Thomas Poinot

Alexandre Govin
email: govin@emse.fr

Keywords: polysaccharide (D), hydroxypropylguar (D), admixture (D), mortar (E), water retention (C)

published or not. The documents may come

Importance of coil-overlapping for the effectiveness of hydroxypropylguars as water retention agent in cement-based mortars

Hydroxypropylguars (HPG) are etherified derivatives of guar gum, a natural polysaccharide extracted from the seed endosperm of native plant from India Cyamopsis tetragonolobus. Due to their thickening effect, good solubility and thermal stability in solution, HPG are used in numerous industries such as textile printing, oil production or paper manufacturing [START_REF] Chen | Determination of the degree of substitution of hydroxypropyl guar gum at C-6 by Pyrolysis-Gas Chromatography spectrometry[END_REF]. HPG are also used in the field of building construction since they are introduced into mortar formulation as viscosity-enhancing and water retention admixtures.

Water retention (WR) is one of the key properties of mortar at fresh state. Indeed, water retention agent allows enhancing cement hydration by limiting the absorption of mixing water by the substrate. Thus, thanks to the WR agent, the mortar exhibits better mechanical and adhesive properties when it is applied in thin layer or on highly absorbent substrate [START_REF] Bertrand | Wedge Splitting Test: A test to measure the polysaccharide influence on adhesion of mortar on its substrate[END_REF][START_REF] Jenni | Influence of polymers on microstructure and adhesive strength of cementitious tile adhesive mortars[END_REF].

Cellulose ethers (CE) are the most widely used WR agents. Therefore, up to now, research into the understanding of water retention in cement-based material is mainly focused on the effect of cellulose ethers. It appears that polymer molecular parameters, such as nature and content of substitution groups, and molecular weight, have a significant influence on WR [START_REF] Pourchez | Some aspects of cellulose ethers influence on water transport and porous structure of cement-based materials[END_REF][START_REF] Patural | Cellulose ethers influence on water retention and consistency in cement-based mortars[END_REF].

Desbrière studied in the mid-90s the influence of cellulose derivatives to the limitation of the fluid loss in cement slurries during static filtration under strong differential pressure (70 bars) [START_REF] Desbrieres | Cement cake properties in static filtration. Influence of polymeric additives on cement filter cake permeability[END_REF]. The author proposed two predominant mechanisms: the increase in the viscosity of the interstitial solution and the reduction of the permeability of the cement cake. He reached these assumptions considering that the fluid flow through a porous media constituted by filter cake can be described by Darcy's law. Thus the fluid loss volume, the opposite of WR, is a function of the viscosity of the filtrate and the cake permeability.

Other authors use also Darcy's law to describe the flow of water in a porous media under a pressure gradient due to capillary suction or depression. In case of good water retention capacity, they assume that the amount of extracted water is low and hence the permeability of the sample remains almost constant. Thus water-soluble polymers improve WR by increasing the viscosity of the pore solution whatever the polymer molecular parameters and dosage [START_REF] Brumaud | Origines microscopiques des conséquences rhéologiques de l'ajout d'éthers de cellulose dans une suspension cimentaire[END_REF][START_REF] Skaggs | Applications of rheological modifiers and superplasticizers in cementitious systems[END_REF]. In the same idea, many authors interpreted WR as the result of a reduction of the aqueous phase mobility due to an increase in viscosity when the polymer-to-cement ratio increases but without involving Darcy's law [START_REF] Afridi | Water retention and adhesion of powdered and aqueous polymer-modified mortars[END_REF][START_REF] Ohama | Polymer-based admixtures[END_REF][START_REF] Capener | Properties of modern rendering systems based on mineral binders[END_REF]. These authors explain also WR improvement in terms of sealing effect due to the formation of polymer water-impervious films in the paste (i.e. polysaccharides act as diffusion barrier to the water) [START_REF] Afridi | Water retention and adhesion of powdered and aqueous polymer-modified mortars[END_REF][START_REF] Ohama | Polymer-based admixtures[END_REF][START_REF] Capener | Properties of modern rendering systems based on mineral binders[END_REF].

Thanks to pulsed field gradient and NMR imaging investigations, Patural et al. [START_REF] Patural | A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars[END_REF] demonstrated that CE do not modify the water self-diffusion coefficient, even when an osmotic gradient is counteracting the diffusion flux. These results allow discarding the diffusion barrier hypothesis and showing that WR is governed by capillary suction.

Then, by means of NMR dispersion investigations, Patural et al. [START_REF] Patural | Nuclear magnetic relaxation dispersion investigations of water retention mechanism by cellulose ethers in mortars[END_REF] showed that the surface diffusion coefficient of water is not modify by the presence of CE, in spite of strong effect on solution's viscosity. However, CE increase the fraction of mobile water molecules transiently present at the solid surfaces. It was found a linear correlation between this fraction of mobile water and WR.

In the same time, by comparing the flow between a CE solution and a simple oil of similar viscosity from imbibitions experiments, Marlière et al. [START_REF] Marliere | How water retention in porous media with cellulose ethers works[END_REF] have obtained different behaviors with these two solutions. Indeed, the flow stops much before with the CE solutions which demonstrate that WR is not only governed by the fluid viscosity.

The second hypothesis of Desbrière [START_REF] Desbrieres | Cement cake properties in static filtration. Influence of polymeric additives on cement filter cake permeability[END_REF] to explain WR is the reduction of filter permeability.

According to this author, this is the major parameter and it may be explained by (i) a decrease in the porosity due to adsorption of macromolecular chains on cement; (ii) a physical plugging of the pore; (iii) an increase in the cake compaction due to modification of the local structure.

The influence of CE on the mortar's permeability is difficult to determine since it is very difficult to measure it in a direct way at the fresh state and water transport properties of hardened mortars differs strongly from the fresh material [START_REF] Green | The effects of lime and admixtures on the water-retaining properties of cement mortars[END_REF][START_REF] Carter | Measurement of the water retentivity of cement mortars[END_REF][START_REF] Hendrickx | Measuring the water capacity and transfer properties of fresh mortar[END_REF]. Nevertheless, several studies discussed the impact of CE on the microstructure and porous structure of cementbased material in both fresh and hardened state [START_REF] Jenni | Influence of polymers on microstructure and adhesive strength of cementitious tile adhesive mortars[END_REF][START_REF] Pourchez | Some aspects of cellulose ethers influence on water transport and porous structure of cement-based materials[END_REF][START_REF] Knapen | Cement hydration and microstructure formation in the presence of water-soluble polymers[END_REF][START_REF] Silva | Pore size distribution of hydrated cement pastes modified with polymers[END_REF]]. It appears that a specific porous structure is generated by CE.

