# Evaluating the Mechanical Performance of Bio-Based Concrete: The Role of Aggregate Type and Orientation in Compression Cyclic Loading

R. Bardouh, E. Toussaint, S. Amziane, Sandrine Marceau

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R. Bardouh, E. Toussaint, S. Amziane, Sandrine Marceau. Evaluating the Mechanical Performance of Bio-Based Concrete: The Role of Aggregate Type and Orientation in Compression Cyclic Loading. Cleaner Engineering and Technology, 2026, 30, pp.101139. 10.1016/j.clet.2025.101139. hal-05443282

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# Evaluating the mechanical performance of bio-based concrete: The role of aggregate type and orientation in compression cyclic loading

R. Bardouh<sup>a</sup>, E. Toussaint<sup>a</sup>, S. Amziane<sup>a,*</sup> ◆, S. Marceau<sup>b</sup>

## ARTICLE INFO

Keywords:
Bio-based
Orientation
DIC
Usable strength
Floating modulus
Fatigue analysis

## ABSTRACT

This study evaluates the effect of the physical aspects of four bio-aggregates (Hemp, bamboo, rapeseed, and reed) on the mechanical properties of bio-based concrete under cyclic compression loading. The bio-based specimens are assessed with respect to two distinct orientations of bio-aggregates: Perpendicular to compaction direction, and parallel to compaction direction. The digital image correlation (DIC) method permits to monitor strain and displacement fields throughout 19 loading-unloading cycles. Beyond conventional mechanical characterization, DIC data were processed to quantify crack density, crack length, and crack opening displacement (COD) as indicators of fatigue damage evolution.

Hemp concrete exhibited the highest compressive strength and modulus, while bamboo concrete showed the weakest performance despite its higher density. In perpendicular orientation, specimens demonstrated cyclic hardening up to 4.5 % strain before entering a damage phase, whereas parallel-oriented specimens began degrading around 1.8 % strain.

The useable strength of parallel-oriented specimens denotes the particle/binder affinity rather than the actual material's strength. The deformation charts given by the digital image correlation show that specimens tested under perpendicular orientation of bio-aggregates exhibit two horizontal lines of stress concentration whereas specimens tested under parallel orientation show the propagation of a vertical concentration lines.

The failure mode of the perpendicular-oriented specimens is characterized by gradual crushing of bio-aggregates whereas the parallel-oriented specimens fail due to buckling that splits the specimen into two different parts on either side of the vertical concentration lines. Crack analysis revealed that hemp, reed, and rapeseed concretes developed limited and slowly propagating cracks (average rate 0.18–0.23 mm/cycle), with maximum crack lengths below 5 mm and narrow CODs (less than 2 mm). In contrast, bamboo concrete displayed rapid crack propagation (dL/dN around 3.66 mm/cycle), high crack density (32), and wide openings (higher than 4 mm), reflecting weak interfacial bonding.

Hence, hemp shives oriented perpendicularly provides the ideal optimized configuration of bio-based concretes assessed in this study.

## 1. Introduction

The construction industry stands as one of the largest energy consumers globally, and a significant contributor to greenhouse gas emissions, especially carbon dioxide (CO₂) (Huo et al., 2020; Zhang et al., 2020). This underscores the urgent need for sustainable materials with lower embodied energy to mitigate environmental impact (Arnaud and Gourlay, 2012). Therefore, great interest has been directed towards vegetal concrete (Jami et al., 2019), often called as bio-based concrete (Huang et al., 2024). Bio-based concrete consists of the replacement of

coarse aggregates (gravel) and fine aggregates (sand) presented in traditional concrete by bio-aggregates from plants and agricultural sources (Amziane and Sonebi, 2016).

Notably, bio-based concrete, particularly hemp-lime formulations, can sequester up to 35 kg of CO₂ per square meter of a wall that is 25 cm thick, sustained over 100 years (Boutin et al., 2005). Bio-based concrete presents eye-catching hygrothermal and acoustic properties (Amziane and Collet, 2017), whereas their main problem resides in their low mechanical performance (Bouloc, 2006; Sellami et al., 2013; Tronet et al., 2016). Hence, a pronounced evaluation of the mechanical

<footnote><sup>a</sup> Clermont Auvergne INP, Institut Pascal UMR 6602 - UCA/CNRS, 63178, Aubière, France</footnote>

<footnote><sup>b</sup> Université Gustave Eiffel, MAST-CPDM, 77454, Marne-la-Vallée CEDEX 2, France</footnote>

<footnote>* Corresponding author.
E-mail address: sofiane.amziane@uca.fr (S. Amziane).</footnote>
performance of these materials is inherently indeed before any potential reinforcement (Bardouh et al., 2024). While hemp is the primary type of bio-aggregates to be incorporated in case studies for bio-based concrete, its availability on the market is less than other bio-aggregates (Ratsimbazafy et al., 2021). Therefore, evaluating alternative bio-based aggregates is of significant interest, as they may serve as viable substitutes for hemp shives in construction applications.

The compressive strength of bio-based concrete is typically below 5 MPa, rendering it unsuitable for load-bearing applications (Arnaud et al., 2013; Nguyen et al., 2010; Tronet et al., 2016). Various mechanical tests have been employed to characterize bio-based concrete, including monotonic compression tests (Arnaud and Gourlay, 2012; Elfordy et al., 2008) and cyclic ones (Mazhoud et al., 2017; Niyigena et al., 2016). While monotonic tests provide basic mechanical parameters such as apparent modulus, cyclic tests provide a more advanced understanding, enabling the calculation of 3 types of modulus: apparent, elastic, and hardening (Niyigena et al., 2019). For instance (Niyigena et al., 2019), found that the methodology applied in cyclic compression was more accurate and helpful in optimizing the actual rigidity of the material.

The mechanical properties of bio-based concrete are influenced by numerous factors, including formulations (Arnaud and Gourlay, 2012; Véronique, 2005), binder type, aggregate characteristics (Niyigena et al., 2018), casting process (compaction energy) (Nguyen et al., 2010; Tronet et al., 2016), curing conditions (Arnaud and Gourlay, 2012), and particle/matrix physicochemical interactions (Diquélou et al., 2015).

Bio-aggregate properties such as water absorption, particle size, and bulk density play a major role in determining the material's performance by altering the green density of the bio-based concrete (Elfordy et al., 2008; Niyigena et al., 2016; Walker et al., 2014). Larger bio-aggregates, though capable of absorbing more water (Niyigena et al., 2016, 2018), exhibit a weak interfacial bonding with the matrix, leading to poor mechanical performance (Niyigena et al., 2018). In this regard (Akkaoui et al., 2017), found that longitudinal Young's modulus of wood aggregates decreased from 12,760 MPa to 10,300 MPa when water content of aggregates increased from 12 % to 25 %, respectively. However, smaller bio-aggregates improve compressive strength by enhancing binder adhesion and reducing voids (Sáez-Pérez et al., 2020), alongside the mitigated effect of released polysaccharides on the hardening of the binder due to small surface area (Gourlay and Arnaud, 2011; Nozahic et al., 2012).

The nature of the bio-aggregate also significantly affects the mechanical performance of bio-based concrete at the composite scale. Comparative studies have shown that hemp shives outperform other plant aggregates, such as rice husk, sunflower pith, and bark, in terms of compressive strength (Brouard et al., 2018; Chabannes et al., 2015; Magniont et al., 2012). For instance, under identical formulation and curing condition, hemp concrete achieved a compressive strength after 4 months of 0.75 MPa compared to 0.45 MPa for rice husk concrete (Chabannes et al., 2015).

Due to the anisotropic behavior of bio-based concrete, the orientation of the bio-aggregates relative to the compaction direction has a substantial impact on its mechanical properties (Nozahic et al., 2012; Youssef et al., 2015). Two distinct orientations arise during compaction: parallel and perpendicular to the compacting direction (Huang et al., 2022; Williams et al., 2016). Perpendicularly oriented bio-aggregates enhance rigidity and distribute stress more effectively, reducing the likelihood of localized failure. In contrast, parallel orientations, while initially more robust, are more susceptible to stress concentrations and damage (Huang et al., 2022).

Additionally, bio-based concrete is highly sensitive to variations, making the prediction of its mechanical properties heavily dependent on the testing methodology (Niyigena et al., 2016, 2019) explored 3 computational methods for determining Young's modulus of hemp concrete: tangential, cyclic, and floating. The floating technique was identified as the most reliable thanks to its low dispersion and

consistency over time, making it a preferred approach for evaluating bio-based concrete.

Curing conditions can significantly alter the stability and the mechanical properties of bio-based concrete. In this context, the porosity of hemp concrete increased by 3 % when subjected to severe aging conditions (immersed in water for 48 h and dried in oven at 50 °C for 72 h, over 40 days) compared to specimens stored in climatized chamber at 23 °C and 50 % RH (Benmahiddine et al., 2020a). Interestingly, moisture buffer value (MBV) and compressive strength decreased by 51 % for specimens subjected to severe aging (Benmahiddine et al., 2020a). In addition (Asli et al., 2021), found that MBV decreased by 40 % between hemp concrete stored for different surface transfer at indoor conditions (T = 21.55 °C and RH = 58.8 %) compared to ambient conditions (T = 20.19 °C ± 10 °C, and RH = 73.9 % ± 25 %). To this end (Arnaud and Gourlay, 2012), demonstrated that ultimate mechanical performance can be obtained at 50 % RH, meanwhile high relative humidity (75 % and 98 % RH) were not suitable for binder setting, and low relative humidity (30 % RH) slowed down very sharply the setting process.

Digital image correlation (DIC) has proven to be a valuable tool for non-contact analysis of material's deformation and crack propagation due to its numerous advantages such as cost efficiency, simplicity, and accuracy (McCormick and Lord, 2010). Recent study using DIC revealed heterogeneous deformation patterns in hemp concrete, emphasizing the role of aggregate characteristics and orientation in mechanical performance (Bardouh et al., 2023).

