Ch Person

E Rius

J Ph

Coupez

Hybrid 30 integrated circuits at millimeter-wave frequencies -advantages and trends

Keywords: Microwave integrated circuit, Hybrid integrated circuit, Millimetric wave, Three-dimensional integration, Coplanar technology, Microstrip line, Antenna array, Patch antenna, Dielectric substrate, Multilayered structure. INTEGRATION HYBRIDE 3D AUX FREQUENCES MILLIMETRIQUES : Circuit integre hyperfrequences, Circuit integre hybride, Onde millimetrique, Integration tridimensionnelle, Te chnolo gie coplanaire, Ligne microruban, Antenne reseau, Antenne plaque, Substrat dielectrique, Structure multicouche

The implementation of passive RF functions is usually done by means of coplanar waveguides at mm-wave fre quencies. We propose original solutions for improving their reliability, by integrating the parasitic mode filters as dielectric bridges through a 3D integration approach. Furthermore, original compensating methods are propo sed in order to control the cut-off frequencies induced by the bounding networks.

The 3D hybrid approach can also be efficiently used so as to accommodate the association between active, passive and radiating components. The 3D-CPW techno logy is therefore completed with Thin Film MicroStrip configurations (TFMS), while composite ceramic/foam architectures become thinkable for radiating arrays.

I. Introduction II. Emerging integration techniques for mm-wave applications III. 3D integration techniques for improving the unipla nar technology IV. Hybrid integration: Benefits of a 3D approach V. Conclusion

I. INTRODUCTION

Wideband and multimedia services contribute today to the development of millimetre-wave applications regarding required data bit rates and point to multipoint distribution constraints. The maturity of the technology at such fre quencies still remains a critical issue for the manufacturers. They need to deliver to private and public users, commer cial systems at reduced cost and with multiple possibilities.

In this way, significant efforts are provided to improve and accommodate new integration technologies for both the operators delivering new competitive services with res pect to the cable (Local Access Network, Wireless Local Loop, ... ), and the customers (interactivity, voo., ... ).

The integration reproducibility and reliability become one of the key parameters, as well as the problem of compatibility between the sub-modules constituting the RF front-ends of such wireless equipments.

In this paper, we focus on original technological solu tions which can be proposed in order to meet the pre vious described requirements.

*Laboratoire d'Electronique et Systemes de Telecommunications, LEST -FRE CNRS n° 2269, ENST Br./UBO -BP832 -29285 Brest Cedex -France, e-mail: Christian.Person@enst-bretagne.fr

II. EMERGING INTEGRATION TECHNIQUES FOR MM-WAVE APPLICATIONS

In the past few years, the intensive development of mm-wave applications has significantly contributed to the evolution of integration technologies and techniques.

The waveguide solution is certainly the best approach in terms of electrical performances (losses), but brings also drastic constraints regarding compatibility (in parti cular with active MMICs), size, and as a consequence cost. For these reasons, the traditional machined copper struc tures have been replaced by plastic injected or molded waveguides [I]. This is a significant contribution for the reduction of cost and weight, with the condition that spe cific metallisation procedures are used (loss considera tions), but also for design flexibility (complex geometrical shapes available).

For instance, such an approach has been efficiently used for implementing high performance filters, diplexers and antenna arrays [START_REF] Meinel | Commercial applications of millimeter waves: History, present status, and future trends[END_REF] . Also, the reliability with active mm-wave components induces the use of res trictive transitions and interconnecting structures for according volume (waveguide) and printed MMIC (Mono lithic Microwave Integrated Circuits) technologies. For such reasons, the crw technology has been considered for a long time as the only solution for developing mm wave integrated functions, with reasonable tolerances in terms of cost, performances and reliability. Indeed, in comparison with microstrip architectures, the proposed one brings interesting properties, especially about design flexibilities (according to the additional design variables) and losses, the electric and magnetic fields being concen trated at the dielectric-air interface instead of within the dispersive dielectric substrate.

The major drawbacks of this emerging technology lie in the control of the fundamental parasitic dispersive modes rejection by means of filtering air-bridges, as well as microstrip or parallel plate modes potentially genera ted when a metallic packaging is used.

Such constraints have motivated the numerous efforts of MMIC designers for developing integrated air bridges (for reproducibility and modelisation reasons) [3]. This way, the achievable reduced dimensions (crw slot widths) are considered for reducing backward radiations in expectation of packaging incidences.

On the other hand, the MMIC technology is really dedicated to component mass-production. The actual trend followed by designers is rather oriented towards hybrid technologies for meeting cost, reconfigurability and other drastic requirements specifically imposed by emerging mm-wave markets. In addition, RF filters and antennas can not reasonably be implemented on GaAs or InP semi-conductor substrates for size and cost motiva tions, as well as for efficiency reasons.

The achievement of hybrid technology for mm-wave wireless services appears today as a real opportunity, provided that interconnections with active MMICs can be optimised and electrically well controlled. Furthermore, hybrid technologies are based upon various technological configurations depending on the process and materials involved. This allows one to improve the performances of hybrid systems and sub-systems as original and opti mal combination of integrated functions can be manufac tured along several methods.

Two fundamental questions have to be answered: -Which processes (materials, procedure, .. . ) can be used simultaneously, depending on the basic func tions to be associated and integrated? -How these processes can be combined to lead to an universal hybrid integration procedure for mm wave products?

III. 30 INTEGRATION TECHNIQUES FOR IMPROVING THE UNIPLANAR

TECHNOLOGY

Many studies have shown that the coplanar wave guide can be considered as a good alternative to micros trip lines in the millimeter frequency range [START_REF] Houdart | Coplanar lines: application to broadband micro wave integrated circuits[END_REF] (high integration level, interconnection facilities, direct ground connections, great flexibility for circuit design). Howe ver, there is one major drawback in dealing with coplanar designs: The parasitic and strongly dispersive coupled slotline mode may be excited by non-symmetrical copla nar waveguide-discontinuities, such as bends or T-junc tions for example. The suppression of such perturbing modes is achieved by the insertion of bridges over the centre conductor that enforce identical potential values between lateral ground planes.

Generally, bridges in microwave-integrated-circuits (MIC'S) are implemented by thin metallic wires (<I> "" 17 µm), which are bonded to the ground planes as shown on figure 1. However, such an operation induces numerous problems in reproducibility and positioning, which deteriorate the circuit performances. Moreover, the mechanical resistance of such bridges does not gua rantee circuit reliability.

In this paper, we present how these problems can be overcome by using a new multi-layer approach for bridge integration. However, dielectric bridges appear as strong discontinuities on coplanar circuits, and must be compensated. Thus, we propose a way to reduce the parasitic influence of the bridges by locally tuning the dimensions of both the bridges and associated compen sating lines.

Thick-film multilayer technology for dielectric bridges

The basic idea consists in using a protective dielec tric layer for interconnecting ground planes by means of a thin conductive printed strip. MMtcs solutions have been proposed in the literature, leading to suspended bounding strips as mentioned on figure 2. Unfortunately, this can not be used in MIC technology, because of etching possibilities that are presently available. Pont dielectrique integre en technologie multicouche.

To overcome such limitations and insure a better mechanical resistance, well-known multilayer hybrid technologies (MHIC) can be employed for implementing dielectric bridges on MICS. Indeed, in accordance with the significant progress accomplished by dielectric ink manufacturers, it becomes thinkable to use standard pro cess, like the Thick Film technology, for realising inte grated mode filtering structures among uniplanar guides. As illustrated in figure 3, dielectric layers can be locally or globally laid down over a first conductive level. Via holes are made in this dielectric layer, to ensure electrical connections between the two lower ground planes and the bridge structure. The second conductive level is directly screen-printed and etched on the upper side of the dielectric layer.

At mm-wave frequencies, photo-printed dielectric materials (HERAEUS ink KQ 110 [START_REF] Barnwell | High performance material technologies for advanced circuit assemblies and MCMs" -Microelectronics In ternational Jo urnal of ISHM Europe[END_REF] ) are employed to ensure punctual interconnections between the different layers by means of miniature via holes (diameter: 40-50 µm). Notes that laser drilling can also be considered to achieve smaller diameters (#10 µm). This requires a tunable ligthwave equipment (excimer laser) for selec ting the materials to be drilled.

The proposed thick film process enables us to obtain robust integrated bridges and eliminate potential pro blems due to positioning and reproducibility. In addition, this process provides an easy control of bridge dimen sions and shape.

Parasitic influences of the dielectric bridges

Due to the reduced height of the dielectric layers (typically between 10 µrn and 20 µm, for the MIC process used), an additional and significant capacitive parasitic coupling effect is introduced between the inner conduc tor of the coplanar waveguide and the upper short-circui ted strip. Obviously, the characteristic impedance and the effective permittivity are significantly modified in the bridge region. In this study, bridges are modelled by equivalent cascaded transmission lines, taking into account the modified electrical parameters of the local multi-layer structure.

Figure 4 shows the influence of the dielectric layer (B) and of the dielectric bridge (C) on the electrical para meters of a 50-Q coplanar waveguide (A). It should be noted that this type of dielectric bridge presents a sharp discontinuity for the coplanar waveguide. Fortunately, the overall length of a dielectric bridge is usually small, i.e. about 50 µm. As a result, the influence of one single bridge on the transmitted RF signal remains negligible below 40 GHz.

However, practical uniplanar designs require a large number of bridges. This results in an accumulation of parasitic effects on the coplanar circuit and, conse quently, deteriorates the electrical responses. .

, I

Influence du pant sur une ligne de transmission avec/sans pant.

ill.3. Compensating dielectric bridges

Specific procedures must be found for minimising the distortion due to the low characteristic impedance nearby the bridge discontinuity.

The basic idea is to associate high characteristic impedance sections on each side of the bridge, to com pensate its low characteristic impedance and recover pro per matching conditions [START_REF] Koster | Investigations on air bridges used for MMICS in coplanar waveguide technique[END_REF][START_REF] Stephan | Integration of various types of compensated die lectric bridges for mm coplanar applications[END_REF][START_REF] Rrus | Theoretical and Experimental Study of Various Types of Com pensated Dielectric Bridges for Millimeter-Wave Coplanar Applications[END_REF]. This is achieved by reducing, in an appropriate way, both the width of the inner conductor under the bridge and the distance (D) it extends each side (Fig 6).

