Daniel Maxime

Estia

Designing and Evaluating Ambient Tangible Interfaces for Shifting Energy Supply in the Workplace

Keywords: ACM Classification H.5.2 User Interfaces: Interaction styles, Theory and methods, H.1.2 User/Machine Systems: Human factors Sustainable HCI, Persuasive Technology, Calm Technology, Peripheral interaction, Tangible interaction, Energy, Workplace

Tiree Energy Pulse [18] (c) eForecast [13] (d) Conversation Washing Machine [3] Figure 1: Four graphical interfaces for shifting energy supply in households.

INTRODUCTION

In 2013, the world capacity to generate renewable energy increased by 8.5%. As a result in 2014, the renewable energy share of global electricity production reached 23.7% [START_REF]Renewables 2016 Global Status Report[END_REF]. As energy generation shifts to renewables and microgeneration, the interplay between supply and demand will be increasingly difficult. Critical problems such as peak demand can lead to power outages, and make current grids inefficient [START_REF]Demand side response in the domestic sector-a literature review of major trials[END_REF]. These issues will be exacerbated due to the supply of renewables fluctuating with the weather (e.g., sun, wind, wave, tide) and limited storage capacity [START_REF] Hafemeister | Sustainable energyâ ȂŤ without the hot air[END_REF]. Unless storage of energy becomes viable to avoid blackouts becoming normality [START_REF] Trentmann | Disruption is normal: blackouts, breakdowns and the elasticity of everyday life. Time, consumption and everyday life: Practice[END_REF], we will need to synchronize our consumption with the availability of this resource.

Recently, interactive systems have been designed to encourage users in using renewable energy when there is plenty of it rather than when there is little. This strategy called Shifting Energy Supply is being actively studied by HCI (e.g., [START_REF] Costanza | Doing the Laundry with Agents: A Field Trial of a Future Smart Energy System in the Home[END_REF][START_REF] Kjeldskov | Facilitating Flexible Electricity Use in the Home with Eco-Feedback and Eco-Forecasting[END_REF][START_REF] Schrammel | FORE-Watch -The Clock That Tells You When to Use: Persuading Users to Align Their Energy Consumption with Green Power Availability[END_REF][START_REF] Simm | Tiree Energy Pulse: Exploring Renewable Energy Forecasts on the Edge of the Grid[END_REF]). For now, most of the designed systems help households to plan their everyday energy usages with forecast information on the availability of renewable energy. These systems use a variety of ambient interfaces to subtlety inform inhabitants without being intrusive. Yet, it is unclear which ambient interfaces are effective in enhancing ambient awareness and maintaining new practices for shifting energy supply. In our work, we wish to study the influence of parameters such as perceptible artefacts (shape-changing (e.g., Figure 2-a), color-changing (e.g., Figure 2-b)) on the effectiveness of ambient interfaces. To answer this concern, we will design and evaluate in the wild an ambient tangible interface for encouraging employees to shift energy supply with laptops in the workplace.

In this paper, we describe our motivation behind designing and evaluating ambient tangible interfaces for shifting energy supply. We then explain our interest for studying shifting energy supply with laptops in the workplace. Because such practices have yet to be identified, we introduce a tool for simulating and exploring practices for shifting energy supply with laptops. Furthermore, we evoke the design of a dynamic bar chart for visualizing renewable energy forecasts. Using different ambient parameters, we will study how this design enhances ambient awareness and helps employees to shift energy supply with laptops in the workplace. Finally, we present our future works.

RELATED WORKS

Our work intersects two main fields: Peripheral Interaction and Ambient Persuasive Technology.

Peripheral Interaction

Ambient Awareness makes human beings aware of (changes in) surrounding information [START_REF] Wisneski | Ambient Displays: Turning Architectural Space into an Interface Between People and Digital Information[END_REF]. In 1996, Weiser and Brown [START_REF] Weiser | Designing calm technology[END_REF] defined Calm Technologies as technologies able to move from the peripheral attention to the central attention of users, and backwards. They affirmed that calm technologies enhance ambient awareness by bringing more details into the periphery: it makes users aware of what is happening around them, what is going to happen, and what has just happened [START_REF] Weiser | Designing calm technology[END_REF]. In line with calm technologies, Ambient Interfaces use perceptible artifacts (e.g., shape, motion, sound, color, light, smell, air) to represent unobtrusively (changes in) digital information. In 2015, Bakker et al. [START_REF] Bakker | Peripheral interaction: characteristics and considerations[END_REF] noted that recent studies have been conducted under the term peripheral interaction, aiming to broaden the scope of calm technology by designing not only for the perceptual periphery but also enabling users to physically interact with the digital world in their periphery.

Ambient Persuasive Technology

In 1998, Fogg [START_REF] Fogg | Persuasive Computers: Perspectives and Research Directions[END_REF][START_REF] Fogg | Persuasive Technology: Using Computers to Change What We Think and Do[END_REF] defined Persuasive Technologies as interactive systems intentionally designed to change attitudes and/or behavior, without using deceit or coercion. In 2010, Ham [START_REF] Ham | Ambient Persuasive Technology Needs Little Cognitive Effort: The Differential Effects of Cognitive Load on Lighting Feedback Versus Factual Feedback[END_REF] described Ambient Persuasive Technologies as interactive systems intentionally designed to change attitudes or behavior or both, that can be integrated unobtrusively into the environment and exert an influence on people without the need for their central attention.

MOTIVATION

The following subsections described our motivation behind designing and evaluating ambient tangible interfaces.

