Pavement design: how to have a more engineering-based approach
Abstract: PERFORMANCE TESTS ON BITUMEN AND BITUMEN MIXES
In this page we present an extract from the article "Performance tests on bitumen and bitumen mixes" by Pietro Ferrari.
In the whole article, the Author intends to highlight how it is now possible to have a more engineering-based approach to design and construct bituminous road pavements, intervening both in definition of the bitumen mix and in choice of the binder which is its active component.
The empirical tests used until now - and which have given excellent results over time, keeping under control groupings of requisites obtained from the experience gained over the years - are no longer sufficient for supporting the leap in quality of paving. Roads now must have increasingly extreme and durable performance over time, due to the continual increase of road traffic (in terms of frequency, weight and load time).
Test methods and the relative laboratory equipment currently exist in PAVELAB® SYSTEMS and IPC Global® ranges (Read more...), all complying with standards, which allow to identify the performance requisites which are necessary to guarantee extreme and durable quality of the surfacing.
This approach, which has been used in the USA for over two decades through the SuperPave system, as part of the SHRP research program, should hopefully completely adopted and consolidated by other countries.”
As well known, SuperPave (Superior Performing Asphalt Pavements) is a product of the SHRP (Strategic Highway Research Program) asphalt research. The system , as Asphalt Institute SP-2 mentions, "incorporates performance -based, asphalt material characterization with design environmental conditions to improve performance by controlling rutting, low temperature cracking and fatigue cracking."
Click the link below to download the whole version of the article available in english and italian.
The final intention of SuperPave is that the specifications of bitumen mixes are given in terms of fundamental properties, identified on the basis of product performance during use, according to two approaches:
- An empirical approach, perfomance related
- A fundamental approach, performance based.
However the two approaches share several general requisites, of which the most significant ones are:
- the granulometric composition,
- the percentage of air voids in the compacted mix,
- susceptibility to deform.
Complex Modulus E* (bitumen mixes)
The specificity of the approach is provided by the introduction of tests which measure the physical magnitudes directly linked with the performance characteristics, as:
- Resilient modulus
- Dynamic modulus and phase angle
- Resistance to fatigue
- Resistance to permanent deformation
- Low temperature cracking
- Fracture energy
These dynamic tests are conducted by automatic machines which operate both under load control and displacement control and which are contained in a thermostatic cabinet.
It should be emphasised that the bitumen mix, as a viscoelastic material, behaves in a manner heavily dependent upon temperature and load in terms of load type, frequency and application time; these parameters must therefore be well-defined in the tests.
Dissipated energy versus numbers of cycles
As known, bitumen is the active binder of the bituminous mix. It is present on average as 5% of the mixes and determine their rheological behavior to a large extent.
In the past, up to about twenty years ago, asphalt binders were classified by mean of traditional empirical tests.
Superpave, since more than two decades, has defined a mechanistic empirical system that includes some empirical tests and fundamental tests where the results are expressed in engineering units at a defined temperature. The system, "PG classification" has been standardized as AASHTO M320 and ASTM D6373. Binder is classified according to two critical temperatures, where the classification assigns a performance grade delimited by two temperatures that correspond to the average seven day maximum temperature of the year and to the minimum temperature of the year of the site where the binder has to be used.
The tests adopted are:
- Flash Point, Rotational Viscosity (ASTM D4402 and AASHTO T316),
- DSR (dynamic shear rheometer test (ASTM D7175/ASTM D7405 and AASHTO T315),
- Rolling Thin Film Oven Test RTFOT (ASTM D2872 and AASHTO T240),
- DSR after RTFOT,
- Pressure Aging Vessel PAV (ASTM D6521 and AASHTO R28),
- DSR test after PAV,
- Bending Beam Rheometer test (BBR, ASTM D 6648 and AASHTO T313) after PAV and Direct Tension test,
- DTT ( AASHTO T314 and ASTM D6723) after PAV.
Overview on some reference points
Regional agencies and bituminous mixture designers often use the MEPDG guide (Mechanistic - Empirical Pavement Design Guide) to asses the performance of pavement with HMA mixtures. This can be realized thanks to the comprehensive and complex calculation program and to the wide-ranging prediction models that have been calibrated by test sites such as LTPP(8) (9) (10) (Long- Term Pavement Performance) test sections with database located throughout the United States, MnROAD(11) (Minnesota Department of
Transportation), WesTrack(12) (Nevada Accelerated Pavement Testing ), FHWA ALF(13) (Federal Highway Administration Accelerated Loading Facility) and other test sites. Stress/ strain laboratory data (E* and Fn), traffic loading, grade of binder, environmental data are inputs to MEPDG guide software for the eleboration of distress prediction, such as rutting and cracking in the pavement.