The complete structure can now be considered as a low-pass filter (L-C-L equivalent model for A-type com pensation) for which the cut-off frequency is related to the parameter D. The dual compensating network can be implemented in a similar way leading to the B-type com pensation solution (Fig. 6). The main advantage of this type of structures lies in the reduction of technological constraints: it does not require neither very narrow strips and an accurate positioning of the bridge. Mode/es equivalents des pants compensis.

Compensated 50-Q transmission lines: experiment

Various dielectric bridge configurations, with and without compensation networks on 50-Q coplanar wave guides, can be designed. The length of each dielectric bridge determines the cut-off frequency of the bridge and, therefore, the return loss level within the operating bandwidth.

"On wafer" measurements have been carried out on compensated 50-Q transmission lines, over the 1 GHz to 50 GHz frequency band. Both types of compensation yield acceptable results with return loss levels better than 24 dB at 40 GHz (see Fig. 7). The compensation tech nique does not introduce significant additional losses (<0,7 dB up to 40GHz). IOOµm coupled slotline-mode on a coplanar structure, especially at high frequencies. Therefore, preliminary measure ments were first carried out on Te-and cross-junctions to verify the integrated bridges efficiency in suppressing the above unwanted mode. All tested circuits, built on an alumina substrate (Er = 9.6, thickness = 635 µm), have 60-µm long bridges. Despite lumped electrical L-C models are usually used [START_REF] Koster | Investigations on air bridges used for MMICS in coplanar waveguide technique[END_REF][START_REF] Rrus | Theoretical and Experimental Study of Various Types of Com pensated Dielectric Bridges for Millimeter-Wave Coplanar Applications[END_REF], we have implemented distributed models based upon cascaded transmission lines (bend-junction: see Fig. 8). The electrical parameters (Zc, £reff) per unit length of the lines are calculated with a static finite diffe rence method (2-D analysis). Jonction en croix optimisee: mesures.

nique can be applied to many cases, e.g. for different bridge lengths and for different numbers of bridges, various topologies of resonators are available for a given response.

Application to wideband millimeter-wave band-pass filter

To validate the above compensated bridges concepts, we designed basic microwave functions such as filters that use integrated bridges. A 5-poles CPW band-pass fil ter with a 19-dB return loss, 23-GHz operating band width centred at 31 GHz (# 75% relative bandwidth) was synthetised. First, a standard topology is considered. It consists in symmetrical and asymmetrical short circuited stubs and A,g/4 series inverters.

The characteristic impedances and dimensions of each section of the structure are presented in the table of Figure 10. Corresponding experimental performances (with classical bonding) are also shown. It should be noted that the same strip width (80 µm) is employed for all resonators. Consequently, only the slot widths are modified to reach the desired characteristic impedances. Differences observed between ideal and experimental values (Fig. 10) are mainly due to the fact that junctions were not taken into account in this design model, as well as the air bridges. The first prototype (A) uses 60-µm-long bridges with respect to the central CPW line. The compensation of the prototype (B) was determined in relation with the 100µm bridge located on the central conductor. Experimental results are in agreement in both magnitude and phase with the desired responses.

Moreover, it should be noted that return losses are about 15 dB in the first case and 19 dB in the second case. Insertion losses are around 1 dB at the central fre quency (31 GHz) and the rejection level is correct.

III.4. CPW technology at MM-wave frequencies:

Physical & Electrical limitations Compared to classical uniplanar technology, the main advantage of the multi-layer uniplanar technology is to provide robust and reproducible integrated bridges. Indeed, with this new technology, it becomes possible to control both the shape and the dimensions of bridges. Thus, it is easier to model their parasitic effects in terms of fundamental propagation mode.

Nevertheless, the coplanar technology can not be uni versally used, especially for future wireless systems because of their required operating frequencies and bandwidths. For instance, extremely narrow operating bandwidths have to be respected f o r wireless point to point or point to multi-point mm-wave links. This imposes specific constraints about RF filters for example. One can still consider a CPW implementation by using separating intermediary ground planes. This achieves low coupling levels between adjacent resonant printed structures. Unfortunately, it has some incidence on the overall dimension (additional losses) of such passive functions, and complicates the modelisation process (full-wave model required).

In this way, additional but complementary guiding structures must be developed and considered to imple ment both efficient radiating elements and loss-less non disturbing miniature narrow-band functions.

The previous described 3D CPW technology is this time supposed to ensure the interconnections between different sub-modules constituting the RF front-ends. It behaves like a transparent intelligent interconnecting mother-board, including at the same time biasing networks.

The 3D integration process can be employed once again, to provide the integration of loss-less interconnec ting or filtering structures by means of Thin Film Microstrip Lines (TFMS), or efficient radiating stacked elements by using ceramic/foam composite substrate. Such tech nological orientation is also well-suited for improving reliability with active MMICs.

IV. HYBRID INTEGRATION: BENEFITS OF A 3DAPPROACH

Considering multi-layer configurations appears today as a good compromise for optimising size, and perfor mances on numerous RF. functions. As shown before, this can be used for example for improving the reliability of the CPW technology. Previous works were also reported at microwave frequencies. They emphasised on the great interest of using overlapped Metal-Dielectric-Metal confi gurations, in particular with the microstrip approach. Such an orientation brings a complete design flexibility and has been commonly generalized for bypassing intrinsic limita tions of basic MIC technologies [START_REF] Person | Design of high performance band-pass filters by using a Multi-layer Thick film technology[END_REF].

At mm-wave frequencies, additional constraints rela ted to the operating wavelength may also encourage desi gners to employ 3D architectures:

-The dimensions of the structures become small, then requiring the use of thin substrates (for preven ting substrate surface modes) -Mounting and bonding operations involved with hybrid integrations (to connect active components and passive functions) may introduce non-negligible perturbations. With the generalisation of chip com ponents and Flip-Chip techniques, the mother-board substrate dimensions should absolutely become com patible with regard to MMIC pads and bumps. Accessing to such low-loss thin substrates has been a major preoccupation in the past few years, with regard to compatibility, measurement, and access conditions.

Unfortunately, thin substrates or standard ceramic sub strates are not really appropriate for integrating radiating elements regarding especially radiation efficiency, gain or achievable bandwidth. On the other hand, high coupling levels can be easily achieved through available overlapping possibilities of multileveled architectures. The combination of multi-layer ceramic substrates and low permittivity foam support appears as an optimal composite substrate for such applications. This is discussed in part IV.2.

IV.1. TFMS technology for MM-wave integration: an attractive and complementary transmission line topology

Thin substrates are recommended at high frequencies for microstrip technology to prevent from parasitic sub strate surface modes. Furthermore, in an hybrid integration approach, MMIC active components dedicated to mm-wave applications are characterised by reduced dimensions, especially about external connecting pads. Compatible interconnection techniques must be proposed to minimise electrical associated parasitic effects. The thick-film pro cess can be used for developing Thin Film MicroStrip lines (TFMS) [10] and, as a consequence, 50-Q feeding line widths comparable to MMIC access pad dimensions.

Figure 12 describes the basic corresponding topology based upon a motherboard alumina substrate, with a first conductive layer (assumed as the TFMS ground plane), a thin dielectric sheet (10 to 40 µm) deposited by a screen printing procedure, and an upper etched layer (microtrip conductor).

In the case of a £r = 4. 3/40 µm dielectric sheet, a 50-Q line can be synthesised by means of a very thin strip (W = 75 µm) in accordance with the dielectric thickness and, consequently, with common MMIC pad dimensions. In addition, the W /h ratio is compatible with a proper description of the technological structure using standard CAD tools based upon transmission line analysis, despite very high operating frequencies.

Avoiding tedious electromagnetic simulations consti tutes a significant advantage. Note that this is not the case when dealing with crw complex architectures or oversized microstrip ones (with respect to wavelengths).

IV.1.1. Accessing to TFMS structures [START_REF] Le Nadan | 3D inte grated narrowband filters for millimetre-wave wireless applica tions[END_REF] The reduced dimensions offered by the TFMS technology are well-suited for designing efficient RF-circuits at mm wavelengths. However, accessing to such technological structures can bring some problems in terms of dimension compatibility with standard feeding technologies ( crw and microstrip), electrical behaviour (parasitic effects, band width & frequency limitations) and mechanical tolerances.

Thus, we have developed high performances crw to TFMS transitions, then keeping in mind the advantage of the CPW motherboard concept in the millimeter-wave range. Such a transition is based upon a Ag/4 overlapped coupled lines separated by a thin dielectric sheet which constitutes simultaneously the TFMS substrate. Note that the TFMS ground plane is directly connected to the CPW ground planes and may also be considered as a potential CPW parasitic mode filtering structure (Fig. 13). Transitions tete-heche CPW -TFMS: masques.

Back to back transition

An optimisation procedure was applied to control the IV.1.2. TFMS approach for designing efficient passive functions lower and upper cut-off frequencies of this series capaci tive coupling structure. Additional inductive and capaci tive sections were cascaded each apart from the MIM (Metal-Insulating-Metal) capacitance to lead to a higher order highpass filter. Quite a wide operating bandwidth can be achieved, as mentioned on figure 14 where experimental results are reported for a 40% relative bandwidth transition (for a -17 dB return loss) centred at 41 GHz. Wider band widths can be considered, using the numerous tuning variables, and taking advantage of the wide range of cou pling achievable with the 3D approach. \;

•-700 GHz � 1

•50dB

I

Mlo.975 97 cJ8 l-+-+-.+--1--1---1----l--1---l-_J 11-000 GHz l-+-+--r.+--l--+--1----l---l--l-_J Transitions tete-beche CPW -TFMS: mesures.

Insertion losses of about 0.8-dB only have been regis tered inside the 15-dB return loss bandwidth for a 1 mm long structure. Such transitions appear really as a conve nient solution for improving interconnections between active and passive modules in future millimeter-wave hybrid front-ends.

With the TFMS technology, the strip width becomes fairly small for a wide set of characteristic impedance values. Thus, it becomes therefore negligible with res pect to wavelengths in the 30-100 GHz range. In this case, TEM approximations can be considered, and the design of passive function like filters and couplers can be directly implemented using traditional topologies and synthesis methods (parallel coupled lines filters, Ag/4 resonators, etc .. ). Furthermore, since the substrate thick ness is kept small, quite low coupling levels are achieved with standard Ag/4 coupled resonators, even when the smallest etchable spacing is reached (the odd and even mode impedances of the coupled lines are quite similar (Zoo #Zoe) because of the ground plane proximity) . This is especially attractive for synthesising very narrow-band band-pass filters at mm-wave frequencies. The figure 15 describes a TFMS parallel coupled lines fil ter designed at 41 GHz, and compared with its corres ponding microstrip version on 254-µm alumina substrate. Filtres faible bande TFMS I microruban: masques.

