# Evaluating the flexural performance of bio-based concrete with different types of bio-aggregate and binder

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

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R. Bardouh, E. Toussaint, S. Amziane, Sandrine Marceau. Evaluating the flexural performance of bio-based concrete with different types of bio-aggregate and binder. Construction and Building Materials, 2026, 509, pp.145176. 10.1016/j.conbuildmat.2026.145176. hal-05453122

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Submitted on 12 Jan 2026

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# Evaluating the flexural performance of bio-based concrete with different types of bio-aggregate and binder

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 concrete

Cracking

DIC

Flexural strength

Floating modulus

## ABSTRACT

This study investigates the influence of bio-aggregate type (hemp, bamboo, and reed) and binder type (CEMIV, LC3, and PNC) on the flexural performance of bio-based concrete, with the objective of identifying optimized formulations for construction applications. The term ‘optimization’ refers to the identification of favorable aggregate-binder combinations based on a comparative assessment of flexural strength, stiffness, deformation behavior, and cracking characteristics. Digital image correlation was employed to analyze deformation and cracking behavior. The results indicate that hemp-based concretes combined with CEMIV exhibit the highest flexural performance, benefiting from effective crack-bridging mechanisms. Reed-based concretes demonstrate a more stable and homogeneous flexural response, particularly when combined with higher-performance binders such as CEMIV and LC3. In contrast, bamboo-based concretes show pronounced interfacial debonding, especially when paired with lower-strength binders (such as PNC), leading to early strain localization. Binders with high fineness, such as PNC, promote the development of finer and more uniformly distributed cracks, despite lower global flexural performance. These findings provide fundamental insights into the intrinsic flexural behavior of bio-based concretes, indicating that the complementary combination of the dimensional stability of reed aggregates and the favorable mechanical performance of hemp shives can form a robust basis for the future development of reinforced bio-based composites, such as fabric-reinforced cementitious matrix (FRCM) systems, where optimized substrate performance is essential.

## 1. Introduction

The rapid expansion of green and alternative construction materials, particularly bio-based building materials, is primarily driven by the urgent need to mitigate global warming, to which the construction sector contributes substantially due to its high environmental footprint [1]. Bio-based materials are widely recognized as environmentally friendly, sustainable, and multifunctional solutions [2]. While the compressive behavior of bio-based concrete has been extensively studied, their flexural performance remains comparatively underexplored [3].

Bio-based concretes are commonly used as infill materials in timber-frame wall systems [4]. Given the high susceptibility of masonry and infill walls to out-of-plane bending failure [5], understanding the flexural behavior of these materials is essential to ensure structural reliability. Previous studies have shown that incorporating bio-based materials, such as hemp concrete, into timber studwork enhances the

racking resistance of timber walls by limiting buckling and improving in-plane load capacity [6]. Moreover, hemp used in both fiber and aggregate forms has been reported to increase the flexural strength of plain concrete by up to 9% when optimized at a fiber content of 0.36% [7].

The flexural performance of bio-based concretes is typically evaluated using three-point [8] or four-point flexural tests [9]. Under flexural loading, these materials generally exhibit an initial linear elastic phase, during which the cementitious matrix predominantly carries the applied stress [10]. This phase continues until the formation of the first visible crack, usually initiating between the lower surface and the neutral axis of the specimen [11], and often coinciding with the peak load. Beyond this point, a sudden drop in flexural strength is commonly observed, which is attributed to the stiffness mismatch between the rigid cementitious matrix and the more deformable bio-aggregates [12].

When fine bio-aggregates are replaced by elongated natural fibers, the flexural response is significantly altered. In such cases, an

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

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

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: sofiane.amziane@uca.fr (S. Amziane).</footnote>
intermediate phase characterized by the development of multiple microcracks emerges, reflecting a gradual transfer of stress from the matrix to the reinforcing fibers [13,14]. The final stage is then governed by progressive interfacial failure at the fiber–matrix interface [15], in contrast to the abrupt post-peak failure typically observed in bio-based concretes incorporating finer aggregates [8].

Accurately capturing flexural deformation and cracking mechanisms in bio-based concretes remains challenging due to their highly heterogeneous and porous microstructure. Conventional point-based measurements, such as linear variable differential transformers (LVDTs), provide global deflection data but offer limited insight into strain localization and damage evolution [16,17]. In this context, digital image correlation (DIC) emerges as a powerful non-contact, full-field measurement technique capable of resolving displacement and strain fields in highly heterogeneous and porous materials [18]. DIC offers several advantages, including cost efficiency and high accuracy, as it is independent of the physical heterogeneity of bio-based concretes [19,20]. This technique has been successfully applied to investigate displacement fields, damage mechanisms, and strain localization in bio-based concretes [19,21,22]. Notably, localized strain bands have been observed at interlayer interfaces formed during casting, emphasizing the critical role of compaction pressure in reducing interlayer voids and enhancing mechanical performance [21].

Reported flexural strengths of bio-based building materials generally range from 0 to 6 MPa, depending on the type and proportion of bio-aggregates used [3]. Nevertheless, significantly higher values have been achieved in specific cases. For instance, Sassoni et al. reported flexural strengths up to 18 MPa in hemp concrete blocks, attributed to the use of high-density composites with densities reaching 1300 kg/m³ [23]. The flexural modulus of bio-based concretes reported in the literature spans a wide range, from approximately 50 MPa to 1500 MPa, reflecting the strong influence of both binder and aggregate characteristics [3].

Although several studies have explored bio-based concretes incorporating low-carbon binders such as limestone calcined clay cement (LC3) [24], existing research has predominantly focused on compressive behavior [25], hydration kinetics [26], or fire resistance [27]. Investigations addressing flexural performance and damage mechanisms remain limited. A small number of studies have examined flexural behavior in LC3-based bio-composites, particularly in mortars reinforced with textile fabrics [28] or natural fibers [27]. However, these studies remain restricted to specific material systems and do not provide a comprehensive comparison across multiple binder–bio-aggregate combinations. Comparative investigations that simultaneously consider different binders and bio-aggregates, while accounting for the distinct physical characteristics of bio-aggregates, remain scarce.

Beyond their current use as non-load-bearing infill materials, bio-based concretes are increasingly considered as substrates for composite reinforcement systems [29–31]. In this context, a thorough understanding of their intrinsic flexural behavior and damage mechanisms is essential for the rational design of reinforced bio-based systems, such as fabric-reinforced cementitious matrix (FRCM) composites, in which the mechanical response of the substrate plays a critical role in overall structural performance [9].

Considering the substantial influence of both binder type and bio-aggregate characteristics on the mechanical performance of bio-based concrete [32], the present study provides a systematic experimental assessment of the flexural performance of bio-based concretes incorporating three mineral binders (CEMIV, LC3, and PNC) and three bio-aggregates (hemp, bamboo, and reed). Formulations were specifically designed to achieve comparable fresh-state consistency and hardened-state density. By combining four-point flexural tests with full-field digital image correlation, this work elucidates the role of binder–aggregate interactions on stiffness, strength, crack development, and ductility, and proposes an interaction-driven approach for optimizing bio-based concrete formulations beyond binder strength alone.

# 2. Materials and methods

## 2.1. Materials

### 2.1.1. Mineral binders

The selection of mineral binders was guided by three main criteria: (i) mechanical performance, (ii) hydration kinetics, and (iii) environmental impact, in line with the objective of designing an optimized bio-based concrete substrate prior to any external reinforcement strategy. Three mineral binders used in this study were supplied by VICAT.

First, a pozzolanic cement commercially available on the French market, NATURAT CEMIV/A (P) 42,5 R CE NF, hereafter referred to as “CEMIV”, was selected due to the beneficial contribution of pozzolanic reactions to strength development when combined with bio-aggregates [33,34]. Previous studies have shown that pozzolanic additions, coupled with water-retaining agents, can significantly enhance early-age compressive strength, with reported increases of up to 100 % through promoted carbonation mechanisms [12].

Second, a prompt natural cement, PNC PM NF, denoted “PNC”, was selected owing to its extensive use in bio-based concrete applications reported in the literature [35,36]. Its rapid setting behavior is particularly advantageous in mitigating the retarding effects induced by soluble sugars released from bio-aggregates during hydration [37].

Third, a calcined clay cement available on the market as CEM type II/ C (Q-LL) 52,5 N, denoted “LC3”, was selected as a low-carbon binder. Partial substitution of clinker by limestone and calcined clay in LC3 can reduce CO₂ emissions by approximately 40–50 % [38]. In addition, its high kaolinite content (up to 79.4 %) enhances hydration reactions, leading to mechanical strength improvements of up to 27 % compared to ordinary Portland cement [39].

The physical properties of the mineral binders were provided by VICAT [40]. Flexural strength and modulus at 28 days were determined on standardized mortar prisms of 40 × 40 × 160 mm³ following NF EN 196–1 [41] for CEMIV and LC3 binders, and NF P15–314 [42] for PNC, to account for its rapid setting behavior. The results obtained after 28 days of curing are summarized in Table 1.

PNC and LC3, owing to their higher fineness, exhibit greater reactivity, which may enhance strength development through improved binding efficiency. In contrast, CEMIV is comparatively coarser, suggesting slower early hydration. This characteristic is beneficial in bio-based concretes, as it reduces early-age instability associated with the release of soluble sugars from bio-aggregates. Among the binders, CEMIV exhibited the highest flexural strength (6.39 MPa) and modulus (113.01 MPa), reflecting the contribution of ongoing pozzolanic reactions to mechanical performance. LC3 showed intermediate mechanical properties, with values comparable to CEMIV at 28 days, indicating synergistic hydration between clinker and pozzolanic phases. Conversely, PNC demonstrated limited strength and stiffness development, with flexural strength and modulus as low as 2.37 MPa and 53.38 MPa, respectively. This result highlights that, despite its high fineness, PNC is less suitable for applications requiring high mechanical performance.

