Alain Rousseau
email: alain.rousseau@seftim.fr

Pierre Gruet
email: pierre.gruet@ineris.fr

Storm Detectors -Tests and Application

Keywords: storm detector, tests, application, risk I

 and is also introduced in the IEC program of work. However, this standard is only a first document that even if very important will not be enough to apply safely the storm detectors in field. First regarding application, it is assumed in the standard that a setting process will be possible for any device installed but in practice, for many industrial applications, this procedure will be very weak and the device should be able to meet the industry requirements in a very short time to be efficient and thus settings procedure should be very straight forward. Second regarding long term withstand, it is very important that the device be reliable and thus tested.

This paper will investigate both sides:

-application from need definition to how to use the storm detectors. This will be illustrated by real case applications.

-tests : laboratory tests as well as open air tests will be introduced

INTRODUCTION

Lightning prevention techniques becomes very popular. The risk evaluation standard IEC 62305-2 will in a near future introduce such measures in its risk calculation. As a matter of fact, this risk evaluation technique means that you should provide protection measures until you decrease the calculated risk below a certain tolerable level. But in some cases, the risk is too high especially in high keraunic areas and standard lightning protection techniques cannot reduce it enough. This is particularly the case for large buildings with high risks, building with explosive atmosphere or place in the world where keraunic level is very high as for example Asia. What to do in such case? Basically, one of the sole remaining options is to reduce the risk duration. This means to implement lightning prevention measures such as storm detectors. If the storm detector is providing an early warning, it is possible to evacuate people from a dangerous zone, to stop a dangerous process or even to disconnect from the network and operate on independent power generators. This is nice in principle but this has a drawback. You can find any type of device on the market. You can find also various technologies such as for example magnetic field sensors or electrical field mills to speak only about local storm detectors as networks exist in many countries. How to be sure that a) the device will warn you enough in advance b) that the warning is reliable c) that the warning will occur at all [START_REF] Wang | Validity Analysis of Electric Field Temporal Difference in Thunderstorm Warning[END_REF], [START_REF] Meng | Application of Detection Data from Electric Field Meter on Ground to Lightning Warning Technique[END_REF], [START_REF] Chen | Lightning monitoring and warning system[END_REF]. This is linked to three main characteristics that such a device should have:

 to be reliable : if you give a false warning too often the user will stop believing in the device and can even in worse case disconnect the alarm. This is also the case is the storm detector is too often faulty or even damaged by pollution.

 to inform in advance : depending on the process you intend to stop or the time you need to react, it is justified to request a certain time between warning and possible first strike on the site. A typical timing which is wanted by users is 30 minutes but this is the limit for many systems.

 to not miss any dangerous event : this is particularly important for dangerous and expensive processes.

The storm detector European standard (future IEC 62793) has defined many parameters that are useful for the user as well as a procedure to define these parameters as well as the covered area precisely. But this is not enough; it is also needed to have tests in the same way than what is done for other devices: a standard to be able to characterize and compare performance of existing devices. Basic tests are needed such as pollution tests or EMC tests but this not enough as the testing means in laboratory are limited and probably not representative of complete field conditions and especially not of long term testing needed to evaluate some characteristics in real environment.

II. APPLICATIONS

The European standard has defined many parameters to allow the use and setting of storm detectors as well as to characterize the various technologies. These parameters are reviewed in the paper by splitting them in 3 categories [START_REF] Gulyas | Theoretical framework of prevention lightning protection[END_REF]:

 parameters to define the appropriate technology for a specific site and use  parameters to allow the setting of the device to meet its goals parameters to be used in risk assessment procedure.

A. Parameters used to define the appropriate technology

There will be mainly a comparison of available product for industrial users i.e. magnetic sensors, field mills and networks of sensors.

The standard defines 4 classes of devices: -class I: they detect a thunderstorm over its entire lifecycle (from initial phase to dissipation phase); -class II: they detect intra-cloud and cloud to ground flashes (from growth phases to dissipation phase); -class III: they detect cloud to ground flashes only (from mature phase to dissipation phase); -class IV: they detect cloud to ground flashes (mature phase only) and other electromagnetic sources with very limited efficiency.

As a matter of fact device that measure only electromagnetic radiation at low frequency have little interest for users, due to the fact that they are not able to provide enough lead time or to provide it with enough reliability. Due to their limited price, such sensors are highly used by the industry but they have limitations. One example will show this; such a storm detector was used on a building located on a top of a hill. This building was dedicated to seismic activity and as such was supposed to work under any meteorological conditions. To achieve this, such a magnetic detector was used in order to switch the power to a power generator and get decoupling from the network and its overvoltages. One day, the first flash occurred right on the top of the hill on the main building and there were of course no warning at all. Since then, they move to another technology: static field sensor.

Network of sensors is an interesting technology because most of the parameters needed for application on site (lead time, Failure To Warn Ratio) have been validated by the provider of this system on a large scale. However, drawbacks can appear if the number of sensors is too small or if the studied site is in a location were accuracy of detection is not so good for any reason. In addition, these networks are generally able to detect the evolution of stormy cloud over a large area such as a country, mainly by detecting the cloud to ground flashes. So this means that, as above, when the first flash appears on the studied site it is too late to give any warning.

Parameters defined in EN 50536 standards and especially FTWR should be published by the provider for its network. It is wise to ask to the network provider an evaluation of all these parameters for the exact location of the studied site as these parameters can take various values over the network covered area. Such networks don't exist in all countries and in some places, the parameters given by the network may not be enough for the desired application (too small lead time for example or bad FTWR). In these cases, a local static field sensor is the good option.

B. Parameters used used in risk assessment

setup of an alarm given in the standard includes three steps: -areas definitions; -alarm triggering criteria; -alarm information delivery

The standard recommends that any new installation need a prior adjustment period before it is considered to be working at its optimum level. This adjustment shall be made by a technician specifically authorized by this manufacturer. It is possible to optimize the warning system parameters and then improve the quality and the reliability. The alarm can thus be better adapted to the end user applications. It is recommended to establish an evaluation procedure. In this procedure the user should provide information about previous experiences (e.g. number of alarms, failure to warn, false alarms, damages, etc.).

In practice this is a quite difficult process for most of the industrial users. If it is clear that the device will be set-up by the manufacturer itself, the adjustment of parameters will be done at installation stage so generally under fair weather conditions. The second step, regarding adjustment of parameters during stormy conditions, will generally be done by the user itself if any. The only existing possibility to adjust the parameters for stormy conditions will be to send a data log to the manufacturer for him to analyze results and propose adjustments. More likely this will be done when the maintenance is made, this means one year after set-up. During one year the user will live with parameters adjusted at installation stage, whatever is the quality of this initial setting. Comparison of data recorded by local detector and lightning location data available from many sources (lightning detection networks, satellite observations, etc) is quite an extensive task especially is performed only after one year. In practice, this procedure is not performed and the setting of parameters is only based on experience of the technician from the manufacturer.

Furthermore, another problem may occur. The user is generally allowed to modify the settings of the device to meet his goal. Even if this procedure is protected by password allowing only one or two people in the site to make the changes, they may not have the background to make this adjustment. Risk is that a user, disturbed by too many false alarms decides to increase warning levels until it becomes not efficient at all.

To illustrate this difficulty in fixing parameters, we will show two examples using static field sensors (field mills).

The first one is a very polluted site in a coastal area. For that site the storm detector is used for people safety. Workers may be present on a 140 m tower and they need around 15 minutes to go down by stairs and ladders. There are many places including the top of tower that are dangerous for them in case of storms and reliable information should be provided to allow them to go down safely. Due to the fact that the site is near the sea and in a very specific topography, many lightning are occurring and most of them without any prior signs and some time without any dangerous clouds in vicinity. To provide this reliable warning a field mill has been installed on shore at the main building. People working on the tower should keep a phone with them to receive warning when storms are approaching. The system has been adjusted over a one year period. Parameters received by the system were transferred to the manufacturer and fine tuning of parameters possible. This worked fine for one year. After a few more years, the user started to complain. In presence of clear storm conditions, the user got no indications from the on the reverse, in clear conditions, there were many false alarms.

The user lost confidence in the device and disconnected it a few months after. It has been shown that the reason was corrosion due to salt fog within less than one year. This is why the proposed tests (see part 3 below) includes corrosion test. A new technology appeared recently: there are no more rotating parts and corrosion effect is then better mastered.

The second case is an industrial site in rural area. Due to the high risk generated by this site and a quite extended site to be covered, the user decided to install 2 field mills at two opposed locations on his site. The two devices are communicating together and decision to warn the user is based on data coming from the two sensors. This has proved to be a good way to reduce the risk in a safe way (even if one device fails the second one can still provide enough detection capability. This was very important due to the dangerous industrial process and the will of the user to protect safely workers and environment) and also to better adjust the settings of the device by comparing recorded data from two sensors. This is based on a PLC box located in a building where of the sensor is installed. This box allows controlling the total warning system. The alarm is given by dry contacts with positive safety able to pilot more than on signalization equipment. In addition, a PC is used to monitor events. The sensors are communicating by an ethernet link.

C. Parameters used to allow setting of the device

The EN 50536 standard has introduced an application guide where 3 steps are defined to establish if a storm detector is needed or not.

-hazardous situations identification; -type of loss determination; -risk control: options to reduce the risk (selection, implementation and follow-up of the proper measures for the control and reduction of risk) Let's take the example given in the standard for a wind turbine farm. Based on the parameters taken for this example, the conclusions is that implementation of adequate thunderstorm warning system is very highly recommended. A wind turbine is a basic service whose continuity, quality or fast recovery shall be guaranteed but also is a workplace with risk in case of a thunderstorm and both situations have to be analysed. The risk to human life can be reduced by avoiding having workers on the wind turbine during thunderstorms. The risk of damages and service losses due to a direct strike cannot be completely eliminated but it is possible to take preventive actions (disconnection of sensitive equipment, park wind turbine in a safe mode, etc.).

