Biocomposite Bridge

Focus and restrictions. This paper focuses on the structural application of the material biocomposite in bridge building. No experimental tests were executed during this research. This paper is the result of two underlying reports: Literature research and applied research. These underlying reports are not published and are in possession of the author of this paper, University of Applied Sciences and Antea Group B.V. Abstract. Biocomposite materials are becoming more interesting to use in infrastructural projects due to their biodegradable, renewable, recyclable and sustainable properties. With a relatively low density, it is an interesting building material regarding a bridge deck. When designing with biocomposite the following factors are important to consider: material design, fibre treatment, coating and manufacturing technique. A PLA-Bamboo biocomposite was applied to an existing design of a bridge deck made out of synthetic composite. Due to its randomly oriented fibres and its equally designed lamellae, the cross section was considered homogeneous and the stresses were calculated according to ‘Hooke’s law’. The unity checks were performed according to ‘CUR 96’ with an own devised material factor of 5,69. This factor was calculated in this study for biocomposites with untreated fibres. The calculations showed that the original material (synthetic composite) was not directly replaceable by the PLA-Bamboo biocomposite. An alternative design of the deck (deck height of 1 meter and doubled thicknesses of the skins and web plates, 40- and 10 mm) showed better results. This design complied for the unity checks for strength.


Introduction
At this moment, most of the bridges in the world are made out of the traditional materials: wood, steel and reinforced concrete. Developments in 'composite bridges' started roughly two decades ago. This new material has proven itself to be a strong and eco-friendly material. Another interesting material, when sustainability is an important factor, is biocomposite. Currently, there is no bridge in the infrastructure which is made out of biocomposite (according to the definition set out in this paper, see below). This brings up the following question: Is biocomposite a good alternative material for structural application regarding a small bridge? Antea Group B.V. is interested in new and innovative materials for infrastructure with the circular design taken into account. With these design considerations, knowledge is gained about what is possible now and perhaps in the future.
In this paper the following definition is used in terms of biocomposite: a composite where all of the elements of the construction (fibres, resins and optional core materials) exist out of 100% organic material, apart from a coating and fibre treatment.

Methodology
adapted from published papers. With the knowledge of this literature review, an applied research was carried out. In this applied research, biocomposite was applied on a design of a synthetic composite bridge deck. The outcome of the calculations showed if biocomposite can be used on this design and where the weak spots are in the material regarding this application.

Pros and Cons
Due to the use of biological materials, biocomposite is biodegradable, renewable, recyclable and sustainable. These are important benefits in regards to the growing environmental awareness (Satyanarayana, 2009;Mohanty, 2000). With an eye on the increasing scarcity of other materials (e.g. oil for synthetic composites), biocomposite can be an alternative material in the future. Next to this biocomposite has a relatively low density and low coefficient of thermal expansion which reduces loads on the structure (Kalia, 2009). The relatively low density makes biocomposite an interesting building material for bridge decks. This reduction in weight could result in less deep foundation piles or even a shallow foundation. Biocomposite also has a wide range of materials (Nijssen, 2015). This variety gives the designer the opportunity to find the materials for his/her biocomposite which are most favourable for the project (low costs on raw materials, high mechanic properties, etc.).
A great disadvantage of biocomposite is the poor adhesion between fibre and matrix. This poor adhesion results in low mechanical properties of the biocomposite (Kalia, 2009). Because biocomposite is, in general, a hydrophilic material, it absorbs water. This water absorption significantly reduces the mechanical properties (Singh, 2000). The mechanical properties can also differ in respect to the climate of origin, harvest method, weather-and soil conditions (Kalia, 2009).

Important factors
Multiple important factors when designing a bridge out of biocomposite are discussed in this chapter.

Material design
When considering the design of the biocomposite the fibre orientation and fibre volume fraction are important aspects which can have a great positive impact on the mechanical properties of the biocomposite. (Shalwan & Yousif, 2013). In comparison with woven or randomly oriented fibres, unidirectionally oriented fibres (bundles) have a significantly higher tensile strength and stiffness in the axial direction. In this case, the strength and stiffness in the radial direction are determined by the resin (Nijssen, 2015). When concerning a bridge deck, the forces in the transverse direction are significantly lower than in longitudinal direction. Another option is to use different orientations for each layer in the biocomposite. In this case, the strength and stiffness in the axial and radial direction of the laminate can be influenced.
The fibre volume fraction is the ratio of the fibre content of the biocomposite. This fibre volume fraction can also be modified to gain a stronger biocomposite. A higher fibre volume fraction results in general in an increase in mechanical properties of the biocomposite; therefore, a high fibre volume fraction can be favourable when the biocomposite has a structural application (Shalwan & Yousif, 2013).