The three binders exhibit distinct chemical and mineralogical compositions as detailed in Tables 2 and 3. CEMIV is characterized by a high clinker content, with alite and belite fractions of 52.1 % and 5.8 %, respectively, and a moderate amorphous content of 15 %. LC3 presents a

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

Physical and mechanical properties of the mineral binders used in this study.</caption><thead><tr><th></th><th>Blain fineness<br>(m²/kg)</th><th>Density<br>(kg/m³)</th><th>Flexural strength<br>(MPa)</th><th>Young's modulus<br>(MPa)</th></tr></thead><tbody><tr><td>CEMIV</td><td>535</td><td>3070</td><td>6.39 ± 0.31</td><td>113.01 ± 5.19</td></tr><tr><td>LC3</td><td>634</td><td>2920</td><td>5.41 ± 0.28</td><td>98.86 ± 4.27</td></tr><tr><td>PNC</td><td>702</td><td>2990</td><td>2.37 ± 0.19</td><td>53.38 ± 7.12</td></tr></tbody></table>
<table><caption>**Table 2**
Chemical composition of the mineral binders used in this study [40].</caption><thead><tr><th rowspan="2"></th><th rowspan="2">PF 950°C</th><th rowspan="2">SiO<sub>2</sub></th><th rowspan="2">Al<sub>2</sub>O<sub>3</sub></th><th rowspan="2">Fe<sub>2</sub>O<sub>3</sub></th><th rowspan="2">CaO</th><th rowspan="2">MgO</th><th rowspan="2">SO<sub>3</sub></th><th colspan="7">Chemical composition (% by mass)</th></tr><tr><th>K<sub>2</sub>O</th><th>Na<sub>2</sub>O</th><th>Ors</th><th>TiO<sub>2</sub></th><th>P2O5</th><th>MnO</th><th>Cl</th><th>Sulfides</th><th>Total</th></tr></thead><tbody><tr><th>PNC</th><td>9.1</td><td>19.6</td><td>8.1</td><td>3.4</td><td>51.4</td><td>3.8</td><td>3.8</td><td>1.3</td><td>0.1</td><td>-</td><td>-</td><td>-</td><td>-</td><td>100.6</td></tr><tr><th>CEM IV</th><td>2</td><td>27.3</td><td>6.7</td><td>5.6</td><td>51.3</td><td>2.2</td><td>2.9</td><td>0.9</td><td>0.6</td><td>-</td><td>-</td><td>-</td><td>0.01</td><td>99.5</td></tr><tr><th>LC3</th><td>6.9</td><td>27</td><td>9.4</td><td>3.8</td><td>45.7</td><td>2.3</td><td>2.1</td><td>1.3</td><td>0.2</td><td>0.1</td><td>0.5</td><td>0.2</td><td>-</td><td>0.3</td><td>99.3</td></tr></tbody></table>

<table><caption>**Table 3**
Mineralogical composition of the mineral binders used in this study [40].</caption><thead><tr><th></th><th>PNC</th><th>CEMIV</th><th>LC3</th></tr></thead><tbody><tr><td>Amorphous</td><td>-</td><td>15</td><td>23.1</td></tr><tr><td>Alite</td><td>-</td><td>52.1</td><td>33.2</td></tr><tr><td>Belite</td><td>30.2</td><td>5.8</td><td>5.7</td></tr><tr><td>Calcium aluminate</td><td>2.1</td><td>3.4</td><td>2</td></tr><tr><td>Ferrite</td><td>6.8</td><td>8.4</td><td>6.8</td></tr><tr><td>Calcite</td><td>15.7</td><td>-</td><td>10.3</td></tr><tr><td>Dolomite</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Free lime + Portlandite</td><td>5.7</td><td>0.6</td><td>0.7</td></tr><tr><td>Periclase</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Quartz alkaline sulphates (Acanite, Lagbeinite, Anhydrite)</td><td>-</td><td>-</td><td>4.6</td></tr><tr><td>Gypsum</td><td>2.1</td><td>0.4</td><td>1.3</td></tr><tr><td>Bassanite</td><td>-</td><td>3.8</td><td>1.1</td></tr><tr><td>Yeelinite</td><td>-</td><td>0.3</td><td>0.9</td></tr><tr><td>Clays</td><td>-</td><td>-</td><td>3.6</td></tr><tr><td>Gehlenite</td><td>7.2</td><td>-</td><td>-</td></tr><tr><td>Spurrite</td><td>13.6</td><td>-</td><td>-</td></tr><tr><td>Other phases</td><td>13.9</td><td>7.7</td><td>5.9</td></tr></tbody></table>

higher amorphous fraction (23.1 %), reduced alite content (33.2 %), and significant contributions from calcined clay (3.6 %) and limestone (10.3 % calcite). In contrast, PNC contains a higher belite content (30.2 %) along with weakly reactive phases such as spurrite (13.6 %) and gehlenite (7.2 %), reflecting a lower intrinsic reactivity despite a comparable CaO content (≈51 %).

### 2.1.2. Bio-aggregates

The selection of bio-aggregates was based on their contrasting physical characteristics and their reported influence on the mechanical performance of bio-based composites. Three bio-aggregates were used in this study: hemp shives (Cannabis sativa), bamboo (Bambusa vulgaris), and reed (Phragmites australis), as illustrated in Fig. 1.

Hemp shives were selected as a reference material, as they are the most extensively studied bio-aggregate in bio-based concrete applications and exhibit favorable mechanical compatibility with mineral binders [3,43]. Reed aggregates were chosen for their fine morphology and high specific surface area. Previous studies have shown that physical refinement of bio-aggregates enhances long-term mechanical performance by promoting better stress transfer within the cementitious

matrix [44,45]. Bamboo aggregates were selected due to their dense and rigid structure, which has been reported to significantly increase stiffness when used as a partial replacement for coarse aggregates in cementitious materials [46]. Moreover, bamboo has been widely investigated in the context of strengthening and reinforcing masonry structures [47], making it particularly relevant for assessing ductility and load redistribution mechanisms.

All 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 particles retained sizes between 1 mm and 10 mm, corresponding to the typical classification of coarse aggregates in construction materials. The bio-aggregates were physically characterized following the protocol of RILEM TC 236-BBM [48]. The measured properties included bulk density, water absorption, and morphological characteristics.

#### 2.1.2.1. Bulk density.
The procedure of determining bulk density involves weighing the bio-aggregates and measuring their corresponding volume with water. The bulk density is calculated according to Eq. (1), by measuring the external volume occupied by the packed bio-aggregates mass using a volumetric substitution method with water. This method includes inter-particle voids and therefore corresponds to bulk density rather than apparent particle density, and presented in Table 4.

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

Where,

* $\rho_{\text{agg}}$: bulk density of the aggregate (kg/m³)

* $m_{\text{agg}}$: dry mass of aggregates occupying the cylindrical mold (kg)

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

* $m_{\text{water}}$: mass of water filling the bulk volume of the packed aggregate (kg)

Among the bio-aggregates, bamboo exhibited the highest bulk density. Its nearly constant bulk density before and after drying indicates low moisture sensitivity and good dimensional stability. Reed was the second densest aggregate but showed higher moisture sensitivity after drying. Hemp shives exhibited intermediate behavior, balancing low density with moderate stability.

#### 2.1.2.2. Water absorption.
Water absorption was determined by drying the aggregates and measuring their mass increase at different immersion times, following the RILEM TC 236-BBM protocol. The water absorption ratio is dependent on immersion times in logarithmic function, as expressed in Eq. (2) and illustrated in Fig. 2.

$$ W(t) = IWC + k \times \log(t) \qquad (2) $$

In this equation, IWC denotes the initial water content (%), while K represents the diffusion coefficient within the aggregate cellular structure, corresponding to the slope of the fitted absorption curves as shown

<figure><img src="image_3.png" /><figcaption>Fig. 1. The different bio-aggregates used in this study.</figcaption></figure>

<table><caption>**Table 4**
Bulk density of the different bio-aggregates before and after drying.</caption><thead><tr><th>ρ (kg/m³)</th><th>Hemp shives</th><th>Bamboo</th><th>Reed</th></tr></thead><tbody><tr><td>Before drying</td><td>118 ± 4</td><td>305 ± 10</td><td>169 ± 8</td></tr><tr><td>After drying</td><td>113 ± 5</td><td>304 ± 7</td><td>145 ± 6</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 2. Water absorption rate of the different aggregates used in this study.</figcaption></figure>

in Fig. 2. The values of these parameters are reported in Table 5.

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

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

Where,

$m_w$ denotes the mass of wet aggregates after being immersed in water (g)

$m_D$ denotes the initial mass of the dry aggregates (g)

Hemp shives exhibited the highest water absorption capacity, absorbing up to 4.5 times their own weight after 48 h, owing to a high diffusion coefficient (0.62). This behavior is consistent with values reported in the literature [49]. In contrast, reed showed a comparable initial water absorption at t = 1 min due to its high initial water content, but its low diffusion coefficient (0.07) limited further uptake, leading to a plateau at approximately 250 % after 48 h. Bamboo was the least absorbent aggregate, with initial water absorption rate as low as 50 % and a diffusion coefficient of 0.073. These differences highlight the necessity of tailoring mix formulations, particularly when using bamboo aggregates in bio-based concretes.

