Strategic Highway Research Program

In 1987, the Congress of the United States of America allocated $150 million over five years to the Strategic Highway Research Program (SHRP) for research to improve the highway network. The research programme, which also sought to improve road safety for both road users and highway workers, focused on four areas:

• Stage 1. Desk Study: assess and evaluate existing information on the material;

• Stage 2. Laboratory Study: test the mechanical properties of materials to allow theoretical predictions to be made of their performance;

• Stage 3. Pilot-Scale Trials: evaluation of construction and performance of materials in small scale trials;

• Stage 4. Full-Scale Trials: full-scale trial on a trunk road to establish whether the previous assessments obtained from Stages 2 and 3 are realized;

• Stage 5. Highways Agency Specification Trials: this stage is necessary to carry out further evaluation of the material and to test the specification under contract conditions.

• high-friction systems;

• overbanding;

• thin surfacings; and

• modified binders for:

• asphalt (the performance of asphalt materials);

The programme is now complete, with 140 ‘products’ (new specifications, test methods, equipment and reports) having been identified, of which 22 are from the Asphalt Program, which had been allocated about $50 million.

The objective of the Asphalt Program was to produce new specifications for both bitumen and mixed materials which were related to the performance of the materials;

however, the short time scale of the project did not allow the development of a rigorous relationship between the material properties and performance under traffic so that, at best, the methods of evaluating materials were performance-related. The list of SHRP products from the Asphalt Program, including their reference numbers, is as follows:

The major ‘product’ of the Asphalt Program has been the SUperior PERforming PAVEment (SUPERPAVE) system for asphalt mixtures. The asphalt concrete mixtures designed by the programme have aggregate gradings passed on the ‘Fuller’ curve (Figure 1.2) with an index of 0.45 in Equation 1.1 (section 1.3).

One important aspect that has been introduced into the structure of the specifications is that the properties are considered in terms of climatic zones. It is no longer necessary to have every binder able to withstand loads at +60 °C and −20 °C; where the climate is more temperate, less onerous requirements may be stipulated. Hence, the binder specification is an attempt to define the behaviour of materials under specific traffic and

• concrete and structures;

• highway operations; and

• long-term pavement performance.

• 1001 Asphalt Binder Specification

• 1002 Bending Beam Test for Asphalt Binders

• 1003 Pressure Ageing Methods for Binders

• 1004 Asphalt Extraction and Recovery Method

• 1005 Direct Tension Test for Low-Temperature Cracking

• 1006 High Temperature Viscosity Test

• 1007 Dynamic Shear Rheometer Test for Binders

• 1009 Binder Chromatographic Test Methods

• 1010 Asphalt Refiners Guide to Binder Performance

• 1011 Asphalt Mixture Specification

• 1012 SUPERPAVE Asphalt Mixture Design System

• 1013 Net Adsorption Test for Asphalt Mixtures

• 1014 Gyratory Compaction for Asphalt Mixtures

• 1015 Rolling Wheel Compaction

• 1017 Shear Test for Asphalt Mixtures

• 1019 Flexural Fatigue Life Test

• 1021 Thermal Stress Restrained Specimen Test

• 1022 Indirect Tensile Creep and Strength Test

• 1024 Environmental Conditioning System

• 1025 Short-Term Asphalt Ageing Method

• 1030 Long-Term Asphalt Ageing Methods

environmental conditions. A set of material tests have been developed to measure the physical properties of both modified and unmodified binders. These tests are much more sophisticated than the penetration and softening point, which are widely used to specify binders.

To support the mixture specification, a suite of test methods have been developed to assess the resistance of mixtures to:

taking into account the effects of:

This suite of tests is currently being used in the United States in pilot trials of the SUPERPAVE specification.

In addition, the US Federal Highway Administration (FHWA) is taking the lead in helping US highway agencies with the implementation of the research outputs. FHWA is also continuing the work of evaluating the long-term pavement performance experiments.

With this commitment to improve and rationalize their specifications, the specification approach and test methods will obviously become a strong influence in Europe and throughout other areas of the world, including through CEN in the further development of their ‘harmonized’ CEN specifications now being prepared.

However, before the SHRP approach is widely adopted by others, the ‘products’ from SHRP need to be validated. The major concern is the robustness of the relationship between the results of the new tests and the performance of the materials in service. Other concerns are the cost of the equipment (about $100k for the binder tests and about $300k for the mixed materials) and the time scale for completing the design of a mixture.

Nevertheless, the evaluation of SUPERPAVE and the associated specifications is taking place and some aspects of them will be introduced into CEN standards at some time in the future.

1.6 REFERENCES

Manual of Contract Documents for Highway Works. Her Majesty’s Stationery Office, London. Volume 1: Specification for Highway Works (MCHW 1).

British Airports Authority (1993) Civil Engineering Section 14: Bituminous Materials — Aircraft Pavements. Quality System Reference Specification SP-CE-014–04–0A, British Airports Authority Technical Service Division.

Comité Européen de Normalisation (1996) Tests for Geometrical Properties of Aggregates; Part 2. Determination of Particle Size Distribution—Test Sieves, Nominal Size of Apertures. BS EN 933–2:1996. British Standards Institution, London.

County Surveyors’ Society (1996) Highway Authorities Standard Tender Specification.

• rutting;

• fatigue cracking; and

• low temperature cracking,

• ageing;

• moisture susceptibility; and

• loss of adhesion.

Cheshire County Council.

Defence Works Services (1995) Marshall Asphalt for Airfield Pavement Works; Hot Rolled Asphalt and Coated Macadam for Airfield Pavement Works. Defence Works Functional Standards. Her Majesty’s Stationery Office, London.

Fuller, W.B. and S.E.Thompson (1907) The laws of proportioning concrete, Trans.

American Society of Civil Engineers 59, 67–172.

Hindley, G. (1971) A History of Roads, Peter Davies, London.

CHAPTER 2