However, Marlière et al. [START_REF] Marliere | How water retention in porous media with cellulose ethers works[END_REF] suggested that WR is not influenced by cement-CE interactions within a given timeframe. Indeed, they have shown that the mortar WR curves (WR vs CE dosage) exhibit similar shape when the cement is substituted by sand or glass beads. Bülichen et al. [START_REF] Bülichen | Working mechanism of methyl hydroxyethyl cellulose (MHEC) as water retention agent[END_REF] demonstrated also that MHEC effectiveness as water retention agent is not linked to its adsorption on cement.

In accordance with Desbrière [START_REF] Desbrieres | Cement cake properties in static filtration. Influence of polymeric additives on cement filter cake permeability[END_REF], the authors cited above considered that WR is explained by a reduction of filter cake permeability due to the formation of hydrocolloidal associated 3D polymer networks at high dosage (i.e. above a certain threshold concentration known as the overlapping concentration) which physically plug the pores of the paste [START_REF] Bülichen | Working mechanism of methyl hydroxyethyl cellulose (MHEC) as water retention agent[END_REF][START_REF] Bülichen | Role of colloidal polymer associates for the effectiveness of hydroxyethyl cellulose as a fluid loss control additive in oil well cement[END_REF][START_REF] Bülichen | Mechanistic study on carboxymethyl hydroxyethyl cellulose as fluid loss control additive in oil well cement[END_REF]. Marlière et al. [START_REF] Marliere | How water retention in porous media with cellulose ethers works[END_REF] described a jamming effect explained by the size and the quantity of polydisperse aggregates. This theory is consistent with the thin CE-enriched filter cake at the mortar/substrate interface suggested by Patural et al. [START_REF] Patural | A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars[END_REF], and with the accumulation of CE observed at the interface with the substrate in tile adhesive mortar [START_REF] Jenni | Influence of polymers on microstructure and adhesive strength of cementitious tile adhesive mortars[END_REF][START_REF] Jenni | Changes in microstructures and physical properties of polymer-modified mortars during wet storage[END_REF] or in the uppermost millimeter of self-leveling flooring mortar layer [START_REF] Gasparo | Quantitative distribution patterns of additives in self-leveling flooring compounds (underlayments) as function of application, formulation and climatic conditions[END_REF]. This short state of the art highlights that the understanding of WR mechanism induced by CE is crucial and interests many authors. The importance of colloidal interactions has emerged recently while the increase in pore solution viscosity is still often advanced. However, few studies have given attention to the impact of HPG on mortar WR although they were presented by Plank in 2004 as promising new class of water retaining agent in building materials [START_REF] Plank | Applications of biopolymers and other biotechnological products in building materials[END_REF]. Indeed, in comparison with CE, HPG exhibit comparable water retention abilities [START_REF] Cappellari | Influence of organic thickening admixtures on the rheological properties of mortars: Relationship with water-retention[END_REF][START_REF] Poinot | Impact of hydroxypropylguars on the early age hydration of Portland cement[END_REF][START_REF] Lasheras-Zubiate | Effect of the addition of chitosan ethers on the fresh state properties of cement mortars[END_REF], similar delayed effect on cement hydration [START_REF] Poinot | Impact of hydroxypropylguars on the early age hydration of Portland cement[END_REF][START_REF] Lasheras-Zubiate | Effect of the addition of chitosan ethers on the fresh state properties of cement mortars[END_REF] and a manufacturing process less polluting [START_REF] Biasotti | Highly substituted hydroxypropylguar: a strong contribution to construction chemistry[END_REF]. Furthermore, the HPG use is already well-established in industrial construction chemistry.

The aim of this work consists in understanding the effectiveness of HPG as water retention agent and comparing its behavior with classical CE. The main issue is to determine if WR is due to an increase in pore solution viscosity or to the formation of hydrocolloidal aggregates in the case of HPG. Thus working with non-cellulosic polysaccharides can also allow us to establish how generalizable are the recent findings concerning CE. For instance, other polysaccharidic admixtures such as starch [START_REF] Patural | Cellulose ethers influence on water retention and consistency in cement-based mortars[END_REF][START_REF] Cappellari | Influence of organic thickening admixtures on the rheological properties of mortars: Relationship with water-retention[END_REF] and chitin derivatives [START_REF] Lasheras-Zubiate | Effect of the addition of chitosan ethers on the fresh state properties of cement mortars[END_REF] were found to act on WR in different ways.

The present work will be divided into two parts. The first one will consist in the study of the influence of pore solution viscosity on WR. The objective is to determine if a master curve appears whatever the admixture when WR is plotted against the pore solution viscosity. The second part will focus on the influence of the agglomerates formation due to coil-overlapping.

To achieve this, the dosage and kind of HPG will be varied and the associate WR and viscosity of the extracted pore solution will be systematically characterized. Results will be compared to those obtained with classical cellulosic water retention agents. The influence of dosage on WR and the viscosity of admixed-mortars pore solutions will be first discussed.

Material and methods

Mineral and organic compounds

Mineral products

The investigated ordinary Portland cement (OPC) was a CEM I 52.5 R CE CP2 NF type cement according to EN 197-1 and NF P 15-318 standards. Oxide composition was determined by X-ray fluorescence spectroscopy (SRS3400, Bruker-AXS). Phase composition was quantified by means of Rietveld refinement method (Siroquant V2.5 software) after XRD analysis (D5000, Siemens). Chemical and phase compositions of the cement used are given in Table 1.

Organic admixtures

Seven polysaccharidic water retention admixtures were selected for this study: two hydroxypropyl methyl cellulose (HPMC1 and HMPC 2) and five hydroxypropylguars (HPG 1, HPG 2, HPG 3, HPG 5 and HPG 6).

Guar gum is a galactomannan consisting of a β(1-4)-linked D-mannopyranose backbone, with random branchpoints of galactose via an α(1-6) linkage [START_REF] Risica | Guar gum methyl ethers. Part I. Synthesis and macromolecular characterization[END_REF]. The ratio of mannose to galactose is about 1.8. HPG are obtained from the native guar gum via an irreversible nucleophilic substitution, using propylene oxide in the presence of an alkaline catalyst. As guar gum exhibits a high chemical reactivity and is soluble in cold water, the chemical modification of the native guar gum does not require hard reaction conditions of temperature and pressure, does not generate large quantity of by-products and needs weak purification procedure [START_REF] Biasotti | Highly substituted hydroxypropylguar: a strong contribution to construction chemistry[END_REF].

In order to compare HPG behavior with classical polysaccharidic admixtures, two cellulose derivatives were also tested. Cellulose is a polysaccharide composed of individual anhydroglucose units linked through β(1-4) glycosidic bonds. Because of strong intermolecular hydrogen bridging bonds, cellulose is not soluble in water. Thereby, cellulose ethers are obtained by pretreating cellulose with base (alkali cellulose) in order to break hydrogen bonds and to make accessible the active groups for etherifying agents. In the case of HPMC, the substitution of the hydroxyl groups takes place by reacting alkali cellulose with the combination of two etherifying agent: propylene oxide and methylchloride.

