Consultation des offres

Offer of PhD studies: Stone masonry structures in fire CDD

Contact

Javad Eslami

95031 Cergy-Pontoise
1
France

Javad.Eslami@cyu.fr

Descriptions

Établissement :
UCP - université de Cergy Pontoise
Laboratoire :
L2MGC - Laboratoire de Mécanique et Matériaux du Génie Civil
Date de début :
02/11/2020
Date de fin :
01/12/2021
Date limite pour postuler :
01/01/2021
Descriptif :

Stone masonry structures in fire
All over the world, masonry is the historic construction material par excellence. Historic masonry buildings are vulnerable to fire due to wooden or metallic decks and roofs, typically high fire loads and
frequent (and inevitable) works of maintenance. The recent, devastating fire of Notre Dame de Paris has enlightened once more the vulnerability of historic architectural heritage and the general concern for the residual stability of the monument. The residual structural capacity of masonry walls, pillars, columns and vaults is difficult to quantify [1]. Generally, only destructive tests (DT) can provide quantitative assessment of mechanical properties; but the need for heritage preservation generally
forces post-fire surveyors to rely mainly upon non-destructive tests (NDT). For instance, P-wave velocity tests can help detecting the damage extension, and colour change evaluations can reveal the
maximum temperatures of exposure [2]. Where possible, calibration between NDT on corresponding DT parameters maximises the knowledge increase, minimising the time, cost and impact of the survey
[3]. On the other hand, even performance-based codes for the fire resistance assessment of masonry structures – like Eurocode 6 [4] and the USA code NFPA 914 [5]– do not contain specific strength
calculation methods for post-fire situation. To enable such methods – e.g. the method of the reduced section (Eurocode 6, Annex C) – it is essential to establish relationships between temperatures and
factors of residual strength for a wide range of masonry materials.
In the field of civil engineering, the high temperature behaviour of stones is much less known and investigated than that of steel or concrete. In particular, most of the available research is focused on a low-medium range of heating (i.e. < 600°C); such temperatures cover the effects of the majority of fires, but might neglect extreme cases. On the other hand, useful scientific information is available in geology and mineralogy context. At high temperature, microscopic physical and chemical transformations trigger macroscopic changes in the physical, thermal and mechanical properties of masonry units and mortars [6]. Among the most relevant processes, the dissociation of metakaolin and transformation of quartz-α to -β produce macroscopic thermal expansion, mostly irreversible, in claybased materials and silicate rocks like sandstone [7, 8]; in carbonate rocks, e.g. limestone, decarbonisation is the cause for material contraction beyond 800°C, while the formation of Portlandite brings on a volume increase during the cooling phases [7]. Damage increase and spalling can show up even days after the fire event [9, 10], having major consequences on the safety of buildings after fire. Little research is available about the post-fire mechanical behaviour of stone masonry, and the very heterogeneous results claim for increasing scientific contributions. This project will investigate the high
temperature behaviour of masonry after high temperature exposure at the material scale. The focus of the investigation will be on limestone (LS), sandstone (SS) and lime mortar (LM), selecting historic
masonry types in France.


PhD studies (the work)
Characterisation of materials at the original state
Ten types of stones will be studied in this task, to ensure the widest possible representativeness of materials used in French historic monuments, in terms of mineralogical composition, physical and mechanical properties. The focus will be on mainly on Lutetian limestones existing in quarries, and comparable to the lithotypes, which were used for the construction of the Notre Dame cathedral
in Paris and lime mortar (LM).
Tests under high temperature
1) Characterization of thermal properties and thermal response of materials. Conductivity, diffusivity and specific heat as functions of temperature up to 800°C will be measured by means of hot disk device. In parallel to such tests, the thermal response of the samples is evaluated by recording the temperature distribution during the heating and cooling, with thermocouples placed inside the stone samples at different depths; such measurements will allow evaluating the latent
heat. Such records will be employed to set up models of the thermal transfer in the stone types.

2) Study of the elementary mechanisms at the origin of the thermal damage of materials. Measurements of the thermal linear deformation up to 1000°C will be carried out by means of a
dilatometer equipped with a furnace. In parallel, the mass loss and heat flux evolution during heating will be investigated with thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC).

3) Assessment of mechanical properties under high temperature. The
compressive strength and modulus of elasticity of the stones and the mortar will be evaluated during heating. The effects of different combinations of heating and loading regimes will be assessed.

Residual tests after high temperature exposure
1) Assessment of mechanical damage and durability indicators with temperature. After cycles of heating-cooling at different maximum temperatures up to 900°C, the mechanical behaviour of stones and mortar will be studied. Compressive and tensile strength will be determined; the residual mechanical behaviour will be characterised, by establishing stress-strain relationships after monotonic as well as cyclic uniaxial compressive tests, and measurement of ultrasonic wave
velocity through the samples. After the heating-cooling cycles, the evolution of durability indicators such as porosity, coefficient of capillary water absorption, water and gas permeability will be investigated. The parameters of interest during this study will be three. a) Initial water
content: absorbed or free water generates thermal and hydraulic gradients during heating, triggering local stress depending on the stone permeability. This effect will be analysed by heating the stone samples at two different moisture state conditions, i.e. pre-dried and saturated. b)
Heating rate: to determine intrinsic mechanical properties without the effect of thermal damage, i.e. to minimise the effect of the thermal gradient (micro-cracking), very slow heating rates (1°C/min) will be applied. On the other hand, to investigate the sensitivity of materials to spalling and the material behaviour at heating rates close to real fires, fast heating cycles (15°C/min) will be performed. c) re-hydration conditions once out of oven, for samples heated beyond 600°C.

2) Correlate the color change to the residual properties of the material. Colorimetry is an indirect and non-destructive analysis technique, which relies on the principle of colour change measurement on a material when it has been exposed to high temperatures. This will allow to evaluate the
temperature attained by the material on site, and thus the decay of its mechanical properties.


Case study: Notre Dame
A set of stone blocks and mortars originating from the collapsed vaults of Notre-Dame will be studied. Assessment methods will include non-destructive (P-wave velocity measurements, colorimetry, magnetic resonance, rebound hardness, portable device for permeability), microdestructive methods (drilling resistance) as well as destructive tests on drilled cores collected from the blocks (porosity, density, thermal and mechanical properties).

Requirements
Master in Civil Engineering
Experience in the following fields : construction materials, thermo-hydro-mechanical behavior of materials, historical building, experimental studies, numerical modeling.

Contacts
- Javad eslami : Javad.Eslami@cyu.fr
- Anne-Lise Beaucour : Anne-Lise.Beaucour@cyu.fr
- Albert Noumowé : Albert.Noumowe @cyu.fr

Mot(s)-clé(s)

  • Comportement physique et mécanique
  • Contrôle non destructif
  • Essais in situ
  • Expérimental
  • Modélisation
  • Ouvrages, patrimoine
  • Thermique du Bâtiment