Ifsttar PhD subject

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Title : Hygrothermal behaviour of cementitious mortars containing bio-based phase change materials (PCMs) and/or miscanthus fibres: material and wall-scale studies

Main host Laboratory - Referent Advisor Navier  -  BENZARTI Karim      tél. : +33 181668251

Director of the main host Laboratory SULEM Jean  -

PhD Speciality Matériaux et Structures

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Marne-la-Vallée

Doctoral affiliation UNIVERSITE PARIS-EST

PhD school SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)

Planned PhD supervisor BENZARTI Karim  -  Université Gustave Eiffel  -  Navier

Planned financing Contrat doctoral  - Ifsttar

Abstract

This doctoral work addresses the general issue of improving the thermo-hygric performance of building envelope materials, with the aim of reducing energy consumption and optimizing occupant comfort. More specifically, it is proposed to compare the thermo-hygric behavior of three different solutions at both the material and wall scales: cementitious mortars incorporating either biobased microencapsulated phase change materials (PCM), micronized miscanthus fibers, or a combination of these two types of inclusions. This latter solution, referred to as a hybrid mortar, explores the possibility of combining the advantages of different components to achieve a composite that exhibits both thermal insulation properties, passive energy storage capacity through latent heat, and humidity regulation capabilities (hygroscopicity).
At the material scale, the first step involved defining formulation protocols suitable for the different mortar solutions considered, allowing for compensation of the loss of workability associated with the introduction of inclusions. A comprehensive program of microstructural, mechanical, and thermo-hydric characterization was then systematically applied to mortar formulations containing varying amounts of PCM and/or plant fibers, after 28 days of curing. Overall, it was found that the mortar with PCM and the hybrid composite exhibit superior thermal insulation properties compared to the reference mortar, but their hygroscopicity is not significantly improved as the presence of PCM tends to restrict moisture transfers within these materials. The introduction of miscanthus fibers, on the other hand, leads to a simultaneous improvement in both thermal and hygric performance. Furthermore, regardless of the type of inclusion used, a significant increase in porosity and a significant drop in the mechanical properties of the mortar are observed compared to the reference. For each of the solutions, an "optimal" content of inclusions, offering a compromise between gains in thermo-hygric properties and loss of mechanical strength, is then selected for investigation at the larger scale.
For the purposes of the study at the wall scale, a specific bi-climatic setup was first designed and developed in the laboratory to simulate different climate scenarios. Walls made from the three mortar solutions previously selected were then equipped with sensors and subjected to cyclic temperature (between 15°C and 40°C) and relative humidity (between 33% and 55% RH) fluctuations. The thermal responses of these different walls were systematically compared to that of the reference mortar wall in order to quantify the performance gains associated with the incorporation of inclusions. For the wall containing PCMs, a relatively modest damping effect of approximately 1°C in temperature was measured at various depths during the heating phase (when the exterior face was subjected to 40°C) compared to the reference. However, numerical simulations conducted at the building scale, taking into account the influence of PCM phase change on heat transfer, predict significant energy savings for winter heating (33%) and air conditioning requirements (27-31%) under different climates. The wall containing miscanthus fibers resulted in a much greater damping of temperature variation at the interior environment during the heating phase, with a difference of approximately 4°C compared to the reference, and also effectively dampened humidity variations. Lastly, the hybrid mortar wall also demonstrated interesting thermal performance, with a reduction of around 4°C in perceived temperature on the interior face during the heating phase compared to the reference. However, this hybrid mortar wall has a limiting effect on moisture transfer, which may compromise indoor comfort.
In conclusion, the mortar formulation containing exclusively miscanthus fibers seems to emerge as the most promising choice among the three solutions considered, demonstrating superior thermo-hygric performance for potential use as an envelope material. Besides, the comprehensive experimental data gathered in this thesis, at both material and wall scales, forms a valuable foundation for future modeling work.

