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Article

Effect of Natural Weathering on the Mechanical Strength of Bamboo Bio-Concrete

by
Vanessa Maria Andreola
1,
Nicole Pagan Hasparyk
2 and
Romildo Dias Toledo Filho
1,*
1
Department of Civil Engineering, COPPE, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro 21941-972, RJ, Brazil
2
Eletrobras FURNAS, Goiânia 74993-600, GO, Brazil
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(11), 3629; https://doi.org/10.3390/buildings14113629
Submission received: 24 October 2024 / Revised: 9 November 2024 / Accepted: 13 November 2024 / Published: 14 November 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Versions Notes

Abstract

The search for solutions that reduce the environmental impact of construction has driven the development of new materials. Bio-concrete represents a significant advance, presenting itself as an alternative to traditional concrete. Recent studies point to durability in outdoor conditions as one of the main challenges in its application. This paper presents natural durability studies performed on bamboo bio-concrete, produced with a bamboo particle volume of 50%. A surface treatment of applying resin externally was tested to reduce water ingress during weathering. The bio-concretes were exposed to natural and outdoor weather conditions for twelve months, and meteorological records were collected during the study period. The effect and influence of the external resin was investigated using visual surface analysis, uniaxial compression, modulus of elasticity and scanning electron microscopy. In terms of visual aspects, the resin was not effective in preventing loss of gloss, while in terms of microstructure, these samples showed better adhesion between the bamboo particles in the matrix. The compressive strength showed significant reductions of 60% (stress) and 73% (Young’s modulus) after twelve months of weathering. External resin could improve microstructures from surfaces to internal portions and more effectively preserve the mechanical strength of bio-concrete.