### 2.1.2.3. Morphological properties.
Morphological characterization of bio-aggregates was performed using image analysis with ImageJ software, as recommended by RILEM TC 236-BBM. This method allows accurate quantification of particle geometry through pixel-based analysis. Two parameters were analyzed: particle length (major axis) and width (minor axis). Fig. 3 displays the cumulative distribution of both axes for each bio-aggregate.

Hemp shives exhibited an elongated morphology, with median lengths ranging from 6 to 20 mm and widths between 3 and 9 mm. Bamboo particles displayed a more equiaxed geometry, with similar length and width distributions and median sizes ranging from 3 to 15 mm. For the minor axis, hemp shives and bamboo exhibited comparable distributions, with approximately 50 % of particles below 3 mm in width. In contrast, reed aggregates presented a markedly finer morphology: their length distribution overlapped with the width distributions of the other bio-aggregates, with lengths not exceeding 9 mm and widths consistently below 1 mm. thereby highlighting an improved particles packing and reducing porosity. This fine morphology promotes improved particle packing and reduced porosity, resulting in better homogeneity when mixed with mineral binders.

<table><caption>**Table 5**
Initial water absorption and diffusion rate of the bio aggregates.</caption><thead><tr><th>Properties</th><th></th><th>Hemp</th><th>Bamboo</th><th>Reed</th></tr></thead><tbody><tr><td rowspan="2">Water absorption</td><td>Initial water content (IWC)</td><td>2.34</td><td>0.51</td><td>2.25</td></tr><tr><td>Diffusion rate (k)</td><td>0.62</td><td>0.073</td><td>0.08</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Morphological properties of the bio-aggregates.</figcaption></figure>

## 2.2. Specimens' preparation

Building upon the physical characterization of the bio-aggregates, the mix design of each type of bio-based concrete was tailored to achieve comparable fresh-state consistency across all configurations. The formulation was primarily governed by the physical properties of the bio-aggregates, while the mineral binder properties acted as amplifying rather than determining factors. The detailed mix proportions are reported in Table 6.

Across all formulations, a constant water-to-binder (W/B) ratio of 0.5 was maintained for the cement paste to ensure consistency and comparability. Bio-based concretes were cast into parallelepiped molds of 400 × 100 × 100 mm³, as described in a previous study [9]. However, a newly developed compaction device applying a pressure of 0.2 MPa was used to compact the material in three successive layers, with 20 blows/layer as shown in Fig. 4.a. The final state of the filled molds is shown in Fig. 4.b.

A total of 9 configurations were produced by combining the three bio-aggregates with the three mineral binders. For each configuration, three specimens were confected.

After demolding, specimens were stored in a conditioned room at 20 °C and 50 % RH for 6 months prior to testing. Density measurements were performed every five days and stabilized after approximately 28 days of curing. The corresponding wet and dry densities are reported in Table 7. Dry density was determined after oven-drying the specimens at 60 °C for 48 h to prevent residual moisture from affecting the mechanical measurements [44].

Bamboo- and reed-based concretes exhibited the highest green densities owing to their aggregates' high bulk densities of 304 kg/m³ and 145 kg/m³, respectively. Despite bamboo having the higher aggregate bulk density, reed-based concrete showed higher fresh and dry densities, attributed to its higher binder content and greater compressibility during casting, which resulted in reduced interparticle voids compared to bamboo-based mixtures.

## 2.3. Bio-aggregates-binders interface-scale visualization

To qualitatively assess the bio-aggregate-binder contact prior to mechanical testing, representative raw images acquired by the DIC cameras were examined at the interface scale. These images are not

<table><caption>**Table 6**
Formulations of the various bio-based concretes.</caption><thead><tr><th rowspan="2">Aggregate materials</th><th colspan="4">Mass (kg)</th><th rowspan="2">A:B:W ratio</th></tr><tr><th>Aggregates (A)</th><th>Binder (B)</th><th>Water (W)</th><th></th></tr></thead><tbody><tr><td>Hemp shives</td><td>0.475</td><td>0.95</td><td>0.95</td><td></td><td>1:2:2</td></tr><tr><td>Bamboo</td><td>1.185</td><td>0.711</td><td>0.711</td><td></td><td>1:0.6:0.6</td></tr><tr><td>Reed</td><td>0.712</td><td>1.068</td><td>1.068</td><td></td><td>1:1.5:1.5</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Fabrication process of parallelepiped specimens, (a): compacting the resulting mixture in layers inside the mold, (b): mixture filled up the mold after leveling the outer surface.</figcaption></figure>

<table><caption>**Table 7**
Fresh and dry densities of the different bio-based specimens.</caption><thead><tr><th rowspan="2">Number of specimens</th><th rowspan="2">Mineral binder &amp; Aggregate used in the specimens</th><th colspan="2">Wet density (kg/m<sup>3</sup>)</th><th colspan="2">Dry density (kg/m<sup>3</sup>)</th></tr><tr><th>Mean</th><th>CV(%)</th><th>Mean</th><th>CV(%)</th></tr></thead><tbody><tr><td>3</td><td rowspan="5">PNC</td><td>Hemp</td><td>700</td><td>3.27</td><td>466</td><td>1.19</td></tr><tr><td>3</td><td>Reed</td><td>775</td><td>4.46</td><td>508</td><td>3.28</td></tr><tr><td>3</td><td>Bamboo</td><td>706</td><td>4.17</td><td>528</td><td>4.21</td></tr><tr><td>3</td><td>Hemp</td><td>627</td><td>3.01</td><td>463</td><td>2.09</td></tr><tr><td>3</td><td>Reed</td><td>767</td><td>3.27</td><td>523</td><td>2.17</td></tr><tr><td>3</td><td rowspan="5">LC3</td><td>Bamboo</td><td>704</td><td>3.35</td><td>567</td><td>3.08</td></tr><tr><td>3</td><td>Hemp</td><td>670</td><td>3.19</td><td>456</td><td>1.31</td></tr><tr><td>3</td><td>Reed</td><td>777</td><td>5.27</td><td>528</td><td>2.13</td></tr><tr><td>3</td><td>Bamboo</td><td>720</td><td>4.49</td><td>536</td><td>2.54</td></tr></tbody></table>

intended to characterize the microstructure but rather to illustrate aggregate morphology, matrix continuity, and potential interfacial features relevant to the observed mechanical behavior. Representative configurations are shown in Fig. 5. Hemp-based concrete (Fig. 5.a) exhibits relatively homogeneous packing with continuous binder coating around the aggregates, suggesting effective interfacial adhesion and load transfer. In contrast, bamboo-based concrete (Fig. 5.b) displays greater heterogeneity, with visible interparticle voids and discontinuous binder coverage, as highlighted in red, indicating preferential crack pathways and weaker interfacial bonding. Reed-based concrete (Fig. 5.c), shows well-coated aggregates with dense packing and continuous binder–aggregate contact, promoting strong interfacial adhesion.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Representative raw grayscale images acquired by the DIC cameras showing the mesoscale morphology and bio-aggregate–binder interfaces for (a) hemp-, (b) bamboo-, and (c) reed-based concretes.</figcaption></figure>

## 2.4. Mechanical testing setup

### 2.4.1. Four-point flexural tests

All bio-based specimens were tested under four-point bending in accordance with NF EN 12390–5 [50]. The span between the roller supports (L) was set to 300 mm, while the distance from each support to the nearest loading point (a) was fixed at 100 mm, resulting in a spacing (l) of 100 mm between the two applied loads. A schematic representation of the specimen geometry and loading configuration is provided in Fig. 6.

Flexural tests were conducted using a Zwick Roell testing machine with a maximum load capacity of 600,000 N. A sensor limiting the force to 10,000 N was used to enhance measurement accuracy. Load data were recorded at an acquisition frequency of 1 Hz, and a constant displacement rate of 1 mm/min was applied throughout the test.

Deflection measurements were obtained using three LVDTs: one sensor with a 25 mm range positioned at mid-span between the loading points, and two sensors with a 10 mm range placed at the supports. The mid-span deflection was corrected by subtracting the average displacement measured at the supports to eliminate rigid-body motion and support compliance effects. In parallel, a non-contact digital image correlation (DIC) technique was employed, showing excellent agreement with the corrected LVDT measurements, thereby validating the accuracy of the deflection measurements. The experimental setup is illustrated in Fig. 7.

A 2D DIC setup was adopted using a Fujinon HF12.5SA-1 C-mount lens (12.5 mm focal length, 5 MP resolution) positioned in front of the specimen. Synchronization between image acquisition and machine data was achieved using CATMAN software, and images processing was performed using 7D software [51]. The subset size and step size were both set to 10 pixels. The spatial resolution was 0.019 pixels for this subset size. The expansion coefficient applied for parallelepiped specimens to convert from pixels to mm was set at 400 mm for 1262 pixels, thereby 0.031 mm/pixel.

## 2.5. Mechanical properties computational methodology

This study assessed two mechanical properties: flexural strength ($\sigma_f$), and flexural Young’s modulus ($E_f$).

The flexural strength ($\sigma_f$) was calculated using classical Euler–Bernoulli beam theory. The maximum flexural stress at failure was determined using Eq. (4),

$$ \sigma_f = \frac{3Fa}{bh^2} \qquad (4) $$

Where,

$\sigma_f$: flexural strength (MPa)

F: maximum applied load (N)

a: distance from support to the point of load application (equivalent

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Schematical representation of the dimensions of parallelepiped specimens under 4-point flexural test.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. Four-point flexural test setup with DIC installation.</figcaption></figure>

to 100 mm)

b: parallelepiped specimens' width (equivalent to 100 mm)

h: parallelepiped specimens' height (equivalent to 100 mm)

The flexural Young's modulus ($E_f$) represents the stiffness of the bio-based concrete beam in the constant-moment zone between the applied loads (2nd zone). The adopted approach ensures that the calculated flexural Young's modulus reflects the actual bending behavior of the beam by accounting for the three distinct moment zones in a four-point flexural configuration.