Fibre treatment
The use of natural fibres in a composite, results in poor adhesion with the matrix. Fibre treatment can clean and modify the surface of the fibre and thus improve the surface roughness. This treatment results in better adhesion with the matrix and ultimately higher mechanical properties of the biocomposite (Kalia, 2009).
Previous research shows that the chemical treatments organosilane and alkali increase the mechanical properties of the biocomposite considerably. Thermal-, plasma-and corona treatment (physical treatments) also have a strong positive influence on the mechanical properties of tensile strength and Young's modulus (Faruk, 2012).

Coating
Biocomposite is, in general, a hydrophilic material. Due to this, biocomposite is likely to absorb moisture. This moisture absorbance decreases the mechanical properties significantly (Singh, 2000). The effect of moisture absorbance on the strengths of a jute fibre composite is shown in Figure 1.
A hydrophobic coating on the biocomposite could prevent the absorption of moisture. The coating needs to resist moisture, UV-light, corrosion and low-and high temperatures in order to be applicable on a bridge. Polyesters are widely used as a coating material because they often have these properties.

Manufacturing techniques
Biocomposite can be manufactured using traditional composite manufacturing techniques, e.g. compression moulding, vacuum infusion, pultrusion and mixing (Faruk, 2012). Of these techniques mixing has a low quality as result, contrary to the others mentioned (Nijssen, 2015).
Vacuum injection or compression moulding is best when the bridge consists of a small number of parts. Figure 2 illustrates the process of vacuum infusion. Pultrusion can be used to make piles for deep foundation or structural profiles. In the latter case, the structure is constructed out of many parts which increase building time.
When a core material is used (in the case of a sandwich biocomposite) wet lay-up, prepreg layup or the adhesive bonding method can be used (Karlsson & Aström, 1996).

Bridge deck design
To test the structural application of the biocomposite material, it was applied to an existing bridge deck design. This design was originally made out of glass fibre reinforced polyester resin (synthetic composite).

Geometry
The deck is made out of a sandwich structure with 20 mm thick skins. The deck has a length of 16 meters and has 5 mm thick (centre distance 200 mm) web plates in both longitudinal-and transverse direction. The cross section of the deck is illustrated in Figure 3.

Materials
Extensive research was done on the mechanical properties of different biocomposites. There was only one biocomposite to apply to this design. This biocomposite exists out of a PLA resin with bamboo fibres. The fibres are untreated and the fibre volume fraction has a value of 38% (fibre weight fraction=30%). The properties, except the shear strength and the interlaminar shear strength (ILSS), were extracted from 'CES Edupack 2016, sheet: PLA(30% natural fiber)'. The shear strength was calculated according to 'Von Mises' theory (τmax=σY/√3) (Anderson, 2005). The ILSS of a PLA-Bamboo biocomposite or any other biocomposite, could not be adopted from the available literature. This value was adapted from a synthetic composite (glass fibre reinforced polyester), knowing that the actual value can be much lower. An assumption could not be made for this value due to the missing of research in this area. This makes the calculations in this research incomplete. Research has shown that the adhesion between core material and skins is poor. The core material is therefore not included in the calculations. The function of the core material is only practical (mould in manufacturing). Balsa wood can be used as core material. This and the PLA-Bamboo biocomposite is shown in Figure 3.

Calculation method
It has been proven that the traditional 'rules of mixtures' (ROM) are not directly applicable on biocomposite (Facca, Kortschot, & Yan, 2006) (Facca, Kortschot, & Yan, 2007). The calculated values differ significantly from the experimental values. Because of this, the use of experiments for determining the mechanical properties of a biocomposite is inevitable. The next problem in designing with biocomposite is to calculate the occurring stresses. Three methods where considered.

Classical laminate theory
The 'classical laminate theory' (CLT) is a theory that calculates the elongations and stresses in lamellae individual. The fibre orientations may differ from one another. For the use of the CLT, the thickness in the cross section must remain the same. This is not the case in the design of the deck (Figure 3). In addition, the shear forces are not included in the CLT (Nijssen, 2015). Due to these reasons, the classical laminate theory cannot be used for this research.

Inhomogeneous cross section
Because the cross section of biocomposite exists out of two different materials with different strength and stiffness properties, it can be considered an inhomogeneous cross section. The adhesion between fibre and matrix must be perfect for this method. This is because the strength and stiffness of the materials are taken into account separately. The interaction between fibre and matrix is not included. The adhesion between fibre and matrix is very unpredictable for biocomposites (Faruk, 2012). Because of this unpredictability, the method based on a inhomogeneous cross section is not a good method to calculate the stresses in a biocomposite.