1. Introduction

The construction sector is one of the largest consumers of energy on the planet, accounting for around 30% of total consumption [1]. The production of conventional materials and the operation of buildings contribute significantly to greenhouse gas emissions [2,3,4,5,6,7]. In this context, the use of plant waste in the production of new building materials has emerged as a promising alternative for reducing the environmental impact of the sector, providing materials with better thermal performance and lower energy consumption [1,3,4,5,6,7,8,9,10]. These materials have the potential to sequester more greenhouse gases than they emit during their life cycle, making carbon negative [3,4].
In recent decades, bio-based cement concretes (BBCCs) have become popular in the construction industry, mostly because they are materials produced from renewable raw materials and have the potential to substitute conventional building materials [1,2,3,4,5,6,7]. BBCCs are composite materials made by a mixture of a cement-based or lime-based matrix with vegetable waste (bio-aggregates), chemical additives and water [5,8,9,10,11]. Different types of bio-aggregates have been used in production, such as peach shells, apricot shells [12], corn stalks [13], sugarcane bagasse [14,15], wood shavings [8,10,16], bamboo waste [1,16,17,18,19], hemp [5,11,20,21], rice husks [10], etc. BBCCs are produced with the incorporation of high volumes of bio-aggregates, usually above 40% [6,8,13]. The porosity and lightness of bio-aggregates contribute to improving the thermal and physical properties of bio-concrete, providing a material with lower density and greater thermal insulation capacity [2,4,16].
In previous studies, Andreola et al. [2] have pointed out that, in the Brazilian context, the use of bamboo is driven by the country’s biodiversity, which includes different native species, as well as the plant’s rapid growth cycle [22,23]. However, the industrial processing of the culm generates a considerable amount of waste, mainly due to the selection criteria to produce laminated pieces [24]. Cutting and leveling to obtain uniform pieces results in losses that can vary from 64% to 84% of the initial material, depending on the species and the product [25].
Currently, research on BBCC (also referred to as “bio-concrete”) mainly focuses on physical (density, porosity) and microstructural properties [12,13,21,22,23,24,25,26,27,28], mechanical properties (compressive and flexural strength) [8,12,13,14] and thermal and hygroscopic properties (moisture buffer value, thermal conductivity, water absorption by capillarity) [1,8,13,21]. In addition, other studies on durability mechanisms are also carried out using accelerated aging methods in controlled laboratory environments [11,13,27,29]. Analyses are reported by biological testing [13,19,21,29] and recording reactions to fire [1], salt exposure [9,11,28], freeze–thawing [9,12,28] or immersion weathering [14,26,27].
However, when accelerated aging is carried out in controlled environments (using climate chambers), no matter how modern the equipment, there are limitations in the studies. The exposure period is reduced to intervals that do not correspond to the actual conditions in which the materials are used. Authors use shorter cycles, such as Arizzi et al. [11], who reduced one year of real weathering to twelve days of weathering in a climate chamber. Marceau et al. [27] and Viel et al. [29] kept samples of hemp bio-concrete for three months in a climate chamber, using constant temperature and relative humidity conditions, which does not correspond to the reality.
One of the challenges of using BBCC is its durability in outdoor environments. Bio-concretes are highly porous [5,9,26,28], with a structure composed of voids [14,21], and can show changes in their properties along time, depending on their conditions of use [26,30]. Furthermore, bio-concretes are bio-based material and are more susceptible to humidity and biological degradation [19]. Once the organic components of BBCC are degraded, they can compromise the structural integrity and longevity of the material [7].
The analysis of durability through outdoor weathering, despite being a process that requires long periods of time, provides information on the material’s behavior under real exposure conditions. So far, few studies have reported on the behavior of BBCC exposed to external weathering. Shea et al. [20] analyzed a building produced with hemp bio-concrete and observed good hygrothermal behavior, which reduced humidity levels inside the building. Piot et al. [31] analyzed two bio-concrete walls produced with hemp blocks and a lime-based coating of the internal wall and a lime and cement-based coating of the external wall. The authors highlighted the importance of the choice of coating, since a difference in moisture absorption between them was detected during external weathering. Sheridan et al. [26] analyzed the performance of hemp and rapeseed straw bio-concretes, and no significant difference was observed between the weathered and un-weathered samples. Ahmad et al. [9] investigated the natural durability of bio-concretes made from corn stalks and observed a loss of mass and mechanical strength in all weathered samples.
The effects of outdoor weathering are most frequently analyzed in lignocellulosic materials such as wood [32,33,34]. Contact with radiation and humidity (exposure to water) causes fissuring and changes in appearance and color, with sunlight being the main factor causing the changes [35,36,37]. In this context, the chemical components, such as cellulose, hemicellulose, lignin and extractives, are sensitive to natural sunlight, and lignin absorbs ultraviolet radiation more easily (UV) [32,33,36]. Photodegradation occurs mainly by UV light, chemically altering the lignin and causing superficial changes in the material to a depth of 0.5–2.5 mm, changing the color of the wood [34,36,37].
As explained, few studies have investigated the durability of BBCC, and none have studied the outdoor weathering of bio-concrete produced with bamboo waste. In this context, this article analyzed the effects of outdoor weathering on bamboo bio-concrete produced with fine particle waste from a Brazilian company. Bio-concretes were then produced with bamboo volumes of 50%, and analyses were carried out on natural samples and on samples treated with external resin. The aim was to investigate natural durability and the influence of the climate in the city of Rio de Janeiro (Brazil). The influence of weathering over nine and twelve months was reported through visual analysis by photographs, mechanical strength tests and microscopic analyses. The study presents a significant innovation in addressing the effects of external weathering on bamboo bio-concrete.

2. Materials and Methods

2.1. Materials

This study used waste culms of the species Dendrocalamus Asper, Phyllostachys aurea and Phyllostachys pubescen with different dimensions varying from 700 to 1200 mm, obtained in the municipality of Petrópolis (RJ, Brazil). To obtain bamboo particles (BPs), the culms were sequentially processed using a table saw, an industrial crusher, a knife mill and a mechanical agitator. The first piece of equipment reduced the waste to sizes between 100 and 400 mm. The second crushed the waste into sizes between 5 and 20 mm. The third piece of equipment produced smaller particles, and after mechanical agitation for 10 min, the bamboo particles that passed through a 4.00 mm sieve were used (see Figure 1a).
As pointed out by Andreola et al. [2] and Wu et al. [12], the extractives present in bio-aggregates have a negative effect on cement hardening, but washing BPs in water (as a treatment) reduces the inhibition or retardation. Therefore, the BPs were previously washed in hot water of 80 °C for 1 h and then were air-dried for 72 h, which was enough time to reach the equilibrium moisture content of 14%. Subsequently, the water absorption of the BPs was quantified, considering the mixing time required to produce the bio-concrete, using the technique suggested by Da Gloria et al. [10]. The apparent density and moisture content tests were determined with Brazilian and ISO standards [38,39]. The apparent density, water absorption and moisture content obtained were 590 kg/m3, 81.1% and 10.2%, respectively. These results are in accordance with those in the literature, where bio-aggregates are characterized by high water absorption due to their characteristic porous structure [8,9,12,16]. This is evident in illustrations of the surface and internal morphology of BPs observed under SEM (Figure 1b,c), showing that BPs possess a highly porous structure and interconnectivity of pores.
A high early strength Brazilian Portland cement (PC)—CP V-ARI type—from Lafarge-Holcim (Rio de Janeiro, Brazil), metakaolin (Mk) supplied by Metacaulim do Brasil (Jundiaí, Brazil) and fly ash (FA) from Pozo Fly Comércio de Cinzas Lima LTDA (Capivari de Baixo, Brazil), were used to produce the bio-concretes. The density and chemical composition are shown in Table 1.