The objective of this work is to evaluate the effect of aggregate type and orientation on the mechanical properties of bio-based concrete under cyclic loading. Indeed, digital image correlation allows further investigation to what extent each aggregate type and orientation can affect the displacement of bio-based concrete at optimal mechanical performance. Life cycle fatigue analysis is applied where cracks exported from DIC data are processed. Hence, the coupling between image analysis and mechanical testing offers deeper insights into the behavioral law governing bio-based building materials. By understanding this coupling, this work aims to identify configurations that optimize strength, rigidity, and energy dissipation, ultimately advancing the development of bio-based concrete as a viable sustainable construction material.

## 2. Materials and methods

This section describes the raw materials used and their physical characterization, the specimens' preparation procedure, the test setup, and the mechanical properties computational methodology.

### 2.1. Materials

#### 2.1.1. Mineral binder

A pozzolanic mineral binder available on the French market, NATURAT CEMIV/A (P) 42,5 R CE NF, called “CEMIV” was used in this study. The compressive strength of the mineral binder was determined at different testing durations based on normalized mortars of 4 × 4 × 16 cm³ and listed in Table 1. The values were given by the cement provider VICAT (VICAT).

#### 2.1.2. Plant aggregates

The four bio-aggregates and their corresponding scientific names

<table><caption>**Table 1**

Compressive strength of the mineral binder used in this study.</caption><thead><tr><td>Duration (Days)</td><td>Compressive strength (MPa)</td></tr></thead><tbody><tr><td>1</td><td>18.5</td></tr><tr><td>2</td><td>29.2</td></tr><tr><td>7</td><td>41.2</td></tr><tr><td>28</td><td>52.8</td></tr></tbody></table>
used in this study are as follow: hemp shives (cannabis sativa), bamboo (bambusa vulgaris), rapeseed (brassica napus), and reed (Phragmites australis) as illustrated in Fig. 1. The bio-aggregates were dried at 60 °C to constant mass and sieved using a 500 µm mesh to eliminate dust and fine particles. For the experimental campaign, most of bio-particles selected were retained between 1 mm and 10 mm, aligning with the typical classification of coarse aggregates in the context of construction materials.

The following bio-aggregates underwent physical characterization tests in accordance with the protocol of RILEM TC 236-BBM (Amziane et al., 2017b). As a matter of fact, the physical properties of the studied bio-aggregates such as the bulk density, granulometric properties, and water absorption were elaborated in this section.

### 2.1.2.1. Bulk density.
The procedure of determining bulk density involves weighing the bio-aggregate and measuring its corresponding volume with water (Amziane et al., 2017a). The authors have followed the operating instructions recommended by TC 236 RILEM. The bulk density is calculated according to equation (1),

$$ \rho_{\text{agg}} = \frac{m_{\text{agg}} \times \rho_{\text{water}}}{m_{\text{water}}} \quad (1) $$

where $\rho_{\text{agg}}$: apparent density of the bio-aggregate (kg/m³).

m<sub>agg</sub>: mass of aggregates immersed in water (kg).

$\rho_{\text{water}}$: apparent density of water (997 kg/m³).

m<sub>water</sub>: mass of water (kg).

Hence, the bulk density of the corresponding bio-aggregates before and after drying is listed in Table 2. Bamboo was the densest among other bio-aggregates, the nearly constant bulk density of bamboo before and after drying suggests its low moisture content which is stable over time. Rapeseed was the lightest and sensitive to moisture evaporation, reflecting a more porous structure. Reed was the second most dense bio-aggregate with high moisture sensitivity after drying. Hemp shives exhibit intermediate behavior, balancing lightness and moderate stability.

### 2.1.2.2. Water absorption.
The process of determining water content entailed drying the bio-aggregates and respecting a designated formula to calculate their water content at different immersion times, as specified in a particular RILEM protocol.

The water absorption ratio is dependent on immersion times in logarithmic function as presented in equation (2), and illustrated in Fig. 2.

$$ W(t) = \text{IWC} + K_1 \log(t) \quad (2) $$

where, IWC denotes initial water content (%)

K<sub>1</sub> represents the diffusion rate in bio-aggregates cells, which corresponds to the slope of the curve w(t) as function of logarithmic time.

Initial water content (IWC) was calculated based on the difference in weight between wet and dry aggregates as elaborated in equation (3),

<caption>**Table 2**</caption>

Bulk density before & after drying, and water absorption of the different bio aggregates.

<table><thead><tr><th>Properties</th><th></th><th>Hemp shives</th><th>Bamboo</th><th>Rapeseed</th><th>Reed</th></tr></thead><tbody><tr><td rowspan="2">ρ (kg/m³)</td><td>Before drying</td><td>118 ± 4</td><td>305 ± 10</td><td>100 ± 6</td><td>169 ± 8</td></tr><tr><td>After drying</td><td>113 ± 5</td><td>304 ± 7</td><td>90 ± 7</td><td>145 ± 6</td></tr><tr><td rowspan="2">Water absorption</td><td>Initial water content (IWC)</td><td>2.343</td><td>0.51</td><td>2.52</td><td>2.24</td></tr><tr><td>Diffusion rate (k)</td><td>0.62</td><td>0.07</td><td>0.39</td><td>0.07</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 2.** Water absorption rate of the different aggregates used in this study.</figcaption></figure>

$$ \text{IWC (\%)} = \frac{m_w - m_D}{m_D} \times 100 \quad (3) $$

where, m<sub>w</sub> denotes the mass of wet aggregates after being immersed in water (kg).

m<sub>D</sub> denotes the initial mass of the dry aggregates (kg).

The data's presented in Fig. 2 have been segmented using linear regression lines to compare the different values of IWC and k for each bio-aggregate as reported in Table 2.

Rapeseed straws exhibited the highest initial water absorption, around 250 %, slightly exceeding that of hemp shives, which absorbed 234 % after just 1 min of immersion. Both hemp shives and rapeseed straws exhibited values consistent with those reported in the literature (Laidoudi et al., 2015). Notably, the rapid diffusion rate of hemp shives at 0.62 enabled it to retain the largest amount of water among the tested bio-aggregates, with a capacity of absorption of up to 4.5 times its own weight after 48 h. Rapeseed straws displayed a similar absorption tendency, closely following the absorption behavior of hemp shives. In contrast, reed showed a comparable initial water absorption to hemp shives and rapeseed at t = 1 min, owing to its high initial water content (IWC). However, its low diffusion rate of 0.07 limited further water uptake, resulting in absorption values that plateaued over time, reaching

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The different types of aggregates used in this study.</figcaption></figure>
250 % after 48 h. Bamboo, on the other hand, was the least absorbent of all bio-aggregates, with initial water absorption rate as low as 50 % and a diffusion rate of 0.073.

### 2.1.2.3. Morphological properties.
The measurement of the dimensions of bio-aggregates was performed through image analysis method using ImageJ software as recommended by RILEM TC 236-BBM, as it allows accurate quantification of morphological parameters through pixel-based analysis. ImageJ is an open-source image analysis tool widely recognized and utilized in materials research due to its robust capabilities in particle size and shape analysis (Igathinathane et al., 2008). It must be noted that particles were sieved at 500 µm in order to remove dust before undergoing image analysis. According to the RILEM recommendations, a sample of 3–6 g was taken for each type of bio-aggregates in order to include at least 2000 particles, thereby covering more than 100 pixels. Hence, two parameters were analyzed for each bio-aggregate, the length (corresponding to the major axis of the particles), and the width (corresponding to the minor axis of the particles) (Ratsimbazafy et al., 2021). Fig. 3 displays the cumulative distribution of the major and minor axis for each bio-aggregate. The aspect ratio corresponds to the ratio of length over width of each particle. The aspect ratio of bio-aggregates was computed within the same ImageJ environment by analyzing the binary images of the particles. The software calculates the aspect ratio as the ratio between the major and minor axes of the best-fit ellipse for each particle.

Among the studied bio-aggregate, it can be noticed that bamboo aggregates showed an equiaxed morphology (aspect ratio close to 1), with length and width nearly identical, and a median size ranging from 3 to 15 mm. However, rapeseed straws were the most elongated, with lengths reaching up to 27 mm. This finding is in line with the literature about the morphological characteristics of straws compared to other plant by-products (Laborel-Préneron et al., 2018). Hemp shives showed intermediate lengths ranging between 6 and 20 mm. For the minor axis (width), hemp shives, rapeseed straws, and bamboo chips displayed comparable distributions, with approximately 50 % of particles measuring below 3 mm. In contrast, reed aggregates demonstrated a markedly different profile: their length distribution overlapped with the width distributions of the other three bio-aggregates, indicating short particles with lengths not exceeding 9 mm. Moreover, reed aggregates were the finest as their width remained below 1 mm.

Hemp shives and rapeseed straws were characterized as elongated particles with aspect ratio of 2 and 2.5, respectively. As for reed

aggregates, they exhibited the highest aspect ratio of 7.14. This is consistent as higher specific surface area is obtained for fine aggregates.