The schematic structure of both molecules is shown in Fig. 1 (substituent positions are arbitrary). It appears that HPMC are linear polymers whereas HPG exhibit branched-chain structure. Table 2 presents a qualitative description of the admixtures used. The qualitative substitution degrees are provided by the manufacturers. The molar substitution ratio (MS HP ) is defined as the number of moles of hydroxypropyl groups per mole of anhydroglucose unit and is less than 3 for our molecules. The degree of substitution, noted DS M , represents the amount of methoxyl groups per anhydroglucose unit and is about 1.8 for HPMC 1 and HPMC 2 according to the manufacturer.

All the HPG samples, provided by Lamberti S.p.A, have similar molecular weight, about 2.10 6 g.mol -1 . The only difference between HPG 1, 2 and 3 is the molar substitution ratio. HPG 5 and 6 exhibit additional substitution (short or long alkyl chains). The two HPMC samples exhibit the same substitution degrees. The only difference between HPMC 1 and HPMC 2 is molecular weight: about 0.25.10 6 and 1.10 6 g.mol -1 respectively. cement, sand and filler) was homogenized in a shaker (Turbula, Wab) for 10 min. Admixture solution was added in order to obtain a water to cement ratio W/C = 1. Mortar mixing procedure was in accordance with EN 196-1 standard [START_REF]Methods of testing cement -Part 1: Determination of strength[END_REF]. All tests were carried out at controlled temperature of 23°C because mortar fresh properties are temperature-dependent. A control test was performed with a mortar without admixture.

Water retention measurements

The water retention capacity of freshly-mixed mortar can be determine by means of different tests. Most of them are based on the measurement of the removed water after suction or depression. ASTM C1506-09 standard [START_REF]Standard test Method fo Water Retention of Hydraulic Cement-Based Mortars and Plasters[END_REF] describes one of these tests. ASTM measurements were performed 15 min after mixing in order to measure the water loss of mortar under depression. The standardized apparatus was submitted to a vacuum of 50 mm of mercury for 15 min. Then, water retention capacity, noted WR, was calculated using the following equation:

100 (%) 0 1 0 × - = W W W WR (1)
W 0 represents the initial mass of mixing water; W 1 is the loss of water mass after aspiration.

Viscosity of the pore solution

The mortar pore solution was extracted from the mortar paste, by means of centrifugation, 15 min after mixing in order to be in the same conditions than WR test. A large volume of mortar (about 150 mL) was centrifuged (Thermo Scientific, Sorvall Legend XF) for 10 min at 4,500 rpm. The large volume allows having a representative sample. Then, the supernatant was collected and centrifuged again (Eppendrof, MiniSpin Plus) at higher speed (14,500 rpm) for 5 min in order to remove residual particles. Finally, rheological properties of the final supernatant is investigated by means of a rheometer (MCR 302, Anton-Paar) equipped with cone and plate geometry (2° cone angle, 50 mm diameter), thermostated at 20°C. The steady shear flow of pore solutions have been investigated using decreasing logarithmic ramps in the 10 3 -1 s -1 range.

Each mortar studied was divided into two parts after the mixing (section 2.2.1). One part allowed measuring WR (section 2.2.2) while the other one was centrifuged in order to determine pore solution viscosity (section 2.2.3). Each formulation (one polymer at one dosage) was prepared and studied twice.

Results and discussions

Influence of polymer dosage on WR

According to the NF DTU 26.1, mortars can be divided into three classes depending on their water retention capacities [START_REF]Travaux d'enduits de mortiers[END_REF]. The first one (low retention) is for mortars that have water retention lower than 86%. The second category (intermediate water retention) corresponds to values ranging from 86% to 94%. The last one is defined by water retention higher than 94%.

This class is related to strong water retention mortars and is desired by formulators of industrial mortars. Finally, it is worth mentioning that WR capability of HPG 5 is closed to HPMC 2 which is the best WR agent studied. With an enough dosage, around 0.6% bwoc, all the HPG allow achieving strong WR. On the contrary, HPMC 1 needs a very high dosage (about 1.4% bwoc) vto provide strong WR capacity to the mortar. These results confirm that HPG are good water retention agents, with efficiency comparable to CE.

Rheological properties of admixed-mortars pore solutions

Steady shear properties

Typical flow curves of mortar pore solutions at different polymer dosages, obtained in case of HPG 5 admixed mortars, are shown in Fig. 3. In all cases, flow curves exhibited typical shear thinning behavior with a Newtonian region in the low shear rate range. The shear thinning nature of random coil polysaccharide solutions under flow is well known and due to entangled network systems [START_REF] Graessley | The entanglement concept in polymer rheology[END_REF][START_REF] Morris | Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions[END_REF]. At low shear rates, the disruption of entanglements by the imposed shear is balanced by the formation of new interactions, resulting in no net change in the extent of entanglements and hence no effect on the viscosity. This corresponds to the Newtonian plateau region, where the viscosity has a constant value called zero-shear rate apparent viscosity η 0 . However, for higher shear rates, the rate of forced disentanglement predominates over the rate of entanglement of polymer chain. As a consequence, chains align in the flow direction and the viscosity decreases with the increasing shear rate.

The experimental data can be conveniently correlated with the Cross model:

) ( 1 ) ( 0 n γ τ η η η γ η & & + - + = ∞ ∞ (2)
where η 0 is the zero-shear rate viscosity, η ∞ is the infinite rate viscosity, τ is a relaxation time and n a non-dimensional exponent.

Among Cross parameter, the zero-shear rate viscosity η 0 will be used in order to characterize the viscosity of the mortars pore solutions. Nevertheless, experimental data deviate from the model at low shear rate, especially for low dosage (Fig. 3). According to Clasen and Kulicke [START_REF] Clasen | Determination of viscoelastic and rheo-optical material functions of water-soluble cellulose derivatives[END_REF], the hydrophobic character of the side chains (hydropropyl for HPG, hydroxypropyl and methyl for HPMC) leads to strong intermolecular interactions and to the formation of 3D physical cross-linking (called superstructures) due to association. These associations determine the flow behavior at low shear rates and induce higher viscosity. When the shear rate increases, it becomes possible to overcome these hydrophobic interactions and thus to break the associated superstructures. This induces first a viscosity drop and then the classical flow behavior, with the Newtonian plateau at the zero-shear viscosity value and shear thinning at higher shear rates. Therefore the Cross model can be adjusted and the zero-shear viscosity can be determined by not taken account of data before the Newtonian regime.

