performance pavements with a long service life and low maintenance requirements.

An increasing number of premature asphalt pavement failures in the United States during the 1980s prompted the development of a co-ordinated, well-funded national research effort to develop improved specifications for bitumen and asphalt mixtures.

In 1987, the U.S. Congress established the Strategic Highway Research Program (SHRP) as a five-year, $150 million product-driven research effort. It was developed in partnership with the Federal Highway Administration (FHWA), the states, the Transportation Research Board, and industry.

The final product in the asphalt arena was published in 1993 as Superpave (SUperior PERforming PAVEments). It included: a new performance-based grading system for asphalt binder, consensus properties of aggregate, new mix design procedures, and mixture analysis procedures.

Performance testing in Europe

Performance testing and evaluation, however, is not a Superpave exclusive. In Europe, mechanical tests and performance evaluation have been the subject of research and practical work for many years.

In the 1980s in France, the road authorities and the industry worked in conjunction to develop a novel concept of performance-based test procedures correlated to field experience using the full-scale test facility in Nantes. The France experience goes back 20 years. In other European countries, such as the Netherlands, Finland, the United Kingdom, and Sweden, similar initiatives have been undertaken as well.

European standards and experiences are currently being transformed into harmonised European standards. As appropriate test methods and specification systems to assess mechanical properties, such as Stiffness Modules, Fatigue Resistance, and Resistance to Permanent Deformations, are put into context and are combined with the work done by CEN, a performance-based system for design and assessment of bituminous mixtures will eventually emerge.

It is in this context that the European Asphalt Pavement Association (EAPA) reviewed the Superpave technology as it is available today to learn the significance of the Superpave system for the European market. This article presents an abstract of that review report.

Implementing Superpave in the U.S.

The release of the SHRP findings in 1993 and the initiation of the Superpave implementation plan was recognised in many countries, and expectations for this new innovative system were very high.

Today, five years after the conclusion of SHRP, the significance of the Superpave technology begins to be seen.

The principle of volumetric design using the Superpave Gyratory Compactor and the innovative performance-based bitumen specification and associated test methods quickly received acceptance throughout the U.S.

The progress of the implementation plan within the U.S., however, has demonstrated that not all of the SHRP products were developed to a degree where they could be implemented.

In 1995, the FHWA reported inconsistencies in the performance models associated with Superpave Level 2 and 3, and further validation and refinement was necessary.

Consequently, implementation of performance testing of bituminous mixes was put on hold, leaving the binder specification and the volumetric design principle using the gyratory compactor described in Superpave Level 1 as the only active part of the Superpave system.

Surprisingly, Superpave did not include design procedures for including recycled asphalt pavement (RAP). During implementation, however, the Superpave Expert Task Group for Asphalt Mixtures took the initiative to develop interim guidelines for including RAP in the Superpave system.

Evaluating binder specifications

The innovative Superpave binder specification system uses a new integrated climatic decision system based on the reliability approach for selecting the optimal bituminous binder for any given project.

Road authorities, however, have a careful and conservative attitude, and consequently, most states are adopting two or three performance grades. The first is a “normal” grade which corresponds to an unmodified bitumen traditionally used in the state. In most of the U.S., this is usually a PG 64-22, corresponding to a 60/70 pen in Europe. The states also adopt one or two “super grades,” corresponding to their usual polymer-modified bitumen, typically a PG 76-22.

The validity of the Superpave Binder Specification is being evaluated. The requirements described in the BBR stiffness and m-value have been very well correlated to field performance of neat binders by studies carried out in Canada. There is much more argument about G*/sind and G*sind .

The requirements of G*/sind of 1 kPa for initial binder and 2,2 kPa for the RTFOT aged binder is generally accepted, with the reservation that aggregate properties make a major contribution to the resistance of rutting of an asphalt mixture. The requirements concerning G*sind is much more controversial and complex. This is currently being worked on, and may be changed to put more emphasis on phase angle „.

The procedure of jumping the performance grade of the binder when dealing with slow moving or heavy traffic is currently under discussion. Obviously, the procedure can only be used for bituminous binders where the time-temperature supposition principle is valid. However, several modified binders will not exhibit this property.

Learning from field verification

The Federal Highway Administration has developed an extensive system to verify the Superpave system in the field, ranging from actual roadway construction to accelerated loading facilities.

The first Superpave test sections were constructed in 1992 using the draft specifications as they existed then in four states. All but one of the sections have performed will to date.

The FHWA’s accelerated loading test site in Virginia has been evaluating Superpave test sections since 1993. Loading of five test sections of different Superpave binders indicated that the Superpave binder specification does rank the binders well, but more work is needed to better classify modified materials.

Testing was also performed at the FHWA’s WesTrack, a 2 kilometer oval track loaded with 75 metric ton trailers, to evaluate the effect of construction variables on performance. The variables included binder content, layer density, and filler content.

Not all of the Superpave mixes placed at WesTrack have performed as expected. Evaluations indicate that the coarse-graded mixes were very sensitive to construction variables which made them unstable and very susceptible to rutting. The fine-graded mixes performed very well with minimal rutting. Although the fine graded mixes with low binder content and extra filler displayed fatigue cracking as expected.

Through the validation programs, many things have been learned to improve upon the initial Superpave System delivered in 1993 by SHRP. Overall, the Superpave system is performing well, but there is always room for improvement.