Keywords : PCM, Concrete, Thermo-hygric Behaviour, Building structures, wall, energy efficiency, modeling, heat and mass transfer

Main host Laboratory - Referent Advisor	Navier - BENZARTI Karim tél. : +33 181668251
Director of the main host Laboratory	SULEM Jean -
PhD Speciality	Matériaux et Structures
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Marne-la-Vallée
Doctoral affiliation	UNIVERSITE PARIS-EST
PhD school	SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)
Planned PhD supervisor	BENZARTI Karim - Université Gustave Eiffel - Navier
Planned financing	Contrat doctoral - Ifsttar
Abstract This doctoral work addresses the general issue of improving the thermo-hygric performance of building envelope materials, with the aim of reducing energy consumption and optimizing occupant comfort. More specifically, it is proposed to compare the thermo-hygric behavior of three different solutions at both the material and wall scales: cementitious mortars incorporating either biobased microencapsulated phase change materials (PCM), micronized miscanthus fibers, or a combination of these two types of inclusions. This latter solution, referred to as a hybrid mortar, explores the possibility of combining the advantages of different components to achieve a composite that exhibits both thermal insulation properties, passive energy storage capacity through latent heat, and humidity regulation capabilities (hygroscopicity). At the material scale, the first step involved defining formulation protocols suitable for the different mortar solutions considered, allowing for compensation of the loss of workability associated with the introduction of inclusions. A comprehensive program of microstructural, mechanical, and thermo-hydric characterization was then systematically applied to mortar formulations containing varying amounts of PCM and/or plant fibers, after 28 days of curing. Overall, it was found that the mortar with PCM and the hybrid composite exhibit superior thermal insulation properties compared to the reference mortar, but their hygroscopicity is not significantly improved as the presence of PCM tends to restrict moisture transfers within these materials. The introduction of miscanthus fibers, on the other hand, leads to a simultaneous improvement in both thermal and hygric performance. Furthermore, regardless of the type of inclusion used, a significant increase in porosity and a significant drop in the mechanical properties of the mortar are observed compared to the reference. For each of the solutions, an "optimal" content of inclusions, offering a compromise between gains in thermo-hygric properties and loss of mechanical strength, is then selected for investigation at the larger scale. For the purposes of the study at the wall scale, a specific bi-climatic setup was first designed and developed in the laboratory to simulate different climate scenarios. Walls made from the three mortar solutions previously selected were then equipped with sensors and subjected to cyclic temperature (between 15°C and 40°C) and relative humidity (between 33% and 55% RH) fluctuations. The thermal responses of these different walls were systematically compared to that of the reference mortar wall in order to quantify the performance gains associated with the incorporation of inclusions. For the wall containing PCMs, a relatively modest damping effect of approximately 1°C in temperature was measured at various depths during the heating phase (when the exterior face was subjected to 40°C) compared to the reference. However, numerical simulations conducted at the building scale, taking into account the influence of PCM phase change on heat transfer, predict significant energy savings for winter heating (33%) and air conditioning requirements (27-31%) under different climates. The wall containing miscanthus fibers resulted in a much greater damping of temperature variation at the interior environment during the heating phase, with a difference of approximately 4°C compared to the reference, and also effectively dampened humidity variations. Lastly, the hybrid mortar wall also demonstrated interesting thermal performance, with a reduction of around 4°C in perceived temperature on the interior face during the heating phase compared to the reference. However, this hybrid mortar wall has a limiting effect on moisture transfer, which may compromise indoor comfort. In conclusion, the mortar formulation containing exclusively miscanthus fibers seems to emerge as the most promising choice among the three solutions considered, demonstrating superior thermo-hygric performance for potential use as an envelope material. Besides, the comprehensive experimental data gathered in this thesis, at both material and wall scales, forms a valuable foundation for future modeling work.
Keywords :	PCM, Concrete, Thermo-hygric Behaviour, Building structures, wall, energy efficiency, modeling, heat and mass transfer

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