Concentration dependence of zero-shear viscosity

As seen above, polymer concentration is an important parameter in controlling the rheological properties of polysaccharide solutions. At low polymer concentration, individual polymer molecules are present as isolated coils which provide their individual contribution to the viscosity. As the concentration of polymer is increased, the coils begin to bring into contact with one another. The corresponding concentration is called coil-overlap concentration (often noted C*). Above this concentration, a dramatic change in flow behavior occurs which results in a sudden increase in the concentration-dependence of zero-shear rate viscosity [START_REF] Graessley | The entanglement concept in polymer rheology[END_REF][START_REF] Morris | Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions[END_REF][START_REF] Robinson | Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan[END_REF].

Therefore a double logarithmic plots of η 0 versus the concentration, as shown in Fig. 4, clearly evidences the onset of significant coil overlap and interpenetration. Indeed, two welldefined linear dependencies can be observed. The dependence of η 0 on concentration is described by two power-law type correlations (η 0 = aC b ) where the exponent b differs depending on the regime considered. In our case, values of exponent obtained are consistent with the values found in literature [START_REF] Morris | Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions[END_REF][START_REF] Robinson | Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan[END_REF][START_REF] Lapasin | Flow properties of hydroxypropyl guar gum and its long-chain hydrophobic derivatives[END_REF][START_REF] Oblonšek | Rheological studies of concentrated guar gum[END_REF][START_REF] Sittikijyothin | Modelling the rheological behaviour of galactomannan aqueous solutions[END_REF][START_REF] Risica | Rheological properties of guar and its methyl, hydroxypropyl and hydroxypropyl-methyl derivatives in semidilute and concentrated aqueous solutions[END_REF]. Thus the polymer dosage corresponding to the Increasing dosage intersection of the two straight lines for each admixture can be attributed to the boundary between semi-diluted and concentrated regime. This critical polymer dosage corresponds to the formation of polymer agglomerates in the pore solution. These data are collected in the Table 2.

It is worth noting that these critical dosages are not coil-overlap concentrations as defined in literature. Indeed, the real concentration of polymer present in pore solution is unknown for a given initial dosage, because (i) a certain amount of polymer is retained into the mortar paste or adsorbed onto cement phases and (ii) a non-negligible amount of water is lost due to cement hydration and mortar drying. The only known concentration is the introduced concentration, given in percentage by weight of cement (called polymer dosage) or in grams per liter (called introduced concentration) in the following.

Since η 0 was chosen to characterize the viscosity of mortars pore solutions, Fig. 4 allows also comparing the viscosifying properties of admixtures studied. It is obvious that HPMC 2 exhibits the stronger viscosity-enhancing effect, far ahead HPG, while HPMC 1 is the worst viscosifying agent. The difference observed between HPMC 1 and 2 is attributed to the difference in molecular mass.

Among HPG, pore solution viscosities of mortars admixed with HPG 1, 2 and 3 are similar.

The only difference between these HPG is the substitution degree. In literature, it is indicated that η 0 decreases when DS increases [START_REF] Kulicke | Characterization of aqueous carboxymethylcellulose solutions in terms of their molecular structure and its influence on rheological behaviour[END_REF]. The same effect was observed when polymers were dissolved in water. However, in mortars, increasing substitution degree leads to a decrease in polymer adsorption on cement phases [START_REF] Poinot | Impact of hydroxypropylguars on the early age hydration of Portland cement[END_REF][START_REF] Pourchez | Changes in C3S hydration in the presence of cellulose ethers[END_REF] and hence an increase in polymer amount in pore solution. Finally, the effect of substitution degree on viscosity is compensated by the effect of adsorption ability. Thus pore solutions have comparable viscosities. Therefore the formation of polymer aggregates occurs at similar polymer dosage of about 0.25% bwoc (which corresponds to an introduced concentration of 2.5 g.L -1 ) for HPG 1-2-3 (Table 2).

On the contrary, the presence of additional alkyl chains (HPG 5-6) leads to the formation of polymer associates at slightly lower polymer dosage (Table 2). The interconnection between alkyl chains creates intermolecular interactions through specific hydrophobic interactions which cause a decrease in the coil-overlapping concentration [START_REF] Simon | Amphiphilic polysaccharides. Evidence for a competition between intra and intermolecular associations in dilute system[END_REF].

The critical polymer dosage for HPMC 2 was found to be similar to HPG 5 (0.2% bwoc or 2 g.L -1 ) while the viscosifying effect is stronger. A relatively high dosage of almost 0.6% bwoc (6 g.L -1 ) is necessary for the overlapping of HPMC 1 coils.

Working mechanism of HPG as water retention agent

The previous section highlights that an increase in polymer dosage leads to an increase in WR and in pore solution viscosity, and to the formation of agglomerates. The remaining question is to determine which parameter governs WR induced by HPG: viscosifying effect on pore solution or entanglement of HPG coils (i.e. formation of agglomerates)? However, the main difficulty was to discriminate these two factors, since the entanglement of HPG coils leads to a sharp increase in viscosity.

Influence of pore solution viscosity

In order to determine the impact of HPG thickening properties on WR, we have plotted WR versus the related pore solution viscosity, for each admixture studied (Fig. 5). It actually appears that WR increases with the pore solution viscosity (Fig. 5-a). From very low viscosities, near water (i.e. 10 -3 Pa.s), viscosities up to a thousand times higher while WR begins at 60%, equivalent to the control without admixture, and increases until reach the plateau near 100%.

However, by zooming, it becomes clear that for a given pore solution viscosity, there are huge differences in WR, and a given WR could be reached with pore solutions of different viscosities. For instance, Fig. 5-b shows WR in the 0.001-0.01 pore solution viscosity range.

All data points are around WR values of 60-65% which corresponds to the control, except for HPG 1, HPG 6 and especially HPG 5. Indeed, with pore solution viscosity lower than 0.003 Pa.s, all admixtures induce low or no WR in comparison to the control, while HPG 5 induces WR close to 80%. In the same way, when HPMC enhances pore solution viscosity up to 0.01 Pa.s, the related WR is only of 70%. But for the same viscosity, HPG 1 and HPG 6 exhibit a WR of 80% and HPG 5 a WR of 87%. On Fig. 5-c, where pore solution viscosity is ranging from 0.01 to 0.1 Pa.s, a strong difference in WR is observed for a given viscosity, from 5 to 18%, whatever the admixture. Here again HPG 1, 5 and 6 allow obtaining the best WR.

These figures show also that it is possible to obtain the same WR with pore solutions exhibiting different viscosities. In fact, viscosity can be multiplied by 5 while WR remains lower than 65% (Fig. 5-b). Mortars can exhibit WR of about 80% with pore solution viscosity varying strongly from 0.003 to 0.02 Pa.s (Fig. 5-a). For high WR ranging from 90 to 95%, viscosities obtained vary in the range 0.015 -0.090 Pa.s (Fig. 5-c). For the same WR (80%), pore solution of mortars admixed with HPMC 2 and HPMC 1 exhibit viscosities of 0.011 and 0.023 Pa.s respectively (Fig. 5-c).