European response to future needs

Pavement loading has reached a very high level; nevertheless, the annual increase is expected to continue into the next decades. It is not only the number of heavy trucks showing an increase. Changes in axle loading, axle configuration, and tire pressure contribute significantly to the major increase in pavement loading.

In addition, the environment together with the health and safety issues will become increasingly important and impact how asphalt pavements are designed and constructed.

At the road authority level, the trend to focus on traffic management, together with decreasing budgets, highlights a demand to re-evaluate the traditional approach to pavement management. Privatising and innovative contractual relationship between road authorities, asphalt producers, and contractors will become increasingly important in the future.

The effect of the harmonised European standards as organised by CEN will be crucially important. The European standards will be the common base for all Europe on which future developments will take place.

The European asphalt industry understands its responsibility and position in regard to the driving public and road authorities. In this perspective, EAPA has presented a research paper, Asphalt Research and Development in Europe. In this paper, EAPA states an inventory of areas of future research.

Some of those research areas are relevant to this report. They are:

* Development of reliable test methods to control and demonstrate the properties of asphalt mixes either by functionality based on laboratory tests or field tests of surface characteristics.

* Assessment of the potential benefits and limitations of a decision model for the optimal use of modified binders and additives to asphalt mixes.

* Development of contractual relationships. Functional contracts or warranty pavements are one of the important contractual concepts of the future.

In functional contracts, the contractor establishes the job mix formula, selects all materials to be used, and, in return, will warrant and maintain the performance of the pavement for a specified period.

Oil industry developing new tests

Binder specifications in Europe are currently based on empirical tests which have proven satisfactory for conventional bitumen, but inadequate for modified bitumen. Many believe that Superpave is moving in the correct, long-term direction towards rational performance-based specifications.

The European Bitumen Association (Eurobitume) is actively promoting and facilitating debate in Europe on this issue, and the European Standards Committee for Bitumen Binder (CEN TC19 SC1) has recently approved a project of the development of rational, performance-related specifications.

Particular issues which have been highlighted by Eurobitume are:

* Superpave may eliminate binders which are suitable for European applications and many introduce other binders which are not suitable.

* Superpave has been developed for conventional asphalt concrete applications. Many other European applications need to be taken into account, e.g., Gussasphalt, Porous Asphalt, and Very Thin Layers.

* Superpave test procedures have limitations in their ability to cover all performance requirements.

Eurobitume has adopted a widely agreed-upon set of principles for developing new tests summarised as follows. The tests should be valid in the field. Europe should not change what it currently uses in favour of new tests and test methods unless significant benefits of change have been shown in results from field trials held within a sound framework of experimental design.

It is advised that the repeatability and reproducibility of many of the Superpave tests are not yet considered satisfactory, and there is a need to develop tests and test methods which are suitable for field quality control. Eurobitume will hold a workshop in Spring 1999 on Performance Related Properties of Bituminous Binders. This will include a PIARC Seminar on SHRP/Superpave. The results of this workshop will prove a strong foundation for the development of performance-related specifications in Europe.

Contractors need useful test

In Europe, quality and enhancements are highly emphasised at the contractor level. European asphalt contractors have developed a high level of intercompany quality assurance and long-time experience with field performance of bituminous paving mixtures.

Given this background, a trend toward functional mixture requirements appears the logical path of the future. Designing the asphalt mixture and the performance of the final pavement is the contractor’s responsibility. To do this, the contractor needs reliable accelerated test methods and performance models on which he can rely his warranty.

The Superpave system, as it is today, does not provide the necessary information for proper evaluation of mixture performance. SHRP indeed did develop a full set of such tests. However, at this moment the practical suitability of the proposed Superpave tests for bituminous mixtures are questioned.

Superpave’s innovative performance- based binder specification presents a very interesting approach, especially when dealing with modified binders. It is the impression of the European asphalt industry that the validation and correlation to field performance, however, is not sufficiently documented in the work done by SHRP.

Numerous questions arise on how the specific tests and the specification limits will interact with the modified binders typically used in Europe as mentioned previously by the oil industry. These questions must, therefore, be answered before using the new tests.

The European asphalt industry believes that the role of the contractor must be stimulated. Developing innovative products and concepts is the task for the industry. To be able to do so, adequate and reliable test methods and principles of mix design and type testing must be available and sufficiently correlated to field experience.

Test methods must be suitable for use in the specific contractual relations used, including warranty projects. Consequently, for the European situation, test methods must be practical, implementable in daily operations, and suitable for the goal. Methods that are too complex, too expensive, and too slow, are not usable and efficient when managing product quality on a daily basis.

For quality control, tests and instruments are required that can be used within the regular production and construction of asphalt pavements. The structure in the European asphalt industry requires reliable and secure specifications in regard to mix design, type testing, and quality control.

Questions must be answered

The scientific basis of Superpave is recognised, but the translation into test methods requires specific adaptation to European contractual relations.

It is the intention of the asphalt industry to pro-actively address the format of the required system. The Technical Committee of EAPA is continuously working on this subject, including a recommendation of a workable system of product quality management.

The most important conclusion is that without the Superpave Mix Analysis & Performance Prediction, the Superpave system does not represent a superior technology as compared to current European technology. Furthermore, extensive validation programs will be required before any recommendations can be made on the usefulness of the Superpave system for the European market.