A wind turbine has also been studied according to IEC 62305-2 Ed. 1. This is based on a real case wind turbine farm. The wind turbine is having following characteristics :

-blades : 33 m long -rotor + blade : total length 70 m -height of rotor : 70 m -1500 kW Site has a Ng = 2. The design has defined three zones: -Z1 outside -Z2 tower -Z3 nacelle and three lines: -L1 power line connected to the power shelter -L2 data line coming from the meteo sensor on the top of the wind mill -L3 power line from rotor to the electrical cabinet in the nacelle.

The result is as follows: The red line of the figure above is the tolerable risk RT that should not be exceeded and the various color columns correspond to risk components. The right brown column is the total risk (some of all components).

The risk component exceeding RT are : RA (dark blue): safety risk for people near the tower base RB (red): direct strike on the wind mill RU (light blue): risk for people inside the nacelle RV (black): risk for equipments inside the wind mill

By implementing lighting protection level 1 we get the following figure:

It can be observed that to decrease risk RA for people in vicinity of the wind mill is not possible even by using the highest protection level of the standard.

In addition, when analyzing risk per zone it appears that the risk in nacelle cannot be reduced as well.

So risk can be reduced neither for people around the wind mill nor for the nacelle zone.

This means that a limiting warning system needs to be implemented as well.

As a conclusion, there is no need for structures to apply the 3 step procedure defined for risk control in EN 50536 as the existing risk method defined in IEC standard 62305-2 is fully applicable. On the opposite, for open areas 62305-2 is not relevant and the three step procedure will show its benefit.

To cover fully the need, IEC 62305-2 will soon introduce a way to incorporate the storm detection in its method. This will be based on FTWR and will apply mainly on the losses by reducing time of presence in dangerous areas. When a storm detector is used to disconnect a line from an external source and provided that this disconnection is made in such a way that flashover from an outside source to an internal circuit cannot occur (enough distance or insulation is then necessary), collection areas for these lines can be reduced by applying the FTWR.

D. Applications

Some applications are reviewed below to show what is needed by industrial users and what are the possible problems encountered in field:

 application to a site under construction in Brazil [START_REF] Hélio | risk analysis associated to lightning refinery construction sites[END_REF]  application to a petroleum company in a tropical climate  application to an ammunition storage methodology used to validate a magnetic sensor installed in a explosive manufacturing company 1) Applications

In Brazil [START_REF] Hélio | risk analysis associated to lightning refinery construction sites[END_REF] has shown that storm detection combined with protective measures as well as risk assessment can be applied to a petrochemical site under construction to define places were workers should stand during stormy period. This original approach could be applied in future to many sites where construction stage can be long and lightning risk high (tall towers, extended sites, scaffolding higher than roof level, tent structure above building to protect people against climatic conditions …). This applies also to renovation of building where very often lightning protection systems have be partly dismantle for a quiet long duration.

A petroleum company in tropical climate needs lightning protection due to risk (explosive areas, highly flammable materials etc.) and large flash ground density. However, this is not enough, as it is necessary to avoid presence of people on tank roofs, near explosive areas or even on metallic structures. Storm detection is then needed and a static field sensor has been implemented locally (there is no lightning location network in this area). However, due to salt fog and industrial corrosion problems as well as humid conditions and small animal possible ingress, it has been decided to use a technology without rotating parts. This has been selected based on previous good experience in coastal areas. Ammunition storage is generally well protected by lightning protection based mainly on natural component (for example concrete structure will well protect these explosives areas). However, the ammunition should one day or another move from these safe shelters and to avoid that this is done under stormy conditions, this is generally monitored by a storm detector. Maintenance is an important issue and is made on a yearly basis as suggested by the manufacturer but experience has shown that many problems may still occur in the one year period: connection between sensor and based damaged by people working on roof, degradation of internal batteries, damage to electrical circuit due to poor power regulation. To covers this it is important that maintenance be done by a qualified personal and be extensive enough and that communication between users and specialist is good. Also preventive actions are needed to avoid operation disruption including battery change at an early stage. An UPS may then be a good option especially when power regulation is poor.

2) Tests to validate magnetic sensors on site An explosive manufacturing company was using a storm detector made of a magnetic sensor installed on top of the main administrative building to stop work in case of stormy conditions. At this time there was no standard for such devices and it has been necessary to check its efficiency.

In order to control the detection efficiency of such a device on site without waiting for real lightning conditions, it is necessary to generate an electromagnetic field for which the level at the detector sensor is equivalent to the one generated by a real lightning in the monitoring area.

To achieve this and in order to use testing means with limited dimensions, it is necessary to generate a high impulse current near the sensor. We used an impulse current as we had not information regarding how the sensor and device was designed.

We calculated the field generated at some distance by a lightning current, and we compared it to what the French draft standard proposed for magnetic storm detectors as a minimum level to detect: 0,06 A/m. This shows that a magnetic sensor needs to have detection efficiency sufficient to detect lightning current as a distance of a few kilometers.

To maximize the field level generated by the test means, we decided to use a loop. It was then necessary to evaluate the field generated by this loop at some distance of the testing means. In addition, the generator impedance as well as the loop impedance were considered for determining the generator current to be used. This led to a 35 cm loop and a distance between sensor and testing means of 10 m as well as 4 kA testing current. The generator is a combination wave generator connected to the loop by a coupling circuit. This circuit was necessary to maximize the radiation at frequency proposed by the French draft standard: 220kHz. Tests have shown that the device was not linear and that sensitivity of the sensor was different under various axes. This was certainly due to the antenna technology. Globally anyway, these tests have shown that the storm detector was working relatively well and was corresponding to principles defined in the French draft standard : its indicator changed from green to orange for a field a little below the level defined in the document.

However, the risk assessment method used on that site has shown that the user needed a longer lead time and a better Failure To Warn Ratio. Thus he moved to a static field sensor a few months after the study has been made.

III. TESTS

This will described minimum tests that seem to be needed to demonstrate long term reliability and efficiency of local sensors.

A. EMC tests

EMC tests as per IEC 61000-4-6 are needed as the storm detectors are used in harsh electromagnetic environment and need to send reliable warning.

B. Pollution tests

Tests described in EN 50164-1 need also to be applied to storm detectors sensors. The specimen shall be subjected to corrosion tests consisting of a salt mist treatment in accordance with IEC 60068-2-52 followed by a humid sulphurous atmosphere treatment ISO 6988. It is also important to test the resistance to UV radiation tests (exposure to Xenon arc lamp for a duration of 1 000 hours or alternatively to Carbon arc lamp or to Fluorescent lamp) for non-metallic sensor housings.

C. Electrostatic tests

The sensor is mounted below a testing plate with dimensions so that the electrical field in the area centred below it is homogeneous (variation around the linear electrical field by less than 2%). The sensor should be mounted as in normal use and should be located so that its highest point is at 1 m above the ground plate located below the testing plate.

The testing plate should be at 2 m (so 1 m above the highest point of the sensor) at ± 1 cm. The voltage applied on the plate should be a DC high voltage (negative polarity) to obtain an electrical field at sensor head varying from 1 kV/m to 10 kV/m. The voltage is increased to obtain at the sensor head 10 values equally distributed between 1 kV/m and 10 kV/m.

D. Electromagnetic tests

The sensor is mounted as in normal use and located at least 5 m from a discharge path created between two electrodes with a distance of at least 1 m between them and with a Marx generator, organized in such a way that the impulse current meets the criteria of a 8/20 wave. The generator should be at adjusted to obtain a current à < 1 kA and ≥ 3 kA 8/20.

E. IP and mechanical tests

These tests with IEC 60529 are also needed on the sensors. The sensor should also be subjected to mechanical test by applying mechanical impacts. The impacts are carried out on the accessible parts of the sensor which may be mechanically stressed accidentally.

F. Open air tests

Based on long term cooperation between China and France regarding lightning studies, an idea raised in late 2005: use existing field facilities to create a field testing for such devices. The Shanghai Lightning Protection Centre originally created to test SPD was then the appropriate place for creating this facility. It is also a severe area for lightning occurrence [START_REF] Rousseau | Testing facility and methodology for local detectors -basis for scientific and standardization discussions[END_REF].

Figure 5. Nanhui station and information transmitted by the system A Vaisala SAFIR system was already used at the place to study lightning activity in Shanghai area. As we needed a proved system to be used as a reference to compare the other local storm detectors, we decided to use the well proven SAFIR system as a reference. This system has three branches which are set in three corners of Shanghai.

This system can not only inspect the lightning between clouds but also the lightning between clouds to earth, even the lightning temporal density. It also can inspect the lightning density in the given area or in the given time. For example, it can inspect the lighting density in about 10 square kilometers or inspect the lightning density in the given twenty minutes.

The facility is now operational and field mill local storm detectors have been tested and have proved the efficiency of such a testing platform.

Figure below shows the variation curve of electric field measured by two different sensors (A and B) during the same lightning event.

We can first note that even in fair weather, the fields recorded are not always exactly the same. In the case of thunderstorms the difference becomes larger. The software and techniques used by the sensors is proprietary but the warning provided to the user should be similar or at least consistent with what is declared in the sensor data sheet.

For example, during the above event, there were obvious distinctions between data from the two devices under test. The maximum and the minimum of the electric field observed by one of them were 2.5 kV/m and -12.7 kV/m respectively when for the other they were ranging between 17 kV/m and -17 kV/m respectively during the same event.

