Imen Abed
email: imenabed@hotmail.com

Maria Paraschiv

Khaled Loubar

Fethi Zagrouba

Mohand Tazerout

Thermogravimetric investigation and thermal conversion kinetics of typical North African and Middle Eastern lignocellulosic wastes

Keywords: Renewable energy, Pyrolysis, Combustion, Thermogravimetric analysis, Lignocellulosic byproducts, Kinetics

The aim of this work was to thermally characterize the renewable lignocellulosic bioresources derived from palm trees in order to highlight their energy potential. Pyrolysis and combustion behaviours of date stones (DS) agricultural by-products were tested by thermo-gravimetric analysis, and the main chemical compositions were analyzed. The work has also been conducted to identify their most important physical characteristics. The study of the sizes and heating rate effects constitute the first part of the experimental work. Inert atmosphere and three heating rates: 10, 20, and 50 °C/min, were applied to various particle sizes of DS. In the second part, tests were carried out in an oxidizing atmosphere (21% O 2 ) by varying the size of the DS. The kinetic parameters such as pre-exponential factor and activation energy were determined. Increasing the particle sizes and the heating rates didn't have an appreciable influence on the global weight losses. However, degradation rates were significant with the porous structure of the DS. Weight losses in inert and oxidizing atmospheres were found to occur in two stages (drying and devolatilization) and in three stages (drying, devolatilization, and oxidation of the char).

INTRODUCTION

The recognised effects of climate change have prompted an urgent appraisal of realistic alternative power generation options. There are various forms of alternative energy sources, such as solar, wind, biogas/biomass, tidal, geothermal, fuel cell, hydrogen energy, etc. The attraction of these sources lies primarily in their abundance and ready access. The Renewable Energy Technologies (RETs) for exploiting these sources include biogas plants, solar lanterns, solar home lighting systems, improved cook stoves, improved kerosene lanterns, solar water pumping systems, solar water heating systems, and water mills. Currently, these options cannot replace the totality of fossil fuel exploitation, but the basic concept of alternative energy relates to issues of sustainability, renewability, and pollution reduction.

In this context, one of these options is to replace a part of fossil fuel with a renewable energy source such as lignocellulosic biomass. Fossil fuels -first coal, then petrol, diesel, and natural gas -have dominated our fuel market. In spite of this, biomass in the form of fuelwood continues to be a major source of energy to this day.

Nearly 70% of India's cooking energy requirement and 32% of its primary energy requirement is met with biomass [START_REF] Mnre | Booklets on Renewable Energy, Ministry of New and Renewable Energy[END_REF]. Globally, fuelwood in its various forms accounts for about 64% of the estimated total world supply of combustible renewable energy sources [START_REF] Demirbas | Potential contribution of biomass to the sustainable energy development[END_REF]. The estimates of worldwide annual generation of electricity from biomass amount to about 185 TWh, of which nearly three quarters are produced from solid biomass, 14% from biogas, and 12% from municipal solid waste. If we consider the portion contributed by biomass to the world's total energy production, it comes to only less than 2% (World Energy Council 2007).

For some regions in the world, namely North African and Middle Eastern countries, fossil fuel sources of oil, gas, and coal are predictably running out. Pressures exerted by population and economic growth as well desalination needs increase demand for electricity [START_REF] Rogner | Prospects for nuclear energy in west Asia and North Africa[END_REF]. In fact, Tunisia and Morocco are already energy importers, while resources in Algeria, Libya, and Egypt are likely to be exhausted in a couple of generations. These countries must research other sources of energy recovery, with a special emphasis on the usage of agricultural waste.

The date palm tree has been considered an important resource for thousands of years. Dates are used as a raw material in many food industries, as well as a constituent in some animal feeds. In addition to the fruit itself, almost every part of the tree is put to use. Wastes of palm trees, which are produced in huge amounts in North Africa and the Middle East, have proved to be excellent biofuels [START_REF] Hamada | Preliminary analysis and potential uses of date pits in foods[END_REF]. [START_REF] Haimour | Utilization of date stones for production of activated carbon using phosphoric acid[END_REF] found that date stones constitute approximately 10% of the fruit's weight, while [START_REF] Al-Badri | The thermogravimetry and pyrolysis of date stones[END_REF] found that about 14% of the fruit's weight is waste material. Adapted from a number of literature references according to Food and Agriculture Organization (FAO) to agricultural services, carbohydrates are the major components of date stones [START_REF] Barreveld | Date palm products[END_REF]. Indeed, these palm trees by-products are composed of cellulose (42%), hemicellulose (18%), lignin (11%), ash (4%), sugar and other compounds (25%).

Energy processes based on lignocellulosic biomass can be divided into two broad categories: biological (fermentation and anaerobic digestion) and thermochemical (combustion, gasification, and pyrolysis). These latter processes can significantly and immediately reduce the mass and volume of wastes, and allow for energy recovery.

In order to evaluate the thermal decomposition of the material, thermoanalytical techniques, in particular thermogravimetry (TGA) and derivative thermogravimetry (DTG), provide basic information in a relatively simple and straightforward manner. Generally, neither a systematic classification of biomass fuels, based on thermogravimetric analysis, nor a general mechanism to interpret such measurements are available, especially for such specific types of lignocellulosic matter.

The aim of the present work was to study the pyrolytic and combustion behaviour of lignocellulosic material (palm by-products) and to obtain a variety of pyrolysis data by changing some factors, namely particle size, heat flow, and reaction atmosphere, with the ultimate purpose of using this kind of biomass for energy recovery and to promote the possibility of developing an uncomplicated thermal conversion technology system for copyrolysis, co-combustion, or co-gasification with wood. Thereafter, it will be interesting to develop new strategies on a larger scale, such as setting up facilities near small farms, making it possible to investigate the practical usage of lignocellulosic bioresources.

For the purpose of this research, date stones from palm trees were investigated: (named hereafter DS). Palm trees produce also another type of lignocellulosic byproduct, which is palm stalk (PS). These agricultural wastes originated from Djerid, an area located in the south of Tunisia. A particular attention has been paid to investigate thermal analyses for DS after confirming the close similarity of these two lignocellulosic by-products. After drying at a temperature that didn't exceed 50 °C during 72 hours, these palm by-products were ground to a desirable particle size and sieved to a powder of less than 1mm in diameter and kept in a desiccator to protect them from recapturing the hygroscopic moisture. DS was concerned for TGA/DTG analysis and kinetic parameter determination.

EXPERIMENTAL SETUP AND CHARACTERIZATION

In order to characterize DS and PS in a thermal setting, different analyses were conducted (Table 1).

Proximate Analyses

Moisture was determined by steaming at 105 °C for 24 hours, until a constant mass is achieved. The mineral matter was determined by calcinations at 550°C during 15 min, and the fat matter was quantified by the Soxhlet method.

The higher heating value of ground samples was measured using an adiabatic oxygen bomb calorimeter (Prolabo model) under 25 bars. The energy content of the dry matter was then calculated.

Ultimate Analyses

Carbon, hydrogen, and nitrogen compositions were determined by total combustion at 1050 °C under an oxygen flow. Carbon and hydrogen were transformed respecttively into quantified carbon dioxide and water, either by catharometry or by specific infrared detectors. Nitrogen from the samples was transformed into various oxides of nitrogen, which were reduced in molecular nitrogen before being quantified by thermal conductemetry. Sulphur content was determined by an analyzer type LECO (model SC 144) performing total combustion at 1350 °C under an oxygen flow. It was transformed into sulphur dioxide, quantified using an infrared-specific detector.

Chlorine was determined by mineralization in an oxygen atmosphere by a combustion type SCHONIGER. Quantitative obtaining of the element in the form of chlorine allows proportioning with silver nitrate by an ordinary potentiometer. Oxygen was determined by the difference. All the experimental tests were carried out in triplicate.

Structural Characterization

The morphology and the chemical distribution of the elements on the surface of raw material were examined using scanning electron microscopy, combined with the ability to generate localised chemical information through Energy dispersive X-ray spectroscopy (EDX) under ultra-vacuum (Fei Quanta 200).

X-ray patterns were recorded with a Panalytical X'Pert PROMPD diffractometer with a wavelength of 1.789 °A. The X-ray patterns were recorded in the 2θ range 0 to 70°w ith a scan rate of 0.017° min -1 . All the samples were films with similar thickness of 0.3 mm.

Thermogravimetric Analyses

The first part of TGA/DTG experiments was performed using a SETARAM/ SETSYS Evolution balance. Approximately 27 to 53 mg of DS samples were placed in the pan of the TGA microbalance, which was operated under a constant purge flow rate of 20 ml/ min of Argon under a pressure of 1032 mbar.

Three heating rates were applied (10, 20, and 50 °C/min) for 1.18 mm, 2.36 mm, and 4.75 mm of DS and the powdered form. Residual weight of these samples and the derivative of weight, with respect to time and temperature (differential thermogravimetry analysis, DTG), were recorded using TGA software. Temperature instructions went from 40 to 1000 °C.

The second part of the TGA/DTG experiments was carried out with a thermo standard balance SETSYS 1750 devoted to combustion under 21% of 0 2 at 10 °C/min.