Designing Ambient Tangible Interfaces for Shifting Energy Supply

Most of the interactive systems designed for shifting energy supply use graphical interfaces (pixel-based interfaces, Figure 1). To the best of our knowledge, only one work [START_REF] Quintal | Watt-I-See: A Tangible Visualization of Energy[END_REF] studied ambient tangible interfaces (object-based interfaces, Figure 2) for shifting energy supply. Yet as evoked by Zuckerman [START_REF] Zuckerman | Objects for Change: A Case Study of a Tangible User Interface for Behavior Change[END_REF], tangible interfaces have strengths that can contribute to "change what people think and do" [START_REF] Fogg | Persuasive Computers: Perspectives and Research Directions[END_REF] : Visibility and Persistance; Situatedness; Tangible representation; and Affordances.

Evaluating Ambient Interfaces in the wild

In 2011, Hazlewood et al. [START_REF] Hazlewood | Issues in Evaluating Ambient Displays in the Wild: Two Case Studies[END_REF] discussed the complex task of evaluating ambient interfaces. They described how these technologies have been evaluated in lab settings, where the focus has been primarily on issues of usability. They argue strongly for the necessity of in-situ evaluation. For example, they proposed to evaluate ambient interfaces by logging the number of times the system is used by conducting interviews or by assessing the effects (e.g., by counting how many people take the stairs instead of the elevator as a result of installing peripheral displays that promote using the stairs).

APPLICATION DOMAIN : SHIFTING ENERGY SUPPLY IN WORKPLACES WITH LAPTOPS

The household is the most common target of the systems designed for shifting energy supply (Figure 1). Yet, collective and public spaces such as workplaces seem to be privileged spaces for encouraging users in shifting energy supply: in 2014, services, transports, and industries counted for 13.3%, However, shifting energy supply in workplaces can be frustrating: when little renewable energy is available, employees may have no other choice but to use energy to achieve their everyday tasks. A recent study [START_REF] Kjeldskov | Facilitating Flexible Electricity Use in the Home with Eco-Feedback and Eco-Forecasting[END_REF] mentions that shifting energy supply in households was more realistic with appliances that could be de-coupled from everyday routines such as washing machines, dishwashers, or battery chargers. In 2014, 66% of the companies of the UE-28 had equipped their employees with laptops, smartphones, and others mobile devices [START_REF]Statistiques sur la société de l'information -entreprises[END_REF]. Mobile devices such as laptops can run on battery, allowing employees to shift energy supply without interrupting their everyday tasks that require computation: when there is plenty of renewable energy, employees can plug their laptop to consume and store renewable energy. On the contrary when there is little of renewable energy, employees can unplug their laptop and proceed with their laptop running on battery.

WORK IN PROGRESS: IDENTIFYING PRACTICES FOR SHIFTING ENERGY SUPPLY WITH LAPTOPS

Before encouraging employees for shifting energy supply with their laptop, we need to identify appropriate practices to promote. For the moment, most of the laptops available on the market come with Lithium-ion (Li-ion) batteries which are widely used as power supply for consumer electronics. However, Li-ion batteries do not have linear (dis)charge curves, meaning that specific levels of battery take more time to charge and less time to discharge than others: several short (dis)charges between two specific levels of battery could be more appropriate for shifting energy supply than a deep (dis)charge.

To answer this concern, we designed a tool for simulating laptop practices for shifting energy supply 1 . The tool implements a Li-ion Battery Dynamic Model [START_REF] Tremblay | Experimental validation of a battery dynamic model for EV applications[END_REF]. It takes in input: the energy production mix (e.g., 10:00-11:00 -90% renewable and 10% nonrenewable); the laptop configuration (e.g., nominal power consumption, nominal current of charge, current state of charge), and the laptop practices (e.g., 10:00-11:00 -laptop with battery plugged to the grid, 11:00-12:00laptop with battery unplugged from the grid). The tool gives in output the consequences of laptop practices on the sector energy consumption (e.g., 100 Wh taken from the grid at 40% renewable and 60% nonrenewable) and on the battery energy storage (e.g., 80 Wh stored in the battery at 30% renewable and 70% nonrenewable).

WORK IN PROGRESS: DESIGNING A DYNAMIC PHYS-ICAL BAR CHART FOR RENEWABLE ENERGY FORE-

CASTS

The stability of the electrical grid requires power supply to constantly meet power demand. Since energy generation started to shift to renewables and microgeneration, forecasting the fluctuation of renewable energy sources has become essential for balancing supply against demand. Such forecasts are commonly used by the interactive systems we identified, helping users to plan their every day energy usages according to the avaibility of renewable energy [START_REF] Costanza | Doing the Laundry with Agents: A Field Trial of a Future Smart Energy System in the Home[END_REF][START_REF] Kjeldskov | Facilitating Flexible Electricity Use in the Home with Eco-Feedback and Eco-Forecasting[END_REF][START_REF] Schrammel | FORE-Watch -The Clock That Tells You When to Use: Persuading Users to Align Their Energy Consumption with Green Power Availability[END_REF][START_REF] Simm | Tiree Energy Pulse: Exploring Renewable Energy Forecasts on the Edge of the Grid[END_REF]. These systems use a variety of dynamic graphical visualizations of renewable energy forecasts: FORE-Watch (Figure 1 Bar charts are data visualizations commonly used for showing the differences, or making comparisons, between different variables. Inspired by several works on dynamic data visualization such as EMERGE [START_REF] Taher | Exploring Interactions with Physically Dynamic Bar Charts[END_REF] (Figure 3a) and inFORM [START_REF] Follmer | inFORM: Dynamic Physical Affordances and Constraints Through Shape and Object Actuation[END_REF] (Figure 3b), we will design a dynamic physical bar chart for visualizing renewable energy forecasts of each hour of the workday. We think such design can help employees to quickly identify hours of the workday when there will be plenty of renewable energy.