## 2.2. Specimens' preparation

The formulation of bio-based specimens varies based on bio-aggregate type, where a total of four specimens were prepared for each bio-aggregate. Among them, two specimens were tested with bio-aggregates oriented perpendicularly to the loading direction, while the other two were tested with bio-aggregates oriented in parallel. Thus, for each bio-aggregate type, both orientations were represented with two replicates each. The mix proportions detailed in Table 3, were cast in 15 cm cubic molds and prepared identically, as described in previous study (Bardouh et al., 2023). The mix proportions for each bio-based concrete were primarily governed by the intrinsic physical properties of the bio-aggregates, with the main objective being to ensure standardized workability and comparable wet density across all configurations. In the case of bamboo aggregates, their markedly lower water retention capacity (about 50 % after 48 h, as shown in Fig. 2) and high bulk density (about 304 kg/m³, as shown in Table 2) significantly affected mix proportions. To compensate for this property and to achieve a consistency similar to that of other bio-based mixes, the bamboo concrete formulation was deliberately adjusted by reducing both the water and binder contents. Therefore, the ratio (A: B: W = 1: 0.6: 0.6) reflects an optimization step specific to bamboo, designed to normalize fresh-state behavior and maintain comparable densities across the different bio-based concretes. Similarly, in the case of reed mixes, the fine morphology of reed aggregates along with their low diffusion rate requires less proportional water to the formulation of reed concrete compared to that of hemp concrete. In this context, higher reed specific area with a median water absorption, requires higher binder content to achieve homogeneous proper coating and strong interfacial bond

<table><caption>**Table 3**
Formulations of the different bio-based concrete fabricated in this study.</caption><thead><tr><th colspan="5">Mass (g)</th></tr><tr><th>Aggregate materials</th><th>Aggregates (A)</th><th>Binder (B)</th><th>Water (W)</th><th>A: B: W ratio</th></tr></thead><tbody><tr><td>Hemp shives</td><td>400</td><td>800</td><td>800</td><td>1:2:2</td></tr><tr><td>Bamboo</td><td>1000</td><td>600</td><td>600</td><td>1:0.6:0.6</td></tr><tr><td>Rapeseed straws</td><td>400</td><td>800</td><td>800</td><td>1:2:2</td></tr><tr><td>Reed</td><td>600</td><td>900</td><td>900</td><td>1:1.5:1.5</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Morphological properties of the different aggregates used in this study.</figcaption></figure>
between the binder and reed aggregates (A: B: W = 1: 1.5: 1.5). Since hemp shives and rapeseed straws exhibited comparable physical properties, their formulations were kept identical (A: B: W = 1: 2: 2).

After demolding, specimens were stored in a conditioned room under 20 °C and 50 % RH for 12 months to track density variations over long term period. This parameter was stabilized starting from 28 days of storage.

During the fabrication, the bio-aggregates naturally orient themselves perpendicular to the compaction direction due to the applied pressure. This behavior has been widely reported in the literature for heterogeneous bio-based concrete (Huang et al., 2022; Lagouin et al., 2019). In this regard, the inclusion orientation was quantified using two angles Θ and φ. Since the orientation is mainly influenced by the compaction direction, angle Θ is the main indicator of whether the particles are distributed perpendicular (Fig. 4a) or parallel (Fig. 4b) to the compaction direction. The dotted lines on Fig. 4a & Fig. 4b represent the interfaces between layers during fabrication. Given the inherent heterogeneity of bio-based concretes, perfect alignment cannot be achieved; hence, θ was considered to vary within the range 40° < θ < 90°, corresponding to predominantly perpendicular orientation, and within a range 0° < θ < 40°, corresponding to parallel orientation. To quantify θ at the composite scale of the specimen, high resolution images of specimens' faces were processed in Fiji ImageJ software. Faces were imaged with the compaction axis aligned vertically (with respect to Z axis), calibrated at a 0.08 mm/pixel ratio, and inclusions (bio-aggregates) segmented by threshold and watershed (see Fig. 4c). Fitted ellipses were then obtained by analyzing particles with a reported angle α, representing the inclination of the particles relative to the X-axis of the image (image axis in Fig. 4c), where α = 0° indicates particles perfectly aligned horizontally within their major axis, and α = 90° indicates particles perfectly aligned vertically within their major axis. Now to link between angle θ (orientation angle with compaction Z axis) and angle α, this latter is transformed relative to compaction Z axis, by subtracting 90° from α. Therefore, θ = 90° - α. Taking the irregularities of particles into consideration, angle α might turn negative, in such case angle φ indicates to what extent each particle is stratified out of the compaction ZY plane (Huang et al., 2022), thereby angle α is normalized by adding 180°

(bi-directional). Therefore, θ = |90° - α|. By rotating the specimens 90° after demolding (case of parallel orientation), angle α increased as particles are aligned vertically to the horizontal image axis. In perpendicular orientation, average angle α was around 24°, meanwhile in parallel orientation average angle α was around 67°.

Dry density, measured after drying bio-based specimens for 48 h in an oven at 60 °C (Table 4) in order to prevent the saturation water from disturbing the mechanical properties measurement (Arnaud and Gourlay, 2012). Bamboo (425 kg/m³) and reed (448 kg/m³) concretes exhibited the highest green densities owing to their aggregates' high bulk densities of 304 kg/m³ and 145 kg/m³, respectively. However, reed concrete was denser than bamboo concrete at both fresh and dry states, attributed to reed's higher binder content and greater compressibility during casting, which reduced voids more effectively than bamboo. Hemp and rapeseed concretes demonstrated similar range of density at dry state (356–359 kg/m³) owing to same formulations and identical physical properties of their corresponding aggregates.

## 2.3. Test setup

### 2.3.1. Compression test

Bio-based concrete specimens were tested 12 months after their fabrication. The specimens were subjected to cyclic compression test using a 250 kN Zwick Roelle machine. However, the force applied by the machine was limited to 10 kN using a force sensor attached to the machine, and registering values at an acquisition frequency of 10 Hz. Zwick software was programmed to stop the cyclic compression test at a drop

<table><caption>**Table 4**
Fresh and dry densities of the different bio-based specimens.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Fresh state</th><th colspan="2">Dry state</th></tr><tr><th>Mean (kg/m³)</th><th>CV (%)</th><th>Mean (kg/m³)</th><th>CV (%)</th></tr></thead><tbody><tr><td>Hemp concrete</td><td>564</td><td>0.08</td><td>356</td><td>0.13</td></tr><tr><td>Bamboo concrete</td><td>621</td><td>0.24</td><td>425</td><td>0.72</td></tr><tr><td>Rapeseed concrete</td><td>575</td><td>0.17</td><td>359</td><td>0.08</td></tr><tr><td>Reed concrete</td><td>679</td><td>0.31</td><td>448</td><td>0.12</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Bio-aggregates orientation investigation, (a): perpendicular to the compaction direction, (b): parallel to the compaction direction, (c): Binarization of the specimens face to quantify θ</figcaption></figure>
of 80% from the ultimate load reached. The modified protocol adapted in this research was based on a previous cyclic compression protocol on hemp concrete (Niyigena et al., 2016). The modified protocol consists of 19 loading-unloading cycles, with a global strain increasing from 1% to 10% in 0.5% increments between each cycle. Loading and unloading rates were set at 3 mm/min and 10 mm/min, respectively. The compression test setup is presented in Fig. 5a. Wooden plates were used between the machine jaws and specimens to ensure smooth contact and minimize surface disturbances (Véronique, 2005).

### 2.3.2. Digital image correlation

2D digital image correlation was employed to analyze deformation on the tested surfaces of bio-based specimens. This non-contact technique measures material's deformation with high precision (Khoo et al., 2016). The setup of DIC is shown in Fig. 5a, where a PCO 2000 digital camera with a 2048 × 2048 pixels² CCD sensor, operating at 14 bits, equipped with a 105 mm lens, positioned to face specimen's center of gravity, and illuminated by LED lamps. Camware64 software recorded images at a frequency of 1 frame per second, synchronized with the Zwick machine's mechanical loading via an acquisition box connected to Catman software. The displacement and deformation fields between two images were calculated using SeptD software, based on the gray level distribution between two images at the relevant points (Cuynet et al., 2019). The criterion on which the correlation was performed consists of a normalized cross-section using the bilinear interpolation technique.

Correlation analysis was configured with a step size and subset size of 10 pixels each, corresponding to the grid pitch and pattern size, respectively (Fig. 5b). This setup ensures accurate point tracking between a reference and deformed image. The size of this relevant point consists of the intersection of the 4 nodes of the surrounding grid step size. In this regard, it is worth noting that larger subsets would result in over-smoothed displacement fields and longer computation times, while smaller subsets could not reliably capture the speckle dots of our pattern. In particular, the subset size was chosen to be slightly larger than the average speckle diameter to guarantee sufficient grayscale contrast within each subset, thereby ensuring accurate correlation while minimizing loss of spatial detail (Khoo et al., 2016; Huang, 2002). Consequently, 3 random subset sizes of 5 × 5, 10 × 10, and 15 × 15 were sampled from the specimens' images to begin with. The sum of squared subset intensity gradients (SSSIG) (Ueno et al., 2024) was determined according to equation (4),

$$SSSIG = \sum_{i=1}^{N} \sum_{j=1}^{N} \left( G_{x}(i, j)^{2} + G_{y}(i, j)^{2} \right) \quad (4)$$

where $G_x$ and $G_y$ represent the first-order derivatives of grayscale intensities, and calculated by central difference of neighboring points in x and y directions, respectively. N denotes the subset window. The mean SSSIG values, standard deviations, and mean per pixel are reported in Table 5. The high observed standard deviation can be explained by the heterogeneous aspect of bio-based materials, where some windows are strongly textured (high SSSIG), others at pores and interfacial transition zones are low. Although, all three subsets choices validate the algorithm of subset size optimal selection proposed by (Pan et al., 2008) for a SSSIG threshold error of $1 \times 10^5$, the 10 subset size contains the highest intensity gradient energy (mean per pixel) for reliable correlation while preserving spatial resolution. Therefore, a subset size and step size of 10 pixels was chosen to ensure an optimal balance between measurement precision and reliable pattern recognition.

To convert pixel measurements into millimeters, an expansion coefficient of 150 mm for 1762 pixels (0.08 mm/pixel) was applied.

Displacement resolution was assessed by determining the standard deviation derived from reference images of the specimen before any motion (expected to remain constant) and was equivalent to 0.021 pixels on a subset size of 10 pixels. Vertical displacement values below this standard deviation error were considered noises, and were excluded from analysis. The spatial resolution, defined as the shortest distance over which two independent measurement points can be distinguished, is governed by the step size. With a step size of 10 pixels and a physical scale of 0.08 mm/pixel, the spatial resolution was equivalent to 0.8 mm.