2.2. Bamboo Bio-Concrete (BBC) Production and Surface Treatment

Bamboo bio-concretes were produced with a matrix composed of 30% PC, 30% Mk and 40% FA, based on the good results of previous works [18]. A volumetric fraction of BPs of 50% was used, and the water-to-binder ratio was set at 0.30, based on previous studies [8,19]. As suggested by da Gloria et al. [10], in addition to the water of hydration (Wh), an additional water of compensation (Wc) that was based on the absorption capacity of the BPs was also incorporated. Following previous studies by Caldas et al. [17], to minimize the possible inhibitory effect of the BPs and accelerate cement hydration, 2% calcium chloride (CaCl2) was used in relation to the mass of the cementitious materials. In addition, superplasticizer (Sp) and viscosity modifying agent (VMA) were added to reduce the amount of water in the mix, control exudation and prevent segregation. The additives CaCl2, Sp and VMA were incorporated at contents of 2%, 1% and 0.1% in relation to the mass of the cementitious materials, respectively. The proportion of the mixtures produced is summarized in Table 2.
The BBCs were produced in a 20 L mixer using two speeds of 125 and 250 rpm. Initially, the cementitious materials were homogenized with the BPs for 1 min. The CaCl2 and Sp were previously dispersed in the Wh and Wc, respectively. Subsequently, both waters were then gradually added to the mixture (2 min). The mixer then was paused to remove excess material from the sides and increase the speed (250 rpm). After this, the VMA was incorporated, and the mixture was homogenized until the 8th minute.
In the fresh state, the workability test was carried out using the flow table test, according to the National Brazilian Standard [40]. In the fresh state, the workability of BBC was tested four times.
The specimens were cast in prismatic molds with dimensions of 400 × 100 × 100 mm3 (length × width × thickness) and cylindrical molds (diameter 75 mm–height 100 mm) in three layers. Each layer was mechanically vibrated on a vibratory table (68 Hz) for 10 s. After casting, the bio-concretes were kept in the molds and protected against moisture loss until they were demolded after 24 h. Afterwards, the cylindrical samples were cut at the ends. All the prismatic samples had their tops faced. One of the prisms was reduced to 100 × 100 × 100 mm3 (length × width × thickness) using a circular saw. This sample was cut to analyze visual images using photographs. All the samples were kept in a conditioned chamber (21 ± 2 °C; 55 ± 5%) for 28 days.
After curing was complete, the specimens were subjected to a surface treatment. The aim was to test the efficiency of an external coating and reduce water entry during natural aging tests. This surface treatment, named external resin (ER), was applied with a brush (Figure 2) in three layers with a four-hour interval between applications. The resin had a colorless finish and low volatile emissions, and the total consumption for each BBC was 554.6 mL/m2. After application, the samples with ER had a bright appearance, with a more intense and saturated color compared to the bio-concrete without resin. The bio-concrete with external resin (BBC-ER) was compared with natural bio-concrete, i.e., without external resin, and considered a reference (BBC-WR).