The flexural Young's modulus was derived by integrating the Euler-Bernoulli bending moment equations over the three beam zones illustrated (Fig. 8). For each zone, the bending moment differs, leading to three separate moment equations (Eqs. 5-10). By integrating each moment equation twice with respect to the longitudinal coordinate, the corresponding deflection equations for each zone were obtained. Upon its identification, the flexural Young's modulus (Eq. 11) was calculated mainly based on Eq. 7 in the middle zone between the applied points. In order to determine the constants of the central deflection in Eq. 7, boundary conditions and continuity at the zones interfaces were applied on all three equations (displacement: $v$; slope: $v'$; curvature: $v''$). The basic equations at each interval are presented below.

For the first zone, where $0 \le x < a$, and $M_1(x)=Fx/2$, Eq. (5) led to first vertical displacement,

$$v''_1(x) = \frac{Fx}{2EI} \rightarrow v'_1(x) = \frac{Fx^2}{4EI} + A \rightarrow v_1(x) = \frac{Fx^3}{12EI} + Ax + B \quad (5)$$

Where E: Young's modulus; I: inertia moment, A and B constants from mathematical integration of $M_1(x)$. By applying boundary conditions and mathematical resolution, the constants A and B were known, $A = Fa^2/4EI$; $B = 0$. Therefore, displacement from Eq. (5) can be simplified to Eq. (6),

$$v_1(x) = \frac{Fx^3}{12EI} + \frac{Fa^2}{4EI}x \quad (6)$$

<figure><img src="image_10.png" /><figcaption>Fig. 8. Schematical representation of the three zones in the moment diagram of the parallelepiped specimens.</figcaption></figure>

For the second zone, where $a \le x < L-a$, and $M_2 = Fa/2$, Eq. (7) led to second vertical displacement,

$$v''_2(x) = \frac{Fa}{2EI} \rightarrow v'_2(x) = \frac{Fax}{2EI} + C \rightarrow v_2(x) = \frac{Fax^2}{4EI} + Cx + D \quad (7)$$

Where C and D two constants from mathematical integration of $M_2(x)$. By applying boundary conditions and mathematical resolution, the constants C and D were known, $C = 0$;

$D = Fa^3/12EI$. Therefore, displacement from Eq. (7) can be simplified to Eq. (8),

$$v_2(x) = \frac{Fax^2}{4EI} + \frac{Fa^3}{12EI} \quad (8)$$

For the third zone, when $L-a < x < L$, and $M_3(x)= F/2(L-x)$, Eq. (9) led to third vertical displacement,

$$v''_3(x) = \frac{F(L-x)}{2EI} \rightarrow v'_3(x) = \frac{F}{2EI} \left(Lx - \frac{x^2}{2}\right) + G \rightarrow v_3(x) \\ = \frac{F}{2EI} \left(\frac{Lx^2}{2} - \frac{x^3}{6}\right) + Gx + H \quad (9)$$

Where G and H two constants from mathematical integration of $M_3(x)$. By applying boundary conditions and mathematical resolution, the constants G and H were known, $G = -Fa^2/EI$; $H = 3Fa^3/4EI$. Therefore, displacement from Eq. (9) can be simplified to Eq. (10),
$$v_3(x) = \frac{F}{2EI} \left( \frac{Lx^2}{2} - \frac{x^3}{6} \right) - \frac{Fa^2}{EI} x + \frac{3Fa^3}{4EI} \quad (10)$$

By substituting the dimensions of the given parallelepiped specimen and applying boundary conditions on the above equations, the flexural Young's modulus in the middle of the specimen (at x = 3a/2) is determined based on Eq. (11),

$$E_f = \frac{31 Fa^3}{48 I \nu_2} \quad (11)$$

Where,

E<sub>f</sub>: flexural Young's modulus (MPa)

31/48 is a mathematical constant,

F: applied force (N)

a: distance from support to the point of load application (equivalent to 100 mm)

I: inertia moment of the parallelepiped specimen (equivalent to 8.33 × 10⁶ mm⁴)

ν<sub>2</sub>: Displacement determined at the middle of the specimen through 25 mm gauge sensor (mm)

The flexural modulus was determined from the initial linear portion of the load–deflection response, corresponding to deformation levels below approximately 1 %. At this level, geometric nonlinearities are negligible and the small-deflection assumption underlying Euler–Bernoulli beam theory remains valid. Moreover, previous investigations on bio-based concretes [19] have shown that material damage initiates at significantly higher strain levels (≈3 %), indicating that the measured response corresponds to an elastic regime. The calculated flexural modulus is therefore intended as a comparative indicator of initial stiffness rather than a full nonlinear material characterization.

The Young's modulus is calculated using the floating technique described by Niyigena et al. [52]. In this technique, the modulus is calculated over a moving strain window (0.4 %) within the linear portion of the stress–strain curve as shown in Fig. 9, and the maximum value is retained as the Young's modulus as expressed in Eq. 12.

$$E_f = \text{MAX}\{E_1, E_2, E_3, E_4, \dots, E_n\} \quad (12)$$

Although the experimental results are presented in terms of force–mid-span deflection curves, the strain interval used for the floating modulus determination was defined based on beam theory. For a four-point flexural configuration, the axial strain at the extreme tensile fiber can be expressed as:

<figure><img src="image_11.png" /><figcaption>Fig. 9. Floating technique methodology of modulus calculation.</figcaption></figure>

$$\epsilon = \frac{24h\delta}{2(4a^2 - 3L^2)} \quad (13)$$

Where,

h: height of the parallelepiped specimen (mm)

δ: mid-span deflection computed by the 25 mm gauge sensor (mm)

a: distance from support to the point of load application (mm)

L: span distance between the supports (mm)

By substituting the parameters in Eq. 13 with the dimensions of the parallelepiped specimens, the resulting elastic strain will be ε = 0.00522 δ. The floating technique was used since it led to higher values compared to other computational methods such as tangent and cyclic, with less dispersion and variability [52,53].

## 3. Results and discussion

### 3.1. Force Mid-span deflection

For clarity, the flexural responses are presented by binder type in the following subsections. For each configuration, an average force–mid-span deflection curve is shown.

#### 3.1.1. CEMIV-bio-based specimens

Fig. 10 represents the flexural response of CEMIV-bio-based concretes. All three configurations exhibited a quasi-elastic response characterized by two main phases. The first phase corresponds to a nearly linear elastic regime up to the peak load, during which the mineral matrix primarily carried the applied stress despite the composite nature of the material [10,54]. The peak load values were consistent with those reported in the literature for bio-based concretes [3]. The elastic phase was followed by a second phase associated with damage initiation and failure, governed by the stiffness mismatch between the rigid cementitious matrix and the more deformable bio-aggregates. Notably, CEMIV-Hemp and CEMIV-Reed reached their peak loads at nearly identical mid-span deflections, indicating a similar threshold for critical crack initiation, albeit with distinct load levels and post-peak responses.

In this context CEMIV-Hemp exhibited an abrupt post-peak drop, reflecting a stiff but brittle behavior. This sudden loss of load-bearing capacity was associated with the formation of a dominant internal crack immediately after peak load. However, no macroscopic crack visible at the specimen surface was observed for hemp-based concretes with any of the three binders, in contrast to previous observations on hemp concrete made with sulfoaluminate cement (CSA-Hemp) subjected to four-point flexural loading [9]. Fig. 11.a illustrates the final state of CEMIV-Hemp specimen compared with a CSA-Hemp specimen from a previous study [9] (Fig. 11.b), where complete specimen splitting occurred at a flexural load as low as 50 N. This comparison highlights the critical influence of binder selection on the failure mode and overall

<figure><img src="image_12.png" /><figcaption>Fig. 10. Force Mid-span deflection of CEMIV-bio-based specimens.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 11. Failure mode difference, (a): CEMIV-Hemp tested in this study, (b): CSA-Hemp tested in a previous study [9].</figcaption></figure>

mechanical performance of bio-based concretes.

In contrast, both CEMIV-Reed and CEMIV-Bamboo exhibited pro-
gressive post-peak behavior without abrupt failure. This enhanced
ductility is attributed to the higher volume fraction of reed and bamboo
aggregates compared to hemp shives in their respective formulations
(see Section 2.2). This observation aligns with findings by Murphy et al.
[10], who reported improved ductility with increasing hemp content,
albeit at the expense of reduced flexural strength.

Among the CEMIV-based configurations, CEMIV-Bamboo exhibited the weakest flexural performance, consistent with its compressive behavior [19]. The specimen reached an ultimate load of approximately 250 N at a mid-span deflection below 1 mm, indicating limited ductility. This poor response is attributed to the intrinsic physical properties of bamboo aggregates, which require tailored mix designs to achieve effective stress transfer. The weak interfacial bonding between bamboo aggregates and the CEMIV matrix limited deformation capacity and load redistribution. In contrast, CEMIV-Reed demonstrated a favorable balance between moderate flexural strength and high ductility, suggesting strong compatibility between reed aggregates and pozzolanic phases in the CEMIV binder.

3.1.2. *LC3-bio-based specimens*

Fig. 12 illustrates the flexural response of LC3-bio-based concretes. LC3-Reed exhibited a flexural behavior very similar to that of CEMIV-Reed (Fig. 10), reflecting the high mechanical performance of both binders and the strong interfacial bonding promoted by the fine morphology of reed aggregates. This synergy resulted in a homogeneous internal structure and a stable flexural response.

