Subject matter

Bituminous mixtures (BM) exhibit a complex thermomechanical behaviour, widely influenced by temperature and loading frequency. Conventional testing methods that are used to characterize these materials in laboratory are often time-consuming and expensive, as they require the use of advanced equipment such as hydraulic presses and they do not offer a direct link with field evaluation. Meanwhile, for in situ auscultation, most of the existing methods rely on surface condition evaluation and they do not directly assess the structural condition of the pavement. Falling Wight Deflectometer (FWD) measurements offer this possibility, but they require a good knowledge of the structure design and remain expensive. Worldwide, rational design of pavement structures is incorporating more and more mechanistic considerations, and control strategies should accordingly incorporate determination of fundamental properties instead of only compositional characteristics. Such improvements should contribute to make pavements endure approximately their designed lives, thus facilitating meeting durability criteria, necessary for the sustainability (including environmental sustainability) of those structures.   

In this context, there is a need to develop alternative testing methods to improve the characterization of BM both at the laboratory scale and real scale. Non-destructive methods offer many advantages: they are more cost-efficient than conventional methods and they can be used in laboratory and in situ. For example, methods based on mechanical wave propagation (MWP) have been widely used for many years to successfully characterize and assess the structural condition of elastic materials such as concrete or steel. At the laboratory scale, ultrasonic testing (UT) and impact resonance testing (IRT) are amongst the most common MWP methods in the academic literature. UT is based on the measurement of the time-of-flight of ultrasonic waves through a specimen. It can be used to check the density, evaluate the stiffness or detect internal defects such as cracks and voids. Meanwhile, IRT relies on the principle of resonance. It consists in vibrating a specimen with an external excitation such as an impact, and measuring the resulting vibrations to identify the natural frequencies of the specimen. These frequencies are directly related to the mechanical properties of the material, such as the stiffness or the damping. Several international standards already exist for the characterization of elastic materials with UT (ASTM C597, EN 12504-4, ISO 1920-7, ASTM D6760, EN 15317, etc.) and IRT (ASTM C215, ASTM E1876, EN 14146, etc.). However, while elastic material specimens may exhibit very specific resonance frequencies, with well-known elastic solutions to relate them to the intended calculation of modulus, the same is not true for viscoelastic materials.  For these materials, such as BM, it is necessary to perform inverse analysis calculations, which makes the interpretation of the results much more difficult and still an open subject in the literature.

For bituminous mixtures, MWP methods are not commonly used because of the lack of guidelines to properly use them specifically for this type of materials that exhibit a complex thermomechanical behaviour. Indeed, there is no existing standard for UT or IRT dedicated to the characterization of BM. In addition to the missing testing procedures, there is also a lack of guidelines for the interpretation of the experimental data gathered from UT and IRT to determine the properties used in pavement design and quality control for example. Therefore, this new RILEM TC will intend to provide guidelines to use UT and IRT to characterize bituminous mixtures at the laboratory scale. More specifically, the main objectives of the TC are:

• To collect and summarize the current practices about the use of UT and IRT for BM (literature study)
• To propose and validate laboratory procedures for performing UT and IRT on BM (experimental development and round-robin test)
• To define basis candidate protocols for transducer characterization and equipment calibration for MWP methods (experimental and numerical developments)
• To evaluate different methods of interpretation of the results of UT and IRT for BM (analytical and numerical developments)
• To establish a first internationally shared database of UT and IRT data on different BM (experimental developments)

Terms of reference

The proposed TC will be active for five (5) years. The TC is sought to be inaugurated at the RILEM Cluster F annual meeting in October 2024, where a wider audience will be exposed to the intentions of this TC and have the opportunity to join in on the efforts. A meeting of part of the TC leaders occurred during the last ISAP Conference (June 2024), in Montreal. In addition, the current experts in the field have already been invited to participate through mailings. To this day, 10 research laboratories in North and South America and Europe, including 7 universities and 3 companies, have confirmed their interest to actively participate to the TC activities. The TC will conclude its work in 2029 and the findings will be presented at a subsequent RILEM event.