2.3. Natural Durability Test Location and Weather Analysis

The bio-concrete samples were exposed to outdoor and natural weathering in the municipality of Rio de Janeiro (Brazil) for 12 months (December 2019 to December 2020). The test location was a laboratory rooftop at Cidade Universitaria, Federal University of Rio de Janeiro on Ilha do Fundão (NUMATS/COPPE/UFRJ), with coordinates 22°51′40.514″ S, 43°13′48.994″ W. The roof structure of the laboratory is made of concrete and coated with an aluminized blanket. As the blanket reflects the sunlight and can increase the temperature levels in the samples, a 30 mm thick expanded polystyrene board was used to provide insulation (see Figure 3, the expanded polystyrene above the aluminized blanket). The samples were placed on polystyrene with a spacing of approximately 50 mm between them, allowing sunlight and air circulation on all sides.
The state of Rio de Janeiro is located in the southeast region of Brazil, and the climate of the municipality of Rio de Janeiro is described as tropical hot and humid, with short rainfalls in the summer and a dry winter, with local variations due to the presence of vegetation, altitude and proximity to the ocean [41]. The climatic conditions of the municipality during the period studied were analyzed based on data obtained from the Instituto Nacional de Meteorologia (INMET). The analysis is fundamental to understanding the performance of exposed bamboo bio-concrete in the climatic conditions of the municipality of Rio de Janeiro. The data allow the results and conclusions to be contextualized in relation to the actual conditions in which the bio-concrete is used.

2.4. Testing Methods

2.4.1. Visual Surface Analysis

Visual changes during outdoor weathering were observed through visual analysis using photographs. The change in appearance and color was analyzed using the photographed surface. To compare the differences, photographs of the same sample were taken on different dates. This method was used in previous studies [18,19] with the photographic recording carried out on a fixed experimental bench (Figure 4). The bench was configured to receive a glass container to position the sample in its center. Two LED spotlights were used on the sides and a third ceiling lamp was fixed 80 cm above the camera. The configuration of the bench and the camera were identical (D3400, 24.2 megapixels, Nikon, Tokyo, Japan) for all ages photographed. The first image of the bio-concrete (BBC-Control) was taken before outdoor weathering. After 9 and 12 months of weathering, the BBC was removed from the roof of the laboratory, photographed and immediately replaced on the roof. The same sample measuring 100 × 100 × 100 mm3 (length × width × thickness) was used for each condition (BBC-WR and BBC-ER).

2.4.2. Scanning Electron Microscopy

Analyses by scanning electron microscopy (SEM) were carried out to investigate the degradation process during outdoor weathering. To prepare the samples for SEM, the bio-concrete with dimensions of 400 × 100 × 100 mm3 (length × width × thickness) was removed from the natural weathering location and reduced to the appropriate dimension of 400 × 50 × 20 mm3 (length × width × thickness) using a circular saw (Figure 5). After that, the samples were cut in half and divided into two types: cut with a saw, resulting in a plain surface (Figure 5a), and broken manually, resulting in a fractured surface (Figure 5b). The two surfaces were analyzed to understand how different preparation methods can influence the observation and interpretation of the results. Both were kept in a room (21 °C; 55% RH) for 72 h and then examined by SEM (VEGA3, Tescan, Brno, Czechia) with secondary electrons and backscattered electrons (SE-BSE). SEM was performed with the following parameters: 30 kV acceleration voltages, 10 nm beam current intensity and a 15 mm working distance.

2.4.3. Uniaxial Compressive Test

The uniaxial compressive strength test was carried out on five cylindrical specimens (diameter 75 mm and height 150 mm) after 28 days on the control sample and after 9 and 12 months on outdoor weathered samples. The test was measured using a Shimadzu-1000 kN universal testing machine (Shimadzu, Kyoto, Japan) at a speed of 0.3 mm/min. During the test, two linear variation displacement transducers (LVDTs), diametrically opposed, measured the axial displacement over a gage length of 75 mm at the mid-height of the specimen, in order to determine Young’s modulus. The compressive strength and the modulus of elasticity were performed according to the Brazilian standards NBR 5739 [42] and NBR 8522 [43], respectively.

3. Experimental Results

The following abbreviations were used in the presentation of the results:
  • BBC-Control: BBC control, analyzed after curing for 28 days (21 ± 2 °C; RH 55 ± 5%).
  • BBC-WR: BBC without external resin, analyzed after 9 and 12 months of weathering.
  • BBC-ER: BBC with external resin, analyzed after 9 and 12 months of outdoor weathering.