In contrast, LC3-Hemp followed the same general mechanical trend as CEMIV-Hemp but showed an approximately 50 % reduction in ultimate flexural load. This reduction is primarily attributed to the high water absorption of hemp shives, which alters the hydration kinetics of the LC3 paste and promotes heterogeneities within the interfacial transition zone (ITZ). This interpretation is further supported by the fact that the flexural strength of the neat CEMIV paste was approximately 11 % higher than that of LC3. LC3-Hemp also exhibited a more brittle response, with a maximum mid-span deflection of 1.8 mm compared to 2.7 mm for CEMIV-Hemp.

<figure><img src="image_14.png" /><figcaption>Fig. 12. Force Mid-span deflection of LC3-bio-based specimens.</figcaption></figure>

LC3-Bamboo again exhibited the weakest flexural strength within the LC3 series. However, unlike the brittle failure observed for CEMIV-Bamboo, LC3-Bamboo showed a more progressive and ductile response, reaching mid-span deflections of up to 1.6 mm. This behavior indicates a modest improvement in deformability despite the limited flexural capacity. Overall, LC3-Reed emerged as the most mechanically efficient configuration within the LC3 group, highlighting the optimal compatibility between reed aggregates and LC3 binder.

3.1.3. PNC-bio-based specimens

Fig. 13 represents the flexural response of PNC-bio-based concretes. PNC-Hemp exhibited a mechanical trend similar to those observed for LC3- and CEMIV-based hemp concretes. However, the lower intrinsic flexural strength of the PNC binder significantly limited the maximum load-bearing capacity. Despite the flexural strength of PNC paste being approximately 56 % lower than that of LC3, the ultimate flexural loads of PNC-Hemp and LC3-Hemp were comparable (≈1000 N). This suggests that the rapid setting behavior of PNC may partially offset the adverse effects of hemp's high water absorption.

PNC-Reed and PNC-Bamboo displayed markedly different behaviors compared to their CEMIV and LC3 counterparts. While reed-based concrete remained the most ductile configuration, its ultimate load was significantly reduced due to the weak intrinsic strength of the PNC binder. Notably, PNC-Bamboo exhibited a substantial increase in ductility, with mid-span deflections exceeding 4 mm, compared to less than 1 mm for CEMIV-Bamboo.

In fact, the flexural response of bamboo-based concretes was found to be strongly governed by the quality of the paste-aggregate interface. In the case of PNC-Bamboo composites, full-field deformation measurements revealed early strain localization (see Fig. 22.c) at the paste-aggregate interface, indicating premature debonding of the interfacial transition zone (ITZ). This observation is consistent with previous microstructural investigations [55] reporting the presence of an extended ITZ halo around bamboo aggregates (area of 225 mm²) embedded in a PNC matrix, characterized by diffuse and poorly defined boundaries. Such a “fuzzy” halo morphology suggests the development of a partially hydrated interfacial zone with limited bamboo-PNC paste adhesion.

As a result, load transfer progressively shifted from the PNC matrix to the bamboo aggregates, whose intrinsic deformability governed the post-elastic flexural response. This redistribution of stresses explains the enhanced ductility and deformation capacity of PNC-Bamboo, albeit at the expense of reduced ultimate flexural strength.

3.2. Mechanical properties

3.2.1. *Flexural strength*

Fig. 14 summarizes the flexural strengths of all bio-based concrete configurations. The values reported inside each bar correspond to the

<figure><img src="image_15.png" /><figcaption>Fig. 13. Force Mid-span deflection of PNC-bio-based specimens.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 14. Flexural strength of the different bio-based concrete configurations.</figcaption></figure>

dry density of the specimens (kg/m³).

Overall, irrespectively to bio- aggregate type, CEMIV-based concretes demonstrated the highest flexural strengths. CEMIV-Hemp achieved the highest value (0.6 MPa) nearly double that of its LC3- and PNC-based counterparts (≈0.3 MPa). This superior performance is attributed to the higher intrinsic strength of the CEMIV binder and the strong interfacial bonding developed between hemp shives and pozzolanic phases. The measured value aligns with flexural strengths reported for hemp–lime concretes of similar density (≈430 kg/m³) produced by projection processes [8].

In contrast, PNC-based specimens consistently showed the lowest flexural strengths. PNC-Hemp and PNC-Reed exhibited reductions of approximately 50 % and 70 %, respectively, relative to their CEMIV counterparts, primarily due to the limited late-age strength development of the PNC binder.

Interestingly, LC3-Hemp exhibited flexural strength values similar to PNC-Hemp, despite the higher intrinsic strength of LC3. This unexpected similarity is likely linked to deleterious chemical interactions between LC3 phases (Table 2) and the extracted sugar components of hemp shives. In fact, phenolic and lignin-like compounds presented in hemp shives disrupt the hydration mechanism by complexing Ca²⁺ ions and interfering with the hydration of reactive aluminous and silicate phases at the interfacial transition zone (ITZ) [56,57]. More particularly, higher Al₂O₃ content (9.4 %), and the significant amorphous fraction (23.1 %), together with the presence of calcined clays (3.6 %), and a reduced alite content (33.2 %) (Table 3), make LC3 sensitive to chemical interactions with organic compounds released by hemp shives, thereby leading to a pronounced hydration disruption and a reduction in effective hydrate formation. This evidence is consistent with previous calorimetric studies [58,59], showing that the addition of hemp shiv reduces the cumulative heat of hydration of LC3 to approximately 92 % of that of the neat binder.

Existing data on LC3-Hemp concretes are predominantly limited to compressive strength measurements. In particular, previous work reported a compressive strength of approximately 0.25 MPa at 28 days for LC3-Hemp concrete with a dry density of about 550 kg/m³ [25]. Despite the lower dry density of LC3-Hemp (463 kg/m³) in this study, the measured flexural strength reached approximately 0.3 MPa, exceeding reported compressive strength. This difference can be attributed to the distinct stress states mobilized under four-point flexion, where the mechanical response is governed by the tensile behavior of the bottom fiber of the specimen, while compression remains limited to the upper zone. This finding suggests that LC3-Hemp composites exhibit a more efficient load-bearing capacity in tension than in compression, which is consistent with the crack-bridging and deformation mechanisms activated by hemp shives under flexural loading.

Reed-based concretes exhibited relatively stable flexural strengths for both CEMIV and LC3 binders, indicating low sensitivity to binder

type due to the fine morphology and moderate water absorption of reed aggregates. However, the weaker PNC binder significantly reduced the flexural strength of PNC-Reed. Notably, the flexural strength of 0.3 MPa obtained for CEMIV- and LC3-Reed is considerably higher than the 0.18 MPa reported for hemp concrete mixed with additives and pozzolans after one year [12].

Bamboo-based concretes consistently exhibited the lowest flexural strengths across all binders (< 0.1 MPa), reflecting weak interfacial adhesion between bamboo particles and cementitious matrices, as previously evidenced in Section 2.3.

Overall, the flexural strength range obtained in this study (0.05–0.6 MPa) is significantly lower than 4.5 MPa reported for LC3-based mortars reinforced with waste rockwool fibers [27]. This discrepancy can be primarily attributed to the markedly different reinforcement strategies and material proportions. While the referenced study used only 5 wt% of rockwool fibers relative to binder [27], the present work incorporated bio-aggregates at much higher contents (≈50 % for hemp, 167 % for bamboo, and 66 % for reed relative to binder mass), inherently reducing the load-bearing capacity of the cementitious matrix.

### 3.2.2. Flexural Young's modulus

Fig. 15 shows the maximum flexural Young's modulus of the bio-based concretes as function of the mid-span deflection at which the modulus was reached. Dry densities are also indicated for each configuration inside bars (kg/m³).

Overall, CEMIV-based specimens exhibited the highest flexural rigidity. CEMIV-Hemp showed the highest modulus (44 MPa) at a relatively large mid-span deflection of 2.6 mm, reflecting strong interfacial bonding and efficient stress transfer in the elastic regime. CEMIV-Reed also demonstrated high rigidity (≈36 MPa at 1.7 mm), highlighting the compatibility between fine reed aggregates and pozzolanic binders. Notably, CEMIV-Reed exhibited higher rigidity than LC3-Hemp and PNC-Hemp, suggesting that reed aggregates can provide an effective alternative to hemp in certain binder systems.

CEMIV-Bamboo showed markedly lower rigidity (≈7 MPa at 0.5 mm), confirming the limited stress transfer capacity associated with weak bamboo–matrix bonding. Similar trends were observed for LC3- and PNC-based bamboo concretes.

LC3-based concretes exhibited lower rigidity than CEMIV-based composites. LC3-Hemp reached an intermediate modulus of 33 MPa at 1.6 mm of deflection comparable to that of PNC-Hemp, indicating incompatibility between hemp shives and LC3 under flexural loading. This finding can be explained by the same considerations elaborated in Section 3.2.1. In addition, LC3-Bamboo demonstrated the highest ductility among bamboo-based concretes, with a modulus of 7 MPa at a deflection of 1.5 mm, suggesting improved energy dissipation compared to CEMIV-Bamboo.

<figure><img src="image_17.png" /><figcaption>Fig. 15. Maximum Young's modulus of the different bio-based concrete configurations as function of their mid-span deflection.</figcaption></figure>
Reed-based concretes showed intermediate behavior, with rigidity primarily governed by binder performance. LC3-Reed reached 27 MPa at 1.5 mm deflection, reflecting reduced stiffness relative to CEMIV-Reed due to the lower mechanical contribution of the LC3 binder.

PNC-based concretes displayed the lowest overall rigidity. PNC-Hemp reached 29 MPa at 1.9 mm, while PNC-Reed exhibited a sharp reduction to ≈ 9 MPa at 2.5 mm. Despite its highly ductile flexural response, PNC-Bamboo reached a maximum modulus of only 5 MPa at 0.5 mm deflection, highlighting early interfacial debonding and limited binder contribution, in line with its flexural response (Fig. 13).