## 2.4. Mechanical properties computational methodology

This study assessed three mechanical properties: Compressive strength ($\sigma_c$), Young's modulus (E), and useable strength ($\sigma_u$), with computational methodologies illustrated in Fig. 6.

As no standard method currently exists for computing the compressive strength (Mazhoud et al., 2017; Walker et al., 2014), the authors have attributed this parameter to a strain rate of 7.5% as elaborated in previous studies (Lagouin et al., 2019; Nguyen et al., 2010; Tronet et al.,

<table><caption>Table 5
Quantification of the SSSIG in DIC.</caption><thead><tr><th>Subset size (pixel)</th><th>Mean SSSIG</th><th>Std SSSIG</th><th>Mean per-pixel (SSSIG/N<sup>2</sup>)</th></tr></thead><tbody><tr><td>5 × 5</td><td>139,430.0</td><td>25,877.2</td><td><strong>5577.2</strong></td></tr><tr><td>10 × 10</td><td>654,053.9</td><td>103,155.8</td><td><strong>6540.5</strong></td></tr><tr><td>15 × 15</td><td>1,373,126.7</td><td>369,803.0</td><td><strong>6102.8</strong></td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a): Compression test and DIC setup, (b): identification of subset size, step size, and speckle pattern.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6. (a):Measurement methods of mechanical properties, (b): second strategy adapted in modulus evaluation (Niyigena et al., 2019).</figcaption></figure>

2016).

Young's modulus (E) was calculated using the floating technique (Niyigena et al., 2019), measuring the maximum value at a strain difference of 0.1 % during the re-loading phase of each cycle as shown by the orange line in Fig. 6a. In this context (Niyigena et al., 2019), showed only 4 cycles with three computational methodologies (floating, cyclic, and tangent), where the initial modulus at the first cycle was considered apparent modulus. However, this study investigates deeper cyclic behavior, where 19 loading-unloading cycles were performed on bio-based concrete. There was no apparent modulus determination in this study since the modulus was calculated at the re-loading section, referring to Young's modulus. The cyclic methodology was excluded since it led to an underestimation of elastic modulus, as values reported after 180 days of curing were reduced by 30 % and 23 % compared to those determined by floating and tangent techniques, respectively (Niyigena et al., 2019). As for tangential and floating methods, both methods provided comparable results, with tangential modulus being a particular case of floating modulus producing higher variability with coefficient of variation of 16.31 % compared to 15.73 % for floating technique (Niyigena et al., 2019). Therefore, the methodology adapted for assessing the Young's modulus in this study was the floating technique, more particularly with second strategy (Fig. 6b) as slight steep slope can be obtained between end-points, leading to higher modulus values.

Useable strength ($\sigma_u$), a novel parameter represents the stress at the cycle where Young's modulus is at its maximum. This parameter, derived from the envelop curve superimposing monotonic and cyclic behaviors, evaluate the material's peak mechanical, linking strength to maximum rigidity at a given strain (Nguyen et al., 2010).

## 3. Results and discussion

### 3.1. Mechanical behavior

#### 3.1.1. Perpendicular oriented specimens

Fig. 7 displays the cyclic compressive behavior of bio-based concrete specimens with bio-aggregates oriented perpendicular to the compaction direction. The mechanical response comprises 2 main zones: A first quasi-elastic zone (strain up to 2 %), where the cement matrix absorbs the load, followed by a second stress hardening zone, characterized by gradual increase in stress with strain. This hardening process was so called densification in literature which prevents abrupt failure, and impose difficulties in identifying a criteria for computing the compressive strength (Elfordy et al., 2008; Walker et al., 2014; Youssef et al., 2015).

Among the tested specimens, bamboo concrete exhibited a very poor mechanical performance due to its unique formulation (A:B:W ratio of

<figure><img src="image_8.png" /><figcaption>Fig. 7. Mechanical behavior of the different bio-based concrete assessed in the perpendicular orientation of bio-aggregates to the compaction direction.</figcaption></figure>

1:0.6:0.6), resulting in low binder content and weak water retention capacity of bamboo aggregates (Fig. 2). This led to weak bamboo-CEMIV adhesion, resulting in early crushing of the corresponding specimen. However, hemp concrete demonstrated superior mechanical performance, attributed to strong interface bonding between hemp shives and CEMIV alongside structural skeleton resilience of hemp shives under mechanical loading. Rapeseed concrete exhibited lower strength than hemp despite similar formulations and water absorption, highlighting differences in bio-aggregate performance. CEMIV-Reed emerged as second outstanding configuration owing to fine morphology of reed aggregates along with higher specific surface area and aspect ratio promoting strong interfacial bonding with the CEMIV matrix, thereby enhancing matrix densification. Although increasing aspect ratio (reed aggregates) enhances the mechanical performance at long term, this improvement was limited taking into consideration other physical properties of bio-aggregates.

#### 3.1.2. Parallel oriented specimens

Fig. 8 illustrates the mechanical behavior of bio-based concrete with aggregates oriented parallel to the compaction direction.

The mechanical behavior is characterized by 4 zones: A first linear zone up to 1.5 % of strain where the matrix absorbs the mechanical loading; A second elastoplastic zone of strain hardening, with rapid stress increases up to 2.5 % of strain, extending to 4 % for rapeseed and reed concrete due to high compressibility and ductility, as noted in prior studies (Brouard et al., 2018); Stress peaks mark the end of this zone, transitioning into the third zone where stress progressively decreases
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Mechanical behavior of the different bio-based concrete assessed in the parallel orientation of bio-aggregates to the compaction direction.</figcaption></figure>

until a strain of 10 %, indicating bio-aggregate crushing and failure; Finally, a plateau zone follows, with stress remains almost constant as strain increases. According to (Akkaoui et al., 2017), this plateau effect is linked to the compression of the granular framework, leading to a rise in material's density and the closure of intergranular and potentially intragranular pores.

Among all bio-based concretes, hemp concrete displayed the highest mechanical efficiency, in contrast to bamboo concrete, which exhibited the poorest performance. These findings can be explained by the same considerations elaborated in the previous section. The inferior mechanical performance of parallel-oriented specimens compared to perpendicular ones is attributed to the limited load-bearing contribution of bio-aggregates in the parallel configuration (Nozahic et al., 2012). In contrast to what was obtained for perpendicular configuration, progressive failure began at 2.5 % of strain in parallel configuration. This can be explained by the weak ability of bio-aggregates to endure loading along their minor axis, causing early failure at the matrix-bio-aggregate interface.

According to (Nozahic et al., 2012) this can be due to two hypothesis: (1), bio-aggregates are crushed, leading to a significant deformation, and (2) the interface between bio-aggregates and binder is not perfect where bio-aggregates are surrounded by holes or powdered binder in the case of parallel-oriented specimens. Their first hypothesis can be related to the specific area of bio-aggregates in contact with the compression plates during testing. As a matter of fact, bio-aggregates endure the mechanical loading on their major axis (length), allowing a better flexibility of the particles to deform when oriented perpendicular to the compaction direction. In contrast, the parallel-oriented bio-aggregates are loaded along their minor axis (width), resulting in a stress concentration at the contact surface with the machine plates, thereby causing premature particle crushing at relatively low deformation levels.

Notably, a short relaxation was observed at peak stress during cycles starting at 2.5 % of strain (highlighted in red ellipse Fig. 8). Consequently, a faster speed can be noticed during the unloading phase regardless the speed of 10 mm/min programmed by the software. These 2 observations can be linked to high longitudinal stiffness of bio-aggregates while collapsing on a relatively smaller area, thereby creating stress concentration on the contact area with the plates. This in turn led to a faster upward return of the compression plates (unloading phase), since the condition of the unloading section was set according to a decrease of the applied force until 100 N. In this context, the stress increases at a faster rate in the first four cycles (up to 2.5 % of strain) in the case of parallel oriented specimens compare to those tested in perpendicular orientation. For instance, the stress at the peak of the 4th cycle for parallel oriented hemp concrete was of 0.35 MPa higher than the one reported for hemp concrete in perpendicular orientation which

was about 0.31 MPa. As a matter of fact, the collapse of parallel oriented bio-aggregates seemed to be more effective than perpendicular oriented aggregates during the elastic zone and the debut of the elastoplastic plateau.

## 3.2. Mechanical properties

### 3.2.1. Compressive strength

As explained in a previous section, the compressive strength was computed on a strain of 7.5 %. Fig. 9 shows the values of the compressive strength of the different bio-based specimens tested in the two aggregate orientations. The compressive strength of hemp concrete was in similar range to what was obtained in a previous study (Véronique, 2005). As described in the mechanical behavior of bio-based concrete, hemp concrete was expected to reach the highest compressive strength while this latter registered the lowest value for bamboo-concrete. In fact, the only exception that prevents a direct relationship between the bulk density of the specimens and their compressive strength was that of bamboo concrete. This exception is related to the unique shape of bamboo-aggregates besides the formulation of bamboo mixes. This latter was mainly described by the low binder content. As a matter of fact, lower compressive strength was reached for formulations with lower binder content (Arnaud and Gourlay, 2012; Murphy et al., 2010). This can be explained by the insufficient role of mineral binder to contribute to the mechanical response (Arnaud and Gourlay, 2012), thereby leading to weak interface bond between bio-aggregates and mineral binder.

In addition, reed concrete demonstrated relatively higher compressive strength compared to rapeseed and bamboo concretes. This evidence can be related to the finer shape of these bio-aggregates which provided a better coating with the mineral binder. Indeed (Chen et al., 2017), observed that the grinding process, intended to achieve finer aggregate, can modify the porous structure of miscanthus concrete, resulting in higher apparent density and improved mechanical performance.