3.1. Weather Variations

Figure 6 shows the recorded annual meteorological variations during the outdoor weathering period studied. The temperature ranged from 15.4 °C to 37.4 °C and RH ranged from 26% to 100%, while rainfall and maximum radiation were 23.8 mm and 4200 kj/m2, respectively. The lowest temperature levels were in July, August and September. In all months of the year, temperature gradients were observed throughout. The month with the most stable temperature was January, while the lowest RH values were observed in May, June and July. The lowest precipitation gradients were in April and June, while the highest rainfall volumes were recorded in February, September and October. The highest solar radiation levels were in February and December, while the lowest were between April and July. The analysis showed that the weather fluctuated mainly during the summer, which was the most critical season of the year [18]. During the seasons, the bio-concrete remained in conditions of constant weather variations, which occurred at short intervals, with intense rainfall and summer conditions of high temperatures.

3.2. Workability

Figure 7 shows the spreading obtained for the fresh BBB-50 mix, and the consistency index obtained was 195 mm. Despite the high volume of BPs (50%), the mixture was homogeneous and allowed easy molding. This consistency index is compatible with the normative standard defined for mortars with good workability, as well as being an indication of the good moldability of bio-concrete [1,4,10].
Previous studies [10] showed that the consistency indices of bamboo, wood and rice husk bio-concrete ranged from 260 mm to 290 mm. Compared to the bio-concrete in this article (195 mm), the increased spreading obtained in the study by Andreola et al. [2] and Gloria et al. [10] can be attributed to the cement matrix, which had no pozzolanic additions.

3.3. Visual Surface Analysis

Photographs of the bio-concrete without resin (BBC-WR) and with resin (BBC-ER) are shown in Figure 8 and Figure 9, respectively, and show the aggressive effects of outdoor weathering. On the surface of the BBC-WR/Control (Figure 8a), it is possible to notice the vibrant shades of the image, with a predominance of yellow. This was also observed on the BBC-ER/Control (Figure 9a), with a darker and brighter shade of yellow, as an effect of the external resin. In the bio-concretes without external resin, after 9 months (Figure 8b) and 12 months (Figure 8c) of weathering, the vibrance of the shades decreased and the predominant shade was brown. This change in the appearance was also observed for the BBC-ER (Figure 9b,c), showing that the three layers of external coating were not effective and did not avoid the glow loss in the samples.
The bamboo particles of the control samples (white circles in Figure 8a and Figure 9a) had a light-yellow color. As the weathering time passed, the color of the BP showed an even lighter shade, showing an aged and faded appearance (red circles, Figure 8b,c and Figure 9b,c). Additionally, when comparing the BBC-Control with the other samples, it can be seen that the pre-existing pores (white squares, Figure 8a and Figure 9a) became larger (red squares, Figure 8b,c and Figure 9b,c), due to the cleaning effect (rain cycles) on the sample surface, which visibly removed a thin layer of cementitious paste, leaving the BPs more exposed and visible. Consequently, all the weathered samples showed a surface with an old and discolored appearance. This indicates that outdoor weathering and meteorological variations (Figure 6) washed the surface (rain cycles) and, simultaneously, exposure to radiation intensified the aging of the bamboo particles (action of the sun).
In the literature, although the phenomenon of photodegradation has only been described for natural wood and the surface changes (depth of 0.5–2.5 mm) observed in weathered material [32,33,34,35,36,37], it was decided to analyze this information for bamboo bio-concrete exposed to outdoor weathering (BBC-WR/12m). Figure 10 shows that when a ±2 mm deep blade is removed from the surface, the appearance of the bio-concrete returns with an appearance similar to that of the outdoor weathered surface (see Figure 8a). The external appearance was aged and faded (Figure 10a), as opposed to the internal appearance, which showed vibrant shades (Figure 10b). This suggests that the bio-concrete was photodegraded during outdoor weathering and, consequently, changes in the appearance and color of the surface were observed.