CONCLUSION & FUTURE WORK

In order to reduce environmental impact, interactive systems have been designed to encourage users in using renewable energy when there is plenty of it rather than when there is little (i.e. shifting energy supply). These systems use a variety of ambient interfaces to unobtrusively inform users on the availability of renewable energy. Yet, it is unclear which ambient interfaces are effective in enhancing ambient awareness and maintaining new practices for shifting energy supply. We think that parameters such as perceptible artefacts (e.g., shape-changing, color-changing) influence the effectiveness of ambient interfaces. To answer this concern, we will design a dynamic physical bar chart for visualizing renewable energy forecasts, helping employees to shift energy supply with laptops in the workplace. However, laptop practices for shifting energy supply have yet to be identified. Therefore, we developed a tool for simulating such practices. Using this tool, We will identify laptop practices to promote with the dynamic data visualization we proposed.

Through a longitudinal approach in a workplace, we will evaluate the effects of different parameters such as perceptible artefacts (shape-changing (e.g., Figure 2-a), color-changing (e.g., Figure 2-b)) on the effectiveness of our design in enhancing ambient awareness and maintaining practices for shifting energy supply with laptops.

  (a) Flower Lamp[START_REF] Backlund | Static! The aesthetics of energy in everyday things[END_REF], using a shape-changing artefact.(b) Energy Local Lamp[START_REF] Pierce | Materializing Energy[END_REF], using a color-changing artefact.

Figure 2 :

 2 Figure 2: Two ambient tangible interfaces using different perceptible artefacts.

Figure 3 :

 3 Figure 3: Two dynamic physical bar charts.

  -a) uses a graphical clock for each minute of the next hour and a graphical timeline chart for the next 24 hours; Tiree Energy Pulse (Figure 1-b) uses a graphical line chart for the next 12 hours; and eFORECAST (Figure 1-c) uses a graphical clock for each minute of the next hour and a graphical line chart for the next 12 hours.

http://itame.estia.fr/simulator/


T Pertin

F Buosi

G Minatchy

A L Flory

Laurence Romana

P Jean-Patrick

USED OF NANOINDENTATION TECHNIQUES TO MEASURE POLYPROPYLENE AGEING

published or not. The documents may come

INTRODUCTION

Polypropylene is a commonly used thermoplastics in various applications ranging from packaging, automotive industry, textiles…Polypropylene production has increased a lot during the last decades thanks to many useful attributes such as low density, high melting temperature (170 °c) and good mechanical properties (high Young's modulus). Despite these inherent advantages, there are concerns regarding the long-term durability of these materials, especially their capacity for sustained performance under harsh environmental conditions. Furthermore, the degradation of polypropylene has environmental impact since toxic residual chemicals and degradation products can potentially be released into the environment [1]. It is therefore important to understand the ageing mechanisms of such polymer in order to design product with good environmental resistance. In this work, nanomechanical techniques have been used to characterize ageing behavior of PP. Advantages of this technique are to measure the mechanical properties with a high spatial resolution allowing to characterize the mechanical degradation at the early stage of ageing. Polypropylene samples have being ageing in an ARTAAC chamber (SEVAR Company) under dry environment. The samples were submitted to UV-irradiation issued from a pressure mercury vapor lamp (irradiance level equivalent to 2 suns), the chamber temperature was kept at 60°C and ageing duration varied from 0 to 20 days (480 hours). The kinetic of chemical degradation was studied by infra-red spectroscopy. Evolution of both carbonyl groups ( absorbance at 1735 cm -1 ) and hydroxyl groups (3400 cm -1 ) as a function of ageing time has been studied in order to characterize oxidation rate of the polypropylene [2]. The evolution of the Young's modulus and the brittleness of PP with ageing time was characterized by means of nano indentation and nano scratch techniques. During the scratch test the normal force increases up to 200 mN with a loading rate close to 7 mN.s -1 . A topographic profile of the surface is performed during and post scratch in order to follow the elastic polymer recovery. The evolution of carbonyl index (CI) and hydroxyl index (HI) as a function of ageing time is shown in figure 1. We can clearly see an induction period (up to 100 hours) corresponding to the consumption of anti-oxidant additives, then a rapid increase of the CI due to the carbonyl built-up (C=0) during oxidation. After 350 hours, the CI stabilized at around 10 -3 which corresponds to a full oxidation of the sample. The evolution of the Young's modulus as a function of the indentation depth is shown in figure 2a for 0 day and 18 days aged samples. Young's modulus value increases significantly with ageing indicating a higher rigidity of the ageing polypropylene probably due to cross-linking induced by UV irradiation. Figure 2b presents the profiles recorded during scratch tests (solid lines) or after the scratch (dashed-line). The higher scratch depth as well as the lower elastic recovery observed on aged PP clearly indicate that during ageing, the elasticity of PP decreases and that brittleness increases. The evolutions of these different parameters as a function of ageing time will be presented here and nanomechanical analyses shown as a powerful tool to predict the durability of polymer.

ACKNOWLEDGMENTS

The authors would like to thanks the Fond Social Européen and La Région Guadeloupe for their financial support.


Cristel Onésippe
email: cristel.onesippe@univ-ag.fr

Nady Passe-Coutrin

Fernando Toro

Silvio Delvasto

Ketty Bilba

Marie-Ange Arsène

SUGAR CANE BAGASSE FIBRES REINFORCED CEMENT COMPOSITES: THERMAL CONSIDERATIONS

Keywords: A. fibres, A. composites, B. Thermal properties, D. Thermal analysis 1. Introduction

This paper examines the thermal properties of cement composites reinforced with vegetable bagasse fibres (1.5 and 3% wrtc).

Thermal properties have been correlated to macroscopic density and porosity in order to estimate thermal conductivity of fibres using a calculation inspired from Maxwell Eucken modelling.

Experimental investigations reveal that adding retified bagasse fibres reduces composites thermal conductivity and yields a weaker specific heat in comparison with composites made with alkaline bagasse fibres. Moreover, the more the fibres, the lighter the specimen; lower its thermal conductivity and lower its specific heat. Thermal conductivity of alkaline fibres is lower than retified fibres one.