H=254µm

Note that the L/W and LIS ratios (W, L: Width, Length of the resonators, S: Spacing between adjacent resonators) for each resonator allows a transmission-line description with the TFMS solution.

Figure 16 gives some experimental results obtained with the TFMS filter, including CPW to TFMS transitions for probe testing facilities. No tuning operation is required after manufacturing despite the severe requirements. Some fair agreement is obtained between simulation and measurement given the fact that the transmission losses are not accounted for the model. Some specific effort must be provided by both manu facturers and designers for decreasing conductive losses with TFMS topologies with the reduced width and thick ness of the conductors. Another solution, proposed in Filtre passe-bande TFMS : simulations & mesures.

[11] consists in artificially lowering the filter order by using an adequate and original synthesis technique. The CPW to TFMS transition previously described can be consi dered as an overcoupled resonator (overlapped struc ture). It can be introduced within the filter which contributes to reduce its size, losses and technological sensitivity.

IV.1.3. TFMS access for improving compatibility with MMIC's

The achievement of commercial mm-wave MMIC's and chip is a key factor for the emergence of hybrid technolo gies in future applications. The critical dimensions of these new products (typically 200*300 µm -thickness 120 µm) become also critical in an electrical point of view.

The connecting bounding wires usually induce important parasitic effects depending on their length, dia meter and contact quality with pads when using micros trip or CPW technologies. Compatible dimensions between the feeding line network and the MMIC pads are not really respected, as well as the common metallic sup port manufactured for compensating thickness disconti nuities.

The TFMS configuration is undoubtly an attractive solution for accessing to MMICs and chips with lowest perturbation phenomena:

-Dimensions are quite similar between active and passive feeding ports.

-Bounding operations can be made less perturbing by minimizing the number of wires and their length. Since the MMIC is usually mounted on a ground plane, the thin dielectric sheet can be etched, thus ensuring the perfect control of the chip location when reported on the motherboard alumina substrate. In addition, the ground plane continuity is readily obtained, and no additional bounding wires are required for interconnecting MMIC and CPW or microstrip ground planes (see Fig. 17). TFMS structures) will be found in number of applications that involve future mm-wave Flip-chip components. Photo-imageable dielectric materials are of great interest to realise discrete connections between lower shielded printed structures and upper surface-mounted Flip-Chip components.

Figure 18 depicts a beam-lead schottky diode moun ted across a 30 µm width slotline, with a separating 20 µm dielectric layer. The interconnection between the diode pins and the slotline arms is performed through 40-µm via holes. Numerous advantages appear:

-Using 3D technological process, the passive circuit is completely shielded and protected, thus simpli fying mounting -Such an implementation also contributes to sim plify the structure analysis. Indeed, the input impe dance can be extracted at the diode connecting ports, using either Agilent-Mos™ multilayer library or 2.5D Agilent-Momentum™ simulator. Conse quently, the non-linear modelisation can be done on Mos © once the complete passive structure is mode led as a multiport network. This is illustrated in figure 19 for a balanced mixer based upon an annu lar slotline structure [START_REF] Hettack | Practi cal Design of uniplanar broadband sub-systems; application to wideband hybrid magic tee[END_REF] (Fig. 18).

IV.2. Mixing technologies for efficiency and miniaturization

Cost reduction and miniaturization efforts will be achieved in the future if global synthesis and integration techniques are considered. Hence, the radiating network should be located as close as possible to the RF-front-end modules. In addition, this will also result in improving the c/N ratio as well as the reconfigurability of such a module.

With the selected hybrid integration approach, active and passive functions are integrated on alumina mother board substrate, i.e. high permittivity substrate, for size and sub-modules compatibility requirements. On the other hand, radiating elements must be integrated on low permittivity thick substrates for radiation efficiency and electrical performances (e.g. bandwidth, ... ). The hybrid process should accommodate the above requirements which enforce opposite trends in terms of permittivity values.

Once again, the flexibility brought by the 3D concept leads to new prospects for mm-wave applications:

• Constructing composite Ceramic/Foam substrates by combining press-moulding and screen-printing process on alumina motherboard substrate. •Translating and matching the well-known Mem brane process to standard 3D hybrid techniques.

IV.2.1. Composite Ceramic/Foam substrate for global optimisation procedure [START_REF] Lattard | High performance miniaturized microstrip anten nas on locally non-homogeneous substrates[END_REF] Works on mm-wave antennas are focused today on the used of new well-suited materials, based upon foam (Er= 1. 07) or honey-cell structures which offer excellent performances up to 60 GHz. The metalisation and etching process remain critical procedures. However, few solutions have been recently proposed through press moulding or chemical deposition techniques.

Printed patch arrays have been implemented on such a support, with specific efforts concerning the miniaturi zation of the basic radiating elements (better control of the antenna array response), by dielectric insert tech nique. Difficulties arise when considering the microstrip array feeding network, because of the (Er,h) values of the substrate: (wide strips '"* parasitic coupling "* side lobes .I', cross-polarisation .I').

Therefore, stacked structures and aperture coupled antenna arrays are used rather than planar configurations, emphasising once again on the benefit of 3D approaches. The figure 20-a Antenne plaques sur un substrat composite.

made on foam substrate fed by a non-resonant slotline located in the ground plane. This slot is excited through a CPW structure made on alumina substrate to obtain effi cient coupling. At mm-wave frequencies, the 3D passive and active functions described before can be directly combined wi th such antenna arrays on foam material, through the com mon ceramic mother-board substrate and an intermediate slotline feeding network. Such technique has been vali dated up to 20 GHz only (see fig. 20-b [START_REF] Le Nadan (th | Optimization and miniaturization of a filter/antenna multi-func tion module using a composite ceramic-foam substrate[END_REF]), but the concept can be easily extended to mm-waves. This origi nal association is obviously a good compromise for com bining efficiency and miniaturization criteria in a global integration approach. Notes that specific synthesis tech niques have been proposed in order to introduce techno logical constraints during the design task (global synthesis). This leads to increase the bandwidth of the radiating patch.

IV.2.2. Membrane technology as an alternative solution

The other opportunity, initially proposed by [START_REF] Rebeiz | Monolithic Millimeter-Wave Tw o-Dimensional Horn Imaging Arrays[END_REF], and considered by numerous authors consists in dev eloping membrane antennas. The thin film membrane having a permittivity close to the one of the air, radiation efficiency is increased. In addition, optimal coupling bet ween guided and radiating modes is obtained, which yields wi deband high effi ci ency antenna arrays.

Interconnecting such a technology with standard ones has been a great challenge during the last few years. Various possibilities are now considered [START_REF] Perrot | Proposed design solutions for low-cost flip-chip membrane devices for mm-wave applications[END_REF] , by using electromagnetic coupling (comparable to the method mentioned above) or suited mounting techniques. A 4*4 aperture fed membrane antenna array designed at 40 GHz is proposed on figure 21. Each basic radiating patch is designed on a membrane section (for radiating effi ciency), while the microstrip feeding network is imple mented on a backside substrate for which the ground plane acts as a shielding plane. The excitation of the dif ferent elements is therefore made easier through the slot apertures, while parasitic coupling are properly rejected, each antenna being located on an electromagnetic shiel ded cavity.

Ultra-low cost membrane structures on copper sup port, processed as Flip-Chip components, and reported on an alumina motherboard substrate were reported in a second time [18]. This is illustrated on figure 22 in the case of a membrane 50-Q line mounted on an alumina motherboard substrate with CPW access, through minia ture bumps (50 µm diameter). This way, one can easily combine 3D functions which are integrated on alumina substrate with mem brane modules. These are used for critical sections like antennas or narrow-band filters for which the membrane characteristics should be efficiently selected (losses, radiation effi ciency, .. . ).

V. CONCLUSION

The main objective of this paper was to demonstrate that hybrid integration techniques appear really today as attractive solutions for developing low-cost high effi ciency mm-wave modules and sub-systems, especially when 3D integration techniques are considered.

On one hand, the reliability of the CPW technology, which can be assumed as the reference technology at such frequencies, can be really improved by implemen ting integrated di electric bridges for parasitic modes sup pression. Original compensation techniques can be used for controlling the corresponding operating bandwidth.

On the other hand, the uniplanar approach can not be considered as an universal response for integrating any mm-wave functions, with any requirements. Using the hybrid ori entation, interconnections between basic func tions or elements remain sensitive operations, as well as the development of high performances passive devices (narrow-band filters, antennas network, ... ). The fl.exibi-Iity of the 3D integration techniques was therefore inves tigated, by using various available technological architec tures, leading to original structures with improved performances with respected to standard approaches. Some efforts remain to be done concerning the mounting techniques, especially the flip-chip ones. However, the actual evolution of both the chemical industry (for delivering high performance dielectric or conductive inks) and the electromagnetic simulators should help designers to find optimal integrated hybrid 3D modules. These will remain competitive with regard to MMIC approaches up to 80 GHz [19].

ACKNOWLED GEMENTS

Have also contributed to the different results presen ted in this paper T. Le Nadan and S. Perrot from LEST/ENST Bretagne (France). Thanks are also due to Pro fessor Serge Toutain, from IRCCYN Nantes (France) for discussions and helpful suggestions concerning mm wave aspects.

  FIG. 1. -Bonding wires on a MIC T-junction.

Filtres

  de modes sur une jonction en T.

FIG. 2 .

 2 FIG. 2. -MMIC bridges (courtesy of IEMN-Lille [3]).Pants integres en technologie MMIC(contribution IEMN Lille {3 /).

FIG. 4 .

 4 FIG. 4. -Electrical parameters of the integrated bridges. Parametres ilectriques des pants intigris.

T

  FIG. 6. -Compensated bridge models.

  FIG. 7. -B-type compensation. Compensation de type B.

  FIG. 8. -Distributed bend model. Circuit equivalent riparti pour un coude.

Figure 9

 9 Figure9describes the behaviour of a cross-junction once all parasitic effects has been properly described and compensated. The measured resonant frequency is equal to 30.4 GHz (30 GHz predicted). According to these results, it thus becomes possible to accurately design a resonator at a fixed frequency. As the compensation tech-

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 4 FIG. 10. -Band-pass filter in classical coplanar technology.Filtre passe-bande en technologie coplanaire standard.