RESULTS AND DISCUSSION

Ultimate and Proximate Analyses

The higher heating values of DS and PS were respectively 22.27 MJ.kg -1 and 19.66 MJ.kg -1 , which were higher than the energy content of a multiple agricultural byproduct such as rapeseed straw (17.64 MJ.kg -1 ) [START_REF] Karaosmanoglu | Determination of reaction kinetics of straw and stalk of rapeseed using thermogravimetric analysis[END_REF], pine chips (18.98 MJ.kg -1 ) [START_REF] Sensoz | Pyrolysis of pine (Pinus brutia Ten) chips: 1. Effect of pyrolysis temperature and heating rate on product yields[END_REF], rice husk (14.42-18.31 MJ.kg -1 ) (Mansaray and Ghaly 1997a), and wood (19.10 MJ.kg -1 ) [START_REF] Kastanaki | A comparative reactivity and kinetic study on the combustion of coal-biomass char blends[END_REF]. Moisture related to the rough weight is about 8.00% and 5.8% for DS and PS, compared to 20% for dried wood and 45% for fresh wood. Table 1 summarizes the composition of DS and PS.

Scanning Electron Microscopy/ EDX Ultra-Vacuum

Scanning electron micrographs of raw DS and PS are shown in Fig. 1. The examination of the initial structure of the DS shows the presence of a few macropores (Fig. 1a) of various size and geometry on the surface. However, it is a fibrous form of PS (Fig. 1b). More than one zone of the surface was analysed, since the chemical elements distribution is heterogeneous (Table 1).

These surfaces were the first contact field for many thermal transfer interactions during the thermal conversion process of the matter. With regard to the mineral content (Tables 2 and3), it was found that the external surfaces of DS show appreciable amounts of carbon, and oxygen. Among the elements present in a few amounts in the DS matter are cobalt, aluminium, silicon, etc. Iron, chlorine, phosphorus, magnesium and others element were considered as traces. Similar to the DS sample, external surfaces of PS show appreciable quantities of carbon, and oxygen, rather a significant amounts of chlorine and potassium. Silicon, calcium, phosphorus and potassium are known to be plant nutrients and soil improvement agents [START_REF] Steenari | Stabilisation of biofuel ashes for recycling to forest soil[END_REF]. The distribution of chemical elements shows that the raw materials are highly heterogeneous depending on the form of the ground DS and PS. The potassium ion is one of the most abundant ions that naturally occur in native material.

It is important to identify the inorganic element distributions since they have an important effect on the pyrolysis temperature and on the products' composition (char, tar, and gases).

On the other hand, when some research opts for a large scale application to investigate this lignocellulosic matter, it is necessary to highlight that high amounts of inorganic constituents in some lignocellulosic biomasses contribute to adverse impact on the different elements of the conversion systems. The chlorine and potassium in biomass are water-soluble, they can largely be removed through leaching, thus mitigating their impact on high temperature conversion devices. Water washing is a means to eliminate some metals, thus improving behaviour during thermal treatments [START_REF] Jenkins | On the properties of washed straw[END_REF]. Ash constituents, especially potassium, sodium, and calcium, act as catalysts for the decomposition process and favour char formation [START_REF] Jensen | TG-FTIR study on the influence of potassium chloride on wheat straw pyrolysis[END_REF][START_REF] Fahmi | The effect of alkali metals on combustion and pyrolysis of Lolium and Festuca grasses, switchgrass and wilow[END_REF]. Chemical analyses of inorganic elements in the raw materials are given in Table 4. It can be noticed that this type of by-product shows an absence of heavy metals.

XRD Analysis

X-ray diffraction patterns of natural DS and PS are given in Fig. 2. The diffractogram of natural DS does not exhibit a horizontal basic line. This shows that the major part of the matter is amorphous. However, a few diffraction peaks emerge from the basic line, indicating the presence of a small amount of crystalline matter. The XRD pattern of natural date pits has been compared to those of native cellulose (C 6 H 12 O 6 ), xylane dehydrate (C 10 H 12 O 9.2 H 2 O), or hemicellulose dehydrate given in the JCPDF crystallographic data base (Table 5) [START_REF] Zhao | Interactions between cellulose and Nmethylmorpholine-N-oxide[END_REF]). Characterization studies on biomass samples are quite important to judge suitability of feedstock for thermochemical conversion. The two lignocellulosic materials presented and characterized above showed a very close similarity. However, because of the important higher heating value of DS sample, we have chosen DS to investigate thermal gravimetric analyses and to show more detail for the thermal degradation behaviors.

Inert Atmosphere

TGA analyses were operated under a constant purge flow rate of 20 mL/min of Argon. Three heating rates were applied at 10, 20, and 50 °C/min.

Typical representations of TGA/DTG diagram for DS are shown in Fig. 3 under inert atmosphere and with a heating rate of 10 °C/min. The raw DS diagram shows three stages; drying, fast pyrolysis, and slow pyrolysis.

The first stage of weight loss ranged from 47 °C to around 160 °C (Fig. 3). The derivative plot (DTG) had a separate peak for this step of thermal process, which was attributed to the moisture loss and probably some volatile compounds in the biomass sample so that the low moisture content resulted in low weight loss during this stage.

The second stage corresponding to fast pyrolysis had three zones. Following this stage, the first zone (Z 1) showed a negligible weight loss (≈ 5%) in the temperature range of 135 °C to 230 °C, this phase was identified as the beginning of the decomposition of cellulose and hemicellulose. The second zone (Z 2) started around 230 °C and extended to 330 °C. Weight loss during this stage was rapid; it was the active pyrolysis step so the second stage was designated as the fast pyrolysis.

Zone 3 of the fast pyrolysis stage showed a rapid weight loss, but relatively less important (20%) than the previous stage, which probably corresponds to the first step of lignin decomposition; this indicates that the amount of lignin is significant in the raw DS.

It was concluded that the most distinguished peak in fast pyrolysis stage is related to the complete pyrolysis of hemicellulose, which took place until 350 °C, while the pyrolysis of cellulose occurred from 250 °C to 500 °C (Fig. 3). The same results have been reported in the literature concerning thermal degradation of rice husks (Manasary and Ghaly 1998c; [START_REF] Varhegyi | Kinetic modelling of biomass pyrolysis[END_REF]. Kilzer and Broido (1965) have proposed a general kinetic scheme for the pyrolysis of pure cellulose. Its decomposition would occur according to two competitive reactions occurring directly from cellulose. The first one (473 to 553 °C) is a slightly endothermic reaction of dehydratation, followed by an exothermal process that produces char and light gaseous species. In the second one (553 to 613 °C), cellulose is postulated to be transformed into an intermediate and unstable compound. A similar scheme has been proposed by Arsenau (1971).

The derivative plot of the region between 230 °C and 500 °C showed two observable peaks but a slow degradation of biomass continued until 900 °C, at which point complete devolatilization occurred.

It can be seen that the lignin degradation was slow and occurred over a wide range of temperatures (Vamvuka et al. 2003a). Following this stage, there was a continuous and slow weight loss from 500 °C to 1000 °C; this stage was so-called "slow pyrolysis," and could be a thermal continuity of decomposition of lignin and other compounds with high molecular weights. Such thermal behaviour can be explained by the structural differences in lignin, cellulose, and hemicellulose. Cellulose and hemicellulose are basically composed of polysaccharides. On the other hand, lignins are complex racemic polymers derived mainly from three hydroxycinnamyl alcohol monomers that differ in their degree of methoxylation and are heavily cross-linked. Lignin polymerization is characterized by some typical inter-unit linkages.

Thus, lignin has a high thermal stability and it is difficult to decompose [START_REF] Yang | In-depth investigation of biomass pyrolysis based on three major components: Hemicellulose, cellulose and lignin[END_REF]. Lignin pyrolysis takes place at a pyrolysis temperature range from 150 °C to 900 °C, and no sharp weight loss peak appears [START_REF] Li | Fast pyrolysis of biomass in free-fall reactor for hydrogen-rich gas[END_REF]). Since the cellulosic compounds have the structure of branching a chain of polysaccharides and no aromatic compounds, that's why their volatilization is easy [START_REF] Gani | Effect of cellulose and lignin content on pyrolysis and combustion characteristics for several types of biomass[END_REF]. [START_REF] Park | Experimental and theoretical investigation of heat and mass transfer processes during wood pyrolysis[END_REF] reported that wood consists of three major components: hemicellulose (25-35%), cellulose (40-50%), and lignin (16-33%), a composition range similar to DS. Each component displays different characteristics during its thermal decomposition. Hemicellulose decomposition occurs at 200-260 °C and produces acetic acid. Cellulose decomposes to levoglucosan and dehydrocellulose at 240-350 °C. Lignin decomposes over a broad temperature range of 280-500 °C and produces more char than the other two components. These temperature ranges for degradation also describe the thermal behaviour of DS, so it is interesting to plan for copyrolysis of wood and DS at a large scale.

Effect of heating rate during pyrolysis of date stones

Figures 4 and5 illustrate the influence of the heating rate on the degradation process of DS (˂100 µm and initial weight ˂ 40 mg). Few workers have discussed testing the effect of heating rate on the devolatilization behaviour of biomass. The most common ramp rate reported in the literature for TGA characterisation of biomass appears to be 10 °C/min [START_REF] Vamvuka | Devolatilization and combustion kinetics of low-rank coal blends from dynamic measurements[END_REF]), 20-30 °C/min [START_REF] Peralta | Development of a reactivity test for coal-blend combustion: The laboratory-scale suspension-firing reactor[END_REF], 50 °C/min [START_REF] Moghtaderi | Pyrolytic characteristics of blended coal and woody biomass[END_REF]), or 100 °C/min [START_REF] Pan | Pyrolysis of blends of biomass with poor coals[END_REF], with relatively few using lower heating rates. It is therefore useful to investigate different heating rate profiles. Heating rate had an effect on the temperature range of the stages of the reaction (Fig. 4). Increasing the heating rate appeared to increase the range of temperatures of each stage, which may have been due to the heat transfer limitations.