Overall, the flexural rigidity was primarily governed by bio-aggregate type, with binder type acting as an amplifying factor rather than altering the intrinsic ranking of mechanical performance. Hemp-based concretes consistently showed the highest rigidity over a broad deflection range, reed-based concretes offered a balanced combination of rigidity and ductility, and bamboo-based concretes exhibited limited stiffness, except in the case of LC3-Bamboo.

### 3.3. DIC analysis of bio-based concretes under flexural loading

DIC maps were generated at mid-span deflection levels corresponding to the points of maximum flexural Young's modulus ($E_f$). The analysis therefore focuses on the elastic regime at peak rigidity in order to investigate displacement fields, deformation patterns, and crack development at the most mechanically relevant stage of loading.

The vertical displacement field ($U_y$) represents the elastic vertical displacement at maximum rigidity, while the vertical strain field ($e_{yy}$) corresponds to the elastic tensile strain at the same loading level.

It should be noted that the color scale of the DIC maps was harmonized only for hemp- and reed-based concretes across binders, and for all LC3-based specimens. This harmonization was possible because LC3-

Bamboo reached relatively high mid-span deflections at maximum rigidity, allowing a consistent scale to be applied. In contrast, the very low deflection values at maximum rigidity for bamboo-based concretes made with CEMIV and PNC binders (≈0.5 mm) required adjusted color scaling to avoid uniform coloration and to ensure sufficient spatial resolution of displacement and strain gradients.

Since the load and deflection levels differ between configurations, the comparison criterion for DIC analysis was fixed at the point of maximum flexural Young's modulus rather than at identical load or deflection levels. Accordingly, the DIC results are presented following the same binder-bio-aggregate grouping adopted for the flexural behavior analysis.

#### 3.3.1. CEMIV-bio-based specimens

Based on the flexural responses shown in Fig. 10, displacement and strain maps were computed at mid-span deflections ranging from 1.3 to 2.6 mm for CEMIV-Hemp; 1 to 1.7 mm for CEMIV-Reed; and 0 to 0.5 mm for CEMIV-Bamboo.

##### 3.3.1.1. Vertical displacement maps.
Fig. 16 shows the vertical displacement maps for CEMIV-bio-based concretes. All configurations exhibit a symmetric displacement distribution with respect to the specimen mid-span, reflecting stable loading and boundary conditions. Negative displacement values correspond to downward deflection. As expected, maximum displacement occurs in the central region between the applied loads, while quasi-static zones appear near the supports. This distribution is consistent with the four-point bending configuration, where the roller supports were positioned at 50 mm and 350 mm along the specimen length, defining a 300 mm span.

Among the configurations, CEMIV-Hemp (Fig. 16.a) demonstrated the most concentrated and pronounced displacement field, with $U_y$

<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Vertical displacement maps of CEMIV-bio-based specimens tested under flexural loading, (a): CEMIV-Hemp (at 1.3–2.6 mm), (b): CEMIV-Reed (at 1–1.7 mm), (c): CEMIV-Bamboo (at 0–0.5 mm).</figcaption></figure>
reaching approximately 1.3 mm. Despite its high $E_f$, this relatively large elastic displacement indicates a high elastic limit, allowing substantial deformation before yielding. This behavior explains the abrupt post-peak response observed in flexural testing, where most of the energy was dissipated within the linear elastic regime. This is consistent with CEMIV-Hemp reaching maximum rigidity at a relatively high mid-span deflection of 2.6 mm.

In contrast, CEMIV-Bamboo (Fig. 16.c) showed the lowest elastic displacement ($U_y \approx 0.4$ mm), indicating that maximum rigidity was reached at an early stage of loading. This behavior is fully consistent with its poor flexural performance and limited ductility.

CEMIV-Reed (Fig. 16.b) exhibited an intermediate elastic displacement of approximately 0.7 mm, accompanied by a notably more homogeneous displacement field. This uniformity supports the existence of strong interfacial bonding between reed aggregates and the pozzolanic phases of the CEMIV binder, enabling efficient stress redistribution throughout the specimen.

### 3.3.1.2. Vertical strain maps.
Fig. 17 presents the vertical elastic strain ($\epsilon_{yy}$) maps for CEMIV-based specimens. The strain fields are inherently heterogeneous due to the irregular morphology of bio-aggregates embedded in a composite microstructure. Two horizontal strain concentration bands are clearly visible at heights of 0–50 mm and 50–100 mm for both CEMIV-Hemp and CEMIV-Reed, extending along the specimen length. In contrast, these bands appear less pronounced (green rather than blue) in CEMIV-Bamboo, reflecting its limited deformability.

These strain concentration bands correspond to the interfaces between successive casting layers, as the specimens were compacted in

three distinct layers. This observation highlights the critical influence of casting and compaction procedures on the mechanical response of bio-based concretes, particularly regarding interlayer continuity and stress transfer [60]. The use of a high-pressure compaction device in the present study effectively mitigated localized strain concentrations at interlayer interfaces, in contrast to a previous investigation [21], where manual compaction resulted in pronounced eccentric strain localization zones, as illustrated in Fig. 18.

### 3.3.2. LC3-bio-based specimens

Based on the flexural responses shown in Fig. 12, displacement and strain maps were computed at mid-span deflections of 0.7–1.6 mm for LC3-Hemp, 0.8–1.5 mm for LC3-Reed, and 0.7–1.5 mm for LC3-Bamboo.

#### 3.3.2.1. Vertical displacement maps.
Fig. 19 illustrates the vertical displacement maps for LC3-based specimens, again displaying symmetric displacement profiles. LC3-Hemp (Fig. 19.a) exhibited the largest elastic displacement among LC3-based concretes ($U_y \approx 0.9$ mm). While following the same trend as CEMIV-Hemp, LC3-Hemp showed reduced displacement capacity, reflecting weaker compatibility and adhesion between hemp shives and the LC3 paste, which limited the extent of the elastic regime.

LC3-Reed (Fig. 19.b) exhibited a displacement field nearly identical to that of CEMIV-Reed, despite its lower rigidity (see Fig. 15). This similarity confirms the stable mechanical behavior of reed-based concretes and the strong bonding between fine reed aggregates and both binders.

In contrast, LC3-Bamboo (Fig. 19.c) showed a pronounced in elastic

<figure><img src="image_19.png" /><figcaption>**Fig. 17.** Vertical strain maps of CEMIV-bio-based specimens tested under flexural loading, (a): CEMIV-Hemp (at 1.3–2.6 mm), (b): CEMIV-Reed (at 1–1.7 mm), (c): CEMIV-Bamboo (at 0–0.5 mm).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. 18. Vertical strain map of hemp concrete at 2 % deformation with manual compaction [21].</figcaption></figure>

displacement at maximum rigidity. This behavior is consistent with its flexural response (see Fig. 12), where yielding occurred at deflections exceeding 1 mm, extending the elastic limit and significantly influencing the displacement field.

#### 3.3.2.2. Vertical strain maps.
Fig. 20 presents the vertical strain maps for LC3-based specimens. The same interlayer strain bands observed in CEMIV-based specimens are visible, corresponding to casting interfaces. However, the strain concentration is less intense for LC3-Hemp (Fig. 20. a) than for CEMIV-Hemp (see Fig. 17.a) consistent with the lower rigidity and elastic displacement of LC3-Hemp (see Fig. 15).

LC3-Reed (Fig. 20.b) showed deformation patterns almost identical to those of CEMIV-Reed (Fig. 17.b), confirming the robustness of reed-binder interactions across different mineral binders.

In particular observation, LC3-Bamboo (Fig. 20.c) demonstrated a more pronounced strain localization than CEMIV-Bamboo (Fig. 17.c), with blue strain bands replacing the predominantly green fields observed for CEMIV-Bamboo. This reflects the enhanced ductility and improved stress redistribution capacity of LC3-Bamboo compared to the brittle response induced by the stiffer CEMIV paste.

### 3.3.3. PNC-bio-based specimens

Based on the flexural responses shown in Fig. 13, displacement and strain maps were computed at mid-span deflections of 1.2–1.9 mm for PNC-Hemp, 1.6–2.5 mm for PNC-Reed, and 0–0.5 mm for PNC-Bamboo.

#### 3.3.3.1. Vertical displacement maps.
Fig. 21 shows the vertical displacement maps for PNC-bio-based specimens. Unlike previous binder systems, PNC-Reed (Fig. 21.b) exhibited the largest elastic displacement (Uy ≈ 0.9 mm), exceeding that of PNC-Hemp (Uy ≈ 0.7 mm) (Fig. 21.a). This can be attributed to two reasons: the very high rigidity of PNC-Hemp compared to PNC-Reed (see Fig. 15); and the particular flexural response of PNC-Reed (see Fig. 13), which reached maximum rigidity at a relatively high deflection of 2.5 mm.

<figure><img src="image_21.png" /><figcaption>Fig. 19. Vertical displacement maps of LC3-bio-based specimens tested under flexural loading, (a): LC3-Hemp (at 0.7–1.6 mm), (b): LC3-Reed (at 0.8–1.5 mm), (c): LC3-Bamboo (at 0.7–1.5 mm).</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. 20.** Vertical strain maps of LC3- bio-based specimens tested under flexural loading, (a): LC3-Hemp (at 0.7–1.6 mm), (b):LC3-Reed (at 0.8–1.5 mm), (c): LC3-Bamboo (at 0.7–1.5 mm).</figcaption></figure>

Despite its highly ductile flexural behavior, PNC-Bamboo (Fig. 21.c) again exhibited the lowest elastic displacement at maximum rigidity (Uy ≈ 0.35 mm), consistent with its very early elastic limit and weak bamboo–PNC interfacial bonding (see Fig. 5.b).