Considering bio-aggregate orientation, it can be clearly seen that bio-based specimens with bio-aggregates oriented perpendicularly exhibited the highest compressive strength compared to parallel ones. For instance, the compressive strength of hemp concrete at 7.5 % of strain decreased by 133 % between perpendicular and parallel orientations. This tremendous decrease is in accordance with the mechanical behavior aspect of each configuration. In this regard, it is worth noting that perpendicular-oriented specimens exhibited quasi-pure uniaxial compression test whereas parallel-oriented specimens experienced buckling effect due to tensile collapse of the mineral binder. This is due to the geometry of the specimen when oriented parallel to the compaction direction (see Fig. 4b), where the interfaces between layers

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Compressive strength at a strain of 7.5 % of the bio-based specimens evaluated in the two bio-aggregates orientations.</figcaption></figure>
are in direct contact with the applied load.

### 3.2.2. Young's modulus

Fig. 10(a) and (b) shows the evolution of the rigidity for specimens tested in perpendicular and parallel orientation, respectively. Irrespective of the bio-aggregate's orientation, hemp concrete was the most robust type of bio-based concrete while bamboo was the most delicate. For instance, the maximum Young's modulus for parallel-oriented hemp concrete was about 62 MPa higher than 8 MPa which corresponds to the rigidity of the bamboo concrete at the same strain level (1.4 %). It is worth mentioning that the green density of bamboo concrete was 425 kg/m³ whereas that of hemp concrete was 356 kg/m³ as previously explored in Table 4. Indeed, no direct relationship can be drawn between Young's modulus and the bulk density of the specimens unlike what was found in the literature (Elfordy et al., 2008; Hirst et al., 2010; Mazhoud et al., 2017).

Nevertheless, a distinct behavior during the rigidification phase of rapeseed concrete tested in the perpendicular orientation can be noted where the Young's modulus continued to increase until advanced strain level at 8.4 %. The same trend in modulus obtained by (Niyigena et al., 2019) can be noticed in bio-based concrete with perpendicularly-oriented bio-aggregates in this study, where Young's modulus progressively increased but at a more pronounced strain rate, especially for Rapeseed concrete. It is worth noting that (Niyigena et al., 2019) applied cyclic compression with 1 % strain increments per cycle, whereas in this study the increments were set to 0.5 %. These short loading phases induced a higher number of cycles within the same strain

range, which in turn promoted progressive particle rearrangement and densification. As a result, the rigidification phase appeared more pronounced in the present study, reflecting enhanced consolidation of the bio-based concrete under cyclic compression.

The rapeseed concrete reached its maximum Young's modulus of 15.85 MPa at a strain rate of 8.4 %. This can be explained by the morphological characteristics of rapeseed straws compared to other aggregates, since these aggregates were lengthier, thereby leading to a high flexibility of deformation on their major axis. Similar findings were reported in the study of (Brouard et al., 2018), where rape straw concrete was described by higher ductility due to its continuous ascending mechanical behavior. Despite their high ductility, rapeseed concrete was still less rigid than reed concrete as the maximum Young's modulus of reed concrete was always higher than rapeseed concrete regardless of the bio-aggregate's orientation. The authors can attribute this evidence to two hypotheses: (1) the high bulk density of the reed concrete compared to rapeseed concrete (see Table 4), and (2) the smaller and finer shape of reed aggregates compared to rapeseed aggregates (see Fig. 3). The second hypothesis is in line with what was found in previous studies where finer aggregates exhibited higher mechanical properties when employed in bio-based concrete (Benmahiddine et al., 2020b; Niyigena et al., 2018).

Drawing from Fig. 10 (a), it can be noticed that the Young's modulus tends to increase at relatively higher strain rate compared to parallel-oriented specimens. For instance, the highest Young's modulus for hemp concrete tested perpendicularly was recorded at a strain of 2.8 % whereas the ultimate modulus in the case of parallel-oriented hemp

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Evolution of the Young's modulus (E) and the damage coefficient (D) of the different bio-based concrete in: (a): Perpendicular orientation, (b): Parallel orientation.</figcaption></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
concrete was limited at 1.8 % as seen in Fig. 10 (b). However, parallel-oriented specimens were more rigid than perpendicular-oriented specimens. For instance, the maximum modulus of hemp concrete was around 62 MPa for parallel-oriented configuration higher than that of perpendicular-oriented specimen which was around 39 MPa. In this regard (Niyigena et al., 2019), reported maximum floating modulus in perpendicular orientation of 76.47 MPa at 2 % of strain higher than 39 MPa in present study. This can be largely attributed to mix design where (Niyigena et al., 2019) employed a lower bio-aggregate-to-binder ratio (0.38) compared to 0.5 in this study, resulting in a higher binder content and consequently a stiffer composite.

According to (Nozahic et al., 2012), the higher rigidity observed in the case of parallel-oriented specimens is due to the stratum parallel direction of particles under compression test, where these particles are met in the longitudinal direction, leading to a more robustness of the bio-based concrete. The same authors mentioned a similarity in observations between bio-aggregates and wood. As a matter of fact (Akkaoui et al., 2017), found that the elastic modulus of spruce wood in the longitudinal direction (parallel orientation in this study) is 13,700 MPa higher than 1090 MPa which corresponds to the elastic modulus of spruce wood in radial direction (perpendicular orientation in this study). The maximum Young's modulus of hemp concrete obtained in this study (62 MPa) was still markedly lower than the modulus of wood concrete (409.7 MPa) reported by (Akkaoui et al., 2017) for a cement/wood ratio of 1.75 after 104 days of curing. This discrepancy can first be attributed to the much higher density of wood concrete (14,870 kg/m³) and the inherent physical properties of wood, which has a bulk density of 230 kg/m³, compared to the significantly lighter bio-aggregates except bamboo used in this study (Table 2).

Furthermore, the higher rigidity reported by (Akkaoui et al., 2017) can also be explained by the higher loading rate applied during cyclic compression tests (10 mm/min versus 3 mm/min in this study). A higher loading rate reduces the time available for deformation mechanisms—such as pore water redistribution, which are particularly pronounced in porous bio-based concretes. As a result, the material responds more elastically under fast loading, leading to an apparent increase in stiffness.

In a parallel study (Godchaux et al., 2025), applied the same cyclic compression protocol followed in this study for wood concrete in parallel orientation. They found an average elastic modulus of 543 MPa for an average density of 773 kg/m³. Interestingly, the authors linked the higher elastic modulus in cyclic loading compared to monotonic loading to the fact that slope of unloading curve of cycle n-1 was lower than loading curve of cycle n, leading to a progressive evolution of the material's stiffness over the cycles.

Fig. 10 (a) and (b) also display the damage phase of the different bio-based concrete tested in perpendicular and parallel orientations of bio-aggregates, respectively. Drawing from the elastic damage model of Mazars for ordinary concrete (Mazars and Pijaudier-Cabot, 1989), the authors have predicted a damage model of bio-based concrete, where the damaged modulus be described as noted in equation (4), and damage coefficient in equation (5).

In this study, the Mazars model was not applied in its full complexity but rather adapted as a phenomenological tool to quantify rigidity degradation. Specifically, the scalar damage coefficient D was defined through the reduction of the Young's modulus:

$$E_{end} = E_{0}(1 - D) \qquad (4)$$

$$D = 1 - \frac{E_{end}}{E_{0}} \qquad (5)$$

### 3.2.2.1. Model parameters.
E<sub>end</sub>: The damaged modulus, determined at the reloading segment of each cycle at strain difference of 0.1 % after residual strain had accumulated.

E<sub>0</sub>: The initial (undamaged) modulus, here defined as the maximum

reloading rigidity measured just before the onset of damage phase.

D: The damage coefficient, where D = 0 denotes the undamaged state, and D = 1 represents complete rigidity loss (ultimate crushing). This scalar representation allows tracking the progressive rigidity loss, even in highly porous, ductile composites.

#### 3.2.2.2. Basic assumptions.

* Damage in bio-based concretes is not driven by brittle crack propagation, but by progressive accumulation of residual strain due to pore collapse, matrix-bio-aggregate debonding, and microcracking.

* The material response is considered isotropic at the composite scale, even though the inclusions are anisotropic. The model therefore evaluates a global rigidity degradation rather than local heterogeneity.

* The evolution of damage is assumed to be continuous and monotonic with increasing cycles (no rigidity recovery).

* The rigidity reduction is assumed to depend only on residual strain, not directly on stress level, which is consistent with the cyclic compression protocol.

With these adaptations, the Mazars model to bio-based concrete does not assume brittle failure, but rather captures the gradual modulus decay associated with ductile, highly porous composites.

The values of E<sub>0</sub> for the different bio-based specimens tested in two orientations of bio-aggregates are reported in Table 6. A direct relationship can be found between the mechanical strength and the maximum modulus before the damage phase.

From a general perspective, hemp concrete was the most damaged type (describe by loss of inherent rigidity) of bio-based concrete evaluated in this study as noted in Fig. 10 (a) and (b). This is consistent when taking into consideration the difference between the maximum and minimum mechanical stress of hemp concrete. For instance, the stress of hemp concrete tested in parallel orientation decreased from 0.36 MPa to 0.18 MPa between a strain of 2.8 % and 10 %, respectively. Rapeseed concrete was the least damaged type of bio-based concrete in perpendicular orientation thanks to its rapeseed straw's high flexibility which maintains the mechanical load until an advanced strain level as shown in Fig. 10 (a). Despite the common damaging behavior between reed and bamboo concrete in perpendicular orientation up to 8 % of strain, bamboo concrete was more damaged by a factor of 1.5 than reed concrete at 10 % of strain. This evidence can be explained by the smaller and finer shape of reed aggregates which remained less damaged compared to bamboo aggregates. However, bamboo concrete was the least damaged type of bio-based concrete tested in the parallel orientation of aggregates as shown in Fig. 10 (b). This finding is coherent with the relationship between the damage behavior and the accumulated strain as previously explained. As a matter of fact, bamboo aggregates in their parallel orientation did not significantly contribute to the mechanical response of bamboo concrete as shown in Fig. 8, thereby bamboo concrete remained less damaged compared to other types of bio-based concrete.