Three temperatures were recorded: T ini is the temperature where weight loss first reached 0.5 mg/min; T peak is where the weight loss reached its maximum; and T end is where the weight loss fell below 0.5 mg/min. The limit of 0.5 mg/min was established using actual profiles as an arbitrary limit for the start and end for peaks (Fig. 4). It was expected that the peak temperature would increase with an increasing heating rate. However, in this study, for 50 °C/min there was a lower peak temperature than that registered for 10 °C/min. In fact, high heating rates and large mass loading tend to deflect the measured temperature from the actual sample temperature. It is clear that by increasing the heating rate, a longer time may be required for the purging gas to reach equilibrium with the temperature of the furnace or the sample because of the heat transfer limitations.

Char characterization

Characterization studies on biomass samples are quite important to express suitability of feedstock for thermochemical conversion. High volatile matter content of biomass with low ash and sulphur content is the main criterion for pyrolysis conversion. From the main characteristics of the raw material (Table 1), it appears that the high volatile content of lignocellulosic biomass favours pyrolysis conversion.

Scanning electron microscopy (SEM) was used to characterize the size and the shape of the char particles, as well as their porous surface structure (Fig. 6). Pyrolysis temperature and heating rate influenced the size and shape of particle due to a general increase in size and proportion of voids and a decrease in cell wall thickness. For DS biomass it seemed be more favourable to apply pyrolysis at 500 °C, where the most highly porous and spherical particles were observed. Fast pyrolysis of DS does not favour the thermal conversion of the matter. At low pyrolysis temperature (Fig. 6b) and low heating rate, no porous structure was seen; the particles were thick-walled and covered by tar agglomerates. The wood sample presented a slightly porous surface at 600 °C and at a slow heating rate; wood samples maintain a rigid aspect, which is due to a natural lignin-developed structure. The diffractogram of DS having undergone pyrolysis under inert gas (Fig. 7) showed significant effects of pyrolysis: several peaks disappeared due to the decomposition of cellulose and hemicellulose during thermal conversion. After pyrolysis, the small amount of crystalline matter was destroyed and no peak of diffraction remained on the baseline of the diffractogram. The nanometric composition of pyrolysed matter was dominated by the presence of carbon. Similarly to wood, DS is mainly composed of the cellulose (≈ 50%). A study has been made on the kinetics of the thermal decomposition of the crystalline cellulose in wood by in situ X ray diffraction [START_REF] Zickler | In situ X-ray diffraction investigation of thermal decomposition of wood cellulose[END_REF]; in this case the fiber diffraction patterns were studied as a function of heat treatment under temperature ranging from 573 to 633K.

The results show a thermally activated reduction in the microfibril diameters with random breaking of the fibers into shorter species due to anisotropic nature of the material. In our case, we have applied a pyrolysis temperature which increases from the ambient temperature to 1000 °C. The material became amorphous and no peak was discerned.

Effect of particle sizes during pyrolysis of date stones

Figures 8 and9 show, respectively, mass loss rates and weight loss for different DS sizes with a fixed heating rate of 10 °C/min. The effects of biomass particle size on its pyrolysis behavior can be directly assessed only for the particles prepared using similar milling and sieving methods for similar types of biomass. Broadly, samples show the same curves for different particle sizes. Particle sizes slightly influence temperature ranges, but more so influence the mass loss rate (Fig. 8). 3) and on the same range of temperature for all particle sizes, curves of mass losses show that mass loss rates increased with increasing particle sizes. In this stage, the major compound degrading is hemicellulose, particularly cellulose. Figure 9 shows that the mass losses during active pyrolysis of different particles size varied slightly; however, the variation was more significant when comparing DS powder with DS of 4.75 mm. This presents a difference of around 7% of mass loss. In fact, the degradation rate was favored by the macroporous structure of DS, which remains increasingly intact when the shape and the size of the particles approaches the rough state. As for the lignin decomposition, starting from 500 °C to the end of decomposition, mass loss increased with increasing particles size (Fig. 9), and no effect over the total duration of the decomposition process was observed. The decomposition of lignin was complete around 900 °C.

Air Atmosphere

Figures 10 and 11 show weight loss and loss rate for three various particles size of DS under an oxidizing atmosphere with 10 °C/min of air. TGA plots suggested that there were three stages of weight loss. For 1.18 mm particles of DS, the first stage of weight loss from 48 to 175-180 °C was similar to the first stage in the inert atmosphere. The loss of water and the volatilization of light molecules may have contributed to weight loss in this stage. After that, in the second and third stage, there was minimal variation in the characteristic profiles for each size fraction. For 4.75 mm particles of DS, there were no significant differences compared to the other sizes. Indeed, the mass loss rate reached a maximum of about 0.1 min -1 at 295 °C vs. 0.06 min -1 and 0.09 min -1 , respectively, for 1.18 mm and 2.36 mm DS particles for the same temperatures. From Fig. 10 it is clear that our TGA experiments were mass transfer limited; however, DTG experiments (Fig. 11) did not show any real influence of increasing particle size in the conditions of 10 °C/min of air and an important initial weight of raw material, so we have to identify the optimum heating rates.

Hence, the effects as a result of particle size may have been negated by the choice of a very slow heating rate, allowing the particles to swell and devolatilize being controlled only by furnace temperature. Moreover, for 2.36 mm and 4.75 mm particles, it is clear that following water vaporization in the first phase, two other stages of thermal degradation are significant. The first stage can be identified with the degradation of hemicellulose, and the second with cellulose. The decomposition of lignin proceeds in the last stage (slow degradation) (Fig. 10). The second phase of weight loss ranged from 240 °C to 400 °C, which was more rapid than the other phases. In this region, there was a very important mass loss for all particle sizes, corresponding to the removal of volatile matter. Similar observations have been made, confirming the low reactivity of lignin solid matrix when the temperature of degradation is below 350 °C to 400 °C [START_REF] Sharma | Characterization of chars from pyrolysis of lignin[END_REF].

can be concluded that at a slow heating rate of 10 °C/min in an air atmosphere, the weight loss in the first stage was less than 5%, which is similar to that obtained in the inert atmosphere conditions. However, in the second stage of thermal degradation, there was a very large weight loss (˃ 85%) as compared with the value registered in the inert atmosphere (≈ 65%), which is the result of char oxidation.

Kinetic Study

Chemical kinetics of biomass pyrolysis and combustion degradation coupled with the description of transport phenomena provide advanced computational tools for the design and optimization of chemical reactors used for thermochemical conversion of wood and biomass. The knowledge of the fuel reactivity is also needed for the formulation of simple design and scaling rules.

Thermo-chemical conversion of biomass results from a strong interaction between chemical and physical processes at the levels of both the single particle and the reaction environment. This why kinetic studies of this lignocellulosic material are needed as a tool for the prediction of reactor performance. The literature does not provide any information relating to the kinetic parameters of DS wastes. Nevertheless, a comparison with the values generally recorded for other biomasses give a rough estimate. Several models are applied in the literature.

The kinetic parameters of DS degradation reactions during pyrolysis are based on the Arrhenius equation, which is also used by many researchers (Karasosmanoglu et al. 2001). This model doesn't fix the reaction order (n); rather, this parameter is determined by linear regression.

The overall kinetic reaction is written as follows,

) (x kf dt dx = (1)
where x is the weight conversion ratio defined as,

        - - = f f w w w w x 0 (2)
During decomposition, w 0 is the initial biomass sample decomposed to sample mass w at time t, where k is the rate constant and f(x) = (1-x)n and n is the order of reaction. According to the Arrhenius correlation the rate constant k is defined as:

RT E Ae k - = (3)
In Eq. 3, A (s -1 ) is the frequency factor, E (KJ/mol) is the activation energy, T (K) is the reaction temperature, and R (J/K/mol) is the gas constant. Thus we can write the following equation:

( )         - +       - =         - - f f f w w w w n RT E A dt dw w w 0 0 ln ln 1 ln (4)
The linear form of this equation is,

Dz Cx B y + + = (5) where       - - = dt dw w w y f 0 1 ln , ( ) n D R E C A B w w w w z T x f f =       - = =       - - = = ln ln 1 0
The constants B, C, and D were estimated by multi-linear regression of the TGA for each stage using Microsoft Excel. Through previous analysis of thermal degradation behaviour, the weight loss is more important during fast pyrolysis (active degradation), which is why kinetic parameters of this phase must be specified. Beyond the fast pyrolysis, lignin decomposition continues until over a broad temperature range of 700 to 900 °C. It is a slow degradation, characterized by an order 0 of the reaction, indicating a low rate of weight loss.

A difficulty in kinetic analysis also exists in separating the effects of chemistry and transport phenomena. One of the key points in relation to intrusion of heat and mass transfer processes in kinetic analysis is the effect of sample sizes/masses during pyrolysis, which cause spatial gradients of temperature (a process taking place under nonnegligible effects of internal heat transfer) or significant differences of temperature between the controlling thermocouple and the sample, especially when these are not in close contact.

Morphology as well as the chemical structure of lignocellulosic sample are the indicative intrinsic parameters of the nature of thermal behaviour during degradation. Indeed the porous structure of DS and the significant quantity of cellulose (42%) enhance thermal degradation.