This figure shows the benefit of an open air testing station. Testing duration is usually between 6 months to one year. Parameters measured at the open air testing platform are :

-time between warning and event in minutes, -number of warnings which occurred without any event (as shown by the reference system) in % of the total number of alarms, -FTWR (events not detected, in spite of the events being registered by the reference system), in %.

IV. CONCLUSIONS

Tests to prove long term efficiency of storm detectors are proposed and this could be a basis for a future addition to the storm detector standard.

Application of storm detectors is also presented with regard to risk assessment as well as need to simplify the selection for industrial users.

Figure 1 .

 1 Figure 1. Corrosion marks of the rotating as well as fix part. Note that blades are damaged.

Figure 2 .

 2 Figure 2. Risk assessment before implementation of protection and prevention measured.

Figure 3 .

 3 Figure 3. Risk assessment after implementation of protection at level 1 (highest level).

FIELDFigure 4 .

 4 Figure 4. On site testing for a magnetic field sensor

Figure 6 .

 6 Figure 6. Difference of measured electrical field during one thunderstorm event

TABLE I .

 I


Amandine Lecocq

Marie Bertana

Benjamin Truchot

Guy Marlair

Comparison of the Fire Consequences of an Electric Vehicle and an Internal Combustion Engine Vehicle

Keywords: electric vehicles, battery, fire, safety, experimental measurements

Since energy storage systems represent key new technologies in the development of electric vehicles (EV), risks pertaining to them have to be examined closely. Lithium-ion (Li-ion) batteries powering EV contain highly energetic active materials and flammable organic electrolytes, which raise safety questions, different to conventional cars. In case of EV fire, concerns remain about batteries fire behavior, about their impact on the fire growth, about their fire-induced potential toxicity, especially in confined spaces and underground car parks and about their reaction with water in case of firemen intervention. Fire tests were therefore achieved for two French car manufacturers on two battery units, on a full battery pack, on an EV and on an analogous internal combustion engine (ICE) vehicle. Thermal and toxic threat parameters governing the fire risk were quantified. For this purpose, the heat release rate and the effective heat of combustion were determined to qualify the thermal impact whereas the main emitted gases governing the toxic potency of the fire effluents were measured. Fire consequences of an EV and the corresponding ICE vehicle were compared. This paper aims at presenting the main results of these fire tests.

INTRODUCTION

In 2005, the transport sector was responsible for approximately 15% of global greenhouse gas emissions, to which road transport contributes as high as 73% [START_REF]Reducing Transport Greenhouse Gas Emissions. Trends and Data[END_REF]. As part of emissions reduction policy, research devoted to alternative and decarbonated energy sources in substitution of fossil fuels in the transportation sector is crucial. In this field, making use of electric energy provided by a powerful battery is an innovative way. The high energy Li-ion battery is indeed one of the emerging new systems of electric storage [START_REF] Armand | Building better batteries[END_REF][3] [START_REF] Etacheri | Challenges in the development of advanced Li-ion batteries: A review[END_REF] proposed in industries for innovative applications, in particular in the automotive sector (e.g. for Battery EV & plug-in EV), thanks to its high energy density.

Due to the reactivity of the materials and the high energy density involved, the Li-ion system may be subject to failures like thermal runaway leading to leakage, gas venting, fire and, in the worst case, explosion [START_REF] Bitsche | Systems for hybrid cars[END_REF]. For this reason, more but still affordable safety precautions are needed: safety vent on each cell, fuses, battery and cell electrical management capable of single-cell supervision and control, etc. In general, most of the inherent hazards trigger accidental scenario when batteries are misused or facing abnormal environmental conditions. When operating out of the stability domain of the system (in terms of temperature or voltage), a series of undesirable reactions (varying according to the type of electrochemistry involved) may occur [START_REF] Ribière | Investigation on the fire-induced hazards of Li-ion battery cells by fire calorimetry[END_REF]. These side reactions can lead to the release of heat and gases, and then subsequently cause thermal runaway [START_REF] Yang | Investigations of the exothermic reactions of natural graphite anode for Li-ion batteries during thermal runaway[END_REF] that entails significant threats including fire phenomena or even explosion as a result of the combustion of the electrolyte and other combustible components after rupture of battery confinement.

External fire of Li-ion batteries and more globally of Li-ion powered EV represents a scenario likely to occur during battery or vehicle life. Indeed, in France, 60 832 ICE vehicle fires required assistance from the rescue services in 2011 [START_REF]Direction générale de la sécurité civile et de la gestion des crises[END_REF].

To ensure the safe development of EV, French public authorities conducted different working groups with regard to EV safety management. To compensate a lack of technical knowledge on real EV fire behavior, the decision to proceed to EV full scale fire testing was taken with the aim of adjusting, as needed, some regulations, more particularly the ones regarding recharge station for EV in underground car parks or petrol stations. Public authorities led a group of experts, car manufacturers and emergency services to define an experimental procedure able to assess, in a suitable manner, the effects of EV fires and their consequences in confined spaces. The main objectives of this procedure were to characterize the general behavior of batteries and vehicles in case of external heat stress, to characterize batteries behavior in contact with water in case of firemen intervention, to identify and quantify emitted gases and energies and to compare fire growth. The defined protocol included five different tests on: two battery units (with and without fire fighting operation), one full battery pack, one EV and one analogous ICE vehicle. INERIS was commissioned to conduct these fire tests for two French car manufacturers in its fire gallery where many parameters were measured throughout the tests. This paper presents the main results and comparisons of the tests between an EV and the corresponding ICE car. A special attention was paid to the analysis of emitted gases, specifically toxic gases as HF and CO.

EXPERIMENTAL Test procedure specifications

Full scale tests were achieved in the INERIS fire gallery, according to fire test procedure specifications developed by the EV safety group, which was mentioned above.

The procedure included five different tests:

-A fire test on a battery unit, a modular assembly of some elemental cells, which was a representative element of the full EV battery. (A battery unit represented at least ten percent of the full battery mass or its energy was at least 1.5 kWh.) -A fire test on a battery unit, with fire fighting operation.

-A fire test on a full battery pack (which mass was around 250-300kg), with late fire fighting attempt. -A fire test on an EV with a fully charged battery.

-A fire test on an analogous Diesel vehicle with a full gas tank.

In order to quantify emitted gases and energies, the tests had to be performed in a confined space which could be operated like a large-scale fire calorimeter.

The ventilation had to be slightly forced and monitored to fully extract combustion gases in the exhaust system and to carry out measurements.

The calibrated ignition source had to ensure a self-sustained fire of the tested elements. It was important to be able to stop the ignition source once the fire was self-sustained, in order to quantify the energy produced by the fire of the tested element without any external contribution of energy. The impact of the transitional ignition phase had to be minimised to get as accurate measurements as possible and not to interfere with emission of gases from the tested element. Therefore a 6 kW propane burner was used to set fire to the vehicles.

Testing facilities and equipments

These fire tests were achieved in the INERIS fire gallery. This gallery, schematized on Figure 1, is 50m long, 3.5m high (on the top of the vaulted ceiling), 3m wide (10 m 2 cross section), with a tower were the main sensors and samplers are set up. The tower is 2m long, 3m wide and 10m high. This gallery has a monitored ventilation system and a gas scrubber system which enables to canalize and clean up combustion smoke before rejection in the atmosphere.

Controlled conditions are generated in the fire gallery with the opportunity to analyze standard decomposition and combustion gases and therefore to quantify thermal and toxic parameters. The online gas analysis instrumentation, including a Fourier-Transform Infra-Red (FTIR) equipment, conjugated to flow rates measurements enable to determine the nature and yields of toxic combustion or decomposition products.

Procedure dedicated to fire tests on vehicles

For each car manufacturer, after preliminary tests on battery units and pack, a fire test was carried out on an EV and another test was carried out on an analogous ICE vehicle. In total, 4 large scale car fire tests were achieved with an identical experimental procedure.

The flow rate in the gallery was approximately 25 000m 3 /h and it was measured throughout the fire tests. The ventilation system is an extraction one, which means that the fresh air gets in the gallery through the section under the door (section of 3m by 30cm) and it is extracted in the tower. For each test, the vehicle was set up in the tunnel of the fire gallery (Figure 1). Experimental set-up for the fire tests on a vehicle.

A gas burner of approximately 6 kW was used to set fire to the vehicle.

To ensure a sustained fire of the vehicle, the left front seat had been lacerated and the car windows had been opened before the test. The gas burner was activated during 1 minute, orientated to the left front seat, inside the passenger cell.

Online gas analysis was performed by several methods: -classical analytical methods using non-dispersive infra-red spectroscopy (NDIR) for CO 2 and CO, paramagnetic measurement for O 2 , chemiluminescence for nitrogen oxides (NO x ) and flame ionization detector (FID) for total hydrocarbons (THC); -a method based on an online Nicolet 6700 FTIR spectrometer, using a 2m gas cell of a volume of 200mL for further analysis of gases and vapors including HF, HCl, HBr, HCN, SO 2 , CO, CO 2 and NO x . The sampling probe and installation was set up to be compatible with the HF specificities, in the above mentioned operational conditions.

The gas analysis methods followed the principles of ISO 19701 [START_REF]Methods for sampling and analysis of fire effluents[END_REF] and ISO 19702 [10] standards.

The other online measured parameters were:

-thermal flux with two fluxmeters located 5m and 8m upstream the vehicle, -temperature inside and on the surface of the vehicle, -smoke temperature, -flow rate (smoke exhaust rate), -video and thermal IR camera, -online gas analysis (CO, CO 2 , O 2 , THC, NO x , HF, HCl, HBr, HCN, SO 2 , etc.).

Off-line measurements and analysis were also carried out, including soot analysis and mass loss measurement.

The total effective heat of combustion and the fire growth were determined using the method of O 2 consumption.