In order to reduce electricity consumption in houses, due to air-conditioning, cement/vegetable fibres composite materials have known an increasing interest during the past few decades [START_REF] Tolêdo-Filho | Development of vegetable fibremortar composites of improved durability[END_REF][START_REF] Savastano | Brazilian waste fibres as reinforcement for cement-based composites[END_REF][START_REF] Arsène | Chemically and thermally treated vegetable fibers for reinforcement of cement-based composites[END_REF][START_REF] Bilba | Sugar cane bagasse reinforced cement composites. Part I. Influence of the botanical components of bagasse on the setting of bagasse/cement composite[END_REF][START_REF] Bilba | First step of valorization of dictame fibers by elaboration of cementitious composites[END_REF][START_REF] Coutts | From forest to factory to fabrication[END_REF][START_REF] Gutierrez | Effect of pozzolans on the performance of fiberreinforced mortars[END_REF][START_REF] Savastano | Performance of low-cost vegetable fibre-cement composites under weathering[END_REF][START_REF] Savastano | Mechanically pulp sisal as reinforcement in cementitious matrices[END_REF]. These materials display many advantages such as they are environmentally friendly, they are cheap [START_REF] Coutts | From forest to factory to fabrication[END_REF] and they allow reduction of electrical consumption by air conditioning. In developing countries, there is a large availability of tropical plants and agricultural wastes and the use of cheap vegetable fibres in replacement of expensive synthetic fibres is a good alternative. Since 1973, due to health reasons, various regulations are applied to restrict and ban the use of asbestos in France and other developed countries [START_REF] Bilba | First step of valorization of dictame fibers by elaboration of cementitious composites[END_REF].

In our research group, bagasse fibre reinforced cement composites have been studied for more than 10 years [START_REF] Bilba | Sugar cane bagasse reinforced cement composites. Part I. Influence of the botanical components of bagasse on the setting of bagasse/cement composite[END_REF][START_REF] Bilba | First step of valorization of dictame fibers by elaboration of cementitious composites[END_REF], the fibres having a random distribution in the composites. They have been prepared for being used in construction area in tropical regions such as Guadeloupe (French West Indies). Bagasse is the solid lignocellulosic residue left after extraction of juice from the sugar cane stalk and is cheap compared to synthetic fibres.

A low thermal conductivity [START_REF] Xu | Effect of sand addition on the specific heat and thermal conductivity of cement[END_REF] and a low specific heat (in our country) are required to have a good thermal insulation of buildings. Many studies have shown that, regarding the mechanical behaviour, vegetable fibres are an interesting alternative to asbestos in fibre reinforced cement products manufactured by the Hatcheck process [START_REF] Savastano | Brazilian waste fibres as reinforcement for cement-based composites[END_REF][START_REF] Savastano | Performance of low-cost vegetable fibre-cement composites under weathering[END_REF][START_REF] Savastano | Mechanically pulp sisal as reinforcement in cementitious matrices[END_REF]. But few works have demonstrated the low thermal conductivity of such vegetable fibre/Portland cement based composite materials resulting from effect of the mixture fibre/matrix [START_REF] Tolêdo-Filho | Development of vegetable fibremortar composites of improved durability[END_REF][START_REF] Savastano | Performance of low-cost vegetable fibre-cement composites under weathering[END_REF] but, to our knowledge, there is no published result concerning specific heat of bagasse reinforced cement composites.

These composites are well known for durability problem that hinders their development [START_REF] Tolêdo-Filho | Development of vegetable fibremortar composites of improved durability[END_REF]; indeed, the lignin in natural fibres is attacked by alkaline cement, hence degradation in composites strength [START_REF] John | Durability of slag mortar reinforced with coconut fibre[END_REF]. Use of pozzolans for reduction of matrix alkalinity [START_REF] Gutierrez | Effect of pozzolans on the performance of fiberreinforced mortars[END_REF], immersion of fibres in slurred silica fume [START_REF] Tolêdo-Filho | Development of vegetable fibremortar composites of improved durability[END_REF] or carbonation of the matrix represent ways to solve this problem and have been investigated. In this paper, in order to improve the strength of bagasse fibres and also try to limit the reaction between fibres and matrix, thermal treatment [START_REF] Arsène | Chemically and thermally treated vegetable fibers for reinforcement of cement-based composites[END_REF] and chemical treatment have been applied to the fibres before incorporating them into the cement matrix.

2. Materials and methods.

Materials

Bagasse fibres used in this study have been obtained from Montebello sugar cane factory (Petit-Bourg, Guadeloupe). This waste has been cut and fibres with diameters ranging from 0.4 to 1 mm have been selected and thermally or chemically treated, they are named "BAG".

Two fibres treatments have been performed at COVACHIMM laboratory (Université des

Antilles et de la Guyane, Guadeloupe):

Heat-treatment: pyrolysis under controlled atmosphere (N 2 flow, 2L/h) during 2 hours at 200°C which is the best compromise between mass loss and degradation of sugars [START_REF] Bilba | Fourier transform infrared spectroscopic study of thermal degradation of sugar cane bagasse[END_REF]. The pyrolyzed fibres are named "BAGP". Matrix is made of cement, sand, water, CaCO 3 , bentonite, silica fume, acrylic styrene polymer and cellulose pulp.

Composites preparation

Composites have been elaborated in GMC laboratory (Universidad del Valle, Colombia).

The process used is inspired from Hatcheck process and is well established in Universidad del Valle [START_REF] Gutierrez | Effect of pozzolans on the performance of fiberreinforced mortars[END_REF]. The composites will be noted "CBAGP" and "CBAGB" for composites reinforced with retified bagasse fibres and composites reinforced with alkaline bagasse fibres, respectively.