Table 6 illustrates the variation of the kinetic parameters of the DS sample during pyrolysis in an inert atmosphere in Argon with 10 °C/min for various particle sizes and heating rates. Kinetic parameters of DS degradation reactions for different particles size showed an order of 2.5 and activation energy of about 58 KJ/mol with a value of R 2 ≥ 0.8 in all cases. On one hand, these results constitute a good approximation. On the other hand, it is clear that in pyrolysis degradation particles size has no important influence on the kinetic parameters.

Table 6. Kinetic Parameters during Active Pyrolysis

It should also be noted that, for the conditions of the thermal analysis, the process of lignin and cellulose decomposition are considered to be globally exothermic and endothermic, respectively [START_REF] Bilbao | Temperature profiles and weight loss in the thermal decomposition of large spherical wood particles[END_REF]. During this primary decomposition, secondary reactions effect was kept at a minimum by maintaning the reaction temperature above 800 °C [START_REF] Di Blasi | Pyrolytic behavior and products of some wood varieties[END_REF].

In this global thermal process the data were examined by taking into account the temperature range investigation, which has an upper limit of 900 to 1000 °C. The limited range of temperature of 500 to 560 °C was needed to determine activation energy. This range of thermal process describes the fast pyrolysis phase (Fig. 3). In this case, heating rate had a significant effect on the activation energy variation.

From Table 6 it can be observed that a higher heating rate decreased the activation energy. As it was explained through the pyrolysis thermal degradation process, heating rate influenced the temperature ranges of the pyrolysis process. Also, the thermal degradation rates increased with the heating rate (Fig. 4), which means that global kinetic evolution was favored in this thermal conversion conditions. Combined effects of changes in decomposition temperature ranges at the different heating rates and the kinetics of the devolatilization were observed.

Contrary to the results shown for pyrolysis kinetic degradation (Table 6), for which activation energy was not influenced by particle size, combustion degradation behaviour (Table 7) showed an increasing value of activation energy with increasing particle size.

It seems that the thermal degradation of the DS under oxidizing conditions was influenced by thermal and mass transfer limits.

CONCLUSIONS

1. The large amount of available palm trees makes palm biomass resources ideal candidates as future feed stock for bioenergy production with a low cost of pretreatment experiments. 2. This study addresses for the first time (on a scientific and technical basis) the feasibility of using palm tree by-products, i.e. date stones and palm stalk, as new renewable sources of energy. Indeed, it is important to identify the main physicochemical characterization, and the thermal behavior of these palm trees byproducts, in order to extrapolate their energy conversion by combustion and pyrolysis. 3. Properties of these biomasses are interesting from a thermal point of view. However, heating values are even greater than the HHV of wood and for other biomasses quoted in the literature. 4. All of the physico-chemical and thermogravimetric analyses affirm that the thermal valorization of these kinds of palm by-products constitutes a significant alternative to implement a potential new renewable energy source. 5. The physico-chemical and morphological study of these wastes shows that they are characterized by a relatively low moisture content, a high HHV, and an absence of heavy metals, compared to other biomasses. 6. Through thermogravimetric analyses, for both samples and other biomasses, weight losses in inert and oxidizing atmospheres were found to occur respectively in two and three stages (drying -devolatilization and oxidation of the char in combustion tests). 7. The major components of DS (hemicelluloses, cellulose, and lignin) influence the mass loss rate during thermal degradation.

Fig. 1 .

 1 Fig. 1. Scanning electron micrographs of (a) DS and (b) PS samples

Fig. 2 .

 2 Fig. 2. XRD patterns of (a) (DS) and (b) (PS)

Fig. 3 .

 3 Fig. 3. Typical TGA diagram of DS (powder) in an Argon atmosphere (10 °C/min)

Fig. 4 .

 4 Fig. 4. Effect of heating rate on DTG during pyrolysis of DS

Fig. 6 .

 6 Fig. 6. SEM photomicrographs of char obtained at (a) 500 °C (50 °C/min), (b) 400 °C (10 °C/min), (c) 600 °C (100 °C/min) for DS and (d) 600 °C (10 °C/min) for wood

Fig. 7 .

 7 Fig. 7. Diffractogram of pyrolysed DS

Fig. 8 .

 8 Fig. 8. Mass loss rate of the different particles size of DS

Fig. 9 .

 9 Fig. 9. Effect of particle sizes on TGA during pyrolysis of DS

Fig. 11 .

 11 Fig. 11. Effect of particle size on DTG during combustion of DS

Fig. 10 .

 10 Fig. 10. Effect of particle size on TGA during combustion of DS

Table 1 .

 1 Proximate and Ultimate Analyses of Palm Trees By-products

	Ultimate analyses (wt % dry)	DS sample	PS	Wood (Kastanaki et al. 2006)
	C	45.80	44.52	49.60
	H	6.28	5.73	6.58
	O	46.60	48.08	37.35
	N	0.81	0.17	4.19
	S	0.22	0.00	0.12
	Cl	<0.20	nd	nd
	Proximate analysis (wt% dry) Ash	1.12	1.50	2.16
	Fat	8.00	2.00	-
	Volatile matters	45.08	48.18	76.33

Table 2 .

 2 EDAX Quantification of DS and PS surface (Powder)

		DS			PS
	Elements (wt%) C O Co Mg Al Si P S Cl K	Zone 1 64.53 33.63 0.93 0.08 0.17 0.28 0.04 0.03 0.03 0.14	Zone 2 62.26 35.85 0.97 0.09 0.17 0.27 0.04 0.04 0.04 0.15	Zone 1 66.80 31.14 0.82 0.12 0.19 --0.06 0.34 0.47	Zone 2 62.52 30.37 -0.16 0.11 0.09 -0.18 2.65 3.92	Zone 3 71.34 27.48 0.84 0.10 0.06 0.03 -0.02 0.04 0.09
	Ca Fe Na	0.07 0.06 -	0.08 0.05 -	--0.06	---	---

Table 3 .

 3 EDAX Quantification of DS and PS surface (Particle)

	DS	PS

Table 4 .

 4 Elemental Compositions of the Raw Materials

	Elements mg/g	DS	PS
	Cu Mg Fe K Ca Mn Cd Zn Al	0.06 1.26 1.2 6.36 1.73 0 0 0 0.4	0.06 2.43 0.2 27.06 7.27 0 0 0.56 0.06

Table 5 .

 5 Bragg Diffraction Angle (°2Th) and Reticular Distance (d) of DS and their Corresponding Compounds Identified in the JCPDF Crystallographic Data Base (N.Cl : native cellulose, HCel : hemicellulose dehydrate, C : carbon)

	Pos. [°2Th.]	d( Å)	Corresponding compounds
	18.4921	5.57122	N.Cel and HCel
	22.9887	4.49214	N.Cel and HCel
	27.4209	3.77678	C
	30.8246	3.36824	HCel
	38.5353	2.71274	N.Cel and HCel

Table 7 .

 7 Kinetic Parameters during Active Combustion/Particle Size Effect

		Active pyrolysis (°C)	A (s	-1 )	E (kJ/mol)	n	r	2
	Heating rate (°C/min)
	10 20 50 Particle sizes (mm) at 10°C/min 1.18 2.36 4.75	165-500 170-490 150-500 170-560 170-470 170-480	720.53 219.20 88.23 830 586 1040	58.4 52.4 44.0 58.3 57.5 59.8	2.83 2.28 2.22 2.78 2.43 2.52	0.9 0.8 0.9 0.9 0.8 0.8

20.0 µm 20.0 µm

ACKNOWLEDGMENTS

This work was realised because of the contribution by the Institut National de Recherche et d'Analyse Physico-chimique, Tunisie. The authors want to express their thanks to Mr. Naser Hamdi, assistant professor at Institut Supérieur des Sciences et Technologies de l'Environnement, for his helpful discussion during the work.


Julien P Chanal
email: julien.chanal@pse.unige.ch

Philippe G Sarrazin
email: philippe.sarrazin@ujf-grenoble.fr

Frédéric Guay

Julie Boiché

Verbal, Mathematics, and Physical Education Self-Concepts and Achievements: An Extension and Test of the Internal/External Frame of Reference Model

Keywords: Academic Achievement, Academic Self-Concept, Internal/External Frame of Reference Model, Physical Activity, Physical Education

 posits that verbal achievement (Ach) produces a lower mathematics self-concept (SC) and that, conversely, mathematics Ach fosters a lower verbal SC after controlling for prior Ach in each domain. The present study extended this model by adding another school subject that has not been previously investigated, namely physical education (PE). The central goal was to verify whether Ach in PE positively or negatively predicts academic SC in the mathematics and verbal domains. Design: Prospective study over one school year. Methods: Participants were 451 French high school students (mean age = 13.5). Ach scores in mathematics, French and PE were gathered at the beginning of the school year. At the end of this one, students completed a questionnaire to assess their SC in the three school subjects. Results: Structural Equation Modelling results revealed, according to the I/E model, negative paths between mathematics Ach and verbal SC, and between verbal Ach and mathematics SC. Results also revealed singular relationship patterns between the two traditional school subjects and PE. A positive path between PE Ach and mathematics SC was observed whereas the path with the verbal SC was non-significant. Conclusion: PE could have benefits to increase SC in other school subject such as mathematics. Broader implications of results for the I/E model are discussed.

