Performance of eco-composite sandwich panels manufactured by vacuum bagging infusion

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<big>'''Performance of eco-composite sandwich panels manufactured by vacuum bagging infusion'''</big></div>

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M. Helena C. Teixeira<sup>1,*</sup>, Luís M.M. Amorim<sup>1</sup>, A. Araújo<sup>1,2</sup>, R. M. Santos<sup>1,2</sup>, Susana P.B. Sousa<sup>1,2</sup></div>

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<sup>1</sup> Institute of Science and Innovation in Mechanical and Industrial Engineering, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal – [mailto:ssousa@inegi.up.pt ssousa@inegi.up.pt]</div>

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<sup>2</sup> Associated Laboratory for Energy, Transports and Aeronautics, Porto, 4200-265, Portugal</div>

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* [mailto:hteixeira@inegi.up.pt hteixeira@inegi.up.pt]</div>

'''RESUMEN:''' La industria de la construcción utiliza normalmente una combinación compleja de materiales con una elevada huella de carbono. Recientemente han surgido nuevos sistemas y soluciones de construcción, con una mayor sostenibilidad y eficiencia energética durante su vida útil, como las estructuras ligeras y ajustables tipo sándwich una de las estrategias más prometedoras. Los procesos más comunes para producir compuestos de polímeros reforzados con fibra (FRP), de grandes dimensiones, son la laminación manual con bolsas de vacío y la infusión al vacío. Además, los paneles compuestos tipo sándwich suelen estar compuestos por fibra de vidrio y resinas termoendurecibles, sin embargo, las fibras de basalto pueden ser un sustituto interesante, ya que son más sostenibles y tienen mejor desempeño. A pesar de la gran cantidad de trabajos de investigación realizados sobre paneles tipo sándwich en compuestos avanzados, se han publicado pocos estudios sobre el uso del proceso de infusión al vacío y el uso fibras de basalto para desarrollar paneles compuestos tipo sándwich. Por lo tanto, este trabajo tiene como objetivo diseñar, fabricar y comparar el comportamiento mecánico, físico, térmico y ambiental de estos paneles compuestos tipo sándwich con fibras de vidrio y basalto, que serán utilizados en la construcción modular. Los resultados revelaron que es factible producir paneles sándwich compuestos a través de este proceso y la solución más prometedora se basa en fibras de basalto.

'''Palabras clave:''' eco-compuestos, construcción modular, multifuncionalidad, prefabricados, sostenibilidad.
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'''ABSTRACT:''' The construction industry uses a complex combination of materials associated with a high carbon dioxide footprint. Therefore, several new building systems and solutions have emerged with higher sustainability and improved energy efficiency over their service lives, with the development of customizable lightweight sandwich structures, being one of the most promising strategies. The common processes for manufacturing fibre-reinforced polymer (FRP) composites, with large dimensions, are both hand layup-assisted vacuum bagging and vacuum bagging infusion. Besides, composite sandwich panels are traditionally composed by glass fibres and thermoset resins. However, basalt fibres can be an interesting substitute, since they are more sustainable, and have a higher performance. Despite the large amount of research work carried out on advanced composite sandwich panels, only a few studies were focused on the manufacturing of these structures based on basalt fibres assisted by vacuum infusion. Therefore, this work aims to design, manufacture and compare the mechanical, physical, thermal and environmental performance of these composite sandwich panels, comprising either glass or basalt fibres, which will be further applied in modular construction. The results revealed that it is feasible to produce composite sandwich panels through this method, and the most promising solution attained through this study is based on basalt fibres.

'''Keywords: '''eco-composites, modular construction, multifunctionality, prefabrication, sustainability.