Figure 11

 11 Figure 11 deals with the same filter theoretical cha racteristics as above, but implemented this time in a multi-layer technology. This allows the implicit integra tion of compensated dielectric bridges. Equivalent distri buted scheme of bridges and junctions are included in the design model. Each discontinuity can now be perfectly controlled and matched.

I

  FIG. 11. -Experimental and theoretical results of compensated multilayer coplanar band-pass filters. Resultats theoriques et experimentaux pour des filtres passe bande multicouches compenses.

FIG. 12 .

 12 FIG. 12. -Transverse geometry of the TFMS topology. Description transverse de la configuration TFMS.

Ill

FIG. 13 .

 13 FIG. 13. -Design of a Back to Back CPW to TFMS transition.

FIG. 14 .

 14 FIG. 14. -Back to back CPW to TFMS transitions: experiment.

FIG. 15 .

 15 FIG. 15. -TFMS I microstrip layouts of a narrow-band filter.

FtG. 16 .

 16 ro-t .. . l Gll.1. I---!.. II _,__,__ --- -TFMS bandpass filter: simulation & measurement.

FIG. 17 .

 17 FIG. 17. -Interconnection between MMIC & TFMS technology. Interconnexion entre les technologies MMIC & TFMS.

9 FIG

 9 FIG. 18. -Beam-lead Schottky diode across a slotline ring. Diode de Schottky a conducteurs poutres montee sur anneau a fente.

a-

  FIG. 19. -Non linear analysis of a 3D CPW mixer: Procedure. Analyse non-lineaire d'un melangeur coplanaire 3D: Demarche.

  FIG. 20. -Patch antenna on a composite substrate.

  , ...... .. .... �--

I 1 .

 1 FIG. 21. -Aperture fed membrane antenna array at 40 GHz. Antenne en reseau a technologie de membranes avec alimentation par fe ntes a 40 GHz.

  FtG. 22. -Membrane Microstrip line mounted on an alumina motherboard.Ligne microruban sur membrane montie sur une carte mere en alumine.


C Quendo

J Ph Coupez

C Person

E Rius

M Le Roy

S Toutain

BAND-PASS FILTERS WITH SELF-FILTERING RESONATORS: A SOLUTION TO CONTROL SPURIOUS RESONANCES

We propose original procedures for the design of high performance filters with simultaneously controled band-pass and band-reject responses. Resonators are des cribed as self-filtering structures and contri butes to the in-band and out-of-band trans fer function. The control is achieved either by specifying the resonators synchronization, or by inserting low-pass structures inside resonators

INTRODUCTION

The synthesis of microwave integrated filters is basically based upon equivalences bet ween the ideal electrical scheme -depending on the selected mathematical transfer function-and corresponding design topologies. In this way, approximations are commonly used, so as to fit both ideal and synthetized responses around a resonant frequency (f o r bandpass filters), using for example the slope parameter method.

Consequently, in-band and out-of-band specifications are usually respected when im plementing microstrip or coplanar bandpass filters. but parasitic responses appear, mainly at I", znct and 3 " 1 harmonic frequencies; indeed, the basic resonant structure produces uncontrolled additional poles and zeros outside the operating bandwidth.

In many applications, low-pass filters are employed in order to reject such spurious res-ponses, thus increasing both the size of the complete filter and transmission losses. In addi tion, such a technique is only efficient in sup pressing the first higher order parasitic resonan ces, the same synthesis limitations being en countered for the low-pass filter (in terms of spurious harmonics).

In this paper, two alternative procedures are proposed to improve band-pass filter per formances. The main idea lies in the possibility of designing resonators fundamentally resonant at a center frequency fO, including simultaneou sly self-filtering harmonic structures.

CONTROLLING HARMONIC RESPONSE

OF INTEGRATED RESONATORS

Integrated filters are usually based upon the use of halfwavelength resonant structures.

The geometry of such an element can be modi fied so as to improve its design fiexibility, the refore leading, for example, to SIR topologies (Stepped Inverter Resonators : see figure 1) where the various parameters can be efficiently used to improve the filter specifications.

• Non conventional stepped impedance resonators - In this way, we have previously shown

[ 1] that such resonators can be analyzed through their input admittance, which is equal to zero at resonance.

y = • y 2(K tan{\ +tan0i)(K-t a n {\ tan 0i) . 1 J 2 K( l-t a n 2 (\)( l-t a n 2 0i) -2 ( 1 +K 2 )tan(\ tan0i
(Eq :1)

with K= Zi : Impedance ratio

Z 1
The fundamental resonance depends on the following relation : Ktan 0 1 tan 02 = 0 while the location of the higher order resonant frequencies is determined with respect to the other poles and zero of the global equation. In this way, a high rejection level is obtained if the resonators are correctly synchronized at harmo nic frequencies.

Nevertheless, the technique is quite sen sitive to technological dispersion.

INCLUDING LOW-PASS FILTERS INTO RESONANT STRUCTURES

We propose a new method for impro ving the out-of-band rejection [2] [3] [4]. In this case, low-pass structures are directly integrated in the different resonators of a band-pass filter and can be synthetized in order to ensure a high rejection level of higher resonant frequencies.

This second solution is applied to the well-known band-pass filter based upon quarter wavelength shunt stubs and series impedance inverters depicted on figure 3 [5]. The associa ted electrical response is presented on figure 2, and the numerous spurious resonances around harmonic resonant frequencies can be observed.

A low-pass filter can be basically consi dered as a L-C-L or C-L-C structure, and is im plemented using cascaded thin and wide mi crostrip lines. So, we have modified the geome try of the impedance inverter of a 2nd order fil ter, so as to insert an equivalent low-pass structure. The electrical response of the com plete filter is modified in the out-of-band region (see figure 2) while it remains quite similar in the desired operating bandwidth. . : ..'. .

; I ii ! ""' ': : : i The same procedure can be used for shunt stµbs, to further improve the stop-band rejection (fig. 4).

L' L • L' t'• L • L • t'• L • L •
Fig. 4: The modified prototype

Equations (1) and (2) are used in order to synthetize the low-pass filter (3rd order filter) respectively around f0 and near the first trans mission zero. q can be considered as a synchro nization parameter between inverters and stubs, which improve the near out-of band rejection (fig. 5 and fig. 6).

Li =-1 -arcta { Ktan( q {3 0� ) +Ktan( q {3 0 4) ] q/30 K( K tan (qf30� )tan (q {30 4) -l )

with K = Z L impedance ratio Z c
(Eq: 2)

Z L : high characteristic impedance -Very low characteristic impedance levels are obtained by using a microstrip configuration on a thin dielectric layer (typically 30 µm) (Fig. 7a).

-High characteristic impedance levels are available by using multilevelled co planar configurations with low permitti vity (£r=4) dielectric sheets (Fig. 7b).

Finally, very low and very high characteristic impedances may be both synthetized while minimizing the series step discontinuities.

�Fig. 1 :

 1 Fig. 1 :Stepped impedance resonators : description

  Fig.3: The basic idea

F1g. 6 :

 6 Fig.5 /Theoretical res po \ses I \ • • " ,_ ' q < 2 : inductive junctions q>2 : capacitive junctions F1g.6 : The diffe re nt topologies The K ratio determines the step dis continuity between inductive and capacitive sections of each low-pass architecture. The hig her this K factor is, the better the spurious reso nances attenuation will be. Nevertheless, the K values are unfortunatly limited, according to technological limitations. For this reason, we investigate on a multilayer technology, based upon thick-film technological process, in order to increase this factor [6]. As mentioned on figure 7, Various technological configurations can be expected, through different combinations of dielectric and conductive layers on an alumina substrate.

Fig. 8 :

 8 Fig.7 : Multilayer technology

  ,.� ...... ,

	_, ,	•:	; :	;;	:.	,:	,�

!".' ;; �; : : ; ;; ;; ,., ;1 � I : I ,. : \/ : i .


Rodolphe Sonnier
email: rodolphe.sonnier@mines-ales.fr

Oumaima Belkhane

Laurent Ferry

Laurent Aprin

Pierre Delot

Coralie Garcia

Arthur Hellouin De Menibus

Hélène Lenormand

Marc Potin

Fire behaviour of hemp, clay and gypsum-based light biobased concretes and renders

Keywords:

Greenhouse gas emissions from cement manufacturing account for about 8 to 10 percent of total CO 2 emissions worldwide. To reduce these emissions, researchers are developing new concrete manufacturing techniques and processes to reduce high energy consumption and environmental impacts. Biobased concretes are eco-friendly insulating materials for building industry that can respond to this problem. However, there is a lack of knowledge regarding how these plant-based fillers ignite and contribute to heat release in case of fire. In this work, the fire behaviour of a series of hemp-based earth and/or gypsum concretes covering a large range of densities (180-1500 kg/m 3 ) is investigated using the cone calorimeter at an incident heat flux of 50 kW/m 2 . The fire performances are mainly monitored by the thermal inertia of the materials. Only the lightest concretes ignite with a density threshold for ignition occurrence around 500 kg/m 3 . For a density of 261 kg/m 3 , the critical heat flux of an earth-hemp concrete was found to be close to 27 kW/m 2 . The flaming period remains very short in all cases.

Introduction

Due to environmental concern, professional building workers, researchers and companies are currently devoting great efforts to reduce the environmental impact of building materials. For this purpose, using bio or georesources rather than fossil resources is part of the solution. A complementary way is to reduce the energy to heat or cool buildings during their service life by using insulating materials.

Raw earth offers a set of advantages, including its local availability, the capability to fully reuse the material after a building end-of-life, and its low embodied energy, in comparison to other binders like cement or lime [START_REF] Gallipoli | A geotechnical perspective of raw earth building[END_REF][START_REF] Gallipoli | Raw earth construction: is there a role for unsaturated soil mechanics?[END_REF][START_REF] Giada | Hygrothermal Properties of Raw Earth Materials: A Literature Review[END_REF]. Raw earth is a traditional material which can be used according to a variety of processes for various structural and nonstructural applications in modern buildings [START_REF]Modern Earth Buildings -Materials, Engineering, Constructions and Applications[END_REF]. Of course, the choice of raw earth is of first importance [START_REF] Lagouin | Effects of organic admixtures on the fresh and mechanical properties of earth-based gypsums[END_REF][START_REF] Rivera-Gomez | Sample key features affecting mechanical, acoustic and thermal properties of a naturalstabilised earthen material[END_REF][START_REF] Ngo | Design of a soil concrete as a new building material -Effect of clay and hemp proportions[END_REF], and the variability of obtained properties requires a special attention [START_REF] Barnaure | Earth buildings with local materials: Assessing the variability of properties measured using non-destructive methods[END_REF]. Several studies investigated how raw earth can be stabilized by cement, lime or gypsum [START_REF] Ashour | Equilibrium moisture content of earth bricks biocomposites stabilized with cement and gypsum[END_REF]. Global warming impact of these binders depend on several parameters such as the energy used or nature of cement and lime.