To keep this TC focused, the TC will comprise two task groups (TG): TG1 – Ultrasonic testing to characterize bituminous mixture (TG Leader: Pejoohan Tavassotti, University of Waterloo) and TG2 – Impact resonance testing to characterize bituminous mixture (TG Leader:Yasmina Mahmoudi, ENTPE). The activities of both TG will start with a thorough literature review about the current practices regarding the use of UT (TG1) and IRT (TG2) on BM. The goal of this first step is to identify the existing laboratory procedures as well as the analysis methods commonly used to convert experimental data to BM mechanical properties such as stiffness and damping, and other applications such as damage identification. Each TG will then define laboratory procedures (e.g., test configuration, equipment, specimen geometry, testing temperature range, number of repetitions, sensors positioning and mounting requirements, etc.) that will be evaluated by performing round-robin testing with multiple BM types. The outcomes will help to identify precision and repeatability/reproducibility of the different procedures, and to select the most accurate one(s). It should be noted that the same mixtures’ types will be used by both TG to facilitate the synergy between them. In parallel of the round-robin testing phase, the 2 TG will identify promising methods to interpret the experimental results from UT and IRT to obtain mechanical properties from them. These methods may include analytical equations, finite element modelling, and also the use of artificial intelligence (AI). They will be used to analyse the results of the round-robin testing in order to determine the limitations of these methods and the most promising one(s) for mixtures’ properties determination. It should be noted that even laboratories that are not necessarily ready to install equipment for the tests themselves but want to contribute in the modelling and/or data analysis activities are welcome. This will be accomplished by sharing data in a database of UT and IRT and with the focus of one of the objectives, which is the results interpretation. The work program is detailed below:

Year 1:

• Literature review and identification of main research gaps on testing and on interpretation of MWP tests
• Establishment of consensus on the general strategy and identification of work limitations in the different groups
• Collection and discussion of published data
• Identification of ongoing projects relating to the use of UT and IRT with BM
• Definition of basic candidate protocols for carrying out tests in several laboratories with similar equipment
• Definition of basic candidate protocols for interpreting test results
• Definition of a common platform for sharing data within the project, produced by TG1 for interpretation by TG2
• Sharing of the platform pilot program

 Years 2 - 4:

• Execution of a round-robin test at the participating institutions with analysis of test execution conditions and on different materials
• Construction of a general database for all tests carried out by all partner institutions, by populating the data sharing platform and identifying necessary improvements in the platform
• Interpreting data generated in different locations and laboratories and shared in the shared platform
• Establishment of the data analysis methods most used by each institution and comparison between the methods
• Discussion of the new results with the active participants
• Symposium planning, organization and event

Year 5:

• Final draft of the terms of reference for UT and IRT implementation (laboratory protocols and interpretation method)
• Final draft of the state-of-the-art (STAR) report summarising the findings of each TG
• Final draft of the journal publications

Detailed working programme

The working programme of the TC will consist of the following activities:

• November 2023: A preliminary presentation of the TC was made during the annual meeting of RILEM cluster F in Lyon. The feedback and inputs received were used to refine the TC objectives and the proposed workplan
• Winter 2023 and spring 2024: call for participants (academics and industries) through mailings; preliminary meeting with some of the TC leaders during the last ISAP Conference (June 2024), in Montreal; writing of the official TC proposal; online meetings with TC chair, deputy chair and future TG leaders
• Summer 2024: Approval of new TC by TAC
• Fall 2024: Kick-off meeting in Delft, during the annual meeting of RILEM cluster F. TC objectives and outcomes will be presented along with preliminary version of work plans for each TG. The first meeting of each TG will be fixed within 6 months.
• Winter and spring 2025: Literature review and gathering of state-of-the-art about the use of MWP methods on BM (UT: TG1 and IRT: TG2). Individual meetings by TGs; finalizing proposal of working plan, evaluation of other possible synergistic activities; identification of missing expertise/experts that could be affiliated with the TC.
• Fall 2025: Annual TC meeting during the annual meeting of RILEM cluster F. Synthesis of the literature review.
• 2025 to 2026: Individual meetings by TGs; establishment of the laboratory procedures to evaluate in round-robin tests; selection of the mixtures’ types and beginning of round-robin tests
• Fall 2025: Annual TC meeting during the annual meeting of RILEM cluster F.
• Fall 2026: Annual TC meeting during the annual meeting of RILEM cluster F
• 2026 to 2027: Individual meetings by TGs, finalizing of round-robin tests; definition of the workplan for the data interpretation phase (selection of the methods to use); start of data interpretation.
• During 2027: Organization of a TC workshop bringing together experts and industrials from the asphalt community to present preliminary findings of the TC and to publicize the potential of MWP methods to characterize BM.
• Fall 2027: Annual TC meeting during the annual meeting of RILEM cluster F.
• 2028 to 2029: End of the data interpretation work; preparation of journal articles; redaction of the STAR and of guidelines than can serve as a basis for further standardization of UT and IRT for BM.
• Fall 2028: Annual TC meeting during the annual meeting of RILEM cluster F.
• Fall 2029: Official closure of the TC during the annual meeting of RILEM cluster F.