3.4. Scanning Electron Microscopy (SEM)

SEM images of plain surfaces from BBC-Control and also of the outdoor weathering samples (12 months) can be seen in Figure 11 (WR) and Figure 12 (ER).
The application of resin in the bio-concrete surfaces promoted the better connection of the BPs in the matrix (M), as can be seen in the control samples’ (Figure 12) micrographs, and it afforded some protection from moisture entry as well as from the degradation of fibers in the transition zone (TZ) with the cement matrix (arrows). Those characteristics can be observed from the SEM images taken from bio-concrete plain surfaces. BBC-WR featured some weakness in the TZ after outdoor weathering (Figure 11), while BBC-ER exhibited better adhesion between bamboo particles and the cement matrix. Thus, the overall BBC-ER microstructure was more preserved in the same period due to resin protection. The effect of surface protection with resin promoted the conservation of fibers attached to cement paste, thus enhancing the internal portion of bio-concretes as well as leading to a denser matrix (Figure 12c). Cement paste involves BPs, as can be seen in the images. On the other hand, bamboo particles present in BBC-WR weathering (Figure 11c) lost their adherence, and several BPs detached from the paste in the TZ.

3.5. Uniaxial Compressive Strength

The stress–strain curves are illustrated in Figure 13, while the compressive strength values and Young’s modulus are summarized in Table 3. According to the results, a decrease in strength and stiffness was observed after outdoor weathering. The influence of the resin was observed in the BBC-ER, as the samples showed the lowest decreases in performance when compared to BBC-WR.
The stress–strain curve (Figure 13) showed that the BBC-Control had a strength of 8.06 MPa and initial elastic linear behavior until 30% of the maximum stress was reached. The increasing behavior of the curve was observed up to approximately 50,000 με, and the test was ended as it reached the reading limit of the LVDTs. Consequently, the post-peak was characterized by an increase in stress, without any reduction in deformation. In the literature, bio-concretes with a higher volume of bio-aggregates (≥50%) and lower mechanical strength are characterized by continuous tension after the linear phase [6,9,12,13] because the bio-aggregate has a porous structure and lower density than cementitious materials [28], characterizing the ductile behavior of the bio-concrete [26].
Comparing the stress–strain curves and Young’s modulus (E) of BBC-Control with BBC-WR, there was a significant decrease in mechanical performance after weathering. After nine months, there was a 54% reduction in stress and a 69% reduction in Young’s modulus. However, more significant decreases were observed after twelve months of weathering, with reductions of 60% (stress) and 73% (E). In other words, extending external weathering by three months (from 9 to 12 months) reduced stress by a further 9% and Young’s modulus by 5%. In the case of BBC-Control with BBC-ER, decreases were also observed to a slightly lesser degree than in BBC-WR. After 9 and 12 months of weathering, there was a 45% and 52% decrease in stress and a 41% and 46% decrease in Young’s modulus, respectively, for each age. This behavior shows that the BBC-ER, although reduced in performance, had 16% greater stress and a 50% greater Young’s modulus than the BBC-WR (after 12 months). After weathering, the resin preserved more of the mechanical strength of the bio-concrete.
The variations observed in the meteorological records (Figure 6) can explain the significant decrease in the mechanical performance of bio-concretes. Sheridan et al. [26] and Piot et al. [31] observed that during outdoor weathering, bio-concrete was exposed to natural sunlight and short-term rainfall, modifying the climatic conditions around the exposed location. Ahmad et al. [13] also associated the decrease in resistance with outdoor and natural conditions, in which intense rainfall caused a cleaning of the samples. Regarding the tropical hot and humid climate, Andreola et al. [18] and Zukowski et al. [30] comment that meteorological variations are more accentuated in the summer season, with combinations of macrocycles, microcycles and mesocycles, characterized by abrupt changes in climatic conditions.
Considering the summer season in Rio de Janeiro, while the specimens were exposed to natural sunlight, a fine and rapid rain typical of the season caused temperature and humidity variations on the surface of the bio-concretes. The continuous wetting and drying of the BBCs happened all season long. Thus, while the temperature and RH (of the surface of the samples) adjusted with natural sunlight and rain, the internal conditions of the bio-concretes were slowly modified. In addition, the reduction in mechanical performance was also influenced by microstructural alterations at the fiber–matrix interface, and this phenomenon was intensified over time, as the weathering period increased.