It can be noticed that bio-based concrete specimens tested in parallel configurations were more damaged than those tested in perpendicular configurations. This can be mainly explained by two reasons: firstly, the stress concentration induced on a smaller surface area of aggregates in parallel orientation, and secondly the failure mode of bio-based concrete in parallel orientation of aggregates which will be investigated in details in section 3.3.3.

### 3.2.3. Useable strength

Fig. 11 shows the values of the useable strength (σ<sub>u</sub>) among different
<table><caption>**Table 6**
Maximum modulus for the different bio-based concrete assessed in both orientations of aggregates.</caption><thead><tr><th rowspan="2">Specimens</th><th colspan="4">Perpendicular orientation</th><th colspan="4">Parallel orientation</th></tr><tr><th>CEMIV-hemp</th><th>CEMIV-bamboo</th><th>CEMIV-rapeseed</th><th>CEMIV-reed</th><th>CEMIV-hemp</th><th>CEMIV-bamboo</th><th>CEMIV-rapeseed</th><th>CEMIV-reed</th></tr></thead><tbody><tr><td>E<sub>0</sub> (MPa)</td><td>38.57</td><td>6.17</td><td>15.84</td><td>20.03</td><td>61.98</td><td>6.31</td><td>17.81</td><td>28.42</td></tr></tbody></table>

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Useable strength of the different bio-based concrete as function of strain rate in the two bio-aggregates orientations.</figcaption></figure>

bio-based specimens tested in the two orientations of bio-aggregates. The useable strength measurement was quite helpful since it gives the strength of the material at its 100 % mechanical performance. For example, the compressive strength at 7.5 % of strain for perpendicular-oriented specimens is higher than their useable strength except for the rapeseed concrete specimens. However, this increase in strength does not serve as a reliable result since the material was almost crashed at 7.5 % of strain. Similar investigations were reported by (Ratsimbazafy et al., 2021) who assumed that the rise in compressive stress was not an effective measure for evaluating bio-based composites, as it was coupled with a corresponding increase in strain. Since the methodology of assessing this parameter is related to the maximum Young's modulus (E₀), the highest useable strength was that of hemp concrete. In this regard, rapeseed concrete demonstrated a relatively higher useable strength in perpendicular orientation, due to its high ductility in deforming as can be seen in Fig. 10 (a) where its corresponding useable strength of 0.24 MPa was computed at a strain of 8.3 %.

Generally, it can be remarked that the useable strength was reached at a higher strain rate for perpendicular orientation than parallel one. For instance, the useable strength for bio-based specimens tested in parallel orientation was never reached beyond a strain of 2 %. However, perpendicular-oriented specimens reached their maximum mechanical performance at a strain rate ranging between 2.8 and 4.4 % depending on the type of bio-aggregate used. This strain can even increase up to 8.3 % for rapeseed straws. This evidence can be explained by the same consideration regarding the effect of bio-aggregate orientations on Young's modulus. Furthermore, the useable strength of parallel-oriented specimens is much lower than that of perpendicular-oriented specimens except for bamboo concrete. This evidence is due to the same considerations regarding the quasi-pure uniaxial compression test (perpendicular orientation) and buckling effect (parallel orientation), which were already elaborated for compressive strength (see section 3.2.1). In this regard, useable strength in parallel orientation refers to the bio-aggregate/binder affinity rather than the inherent strength of the bio-based composite. Indeed, the coating strength (case of parallel orientation) between hemp shives and CEMIV is higher than other bio-based specimens which denotes a strong hemp shives/CEMIV affinity. However, the exception of the above-mentioned evidence (case of bamboo

concrete) exhibited the lowest coating strength in the order of 0.04 MPa, thereby resulting in the weakest particle/binder affinity among other configurations. This finding can be explained by the distinct formulation of bamboo-concrete (see Table 3). Indeed, the coating strength of bamboo concrete was very close in range to its useable strength under a quasi-pure uniaxial compression test (case of perpendicular orientation). Hence, this finding gives a spotlight on the very weak bamboo particle package optimization in contributing to the overall specimen's mechanical strength since its actual mechanical strength is very close to its coating strength. In addition, the particle/binder affinity of relatively smaller and thinner aggregates (case of reed concrete) was found to be stronger than lengthier particles (case of rapeseed concrete).

### 3.3. Evaluation of the mechanical deformation of bio-based concrete by DIC

The section aims to investigate the effect of bio-aggregate type and orientation on the displacement and deformation fields of bio-based concrete. In addition, the coupling between life cycle fatigue analysis along with crack patterns and damage behavior provided an advanced understanding of the mechanical response of these materials under cyclic compression loading. In this section, DIC fields of bio-based composites are assessed according to the bio-aggregate type and orientation during zones of maximum rigidity.

The vertical displacement maps of the bio-based specimens that were evaluated at maximum rigidity consist of a strain difference of 0.1 %, similar to that used for computing the modulus of elasticity (see section 3.2.2). In this regard, it must be noted that the maximum rigidity of perpendicular-oriented specimens of hemp concrete, rapeseed concrete, reed concrete, and bamboo concrete was reached at a strain of 2.9 %, 8.35 %, 4.4 %, and 4.4 %, respectively. Consequently, the maximum rigidity of parallel-oriented specimens was reached at a strain of 1.48 % among the different bio-based concrete except that of reed concrete where the maximum rigidity occurred at 1.9 % of strain.

Owing to the mechanical tendency of bio-based concrete in Figs. 7 and 8, the displacement maps of bio-based concrete at maximum rigidity in perpendicular orientation were computed between a stress range of 0.17 MPa–0.21 MPa (hemp concrete), 0.17 MPa–0.19 MPa (rapeseed concrete), 0.15 MPa–0.17 MPa (reed concrete), and 0.025 MPa–0.03 MPa (bamboo concrete). Likewise, the same displacement maps at maximum rigidity in parallel orientation were assessed within stress levels of 0.21 MPa–0.27 MPa (hemp concrete), 0.05 MPa–0.07 MPa (rapeseed concrete), 0.08 MPa–0.11 MPa (reed concrete), and 0.012 MPa–0.019 MPa (bamboo concrete).

#### 3.3.1. Effect of aggregate type and orientation on the vertical displacement of bio-based specimens at maximum rigidity

Fig. 12 illustrates the vertical displacement maps of bio-based concrete at their maximum rigidity for perpendicularly-oriented specimens. From a general aspect, it can be seen that a homogeneous distribution of the vertical displacement where usually 3 different zones of concentration govern the displacement of the material. First of all, the upper portion of the bio-based specimens is the most displaced zone (blue color) since it consists of a stress concentration zone in contact with the machine jaws (Bardouh et al., 2023). The second zone located in the central portion of the specimen is characterized by less displacement than the upper portion (green color). This is consistent with the absorbed energy of the bio-aggregates as the bio-based specimens were
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Vertical displacement maps of bio-based concrete tested in perpendicular orientation of bio-aggregates at maximum rigidity, (a): hemp concrete (at 2.9 % of strain), (b): rapeseed concrete (at 8.35 % of strain), (c): reed concrete (at 4.4 % of strain), (d): bamboo concrete (at 4.4 % of strain).</figcaption></figure>

displaced downward. This was followed by a third zone at the bottom of the specimen (yellow color) where the specimens were the least displaced. The three aforementioned zones correspond to the 3 zones between layers during the fabrication process. However, rapeseed concrete (Fig. 12b) demonstrated a relatively more pronounced concentration zone in the third zone (orange-red colors). This evidence can be explained by the relatively higher strain rate (8.35 %) at which the vertical displacement map was assessed for rapeseed concrete. In addition, bamboo concrete (Fig. 12d) exhibited an almost rigid zone which was very low displaced at the right bottom corner of the specimen. This finding can be attributed to the very low-stress difference of 0.005 MPa relative to a strain difference of 0.1 % used to generate the displacement map of bamboo concrete, resulting in less motion on the bottom part of the specimen due to the weak load applied. As a matter of fact, a direct relationship can be drawn between the increase of the applied load (stress difference used to generate the displacement maps) and the high vertical displacement (described by the degree of color as shown in Fig. 12).

On the other hand, Fig. 13 represents the vertical displacement maps of bio-based concrete at their maximum rigidity for parallel-oriented specimens. A more uniform distribution of the vertical displacement can be noticed in the maps of hemp concrete (Fig. 13a) and rapeseed

concrete (Fig. 13b) compared to reed concrete (Fig. 13c) and bamboo concrete (Fig. 13d). This evidence can be explained by the similarity in shape of elongated particles of hemp shives and rapeseed straws. However, reed and bamboo concrete exhibited almost a similar response of vertical displacement between the two orientations of bio-aggregates with more pronounced concentration zones at the bottom of the specimens for parallel-oriented configurations. This finding can be attributed to an aspect ratio equivalent to 1 for bamboo aggregates where the major and minor axis are almost constant. As for reed concrete, the strong affinity between finer reed and CEMIV induce a homogeneous distribution along the composite material, mitigating any disturbance coming from bio-aggregate orientation. Indeed, the degree of flexibility of bio-aggregates in parallel orientation is limited to some extent as particles can slip due to stress concentration.