Self-concept theory suggests that the perceptions students have of ability in a given school subject are not an epiphenomenon, but rather an important cognitive state that drives behavior and academic achievement (e.g., [START_REF] Marsh | Theoretical perspectives on the structure of selfconcept[END_REF]. In the past decades, researchers have explored the relations between achievement and self-concept in various school subjects (e.g., [START_REF] Byrne | Self-concept/academic achievement relations: An investigation of dimensionality, stability and causality[END_REF]. Studies have highlighted the paradoxical finding that students' achievements can simultaneously enhance and decrease selfconcept. Specifically, self-concept studies generally indicate that mathematics and verbal achievements are strong predictors of corresponding self-concepts in these school subjects, but at the same time, achievement in one school subject negatively predicts selfconcept in another subject after controlling for previous achievement in the matching domain. This indicates that self-concept in a given subject is influenced not only by specific achievement in that subject, but also by achievement in other subjects. These patterns of relations have been investigated under the well-known Internal/External Frame of Reference Model (I/E model, [START_REF] Marsh | Verbal and mathematics self-concepts: An internal/external frame of reference model[END_REF], which has proven quite robust in many different cultures [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF].

Up to now, most studies on the I/E model have focused on the verbal and mathematics domains, but, as argued by Marsh, [START_REF] Marsh | Extension of the internal/external frame of reference model of self-concept formation: Importance of native and non-native languages for Chinese students[END_REF], "… the theoretical underpinning of the model suggests that this frame of reference should be a function of all different school subjects (although the amount of weight put on each may vary)" (p. 545). However, could achievement in certain less academic school subjects produce a different picture? For example, could achievement in physical education have positive effects on mathematics and verbal self-concepts, after controlling for achievement, instead of negative effects, as predicted by the I/E model? In fact, a host of studies have shown that physical activity has beneficial effects on students' grades, coursework selection, and academic self-concept (see [START_REF] Marsh | School athletic participation: Mostly gain with little pain[END_REF].

We therefore focused on answering these questions by assessing achievement and self-concepts in verbal, mathematics, and physical education subjects. In the following sections, we present a more detailed discussion of the I/E model and the empirical findings to date, followed by some studies that focus on the effects of physical activity on academic self-concept and achievement. Finally, we present the specific goals of this study.

The I/E model

According to the I/E model, students' academic self-concepts are influenced by both internal and external comparisons. External reference refers to a social comparison process by which students compare their self-concept of ability in a particular school subject with the accomplishments of other students in the same school subject. This is based on the assumption that social membership provides a frame of reference, or standard of comparison, which is used by the group members in their self-evaluations (e.g., [START_REF] Chanal | Big-Fish-Little-Pond effects on gymnastics self-concept: Social comparison processes in a physical setting[END_REF]. Internal comparison refers to the process by which students compare their performance in a particular school subject to their performance in another subject, independently of social comparison (e.g., [START_REF] Marsh | Multidimensional self-concepts: Relationships with age and academic achievement[END_REF]. The external comparison process predicts that better accomplishments in one domain would lead to higher self-concepts in the matching domain. According to the internal comparison process, however, better accomplishments in one domain should lead to lower self-concepts in the non-matching domain, once the positive effects of matching accomplishment are controlled for. For example, students with average achievement in mathematics would gauge their mathematics self-concept depending on whether their achievement in the verbal domain is below or above average. Below average achievement in the verbal domain would lead students to have slightly above average mathematics self-concepts, because the internal comparison process would lead them to slightly overestimate their abilities in mathematics. Conversely, above average achievement in the verbal domain would lead students to have slightly below average mathematics self-concepts. Thus, the I/E model hypothesizes: 1) a strong and positive correlation between mathematics achievement and verbal achievement; 2) a positive path between academic achievement and self-concept in the same domain; 3) a weak correlation between verbal and mathematics self-concepts; and 4) a negative path between achievement in one school subject to self-concept in another subject (see [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF].

Several studies conducted in different countries have supported postulates of the I/E model for the verbal and mathematics domains (e.g., [START_REF] Abu-Hilal | Self concept: The generalizability of research on the SDQ, Marsh/Shavelson model and I/E frame of reference model to the United Arab Emirates students[END_REF][START_REF] Dai | Incorporating parent perceptions: A replication and extension study of the internal-external frame of reference model of self-concept development[END_REF][START_REF] Kong | Chinese students' self-concept: Structure, frame of reference, and relation with academic achievement[END_REF][START_REF] Lee | Academic self-concept of talented students: Factor structure and applicability of the internal/external frame of reference model[END_REF][START_REF] Marsh | Extension of the internal/external frame of reference model of self-concept formation: Importance of native and non-native languages for Chinese students[END_REF][START_REF] Skaalvik | A test of the Internal/External Frame of Reference Model at different levels of mathematics and verbal self-perception[END_REF]. For example, [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF] performed a cross-cultural study in 26 countries among 55,582 participants, revealing that, despite some disparities among countries, invariance tests supported the generalizability of the model across cultures. Some studies have attempted to extend the I/E model to other academic subjects.

For instance, in a 5-year longitudinal study based on a large representative sample of high school students in Hong [START_REF] Marsh | Extension of the internal/external frame of reference model of self-concept formation: Importance of native and non-native languages for Chinese students[END_REF] tested the I/E model using three school subjects: Chinese, English, and Mathematics. Results showed that Mathematics, English, and Chinese achievements had positive effects on matching self-concept dimensions, but, as expected, these achievements also had negative effects on non-matching domains. Similarly, [START_REF] Marsh | An extension of the internal/external frame of reference model: A response to Bong[END_REF] demonstrated that Mathematics, English, and Spanish achievement all had positive effects on matching selfconcepts, but negative effects on self-concepts in other dimensions. [START_REF] Sparfeldt | Block versus randomized format of questionnaires: A confirmatory multigroup analysis[END_REF] tested the I/E model using Mathematics, Physics, German, and English. Results confirmed that, among the twelve path coefficients between grades and self-concepts, seven were negative, four were not statistically significant, and only one coefficient was positive, between grades in mathematics and self-concept in physics.

In sum, although one study demonstrated a positive effect of achievement in one school subject on students' self-concept in a non-matching school subject, negative effects appear to be the most common result found in studies testing the I/E model for school subjects other than Mathematics or Verbal.

Physical activity, academic self-concept, and academic achievement A number of studies have tested the effects of physical activity on a host of educational outcomes. Participation in interscholastic sports, intramural sports, or athletics was associated not only with reduced dropout rates (e.g., [START_REF] Mahoney | Do extracurricular activities protect against early school dropout?[END_REF] and higher grades (e.g., [START_REF] Marsh | School athletic participation: Mostly gain with little pain[END_REF], but also with higher social and academic self-concepts (e.g., [START_REF] Marsh | The effects of participation in sport during the last 2 years of highschool[END_REF]. For example, [START_REF] Marsh | School athletic participation: Mostly gain with little pain[END_REF] showed that the more students participated in physical activity, the higher their grades and self-esteem. Studies have also established a positive relationship between physical fitness and academic achievement. For example, [START_REF] Castelli | Physical fitness and academic achievement in Third-and Fifth-Grade Students[END_REF] showed that physical fitness indices (i.e., aerobic capacity and body mass index) were related to academic achievement in third-and fifth-grade students. More precisely, children who displayed better physical health (as assessed by high aerobic capacity and low body mass index) showed, among others, better performance in mathematics and reading. Despite these findings, no studies to our knowledge have investigated the effects of achievement in physical education on mathematics and verbal self-concepts. Although various positive effects of physical activity on academic achievement and self-concept have been found, the results were mostly observed in contexts where physical activity was not a compulsory academic school subject for all students. Therefore, when assessing the effects of achievement in physical education on self-concepts in other school subjects, would we observe negative or positive effects? In fact, students might form their selfconcepts in various school subjects using the internal and external comparison processes of the I/E model, for a negative effect of achievement in physical education on mathematics and verbal self-concepts. Alternatively, we could argue that students might use their achievement in physical education to foster their mathematics and verbal selfconcepts. Specifically, some models (e.g., [START_REF] Broh | Linking extracurricular programming to academic achievement: Who benefits and why?[END_REF] suggest that physical activity may contribute to academic skills by fostering work ethics, respect for authority, and perseverance. Moreover, the positive effects of physical activity on children's cognitive functioning have been supported in recent meta-analyses (e.g., [START_REF] Etnier | The relationship between physical activity and cognition in children: A meta-analysis[END_REF].

Thus, better grades in physical education could indicate that students have developed cognitive skills (e.g., problem solving skills, attention process, and decision making) that are beneficial for more academic school subjects. In turn, these skills could foster higher self-concepts, thereby leading to a positive relationship between achievement in physical education and self-concepts in other school subjects. Finally, we could assume that students might be unable to use an internal frame of reference based on their achievement in physical education when it comes to assessing self-concept in more academic school subjects. In fact, physical education differs significantly from other school subjects, principally because achievements in this area reflect physical rather than intellectual skills. Therefore, the internal frame of reference effect might be less likely to occur between achievement in this subject and self-concepts in school subjects that require intellectual skills.

The present investigation

The central goal of our study was to verify whether achievement in physical education positively or negatively predicts academic self-concepts in the mathematics and verbal domains. The study was conducted in a French high school where physical education was compulsory for all students. We chose two traditional school subjects that have been investigated in previous studies (i.e., mathematics and verbal: in this case, French) in addition to physical education. The model tested is illustrated in Figure 1.

In line with previous work (e.g., [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF], we hypothesized that correlations between achievements in the various domains would be relatively strong, whereas correlations between self-concepts would be fairly weak. Moreover, we predicted that achievements in each traditional school subject (i.e., mathematics and verbal) would positively predict corresponding self-concepts and negatively predict noncorresponding self-concepts. In addition, we posited that achievement in physical education would positively predict physical education self-concept. We did not make any predictions about the effects of achievement in physical education on mathematics and verbal self-concepts.