4. Conclusions

This study evaluated the durability of BBC exposed to natural weathering and analyzed the effectiveness of a resin in improving the properties of bio-concretes. A twelve-month experimental analysis was carried out, comparing the different samples to assess the effectiveness of the resin in mitigating the effects of the external environment. The conclusions are as follows:
(i)
The variations in weather conditions showed constant changes in temperature and relative humidity, with changing cycles of precipitation, which caused the surface of the bio-concrete to be cleaned.
(ii)
Outdoor weathering caused visual changes on the surface of BBC, a progressive change in color and the faded appearance of the bamboo particles. The changes were just superficial, and after removing a laminate from the surface, the appearance and color returned to the appearance before aging.
(iii)
The results from compressive strength tests showed that the stress and modulus of elasticity decreased significantly after external weathering for all the bio-concretes. The most significant decreases were observed in the BBC without external resin (WR) compared to bio-concrete with resin (ER).
(iv)
In the BBC-WR samples, after nine months of weathering, decreases in stress (up to 54%) and modulus of elasticity (up to 69%) were observed. After twelve months, more significant reductions of 60% (stress) and 73% (E) were registered. For BBC-ER, the decreases in stress and modulus of elasticity after twelve months of weathering were 52% and 46%, respectively.
(v)
The external resin preserved more of the mechanical strength of the bio-concrete but did not prevent visual changes (appearance and color) from being completely preserved. This statement could be verified by microscopic analyses since the integrity of matrix and interface with fibers was relatively well preserved.
(vi)
In general, BBC-ER performed better than BBC-WR since internal integrity was more preserved due to the surface treatment, and besides the transition zone of fibers, the cement matrix also showed improvements compared to BBC-WR.
(vii)
Bamboo bio-concretes have great potential to be used in construction, depending on the environment. Due to their mechanical characteristics, BBCs can be used as internal partitions, false ceilings, retrofitting or open elements such as hollow brick walls. In the case of external exposure to outdoor weathering, some treatments are necessary, and further studies should be carried out on additional surface protections to recommend their use for external applications.

5. Study Limitations and Future Research

One limitation of this study was the short analysis period of one year, which meant that it was not possible to compare the influence of weathering in different seasons, such as between two summers. Another limitation of the study was the exclusive use of a single type of bio-aggregate and a single external resin.
Recent studies [8] show the potential of silane–siloxane as an additive in bio-concrete, acting as an external coating or as a component of the mix, with the aim of minimizing water permeability and optimizing the properties of the material. Based on the study and for future research, the authors suggest carrying out compressive strength tests and scanning electron microscopy on bamboo bio-concrete samples subjected to natural exposure for two or three years. They also suggest applying new surface treatments and conducting comparative studies between different approaches.