TEST RESULTS AND COMPARISON

Fire behavior and heat release rate (HRR) Fire development was found similar for all vehicles; the fire spread inside the passenger cell before propagating to the rear of the vehicle and then to the front of the vehicle. It's worth noting however that fire propagation can be influenced by the ventilation imposed during the test and by the ignition method which is used. The general behavior in case of an external fire initiating event was globally found similar for both types of vehicles. No explosion or projection related to the battery was observed during EV fire tests in our test conditions.

The measured mass loss was close for EV and ICE vehicles. For both car manufacturers, the measured mass loss was around 20% of the initial mass.

The maximal HRR and the overall dissipated effective heat of combustion (integration of HRR profile) were close for both analogous vehicles. The comparison of the evolution of HRR versus time for EV and ICE vehicle for the car manufacturer 1 and for the car manufacturer 2 are respectively represented in Figure 2 and Figure 3. In the present case, HRR computation is based on O 2 consumption corrected for CO and soot production. For the car manufacturer 1, the maximal HRR was 4.2 MW for the EV and 4.8 MW for ICE vehicle. Peaks attributed to the combustion of the battery pack appear at approximately 35 minutes after ignition. For the car manufacturer 2, the maximal HRR was 4.7 MW for the EV and 6.1 MW for ICE vehicle. Data of the literature mentioned that the HRR for a single passenger automobile (ICE vehicle) varies from 1.5 to 8 MW [START_REF] Steinert | Experimentelle Untersuchhungen zum Abbrand-und Feuerubersprungsverhalten von Personenkraftwagen[END_REF] [12] according to its size, but the majority of the tests reported in the literature show HRR values less than 5 MW [START_REF] Lönnermark | Emissions from an automobile fire[END_REF] for medium size cars. Then, measured HRR values during our tests are consistent with data from literature.

The overall dissipated effective heat of combustion was computed at 6300 MJ for EV and 6900 MJ for ICE vehicle for the car manufacturer 1 (Figure 4) and at 8500 MJ for EV and 10000 MJ for ICE vehicle for the car manufacturer 2 (Figure 5). From these values, the effective heat of combustion expressed as heat of combustion (in MJ) per kg of combusted material was evaluated. The effective heat of combustion was around 36-36.5 MJ/kg for ICE vehicles of both manufacturers. This value is consistent with the plastic heats of combustion and with the effective heat of combustion of 35 MJ/kg reported in [START_REF] Lönnermark | Emissions from an automobile fire[END_REF]. The effective heat of combustion was around 30-31 MJ/kg for electric vehicles of both car manufacturers.

Figure 5 Comparison of the effective heat of combustion released vs. time for EV and analogous ICE vehicle tests for the car manufacturer 2

Gas analysis

According to actual measurements, HF was emitted in significant quantities during both electric and ICE vehicles fire tests. This is shown in the graphs hereinafter, Figure 6 and Figure 7, representing mass flow of HF production as a function of time for both car manufacturers. It's worth noting that a significant emission of HF was also measured during ICE vehicle tests. A similar peak of HF emission at 14 min was observed for EV and ICE fire experiments. It may come from fluorinated materials contained in the vehicle (e.g. from a fluorinated refrigerant contained in the air conditioning system; this hypothesis wasn't confirmed). In the case of EV, additional HF emission peaks corresponding to the combustion of the lithium-ion battery pack were observed around 25-30 minutes after triggering vehicle fire, Figure 6 and Figure 7. This is consistent with known existing potential sources of fluorine in a Li-ion battery like the electrolyte (most often LiPF 6 in current technologies) and the binder material of the electrodes (often PVDF). This is also coherent with preliminary tests achieved on battery units and full battery pack. Consequently, HF cumulative mass was measured in higher quantities in the case of EV due to the combustion of Li-ion battery pack. As regards the other emitted gases, fire experiments showed the production of similar cumulative masses of CO 2 , CO, THC, NO x , HCl and HCN for both types of vehicles. No HBr was detected for these 4 tests. The total quantity of emitted gases (limited to measured gas and vapors) is reported in Table 1, in bold. This table doesn't take into account the kinetic of gas emission, which is an important parameter.

The measured quantity of the main emitted gases (CO 2 , CO, THC, etc.) and the thermal effects (HRR, heat of combustion) were higher for the manufacturer 2 due to the presence of a bigger amount of combustible material in its cars which are bigger models. Our tests show that the general behavior of EV and ICE vehicles exposed to the same external heat stress was similar. The maximal heat release rate (HRR), the overall dissipated heat of combustion and the effective heat of combustion were close for both types of vehicles.

The analysis of the combustion gases from car fires highlighted that the cumulative masses of CO 2 , CO, total hydrocarbons, NO, NO 2 , HCl and HCN were similar for both types of vehicles. A significant quantity of HF was measured during EV and ICE vehicle fire tests. To our knowledge, HF emissions from conventional ICE vehicles have not been reported into the literature so far, may be due to recent introduction of fluorine sources in modern cars. The cumulative mass of HF was higher for EV due to the combustion of the Li-ion battery pack.

In addition to HF, a significant quantity of toxic gases including CO and HCl, in relation with the presence of chlorinated polymers, was produced during the fire tests on both types of vehicles. All toxic compounds have to be examined to assess the global toxicity of combustion smokes during EV and ICE vehicle fires. These tests provided source terms, which can be used in modeling work to predict toxic gas dispersion and thermal effects in confined spaces, such as tunnels, underground car parks or other underground facilities.

The results of these tests are only valid for the four tested vehicles of two car manufacturers. Indeed, numerous parameters such as the fire scenario initiating event, the battery technology, its packaging, its design and its position within the vehicle are liable to play a significant role on the overall behavior of an EV exposed to an external fire. Thus, these results cannot be extrapolated to other vehicles, to other car manufacturers, to other potential fire scenarios or to other battery technologies.

These tests only studied the vehicle behavior in the case of a fire outbreak in passenger cell. In the case of a fire outbreak generated in the battery by an internal short circuit or an overcharge, the kinetics of observed phenomena would certainly be different.

  Figure 1Experimental set-up for the fire tests on a vehicle.

Figure 2 Figure 3

 23 Figure 2Comparison of the heat release rate vs. time for EV and analogous ICE vehicle tests for the car manufacturer 1.

Figure 4

 4 Figure 4Comparison of the effective heat of combustion released vs. time for EV and analogous ICE vehicle tests for the car manufacturer 1

Figure 6 2 Figure 7

 627 Figure 6Comparison of HF production vs. time for EV and analogous ICE vehicle tests for the car manufacturer 1


MSc Rania Ghosn

Ph.D Gyorgy Thuroczy

Ph.D Nathalie Loos

M.D. Valérie Brenet-Dufour

Pharm. D. Sophie Liabeuf

Ph.D René De Seze

Ph.D Brahim Selmaoui

Acute exposure to mobile phone and assessment of internal cerebral circulation in young healthy subjects: A Transcranial Doppler study

Keywords: transcranial Doppler sonography (TCD), cerebral blood flow velocity (CBF-V), electromagnetic field (EMF), Global system for Mobile Communication (GSM)

INTRODUCTION

The rapid worldwide increase in the use of mobile phones raises questions about the possible adverse effects of RF fields emitted by these devices. The temporal lobe of brain is closest to the mobile phone. This may lead to relatively high energy deposition in these parts of human head during the use of mobile phone. The cerebral circulation may be potentially affected due to the exposure to RF emitted by mobile phone. Therefore the studies on cerebral blood flow are essential in order to evaluate the possible interaction exposure to RF with the central nervous system. Data in the literature related to the brain circulation are limited and controversial due to the different methods and protocols applied in these studies (i.e. REG, PET, NIRS,) [START_REF] Thuróczy | Human electrophysiological studies on influence of RF exposure emitted by GSM cellular phones[END_REF][START_REF] Mizuno | Effects of W-CDMA 1950 MHz EMF emitted by mobile phones on regional cerebral blood flow in humans[END_REF][START_REF] Aalto | Mobile phone affects cerebral blood flow in humans[END_REF][START_REF] Curcio | Acute mobile phones exposure affects frontal cortex hemodynamics as evidenced by functional near-infrared spectroscopy[END_REF][START_REF] Kwon | No effects of short-term GSM mobile phone radiation on cerebral blood flow measured using positron emission tomography[END_REF][START_REF] Lindholm | Thermal effects of mobile phone RF fields on children: A provocation study[END_REF].

OBJECTIVES

To investigate internal cerebral vascularisation in order to define the possible physiological modifications of the using mobile (cellular) phones. The basic approach of our human study is to compare the cerebral blood flow of the exposed and non-exposed hemisphere before, during and after the mobile phone exposure using non-invasive widely accepted diagnostic procedures.