3.3.3.2. *Vertical strain maps.* Fig. 22 presents the vertical strain maps for PNC-based specimens. Likewise, same concentration lines can be observed in PNC-Hemp (Fig. 22.a) and PNC-Reed (Fig. 22.b) to those observed for other binders. However, PNC-Reed shows more intense localized strain due to its higher elastic displacement combined with low load-bearing capacity.

A distinctive deformation pattern is observed for PNC-Bamboo (Fig. 22.c), where 2 highly localized eccentric strain zones ($\epsilon_{yy} \approx 3 \times 10^{-3}$) appear near the bottom fiber under the applied load. These zones indicate early interfacial debonding, with the load primarily carried by the bamboo aggregates. Since these strain localizations occurred at an early deflection of 0.5 mm, PNC-Bamboo ultimately failed by specimen splitting at mid-span, confirming a bending-dominated failure governed by ITZ weakness.

### 3.3.4. Average displacement profile between analytical and DIC data

This section consists of superposing the analytical data of the average vertical displacement obtained in Eqs. 6, 8, and 10, to the DIC data during the linear section for a particular configuration of bio-based concrete to evaluate the pertinence between both methods. In this context, a horizontal cut was performed at middle height of the specimen to generate the average per column of the vertical displacement values from the DIC map of CEMIV-Reed.

Fig. 23 illustrates the superposition of the analytical data to the DIC data for CEMIV-Reed. Both datasets exhibited the expected parabolic

deflection shape, with maximum displacement at mid-span and near-zero displacement at the supports, confirming that the DIC measurements reliably captured the global flexural response in linear section. Nevertheless, some systematic offsets and local deviations were observed between the curves. The analytical data, based on ideal assumptions of homogeneity, linear elasticity, and perfect boundary conditions, predicts a smoother response, whereas the DIC results display slight asymmetry and local fluctuations which can be explained by experimental imperfections, bio-based materials heterogeneity, and crack-induced stiffness reductions.

### 3.3.5. Cracking mode of bio-based concretes under flexural loading

To quantify the cracking behavior from the DIC measurements, the vertical displacement fields ($U_y$) and the vertical strain fields ($\epsilon_{yy}$) were analyzed. A strain-based crack detection approach was adopted, where regions exceeding a prescribed threshold in $\epsilon_{yy}$ were identified as potential crack zones. In this study, a threshold of $\epsilon_{yy}= 0.002$ was selected based on the noise level of the measurements and the expected tensile strain at crack initiation.

A connected-component analysis of the thresholded strain map was then performed to segment individual cracks and extract their geometrical features. For each detected crack, the crack opening displacement (COD) was evaluated from the displacement field by averaging the vertical displacements of a small band of nodes immediately above and below the crack location, and computing their difference. This procedure provided a local measure of the opening for each crack. The total number of cracks was determined directly from the number of connected crack regions identified, while the average crack opening was calculated as the mean of all individual COD values. Compared to conventional methods such LVDTs, which can only provide crack opening at
<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Vertical displacement maps of PNC-bio-based specimens tested under flexural loading, (a): PNC-Hemp (at 1.2–1.9 mm), (b): PNC-Reed (at 1.6–2.5 mm), (c): PNC-Bamboo (at 0–0.5 mm).</figcaption></figure>

predefined locations, the adopted DIC-based methodology offers a full-field and non-contact measurement of both the number of cracks and their corresponding openings, enabling a more comprehensive assessment of the cracking mode of bio-based concretes under flexural loading.

The crack opening displacement charts are presented based on the mineral binder-bio-aggregate combinations, following the same approach adopted in the displacement and strain maps. The cut was performed on middle height of the DIC maps at maximum rigidity during the elastic zone. The offset in vertical displacement at the vicinity of each crack represents the cracks opening.

### 3.3.5.1. CEMIV-bio-based specimens.
Fig. 24 illustrates the crack opening displacement (COD) profiles for CEMIV-bio-based specimens. The cracks were concentrated with highest crack opening at the middle of the specimens between 100 mm and 200 mm, where the load was directly applied. It is worth noting that not all cracks can be noticed in COD profiles due to the precise low strain threshold taken which is 100 times less than the scale of the vertical displacement field (U<sub>y</sub>).

The COD profiles highlight distinct cracking responses influenced by the type of bio-aggregate. CEMIV-Hemp specimen (Fig. 24.a) exhibited the largest vertical displacement at maximum rigidity, reflecting its higher load transfer capacity and overall rigidity. This behavior was accompanied by the highest number of cracks (117) but with narrowest average crack opening (0.0012 mm), suggesting that crack propagation was more diffuse, with multiple fine cracks contributing to energy dissipation and delaying localization. This can be explained by the positive effect of hemp shives in bridging the initiated cracks at the microstructure of CEMIV-Hemp.

CEMIV-Reed (Fig. 24.b) with intermediate response in terms of rigidity and vertical displacement, has showed lowest number of cracks (81) but with higher average COD of 0.0021 mm. This indicates a more localized cracking mechanism, where fewer but wider cracks developed, leading to higher strain concentration zones. This is consistent with the vertical strain map of CEMIV-Reed (Fig. 17.b) where blue concentration lines can be noticed despite the relatively low vertical displacement. Although the fine reed aggregates synergize effectively with the CEMIV matrix, leading to a strong interfacial bond as evidenced by the lower number of cracks, their finer morphology limits their ability to bridge cracks efficiently, which explains the relatively larger crack openings observed.

In contrast, CEMIV-Bamboo (Fig. 24.c), although displaying the lowest rigidity and the smallest global vertical displacement, generated a total of 99 cracks with highest average COD (0.0028 mm). This finding suggests that despite the low strain concentrated zone of CEMIV-Bamboo (Fig. 17.c), the stress was concentrated into fewer crack planes, leading to larger average COD. This evidence can be explained by the weak interphase bond between bamboo aggregates and CEMIV matrix, limiting the absorption of cracks due to ineffective contribution of bamboo aggregates to the mechanical response.

The apparent contradiction between higher concentration zones in the case of CEMIV-Hemp and the small average COD can be explained by the fact that the concentrated strain zones seen in the DIC fields (Fig. 17. a) correspond to the activation of numerous microcracks (highest for CEMIV-Hemp); however, the strong interfacial bond between hemp shives and CEMIV matrix restricted their widening, ultimately resulting in smaller average COD values.
<figure><img src="image_24.png" /><figcaption>**Fig. 22.** Vertical strain maps of PNC-bio-based specimens tested under flexural loading, (a): PNC-Hemp (at 1.2–1.9 mm), (b): PNC-Reed (at 1.6–2.5 mm), (c): PNC-Bamboo (at 0–0.5 mm).</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Fig. 23.** Average vertical displacement between analytical data and DIC data for CEMIV-Reed.</figcaption></figure>

#### 3.3.5.2. LC3-bio-based specimens.
Fig. 25 represents the COD profiles for LC3-bio-based specimens. Among them, LC3-Hemp exhibited the highest crack count (139) with an average crack opening of 0.0030 mm. The relatively larger number of cracks with wider openings in LC3-Hemp compared to CEMIV-Hemp can be attributed to the lower load-bearing capacity and reduced intrinsic strength of the LC3 matrix. This weaker matrix promotes earlier crack initiation and less effective crack bridging, thereby facilitating both higher crack density and

greater opening widths.

LC3-Reed with lowest vertical displacement showcased a similar cracking response to CEMIV-Reed with a total of 79 cracks with openings of 0.0023 mm. This finding highlights the stable bio-based configuration of fine reed aggregates when mixed with CEMIV and LC3 matrices. This is consistent when comparing the flexural response, displacement and strain fields between both configurations. Interestingly, LC3-Reed demonstrated the lowest number of cracks with finest COD, suggesting an early effective energy dissipation with less strain concentration when compared to other LC3-bio-based configurations.

Similarly, LC3-Bamboo exhibited an intermediate crack density (101 cracks) with an average COD of 0.0027 mm. Notably, LC3-Bamboo reached higher vertical displacements than its CEMIV counterpart, reflecting improved ductility and a more favorable post-cracking response. This suggests that the bamboo aggregates contributed more effectively to the flexural performance when combined with the LC3 matrix. The relatively larger number of cracks, accompanied by narrower average openings, indicates that the dense bamboo aggregates helped distribute and absorb the initiated cracks, thereby enhancing energy dissipation and delaying localized failure.

#### 3.3.5.3. PNC-bio-based specimens.
Fig. 26 shows the COD profiles for PNC-bio-based specimens. Among them, PNC-Hemp, which exhibited an intermediate vertical displacement, developed a total of 134 cracks with the finest average opening (0.0009 mm) compared to its hemp-based counterparts. This behavior can be attributed to the dual effect of PNC cement: while its relatively weak intrinsic mechanical properties and low load-bearing capacity promote the initiation of a large number of cracks, its exceptionally high fineness (702 m²/kg, compared to 535 m²/kg for CEMIV and 634 m²/kg for LC3) enhances bio-aggregates packing and densifies the microstructure. This refinement limits the ability of
<figure><img src="image_26.png" /><figcaption>Fig. 24. Crack opening displacement profiles for CEMIV-bio-based specimens, (a): CEMIV-Hemp, (b): CEMIV-Reed, (c): CEMIV-Bamboo.</figcaption></figure>

cracks to propagate and widen, thereby resulting in narrower average COD despite the overall weaker flexural performance of the PNC-Hemp.