At the production stage, gypsum is about 6 times less impacting than CEM I cement and 5 times less impacting than lime in terms of global warming [START_REF] Zieger | Impact of GHGs temporal dynamics on the GWP assessment of building materials: A case study on bio-based and non-bio-based walls[END_REF][START_REF]Calculated FIGRAmax versus density[END_REF][START_REF] Fořt | Carbon footprint analysis of calcined gypsum production in the Czech Republic[END_REF]. Gypsum bring several advantages: a very fast setting kinetic and likely a more protective fire behaviour than clay-based materials since gypsum is used for a long time as protective panels against fire [START_REF] Just | Gypsum plasterboards used as fire protection -Analysis of a database[END_REF]. Moreover, its variability is easier to control than for clay. Gypsum binder can be used alone with plant particles, or it can be associated with clay in a given proportion.

Many works have already been devoted to assess the properties of plant-based concretes [START_REF]Bio-aggregates Based Building Materials (State-of-the-Art Report of the RILEM)[END_REF][START_REF] Jami | A review of the properties of hemp concrete for green building applications[END_REF], including mechanical properties [START_REF] Elfordy | Mechanical and thermal properties of lime and hemp concrete (''hempcrete'') manufactured by a projection process[END_REF][START_REF] Benmahiddine | Experimental investigation on the influence of immersion/ drying cycles on the hygrothermal and mechanical properties of hemp concrete[END_REF], heat conductivity [START_REF] Elfordy | Mechanical and thermal properties of lime and hemp concrete (''hempcrete'') manufactured by a projection process[END_REF], hygrothermal properties [START_REF] Giada | Hygrothermal Properties of Raw Earth Materials: A Literature Review[END_REF][START_REF] Benmahiddine | Experimental investigation on the influence of immersion/ drying cycles on the hygrothermal and mechanical properties of hemp concrete[END_REF], acoustic properties [START_REF] Kinnane | Acoustic absorption of hemplime construction[END_REF][START_REF] Degrave-Lemeurs | Acoustical properties of hemp concretes for buildings thermal insulation: Application to clay and lime binders[END_REF] or environmental impact [START_REF] Ingrao | Energy and environmental assessment of industrial hemp for building applications: A review[END_REF][START_REF] Ben-Alon | Life cycle assessment (LCA) of natural vs conventional building assemblies[END_REF]. Some papers have focused more particularly on earth-hemp [START_REF] Degrave-Lemeurs | Acoustical properties of hemp concretes for buildings thermal insulation: Application to clay and lime binders[END_REF][START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF][START_REF] Fernea | Hemp-clay building materials -An investigation on acoustic, thermal and mechanical properties[END_REF] or

Materials and methods

The plant particles used in this study are hemp shiv. Hemp shiv comes from Poitou Chanvre production. Shiv length and width are 7.6 (±1.3) and 2.0 (±0.4) mm respectively.

Earth is a fine natural kaolin (trademark: Blankalite 78 from Soka -France) with a true density of 2600 kg/m 3 . 99% of particles have <20 µm-size. Earth 2 was collected from earthworks performed close to Nocé (61340, Normandy, France). It was used in more than 30 hemp-clay operations. A granulometry, sedimentometry, and methylene blue value characterization showed it has a high fraction (37%) of normally active clay. Gypsum was a 0/200 µm natural hemihydrate gypsum plaster (trademark: neige 1R from Plâtres Vieujot/Platre.com France).

Sand was a siliceous 0/4 mm sand coming from sablières de la Seine in Bernières sur Seine (27700). Exfoliated vermiculite with a density ranging from 65 to 160 kg/m 3 comes from Nestaan NV (Belgium).

Fig. 1 shows the different constituents used for preparing biobased concretes.

Samples were prepared as follows:

• Plant particles and binder were weighted to produce 6 to 7 samples for each formulation. Each component weight varied from 500 g to 5 kg depending on the recipe followed. Weighing was done with a ± 0.1 g balance. • Materials were first mixed dried. Then water was added in adequate proportion to obtain a good workability, based on the expertise of craftmen that produced the samples. Water to binder ratio depends on the water absorption of plant particles, on the binder water demand and on the recipe. Water to binder ratio varied from 1 to 1.5 for clay (i.e. earth) based samples and from 0.4 to 1.5 for gypsum-based samples. • A first 10 × 10 x10 cm 3 mould was completely filled by a moderate manually compaction. The mass introduced in the mould was recorded.

• The other moulds with the same mix were filled with the same mass of material, to obtain repeatable samples;

Cardboard moulds were used as it was convenient to produce a large number of samples (Fig. 2). They were used for manufacturing operations by adding a rigid plastic sheet between mixture and cardboard to ensure clean surface conditions. 1 day after manufacturing, mould and rigid plastic sheets were removed so the sample could dry. When gypsum-hemp concretes [START_REF] Charai | Thermal insulation potential of nonindustrial hemp (Moroccan cannabis sativa L.) fibers for green plaster-based building materials[END_REF][START_REF] Babu | Mechanical and thermophysical behavior of hemp fiber reinforced gypsum composites[END_REF][START_REF] Iucolano | Thermo-mechanical behaviour of hemp fibers-reinforced gypsum plasters[END_REF].

Despite their promising future for building industry, there is still a lack of data about the association of mineral binders with plant particles, especially about their reaction-to-fire and their fire resistance. Several trade documents or scientific papers have presented some results, which are generally quite good. Lanos reported several examples of fire resistance tests on biobased materials (based on hemp or straw) [START_REF] Lanos | Bio-aggregate Based Building Materials Exposed to Fire, chapter 7 in Bio[END_REF]. Volhard studied two earth-straw concretes with different densities and concluded that these concretes exhibit a good fire resistance [START_REF] Volhard | Construire en terre allégée[END_REF]. Regarding the reaction-to-fire (i.e. the direct contribution to heat release), Lanos reported SBI (Single Burning Item) rating of various biobased materials, including some earth-hemp concretes [START_REF] Lanos | Bio-aggregate Based Building Materials Exposed to Fire, chapter 7 in Bio[END_REF]. Most of them are rated A2 or B, but some F ratings are also reported without details. Note that the additional rankings concerning smoke and dripping are always at the best level (i.e. s1, d0).

Nevertheless, each report focused on one specific product (or a limited range of products), which does not allow to extrapolate results when the density, the binder or plant nature are different than the ones tested. To the best of our knowledge, no comprehensive study has been published in open literature about the reaction-to-fire of an extended series of biobased concretes. The present study aims to provide an overview of the flammability of these materials in order to estimate the fire hazard from a couple of parameters as the concrete density, the fiber content or the nature of the binder. Around 20 concretes based on hemp and different binders were prepared covering a large range of density (180-1500 kg/m 3 ). Their flammability was assessed using a well-known bench-scale test, namely cone calorimeter. samples were stabilized in mass in room condition, they were packed again in cardboard moulds and sent to the laboratory.

After preparation, concretes were stored in cone calorimeter room up to mass stabilization (weight change over 24 h < 1%). This occurs after several weeks. The atmosphere of the room was not controlled and relative humidity varied in the range 20-35 %. The influence of moisture is discussed below.

Concretes are called as XYZ with X referring to the binder (E for the main type of earth used in this study, E2 for the second type of earth, G for gypsum and GE for a gypsum-earth mix), Y to the filler (H for hemp and V for vermiculite) and Z to the density. For example, GH423 is the concrete based on gypsum and hemp and presenting a density of 423 kg/ m 3 . When the binder was a mix of earth and gypsum, the concretes were called GaE(100-a)HZ with "a" the weight content of gypsum in binder. For example, G75E25H184 refers to a concrete containing a binder based on 75 wt% of gypsum and 25 wt% of earth and presenting a density of 184 kg/m 3 . In some concretes, especially the densest ones, sand was also added. Table 1 lists all the concretes prepared with the water content used during the preparation.

Density was calculated considering a volume of 10x10x10 cm 3 . Nevertheless, for some samples, shrinkage occurs during curing and leads to a slight reduction in volume. This reduction was assessed around 9%. Therefore, the reported densities must be considered as minimum values. Density was calculated after mass stabilization in the conditions already indicated above.

Hemp flammability was tested at microscale using pyrolysiscombustion flow calorimetry (PCFC -FTT, East Grinstead, UK) according to method A (anaerobic pyrolysis) and method B (aerobic pyrolysis) of ASTM D7309 standard. A few mg-sample was heated at 1 K/s up to 750 • C under nitrogen or air. Gases released are sent to a combustor at 900 • C in an excess of oxygen, ensuring a complete combustion. Heat release rate curves are plotted versus pyrolysis temperature. HRR is calculated using oxygen depletion method. Indeed, 1 kg of oxygen consumed corresponds to 13.1 MJ of energy released according to empirical Huggett's relation [START_REF] Huggett | Estimation of rate of heat release by means of oxygen consumption measurements[END_REF].

Cone calorimeter (FTT, East Grinstead, UK) tests were usually performed at 50 kW/m 2 using piloted ignition for 20 min according to ISO 5660 standard [START_REF]Iso Reaction-to-fire tests -Heat release, smoke production and mass loss rate -Part 1: Heat release rate (cone calorimeter method) and smoke production rate (dynamic measurement)[END_REF]. Some additional tests were performed at different heat fluxes or without spark igniter. The 10x10x10 cm 3 samples were directly put on the sample-holder without rockwool or aluminium foil. A frame was used to ensure that heat flux was only absorbed by the surface sample (and not by the side faces) (Fig. 3). When a balance was used during test, mass was recorded each 30 s. Usually two tests were performed for each concrete. Statistical analysis was carried out on concrete EH261. The char front was measured at the end of the test by measuring the black layer thickness using a ruler on the side faces. Usually, the thickness was similar on the four sides. When the thickness was significantly higher on one side, only the thickness on the other sides was considered. In cone calorimeter test, heat front moves through the thickness. When it reaches the sample bottom, the temperature inside the sample increases because heat cannot be removed. This temperature increase accelerates the decomposition. 10 cm-thick samples ensure that they can be considered as semi-infinite. In other words, the heat front does not reach the bottom of the samples during the fire test.