==1. Introduction==

Ongoing climate changes and ecological degradation are challenging for the whole world, especially considering that global primary materials extraction and their use in the construction sector are projected to double in the coming decades [1-3]. This sector is also the one with higher environmental impact, since it is responsible for the consumption of approximately 60 % of the entire volume of natural and critical resources at the global scale [1-3], for high energy consumption, greenhouse gas emissions, solid waste generation, and pollution [4, 5]. In the several phases of the building’s life cycle, a large amount of carbon dioxide (CO<sub>2</sub>) is emitted into the atmosphere. In fact, this sector is responsible for 37 % of global CO<sub>2</sub> emissions and 34 % of energy consumption [6]. The growing concerns with environmental impact and the awareness to save materials, and energy resources have raised the need to develop more sustainable solutions. These include both reducing the amount of resources and using more environmentally friendly materials in construction technologies. The use of advanced composite materials is one of the most promising strategies for the construction of new structures, rehabilitation or expansion of existing ones. Although they present significant advantages, the limited knowledge, and the lack of standards for the application of composite structures in the construction industry are delaying their widespread industrialization [7, 8].

Composite sandwich panels are an example of advanced composite structures, which consist of two thin and rigid laminates separated by a lightweight core [9]. These structures exhibit several advantages, such as high strength-to-weight ratio, thermal and acoustic insulation, vibration absorption, impact resistance and versatility, allowing prefabrication and modularity [9, 10]. The production method and materials selection for the design and development of these panels are crucial, as it determines their overall final properties and sustainability [11]. The most common process for manufacturing fibre-reinforced polymer (FRP) composites, with considerable dimensions and good quality, are liquid moulding techniques, including hand layup-assisted vacuum bagging and vacuum bagging infusion [12-15]. Due to its reliance on manual procedures, the hand layup process is highly labour-intensive and time-consuming. Additionally, the final properties of the structure may be highly affected by the manual procedure, because it is heavily dependent on the skill and experience of the operator [16]. The application of vacuum pressure in this process can improve the composite properties, since it compacts the laminate layers, reducing the number of voids and removing excessive resin [16, 17]. One of the main advantages of this technique is that it can be used for the maintenance and repair of damaged structures [16]. On the other hand, the vacuum infusion process is a more versatile, accurate, and well-established methodology that uses vacuum pressure to drive resin into the composite laminate [11, 18]. The manufactured composite materials by this process exhibit a good fibre-to-resin ratio, and an overall high mechanical performance [19].

Although glass fibres are usually applied on composite sandwich panels, basalt is a promising material for a more sustainable development of these structures, considering that it is an environmentally safe and inert material with no reaction with air and water [20, 21]. These fibres have better properties than glass counterparts, such as higher temperature stability and resistance against corrosion, superior strength, and chemical resistance [21, 22].

<span id='cite-_Ref156983782'></span>[[#_Ref156983782|Table 1]] presents a comparison of the main properties of both fibres.

{| style="width: 100%;border-collapse: collapse;" 
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Fibre'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Density (g/cm<sup>3</sup>'''</span><span style="text-align: center; font-size: 75%;">''')'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Tensile Strength (MPa)'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Young’s Modulus (GPa)'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Thermal Conductivity (W/m·°C)'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Sound Absorption Coefficient'''</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Glass'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">2.55</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1 930 – 2 050</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">72 – 85</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.2 – 1.35</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.8 – 0.93</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Basalt'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">2.67</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1 430 – 4 900</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">71 -110</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.03 – 0.038</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.95 – 0.99</span>
|}


<div id="_Ref156983782" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Table 1. Properties of the glass and basalt fibres [20, 23].</div>

Even though extensive research work has been carried out on advanced composite sandwich panels, only a limited number of studies have been published involving the vacuum infusion method and basalt fibres [21, 24-26]. Therefore, this work aims to design, manufacture and compare the mechanical, physical, thermal, and environmental properties as well as evaluate the performance of two different composite sandwich panels with an extruded polystyrene (XPS) foam core, one with glass fibres and the other with basalt fibres.

==2. Materials and methods==

===2.1. Materials ===

XPS foam core (ifoam®, IMPERALUM, Portugal) was directly bonded to a composite laminate that comprises an epoxy resin (Biresin® CR83/CH 83-6, SIKA Portugal, Portugal) and fibres. Thus, two panels with different laminates were compared, one with glass fibres (450 g/m<sup>2</sup>, Castro Composites, Spain) and other with basalt fibres (450 g/m<sup>2</sup>, Castro Composites, Spain).