### 3.3.2. Effect of aggregate orientation and type on the mean values of vertical displacement at maximum rigidity

For a better interpretation of the effect of bio-aggregate type and orientation on the vertical displacement of bio-based concrete, this section addresses the mean values of the above-mentioned displacement maps at maximum rigidity. In this context, Fig. 14 shows the mean values of the vertical displacement fields by line in perpendicular-
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Vertical displacement maps of bio-based concrete tested in parallel orientation of bio-aggregates at maximum rigidity, (a): hemp concrete (at 1.48 % of strain), (b): rapeseed concrete (at 1.48 % of strain), (c): reed concrete (at 1.9 % of strain), (d): bamboo concrete (at 1.48 % of strain).</figcaption></figure>

oriented bio-based concrete at maximum rigidity. The schematical representation of the specimen can clarify the computational methodology of the mean values of vertical displacement. In fact, the dotted blue lines represent vertical displacement vectors through the entire geometry of the specimen. Thereafter, the mean values plotted in Fig. 14 refer to the average value across each blue dotted line. Consequently, each blue dotted line can be attributed to a specific location among the vertical axis as seen in the schematical representation. The vertical axis was defined between 0 and 150 mm where 0 and 150 represent the terminals at the bottom and the top of the specimen, respectively. Hence, the mean values of vertical displacement were distributed in ascending order, starting from the bottom of the specimen (0 mm at vertical axis) to the top of the specimen (150 mm at vertical axis). It must be noted that the negative sign is due to the downward displacement of the material. Hemp concrete was the most robust type of bio-based concrete in perpendicular orientation of bio-aggregates whilst bamboo concrete was the least rigid type of bio-based concrete. Indeed, it can be noticed that hemp concrete in the perpendicular orientation was the least displaced vertically (Fig. 14a) whereas bamboo concrete was the most displaced vertically (Fig. 14d). Inferring from these findings, a direct relationship can be drawn between the high rigidity of bio-based specimens and the low vertical displacement. This relationship is

consistent with the fact that a rigid material is less deformable than a soft one. In the same regard, it can be seen that all bio-based concrete tested in the parallel orientation underwent less vertical displacement than those tested in perpendicular orientation at their maximum rigidity. For instance, hemp concrete reached a maximum vertical displacement of 0.08 mm less than 0.18 mm which corresponds to hemp concrete's maximum vertical displacement in perpendicular orientation. However, it must be noted that the displacement fields in parallel-oriented configurations were computed at a strain rate of 1.48 % and 1.9 % whereas those of perpendicular-oriented were determined at relatively higher strain rates of 2.9 %, 4.4 %, and 8.35 % (see section 3.3). In addition, the above-mentioned relationship can be also validated in the case of parallel-oriented configurations (see Fig. 15).

### 3.3.3. Effect of bio-aggregate orientation on the vertical deformation of hemp concrete

This section investigates the effect of the bio-aggregate orientation of a particular bio-based specimen (hemp concrete) on the vertical deformation maps of the corresponding specimens. The vertical deformation maps in both configurations were computed between a strain of 0 % and 5.24 %.

Fig. 16(a) and (b) illustrate the vertical deformation maps of hemp
<figure><img src="image_18.png" /><figcaption>Fig. 14. Mean values of vertical displacement by line for bio-based concrete tested in perpendicular orientation of bio-aggregates at maximum rigidity, (a): hemp concrete (2.9 % of strain), (b): rapeseed concrete (8.35 % of strain), (c): reed concrete (4.4 % of strain), (d): bamboo concrete (4.4 % of strain).</figcaption></figure>

concrete for perpendicular and parallel-oriented specimens, respectively. From a general perspective, it can be clearly seen that bio-based concrete exhibits anisotropic behavior contrary to what was considered by (Arnaud et al., 2013) who assumed an isotropic behavior for hemp concrete. Furthermore, heterogeneous approach regarding the mechanical behavior of these materials can be noticed in their vertical deformation. Concerning the vertical deformation map for perpendicularly oriented specimens, a propagation of 2 strain concentration lines (blues lines) can be remarked (Fig. 16a). In fact, these concentration lines correspond to the interface zones between layers during casting since the specimens were filled by 3 different layers. Therefore, the casting process is extremely important in optimizing the mechanical properties of bio-based concrete. Failure mode is governed by interfacial delamination and shear-band formation along casting interfaces. Cyclic loading induces microcrack initiation at weak interlayers, followed by coalescence into through-thickness cracks that promote splitting and loss of structural integrity.

As for parallel-oriented specimens (Fig. 16b), 2 strain concentration

lines can be observed which represent the direct damage of the defect lines (interface between casting layers) under mechanical loading. As a matter of fact, the tensile collapse of the mineral binder in parallel-oriented specimens led to direct collapse of these defect lines which induced a very weak bond interphase between bio-aggregates and mineral binder. This phenomenon is described by the buckling effect around these concentration lines. Indeed, these lines are responsible for splitting the specimen into two detached parts at a relatively higher strain rate. This evidence can explain the relatively higher damage coefficient for parallel-oriented specimens compared to perpendicular-oriented specimens. Similar observations were reported by (Youssef et al., 2015) who obtained a buckling collapse of hemp concrete for configurations in which aggregates were parallel to the compaction direction.

### 3.3.4. Crack and life cycle fatigue analysis

Since the assumptions of Paris law on fracture mechanics are not compatible for heterogeneous bio-based concrete (Renju and Keerthy,
<figure><img src="image_19.png" /><figcaption>Fig. 15. Mean values of vertical displacement by line for bio-based concrete tested in parallel orientation of bio-aggregates at maximum rigidity, (a): hemp concrete (1.48 % of strain), (b): rapeseed concrete (1.48 % of strain), (c): reed concrete (1.9 % of strain), (d): bamboo concrete (1.48 % of strain).</figcaption></figure>

2020), which is characterized by highly porous microstructure, and distributed damage mechanism. Under such conditions, crack initiation and growth occur simultaneously across multiple localized zones rather than a single crack front, making the application of Paris law physically unstable resulting in underestimation of cracking mechanism.

In this context, DIC approach was adopted to quantify damage evolution during cyclic compression. For each of the 19 loading-unloading cycles, vertical strain fields (eps<sub>yy</sub>), obtained from DIC during the re-loading phase were processed in MATLAB to detect regions of high tensile strain corresponding to crack initiation and propagation. Pixels exceeding a critical strain threshold (0.02) were segmented, and structure filtering was applied to remove noise and merge connected crack zones. The crack density was defined as the total number of independent crack regions per cycle, while the crack length was computed as the cumulative perimeter of these regions.

Furthermore, the crack opening displacement (COD) was evaluated directly from the vertical displacement field (U<sub>y</sub>), by computing the average displacement discontinuity across cracks. The mean COD per cycle was then obtained as the average of all individual crack openings detected. The processed DIC images consist of the onset and the end of

each-reloading phase per cycle, matching zones where damage model (based on rigidity loss) was computed.

3.3.4.1. *Perpendicular orientation of aggregates.* Fig. 17 shows the crack density of bio-based concretes across cycles when assessed in perpendicular orientation of bio-aggregates. Hemp and reed concretes exhibited the same trend in cracks initiation with cracks being delayed to appear until 8th cycle for reed concrete compared to 5th cycle for hemp concrete. This finding highlights the effective synergy between fine reed aggregates and CEMIV paste in creating strong interphase bond. 6 and 5 cracks were reported at 19th cycle for hemp and reed concretes, respectively. As for rapeseed concrete, it can be seen that crack initiation were delayed until 10th cycle, with lowest crack density of 4 at the end of loading-unloading cycles. This is in line with the pronounced rigidityation phase and the low damage coefficient reported for rapeseed concrete in perpendicular orientation (see Fig. 10a). Interestingly, bamboo concrete which is the poorest mechanical performant exhibited the highest crack density up to 32 at 18th cycle. The short decrease in crack density of bamboo concrete between 18th cycle and 19th cycle can be explained by the fact that some small crack merged into larger ones
<figure><img src="image_20.png" /><figcaption>Fig. 16. Vertical strain maps of hemp concrete during damage at a 5.24 % of strain, (a): perpendicular orientation of bio-aggregates, (b): parallel orientation of bio-aggregates.</figcaption></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>Fig. 17. Crack density across cycles for bio-based concretes assessed in perpendicular orientation of bio-aggregates.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>Fig. 18. Total cracks length across cycles for bio-based concretes assessed in perpendicular orientation of bio-aggregates.</figcaption></figure>

(increased crack opening), resulting in reducing count. The high crack density in bamboo concrete can be explained by two evidences: the relatively high damage coefficient (D = 0.3 at 10 % of strain), and the poor mechanical properties of bamboo concrete.