Method

Participants and procedure Participants were 451 students (242 boys and 209 girls) attending a high school in France. Average age was 13.5 years (SD = 1.3). The study took place over one school year. Data were collected at the end of the school year (i.e., before the final report card of the year, in early June 2004). Students who agreed to participate in the study completed a questionnaire to assess their self-concepts in three different school subjects: Mathematics, French (i.e., the verbal component) and Physical Education. Questionnaires were administered in class.

Measures

Academic self-concept measures. Mathematics and French self-concepts were measured with 5 items each (e.g., "I find mathematics lessons very difficult.") taken from the French version of the Self-Description Questionnaire II [START_REF] Guerin | Construct validation of the Self-Description Questionnaire II with a French sample[END_REF]. The physical education self-concept was measured with 6 items (e.g., "In physical education lessons, I am good at most physical activities and sports) slightly adapted from the Sports competence subscale of the French version of the Physical Self-Description Questionnaire [START_REF] Guerin | Generalizability of the PSDQ and its relationships to physical fitness: The European French connection[END_REF]. For each item in this dimension, we clearly specified to students that we were interested only in their general perceptions of competence in the sports and activities they practiced under the physical education program. Answers on all items were rated on a 6-point Likert type scale ranging from (1) False to (6) True. Cronbach's alpha coefficients for all three measures were excellent (see Table 1).

Academic achievement measures. For all students who completed the questionnaire, achievement scores on a 20-point scale in the three school subjects at the beginning of the school year (November 2003) were gathered from the official student records. We made this choice because many authors have supported the utility of the prospective design over a cross-sectional design (e.g., [START_REF] Bollen | Structural equations with latent variables[END_REF][START_REF] Rogosa | A critique of cross-lagged correlations[END_REF]. In France, physical education is a compulsory subject for all high school students (see Wang, Papaioannou, Sarrazin, Jaakola, & Salmon, 2006). Students practice different sports and physical activities throughout the school year, following a 10-week cycle (i.e., 10 lessons of 2 hours). Although health is a concern, student evaluations were predominantly based on their performance. Student achievement in our study is determined by the mean of grades received in the various activities performed during the trimester (from two to four activities). Previous studies have demonstrated the relevance of using grades instead of standardized tests (e.g., [START_REF] Guay | Academic self-concept and academic achievement: A developmental perspective on their causal ordering[END_REF], and that grades are a very good indicator of student achievement [START_REF] Jussim | Teacher expectations and self-fulfilling prophecies: knowns and unknowns, resolved and unresolved controversies[END_REF].

Studies carried out in PE in France reported high correlations (rs >.70) between students' performance and grades, in a swimming cycle [START_REF] Trouilloud | The influence of teacher expectations on students' achievement in physical education classes: Pygmalion revisited[END_REF] or gymnastics cycles (Boiché, Sarrazin, Grouzet, Pelletier, & Chanal, in press). andand

Data analysis

Handling of missing data. We observed less than 2% missing data for the overall dataset. Since many studies underscore the inappropriateness of pairwise and listwise strategies for handling missing data (e.g., Graham & Hoffer, 2000), we used the FIML (Full Information Maximum Likelihood) method, using LISREL (version 8.80), to estimate missing values. In brief, this methodology rebuilds the covariance matrix and sample means estimates.

Statistical analysis. We began by assessing a Confirmatory Factor Analysis (CFA) model in order to test the construct validity of the variables included in the model before assessing a Structural Equation Model (SEM). CFA and SEM were conducted with LISREL (version 8.80), using maximum likelihood estimation (for further discussion, see [START_REF] Jöreskog | LISREL 8: Structural equation modeling with the SIMPLIS command language[END_REF]. To ascertain model fit, we used the root-mean-square error of approximation (RMSEA), the chi-square test statistic, the chi-square/degrees of freedom ratio (χ2/df), and the Nonnormed Fit Index (NNFI). A non-significant chi-square indicates that the model adequately represents sample data. [START_REF] Browne | Alternative ways of assessing model fit[END_REF] suggest that RMSEAs less than 0.05 are indicative of a "close fit" and that values up to 0.08 represent reasonable errors of approximation. The Chi-Square/Degrees of Freedom (χ 2 /df) Ratio is a test of model misfit (χ 2 ), compared to the parsimony of the model as indicated by the model's degrees of freedom (df). In general, a χ 2 /df ratio of less than 2 is indicates a relatively good model fit. The NNFI varies along a 0-1 continuum, where values greater than .95 typically correspond to excellent fit to the data.

Results

CFA and SEM

The CFA model tested provides adequate data fit: χ 2 (140, N = 451) = 273.24, RMSEA =.046, χ 2 /df = 1.75, NNFI =.995 1 . All factor loadings are statistically significant and substantial (.66 to .88, mean = .79). Because the CFA and SEM models are equivalent (i.e., the number of parameter estimates are the same), the goodness of fit is the same. The critical parameter estimates are the path coefficients between previous achievement levels and subsequent self-concepts. SEM results are presented in Figure 1.

Grade effects on self-concepts. Results show: a) strong and positive paths between mathematics and verbal achievement and matching self-concepts (β = .83 and β = .76, p <.05); b) substantial correlations between mathematics and verbal achievement (r = .69, p <.05), but not for self-concepts (r = .02, p > .05); and c) negative paths between achievement in mathematics and the verbal domain and non-matching self-concepts in the mathematics and verbal domains (β = -.37 and β = -.23, p <.05).

Turning to the various relations involving physical education, we observe a positive association between achievement and self-concept in physical education (β = .50, p <.05). Physical education achievement correlates with mathematics and verbal achievement (r = .19, and r = .15, p <.05, respectively), whereas physical education selfconcept correlates only slightly with mathematics self-concept (r = .12, p < .05), and does not correlate with verbal self-concept (r = .02, p > .05). Of the four path coefficients between achievement and self-concept in physical education and achievement and selfconcepts in the other two school subjects, only two are significant: the path between verbal achievement and physical education self-concept is negative (β = -.18, p <.05), while the path between physical education achievement and mathematics self-concept (β = .08, p <.05) is positive. 2

Discussion

The main purpose of our study was to test an extension of the Internal/External Frame of Reference Model to physical education. Specifically, we tested possible negative or positive relations between physical education achievement and students' mathematics and verbal self-concepts. A one-school-year study involving 451 students was conducted in a French high school. First, the results of the study reproduced those of previous research on the traditional I/E model (e.g., [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF], corroborating the existence of: (1) a strong relationship between achievements in mathematics and the verbal domain; (2) no relationship between mathematics and verbal self-concepts; and (3) negative paths between mathematics achievement and verbal self-concept, and inversely, between verbal achievement and mathematics self-concept.

Results also revealed that relationship patterns between the two traditional school subjects and physical education differ appreciably. First, while relationships between traditional academic subject self-concepts and physical education self-concepts appear relatively weak or non-significant (cf. Figure 1), correlations between achievements in mathematics and physical education (r =.19) and between achievements in verbal and physical education (r =.15) are weaker than the correlation between mathematics and verbal achievements (r =.69). In other words, students with high achievement in mathematics also had high achievement in verbal, and to a lesser extent, physical education. These positive relationships between academic results and physical education are in line with previous results (e.g., [START_REF] Marsh | Explaining paradoxical relations between academic self-concepts and achievements: Cross-cultural generalizability of the internal/external frame of reference predications across 26 countries[END_REF][START_REF] Marsh | School athletic participation: Mostly gain with little pain[END_REF], and they emphasize the particularity of achievement in physical education in the school context. In fact, albeit positive, the relationships between achievement in physical education and achievements in academic school subjects are low, perhaps because abilities assessed in physical education (i.e., motor abilities) are not related to abilities assessed in academic school subjects.

Concerning the relations between verbal and physical education, results show that achievement in physical education is not linked to verbal self-concept, whereas verbal achievement negatively predicts physical education self-concept. The non-significant relationship between achievement in physical education and verbal self-concept could be explained by the non-academic status of physical education. In effect, there is a hierarchy among school subjects in which physical education (as well as art, music, and other less academic school subjects), is largely considered by students, parents, and teachers as a less important subject than mathematics or verbal subjects. As a result, achievement in physical education does not seem to be involved in the internal comparison process for verbal self-concept. In other words, students do not gauge their verbal self-concept based on their achievement in physical education. In contrast, there is a negative relationship (-.18) between verbal achievement and physical education self-concept, after controlling for physical education achievement. Thus, physical education self-concept appears to be gauged by verbal achievement. This relationship is in line with the predictions made for the I/E model.

Concerning the relationships between mathematics and physical education, results

show that achievement in mathematics is not negatively linked to physical education selfconcept, whereas a positive link between achievement in physical education and mathematics self-concept is found. Two explanations may be advanced to account for this positive association: (1) transfer of learning skills that are applicable to all subjects; and/or (2) transfer of learning skills that are specific to certain subjects. First, learning in physical education might foster some students' academic skills (e.g., [START_REF] Broh | Linking extracurricular programming to academic achievement: Who benefits and why?[END_REF], cognitive functioning (e.g., [START_REF] Etnier | The relationship between physical activity and cognition in children: A meta-analysis[END_REF], and self-regulation skills (e.g., Milosis, & Papaiannou, 2007) that are compatible with learning in more academic school subjects. For example, in an intervention study based on the multidimensional model of goal orientations, Milosis and Papaiannou (2007) found that students who were taught to use and transfer certain self-regulation skills (i.e., goal setting and self-talk) in physical education classes had better self-concepts in mathematics and verbal domains. However, although these explanations support a positive association between achievement in physical education and other school-subject self-concepts, they do not explain why a beneficial effect on self-concept was found only for mathematics and not the verbal domain.