===2.2. Experimental Procedure===

The composite sandwich panels were produced by vacuum infusion, as it can be seen in <span id='cite-_Ref134023098'></span>[[#_Ref134023098|Figure 1]], with two layers of fibres on both sides of the core material. The fibre fabrics with 40 cm × 60 cm were laid up with ± 45° orientation. The epoxy resin (A) was mixed with the hardener (B), at room temperature, in a proportion of 100A:30B, and then, degassed (for approximately 10 minutes) to minimize the presence of air bubbles and, therefore, potential defects. After the application of the release agent, positioning of the peel ply, infusion mesh, spiral, tube, vacuum bag and sealant tape, a vacuum pressure of 500 mbar was set, and a leak test was performed. Afterwards, the inlet valve was opened and the resin starts to flow through the layers of fibre. The panels were cured for 10 h at 60 °C, keeping the vacuum pressure till the end of the procedure. After the removal of all the consolidation materials, the produced composite sandwich panels were machined into different specimens for further characterization.

[[Image:Review_242578956577-image1.png|600px]]

<div id="_Ref134023098" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Figure 1. Infusion process for producing composite sandwich panels: a) application of the release agent and material positioning; b) vacuum application and resin flow progression; c) vacuum infusion process (all material impregnated); d) detailed view of the different layers used in the infusion process (1 – flow mesh, 2 - peel ply, 3 – fibres, and 4 – foam core).</div>

===2.3. Test procedures===

The composite sandwich panels and the laminates were characterized to evaluate physical, thermal, and mechanical properties using several techniques.

====2.3.1 Laminates====

The density and the fibre volume fraction were determined to evaluate the physical properties of both laminates and analyse the influence of the core in the infusion of the top laminate. The density of the laminates was calculated using Method A of ASTM D 792, and the fibre volume fraction and void content were determined, following ASTM D 3171 – G.

Differential scanning calorimetry (DSC) experiments were performed using a Q20 model calorimeter (TA Instruments), to determine if the curing reactions were complete, and evaluate the thermal transitions of the laminates. The experiments were carried out according to ISO 11357. The samples were heated from 30 °C to 110 °C, at a constant rate of 20 °C/min, under a nitrogen flow rate of 50 mL/min.

====2.3.2 Sandwich panels====

The mechanical properties of the sandwich panels were determined by i) three-point flexural, and ii) edgewise compression tests performed using an Instron 5900R Universal Testing Machine, following ASTM C 393/ C3939 M and ASTM C364/ 364 M, respectively.

The dimensions of the specimens for the three-point flexural test were 200 mm × 75 mm. The test was carried out with a 100 kN load cell and at a constant loading rate of 6 mm/min. Aiming at avoiding early failure, the loading bar and support bars had a 25 mm diameter. Edgewise compression tests were conducted with the same load cell and at a speed of 0.5 mm/min. The specimens with 160 mm × 40 mm were subjected to a compression force parallel to the plane of the laminates. <span id='cite-_Ref134517939'></span>[[#_Ref134517939|Figure 2]] a) and b) describe the set-up for the flexural tests, where S is the support span with 150 mm, and P is the load, and <span id='cite-_Ref134517939'></span>[[#_Ref134517939|Figure 2]] c) and d) the set-up for the compression tests.

[[Image:Review_242578956577-image2.png|600px]]

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Figure 2. Set-up for the mechanical tests performed: a) and b) flexural test, and c) and d) compression test.</div>

The composite sandwich panels’ density was calculated based on the ASTM C271, by using a digital calliper (150 mm ProK) and a Radwag Scale AS 220.R2.

To analyse the environmental impact of the two sandwich panels, it was used the GRANTA EduPack eco audit tool (only at material level). With this software, it is possible to analyse the energy consumption and carbon footprint for the whole life cycle, without exploring every parameter of the developed solutions [27].

==3. Results and Discussion ==

===3.1. Laminates ===

The physical and thermal properties of the produced laminates were determined, and the average results for three samples with the respective standard deviations are summarized in <span id='cite-_Ref134180743'></span>[[#_Ref134180743|Table 2]].