For some tests, the distance between radiant cone and the upper surface was increased to 60 mm (versus 25 mm in standard test) to measure the surface temperature using an infrared camera (Optris CT). The mean temperature of the zone at the center of the upper surface was measured on the range 125-900 • C. Emissivity was considered to be equal to 1 which is reasonable for such rough surface materials and for an approximative surface temperature measurement.

Single Burning Item (SBI) simulations were performed using Con-eTools software (FTT, East Grinstead, UK) from the cone calorimeter experiments at 50 kW/m 2 . TTI in SBI simulations is an input data. It was taken to be equal to the value measured in cone calorimeter or 30 s maximum (when no ignition was measured). gypsum) and sand. However, for gypsum-based concretes, the gypsum is hydrated during curing (it changes from semi-hydrate to dihydrate form) and water consumed by this process is considered. In these concretes, sand is also often added to limit the shrinkage during curing, similarly to professional building workers practice for rendering. Densities in the range 180-600 kg/m 3 are reached for hemp content between 15 and 35 wt%. Several parameters may control the flammability. High-density concretes need more time to be heated up to pyrolysis temperature: indeed, the energy absorbed to increase the temperature from ambient to pyrolysis temperature is proportional to the product of density and specific heat. Moreover, higher density usually corresponds to higher heat conductivity. A high conductivity allows a higher heat transfer from the surface to the bulk. A second parameter is the hemp concentration. Indeed, ignition occurs when the amount of fuels in gas phase reaches a threshold concentration. When the hemp concentration is too low, this threshold may not be reached. Fig. 6 plots the hemp concentration (i.e. the hemp density in concrete, in kg/m 3 ) versus the density. For low-density concretes, both parameters appear to be relatively independent. For density ranging from 250 to 500 kg/m 3 , the hemp concentration may vary roughly from 75 to 110 kg/m 3 . Its value is lower than 50 kg/m 3 for the densest concretes. The reason is that the compaction may change the density at a same hemp content. Moreover, the binders (including sand in some formulations) can have different densities.

Results

Hemp flammability at the microscale

The flammability of hemp was analyzed at the microscale using the pyrolysis combustion flow calorimetry. Pyrolysis is mainly anaerobic during a fire, as long as oxygen is consumed in the flame. When the flame vanished, thermo-oxidation, i.e. aerobic pyrolysis, occurs. The flaming period in cone calorimeter is limited for the biobased concretes (see below), however, both conditions were tested using the pyrolysis combustion flow calorimetry.

The results in Fig. 4 show that HRR curve has only one peak in anaerobic pyrolysis. This peak is around 106 W/g and centered at 335 • C. This is in good agreement with previous works [START_REF] Dorez | Effect of cellulose, hemicellulose and lignin contents on pyrolysis and combustion of natural fibers[END_REF][START_REF] Moussa | Toward the cottonization of hemp fibers by steam explosion, Flame retardant fibers[END_REF]. Total heat release is close to 10 kJ/g (a little bit higher than the previously reported values) and residual fraction (i.e. the ratio between the final mass and the initial mass) is 0.19. Heat of complete combustion is 12.8 kJ/g. Even if hemp is mainly composed by hemicellulose, cellulose and lignin, the decomposition steps of these three components merge, resulting in only one peak.

Under aerobic pyrolysis, two peaks are observed. The first one, 170 W/g at 288 • C, results from the decomposition of the main components which occurs earlier in presence of oxygen. The second one, 181 W/g at 383 • C, is due to the thermo-oxidation of the char. Consequently, no significant char is left and THR is much higher (16.4 kJ/g) as well as heat of complete combustion (16.8 kJ/g).

Considering these last values, it is possible to calculate the theoretical maximum heat released during the cone calorimeter test. Since the hemp content never exceeds 35 wt% in the samples studied, thus the heat release cannot be higher than 5.9 kJ per gram of concrete. When the hemp content is 5 wt%, this value is even much lower (<1 kJ/g). Since the piloted ignition is based on the gas phase combustion energy density [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF], it is clear that such samples would hardly ignite. Comparatively, these heat release values are very low: recent results showed that flame out in cone calorimeter occurs in few seconds for an alginate-based foam with a density of 40 kg/m 3 (lower than the current biobased concretes) and exhibiting a heat of combustion of 3 kJ/g [START_REF] Vincent | Fire behavior of innovative alginate foams[END_REF]. Common polymers including those used as insulating materials exhibit much higher heat of combustion (20-40 kJ/g).

Density of biobased concretes

The density of biobased concretes is mainly monitored by adjusting the content of biofillers (here, hemp) and changing the compaction during casting (compaction level was not studied here, all sample were manually compacted in mould). Therefore, biofillers content and density are in close relation (see Fig. 5). Hemp content is calculated on a dry basis, i.e. by considering only the amount of hemp, binder (earth or negligible (around 15 g due to water release from earth and vermiculite decomposition).

Water absorption in a biobased concrete depends on its composition. According to Colinart et al. [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF], water content of hemp-earth concrete (density 196 kg/m 3 ) after drying at 105 • C increases from 2 to 5% (4-10 g) when relative humidity increases from 20% to 50%. For densest concretes, hemp content and then water content (in percentage) is lower. Even if these values are limited, they are not negligible and should be accounted so the mass loss is not fully assigned to hemp decomposition.

The overall mass loss is an indicator but it hides a more complex reality. Indeed, water release, gypsum and hemp decomposition occur at different temperatures. As there is a heat gradient through the thickness of the samples, the front of hemp pyrolysis does not coincide with the front of water release or the front of gypsum decomposition.

The role of water content in flammability must be considered, especially because the relative humidity (RH) has varied in the storage room (in a moderate RH range 20-35%) during the study. Some additional experiments were carried out in Supporting Information S2. Results confirm that the water absorption in the studied range has no significant influence on flammability.

Eight tests were performed on EH261 at different periods of the study to ensure the reliability of measurements. Table 2 shows the mean, minimum and maximum values of TTI, pHRR and residual fraction as well as the standard deviations. Data for pHRR and residual fraction vary in a narrow range. On the contrary, data for time-to-ignition are more scattered even if this concrete ignites quickly in all cases. This observation is also valid for other concretes. Especially, few of them do not ignite in all cases. For example, PH320 ignites only one time over two tests. Of course, the occurrence of ignition has a great effect on pHRR but not on residual fraction as explained below. Two reasons for such variability may be proposed. First, some of these concretes are close to the flammability limit, i.e. their amount of fuels is just high enough to ignite. Second, the observation of the sample surface shows that some hemp fibres are apparent and not fully covered by the binder, especially for lighter concretes. Such free plant particles may promote ignition. This point will be investigated in the future. These variabilities confirmed our choice to test a large number of samples to reach correct findings when testing these kinds of materials with a cone calorimeter.

Influence of heat flux and thermal behavior

EH261 was chosen to study the influence of heat flux. While this material is one of the most flammable ones, it may be considered as a worse case from fire hazard point of view. Fig. 8 shows the HRR curves for EH261 at different heat fluxes. Two curves are shown for heat flux of 30 kW/m 2 . At this heat flux, only one of both tests led to ignition. As usual, time-to-ignition decreases when heat flux increases (69 s at 30 kW/m 2 and <10 s at 70 kW/m 2 ). Peak of heat release rate increases but the change is relatively limited (from around 60 kW/m 2 at heat flux of 30 kW/m 2 to 110 kW/m 2 at 70 kW/m 2 ). The peak is much lower when no ignition occurs (<20 kW/m 2 ).

Residual fraction also decreases when heat flux increases, from 0.90 at 30 kW/m 2 to 0.81 at 70 kW/m 2 ). Residual fraction is similar for both tests at 30 kW/m 2 , confirming that the short flaming period has no impact on mass loss. The char front also increases: 1.5-2 cm at 30 kW/ m 2 to 4.5 cm at 70 kW/m 2 .

Materials can be divided into two categories according to their

Flammability of biobased concretes at bench scale -Preliminary remarks

This work is focused on the ignition, the peak of heat release rate and the mass loss during cone calorimeter test. Smoke production is another important aspect of the fire behaviour. But it was found to be negligible or very low for these materials (in open as well as in close conditions, i.e. in smoke chamber -Figure S1 in supporting information).

Typical heat release rate (HRR) and mass loss curves are shown in Fig. 7. Fig. 7A presents the curves for ignited and non-ignited concretes. For ignited concretes, ignition occurs in <30 s and is followed by a narrow peak of heat release rate. This peak varies depending on density but does not exceed 130 kW/m 2 . It is followed by flame out after only few dozens of seconds. In other words, the flame is not sustained. The HRR curve does not go down to 0 due to the thermo-oxidation of hemp after flame out. The plateau remains lower than 20 kW/m 2 . Nevertheless, the measurement of such low HRR values is not accurate and cannot be used for correct heat release calculations. Indeed, even EV631 containing no fuel exhibits a low but non-negligible heat release rate (noisy curve).

For non-ignited hemp-based concretes, there is no pHRR but the curve stabilizes to a constant HRR value (still lower than 20 kW/m 2 ). Higher is the density, lower is the plateau. For the densest concretes, the HRR remains close to 0 during the whole test.

Fig. 7B shows the mass loss (in grams) during the test. The mass loss increases linearly during the whole test, even for ignited concretes where a short flaming period occurs at the beginning. The mass loss reaches around 45-50 g for the lightest concretes. Its value is still 20-25 g for the densest one. Obviously, the relative mass loss (i.e. in wt%) is much lower for the dense concretes. The mass loss is related to the hemp decomposition, the release of water (either free or constitutive) and the decomposition of gypsum. The mass loss for EV631 is low but non-thermal behaviour [START_REF] Schartel | Development of fire-retarded materials-Interpretation of cone calorimeter data[END_REF]. In thermally-thick materials, a heat gradient is observed through the sample thickness. Heat conductivity allows transferring heat from the surface to the bulk. A linear relation between the reciprocal of time-to-ignition square root and heat flux is observed in this case (Equation ( 1)). The time-to-ignition does not depend anymore on the thickness. On the contrary, for thermally thin materials, the whole volume is heated at the same temperature and burns at the same time. Heat conductivity does not influence the time-to-ignition. The reciprocal of time-to-ignition linearly changes with heat flux (Equation ( 2)).