Fig. 18 illustrates the crack length growth across cycles of the different bio-based concretes assessed in perpendicular orientation. The equation boxes represent the slope of crack length propagation per cycle determined by linear regression analysis dL/dN as an analogue to crack advance per cycle da/dN in Paris law descriptions (Pugno et al., 2006). Hemp, reed, and rapeseed mixes exhibited slow, and steady crack growth (average rates around 0.18–0.23 mm per cycle), consistent with a prolonged initiation stable phase, while crack length was limited to 4.5 mm. The high flexibility of long rapeseed straws reduced the crack length to 2.5 mm at 10 % of strain (19th cycle). In this regard, it is worth noting that the damage behavior illustrated in Fig. 10a represents more precisely the rigidity degradation than the actual damage of bio-based concrete. Indeed, the high damage coefficient of hemp concrete is directly related to the very high maximum rigidity (E₀) of this latter. Nevertheless, the low dL/dN for hemp concrete indicates better resistance to crack propagation under cyclic compression, owing to the

effectiveness of hemp shives in bridging cracks. Same findings can be reported for reed and rapeseed concretes. In contrast, bamboo concrete demonstrated a pronounced transition of crack propagation (average dL/dN of 3.66), where cracks initiation stage occurred at cycle 7, moving to steady stage of propagation (between cycle 9 to 12), ending in an accelerated stage around cycle 12–13 (6.5–7 % of strain). This latter phase of bamboo concrete showed an even more pronounced growth crack length rate starting from 16th cycle, in line with the shift in Damage coefficient at 8 % of strain (see Fig. 10a). This finding suggests that weak interphase bond between bamboo aggregates and CEMIV paste resulted in the most damaged state up to a total crack length of 44.28 mm. In this regard, the lower damage coefficient of bamboo concrete compared to hemp concrete is attributed to the very low maximum modulus of bamboo concrete, making rigidity degradation negligible compared to hemp concrete. Therefore, regardless, hemp concrete entering a damage phase described by a more pronounced drop in its inherent mechanical properties, hemp concrete remains less damaged in terms of fracture mechanics compared to bamboo concrete whose cracks initiation and propagation were the highest (32) and lengthiest (44.28 mm).
Fig. 19 shows the cracks opening across cycles of bio-based concretes in perpendicular orientation of bio-aggregates. Overall, cracks were continuously getting wider as accumulated strain per cycle increased. Bamboo concrete exhibited the widest cracks starting from 2.04 mm up to 4.41 mm. This is in line with the fast-growing crack rate and weak interfacial bond, resulting in a more localized strain around crack zones. Hemp concrete showed an intermediate crack opening up to 1.82 mm, suggesting effective energy dissipation of hemp shives by bridging micro-cracks and allowing better distribution of stresses. The flexible elongated morphology of rapeseed straws reduces even more the crack opening up to 1.34 mm. The narrowest cracks opening was reported in reed concrete (0.92 mm), highlighting the strong bonding between fine reed aggregates and CEMIV paste and a better uniform stress distribution. This latter is in line with the vertical displacement fields where reed concrete showed almost same response regardless of orientation variation (see Fig. 12c & Fig. 13c).

### 3.3.4.2. Parallel orientation of aggregates.
Fig. 20 shows the crack density across cycles in parallel orientation of bio-aggregates. From a general perspective, high cracks density is observed in parallel orientation except for bamboo concrete. This exception can be explained by the morphological equiaxed shape of bamboo aggregates making orientation effect insignificant, and to relatively better contribution of bamboo aggregates to the mechanical behavior in perpendicular orientation, thereby generating more cracks. Rapeseed concrete generated the highest crack density with a continuous increase up to 25 cracks. Bamboo concrete came second with 22 cracks at 19th cycle. A similar trend of crack density evolution can be remarked in hemp and reed concretes, where both reached ultimate crack density of 19 cracks at 16th cycle, followed by continuous decrease to 17 and 15 cracks at 19th cycle, respectively. This can be explained by the same consideration elaborated for bamboo concrete in perpendicular orientation, where nearby small cracks getting wider resulting in a merged crack of wider opening. Interestingly, no new cracks were reported in hemp concrete between cycles 5 and 9, suggesting a more uniform steady propagation stage of cracks. Same findings can be seen for reed concrete between cycles 11 to 12, and 13 to 14.

Fig. 21 displays the crack length growth of bio-based concretes across cycles in parallel orientation of bio-aggregates. Overall, higher growth rates can be remarked in parallel orientation compared to perpendicular one, with the sole exception of bamboo concrete. This is directly related to high damaged bio-based concretes in parallel orientation along with the low vertical displacement, resulting in pronounced crack propagation.

The initiation phase occurred at early strain rate starting from cycle 2 for rapeseed and hemp concretes. The initiation phase was partially delayed for bamboo concrete until 3rd cycle, and for reed concrete until 5th cycle. Rapeseed concrete exhibited the shortest steady propagation

<figure><img src="image_24.png" /><figcaption>Fig. 19. Cracks opening across cycles for bio-based concretes assessed in perpendicular orientation of bio-aggregates.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Fig. 20. Crack density across cycles for bio-based concretes assessed in parallel orientation of bio-aggregates.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Fig. 21. Total cracks length across cycles for bio-based concretes assessed in parallel orientation of bio-aggregates.</figcaption></figure>

phase, where crack length rapidly increased starting from 5th cycle. This was then followed by a progressive increase in cracks length per cycle (dL/dN = 0.92) up to 17.5 mm at 19th cycle. This high variability in life cycle fatigue for rapeseed concrete between both orientations, suggest the poor effectiveness of rapeseed straws when aligned parallel to compaction due to potential slippage. Hemp, reed, and bamboo mixes exhibited almost same tendency in second stage of steady propagation, before bamboo abruptly increased at cycle 10, marking again the highest crack length growth rate (dL/dN = 1.29) among other mixes. Hemp and reed concretes showed the transition into accelerated crack growth stage at nearby cycles 11 and 12, respectively. Nevertheless, reed concrete increased at a faster rate (dL/dN = 0.92) reaching 14.69 mm at 19th cycle compared to 12.78 mm for hemp concrete (dL/dN = 0.87).

In fact, an inversely proportional relationship can be drawn between useable strength (Fig. 11) and crack length growth in parallel orientation, where high useable strength resulted in small crack length, and vice-versa. This finding is in line with the definition of useable strength in parallel orientation, where it refers to the coating affinity between aggregates and CEMIV paste. Therefore, the better is the coating strength, the short is the crack length growth.

Fig. 22 shows the cracks opening of different bio-based mixes across cycles in parallel orientation of bio-aggregates. Likewise, bamboo concrete exhibited the highest crack opening up to 6.49 mm. Contrary to its cracks opening in perpendicular orientation, rapeseed concrete showed higher crack opening where this latter increased abruptly at cycle 17 reaching 4.19 mm at the end of cyclic loading. This finding is proportional to the increase in crack length of rapeseed concrete in parallel orientation, highlighting more localized strain around long rapeseed straws when loading is subjected on their width instead of their length.
<figure><img src="image_27.png" /><figcaption>**Fig. 22.** Cracks opening across cycles for bio-based concretes assessed in parallel orientation of bio-aggregates.</figcaption></figure>

Another reason is that load is directly applied on defect lines (interfaces between layers), increasing stress concentration, thereby widening initiated cracks. Hemp and reed concretes showed almost the same evolution in crack opening across cycles, with reed mixes underscoring again the narrowest crack openings (2.88 mm) whereas hemp concrete reached 3.34 mm.

## 4. Conclusion

Drawing from the coupling between mechanical testing and DIC-based image analysis, this study significantly advances our understanding of the impact of bio-aggregate type and orientation on the mechanical properties of bio-based concrete. As such, the following points can be concluded:

* **Bio-based concrete performance:** Hemp concrete exhibited the best overall mechanical performance, combining high rigidity (E = 62 MPa) and moderate deformability, while bamboo concrete performed the weakest despite its higher density, mainly due to its poor interfacial bond and specific mix design

* **Aggregate size and binder affinity:** Finer aggregates, such as reed particles, improved particle-binder adhesion, enhancing compressive strength and mitigating localized strain. Reed concrete ranked second in mechanical performance after hemp concrete, confirming the advantage of refined particle grinding.

* **Effect of orientation:** The perpendicular orientation of aggregates contributed to package optimization, leading to superior mechanical strength. In parallel orientations, useable strength refers to the coating strength between mineral binder and aggregates rather than the actual material strength.

* **Rigidity and deformability:** Perpendicular-oriented specimens endured rigidification up to 4.5 % strain, while parallel ones rigidified sharply below 2 % strain. High rigidity correlated with reduced vertical displacement.

* **Failure mechanisms:** Perpendicular specimens failed predominantly by particle crushing, whereas parallel ones failed by splitting along interfacial defect planes.

* **Crack initiation and propagation:** Under cyclic compression in perpendicular orientation, hemp, reed and rapeseed concretes exhibited stable crack propagation between 5 and 10th cycle (less than 9 cracks), while bamboo concretes showed pronounced propagation from 7th cycle, with crack density (32 cracks) and length (around 88.6 mm).

* **Crack growth rate (fatigue response):** In perpendicular orientation, crack propagation rates (dL/dN) were low for hemp, reed, and rapeseed concretes (0.18–0.23 mm/cycle), confirming good fatigue resistance. In contrast, bamboo concrete displayed an accelerated

rate (3.66 mm/cycle). Crack propagation rate increased in parallel orientation (0.87–1.29 mm/cycle) with sole exception of bamboo.

* **Crack opening displacement (COD):** Bamboo concrete exhibited the widest cracks (up to 4.4 mm perpendicular and 6.5 mm parallel), while reed concrete showed the narrowest openings (0.9–2.8 mm), highlighting better energy dissipation and interfacial bonding under damage. Hemp concrete displayed intermediate opening (1.83–3.34 mm).

* **Orientation-fatigue coupling:** Parallel-oriented specimens were more damaged, with higher crack growth and opening, indicating that orientation significantly influences fatigue life. The inverse relationship between useable strength and crack length confirms that stronger coating strength reduces crack propagation.

* **Damage interpretation:** High D in hemp concrete reflects rigidity loss from its high modulus rather than real damage, while low D in bamboo concrete in parallel orientation masks severe cracking (22 cracks, 18 mm total length) due to its very low maximum rigidity (E₀).

This work not only contributes to the field of sustainable construction materials but also sets the stage for future investigations on the effects of mineral binder nature and other factors on bio-based concrete's mechanical properties.

The findings underscore that bio-based concrete is recommended to have its particles oriented perpendicularly for better dissipation of mechanical energy through deformation, besides being efficient to be reinforced by external structures up to a strain difference of less than 5 %.

Future work will extend the adapted DIC methodology in evaluating life cycle fatigue by applying Paris-law type analyses to bio-based concrete specimens to derive explicit crack growth rates.

Hence, this study offers a promising avenue for optimizing the mechanical properties of bio-based concrete under compression loading prior to any reinforcement technique, while reducing the environmental footprint of the construction industry.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgement

The authors would like to extend their appreciation to the French national research agency (ANR) for supporting and funding BIOUP project on which this study took a part (ANR-21-CE22-0009).

## Data availability

Data will be made available on request.

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