Study of damage in low carbon cementitious road materials including recycled aggregates – Towards resilient design for sustainable pavements

Context 

The road sector faces a twofold challenge: on the one hand, drastically reducing its carbon footprint during construction, and on the other hand, ensuring the durability of structures under increasingly aggressive traffic and a changing climate.

The French design method for new pavements [1] is based on empirical fatigue models (Wöhler curves) and Miner’s rule. These models were calibrated for traditional materials and standard conditions. However, in the case of rigid pavements (with cementitious base or surface layers), the emergence of new low carbon binders (ternary cements, alkali activated slags) and even geopolymer binders, together with the use of recycled aggregates, fundamentally modifies the microstructure and stiffness of the materials [2], which raises the question of the applicability of the current design approach. In addition, these rigid pavements are not only subjected to traffic loads [3]. The thermal gradient between the surface and the bottom of the layers generates curvatures that modify their mechanical response.

There is therefore a major scientific uncertainty today regarding how these new “eco designed” materials behave under mechanical fatigue [4], thermal fatigue, and the combination of these two mechanisms. This question is valid for road pavements as well as for airfield pavements and special pavements (structures for tramways, industrial surfaces, etc.)

Objectives

This research project aims to address the following scientific challenges :

• Better characterize the fatigue behavior of the new materials, based on four point bending tests at various stress or strain levels. The evolution of the microstructure will be characterized by monitoring the elastic modulus during testing and by using digital image correlation to observe cracking, enabling an energybased analysis;
• Assess the validity of the linear damage accumulation law for these heterogeneous materials under complex loading histories resulting from sequences of real traffic and climatic variations, which induce temperature gradients responsible for slab curling/warping phenomena;
• Determine whether cement substitution and the inclusion of recycled aggregates in rigid, elastic materials influence microcrack initiation and energy dissipation.

The specific objectives of the PhD are:

• Multi physical characterization: analysis of the link between the structure/microstructure of low carbon matrices with recycled constituents and their mechanical properties (moduli, strengths, etc.);
• Using fatigue tests, develop a failure criterion based on dissipated energy in order to better account for material damage and to improve the damage accumulation law;
• Take into account the diversity and statistical nature of in situ loading conditions in order to study sequence effects (variable amplitude, discrete occasional overloads, etc.) in pavement structures;
• Derive a practical design approach: propose new input parameters (fatigue slope 1/b, allowable stress σ6, calibration coefficients, etc.) for the french design software Alizé, so as to account for the behaviour of these new materials.

Planned work programme

The research methodology will be structured into three successive phases.

First, a laboratory experimental campaign will be conducted to develop fatigue tests suited to the new materials. Using four point bending mechanical tests, instrumented with strain gauges, accelerometers and digital image correlation, damage initiation will be detected. The work will first be performed at constant loading level, and then with more or less complex mechanical loading cycles.

Second, this methodology will be applied to emerging rigid pavement materials incorporating new binders with a lower carbon footprint than conventional cement (e.g., LC3 cements) or geopolymers. Beyond standard mechanical characterizations such as modulus and strength, special attention will also be paid to thermal properties.

Finally, using numerical modelling with finite element codes such as ABAQUS, the behavior of a pavement slabs subjected to different combinations of loads and temperatures will be simulated, in order to identify the most critical loadings (locations within the structure and stress levels). The ultimate goal is to define, for the various cases considered, stress correction factors to be used within the classical design method based on a multilayer analytical model. These studies will help optimize the length and thickness of pavement slabs.

From a scientific standpoint, the aim is to establish a fatigue law or a failure criterion capable of better describing the behavior of these heterogeneous media under the combined effect of repeated mechanical and climatic loadings. From a societal standpoint, this project will help validate and secure the operational use of low carbon concretes for pavements subjected to heavy traffic. It will define the key technical data required to update the design method for these materials.

References

[1] LCPC/SETRA (2014). Guide technique : Conception et dimensionnement des structures de chaussée. IFSTTAR / CEREMA.

[2] Thomas, C., et al. (2014). Fatigue limit of recycled aggregate concrete. Construction and Building Materials, 52, 146-154.

[3] Hiller, J. E., & Roesler, J. R. (2005). Determination of critical concrete pavement fatigue damage locations using influence lines. Journal of Transportation Engineering, 131(8), 599-607

[4] Oh, B. H. (1991). Cumulative damage theory of concrete under variable amplitude fatigue loadings. ACI Materials Journal, 88(1), 41-48.

Gustave Eiffel University is a national public multidisciplinary institution for higher education and research. Its primary missions are education (initial and continuing), research and innovation, support for public policies, and community outreach.

It has several sites across the country, including the Nantes campus located in the town of Bouguenais.

The PhD will be carried out within the LAMES (https://lames.univ-gustave-eiffel.fr/en/) and MIT (https://mit.univ-gustave-eiffel.fr/en/) laboratories of Université Gustave Eiffel (Nantes campus), with supervision covering all the issues addressed in the thesis. The MIT laboratory focuses on the characterization of road materials and the
development of innovative materials to improve the durability and resilience of road infrastructures and to reduce their carbon footprint. The activities of the LAMES laboratory concern the study of mechanical behaviour, modelling and design of pavement structures, pavement monitoring and instrumentation, and full scale testing.

We are seeking outstanding and motivated candidates with a Master’s degree in relevant mechanics or materials science disciplines. The candidate should demonstrate motivation, scientific curiosity, and a strong interest in mechanical assemblies, laboratory testing, and numerical modelling. A strong command of the English language (written and spoken) is required.