TTI = π 4 kρc [ T ig -T o ε × HF -CHF ] 2 (1)
TTI = lρc T ig -T o ε × HF -CHF (2)
with k the heat conductivity, c the specific heat, ρ the density, l the thickness, ε the emissivity, HF the heat flux, T ig the temperature at ignition and T 0 the room temperature.

Drawing the reciprocal of √ TTI versus heat flux is a suitable way to identify the thermal behaviour of a material. Moreover, extrapolating these curves to 0 allows calculating the critical heat flux (CHF), i.e. the minimum heat flux for ignition. Nevertheless, in practice, both curves are quite often linear and it is hardly possible to conclude about the thermal behaviour. Another approach is to compare calculated CHF to experimental ones.

Considering Equation (1) (i.e. a thermally thick behaviour), CHF was 7 kW/m 2 . On the contrary, Equation (2) (thermally thin behaviour), CHF was found to be 27 kW/m 2 (Fig. 9). From the tests described above, it is clear that CHF should be close to 30 kW/m 2 . This value is quite high in comparison to many solid fuels as polymers (except halogenated or thermostable ones) [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF]. It is noteworthy that this value is probably a minimum value for biobased concretes, because most of them do not ignite even at 50 kW/m 2 .

This result also means that the light sample tested can be considered as a thermally thin one, despite its high thickness (10 cm). In fact, for insulating materials, heat conductivity is low and cannot contribute efficiently to cool down the surface by transferring heat to the bulk. Therefore, TTI on such materials depends on the thickness directly absorbing the heat flux from the cone, according to Equation (2) [START_REF] Vincent | Fire behavior of innovative alginate foams[END_REF]. This thickness may be assessed if thermophysical properties are known. Heat conductivity was measured close to 0.08 W/m.K. This value agrees with values reported by Labat et al. and Colinart et al. on biobased concretes of similar density [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF][START_REF] Labat | From the experimental characterization of the hygrothermal properties of straw-clay mixtures to the numerical assessment of their buffering potential[END_REF]. Specific heat and emissivity were chosen equal to 1.5 kJ/kg.K and 1 respectively. A temperature at ignition of 450 • C was chosen (see below about the measurements of surface temperature). The best agreement between calculated and experimental TTI is found for a thickness of 2.5 mm (see Fig. 10). TTI calculations with Equation ( 1) are also shown for comparison purpose. Such thickness appears quite reasonable. For a lightest alginate foam (40 kg/m 3 ), the critical thickness above which TTI is constant was found to be around 1.5 cm [START_REF] Vincent | Fire behavior of innovative alginate foams[END_REF]. Calculated TTI is not in agreement with experimental values only for the lowest heat flux, but this heat flux is very close to the CHF. Moreover, ignition was not observed for one test among two.

Influence of density on flammabilty

First, we evaluated if hemp concentration is the main parameter driving the flammability. Figure S3 in Supporting Information shows no clear correlation between reciprocal of time-to-ignition (TTI -1 ) and hemp concentration. Low (<50 kg/m 3 ) as well as high (greater than110 kg/m 3 ) hemp concentrations do not lead to ignition. Ignition occurs for intermediate hemp concentrations. Therefore, the discussion focuses on the role of density on flammability.

On the other hand, Fig. 11 shows a clear correlation between the time-to-ignition and the density for all materials studied. The densest materials do not ignite and thus TTI -1 can be considered to be null. On the contrary the lightest materials ignite. A rough threshold of density can be proposed: above 500 kg/m 3 , no biobased concrete ignites. Ignition occurs for almost all concretes with a density lower than 500 kg/m 3 (except GH423). The lightest concretes exhibit a lower TTI. However, the relation between time-to-ignition and density for ignited materials is rough due to the uncertainties already discussed. Nevertheless, in all cases, time-to-ignition varies in a narrow range (6-29 s).

Flame out occurs after only few dozens of seconds. The burning time does never exceed 60 s. It is the reason why the mass loss rate does not seem to increase during the flaming period (remind that the mass loss was measured each 30 s). Moreover, during the most part of the test (at least 95%, i.e. 19 min over 20 min), pyrolysis can be considered as aerobic while oxygen is not consumed in the flame.

It can be noted that most of concretes undergoing ignition are based on earth or a mix of earth and gypsum. However, it is not possible to conclude about the role of gypsum to prevent ignition since there is only one gypsum-based concrete exhibiting a low density (GH320). Nevertheless, it is noteworthy that it exhibits the highest TTI (29 s) among the lightest concretes (and only one sample was ignited). Further work is needed to assess properly the role of gypsum.

Right after ignition, the heat flux absorbed by the material is increased by the heat flux from the flame. During several dozens of seconds, the hemp pyrolysis rate increases and pHRR is much higher than for non-ignited concretes as shown in Fig. 12. The pHRR of nonignited materials does not exceed 20 kW/m 2 and tends to continuously decrease when density increases. This heat release is partly due to the pyrolysis of biofillers but also to the measurement uncertainties. Indeed, the HRR curve is noisy with positive values even for EV concrete which does not contain any fuel. The comparison between GH423 and EH430 highlights the influence of ignition on pHRR. While GH423 does not ignite, EH 430 exhibits a time-to-ignition of 22 s. Its pHRR is 69 kW/ m 2 versus lower than 20 kW/m 2 for GH423.

The peak of heat release rate increases when density decreases, especially for concretes which ignite. In other words, the mass loss rate reaches maximum values for the lightest concretes. However, these concretes do not contain the highest hemp concentration (see Fig. 6).

Then, it may be assumed that the pyrolysis zone reaches a higher thickness for these concretes, maybe because the radiation from the radiant cone penetrates deeper. This may be also ascribed to their low heat conductivity. Indeed, the heat absorbed cannot be easily removed and then a thicker layer of concrete reaches a temperature high enough for pyrolysis. During burning, the mass loss is due to the pyrolysis of hemp, the release of absorbed water and the decomposition of gypsum. As already explained, hemp pyrolysis is aerobic and leads to a complete filler decomposition (no residue). Dihydrate calcium sulphate releases two molecules of water, losing 20.9 wt% of its mass.

Residual fraction is plotted on Fig. 13. It increases with the density and comes from 0.75 for the lightest concretes to 0.98 for the densest ones. Once again, a critical density threshold can be found around 500 kg/m 3 : above this value, the residual fraction increases slowly with the density. Below it, the decrease in residual fraction is much faster. Note that it is not due to the increase in mass loss rate during flaming period. The mass loss during these few dozens of seconds is not high enough to account for the lower residual fraction of these concretes. See also Supporting Information S4 about this point.

When comparing EH to GH at similar density, residual fraction may front may be slightly higher for E2H259 but it is assumed that the nature of earth is negligible on the reaction-to-fire.

Surface temperature at ignition

Ignition occurs when the concentration of fuels in gas phase reaches a threshold concentration. An equivalent but more convenient criterion for ignition is to consider a threshold temperature at sample surface. This temperature is obviously dependent on the material.

Temperature at ignition was measured for some concretes using infrared camera. Note that the distance between cone and sample surface is increased to 60 mm for these experiments. Therefore, even if the heat flux at the surface is still 50 kW/m 2 , the times-to-ignition are slightly different from those reported previously. Fig. 18 shows the surface temperature during cone calorimeter test for some concretes that undergo ignition, i.e. concretes with a low density. Arrows indicate the occurrence of ignition. It can be observed that ignition occurs for a temperature within the range 400-500 • C.

Heating rate depends on the thermophysical properties of a material, namely specific heat, heat conductivity and density. A low density should lead to a higher heating rate and to an earlier ignition. However, there is no clear correlation between the density and the time to reach this temperature. GEH184 is the lightest concrete and it is heating faster. Its time-to-ignition is the lowest. Nevertheless, ignition occurs earlier for EH430 than for EH261. Moreover, GH320 exhibits the lowest heating rate. This may be ascribed to the release of water due to the decomposition of gypsum. This endothermic phenomenon (around 500 J/g [START_REF] Thomas | Thermal Properties of Gypsum Plasterboard at High Temperatures[END_REF][START_REF] Yu | Thermal properties and microstructure of gypsum board and its dehydration products: A theoretical and experimental investigation[END_REF]) lowers the heating rate. The surface temperature increases after ignition and stabilizes at a temperature ranging from 650 • C (for EH184) to 600 • C (for GH320).

The surface temperature evolution during cone calorimeter test is plotted in Fig. 19 for the series of earth-based concretes. The density of these concretes ranges from 261 to 1297 kg/m 3 . Only the two lightest concretes exhibit ignition. The heating rate as well as the maximum temperature reached decrease when density increases.

Interestingly, EH512 and EH965 reach high surface temperature (500 • C or more) without igniting. This temperature is higher than the temperature at ignition measured for EH261 and EH430. Moreover, EH512 contains a similar hemp concentration as EH261 and EH430 (77 kg/m 3 versus 92 and 86 kg/m 3 ). Therefore, the non-flammable behaviour of EH512 cannot be assigned to a too low hemp concentration or a too low temperature surface. The heating rate is probably the main point which explains why ignition occurs for EH261 and EH430 and not for EH512. Indeed, if the heating rate is too low, the decomposition occurs over a longer period. Then, the fuels are released more slowly and the threshold fuel concentration to reach ignition may not be reached. Since the heating rate depends on the product of specific heat and density, this result highlights once again the role of density as the main parameter monitoring the flammability.

SBI simulations using ConeTools

ConeTools is a software allowing to calculate rating in SBI test from cone calorimeter data. The method was developed and showed a quite good agreement between experimental and calculated ratings from a series of 40 building materials [START_REF] Hakkarainen | Application of a One-dimensional Thermal Flame Spread Model on Predicting the Rate of Heat Release in the SBI Test[END_REF][START_REF] Hakkarainen | Rate of Heat Release and Ignitability Indices in Predicting SBI Test Results[END_REF][START_REF] Hansen | Prediction of Heat Release in the Single Burning Item Test[END_REF]. Later, other papers attempted to use ConeTools to predict SBI rating [START_REF] Van Hees | Use of the Cone Calorimeter and ConeTools software for development of innovative intumescing graphite systems[END_REF]. It is noteworthy that ConeTools allows predicting the rating but is not able to fit accurately the experimental HRR curve in SBI.