The content of sand and calcium carbonate were kept constant at 50% and 30% by weight of cement respectively. The proportions of the matrix compounds were in weight percent with respect to Portland cement (wrtc): sand 50 %, limestone powder 30 %, bentonite 1.5 and 3 %, cellulose pulp 4 %, silica fume 5 %, an aqueous copolymer dispersion of butyl acrylate and styrene from BASF (Acronal ® 296 D) 7.5 %. The content of sand and calcium carbonate was kept constant at 50% and 30% by weight of cement respectively. The amount of fibres was varied from 0 to 3 % wrtc. Slurry of each mix contains a water/cement ratio of 0.9 (weight by weight) being prepared before the pouring on a casting bed that was subjected to vacuum to obtain a flat sheet. After the slurry dewatering process, pads (160 x 50 x 8 mm 3 ) cut from the fresh laminate were cured for 21 days at 100% of relative humidity and left to air dry in the laboratory for 7 days at room temperature.

Five mixes were elaborated: 2 samples for the 2 types of fibres for each fibre contents and 1 as control (reference without any fibre). Proportions of mixes are shown in Table 1.

Thermal conductivity measurements

Thermal tests were carried out under controlled laboratory conditions (temperature = 298±1 K and 70-80% relative humidity) on 365 days old specimen in COVACHIMM group (Université des Antilles et de la Guyane). The apparatus used was a thermal conductimeter "CT-mètre" with a thermal probe commercialized by Controlab (Saint-Ouen, France). At least six measurements per composite were conducted with one hour interval between each measurement in order to evaluate the standard deviation of the results.

Isothermal calorimetry measurements

Isothermal calorimetry has been carried out on a C80 calorimeter (Setaram, France) at Université des Antilles et de la Guyane. Calorimetry is a very strong investigation method and is usually used to determine specific heat of cement composites under air atmosphere [START_REF] Govin | Modification of cement hydration at early ages by natural and heated wood[END_REF].

All measurements were carried out at least two times, as usually proposed in previous work.

The apparatus consists of 4 parts: the calorimeter where the signal comes from, the power module which provides steady power supply, the controller which serves as an interface transforming the voltage signal into digital signal, the computer which records and processes the signal [START_REF] Kabadi | High temperature high pressure thermodynamic measurements for coal model compounds[END_REF].

The C80 calorimeter is the core part of the whole system. The Setaram C80 differential heatflux apparatus is based on the Calvet principle. Two identical wells for inserting the removable confinement cylinders with cells are located in a heated calorimetric block fixed inside a cylinder surrounded by insulating material. The heat evolved or absorbed during the process is transferred between the cell and the calorimetric block through the thermopiles surrounding the wells.

Thermopiles consist of thermocouples connecting the outer surface of the well wall with the block. Due to fast transport of the heat through the thermocouples the difference of temperature between the cells and the block is negligible.

The two thermopiles are connected differentially and the resulting signal is proportional to the difference in the heat exchange between the cells and the block. In C80 system the signal will enter the controller at first, the special program in the computer will process the signal, the computer displays results in the form of a temperature line and a heat flux line which represents the heat flux difference between the two wells.

The C80 calorimeter is a very precise instrument for the enthalpy measurement. It has a resolution of 0.001°C for temperature and a resolution of 0.001 mW for heat flux. Specific heat (J/g.K) is defined as the amount of heat required to raise the temperature of the unit mass (1 g) of a substance by a unit temperature increase (1 K). The specific heat capacity Cp at constant pressure and the specific heat capacity Cv at constant volume for solids at room temperature are equal to within < 1%. This parameter is related to thermal conductivity λ, thermal diffusivity κ and macroscopic density ρ by the equation:

€ = .
Eq. 1

All measurements were carried out at least two times, as usually proposed in previous work.

Macroscopic density measurements

Composites porosity has been measured using helium gas intrusion under helium gas flow with a "Pycnomatic" Thermo Electron Corporation equipment (France) pycnometer. Five measurements were conducted for each composite at 298 K, relative humidity of 70-80%.

Mercury intrusion porosimetry measurements

Porosity of the composites has been studied by mercury intrusion porosimetry from 0.1 to 200

MPa with mercury porosimeters Thermofinnigan (France) Pascal 140 and Pascal 240. This range of pressure corresponds to pore radius from 58 µm to 37 Å.

At least, two measurements were carried out.

3. Results and discussion.

Fibres and matrix characterization

Bagasse is a natural lignocellulosic fibre, that is to say that each unit of bagasse fibre is composed of crystalline cellulose surrounded and cemented together with hemicellulose and lignin [START_REF] Cao | Mechanical properties of biodegradable composites reinforced with bagasse fibre before and after alkali treatments[END_REF]. The sugar cane bagasse chemical composition has already been characterized [START_REF] Bilba | Sugar cane bagasse reinforced cement composites. Part I. Influence of the botanical components of bagasse on the setting of bagasse/cement composite[END_REF],

it is summarized in Table 2.

Pyrolysis does not affect general composition of fibres (weak variation of hemicellulose, cellulose and lignin amounts) except for extractives amounts. The alkaline pre-treatment decreases mainly lignin, extractive and hemicellulose amounts.

Surface of raw and treated fibres has been observed by SEM.

Mild pyrolysis is known to mainly affect hemicellulose (in less proportions, lignin) and creates a new "pseudo-lignin" more hydrophobic and rigid than the initial one [START_REF] Boustingorry | The influence of wood aqueous extractives on the hydration kinetics of plaster[END_REF]. Figure 1 shows the dissolution of hemicellulose and delignification process generated by pyrolysis and alkaline attack [START_REF] Arsène | Chemically and thermally treated vegetable fibers for reinforcement of cement-based composites[END_REF].