Building on the behavior observed for hemp-based specimens, the analysis of PNC-Reed reveals the same cracking pattern. At maximum rigidity, PNC-Reed developed a total of 86 cracks with the finest average opening of 0.0006 mm, while reaching a relatively higher vertical displacement of 0.9 mm. This combination highlights the weak intrinsic mechanical performance of the PNC matrix, which favors the initiation of a larger number of cracks compared to other reed-based concretes. Nevertheless, the extremely fine crack openings, despite the higher vertical displacement and low load-bearing capacity, can be attributed to the high Blaine fineness of PNC cement, which densifies the micro-structure and restricts crack widening.

Extending the analysis to bamboo-based concretes, PNC-Bamboo exhibited the highest number of cracks among its counterparts, with a total of 135, while maintaining relatively fine openings with an average COD of 0.0018 mm at maximum rigidity. The elevated crack count is closely linked to the limited mechanical performance of the PNC matrix, which shifted the flexural response to be largely governed by the bamboo aggregates. Nevertheless, the dense aspect of bamboo aggregates, combined with the high Blaine fineness of PNC cement, contributed to restricting the widening of cracks, thereby mitigating the average COD despite the pronounced cracking.

### 3.3.6. Comparative analysis for identical bio-aggregates with different binders

Reinterpreting the DIC displacement, strain, and cracking data by fixing the bio-aggregate type and varying the mineral binder provides further insight into the role of binder properties on flexural behavior.

#### 3.3.6.1. Hemp-based concretes.
CEMIV-Hemp exhibited the highest rigidity and elastic displacement, accompanied by dense but finely distributed cracking, whereas LC3-Hemp showed reduced displacement capacity and highest crack density with wider openings, reflecting weaker LC3 paste-hemp compatibility. In contrast, PNC-Hemp developed a relatively high crack density with the narrowest openings, indicating early crack initiation governed by the weaker matrix but restrained propagation due to the high fineness of PNC cement.

#### 3.3.6.2. Reed-based concretes.
CEMIV-Reed and LC3-Reed exhibited nearly identical displacement, strain, and cracking responses, confirming strong binder-aggregate synergy. PNC-Reed showed higher displacement but retained fine crack openings, again emphasizing the role of binder fineness in controlling crack widening rather than strength.

#### 3.3.6.3. Bamboo-based concretes.
In bamboo-based concretes, the influence of binder type was most pronounced. CEMIV-Bamboo showed early rigidity loss and limited deformation, consistent with brittle behavior and weak interfacial bonding. LC3-Bamboo exhibited enhanced displacement capacity and more distributed strain fields, indicating improved stress transfer and ductility. Conversely, PNC-Bamboo displayed early strain localization and eccentric cracking, confirming premature ITZ debonding and a flexural response predominantly governed by the bamboo aggregates.

### 3.3.7. Consolidated comparison of bio-based concrete configurations

To facilitate cross-comparison and synthesize the extensive experimental and DIC-based results presented in the previous sections, Table 8 provides a consolidated overview of the nine bio-based concrete
<figure><img src="image_27.png" /><figcaption>Fig. 25. Crack opening displacement profiles for LC3-bio-based specimens, (a): LC3-Hemp, (b): LC3-Reed, (c): LC3-Bamboo.</figcaption></figure>

configurations investigated. The table summarizes key mechanical in-
dicators at peak flexural response, including maximum flexural Young's
modulus, corresponding mid-span deflection, global rigidity level,
ductility near peak response, and a synthetic assessment of binder-bio-
aggregate interfacial behavior.

This consolidated overview highlights clear performance rankings
and trends that are less apparent when results are examined by binder
group. Hemp-based concretes generally exhibit the highest rigidity and
load transfer efficiency, particularly when combined with high-
performance binders such as CEMIV. Reed-based concretes show sta-
ble and intermediate behavior across different binders, reflecting
favorable morphology and interfacial compatibility. In contrast,
bamboo-based concretes consistently demonstrate limited rigidity and
weaker interfacial bonding, with binder type primarily influencing
ductility rather than strength.

4. Conclusion

Drawing from the coupling between the experimental results and
digital image correlation analysis, the following points can be
concluded:

* Within the investigated mix design and specimen dimensions, CEMIV-Hemp exhibited the most favorable overall flexural performance, achieving highest flexural strength of 0.6 MPa, maximum rigidity of 44 MPa, and largest elastic displacement at maximum rigidity of 1.3 mm. This superior response was closely tied to highest crack density (117) but narrowest openings (0.0012 mm), confirming strong bonding and efficient crack bridging

* Reed concretes showed intermediate performance, reaching 0.3 MPa of flexural strength and rigidity of 27–36 MPa, uniform displacement of 0.7 mm, with fewer cracks (around 80) but wider openings (0.0021–0.0023 mm), reflecting localized cracking yet stable structural behavior.

* Bamboo concretes remained weakest, with flexural strength < 0.1 MPa and rigidity < 7 MPa. Crack analysis revealed wider openings under CEMIV (0.0028 mm) but improved ductility with LC3 and high crack density (135) with PNC.

* Bamboo-based DIC maps showed low displacement (0.35–0.4 mm) for PNC and CEMIV mixes, with LC3-Bamboo showing increased ductility but persistent weak bonding. Severe incompatibility in PNC-Bamboo led to high strain localization.

* Binder type amplified aggregate influence: The ranking of aggregate efficiency remained Hemp > Reed > Bamboo across all binders. Strong binders (CEMIV, LC3) enhanced stiffness, while PNC revealed poorest mechanical performance, described by high crack counts but finer openings due to its high fineness.

* High-pressure compaction reduced strain localization by about 100 times, improving stress distribution and limiting crack concentration zones.

It should be emphasized that the flexural strength levels reported in
this study are consistent with the intended use of bio-based concretes as
lightweight infill and insulation materials. Taken together, these find-
ings point toward an optimization strategy that leverages the comple-
mentary attributes of hemp and reed: combining the crack-bridging
efficiency and mechanical robustness of hemp shives with the fine
morphology and stabilizing effect of reed aggregates. Such a hybrid
approach offers a promising pathway toward developing a bio-based
<figure><img src="image_28.png" /><figcaption>Fig. 26. Crack opening displacement profiles for PNC-bio-based specimens, (a): PNC-Hemp, (b): PNC-Reed, (c): PNC-Bamboo.</figcaption></figure>

<table><caption>**Table 8**

Consolidated summary of flexural performance, ductility, and interfacial behavior for the 9 bio-based concrete configurations.</caption><thead><tr><th>Binder</th><th>Bio-aggregate</th><th>E<sub>f</sub> (MPa)</th><th>Mid-span deflection at E<sub>f</sub> (mm)</th><th>Global rigidity</th><th>Ductility near E<sub>f</sub></th><th>Binder-bio-aggregate interface &amp; DIC-based synthesis</th></tr></thead><tbody><tr><td rowspan="3">CEMIV</td><td>Hemp</td><td>≈ 44</td><td>2.6</td><td>Very high</td><td>High (wide linear zone)</td><td>Very strong hemp-CEMIV adhesion; highly efficient stress transfer</td></tr><tr><td>Reed</td><td>≈ 36</td><td>1.7</td><td>High</td><td>Moderate to good</td><td>Good reed-CEMIV compatibility; effective role of pozzolans with fine reed aggregates</td></tr><tr><td>Bamboo</td><td>≈ 7</td><td>0.5</td><td>Very low</td><td>Low (rather brittle behavior at rigidity peak)</td><td>Poor stress transfer; weak bamboo-CEMIV interface</td></tr><tr><td rowspan="3">LC3</td><td>Hemp</td><td>≈ 33</td><td>1.6</td><td>Intermediate</td><td>Moderate</td><td>Unfavorable hemp-LC3 interface due to chemical effect of aluminium on disrupting hydration kinetics</td></tr><tr><td>Reed</td><td>≈ 27</td><td>1.5</td><td>Intermediate</td><td>Moderate</td><td>Stability in deflection and strong bond interphase with LC3 paste</td></tr><tr><td>Bamboo</td><td>≈ 7</td><td>1.5</td><td>Very low</td><td>Highest among bamboo concretes (at rigidity peak)</td><td>higher deflection and better energy dissipation</td></tr><tr><td rowspan="3">PNC</td><td>Hemp</td><td>≈ 29</td><td>1.9</td><td>Intermediate (highest among PNC-based concretes)</td><td>Moderate to good</td><td>Modertae deflection in linear zone and strong interphase bond</td></tr><tr><td>Reed</td><td>≈ 9</td><td>2.5</td><td>Low</td><td>High (large deflection at rigidity peak)</td><td>High ductility; relatively weak interfacial bonding with PNC.</td></tr><tr><td>Bamboo</td><td>≈ 5</td><td>0.5</td><td>Very low (lowest of all)</td><td>Low (limited linear zone)</td><td>Early PNC-bamboo interfacial debonding due to strain localization</td></tr></tbody></table>

concrete configuration that is both mechanically performant and structurally stable, laying the groundwork for its reliable integration in reinforced composite systems such as fabric reinforced cementitious matrix for potential construction applications. Bamboo concrete requires a more tailored formulation at large scale specimens due to the weak bond interphase with the mineral binders.

### CRediT authorship contribution statement

Rafik Bardouh: Conceptualization, Methodology, Investigation,

Formal analysis, Software, Data curation, Writing – original draft. **Evelyne Toussaint:** Methodology, Writing – review & editing, Validation, Supervision, Methodology, Investigation, Software. **Sofiane Amziane:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Formal analysis. **Sandrine Marceau:** Visualization, Supervision, Project administration, Funding acquisition, Writing – review & editing.
# 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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