METHODS

The cerebral blood flow of middle cerebral arteries (MCA) was monitored by transcranial Doppler sonography (TCD). The technique is based upon measurement of the Doppler frequency shift of reflected ultrasonic waves backscattered by moving blood cells [START_REF] Bondar | Cerebral blood flow velocities by transcranial doppler during parabolic flight[END_REF]. The various cerebral arteries each have their own characteristic TCD waveform, depth, location, and flow direction. This allows their unique identification by sonography [START_REF] Petty | Transcranial Doppler Ultrasonography: Clinical Applications in Cerebrovascular Disease[END_REF]. TCD is a noninvasive technique that allows for constant monitoring of mean flow velocity in an intracerebral artery [START_REF] Aaslid | Transcranial Doppler examination techniques[END_REF]. LOOKI 2TC, ATYS Medical Doppler device has been applied in the study, with a helmet support for bilateral recording by two 2 MHz-probes (ATYS 1133) in temporal position on the head. Mean Cerebral Blood flow velocity (CBF-V, cm/s) and percentage of cerebral blood flow (l/min) have been recorded. Middle cerebral arteries PI and RI were calculated from flow velocity waveforms. PI was calculated as (Vmax-Vmin)/Vmean with Vmax = peak systolic blood flow velocity, Vmin = minimum diastolic velocity, and Vmean = time averaged maximum velocity [START_REF] Gosling | Arterial assessment by Doppler-shift ultrasound[END_REF]. In the cerebral vasculature, high PI can indicate higher peripheral vessel resistance concomitant with increased intracranial pressure [START_REF] Schatlo | Clinical applications of transcranial Doppler sonography[END_REF]. RI was calculated as (Vmax-Vmin)/Vmax [START_REF] Pourcelot | Velocimetrie ultrasonore Doppler Seminaire INSERM[END_REF]. Heart rate (HR) was calculated from TCD wave according to the formula: HR=60/R-R (R-R is the interval between an R wave and the next R wave). A voluntary BH (~ 30 seconds) physiological test was carried out as positive control to check the ability of TCD to demonstrate physiological changes in cerebral artery flow velocity. The skin temperature was measured continuously during the study. It was measured on the cheek and behind the earlobe on both sides of the head by four channels with a Luxtron optical thermometer (Luxtron Fluoroptic Thermometer, Model 790, California, US).

PROTOCOL

Twenty-nine healthy subjects were volunteered in randomized crossover double-blind study aged 18 to 35 years. They were selected after routine, non invasive, clinical and laboratory examinations. Selection criteria includes: regular sleep habits, no medication, no chronic disease or disability, no recent acute illness, no smoking, no neurological or respiratory vascular history. Those selected were instructed to abstain from consuming alcohol and coffee for 24 hours before and during each experimental session. Pre-exposure period (10 min): a first baseline (BL1) was recorded, then a first breath holding test (BH1) was applied. Exposure period (20 min): the phone (sham or real) was positioned by the holder attached to the helmet on the left side. The exposure period was 20 minutes. Four recordings were performed during the fifth, the tenth, the fifteenth and the twentieth minute of the exposure period. At the end of exposure, the mobile phone was carefully removed. Postexposure period (20 min): five recordings were performed during the first, the fifth, the tenth, the fifteenth and the twentieth minute of post-exposure period. After post-exposure period a second breath holding test (BH2) was applied (same conditions as BH1). The last recording was the BL2. Each TCD recording was spread over a period of one minute.

EXPOSURE

Commercially available mobile phone was used with exposure duration of 20 minutes. The phone was operated in test mode via PC connections to be tuned into constant and controlled RF power emission at the requested power level. The GSM signal was 900 MHz with GSM modulation at peak power 2 W (0.25 W averages). The measurements of absorbed RF power in the head SAR (W/kg), were made in a standard SAM phantom (Specific Anthropomorphic Mannequin phantom, Antennessa, France) filled with standard brain tissue-equivalent liquid (Satimo, France) according to CENELEC standard EN 50361 [START_REF]Basic Standard for the measurement of Specific Absorption Rate related to human exposure to electromagnetic fields from mobile phones (300 MHz-3 GHz)[END_REF]. The maximum SARs values were measured when the TCD probe was placed on the phantom at the position of cranial window beside the mobile phone according to the protocol of the study. The SARs averaged on 10 g tissue, 1 g tissue, or the peak were 0.53 W/kg, 0.76 W/kg and 1.03 W/kg respectively. The phone was positioned with the touch position to the cheek according to the EN-50361 standard by a helmet holder of TCD probes. The sham or real exposure is realized ntenna connector of the phone [START_REF] Parazzini | [END_REF].

RESULTS:

CBF-V and CBF:

The mean CBF-V measured in both sessions (exposure and shamexposure) are shown in Fig. 1. Comparison has performed within session (sham or exposure) between pre-exposure period (BL1), exposure period (EP) and post-exposure period (BL2) and between sessions (sham vs. exposure). Results showed no significant change between different periods of the recording (p > 0.05). In addition, similar results were observed when comparing the right (control) and left (exposed) sides during sham and real exposure (p > 0.05). Likewise, no differences were observed between the values of CBF at different periods of recording when the data were expressed as percentage relative to baseline BL1 (p > 0.05).

PI and RI:

No significant differences were observed in PI and RI when comparing different exposure periods (BL1, EP and BL2) within session (p > 0.05) and between sessions (p > 0.05). Moreover, when data were compared between left and right side the results were found to be similar suggesting that RF-EMF had no effect on PI or RI.

Breath holding test:

Mean values of different measured and calculated parameters during BH in sham and real exposure sessions showed significant increase (p < 0.001) in CBF-V and percentage of CBF during BH when compared to BL1. A significant decrease was observed in PI and RI during BH1 and BH2 in both sides and in both sessions (p < 0.001). No differences in different parameters were observed during BH when comparing control and exposed side, or sham and real exposure.

Skin temperature:

In the right side (with no handset) temperature did not significantly change in the cheek and under the ear. It seems to be stable during sham and real exposure. However, an increase in temperature was observed in the left (exposed with handset) cheek. Indeed, during sham exposure temperature rose up to 1.4°C in the cheek and 0.4°C under the ear respectively, while it increased up to 2°C and 0.5°C during real exposure. Increase in cheek temperature was significantly higher by 0.6°C with the real phone than with the sham phone (t-test: p < 0.05). Additional measures of temperature were performed on surface of the phones (real and sham) alone. After 20 min of transmission, the temperature of the sham phone rises from 24.45 to 27.33°C while the temperature of the real phone rises from 24.81 to 28.39°C. Heating produced from the real phone was found to be higher of 0.7°C than the sham phone.

CONCLUSION

In conclusion, this study assessed the effects induced by a normal daily exposure (duration: 20 min) to a GSM mobile phone on measures of CBF-V noninvasively and safely evaluated by TCD. Data showed no significant changes in CBF-V, CBF, PI, and RI in middle cerebral arteries during exposure. These negative results should not encourage excessive mobile communication, because minor biological and neurophysiological influences may not be detectable by the current method.

Fig. 1 :

 1 Fig.1: Mean of cerebral blood flow velocity (CBF-V) in both sides during sham and real exposure. No significant variations were observed in CBF-V in the right (control side) and the left (exposed side) middle cerebral arteries before, during and after sham and real exposure.


Sandra Dochez
email: sandra.dochez@ifsttar.fr

Farid Laouafa
email: farid.laouafa@ineris.fr

Christian Franck
email: christian.franck@ineris.fr

Sylvine Guedon
email: sylvine.guedon@ifsttar.fr

François Martineau
email: françois.martineau@ifsttar.fr

Marion Bost
email: marion.bost@ifsttar.fr

WATER WEATHERING IN ROCKS DISCONTINUITIES

Keywords: rock discontinuities, water, alteration, cultural heritage, stone

The behavior of rock discontinuities at different spatial scales remains a major challenge for scientists who want to understand the rock alteration, both in the triggering of rockfall and in the evolution of the cliffs. Our research aim to take into account the geomorphological features related to the slopes and rock discontinuities subject to weathering conditions. Several studies show the physical effect of water in the alteration of rock through cracks and micro cracks. In Cultural Heritage point of view, a rock discontinuity could be described or analyzed as the interface between stone and joint in buildings. The mechanical and physical discontinuity behavior and its time evolution have a strong influence on the onset of instability of the blocks and on their strength. The effects of climate change (increase of rainfall, temperature fluctuation…), may significantly increase the inception of phenomena such rock falls, stone degradation and damages on monuments. The aim of this study is to characterize the chemical influence of water flowing inside the discontinuity and also through the porous stone matrix. This paper focus on experiments performed in order to analyze the effects of physical and chemical alteration on the mechanical behavior of porous limestone. Limestone is a both commonly stone used in construction of cultural heritage buildings and found on French territory. These experiments could have some interesting results in restoration or conservation of building. We'll present the results of the first two experiments performed in quasi-static and dynamic of the flowing process. The observation and the monitoring of the alteration process are performed in order to determine the physical and chemical reactions involved in this phenomenon. This one we'll be linked to the mechanical aspect of the massif and or the building.

Introduction:

Chemical weathering exerts a major control on the Earth's geologic landscape. Most chemicals are solvated; the water becomes a major agent of degradation of rocks and stones. The behavior of rock media is mainly due to their fracturing and crack distribution which occurs at different scales. Failure in the rock masses occur mainly by opening of existing discontinuities and relative sliding blocks or benches [START_REF] Asroun | Stabilité des massifs fissurés[END_REF]. Runoff, which will produce mechanical erosion by removal of particles, is also quite active [START_REF] Kupper | Les vitesses d'érosion du calcaire dans l'eau : étude de phénomènes actuels[END_REF]. Hydrostatic pressure can cause a decrease in the tensile strength of rock bridges fatigue [START_REF] Frayssines | Contribution à l'évaluation de l'aléa éboulement rocheux (rupture)[END_REF] [START_REF] Druon | Aspects expérimentaux des effets de l'eau sur l'altération des discontinuités rocheuses[END_REF]. The study presented in this paper concerns the characterization of the types of chemical alteration appeared during a given period and to quantify the effects of water. Understanding its phenomena is crucial in view for number applications including civil engineering, but also in the study of delicate heritage monuments in order to draw up plans for restoration and conservation. The second part of the paper is devoted to an overview of different materials and experimental methods used. The third part presents the experiments, the forth gives the first results, the fifth announces the discussion; the part six expose the conclusions and perspectives and possibilities of our research in the different fields of applications as civil engineering, cultural heritage conservation.