Thus, the second explanation, transfer of skills applicable to specific school subjects, might be more compelling. In fact, there appear to be linkages between mathematics performance and spatial ability. Numerous studies have demonstrated correlations between mathematics performance and spatial ability (e.g., [START_REF] Battista | Spatial visualization and gender differences in high school geometry[END_REF][START_REF] Kaufmann | Imagery effects on problem solving[END_REF][START_REF] Hegarty | Types of visuo-spatial representations and mathematics problem solving[END_REF][START_REF] Sherman | Predicting mathematics performance in high school girls and boys[END_REF]. Specifically, the better the students' spatial ability, the better they performed in mathematics. From this perspective, experience in spatial activities (e.g., gymnastics) in physical education classes might be an important cause of improved spatial abilities. Therefore, we can speculate that physical education could increase students' spatial abilities, leading to better achievement in mathematics, which would lead in turn to the development of a better mathematics self-concept. This skill transfer might explain why beneficial effects were observed only for mathematics and not the verbal domain. Nevertheless, this reasoning is speculative, and further studies are required to support this notion before any definitive conclusions can be drawn.

Conclusion

Our study contributes to the existing literature by presenting an original effect of academic achievement in an unexplored school subject on self-concept in other school subjects. Perhaps because physical education is less academically oriented, negative relationship patterns between achievement in physical education and self-concepts in other school subjects were not observed in our study, and only a positive link was found.

An effect of self-concept enhancement in mathematics was therefore obtained without a negative counterpart for student's academic self-concept in other subjects. This result has not been previously demonstrated for any other school subjects considered in I/E model studies.

Our results have some implications for educational psychologists and practitioners.

As emphasized in previous works (e.g., [START_REF] Valentine | The relations between self-beliefs and academic achievement: A systematic review[END_REF], better selfconcept leads to better achievement, even after controlling for prior achievement. Thus, the demonstration of positive effects of achievement obtained in a less academically oriented school subject on student self-concept highlights other potential benefits of such school subjects in the educational system. Extensions of the I/E model to other less academically oriented school subjects (i.e., art, music, and technology) might reinforce this result. Some potential limitations to our study have to be pointed out. First, there are limitations in the methodology used. Longitudinal design studies with more than two waves of measurement are needed to validate the causal effect between grades and selfconcept. Moreover, even if grades are good indicators of achievement, they imperfectly capture student's performance. Finally, links found between grades and self-concept in PE have been found to be high and could be very different across countries and cultures where PE goals are not the same than in the French system. Therefore, replications are needed before generalization of our results.

In sum, it appears that physical education has benefits not only to reduce obesity and improve physical and mental health, but also to increase self-concept in mathematics.

This study provides further support for the maintenance of physical education in educational systems at a time when the practice is being brought into question. Footnotes 1. We added some equality constraints in our CFA model to test the differences that might exist in the associations between achievement and self-concept in the various school subjects. Analyses revealed that the correlations between achievement and selfconcept for verbal (r = .50) and physical education (r = .48) domains are not statistically different (χ 2 (141, N = 451) = 274.02, ∆ χ2 = .78, ∆ df = 1, p = .38) and that the correlation between achievement and self-concept for mathematics (r = .68) is higher than for physical education (χ 2 (141, N = 451) = 300.69, ∆ χ2 = 27.45, ∆ df = 1, p < .001) and for verbal (χ 2 (141, N = 451) = 291.71, ∆ χ2 = 18.47, ∆ df = 1, p < .001) domains.

Equality constraints were also used to test the differences that might exist between the associations between achievements in the various school subjects. Analyses revealed that the correlations between achievement in physical education and mathematics (r =

.

  Figure CaptionsFigure1. SEM of the effect of Physical Education (PE) achievement on mathematics and French academic self-concepts and achievements. NS = Non-significant. Curved lines are correlated residuals between latent constructs

Table 1

 1 Cronbach's Alpha Coefficients, Mean Ratings, Standard Deviations and Correlations of Variables of the Model Tested Note. N = 451. All Correlations are significant at the .05 level except those ranging between .10 and -.10.

	Variables	α	M	SD 1	2	3	4	5	6
	1. French grades	-	11.60	2.72	-
	2. Math grades	-	11.78	3.42	.69	-
	3. PE grades	-	13.58	2.44	.15	.19	-
	4. French self-concept	.88	3.95	1.32	.50	.15	.01	-
	5. Maths self-concept	.90	3.93	1.46	.36	.69	.20	.02	-
	6. PE self-concept	.91	4.41	1.20 -.06	.03	.48	.02	.12	-


Ralph-Samuel Samuel Grossmann
email: rsgrossmann@hotmail.comwww.rsg8.com

A case study, Urban Exploration in the works of Robert Smithson and "Stalker" (a collective)

History of Urban exploration : A case study, Urban Exploration in the works of Robert Smithson and Stalker [a collective].

By Ralph Samuel Grossmann, Phd Student, EHESS Paris, Art historian and Fine Art photographer. The conference is held as part of the Seminar, "Diversity as a marker of the temporality of the city" -First Day, Paris, June 1 st 2016.

Robert Smithson is a key figure of the American artistic scene. Not only an artist he's also considered an art theorist, who wrote many texts about art regarded as an important complement to his artworks. We will not enter the details of his curriculum but focus our attention on the years 1967-68 when he developed a set of works devoted to urban exploration.

Considering the cultural situation of the United States in the late fifties, Smithson developed the idea that entropy had gained everywhere and could open new ways of making Art. The premise of his analysis lies in his observation of the American urban sprawl of the fifties where the repetition of similar housing solutions, alongside with the development of suburban malls, movie theatre, highways and roads led to what he described as an entropic landscape1 . Which one should understand as being very dissimilar, in terms of its components, to the concept of "non-lieu" (which could be translated to non-site) developed by the French Anthropologist Marc Augé, in the early nineties. 2 The similarity of concepts can be traced back to a similar type of urban landscape, which originates in the US and was later also developed in European suburban architecture, although to a lesser extent. Both the sociologist and the artist consider also the city within the sprawls of suburbia as interconnected. This is a critical point we will see and develop along the presentation.

Both Smithson and Augé understood the city as having been deeply transformed by places, spaces, situations where one is being in transit, held out from any dynamic action, almost in a form of surrender to the given situation. These "non-lieux" can be understood as all the spaces where the person is considered as in motion in a space emptied out of meaning, built only to conduct the body and the mind in given patterns, dispositions or necessities of the life in a big city. These places are for instance, airports, stations, malls, urban housing, connecting-traffic road systems.

Not only did Smithson recognize this phenomenon, he also connected this issue to Minimal Art, considering that such a background could explain why Minimal Art designed Artifacts, avoiding subjectivity, mainly void of expressive qualities, in order to reinforce the idea of their being present. This concept of actual presence of the artifact emphasized perception as the main part of an artwork. It reversed the paradigm of modernism, shifting the attention from the artwork towards the perception at play when one discovers the artwork.

Why is this relevant to the issue of Urban exploration ?

Because in the flow of its reflection on Art, Smithson, with other artists, insisted on the current reality as a key source of inspiration and used a set of concepts such as displacements, exploration, body motion, mapping and collecting to imagine new art forms. And this event took place with much similar concepts, in geography and history and other Liberal Arts with the situationists movement. Observing some of Smithson works are an interesting case study to understand to which extend his type of urban exploration differs from those of the Surrealists or from Walter Benjamin, for instance. So, let's go back to Smithson. His first productions relate to Minimal Art, as can be seen in this example. They translate the idea of objecthood and presence and are linked by the artist to a set of science-fiction books and architectures and to the concept of entropy. Entropy manifests itself, according to Smithson, in the urban sprawl or in the invasion of printed matters in the daily life. This deconstructive force being an inspiration to Art as well as a new situation in which people have to live. 3 In his point of view the notion of monumentality is central: because Minimal Art has raised again the attention towards the shape of a form and the feeling of void associated to it, both aesthetic qualities that Smithson associates with monumentality. The next step in his exploration of the concept of entropy was to explore sites outside big cities both to expand the field of what could be Art, and to translate the Zeitgeist into his production. Known as Non-Sites a set of works of the years 1967-1968, are based on site-seeing. But unlike the common tourist who visits the monuments of importance, the sites discovered by Smithson are dumps, wreckage sites, abandoned airfields, etc. In these very accessible works, the non-site is presented under two forms : its map and a collection of items found there, usually absolutely useless material. The radicality of the gesture is difficult to understand to us, because Contemporary Art has constantly played with such transgressions since. Nevertheless, in a simple yet radical manner, Smithson, creates with these artworks, (called Earth Works as a generic title) a new form of representation of nature. The action of collecting items on a site can be directly linked to geologists or scientific explorers. Yet, the collected items and the chosen sites do not fit into the category of « scientifically relevant material or sites ». On the contrary they are unseen places, underestimated, bordering spaces of the city or unexploitable natural resources. In both cases their importance is absolutely minimal. By putting the accent on these sites, visiting them and collecting material there, the work underlines the importance of the action to do so, for itself, as well as the importance of such places in a greater scheme of things. In this last sense, one can see the relation to both scientific exploration and to the tradition of urban exploration, as progressively constructed from the Grand Tour to Walter Benjamin, around the notion of random or organized exploration of a given set of reality.