Finally, it is clear that pore solution viscosity cannot govern alone the WR of admixed mortars, as previously shown in the case of CE [START_REF] Marliere | How water retention in porous media with cellulose ethers works[END_REF] and in agreement with the remark of Patural et al. [START_REF] Patural | A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars[END_REF] (i.e. some viscosifying admixtures such as starch, polyethylene oxides or polyacrylamides do not provide WR). For the 5 HPG samples studied, it is obvious that the shape of WR curves is strongly influenced by the formation of polymer associates. Below this threshold dosage, no or low increase (< 3%) in WR is observed in comparison with the control test without admixture.

Above the critical dosage, a sudden and sharp rise in WR is obtained.

Above the considered threshold dosage, the viscosity of the mortar pore solution exhibits also a strong and abrupt increase. Nevertheless, it has been shown above that the increase in pore solution viscosity cannot explain the WR induced by HPG. Therefore, based on these results

and in agreement with literature considering CE [START_REF] Marliere | How water retention in porous media with cellulose ethers works[END_REF][START_REF] Bülichen | Working mechanism of methyl hydroxyethyl cellulose (MHEC) as water retention agent[END_REF], it appears clearly that the increase in WR in presence of HPG is due to the overlapping of HPG coils.

When the mortar is applied to absorbent substrates, few amount of water is lost first. This initial lost water flow leads to the formation of a thin HPG-enriched filter cake at the interface mortar-substrate [START_REF] Jenni | Influence of polymers on microstructure and adhesive strength of cementitious tile adhesive mortars[END_REF][START_REF] Patural | A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars[END_REF][START_REF] Jenni | Changes in microstructures and physical properties of polymer-modified mortars during wet storage[END_REF]. At sufficient dosage, coil-overlapping occurs. HPG associates thus formed can physically plug the porous network and therefore stop the water flow. Once the water flow stopped, it can be considered from a macroscopic point of view that HPG provides WR.

This theory shows that the amount and/or size of polymer agglomerates are the key parameter for the WR mechanism, but the size of the filter cake porous network too. This assumption is consistent with the fact that water transport in fresh mortars is intrinsically related to the particle size distribution of the binder [START_REF] Hendrickx | Measuring the water capacity and transfer properties of fresh mortar[END_REF] and granular media [START_REF] Green | The effects of lime and admixtures on the water-retaining properties of cement mortars[END_REF][START_REF] Carter | Measurement of the water retentivity of cement mortars[END_REF].

The cases of HPG 5 and 6 are interesting. Indeed, for instance HPG 5 exhibits the best WR at a given dosage in comparison with other HPG, and slightly lower WR than HPMC 2 (Fig. 2).

In addition, at a given induced viscosity of pore solution, HPG 5 and 6 are by far the best WR agents even in comparison with HPMC 2 (Fig. 5). Thus a positive impact of the substitution of alkyl chains can be suggested. In previous study [START_REF] Poinot | Impact of hydroxypropylguars on the early age hydration of Portland cement[END_REF], it has been assumed that the presence of the alkyl chains could cause steric hindrance which could reduce the adsorption ability. If the adsorption is reduced, more polymer molecules are actually in solution and hence effective to provide WR capacity. Nevertheless, the adsorption of non-ionic polysaccharide is a matter of controversy. According to Bülichen et al. [START_REF] Bülichen | Working mechanism of methyl hydroxyethyl cellulose (MHEC) as water retention agent[END_REF], the considered adsorption could be actually a physical retention at high dosage. More likely, the alkyl chains enhance the entanglement of coils and thus increase the size and/or the amount of polymer aggregates at low dosage. This may explain the surprising high WR induced by HPG with additional alkyl chains, even with a weak effect on pore solution viscosity. This observation confirms that the relationship between pore solution viscosity and WR is not obvious.

By comparing three similar HPG which differ only in the degree of substitution (HPG 1, 2 and 3, see Table 2), it appears that HPG 1 exhibits better WR efficiency than HPG 2 and 3 for a given induced pore solution viscosity (without regarding the polymer dosage). Thus a low degree of substitution (HPG 1) leads to better WR for a similar effect on pore solution viscosity. During HPG manufacturing, hydroxyl groups are substituted by propylene oxide.

The resulting hydroxypropyl groups replace strong intermolecular polar OH-OH interactions with medium strength hydrophilic-hydrophobic bonds [START_REF] Cheng | homme, Characterization and Intermolecular Interactions of Hydroxypropyl Guar Solutions[END_REF]. Indeed, the hydroxypropyl groups sterically block the hydrogen bondings sites on the guar backbone. Finally an increase in substitution degree induces a reduction of the entanglement between polysaccharide chains.

Thus HPG 1 with low substitution degree exhibits stronger entanglement than HPG 2 and 3 at the same viscosity, and therefore a better WR.

The behavior of HPMC is more difficult to assess because of a lack of WR values below the threshold dosage. It appears nevertheless a significant WR effectiveness before the formation of the agglomerates (around +10% for HPMC 2). According to Bülichen et al. [START_REF] Bülichen | Working mechanism of methyl hydroxyethyl cellulose (MHEC) as water retention agent[END_REF][START_REF] Bülichen | Role of colloidal polymer associates for the effectiveness of hydroxyethyl cellulose as a fluid loss control additive in oil well cement[END_REF], CE are able to achieve WR before the formation of hydrocolloidal associated polymer network by water intramolecular sorption and concomitant swelling.

Conclusions

This study confirms that HPG provide strong WR to freshly-mixed mortars, in the same way than CE. As expected, WR is concentration dependant. Nevertheless, a sharp change in slope is observed on the WR curve of HPG-admixed mortars. This is attributed to the threshold dosage corresponding to the formation of hydrocolloidal associated HPG molecules network.

This critical dosage was determined by studying the rheological properties of mortars pore solutions extracted by centrifugation. Above this dosage, it can be assumed that HPG aggregates stop the water flow by plugging the porous network of a thin HPG-enriched filter cake at the interface mortar-substrate. Below this dosage, a nearly zero or hardly discernible effect of HPG on WR is obtained, which is not the case for CE.

The effect of pore solution viscosity as explanation for WR was rejected. For a given viscosity, admixtures exhibit very different WR ability. Likewise, huge differences in pore solution viscosity allow obtaining similar WR. Clearly, the key parameter to determine the dosage of WR agent in dry-mix mortar is not the induced viscosity of pore solution but the formation of polymer agglomerates. The results highlight moreover the positive impact of additional alkyl chain on WR. By enhancing overlapping, the additional substitution of alkyl chains improves the effectiveness of WR agent at low dosage.

  agent in cement-based mortars Thomas Poinot a -Alexandre Govin a* -Philippe Grosseau a a Ecole Nationale Supérieure des Mines, SPIN-EMSE, Département PROPICE, CNRS:UMR5307, LGF, F-42023 Saint-Etienne 1 Introduction

Fig. 1 :

 1 Fig. 1: Molecular structure of HPMC (a) and HPG (b).

Fig. 2

 2 Fig. 2 presents the water retention of mortars containing increasing dosages of admixture. The

Fig. 2 :

 2 Fig. 2: Effect of polymer dosage on water retention of admixed mortars.

Fig. 3 :

 3 Fig. 3: Flow curves of mortars pore solutions for different polymer dosages -Example of HPG 5 -The full lines represent the calculated values according to the Cross model (Eq. 2).