The surface of pyrolyzed fibres is less damaged than BAGB one, as confirmed by chemical composition showing large weight loss of hemicellulose (around 37 wt% for BAGB) and lignin (around 11 wt%), according to raw bagasse. Ca(OH) 2 reacts with hydroxyl groups of hemicellulose (cementing material) and it contributes to the destruction of the structure and thereby the fibres split into filaments. This fibrillation increases surface area and then increases the contact and adhesion with matrix.

Physico-chemical properties of cement, presented in Tables 3 and4, have also been determined according to technical Colombian norms (NTC).

93.5% by weight of the limestone filler is CaCO 3 . The particle size distribution of the powdered Portland cement and the limestone filler are shown in Figure 2.

50% by weight of limestone powder used has a particle size below 2 micrometers and the Portland cement average particle size is 15 micrometers.

The characteristics of sand are given in Table 5. As we can see, the Portland cement has factory additions that are calcareous materials and the sand seems appropriated for use in cement-based materials. The size distribution was determined using a Malvern laser diffraction analyser.

Thermal conductivity (λ)

Figure 3 reports evolution of 365 days old composites thermal conductivity according to fibre content and fibre treatment.

In our experiment, thermal conductivity of matrix (without fibre) is about 0.6188 W/m.K.

According to Xu and al. [START_REF] Xu | Effect of sand addition on the specific heat and thermal conductivity of cement[END_REF], thermal conductivity of cement paste (Portland type I + water) is 0.53 W/m.K and thermal conductivity of mortar (cement + sand + water) is 0.58 W/m.K.

Here, we can see the effect of all the additives (cellulose pulp, bentonite, silica fume and polymer) in our matrix. They contribute to the 6% of increase of thermal conductivity compared to mortar prepared by Xu et al. [START_REF] Xu | Cement of high specific heat and high thermal conductivity, obtained by using silane and silica fume as admixtures[END_REF]. Comparison of the results on matrix without fibre and those on matrix with fibres shows that generally, fibres addition decreases the thermal conductivity; a quadratic polynomial decrease is correlated for BAGP and BAGB composites. The best effect is observed when adding 3% wrtc of fibres. This result is already known in the literature [START_REF] Asasutjarit | Development of coconut coir-based lightweight cement board[END_REF][START_REF] Chung | Cement-matrix composites for thermal enginnering[END_REF]: by increasing fibre content, there is a progressive decrease of thermal conductivity. The thermal conductivity of composites decreases from 0.62 W/m.K for CBAGB1.5 to 0.46 W/m.K for CBAGP3. This indicates a promising potential for development, indeed Asasutjarit and al. reported that thermal conductivity of cellulose commercial board is 0.68 W/m K [START_REF] Asasutjarit | Development of coconut coir-based lightweight cement board[END_REF].

The effect of fibres treatment on thermal conductivity is stronger for retified bagasse fibres than for alkaline bagasse fibres maybe because of the difference observed between the chemical composition of BAGP and BAGB showing for basic bagasse fibres higher amount of cellulose and lower extractives one for pyrolyzed bagasse fibres.

The most insulating composite is observed with 3% wrtc of bagasse fibres because the resulting thermal conductivity is 25% lower than matrix one. For 3% wrtc addition of bagasse fibres, fibre treatment does not seem to have any influence on thermal conductivity: the observed thermal conductivities of CBAGP3 and CBAGB3 are quite equal according to standard deviation.

Specific heat (Cp)

Figure 4 presents evolution of specific heat according to fibre content and fibre treatment.

Specific heat of matrix without any additive is 0.2673 J/g.K. According to others [START_REF] Xu | Effect of sand addition on the specific heat and thermal conductivity of cement[END_REF], specific heat of cement paste is 0.736 J/g.K and that of mortar is 0.642 J/g.K. We can see that the effect of additives (particularly silica fume [START_REF] Xu | Effect of sand addition on the specific heat and thermal conductivity of cement[END_REF]) in matrix is very important because our specific heat is by 41% lower than mortar one.

When fibre content increases, there is a progressive quadratic polynomial decrease of specific heat for both types of fibres. The lowest value is obtained for CBAGP3. Usually, literature reports that with increasing synthetic fibre content, there is an increase of specific heat [START_REF] Chung | Cement reinforced with short carbon fibers: a multifunctional material[END_REF].

There is no published result concerning vegetable fibres reinforced cement composites in order to compare our results. We thought that with increasing fibre content, we would observe an increase in specific heat according to a classical rule of mixture extrapolating from wood and concrete data. This unexpected result can be explained by presence of porosity due to alveolar of fibres and also by lack of fibre/matrix adhesion. Figure 5 presents SEM pictures of CBAGB and CBAGP with fibre content of 3% wrtc. We can see that there are some fragments of CaCO 3 packed onto fibre surface. This packing is thought to be responsible of a smaller contact zone between fibre and matrix and, as a consequence, loss of specific heat.

True density and porosity

Table 6 reports main results of macroscopic density and porosity measurements of bagasse composites.

Even if the morphologies of pyrolyzed and chemically treated bagasse fibres are different, true density of CBAGP and CBAGB are similar according to standard deviation.

The thermal conductivity is inversely proportional to the voids in the composites. The packing of fibres into matrix generates the voids. This is why, as shown by Table 6, the more the fibre content (3% wrtc), the lower the density. These results confirm the data in the literature: heatinsulating properties of composites is opposite to its density [START_REF] Khedari | New lightweight composite construction materials with low thermal conductivity[END_REF] that is to say that heat-insulating properties are enhanced when density decreases. We notice that the density values of composites reinforced with 1.5% wrtc of fibers are slightly higher than that presented by the reference and that the composites reinforced with 3% wrtc of fibers are only 3-10 % higher than reference density value. This behaviour may be related to the nature of density measured (true density). Indeed, we suppose that the matrix density is lower than the fibers one. It may exist non-connected pores or pores that are not reachable by helium gas.

The porosity of the matrix (control specimen) is decreased with the addition of fibres. In the range of our study, whatever the fibre content, the porosity values are of the same order.