{| style="width: 100%;border-collapse: collapse;" 
|-
|  rowspan='2' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Test'''</span>
|  rowspan='2' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Property'''</span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Glass Laminate'''</span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Basalt Laminate'''</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Top'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Bottom'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Top'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Bottom'''</span>
|-
|  rowspan='2' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Density</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Epoxy resin density (g/cm<sup>3</sup></span><span style="text-align: center; font-size: 75%;">) (according to datasheet)</span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.15 </span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.15</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Composite density (g/cm<sup>3</sup></span><span style="text-align: center; font-size: 75%;">)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.74 ± 0.04</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.73 ± 0.02</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.84 ± 0.04</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.87 ± 0.02</span>
|-
|  rowspan='2' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Constituent content</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Fibre volume fraction (%)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">47.7 ± 1.0</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">47.5 ± 0.9</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">54.1 ± 0.8</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">54.4 ± 0.3</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Void content (%)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">7.2 ± 2.3</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">7.0 ± 1.0</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">5.4 ± 2.6</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">3.5 ± 1.4</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">DSC</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Glass transition temperature (°C) of the 1<sup>st</sup></span><span style="text-align: center; font-size: 75%;"> heating ramp  </span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">76.9 ± 0.1</span>
|  colspan='2'  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">82.6 ± 0.4</span>
|}


<div id="_Ref134180743" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Table 2. Data test results of the developed laminates.</div>

The density of the basalt laminate is slightly higher than the counterparts containing glass fibres (+5.7 % for the top laminate and +8.1 % for the bottom laminate) as expected, considering that the basalt fibres are denser, and the fibre volume fraction of the produced basalt composites is higher (+6.4 % and +6.9 %, for the top and bottom laminate, respectively). The evaluation of void content showed that the glass laminate presented a superior value (+1.8 % for the top laminate and +3.5 % for the bottom laminate). Furthermore, by making the panel in a single step, the impact of a core on the properties of the top laminate was reduced, although in the basalt panel there was a slight increase in the void content (+1.9 %). Moreover, the DSC experiments revealed that the basalt laminate, contrary to the glass counterparts, achieve higher glass transition temperature than the one presented in the resin datasheet (80 °C), what can lead to better properties. The presented results showed that is relevant to control the design and the process parameters, since the performance of the developed composites can be further improved, essentially by reducing the void content.

===3.2. Composite Sandwich Panels===

The mechanical properties and the sustainability of both sandwich panels were evaluated and are summarized in <span id='cite-_Ref134181136'></span>[[#_Ref134181136|Table 3]]. The average and standard deviations for five valid results are reported.

{| style="width: 100%;border-collapse: collapse;" 
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Test'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Property'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Glass'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Basalt'''</span>
|-
|  rowspan='4' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Flexural Test'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Maximum load (N)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">499.5 ± 30.8</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">488.9 ± 22.0</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Core shear ultimate strength (MPa)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.18 ± 0.01</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.17 ± 0.00</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Facing stress (MPa)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">15.7 ± 0.7</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">16.2 ± 0.7</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Modulus of elasticity in bending (MPa)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">350.2 ± 11.0</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">243.1 ± 14.8</span>
|-
|  rowspan='2' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Compression Test'''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Maximum load (kN)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">3.05 ± 0.14</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">3.36 ± 0.29</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Ultimate edgewise compressive strength (MPa)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">47.7 ± 3.6</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">59.8 ± 5.6</span>
|-
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Density '''</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">Composite sandwich panel density (g/cm<sup>3</sup></span><span style="text-align: center; font-size: 75%;">)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.16 ± 0.00</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">0.15 ± 0.00</span>
|-
|  rowspan='3' style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">'''Sustainability '''</span>