Fig. 4 :

 4 Fig. 4: Concentration dependence of zero-shear viscosity for all the admixtures studied

Fig. 5 :

 5 Fig. 5: Effect of pore solution viscosity on water retention from overall view (a), zoom in low viscosities (b) and zoom in middle viscosities (c)

Fig. 6 :

 6 Fig. 6: Effect of polymer agglomerates formation on WR

Table 1 : Chemical and phase compositions of the investigated cement. Chemical composition (% wt) Phase composition (% wt)

 1

	Oxides	XRF	Oxides	XRF	Phases	XRD (Rietveld)	Phases	XRD (Rietveld)
	CaO	66.9 ± 0.8	MgO	1.16 ± 0.01	C 3 S	79.4 ± 0.5	Gypsum	1.3 ± 0.2
	SiO 2	20.9 ± 0.2	TiO 2	0.32 ± 0.03	C 2 S	8.2 ± 0.4	Anhydrite	3.2 ± 0.2
	Al 2 O 3	4.7 ± 0.1	P 2 O 5	0.14 ± 0.01	C 3 A	3.3 ± 0.2	Hemi-hydrate 0.8 ± 0.3
	SO 3	2.4 ± 0.2	MnO	0.04 ± 0.01	C 4 AF	4.1 ± 0.9	Free CaO	0.5 ± 0.2
	Fe 2 O 3	2.6 ± 0.1	K 2 O	0.10 ± 0.01
	LOI	2.1 ± 0.1

Table 2 : Qualitative description of the admixtures used.

 2

	Admixtures	Viscosity	MS HP	DS M	Additionnal substitution
	HPMC 1	Very low	Very low Very high
	HPMC 2	Very high	Very low Very high
	HPG 1	Medium	Low	/
	HPG 2	Medium	Medium	/
	HPG 3	Medium	High	/
	HPG 5	Medium	High	/	Shorter alkyl chain
	HPG 6	Medium	High	/	Longer alkyl chain
	2.2 Experimental methods of investigation
	2.2.1	Preparation of mortars
	Admixtures were previously dissolved in deionized water by magnetic stirring for 24h in
	order to obtain a complete dissolution of all polymers. The concentrations of these polymer
	solutions (from 1 to 16 g.L -1 ) allow obtaining, in mortars, polymer dosages varying from 0.1
	to 1.6% by weight of cement (bwoc).

Then, mortars were prepared according to the following mixture proportions: 30 wt.% of cement, 65 wt.% of siliceous sand and 5 wt.% of limestone filler. The dry mixture (i.e.

Acknowledgments

The authors would like to acknowledge Lamberti S.p.A. for the provided products.


Mohamed Anis Dhuieb
email: mohamed-anis.dhuieb@irccyn.ec-nantes.fr

Florent Laroche

Alain Bernard

Alain Bernard Digital

Digital Factory Assistant: Conceptual Framework and Research Propositions

Keywords: Knowledge Management, PLM, Virtual Assistant, Ubiquitous computing

Introduction

Industrials and researches illustrate different visions of the factory of the future. The shared aspect of these visions is providing the factory's employers with consistent IT systems to simplify and support employee's tasks.

Based on an integrated approach of material-processes-information-human being, our proposition is to develop a customable company assistant that interact with complete information set. The assistant will be inspired by knowledge based systems, contain different decision aided functionalities, and use Virtual engineering technologies techniques. This will allow extending the dimensions of Virtual Technologies models with further data sets. This assistant will assist factory's actors during engineering phases by providing them with the right information. It consists of a knowledge based system which allows the access of different knowledge resources in the factory such as Product Lifecycle management (PLM) CAx models (Computer aided Design/ Manufacturing/engineering CAD/CAE/CAM…), material flows, process chains, simulation data… [START_REF] Laroche | Towards the factory of future An integrated approach of material-processes-information-human being[END_REF] One of the challenges in our proposition is to select the right information from numerous sources and transforms it into useful knowledge.

The first part of the paper presents a state of the art of the evolution in digital tools in the factory. The second part deals with the human being role in the product lifecycle as a decision maker. Some related works are presented in the third part and the fundamentals of ARTUR project, which illustrate our vision of the factory of the future, will be explained in the third part before the conclusion.

2

Evolution of Digital Tools in Factory: Future Information

Systems in Factory

Nowadays, enterprises have acquired various tools that allow them to be very confident with the products they develop. The concept of smart factories proposes them to automate and control most of their tasks [START_REF] Laroche | Towards the factory of future An integrated approach of material-processes-information-human being[END_REF]. Today's factory contains more and more digital tools based on ICT. The digital and virtual factory paradigm can support the design and management of a production environment by addressing various key issues like: reduction of production times and material waste thanks to the analysis of virtual mock-ups, development of a knowledge repository where people can find stored information in different versions, with both advisory role and support to the generation of new knowledge and improvement of workers efficiency and safety through training and learning on virtual production systems [2][3]. Advanced technologies such as ubiquitous computing (i), virtual engineering technologies (ii), and the concept of the Web 3.0 (iii) are supplying the factory with a new wave of intelligence: i)

Ubiquitous Computing was first introduced by Weiser in the end of 80s. It consists of a new model of interaction where the information processing is integrated in our daily life. The context-awareness is the particular aspect of ubiquitous information systems. These information systems have to anticipate the user's needs in a particular situation and act proactively to provide appropriate assistance. Many researchers describe the vision of the factory of things which adopt the basic concepts of the internet of things and implement smart objects that interact basically on semantic services integrating knowledge representations. The main goal of such propositions is to make the information available anywhere, anytime for any user based on Weiser vision of ubiquitous computing.

