More than 95% of European road pavements comprise an asphalt layer
covering structural layers and foundations of unbound granular material.
This granular material, which is made of quarried aggregates, forms, on
average, more than 75% of these road pavements. Yet its behaviour is
poorly understood and has been little researched, especially when compared
to asphalt and concrete. Therefore, this project aims to achieve a real
advance in structural road pavement design and performance by
investigating this material in some depth.
The objectives are:
- To provide a fundamental and universal mechanical behaviour framework for unbound granular materials drawing on functional and simplified laboratory tests.
- To produce a new, rapid and practical assessment approaches based on this framework.
- To deliver a fundamentally acceptable classification system for granular materials.
- To collect measurements of real, in-situ, mechanical performance thereby assessing effects of site and climate variations.
- To produce models of the mechanical behaviour based on the both laboratory and in-situ results.
- To propose better means of use of unbound granular materials using experience from the above objectives.
- To draft guidelines for testing, modelling and incorporation of unbound granular materials into structural and foundation pavement layers.
The purpose of the testing is to describe the materials in ways, which will permit traditional, index results to be compared with descriptions based upon a sound fundamental tests. Thus, the possibility of using index testing as rapid, commercially practical proxies for fundamental assessments will be investigated. The Testing is in three forms:
- Simple - both conventional and unconventional.
- Fundamental - usually more complex.
- assessed UGMs in a variety of ways to determine those test procedures which can deliver useful characterisations;
- determined the variability of in-situ pavement conditions which can have a large affect on actual UGM performance;
- provided an analytical framework for describing performance, which can be used as a basis for more reliable design computations;
- found that in-situ monitoring of UGM condition in pavements reveals that the moisture content of the UGM varies considerably with the season, being dependent on precipitation levels, integrity of the sealed surface, final preparation applied to the shoulders of the pavement, level of the pavement and the ability of the pavement to self drain;
- found that pavement performance, as measured by deflection in-situ, shows a very serious degradation as the moisture content of the UGM rises;
- demonstrated the viability of building pavements with thin surfacings;
- found that empirical laboratory tests, being widely used throughout Europe, can aid material assessment but often fail to clearly differentiate between material qualities due to their simplistic nature;
- found that, employing repeated load triaxial tests (RLT), significant reductions in the stiffness of the UGMs and significant increases in permanent deformation can be observed as the moisture content rises;
- found that the rutting behaviour, as measured in a wheel tracking test facility, confirmed the applicability of the RLT method to characterise the permanent deformation propensity of UGMs;
- confirmed that design procedures which adequately incorporate the behaviour of the UGM layers have shown that:
- the moisture content must be carefully matched for thinly surfaced pavements, given the significance of the moisture content on the UGM and pavement performance,
- linear elastic computations are acceptable for thickly surfaced pavements and for simplistic representations of the pavement structure,
- the Boyce constitutive model of resilient stress-strain behaviour provided an appropriate non-linear representation of the UGM for use in calculating the response to traffic loading for a pavement having an intermediate thickness of bituminous surfacing;
- found that analytical computations, employing the materials data from the RLT, showed that, at design stage, more thinly surface
The COURAGE study recommends that more effort should be given to determining the relevant mechanical properties of unbound granular materials, rather than to determining the properties of material indices which is currently the common European practice. To achieve a reliable characterisation it is recommended that the RLT test is employed, following the draft CEN standard but with modifications. In addition, the testing programme must assess UGMs at the likely in-situ moisture contents which they will have during the life of the pavement. Design of the pavement should utilise an adequate representation of the pavement structure and the UGM. This will necessitate the use of a non-linear approach for the UGM drawing on the data obtained in the RLT. Furthermore, RLT data can be used to check for rutting susceptibility although, at present, direct computation of rutting development with trafficking is not possible.