As shown in Figure 6, thermal conductivity and specific heat are related to true density by a quadratic polynomial equation. An optimal true density, that is to say an optimal amount of fibres for heat-insulation, corresponding to a minimal thermal conductivity and a minimal specific heat, exists. For CBAGP, this optimal density is around 2.56 g.cm -3 and for CBAGB 2.58 g.cm -3 .

The trend of our results is coherent with those of commercial insulating materials that associate low thermal conductivity and low specific heat. For example, commercial cellulose pads show Cp = 1.9 kJ/kg.K and λ = 0.04 W/m.K or polystyrene exhibits Cp = 1.2 kJ/kg.K and λ = 0.032 to 0.038 W/m.K.

4. Estimation of thermal properties of bagasse fibres.

Thermal conductivity

According to Wang and al. [START_REF] Wang | A new approach to modelling the effective thermal conductivity of heterogeneous materials[END_REF], thermal conductivity of cement/fibre composite can be determined using the Maxwell-Eucken equation:

€ K = k m v m + k f v f 3k m 2k m + k f v m + v f 3k m 2k m + k f Eq.2
where K is the effective thermal conductivity of the composite, We assume that

€ v m + v f = 1.
This expression can be formulated as:

€ K -k m k m = 3 k f -k m ( ) v f k m -k f ( ) v f + 2k m + k f Eq.3
If we suppose € v f very small, we can write:

€ K -k m k m ≅ 3 k f -k m ( ) 2k m + k f v f 1- k m -k f ( ) 2k m + k f v f ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ Eq.4
First, we will neglect the right term, in order to find an expression of

€ K -k m k m according to € v f .
If %f is the weight percentage,

€ v f = 1- ρ ρ m 1- % f 100 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Eq. 5
where € ρ is the composite bulk density and € ρ m , matrix bulk density. These values are obtained via mercury porosity measurements.

Modelling results.

If

€ A = 3 k f -k m ( ) 2k m + k f , it follows € k f = k m 3+ 2A 3-A (A is the coefficient director of the curves).
The main results, with a good correlation (R = 0.99) are in Table 7.

We notice that our

€ v f values are quite high, € v f is considerable regards to 1. Indeed, true
density of light wood, as balsa, is approximately 0.1 g.cm -3 (density of balsa ranges from 0.04 to 0.38 g.cm -3 [START_REF] Vural | Microstructural aspects and modeling of failure in naturally occuring porous composites[END_REF]). Considering this theoretical value, we would find corrected € v f values around 0.06 and 0.12 (6% and 12%, respectively). This suggests two hypotheses, either density of vegetable fibre is lower than the density 0.1 g.cm -3 of light wood or either the model is not fully adapted to study this composite material.

As there is no information about thermal conductivity of vegetable fibre, the Maxwell-Eucken equation allows determination of the thermal conductivity of pyrolyzed and alkali-treated bagasse fibres with a good correlation.

k f , thermal conductivity of fibres, is calculated without any approximation and by using our experimental values.

Figure 7 presents the trend of the calculated curve for BAGP composites. The linear trend of the curve shows that the model is convenient for BAGP composites.

We can check that the term

€ B = k m -k f 2k m + k f ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ v f is negligible regards to 1.
For retified bagasse fibres € B max = 0.085 (that is to say a precision of 8.5% on A v f ) and for alkaline bagasse fibres, € B max = 0.057 (precision of 5.7% on A v f ).

Perspectives of application of these materials

The thermal diffusivity of the composites is an important parameter in order to estimate their potential use in housing and building [START_REF] Santos | Effect of moisture and porosity on the thermal properties of a conventional refractory concrete[END_REF].

Thermal diffusivity κ of a material can be obtained by the use of the following calculation:

€ κ = λ ρ C p Eq.6
where λ is thermal conductivity (W/m.K)

ρ is density (kg/cm 3 )
Cp is specific heat (J/ kg.K) Calculated thermal diffusivity for BAGP and BAGB composites are shown in Table 8.

Thermal diffusivity measures the rapidity of heat propagation through a material.

Addition of fibres increases thermal diffusivity of composites. The effect is higher for retified bagasse fibres. When increasing fibre content, there is a decrease of this parameter.

According to these results, we can say that the optimal fibre content is around 1.5 wrtc % because this composition allows the highest thermal diffusivities.

Assuming that our composite is a semi-infinite material [START_REF] Taine | Transferts thermiques[END_REF], initially isotherm at T 0 and we choose to impose a temperature T e at the moment t = 0 on the surface at x = 0.

According to

€ u = x 2 κt
Eq.7 and T + = erf(u) if

€ T + = T -T e T 0 -T e Eq.8
Where x is the material thickness measured from the surface; the thickness is supposed to be infinite, t is the time, T is the temperature corresponding to the material thickness, erf is Gaussian error function.

This approximate model supposes that the real thickness L verifies L 2 >> κ t.

When adding fibres, there is an increase of thermal diffusivity that is to say that for the same material thickness, the same temperature T is reached while diminishing time t. It would mean that when adding fibres, there is no storage of heat or that heat is dissipated more quickly.

Decrease of € λ while € κ increases means that, according to Eq. 6, € ρC p is clearly decreased when adding fibres into matrix.

We suppose that this behaviour could be explained by the raising of porosity due to the incorporation of fibres.