[4][5][6][7][8][9] ii)
Virtual engineering technologies become an emerging field in industry and research. Many industrials use Virtual and Augmented Reality to make advanced simulation of their manufacturing process or for training their employees. The application fields of virtual engineering can be considered for the complete life-cycle of the product, from the earliest design stages of the products to their manufacturing, assembly, use and maintenance phases. This evolution in the field of virtual engineering reflects a real modification of our way of thinking product development. Thus, global coherency has emerged as a result of improved numerical integration and better software functionality for manufacturing preparation as well as for process simulation and part dimensional and geometrical control after manufacturing. [10][11][12] [START_REF] Laroche | Contribution à la sauvegarde des Objets techniques anciens par l'Archéologie industrielle avancée. Proposition d'un Modèle d'information de référence muséologique et d'une Méthode inter-disciplinaire pour la Capitalisation des connaissances du Patrimoine technique et industriel[END_REF] iii) The Web 3.0 is the more recent technology from those mentioned previously. Mills Davis describe his vision of the web 3.0 in his industry roadmap to web 3.0: In order to connect systems, integrate information, and make processes interoperable, the first step is to integrate the knowledge about these systems, content sources, and process flows. Semantically modeled machine executable knowledge lets us connect information about people, events, locations, times, across different content sources and application processes. In Web 3.0, knowledge lives, evolves and is stored transparently. It can be used, validated, added to, and combined with other knowledge at run time by multiple systems. This enables a system to "learn" to do things that the system designer did not anticipate. This is an important shift from IT as it has been practiced until now.

The basic shift occurring in Web 3.0 is from information-centric to knowledge-centric patterns of computing. Web 3.0 encompasses a broad range of knowledge representation and reasoning capabilities including pattern detection, deep linguistics, and ontology [START_REF] Davis | Semantic Wave 2008 Report: Industry Roadmap to Web 3.0 & Multibillion Dollar Market Opportunities[END_REF] [15] [START_REF] Hendler | Web 3.0[END_REF].

In modern product development, as the complexity and variety of products increase to satisfy increasingly sophisticated customers, so does the need for knowledge and expertise for developing products. Today's knowledge-intensive product development environment requires a computational framework which effectively enables capture, representation, retrieval and reuse of product knowledge. [17][18]The factory of the future requires holistic production systems with a learning capability, based on learning effects in all stages of the process chains involved, the goal is to achieve higher production outputs by integrating knowledge modules into the engineering systems concerned [START_REF] Westkämper | EMO Hannover 2011 showcases the factory of the future: New production concepts integrate knowledge and learn from it[END_REF].

As we mentioned in the introduction our proposition is to develop a customizable knowledge book called "Digital Factory Assistant". The crucial point of success of our work will be the building of a comprehensive model that represents most of the elements in the factory, seen as knowledge resources, and to guarantee the interoperability between them, but at first let's see what are the reasons that led us propose it.

The Human Factor in Production Systems

For an enterprise's management point of view, enterprise's managers make a significant effort to reduce the cost of non-quality (CONQ). Measuring and reporting this cost is a critical issue for any manager who aims to achieve competitiveness in today's markets [START_REF] Schiffauerova | Managing cost of quality: insight into industry practice[END_REF]. Studies show that the lack of information, gaps in training, and the unavailability of documentation in production lines are the most important reasons of CONQ rising [START_REF] Benhabib | Les coûts de non-qualité[END_REF]. Despite the evolution of digital tools in factory as seen in the previous section, the role of the human being in product lifecycle is not insignificant. Labrousse [START_REF] Labrousse | Proposition d'un modèle conceptuel unifié pour la gestion dynamique des connaissances d'entreprise[END_REF] highlight the role of the human being as a decisional actor in the factory life. He makes the relation between information, knowledge, and decision: information is not easy to understand by the user, the meaning of the information can be strongly influenced by the user and the context. The knowledge is the emergence of the user and the interpreted information. In many cases elements impacting decisions are not totally mastered and suffer from incompleteness. In the process of decision making, users combine different types of data and knowledge (both tacit and explicit knowledge) available in different forms. The decision making process results in improved understanding of the problem and the process [START_REF] Bolloju | Integrating knowledge management into enterprise environments for the next generation decision support[END_REF].

One of the solutions to reduce the cost of non-quality is developing assistant tools that help factory employees in their tasks. There are several works related to human assistance in different phases of product lifecycle. Projects deal with assistance in production phase is not very common unlike in other product lifecycle phases like design or maintenance. The next section describes few works related to the human being assistance in the factory.

Previous Works Relied on Human Being Assistance in Factories

Many propositions have been encountered in the literature. Despite the diversity of these projects, few of them deal with human assistance in production systems at a "machine level" where information is less available comparing to other departments in the factory. In this section, we discuss a number of works related on our work. In a maintenance context, CARMMI model [START_REF] Espíndola | A model-based approach for data integration to improve maintenance management by mixed reality[END_REF] provide support to operators during maintenance tasks through mixed reality providing information access from different sources. This model allows the acquisition and presentation of different data: CAx data, maintenance, and virtual data. From a decision making point of view, the big quantity of the information presented in the interface can reduce the efficiency of the operator to make the right decision. In fact, the interface contains augmented reality, visualization system, information and historical information data. The human machine interactions are not studied in this project.

A second work in the same context was presented by Toro [START_REF] Toro | Knowledge based industrial maintenance using portable devices and augmented reality[END_REF]. A framework and a system implementation were developed for the exploitation of embedded knowledge in the domain of industrial maintenance. This framework uses a shared ontology designed to model and support pervasive computing applications called SOUPA. A knowledge acquisition system is also developed in this project called Experience Knowledge Structure. This system extends the functionality of SOUPA in a way that formal decision events can be suggested to the user during his work. Both of these two technologies were used to develop the UDKE system (User, Device, Knowledge, and Experience). This UDKE provide a conceptual model of a maintenance system that combines knowledge, user experience and Augmented Reality techniques. The context awareness notion was absent from this project so the possible decisions and information provided to the user were not relied to his task and did not consider his feedback.

Kwon [START_REF] Kwon | UbiDSS: a proactive intelligent decision support system as an expert system deploying ubiquitous computing technologies[END_REF] [27] has developed a framework called ubiDSS. ubiDSS is a framework of multi-agent and context-aware based proactive decision support system. The architecture of this framework is composed by a number of subsystems: context subsystem, dialogue subsystem, knowledge subsystem, model management subsystem, and a database management subsystem. Even that the application field of the ubiDSS system is far from industry, it was very important to study the interactions between the components of its framework and the models behind.

Many other propositions were encountered in the literature relied to the human being assistance in the different product lifecycle stages especially in design process. However, the application of such propositions in a production context is very rare.

ARTUR: A Digital Factory Assistant

Concept

Related to visions of the factory of the future, our work is to offer the factory operator with a so called "Digital Factory Assistant". Through this assistant the operator can have information from distributed knowledge resources relying in different ways of interaction in order to act upon the task that he is doing. So, the main research issue of our work is to find which information to give to the operator, where, when, and how. The basic idea is to allow each person to be able to react in given situation, mainly thanks to high speed simulation and performance evaluation models and methods. [START_REF] Laroche | Towards the factory of future An integrated approach of material-processes-information-human being[END_REF] Fig. 1. Based on knowledge extraction mechanisms, knowledge will be extracted from factory information system elements compared as a lower part of an iceberg. The operator will deal only with a mobile device, compared as the top part of an iceberg, where knowledge will be deployed with intelligent navigation system [START_REF] Carolan | Developing models for enhanced learning in engineering[END_REF].