Homogeneous cross section
In this method it is assumed that the properties in the entire cross section of the structure are equal. In order to ensure this, the lamellae must be equal to each other as well. This includes, fibre volume fraction and fibre orientation. When a material has a homogeneous cross section, 'Hooke's law' can be applied. In order to calculate according to Hooke's law, a linear relation is needed in the stress-strain curve of the material (Welleman, 2011). Studies have proven that the stress-strain curve of a biocomposite increases linearly to a maximum fibre volume fraction of 50% (Ochi, 2007;Akil, 2011;Shin, 1989).
A biocomposite is manufactured in lamellae, due to this the adhesion between these lamellae needs to be strong enough. These lamellae are not included in the design of a homogeneous cross section but the adhesion will be checked according to CUR 96 (see Table 4, interlaminar shear stress). For these reasons, the method based on a homogeneous cross section is a good method to calculate the stresses in a biocomposite. This method was used for this study.

Standards
Several standards where used for calculating the structural safety of the deck. The 'Eurocodes' where used for the design of the loads and the safety factors. The unity checks where performed according to 'CUR 96'. This is a recommendation (not a standard) for designing structures with glass fibre reinforced synthetic resins (CUR, 2003). The reason that CUR 96 was used is because of the absence of a standard (or recommendation) on biocomposite and the absence of a standard on synthetic composite.
For this study, the material factor was adapted for the use of CUR 96. This factor was distracted from differences between calculated values of the tensile strength and young's modulus according to the ROM and the experimental values.
The highest calculated factor in Table 2 has a value of 5.69 and counts for all biocomposites with untreated fibres which are produced according to the following methods: vacuum injection, compression moulding, prepregs and pultrusion. Calculating the inverse of this factor gives a 0.18 strength correction factor.  Table 3 presents the occurring stresses in the deck by position (see Figure 4).  The stresses in Table 4 must be checked according to CUR 96 (CUR, 2003).   Due to the randomly oriented fibres the stresses are checked in longitudinal (1 and 2) and transverse (3 and 4) direction of the deck. With a highest calculated unity check of 17.92, the original design of the bridge deck does not comply when a PLA-Bamboo biocomposite is used as material. The values of the unity checks can decrease when a different biocomposite is used or when fibre treatment is applied. This increases the mechanical properties and gives an opportunity to apply a lower material factor.

Alternative design
An adjustment on the design of the cross section of the deck can decrease the occurring stresses. For this study the thickness of the skins and web plates where doubled (40-and 10 mm) and the height of the deck was increased up to 1.0 meters (+ 117%, see Table 5 and Figure 5). The design of the biocomposite remained unchanged due to the lack of information about the properties of biocomposites. The highest unity check of 0.89 was calculated. This shows that this biocomposite can be applied, but extreme adjustments must be done to the design. A different biocomposite may show better mechanical properties and results in which no (extreme) adjustment to the design of the cross section are necessary.

Conclusions
Several conclusions have been made during this research on the structural application of biocomposite on a bridge. 2. Biocomposite has a linear stress-strain curve up to a maximum fibre volume fraction of 50%.
3. With a safety factor of 5.69, CUR 96 can be used as recommendation for designing with biocomposites with untreated fibres.
4. In comparison with other building materials (steel, concrete, synthetic composite etc.), biocomposite is eco-friendlier, biodegradable and it does not deplete other materials like iron and oil (in synthetic composite). The relatively low density (ca. 1.3 g/cm 3 , steel: ca. 7.9 g/cm 3 ) makes biocomposite an interesting building material for bridge decks. Due to these benefits, biocomposite can be a good alternative building material.
5. The design of the biocomposite (fibre orientation and fibre volume fraction) and fibre treatment are important factors regarding the mechanical properties of the biocomposite.
Biocomposite can be manufactured using traditional composite manufacturing techniques like vacuum infusion, compression moulding and pultrusion.

Recommendations
There are four recommendations made based on mechanical properties, material design, calculation method and time-dependent properties: 1. There is still too little knowledge about the properties of biocomposites. Most research currently done is about the tensile strength and Young's modulus of biocomposite. When designing a bridge, properties such as compressive strength, (interlaminar) shear strength, shear modulus and poisson's ratio are also important. More research on these properties is needed. With more research and thus knowledge about the mechanical properties of the biocomposite, it can also be possible to reduce the material factor calculated in this research.
2. The fibres flax, nettle, hemp, abaca and silk with a PLA resin can be a biocomposite with high mechanical properties. An organosilane-, alkali-, thermal-, corona-or plasma fibre treatment show good adhesion between fibre and matrix and thus results into higher mechanical properties of the biocomposite. The mechanical properties of these combinations must be studied.
3. More research on calculation the occurring stresses in biocomposites is needed. The 'Classical Laminate Theory' can be applied but has its limitations (shear strength and thickness in the cross section). A method based on a homogeneous cross section can only be used when the properties for each lamellae are equal to one another.
A bridge is mostly designed for numerous years. Because of this, the time-dependent properties are important. More research is needed to the creep-and fatigue behaviour of biocomposite. Also the effect of time on the Young's modulus must be studied.