If we consider a sample composite with a total volume V as

€ V = V 1 + V 2 + V 3
Eq. 9 with € V 1 : volume of completely compacted matrix We can write that composite specific heat € VρCp is the sum of specific heat of each component : Eq. 11 We pose:

€ VρCp = V 1 ρ 1 Cp 1 + V 2 ρ 2 Cp 2 + V 3 ρ 3 Cp 3 Eq.10 so € ρCp = V 1 V ρ 1 Cp 1 + V 2 V ρ 2 Cp 2 + V 3 V ρ 3 Cp 3
€ V 2 V = v f volume fraction of fibres € V 3 V = ε composite total porosity
It comes:

€ ρCp = 1-v f -ε ( ) ρ 1 Cp 1 + v f ρ 2 Cp 2 + ερ 3 Cp 3 Eq. 12
And finally

€ ρCp = ρ 1 Cp 1 -v f ρ 1 Cp 1 -ρ 2 Cp 2 ( ) -ε ρ 1 Cp 1 -ρ 3 Cp 3 ( ) Eq. 13
The term

€ ε ρ 1 Cp 1 -ρ 3 Cp 3 (
) comprises the increase of porosity with adding fibres and consequently explains the effect on diffusivity € κ .

It is difficult to estimate this term because it may exist different porosities :

-Fibres internal porosity, -Matrix porosity (excluding fibres presence) -Matrix preexisting porosity (before adding fibres).

Conclusion.

Thermal conductivity of vegetable fibres/cement composites is an important parameter in the use of such materials in construction for saving energy. In this study, particular attention is given to the influence of weight fraction of bagasse fibres and fibres treatment. Composites elaborated with retified bagasse fibres are weaker heat conductor materials than those elaborated with alkaline bagasse fibres. Therefore, CBAGP could be preferred to CBAGB and used to prevent heat transfer into building and consequently to save energy. Regarding the specific heat, the best results are obtained with CBAGB but both types of composites show weaker specific heat than matrix alone. As a result, it is possible to construct a quite good insulating wall for housing using retified or alkaline bagasse fibres (approximately 1.5 % wrtc).

The thermal behaviour of these cement composites reinforced by bagasse fibres show a quite good correlation with Maxwell-Eucken modelling if we assume that the density of bagasse fibres is lower than light wood (balsa) one.

This study has to be supplemented by additional experiments on specimen containing higher fibre content and special attention should be accorded to fibre/matrix interface. This problem could be solved by the addition of silica fume, for example. Indeed, silica fume is well known in the literature to decrease the thermal conductivity.

  Chemical treatment: attack by a 5% by mass alkaline solution of Ca(OH) 2 . The alkali treated fibres are named "BAGB". The cement used was a Portland cement marketed as ASTM 1.

€

  k m and € v m are respectively the thermal conductivity and volume fraction of the continuous phase i.e. the matrix, € k f and € v f are respectively the thermal conductivity and volume fraction of the dispersed phase i.e. the fibres.

3 :

 3 density of air included into pores € Cp 3 : specific heat of air included into pores We assume that € ρ and € Cp are respectively composite density and specific heat.

Figures caption.

  Figures caption.

Figure 1 :

 1 Figure 1: SEM pictures of (a) raw bagasse fibres, (b) retified bagasse fibres and (c) alkali-

Figure 2 :

 2 Figure 2: Particle size distribution of the limestone filler and the Portland cement.

Figure 3 :

 3 Figure 3: Evolution of thermal conductivity according to fibre content and fibre treatment.

Figure 4 :

 4 Figure 4: Evolution of specific heat according to fibre content and fibre treatment.

Figure 5 :

 5 Figure 5: SEM pictures of CBAGP (a) and CBAGB (b) with fibre content of 3%wrtc.

Figure 6 :

 6 Figure 6: Relation between thermal conductivities, specific heat and true densities of

Figure 7 :

 7 Figure 7: Thermal modelling of BAGP composites.

Figures.

  Figures.

Figure 1 :

 1 Figure 1:

  Figure 5.

  Figure 7.

Table 6 :

 6 True density and porosity of cement composites reinforced with bagasse fibres.

		ASTM N° 4 (opening: 4,760 mm)		94.23
		ASTM N° 8 (opening: 2,380 mm)		88.11
		ASTM N° 16 (opening: 1,190 mm)		65.52
		ASTM N° 30 (opening: 0,595 mm)		37.59
		ASTM N° 50 (opening: 0,297 mm)		12.00
		ASTM N° 100 (opening: 0,149 mm)		2.05
	Method Sample		Control	CBAGP		CBAGB
		% wrtc fibre		0	1.5	3	1.5	3
	He	True density (g.cm -3 )	ρ t	2.6648 [±0.0108]	2.8277 [±0.0223]	2.5526 [±0.0091]	2.8316 [±0.0045]	2.5681 [±0.0186]
		€ Bulk density (g.cm -3 )	ρ a	2.49923	1.6155	1.4561	1.5744	1.6024
	Hg	(g.cm -3 ) € Apparent density	3.7246	2.8771	2.6108	3.0707	3.0751
		Porosity		64.98	43.85	44.23	48.73	47.89
		(%)

Table 7 :

 7 Thermal conductivity modelling results.

		Retified bagasse fibres Alkaline bagasse fibres
	% f v f	1.5 0.1909	3 0.2818	1.5 0.2115	3 0.2096
	ρ composite (g/cm 3 ) 1.6155	1.4561	1.5744	1.6024
	k f (W/m.K)	0.1781		0.1092
	€
	€
	€

Table 8 :

 8 Composites thermal diffusivities.

	Composites	Control CBAGP1.5 CBAGP3 CBAGB1.5 CBAGB3
	κ ..10 6 (m 2 s -1 ) 1.1771 1.5856	1.4228	1.5504	1.3114

Acknowledgements.

This work was financially supported by ECOS-Nord Colombia (Project # C05P03 titled "Vegetable fibres-cement composites"), for the Colombian part by the MEN/COLCIENCIAS and for the French part by the region Guadeloupe (FWI).

The authors acknowledge also CAPES COFECUB Project # 5306.

Tables.

Table 1-Proportions of composites mixes.

Composites

Fibre and treatment

Fibres %wrtc

Bentonite %wrtc

Polymer %wrtc

Silica fume %wrtc

Paper pulp %wrtc