Smithson envisions his Non-Sites works as embodying a specific form of actuality, a presence that lasts in the shape given to the artform, by comparison to the experience on site. He has written many texts about the relationships between the site and the Non-site (the artwork). In them he underlines that both aspects of a reality are seen as two polarities bearing relations to each other. One being the source of the other, but also by means of displacement being somehow informing us about its source. The complex aesthetic criteria here at play exceed the present conference and we can't discuss this aspect here.

More important for our thoughts today is to underline, that all these non-sites have been explored in a similar way and represented in the gallery space under a similar pattern : by means of a container, the collected material in the form of rocks, earth, soil, and by a companion map of the 3 Robert Smithson, "Entropy and the New Monuments", Artforum, June 1966, p.66-72.

existing site. The work can therefore be understood as a geological survey, as well an archeology of the contemporary city.

If we replace these works in the lineage of city exploration such as these of Walter Benjamin or the Surrealists around André Breton or these made by the Situationists, which occurred during the lifetime of Robert Smithson, one can understand the gap. Smithson's Nonsites don't show any monument, they don't collect human artifacts, but stones and broken concrete rubbish, the work is only here to manifest in an ironical way how the city produces entropy (in particular Non-site « line of wreckage », Bayonne, New Jersey). On the contrary to former explorers of the city, who insisted and collected aspects of the urban, its shapes, its artefacts, its monuments and its history, as well as the experience within its layered landscape, Smithson insists upon entropy, as some sort of hidden side of the city.

In the same time frame, he reinforces this statement by using the term Earthwork in two dissimilar contexts. Quoting Earthworks, the title of a novel by Brian Aldiss, he discusses the possible meaning of the word in an important text published in Artforum in December 1967: The Monuments of Passaic. The text can be understood as some sort of a prequel to the title for his first Nonsite, which he actually realizes from January 1968 on, and will be presented in his exhibition at the Dwan Gallery, in March 1968 under the title "A non-site (Indoor Earthwork)4 ". The published text is a parody of exploration or travel narrative and describes the discovery of a set of Monuments all located in the outside of the suburban town of Passaic (hometown of Smithson in New Jersey). The idea to announce his upcoming exhibition in a publication by using a similar word in the text and concurrently in the title relates again to the entropic principle: it multiplies the informational signs and layers the meaning of the term to avoid a simple definition.

Initially Smithson had in mind to produce the Monuments of Passaic for the Gallery space, the entire piece is composed of 24 images of different « monuments », all are the elements of suburban industrial landscape found along the Passaic River. Visitors of the show would have been invited to follow a guided tour by the artist, starting in a bus and going towards this forgotten land, just outside New York City. But this part of the work only happened for a small group of friends of the artist and wasn't carried on for real. Therefore, the work exists under two forms: a published text with images and a photographic show. Under both aspects, the "represented monuments" are underestimated, overlooked parts of an urban environment. By describing the travel through the suburbs to the industrial landscape in Passaic, Smithson initiates in fact a new theme in Art : the exploration of debris, leftover land, forgotten spaces. Urban planner, as well as archeologists or historians have been keen to exploit, visit or study such neighboring spaces of the city to better understand, and / or, to define the patterns of organization within an urban area. But Smithson is one of the first artists to explicitly represent such an area as being interesting. Unlike, for instance, pictures by Eugene Atget of the Park of Sceaux (France), taken in the early 1920's, which show this historical monument in a state of actual decay, the monuments of Passaic are transitory elements of urbanism, and have in fact disappeared, to the exception of the bridge, replaced by a concrete bridge. The singular side of Smithson's work is to have not only taken the pictures but also to have described and explained every monument with a text. In this sense, he did what Kevin Lynch suggests in his book American

The image of the city, published in 1960 : « it is to every citizen to inform his environment with information and connections. So it can become a distinct and remarkable space.5 » In an interesting way, by exploring an insignificant part of the city, the artist underlines the importance of the contextualization and of the information in the representation of a monument. The visible shape is only one aspect of this representation, the written information being another. The link to the practice of history or archeology becomes obvious. The industrial landscape of Passaic, from vernacular becomes patrimonial. For two reasons: because of the actual destruction of the so-called monuments, the artwork remains and becomes a collection of souvenirs showing the destroyed scenery; because the text written by Smithson elaborates for each monument a singular commentary it enhances its relevance. He does not only describe his subjective experience about the monuments but also ascribes them to take place in a landscape that he describes in the form of travel literature. We see the connection with the topic of our seminar. The city produces not only meaning within its limits but also within the limits of its territory, be it a territory as space or a territory as time. Every item in the urban fabric can become important, shifting from vernacular to patrimonial in accordance to a changing cultural context.

[Here we read the text of Smithson6 and translate it simultaneously in French]

In the early 1990's, the collective Stalker7 , a group of Italian urban planners, architects and artists, has reaffirmed the importance of Robert Smithson by introducing in their creative practice a set of actions inspired by Smithson's reflections on the non-sites, as well as Marc Augé's « non-lieux ». The basic idea was to question the issue of walking within a city and outside it. The practice of walking is viewed by this collective as being able to inform urban planners about new ways to organize the urban space, be it patrimonial, urban or suburban. By specifically aiming the unconstructed areas in and around cities, the collective has remarkably influenced urban planning, as well as history in particular by enabling a better understanding of the issue of pedestrian exploration and motion within urban and suburban areas.

Insisting in their work on the issue of crossing, crossing a space, a leftover land, crossing a railroad, a fence, the collective has been able to underline the growing uncertainty of actually walking around the city. It enabled urban planner to better focus on the concept of fluidity in their urban design and contributed for instance to a revival of urban restoration of abandoned railroads, tunnels and such unused pieces of land. The Coulée verte (Green Path) in the suburbs of Paris, between Paris and Massy-Palaiseau initiated in the early 2000's and partially inaugurated in 2004 (achieved in 2008) is a example of such a novel use of abandoned territories to reconnect the city center with distant suburbs with a pedestrian and bicycle road. The unused land was reclaimed and organized in a way to bridge various neighborhoods and constitutes an interesting example of how the limits of different territories can become, instead of limits, a new situation for motion and exchanges between various suburbs. Identically the Parc du plateau we are about to visit later today, connects in a very successful way Montreuil and Bagnolet and tends to ameliorate the pedestrian connection between the two cities in a successful way. Although this garden is of course the design of a landscape architect, the groundbreaking research of Stalker was so influential, that it can be seen as a possible a source of inspiration for most urban parks since the mid-nineties. Stalker has notably influenced the practices of urban planning and landscape gardening by emphasizing the experience of walking through the territory.

Similarly, some of the efforts of the group to better understand the experience of walking in and around the city proved useful and suggested an explanation to one peculiar historical mystery. Francesco Carreri, one of the group main figures, proposed to read megalith stones, present in many areas in Europe from Italy to the England, France or Switzerland, as possible markers within a territory of ancient roads. If only an intuition, some aspects of the statement were positively accepted by archeologists. It shows again how History, art history and human sciences can interact when dealing with the issue of the territory or the landscape. What seemed to be only a forgotten fragment, a patrimonial monument, the standing stone, might have actually been a mere utilitarian artefact for practical purpose in historical time. Through the reenactment of an experience, through the creative practice and the observation of given situations, outside the usual pattern or outside the hierarchy of patrimonial importance, the collective Stalker initiated an important methodological renewal. If this renewal concerned mainly urban planning, it was also interesting in terms of archeological understanding and could be inspirational to other practices in the human sciences.

The theme of our two days conference about the territory, the history, the social diversity and the issue of markers in space, and in time, can also be seen as part of a renewal of methodology. In this bigger scheme of things, Stalker as well as Robert Smithson initiated interesting ways to observe the landscape as part of an environment and to link history, space and information (both in theory and in experience) in a constructive way. Their urban explorations opened a fundamental change: the territory or the landscape is no more seen as being self-relevant but considered in connection with its surrounding environment. Making the limits of this territory not the end of it, but also a specific area to study and observe in order to achieve a better understanding of its reality as a complete landscape, inhabited by social practices and history.

Link to the Seminar program: https://ishaparis.tumblr.com/

In a discussion with fellow artist Alan Kaprow, Robert Smithson share thoughts about the urban sprawl outside of cities and remind us they would not want to live there despite of the inspirational qualities of this type of landscape. Smithson says about it: "It is the future -the Martian landscape", in Jack Flam, Robert Smithson, the collected writings, University of CaliforniaPress, 1996, p.45.

Marc Augé, Non-lieux. Introduction à une anthropologie de la surmodernité. Paris, Le Seuil, 1992.

It will later be titled : Nonsite, Pine Barrens and now belongs to the collection of National Gallery of Art, Washington DC.

Kevin Lynch, The image of the City, Cambridge, Massachusetts Institute of Technology Press, coll. « Publications of the Joint Center for UrbanStudies», 1960, p.107

Robert Smithson, « The monuments of Passaic », Artforum, December 1967. We used the reprint of the text in: Jack Flam, Robert Smithson, the collected writings, University of CaliforniaPress, 1996, p.68-74.

Francesco Careri, Walkscapes : la marche comme pratique esthétique, Jacqueline Chambon : Actes Sud, Arles, 2013 : this book writen by the founder of the Stalker Collective explains in its entirety the guiding principles of their work, and comments the work of Robert Smithson (p. 100-105).