<span style="text-align: center; font-size: 75%;">'''(material)'''</span>
|  style="border: 1pt solid black;text-align: center;vertical-align: top;"|<span style="text-align: center; font-size: 75%;">Energy (MJ)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">65.2</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">38.3</span>
|-
|  style="border: 1pt solid black;text-align: center;vertical-align: top;"|<span style="text-align: center; font-size: 75%;">CO<sub>2</sub></span><span style="text-align: center; font-size: 75%;"> Footprint (kg)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">3.4</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">1.8</span>
|-
|  style="border: 1pt solid black;text-align: center;vertical-align: top;"|<span style="text-align: center; font-size: 75%;">Cost of the fibres (€/m<sup>2</sup></span><span style="text-align: center; font-size: 75%;">)</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">6.45</span>
|  style="border: 1pt solid black;text-align: center;"|<span style="text-align: center; font-size: 75%;">12.99</span>
|}


<div id="_Ref134181136" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Table 3. Data test results of the developed sandwich panels.</div>

An overall positive trend was observed for specimens with basalt fibres, where a more pronounced improvement was attained for the ultimate edgewise compression strength (+25 %). However, lower values of core shear ultimate strength (-6 %), and modulus of elasticity in bending (-31 %) were found on the flexural tests performed, in comparison with specimens containing glass fibres. This limitation can be associated to the smaller thickness of the basalt laminate (0.85 ± 0.06 mm and 0.75 ± 0.04 mm for the glass and basalt laminates, respectively) obtained at the end of the vacuum infusion process promoted by the higher fibre volume fraction.

<span id='cite-_Ref134690270'></span>[[#_Ref134690270|Figure 3]] shows the failure mode of the composite sandwich panels in study.

<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 [[Image:Review_242578956577-image3.png|402px]] </div>

<div id="_Ref134690270" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Figure 3. Failure mode of the test sandwich panels: a) and b) flexural tests, c) and d) compression tests.</div>

The failure mode in both sandwich panels was similar, mainly promoted by core crushing in the flexural test, and in the compression experiments, the mechanical failure was due to partial debonding combined with core shear failure [28]. These results indicate that a good adhesion was established between the laminates and the core.

Regarding the sustainability study, since GRANTA Edupack 2021 software does not consider XPS, a similar material, expanded polystyrene (EPS) was used. According to Table 3, it is possible to observe that the highest impacts were found for the glass fibre sandwich panel because of its high-energy consumption and CO<sub>2</sub> footprint (+70 % and +86 %, respectively). Although the basalt fibres have a lower environmental impact, it leads to a higher cost in relation to the glass fibres (+101 %).

==4. Conclusions==

With the goal of finding a sustainable product for the construction sector, it was proposed the use of composite sandwich panels. In this research, the physical and mechanical behaviour of the developed solutions, by vacuum infusion, was evaluated as well as their sustainability. In summary, the concluding remarks can be stated as:

* The laminates of the sandwich panels presented a fibre volume fraction of 48 % and 54 %, and a void content of up to 5.4 % and 7.2 %, for those containing glass and basalt fibres, respectively. The impact of the core on the properties of the top laminate was relatively low. The performed thermal tests revealed that the combination of basalt fibres and resin resulted in a higher glass transition temperature than the one found on the resin datasheet.

* In general, the basalt composite sandwich panel presented better mechanical performance, however, the flexural properties were slightly lower due to the thinner composite laminate. The compression tests showed a good adhesion between the core and the laminate, since occurred rupture of the core combined with a slight delamination of the composite laminate.

* Regarding the sustainability of the developed composite sandwich panels, the one with basalt fibres presented lower energy consumption and CO<sub>2</sub> footprint, yet the cost is relatively higher than the one with glass fibres. This can difficult the utilization and spread of these materials in the construction sector.

The results revealed that it is feasible to produce composite sandwich panels through the vacuum infusion method and the most promising solution is the one with basalt fibres. However, additional studies are anticipated, to optimize the vacuum infusion procedure, and to further decrease the void content. This could enhance even more its global performance and allow the full exploitation of the promising properties of the basalt laminates and composite sandwich panels. In complement, future work can be carried out with more sustainable core materials to find a more ecological and promising structure.

==5. Acknowledgements==

This work was developed in the scope of “iclimabuilt” project, supported by the European Union under the HORIZON2020 Framework Programme for Research and Innovation under grant agreement no 952886.

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