SBI rating was calculated for several concretes using ConeTools. Rating depends on FIGRAmax (i.e. the maximum value of FIGRA -FIGRA is defined as the growth rate of the burning intensity, and is calculated as the maximum value of the function "heat release rate/ elapsed test time" and THR600 (i.e. the total heat release after 600 s). FIGRAmax was systematically found to be the most severe criterion. Fig. 20 shows the FIGRAmax for some flammable concretes (for denser be slightly lower for GH. This may be assigned to the additional water release due to gypsum decomposition. Theoretical maximum mass loss rate can be calculated considering the complete decomposition of hemp and water release from gypsum. Note that two-step dehydration of gypsum occurs at moderate temperature (<300 • C). During the two steps, 1.5 molecule and half a molecule of water are released respectively (i.e. 15.7 and 5.2 wt%) [START_REF] Thomas | Thermal Properties of Gypsum Plasterboard at High Temperatures[END_REF]. The release of absorbed water was not considered; therefore, these calculations should be considered carefully as a rough approximation. Fig. 14 plots the ratio between the experimental mass loss and the theoretical one versus density. It clearly appears that gypsum-based concretes have a lower ratio. In other words, the heating front may move more slowly through the thickness of these concretes, leading to lower decomposition of hemp and gypsum.

Fig. 15 shows some pictures of residues from hemp-based concretes after cone calorimeter tests. The lower part of the samples corresponds to the initial aspect of the concretes. Note that the sample E2H259 has a different colour due to the binder (second earth used called E2). Lower is the density, higher is the hemp content, and more apparent are the hemp particles. The upper part corresponds to the char layer formed during the test. Its mechanical strength is negligible, especially for the lightest concretes. When density increases, the char layer thickness decreases from 6 cm to around 0 cm, evidencing that the front char moves faster in the lightest concretes (Fig. 16). However, the measurements are relatively scattered and do not allow to conclude about the effect of binders (earth vs gypsum).

Influence of binders

Density was found to be the main parameter controlling the flammability. Some results may suggest that the binder has a small influence on reaction-to-fire, especially gypsum may reduce the pyrolysis rate as discussed above (Fig. 14). The comparison of concretes EH261, GH320, G25E75H179, G50E50H254 and G75E25H184 allows assessing the influence of the binders for light concretes with a high amount of hemp. Note that the density of these concretes varies in the range 179-320 kg/ m 3 . Once again, Fig. 17 shows that pHRR as well as residual fraction are mainly controlled by density and the nature of binder does not seem to be influent. Nevertheless, considering only G25E75H179 and G75E25H184 exhibiting the same hemp content and density, the second one shows a slightly lower pHRR and a higher residual fraction. Especially its mass loss (in % of maximum mass loss) is only 55% versus 66% for the first one.

Further investigations are needed to better assess the role of gypsum on flammability.

A last comment can be done about the influence of the earth nature on flammability. Two earths were used (E and E2) and two concretes with the same density were prepared: EH261 and E2H259. Both exhibit similar performances (time-to-ignition, pHRR, residual fraction). Char concretes, rating is systematically found to be A2/B with very low values for FIGRAmax and THR600). Rating ranges from D (>250 W/s for the lightest concretes) to A2/B (FIGRAmax < 120 W/s for GH320 despite its ignition). For EH261, most cone tests lead to a rating C (FIGRAmax < 250 W/s) except one (rating D) due to its low TTI in cone calorimeter test. This result highlights the strong influence of TTI on simulations.

According to these predictions, rating should be B for biobased concretes with a density exceeding 500 kg/m 3 in agreement with most of data provided by Lanos [START_REF] Lanos | Bio-aggregate Based Building Materials Exposed to Fire, chapter 7 in Bio[END_REF]. A gypsum-straw concrete with a density of 550 kg/m 3 and a thickness of 75 mm was also B-rated according to SBI test sponsored by one of us and performed by FCBA. Conversely, a claystraw concrete with a density of 400 kg/m 3 and a thickness of 200 mm was rated C (FIGRA 0.2MJ equal to 130.1 W/s) which is also in agreement with our findings [43].

Note that these predictions should be only suitable for 10 cm-thick concretes. Nevertheless, the front char in cone calorimeter test never exceeds 6 cm. Then the predictions are assumed still proper as long as the material is not too thin.

Conclusion

Fire behaviour of earth and gypsum-based concretes containing hemp was assessed using cone calorimeter for a wide range of density. The flammability is mainly a function of the density. The role of binder seems to be of lesser importance. Especially ignition is observed for lowdensity concretes even if the flame is not sustained more than 60 s. A threshold density roughly close to 500 kg/m 3 is identified. Even if their surface temperature may reach high values, the heating rate of higherdensity concretes is too low and fuel release is spread over a long period of time, preventing that fuel concentration in gas phase reaches a critical value for ignition. While the flame vanishes after <1 min, it does not significantly accelerate the decomposition rate during a long period, and residual fraction is not affected by ignition. Time-to-ignition values appear relatively scattered, and this point needs more investigations.

From ignition point of view, these concretes, at least the lightest ones, can be considered as thermally thin since heat transfer from the surface to the bulk is negligible. Critical heat flux for ignition is high (27 kW/m 2 ) for one of the most flammable concretes. Peak of heat release rate remains low in most cases but reaches 100-130 kW/m 2 in case of ignition for the lightest concretes. Residual fraction and char front after 20 min also depend on the density. Smoke production is always negligible for these concretes.

Finally, SBI rating was predicted using ConeTools. If most concretes exhibit a very good rating (A2/B), some of them among the lightest ones are rated C or D. Further works are required to compare this prediction to SBI tests. Further work will be also carried out to investigate the influence of other bioresources and binders to flammability, especially for low-density concretes.

CRediT authorship contribution statement

Declaration of Competing Interest

The work was supported by some companies or associations dealing with the use of biobased concretes as insulating materials or renders. These companies are represented through some co-authors of the article.

The work has been carried out loyally, not to promote these materials but to identify the properties (density, hemp content) influencing their fire behavior.

Fig. 1 .

 1 Fig. 1. Constituents of biobased concretes.

Fig. 2 .

 2 Fig. 2. Packed samples (at the bottom) and unpacked samples (at the top-left).

Fig. 3 .

 3 Side view of the set-up in cone calorimeter test.

Fig. 4 .

 4 Fig. 4. HRR curves of hemp in PCFC (aerobic and anaerobic pyrolysis).

Fig. 5 .

 5 Fig. 5. Hemp content versus density in casted hemp biobased concretes after mass stabilization.

Fig. 6 .

 6 Fig. 6. Hemp concentration versus density in biobased concretes after mass stabilization.

7 .

 7 HRR curves (A) and mass loss curves (B) for several concretes in cone calorimeter tests at 50 kW/m 2.

Fig. 8 .Fig. 9 .

 89 Fig. 8. HRR curves of EH261 at different heat fluxes.

Fig. 10 .

 10 Fig. 10. Experimental and calculated TTI versus heat flux for EH261 (for thermally-thin model, thickness was fixed to 2.5 mm).

Fig. 11 .

 11 Fig. 11. Reciprocal of the time-to-ignition versus density.

Fig. 12 .

 12 Fig. 12. pHRR versus density.

Fig. 13 .

 13 Fig. 13. Residual fraction versus density.

Fig. 14 .

 14 Fig. 14. Mass loss (in % of maximum mass loss) versus density.

Fig. 15 .

 15 Fig. 15. Some samples after cone calorimeter tests (at 50 kW/m 2 ).

Fig. 16 .

 16 Fig. 16. Char front versus density.

Fig. 17 .

 17 Fig. 17. Peak of HRR and residual fraction versus density for concretes containing a high hemp content and various binders (for GH320, data correspond to the ignited sample).

Fig. 18 .

 18 Fig. 18. Temperature at surface during cone calorimeter at 50 kW/m 2 for several biobased concretes (arrows indicate the occurrence of ignition).

Fig. 19 .

 19 Fig. 19. Temperature at surface during cone calorimeter at 50 kW/m 2 for earth-based concretes with various densities.

Rodolphe Sonnier:

  Conceptualization, Data curation, Methodology, Project administration, Supervision, Validation, Writingoriginal draft, Writingreview & editing. Oumaima Belkhane: Data curation, Investigation, Methodology, Validation, Writingreview & editing. Laurent Ferry: Validation, Writingreview & editing. Laurent Aprin: Validation, Writingreview & editing. Pierre Delot: Conceptualization, Funding acquisition, Validation, Writingreview & editing. Coralie Garcia: Conceptualization, Funding acquisition, Validation, Writingreview & editing. Arthur Hellouin de Menibus: Conceptualization, Data curation, Funding acquisition, Methodology, Supervision, Validation, Writingreview & editing. Hélène Lenormand: Validation, Writingreview & editing. Marc Potin: Conceptualization, Funding acquisition, Validation, Writingreview & editing.

Table 1

 1 List of concretes prepared and their composition.

		Earth	Gypsum	Sand	Water	Hemp	Hemp
		(wt%)	(wt%)	(wt%)	(wt%)	(w%)	after
							curing
							(w%)
	EH261	33	0	0	50	18	35
	EH430	27	0	27	32	14	20
	EH512	42	0	0	50	8	15
	EH965	16	0	63	17	5	5
	EH1297	16	0	64	17	3	3
	GH320	0	32	0	50	18	33
	GH423	0	42	0	41	17	26
	GH649	0	44	0	44	11	18
	GH1070	0	63	0	33	4	5
	GH1328	0	69	0	29	2	3
	G25E75H179	18	6	0	62	14	35
	G50E50H254	20	20	0	39	22	34
	G75E25H184	6	18	0	62	14	33
	G25E75H1333	20	7	52	19	3	3
	G50E50H1445	13	13	53	18	3	3
	G75E25H1453	7	20	52	19	3	3
	E2H259	33	0	0	50	18	35
	EV631	37	0	0	43	20*	35*

* vermiculite (not hemp).

Table 2

 2 Mean values of flammability and standard deviations for EH261 (minimum and maximum values are done in brackets).

TTI (s) pHRR (kW/m 2 ) Residual fraction Mean value 17 (6-23) 91 (82-103) 0.85 (0.83-0.87) Standard deviation 6 7 0.02

Acknowledgments

This study was funded by Plâtres Vieujot, Bâtir en Balles, Le Réseau de la Construction Paille and l'Association des Chanvriers en Circuits en Courts (represented by Eco-Pertica, with the support of the Parc Naturel Régional du Perche). Hokuto Iseki is warmly thanks for his precious contribution for samples making.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.conbuildmat.2022.127230.