Materials:

The main material used in the experiments are limestones from French quarries; oolithic limestone named:Pierre de Lens (Lens stone), Pierre de Caen (Caen stone), Comblanchien and Larrys. Their petrophysics properties are compelled in the following (Table 1).

The Comblanchien and Larrys are from Burgundy, Comblanchien is a famous building stone used in France in many architectural buildings both historic (e.g. The Opéra de Paris).

Lens stone also is a famous limestone in the Gard in south of France whose exploitation dates back to Roman. Used in Nimes, Carcassonne, Montpellier, Perpignan, etc. It has become a benchmark for sculptors and stonemasons. Caen Stone is used in the rehabilitation of cultural heritage and restoration; it will be used also for renovation, decoration and furnishings.

Materials for the experience on static mode alteration :

These different limestones are cored whose dimensions are known :100 mm in length and 12 mm in diameter.

Water : pH 2 (acidified with sulfuric acid (H 2 SO 4 )). weight accuracy ± 0.0001g Beakers (100 ml) SEM (Scanning Electron Microscope) Calcium test JBL Jbl calcium test instructions: 5 mL of water in the tank in the test tube add 5 drops of reagent (1) mix then wait one minute; 1 dose of the powder (2) stir until dissolved then the number of drops needed to change the color of the reagent (3) (transition from pink to purple and blue).the number of drops multiplied by 20 and gives your value of Ca.

Materials for the experience on dynamic mode alteration:

These different limestones are sawed whose dimensions are known :100 mm high and the base is 50 mm square edge.

Water : pH 2 (acidified with sulfuric acid (H 2 SO 4 )). weight accuracy ± 0.0001g SEM (Scanning Electron Microscope) Rugosimetry Laser

2.3

Experiments:

The experiment one: Alteration in quasi-static mode, and the second : Alteration in dynamic mode are made in order to describe the phenomena : alteration of rock bridge (defined as a continuity between a unstable block and the massif) (2.4) and the alteration of the porous matrix (2.5).

Alteration in quasi-static mode:

The goal of the experiment is to simulate the behavior of a rock bridge in a gap filled with stagnant (none flowing) water: alteration is called quasi-static regime (without forced fluid convection).

The experiment consists in the observation and measurement of the degree of limestone sample alteration subjected to acid baths without forced convection of the solution.

Approach: This experiment was carried out initially with several types of limestone (4) different in porosity (1% to 22%) to identify the role of the microstructure. This is to make cycles of alteration with pencil shaped to represent the geometry of a rock bridge. Note that in this experiment there is a natural convection due to gradient density. The velocity induced in the solution remains very small in comparison to the flow phenomena in a discontinuity where the water does not stagnate. The specimens are soaked and left 5 days in beakers containing the acid solution, and left to dry in an oven at 60°C during 2 days, this period of "soaking + drying" constituting a cycle of deterioration. These specimens will undergo several cycles in order to monitor temporal alteration.

Alteration in dynamic mode:

The goal of the experiment is to "reproduce" the alteration in wall surface of a discontinuity where the water flows and a rock bridge: named: alteration in dynamic mode (with pseudo forced convection).

The experiment is to replicate the alteration of hangingwall and to observe and measure the degree of alteration.

Approach: This alteration is caused by an acid water flow through an artificial discontinuity created by sawing a block of limestone at an angle to define a rock bridge and secured to the lower block. This experiment is being developed, especially regarding the shaping of the specimens.

Results:

3.1 Alteration in static mode:

Petrography

In this section we compare by SEM (Scanning Electronic Microscopy) Lens stone (Figure 1) which has a high porosity (15%) and Larrys (Figure 2) which has a small porosity. In order to compare the intensity of the alteration by acid water, we observe by SEM, a first sample unaltered and a second one which underwent 4 cycles of alteration for each type of limestone. We can observe on figure 1B that oolites in Lens stone are altered, they seem to be unstructured, whereas the Comblanchien is not altered as we can see on figure 2. C-D.

In addition of SEM observations, we observe the sample with binocular. Saad [START_REF] Saad | Influence du changement climatique et des conditions extrêmes sur les massifs fracturés : Rôle des fluides dans leur processus d'altération[END_REF], showed appearance of small black spot on the surface of the sample of Pierre de Lens, due to an accumulation of particles of manganese. This kind of black spot are already seen on the Pierre de Caen, but not on Larrys and Comblanchien.

pH and Calcium rate

pH measurement is linked to the concentration of oxonium ion. This concentration allowed us to have a quantification of the calcium dissolved after acid bath as formula explains [1].

CaCO 3 + 2 H 3 O +  Ca 2+ + CO 2 + 3 H 2 O [1]
During the period of imbibitions an increase of the pH is observed, which is linked to an increase of concentration of calcium. This pH increase during the period of imbibitions until it reaches saturation at about 7.3.

Another test has been performed; a simple test usually used monitoring of corals in the aquarium; on water after imbibitions cycle. This test showed an increase of calcium concentration with the porosity. Smaller the porosity is, smaller the rate of calcium is.

Mass evolution

We measure the weight of the different rock sample after the drying period. We could notice a loss of weight for each type of limestone and each sample (Figure 3). Both types of limestone have the same behavior between the beginning and Cycle 3. From Cycle 4, we see a difference. The less porous limestone seems to lose less mass than the other one.

Figure 3: Evolution of the sample weight after drying period between each cycle for two type of limestone (high and low porosity).

Alteration in dynamic mode:

The experience and particularly the shaping of specimens does not allow us until now to have the back and the results corresponding to the experiment.

Discussion:

The reduction in the mass of rock bridges due to weathering poses questions about the implications on the interlocking of the stones. It is expected that the reduced mass and changed shape of single rocks has a detrimental effect on the stability of the rocks and leads to an increased degree of hangingwall's mobility. The more the single rock elements become degraded, the more movement is allowed and the more the rock is prone to rock-to-rock abrasion and hence degradation.

The lake of time to realize the experiment 2 named Alteration in dynamic mode, let us to think about the improvement of the method that we want to develop which is profilometry by laser. We want to observe the influence of water on the surface, and specific ways taken by the fluid.

Concluding remarks:

To conclude this paper, even if we have increased the rate of the phenomenon, the rock alteration is a quite long process. The observed phenomena appeared late. However it should be noted that lower porosity lost less weight than the high porosity.

We can say that porosity influence the degradation. We also note that the measurement of pH is alternative way to measure the calcium concentration. We are at a crossroads on this measure, because we choose a simple and inexpensive, such as classic aquarium test. This calcium concentration measurement should be more efficient with a water analyse by ICP mass spectrometry.

The previous experiments showed important aspects of the alteration of the rock bridge, the discontinuity and also the massif. The following experiment will take into account the previous results in order to improve it.

According to Kupper [START_REF] Kupper | Les vitesses d'érosion du calcaire dans l'eau : étude de phénomènes actuels[END_REF], the weathering is greater in low temperatures, hence the idea is to set up an experiment that will seek to alter in quasi-static regime at a temperature of 5°C.This temperature can be an in situ temperature reported in temperate countries (autumn to winter period) [START_REF] Bost | Altération par le gel des massifs rocheux : Etude expérimentale et modélisation des mécanismes de générations des contraintes dans les fissures[END_REF].

The large-scale component (in-situ, ground) was naturally considered to validate the observations from laboratory experimentation. Several sites are under investigation in Isère and Savoie (France). Laboratory experiments are performed attempting to replicate the best reality on the ground where possible using blocks of the site will be chosen [START_REF] Hantz | Contribution à l'élévation de la durée de vie d'un compartiment rocheux susceptible de s'ébouler[END_REF].

Mainly of cultural heritage monument are weathered because of the environment condition (e.g. pollution by exhaust gas of cars.). All experiments conducted here could be easily applicable on cultural heritage, indeed the size of the simple are quite small. The acid used could simulate an acid rain which replaces the study in a configuration of urban environment.

  Figure 1: A. SEM image of Pierre de Lens unaltered sample. B SEM image of Pierre de Lens after 4 cycles of alteration.

Table 1 :

 1 Properties of different limestones used for the experiments in quasi-static and dynamic alteration mode.

	Limestones	Porosity (%)	Shape for	Shape for
			Alteration in	Alteration in
			quasi static mode.	dynamic mode
	Comblanchien	<1	cylindrical	rectangular box
	Pierre de Caen	~20	cylindrical	rectangular box
	(Caen stone)
	Pierre de Lens	~15	cylindrical	rectangular box
	(Lens stone)
	Larrys	~4	cylindrical	rectangular box

Acknowledgment:

This work has been performed thanks to the INERIS National Institute for Industrial Environment and Risks, and the IFSTTAR French institute of science and technology for transport, development and networks.


Ronan L Hébert

Matthieu Angeli

Eric Doehne

Sandra Dochez

Andrea Hamilton

RECORDING OF THE THERMAL EVOLUTION OF LIMESTONES UNDERGOING EXPERIMENTAL ACCELERATED AGEING TESTS

Keywords: Heptahydrate, Mirabilite, sodium sulfates, accelerated ageing test

Sodium sulfates are widely regarded as the most destructive salts for porous stone, concrete and brick. Thenardite (Na2SO4), mirabilite (Na2SO4, 10H2O) and heptahydrate (Na2SO4, 7H2O) are the common phases that occur under surface conditions. The heptahydrate phase has been largely neglected in most modern work about salt weathering. However, several recent publications suggest that it could play an important role in geochemical and planetary processes. Therefore its role in the Na 2 SO 4 -H 2 O system should be clarified. We present here results of accelerated ageing experiments performed on samples of a micritic limestone under two different ambient temperatures: 20 C (i.e. below the upper limit of metastability of the heptahydrate), and 30 C (i.e. above this limit and also below the upper limit of stability of mirabilite). Thermocouples were placed into the samples in order to follow the evolution of the temperature of the stones during the weathering tests. This method allows the recording of exo and endo-thermic reactions that are linked to physicochemical processes, such as crystallization, wetting, evaporative cooling, etc. The experiments show significant differences in the pattern and degree of damage. In the experiment at 20 C, the loss of material is noticeable, and occurs as crumbling and scaling. In the experiment at 30 C, we observe just efflorescence with no visible loss of material. The recorded thermal evolution is also different for the two experiments, which provides some clues as to the different physicochemical processes occurring in each situation, depending on the "ambient" temperature.