Figure 1 outline the first proposition of the conceptual framework of our work in a high level. The framework will be based on three layers: the first one is related to the human-machine level, where contextual information related to user's working situation can be acquired. The second layer is related to the definition, representation, exploration, and structuring knowledge. The last layer deal with the issue of linking knowledge layer with the whole information system in the factory.

To build our "Digital Factory Assistant", we have to consider many points of view. In the following sections we will try to detail some of them.

Evaluation of Human-Machine Interactions

Interaction between the human being and the machines or environment is a topic that is often underestimated. More satisfactory relations need to take the psychological aspects into account and also improve the factory environment by controlling many factors. The relations between human beings, machines, and their environment play a central role in production systems [START_REF] Zuehlke | SmartFactory Towards a factory-of-things[END_REF].

A cognitive approach will be developed; the information quantity will be different from a beginner operator to an expert operator. With this approach, and in a context of uncertainty, the system will transfer a small amount of knowledge to a beginner user and led him, as his needs, to a larger amount of knowledge and do the opposite for a more expert user: "smooth roads never make good drivers".

Knowledge Exploitation

Knowledge management aims at capturing explicit and tacit knowledge of an organization in order to facilitate the access, sharing and re-use of that knowledge [START_REF] Sriti | Towards a semantic-based plateform to imporove knowledge management in collaborative product development[END_REF]. Foundations of our research project relate to a new proposition for modeling and structuring information, and more generally knowledge. Consequently, dealing with the global data, information, knowledge and know-how, we will be able to contextualize reusing this knowledge in any given situation that would need this knowledge [START_REF] Laroche | Towards the factory of future An integrated approach of material-processes-information-human being[END_REF]. Our assistant will be based on a dynamic knowledge representation: knowledge will be presented to an operator according to some relevant information including his working situation [START_REF] Hasan | Integrating safety into the design process: elements and concepts relative to the working situation[END_REF] [START_REF] Hasan | Proposal of a new design approach integrating the concept of the working situation[END_REF], the maturity level [START_REF] Xu | Proposition d'un système intégrée de référence de connaissances pour l'évaluation de la connaissance dans le développement de produits[END_REF], and the operator experience. In a process of decision making, the actor will combine different types of data and knowledge available in various forms.

The operator will have the possibility to personalize the set of information and knowledge as his need and as his evolution on the process or the service that he works on it. An intelligent knowledge and information system will be deployed in mobile device to make the information available in any context, even in complex working situations. [START_REF] Labrousse | Proposition d'un modèle conceptuel unifié pour la gestion dynamique des connaissances d'entreprise[END_REF][32] [START_REF] Ammar-Khodja | Processus d'aide à la spécification et à la vérification d'application d'ingénierie à base de connaissances expertes[END_REF].

Ubiquitous Environment

As mentioned in the first part of this paper, our system will run in an ubiquitous environment [START_REF] Weiser | The computer for the 21st century[END_REF]. The ubiquitous environment is now emerging as a primary driver to change the decision makers' task environment. This trend certainly has the potential to improve the decision making framework in gathering and processing decision makers' contextual data in an automated way, which intelligently extracts proactive decisions [26][27]. The context-awareness is the particular aspect of ubiquitous information systems. Dey [START_REF] Dey | Understanding and Using Context[END_REF] has defined the context as "any information that can be used to characterize the situation of an entity. An entity is a person, place, or object that considered relevant to the interaction between the user and the application, including the user and the applications them-selves." Humans possess a different perception of contextual information, which is mostly due to the availability of context information on different levels of granularity or abstraction [START_REF] Zimmermann | An operational definition of context[END_REF].

Fig. 2. Basic elements creating the ubiquitous environment

In accordance with the vision of Weiser in ubiquitous computing, our system will be based on a context aware model. The ubiquitous environment is created by the data flow coming from different resources:

• user's input dependent in his working situation (1) (this context information will act like a sensor in our model), • the coupling of the existent information system in the factory and knowledge database (2), • the process simulation (4) based on advanced numeric simulation technics [START_REF] Chinesta | First steps towards an advanced simulation of composites manufacturing by automated tape placement[END_REF],

• the extending of the virtual reality models with a further data set (3) (2).

The system will have the capability to understand the user's need by "compiling" context information and automatically "generate" the relevant information and knowledge to the user.

In addition to context awareness, reactiveness is an important requirement for a ubiquitous information environment. Proactiveness means to process information on behalf a user so an action can be taken without requiring his attention. This implies knowing what a user would want to do with the requested information, and detecting patterns for his behavior. [START_REF] Van Bunningen | Context for ubiquitous data management[END_REF] Therefore, we will define effective context information extraction techniques to identify the working situation of the operator and to determine the information set that will be presented.

Future Model Aspects

Taking into account the main aspects of our idea about the "Digital Factory assistant", our model will be based on several approaches related to enterprise, knowledge, and context modeling. Enterprise modeling is defined by Molina [START_REF] Molina | Reference models for the computer aided support of simultaneous engineering[END_REF] as the art of externalizing enterprise knowledge which adds value to the enterprise or needs to be shared. It consists of making models of the structure, behavior and organization of the enterprise. [START_REF] Labrousse | Proposition d'un modèle conceptuel unifié pour la gestion dynamique des connaissances d'entreprise[END_REF] A new proposition for modeling working situations will be designed. This aspect will be based on Hasan proposition [30] [31] for modeling the working situation at machine level.

Fig. 3. Macro view of the working situation [30] [31]

A level of detail will be employed during the modeling process [START_REF] Laroche | Contribution à la sauvegarde des Objets techniques anciens par l'Archéologie industrielle avancée. Proposition d'un Modèle d'information de référence muséologique et d'une Méthode inter-disciplinaire pour la Capitalisation des connaissances du Patrimoine technique et industriel[END_REF] this aspect will define the granularity of the information to be presented to the user. The desired model should be in one hand extensible to acquire more functionality and more types of data and in another hand interoperable so he can communicate and acquire knowledge and information from other existing systems in the factory.

Conclusion and Future Work

We believe that in the factory of the future the information and the knowledge will have more accessibility with small devices that interact with its user in an intelligent way.

The main aspect for our vision is to extract the right information in the right time and give it to the right user. In a production system, the user may be faced with working situations that require him to interact with the machine in front of him by choosing the right decisions. The decisions can be a force to be applied, a position of a tool in the machine… So, to make his decision the user will need to have such propositions of what he can do or what measure he will take and, using advanced simulation technics, he can anticipate the influence of his choice. To validate all this aspects in our project ARTUR, we will be based on two use cases in aeronautic industry. Currently, we started a survey to identify the different elements that will help us to build our information model.