Author Contributions

Conceptualization, V.M.A.; Methodology, V.M.A.; Software, V.M.A.; Validation, V.M.A. and R.D.T.F.; Formal analysis, V.M.A.; Investigation, V.M.A.; Resources, V.M.A.; Data curation, V.M.A.; Writing—original draft, V.M.A.; Writing—review & editing, V.M.A., N.P.H. and R.D.T.F.; Visualization, V.M.A. and N.P.H.; Supervision, R.D.T.F.; Project administration, V.M.A. and R.D.T.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Conselho Nacional de Desenvolvimento Científico e Tecnológico—Brasil—(CNPq).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to express their gratitude to Conselho Nacional de Desenvolvimento Científico e Tecnológico—Brasil—(CNPq).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) BPs used; (b) surface and (c) internal morphology observed under SEM.
Figure 1. (a) BPs used; (b) surface and (c) internal morphology observed under SEM.
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Figure 2. Surface treatment of bio-concrete with external resin.
Figure 2. Surface treatment of bio-concrete with external resin.
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Figure 3. Distribution of samples on top of expanded polystyrene.
Figure 3. Distribution of samples on top of expanded polystyrene.
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Figure 4. Photo recording on experimental bench: (1) LED spotlights; (2) camera; (3) sample; (4) glass guide; (5) fixed bench.
Figure 4. Photo recording on experimental bench: (1) LED spotlights; (2) camera; (3) sample; (4) glass guide; (5) fixed bench.
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Figure 5. SEM analysis: (a) plain surface, (b) fractured surface.
Figure 5. SEM analysis: (a) plain surface, (b) fractured surface.
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Figure 6. Registration of annual meteorological variations during the outdoor exposure period: temperature (°C), RH (%), rainfall (mm) and radiation (kJ/m2).
Figure 6. Registration of annual meteorological variations during the outdoor exposure period: temperature (°C), RH (%), rainfall (mm) and radiation (kJ/m2).
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Figure 7. Spreading and consistency.
Figure 7. Spreading and consistency.
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Figure 8. Visual analysis of BBC without external resin (WR). (a) BBC-WR/Control (28 days); (b) BBC-WR/9m; (c) BBC-WR/12m.
Figure 8. Visual analysis of BBC without external resin (WR). (a) BBC-WR/Control (28 days); (b) BBC-WR/9m; (c) BBC-WR/12m.
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Figure 9. Visual analysis of BBC with external resin (ER). (a) BBC-ER/Control (28 days); (b) BBC-ER/9m; (c) BBC-ER/12m.
Figure 9. Visual analysis of BBC with external resin (ER). (a) BBC-ER/Control (28 days); (b) BBC-ER/9m; (c) BBC-ER/12m.
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Figure 10. Visual surface analysis (BBC-WR/12m). (a) External surface; (b) internal surface, by Andreola et al. [18].
Figure 10. Visual surface analysis (BBC-WR/12m). (a) External surface; (b) internal surface, by Andreola et al. [18].
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Figure 11. SEM of BBC without external resin (WR). (a) BBC-WR/Control (28 days) plain surface; (b) BBC-WR/12m plain surface; (c) BBC-WR/12m fractured surface.
Figure 11. SEM of BBC without external resin (WR). (a) BBC-WR/Control (28 days) plain surface; (b) BBC-WR/12m plain surface; (c) BBC-WR/12m fractured surface.
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Figure 12. SEM of BBC with external resin (ER). (a) BBC-ER/Control (28 days) plain surface; (b) BBC-ER/12m plain surface; (c) BBC-ER/12m fractured surface.
Figure 12. SEM of BBC with external resin (ER). (a) BBC-ER/Control (28 days) plain surface; (b) BBC-ER/12m plain surface; (c) BBC-ER/12m fractured surface.
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Figure 13. Compressive stress–strain curves.
Figure 13. Compressive stress–strain curves.
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Table 1. Chemical properties and density of cementitious materials.
Table 1. Chemical properties and density of cementitious materials.
Chemical Elements (%)PCMkFA
CaO72.63-2.29
SiO213.2250.9550.96
Al2O34.90442.2234.19
Fe2O33.8761.9825.33
SO33.3551.2021.59
K2O1.0751.9833.52
SrO0.4840.0040.024
TiO20.298-1.24
MnO0.1460.0090.046
Density (kg/m3)305328101885
Table 2. Materials consumption in kg/m3.
Table 2. Materials consumption in kg/m3.
BPPCMkFAWhWcCaCl2SpVMA
BBC-50295182.1182.1242.7182.11239.2715.937.960.79
Table 3. Compressive strength and modulus of elasticity (variation coefficient in brackets).
Table 3. Compressive strength and modulus of elasticity (variation coefficient in brackets).
Compressive Strength (MPa)Young Modulus (GPa)
BBC-Control8.06 (2.92)1.04 (4.44)
BBC-WR/9m3.70 (2.99)0.32 (1.96)
BBC-WR/12m3.27 (5.96)0.28 (3.71)
BBC-ER/9m4.40 (1.99)0.61 (3.73)
BBC-ER/12m3.90 (4.32)0.56 (4.61)
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MDPI and ACS Style

Andreola, V.M.; Hasparyk, N.P.; Toledo Filho, R.D. Effect of Natural Weathering on the Mechanical Strength of Bamboo Bio-Concrete. Buildings 2024, 14, 3629. https://doi.org/10.3390/buildings14113629

AMA Style

Andreola VM, Hasparyk NP, Toledo Filho RD. Effect of Natural Weathering on the Mechanical Strength of Bamboo Bio-Concrete. Buildings. 2024; 14(11):3629. https://doi.org/10.3390/buildings14113629

Chicago/Turabian Style

Andreola, Vanessa Maria, Nicole Pagan Hasparyk, and Romildo Dias Toledo Filho. 2024. "Effect of Natural Weathering on the Mechanical Strength of Bamboo Bio-Concrete" Buildings 14, no. 11: 3629. https://doi.org/10.3390/buildings14113629

APA Style

Andreola, V. M., Hasparyk, N. P., & Toledo Filho, R. D. (2024). Effect of Natural Weathering on the Mechanical Strength of Bamboo Bio-Concrete. Buildings, 14(11), 3629. https://doi.org/10.3390/buildings14113629

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