Introduction

Sodium sulphates are widely regarded as the most destructive salts for porous stones, and other building materials such as concrete or brick [START_REF] Goudie | Sodium sulfate heptahydrate: a synchrotron energydispersive diffraction study of an elusive metastable hydrated salt[END_REF]. It is important to fully understand their crystallization process in porous networks and, in the future, to find ways to prevent or limit their damage to porous materials. Anhydrous sodium sulphates (i.e. Na2SO4) occur in several polymorphic forms labeled I-V (Kracek, 1929;[START_REF] Eysel | Crystal chemistry of the system Na2SO4-K2SO4-K2CrO4-Na2CrO4 and of the glaserite phase[END_REF]Rasmussen et al., 1996[START_REF] Rodriguez-Navarro | How does sodium sulfate crystallize ? Implications for the decay and testing of building materials[END_REF]. Phases I, II and IV are high temperature polymorphs (I >270°C and II >225°C). Phase IV is poorly known (e.g. Rasmussen et al., 1996;[START_REF] Rodriguez-Navarro | How does sodium sulfate crystallize ? Implications for the decay and testing of building materials[END_REF] but it is considered to be metastable with a small temperature range of existence (210-220°C). Phase V, known as Thenardite (Thn), is the stable phase at low temperature (i.e. close to ordinary temperature), low relative humidity and is the naturally occurring mineral. Phase III is metastable and is frequently observed during evaporation of a sodium sulphate solution [START_REF] Amirthalingam | Topotaxic phase change in Na2SO4[END_REF]. Note that there is no indication that phases I, II and IV occur as metastable phases close to ambient temperatures [START_REF] Steiger | Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress[END_REF]. Sodium sulphates can also occur as hydrates: the decahydrate (Mirabilite: Na2SO4.10H2O; Mir) which is stable below 32.4 °C (Fig. 1) and the heptahydrate (unnamed, Na2SO4.7H2O; SS7) which is metastable below 24°C (Fig. 1). These limit temperatures of stability (for Mir) and metastability (for SS7) correspond to peritectic temperatures where the hydrates undergo an incongruent melting to Thn. These temperatures are higher when peritectic reactions produce anhydrous form III (34.06°C for mirabilite, 28.25°C for heptahydrate; [START_REF] Steiger | Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress[END_REF]. Although it is known since the middle of the 19 th century [START_REF] Loewel | Observations sur la sursaturation des dissolutions salines[END_REF], the heptahydrate has been largely neglected in almost all modern works about salt weathering. Heptahydrate has been recently the subject of abundant publication (e.g. [START_REF] Genkinger | Crystallization of sodium sulphate: supersaturation and metastable phases[END_REF][START_REF] Hamilton | Sodium sulfate heptahydrate: direct observation of crystallization in a porous material[END_REF]Hamilton & Hall, 2008;[START_REF] Steiger | Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress[END_REF]Asmussen, 2008, Espinosa Marzal andScherer, 2008;[START_REF] Derluyn | Sodium sulfate heptahydrate I: The growth of single crystals[END_REF][START_REF] Saidov | Nucleation of sodium heptahydrate on mineral substrates studied by nuclear magnetic resonance[END_REF] suggesting that it very likely plays an important role into geochemical and planetary processes, and therefore it should be reconsidered in the Na2SO4-H2O system. Anhydrous Na2SO4 (III and V) can form directly upon evaporation of a sodium sulfate solution above 32.4 °C (Amirthalingen et al., 1977) but also by dehydration of mirabilite [START_REF] Rodriguez-Navarro | How does sodium sulfate crystallize ? Implications for the decay and testing of building materials[END_REF]. Hydrated Na2SO4 can form upon evaporation of a sodium sulfate solution below 32.4°C, as well as upon cooling of the same solution. The relationship between the heptahydrate and the decahydrate is not straightforward. There are several studies that show that mirabilite rarely forms directly from supersaturated sodium sulfate solutions (in the absence of seed crystals), and that the first solid to form is the metastable heptahydrate when cooling or evaporating a supersaturated sulfate sodium solution (e.g. [START_REF] Loewel | Observations sur la sursaturation des dissolutions salines[END_REF][START_REF] Hartley | The Spontaneous Crystallization of Sodium Sulfate Solutions[END_REF][START_REF] Genkinger | Crystallization of sodium sulphate: supersaturation and metastable phases[END_REF][START_REF] Rijniers | Experimental evidence of crystallization pressure inside porous media[END_REF]Espinosa Marzal and Scherer, 2008;[START_REF] Pel | The formation of meta-stable sodium sulfate heptahydrate in porous materials as studied by NMR[END_REF][START_REF] Derluyn | Sodium sulfate heptahydrate I: The growth of single crystals[END_REF][START_REF] Saidov | Nucleation of sodium heptahydrate on mineral substrates studied by nuclear magnetic resonance[END_REF]. The destructive effect of sodium sulfates in porous media is attributed to Mir (rather than Thn) especially when forming from highly supersaturated solution originating from the rapid dissolution of Na2SO4 [START_REF] Chatterji | Efflorescence and breakdown of building materials[END_REF][START_REF] Flatt | Salt damage in porous materials: how high supersaturations are generated[END_REF][START_REF] Tsui | Crystallization damage by sodium sulfate[END_REF][START_REF] Steiger | Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress[END_REF]. SS7, according to [START_REF] Rijniers | Experimental evidence of crystallization pressure inside porous media[END_REF] is considered to be harmless Several studies (e.g. [START_REF] Rodriguez-Navarro | Salt weathering: influence of evaporation rate, supersaturation and crystallization pattern[END_REF][START_REF] Flatt | Salt damage in porous materials: how high supersaturations are generated[END_REF][START_REF] Scherer | Stress from crystallization of salt[END_REF][START_REF] Angeli | Influence of temperature and salt concentration on the salt weathering of sedimentary stones with sodium sulphate[END_REF] have shown that temperature plays a major role on salt weathering of sedimentary rocks. For this reason, we present in this study some results of accelerated ageing experiments performed on samples of a detritic limestone under two different ambient temperatures: 20°C, which is below the upper limit of metastability of SS7, and 30°C, which is above this limit and also below the upper limit of stability of Mir. The goal of this experimental study is to evaluate the possible role of SS7 in the damage of porous material by sodium sulfate crystallization. The originality of this study is that we placed thermocouples into the samples in order to follow the evolution of the temperature of the stones during the accelerated ageing tests. We discuss the temperature curves for each phase of the cycle with the aim of understanding the processes that occur and that are responsible for the damage observed in the two experiments.

Material and methods

Material

Cubic samples (7 cm/side) of a lutetian-aged limestone from the Paris Basin commercially known as "Roche fine" have been used in this study. This rock has been chosen for several reasons: it is relatively homogenous from one sample to another, it has been widely used for construction in Paris (historic buildings and monuments), it is one of the stones required for use in restoration and construction in the protected areas of the "Ile de France" region, and it is also the rock that has been used in previous experiments from which the current study originated (e.g. [START_REF] Angeli | Influence of temperature and salt concentration on the salt weathering of sedimentary stones with sodium sulphate[END_REF]. Roche fine is a fine-grained detritic limestone made of calcite (90%) and quartz (10%) with a very low tensile strength (1.5 MPa), a high porosity (37,2%) with two pore entry sizes (mainly around 20 µm, and another one between 0.03 to 5 µm). Complete hydromechanic properties are available in [START_REF] Angeli | Salt crystallization in pores: quantification and estimation of damage[END_REF] We used four samples per experiment. All samples undergo several cycles of washing by immersion in demineralized water at room T for 6 hours and drying at 105°C until constant weight, in order to remove any possible natural occurrence of soluble salts in the stone. A single thermocouple is placed in each cube at the center of the sample. The drill hole used to insert the thermocouple is filled up with a mixture of lime and limestone powder resulting from the drilling of the sample.

The thermocouples (type K) are connected to a thermal acquisition station (Agilent 34970A) with 22 recording channels. The acquisition station is connected to a personal computer using ® Agilent BenchLink Data Logger software for programming the thermal station and recording data. According to the manufacturer, the accuracy of measurement is ± 0.5°C.

Methodology

The samples are subjected to 24 hours accelerated ageing cycles modified from the EN 12370 standard test. The cycles consist of three stages (Fig. 2): -(i) 2 hours imbibition at "ambient" temperature. The 4 samples are placed in a cooled incubator (® LMS 303NP) where the temperature is fixed at 20°C or 30°C according to the experiment. 2 samples soak in approximately 0.5 mm height of a sodium sulfate solution of 5.5 weight % Na2SO4; the two other samples are used for control and comparison. For one of these samples, the sulfate sodium solution is substituted with demineralized water, and for the other one there is no imbibition solution at all. Each sample is placed into an individual box that is supplied with solution by a system of drop by drop, and with an overflow in order to keep the level of solution constant. The solutions are stored at ambient temperature in the incubator. -(ii) 16 hours of drying. All the samples are placed in a drying oven (® memmert UE600) at 105°C. -(iii) 6 hours of cooling. The samples are replaced into the cooling incubator with fixed T so that they return to their initial "ambient" temperature, i.e. respectively 20°C (exp20) and 30°C (exp30). The samples are weighted and photographed after each stage. Seven full cycles have been performed during which temperature was regularly recorded (every 3 minutes).

Results

Rock decay vs. cycle

Figure 2 shows the different samples at the different stages of the 7 cycles. The reference samples do not show any damage after 7 cycles. The weight monitoring (Fig. 3) confirms it as their weight remains constant through the full experiment whatever the ambient temperature. It suggests that the limestone is inert with respect to demineralized water.

Fig. 2: macroscopic evolution of the samples during the accelerated ageing cycles

Samples subject to Na2SO4 solution at 20°C show intense damage starting with efflorescence (from cycle 1), then contour scaling (from cycle 3), crumbling, and desquamation leading to an important change of shape. The distribution of the damage is heterogeneous. An important loss of material is observed at the bottom of the samples from cycle 4. It seems related to a granular decohesion as suggested by the presence of loose material at the end of the imbibition stage (starting from cycle 3). Vertical faces are more affected by crumbling and the top of the samples by cracking and desquamation. On the opposite, samples in exp30 do not present any major damage but just efflorescence that starts to occur as soon as cycle 1 and increases as the cycles go by. The monitoring of sample weight during cycles (Fig. 3a) shows consistent results with the shape evolution described above. All the samples undergo a similar weight increase during the two first cycles, but the evolution differs from cycle3. Indeed, samples in exp30 show a continuous and progressive weight gain, while samples in exp20 show a smaller weight increase during the third cycle, before undergoing weight decrease from cycle 4. All these results are consistent with previous research [START_REF] Price | The use of the sodium sulphate crystallisation test for determining the weathering resistance of untreated stone[END_REF][START_REF] Angeli | Salt crystallization in pores: quantification and estimation of damage[END_REF][START_REF] Angeli | Influence of temperature and salt concentration on the salt weathering of sedimentary stones with sodium sulphate[END_REF].

Temperature evolution of the control samples

The temperature curves for the "dry" reference samples are shown on Fig. 4. The curves are very regular. They show in particular a constant and high heating rate of the samples during drying until reaching a plateau at 105°C. Then a progressive and constant decrease of T is observed during cooling until reaching the ambient temperature where a second plateau is observed. This indicates that the studied limestone presents an inert behavior with respect to temperature. T curves are consistent with the model of heat diffusion through the rock. The temperature curves of reference samples undergoing imbibition by demineralized water (Fig. 5) are also very regular and similar whatever the ambient temperature. During wetting, the temperature of the sample decreases ( -8 °C in exp20, and -6°C in exp30). This is due to the evaporative cooling. The drying period is characterized by a variation of the heating rate. First we observe a rapid increase of T until a sort of plateau at ~59°C, then a deceleration of the T increase up to an inflection point at ~85°C where it becomes more rapid again (but not as much as the beginning of drying) until reaching the final plateau at 105°C. The variation of the heating rate indicates the occurrence of two endothermic reactions, the first one requiring more energy than the second one. The first one leading to the ~59°C plateau, would correspond to the departure of water on the surface as well as the water trapped in the 20µm size entry pores, the second one (inflection point at 85°C) would correspond to the departure of the water trapped within the small entry size pores. Cooling is characterized by a rapid, progressive and continuous return to ambient T.

Temperature evolution of the samples subject to salt weathering

Before discussing the different stages of the cycles for each experiment, it is worth noticing that the first cycle is always very different from the following cycles that are more repetitive, as if the first cycle is a transitory phase (Fig. 6). The difference between cycle 1 and following cycles is the absence of salt in the porous network of the stone at the beginning of the imbibition. Therefore cycle one will not be discussed in this study, in particular the drying stage on which we cannot rely.

Evolution of the temperature during imbibition

First imbibition is characterized by a significant temperature decrease of several degrees (~ -5.5°C in exp20, and in exp30) that lasts all along this stage. From cycle 2, the imbibition is characterized first by a temperature rise immediately followed by a fall of temperature. The temperature rise matches with the hydration of the system, which leads to the dissolution of the salts (Thn?) formed during former drying period. Depending on the salinity and T, the rehydration of the system can also correspond with the formation of hydrates. This interpretation is besides consistent with the lack of T increase during the first imbibition as there is no salt in the sample at the beginning of the experiment. Temperature decrease is due to evaporative cooling as in the first imbibition.

It is worth noting that the T rise is more important in exp20 than that in exp30 (more Mir crystallized). In a similar way, the T decrease due to evaporative cooling increases progressively with cycles in exp20 whilst it decreases in exp30. Then several chemical phenomena occur during this part of the cycle, which correspond to the major (if not single) phase of damage.

Evolution of the temperature during drying

The drying behavior is very different during the first cycle than during the subsequent cycles. In exp20, the drying period can be decomposed into 4 stages: (i) rapid heating, similar to what happens on the reference samples; (ii) slower heating period; (iii) rapid heating again, same velocity as in first stage; and (iv) temperature step at 105°C (plateau). The differences in the drying period all occur during the lower velocity heating. Comparing this velocity to the dry sample velocity shows that an endothermic reaction occurs at this moment, and that this reaction draws its energy from the heat of the oven. There are several features to be noted: -the drying velocity decreases progressively from the 2nd cycle to the 5th cycle -from the 5th cycle to the end, the temperature remains constant for a period and then starts to increase again the duration of the step at 105°C increases at every cycle It appears this endothermic reaction requires more and more energy at every cycle, showing that there are more and more reactants (salts) at every cycle. Since the salt content in the sample increases at every cycle, we can assume that the hydrate (Mir?) production during the imbibition (damaging) stage increases accordingly. Thus there is a larger amount of hydrate to dehydrate. From this point, the "evaporation duration" is defined as the time necessary to reach 105°C. It is a proxy for the amount of time needed to remove all the water in the sample. The evaporation duration increases until the appearance of decay. This suggests that the efflorescence partially blocks the pores at the surface, thus reducing the evaporative surface. Once the decay starts at cycle 4, the evaporation duration is progressively reduced. This increase in the drying velocity shows that the decay of the surface creates cracks and/or reduces the sealing efficiency of the salt crust.

The drying stage in exp30 is much simpler. The initial drying velocity is similar to the reference sample until water starts to evaporate. Then the evaporation velocity starts to decrease, which corresponds to the evaporation of water in the sample and the precipitation of Thn. The remaining bulk (stone + Thn) is then heated to 105°C. Apparently very little of the damaging hydrated phase occurred during imbibition and no damage has been observed in the sample after 7 cycles. Mir is a mineral that grows easily but nucleates very seldomly. A possible explanation for the lack of Mir crystallization in exp30 could be that since Mir is difficult to nucleate [START_REF] Hartley | The Spontaneous Crystallization of Sodium Sulfate Solutions[END_REF], it could need an intermediate heptahydrate phase nucleation to be initiated and later grow from the heptahydrate nucleus as saturation increases by evaporation or cooling (but T is over the limit of metastability of SS7 in this precise case). Another explanation (according to the phase diagram) is that until supersaturation is reached, no Mir can precipitate, and once it has, only a small amount of Mir crystallizes.

Evolution of the temperature during cooling

The temperature evolution during cooling is more straightforward. It requires about 2 hours for each sample to return to its ambient temperature from the peak of T achieved at the end of the drying stage (Fig. 6). This cooling is fast (~50°C/hour) at the beginning, and slows down when approaching ambient T. We can consider that all the samples have returned to their ambient T after 2 hours and therefore, they do not undergo further temperature variation until the beginning of next cycle.

Conclusion

After 7 complete cycles of 24 hours, the samples at 20°C and 30°C show completely different damage patterns as well as different temperature curves. It illustrates the difficulty of completely removing the water from the rock under certain conditions--as illustrated by the impossibility of reaching 105°C at the end of the drying period for the experiment at 30°C. This suggests that the crystallization of salts can fill the pore entries, trapping sodium sulfate solutions or more likely hydrated phases. It requires either more energy or a longer period of drying to release water from the samples. This result raises questions about the accuracy and the relevance of the standard stone durability tests.

The evaporative cooling phenomenon shows that imbibition is a favorable stage for the precipitation of sodium sulfate hydrates, especially if the concentration of the solution is close to supersaturation. Evaporation during this stage can trigger nucleation, and the cooling favors crystal growth. The temperature profiles of this experiment do not enable the positive identification of two different hydrates. Supersaturation is achieved sooner at 20 than 30°C which drives more hydrates to precipitate during imbibition. In addition, as complete dehydration of the sample is not reached in exp30, there is less (or even no) Thn to transform during imbibition. Therefore the current results do not allow an estimage of the role of SS7 in the weathering of rocks by sodium sulfates. Finally, this study shows that recording temperature cycles can provide a simple and inexpensive method that enables monitoring exo/endothermic physico-chemical processes such as phase crystallization. It can be useful to identify mineral precipitation, sequences of crystallization and the important stages (wetting, evaporation, cooling) that are involved in rock damage.

Fig. 1 :

 1 Fig. 1: Phase diagram of the Na2SO4-H2O system. Stars represent the concentration and Temperature for the two experiments.

Fig. 3 :

 3 Fig. 3: Weight evolution of the samples in dry state at 20°C (A) and 30°C (B)

Fig. 4 :

 4 Fig. 4: Temperature evolution of the "dry" reference during the 7 cycles (A), and for a cycle of 24 hours (B). The Temperature profile is in exp20.

Fig. 5 :

 5 Fig. 5: Temperature evolution of the reference samples undergoing imbibition by demineralized water during the 7 cycles (A), and for a single cycle of 24 hours (B).

Fig. 6 :

 6 Fig. 6: Temperature evolution of samples undergoing accelerated ageing cycles with sodium sulfates at 20°C (A) and 30°C (B).