Utilizing Cob–Earth and Sand-Filled Plastic Bottles to Address Environmental Challenges: A Sustainable Construction Solution

Abstract

The construction industry is a major contributor to global carbon emissions, primarily due to its reliance on cement-based materials. Simultaneously, plastic bottle waste presents a significant environmental challenge. This study aims to address both issues by exploring the integration of plastic bottle waste into cob–earth materials as a sustainable alternative to traditional concrete modules. The research involves testing various mixes with plastic bottles arranged in different patterns to assess their load-bearing capacity and distribution. The cob mix with bottles arranged in a modified pattern demonstrated the highest load resistance, bearing over 47.1 kN, making it suitable for prototype development. The study also investigates the potential of using cob as an exterior finishing layer, reducing the need for cement. The results show that using local earth materials significantly lowers embodied carbon, offering a more sustainable construction solution. This approach helps mitigate plastic waste and supports climate resilience by promoting low-carbon, locally sourced materials, aligning with Egypt’s national sustainability commitments.

1. Introduction

The regeneration of plastic bottle waste significantly contributes to the global solid waste challenge, while the reliance on cementitious structures and red bricks in construction continues to play a major role in carbon emissions [1]. According to the World Green Building Council, buildings are responsible for 39% of global energy-related carbon emissions, with 28% attributed to operational emissions (such as heating and cooling) and 11% to materials and construction. “Upfront carbon,” which is the carbon released before the asset is used, is expected to account for half of the carbon footprint of new construction between 2020 and 2050 [2]. Utilizing local materials and repurposing local waste can help advance more sustainable construction practices and reduce this environmental impact. Therefore, exploring solutions that address both plastic bottle waste and harmful construction practices is critical in responding to the urgency of climate change. This research aims to tackle these two challenges by investigating the use of plastic bottle waste in construction, exploring multiple applications. The methodology involves systematically testing different material samples, evaluating alternatives, and incorporating successful findings into a larger prototype to meet the research objectives. This study is intended as a proof of concept, focusing on demonstrating the feasibility of using sand-filled plastic bottles and cob as a construction system. While the research was conducted in Egypt, the materials, methods, and applications used are not exclusive to the local context and reflect internationally accessible practices.

2. Literature Review

The construction industry is a major contributor to global carbon emissions and waste generation, prompting extensive research into sustainable alternatives. One approach involves repurposing plastic waste, particularly polyethylene terephthalate (PET) bottles, as construction materials to mitigate environmental impact. Studies have explored various methods of integrating plastic bottles into building elements, including filled and unfilled bottles, to assess their structural and environmental viability.

Mansour and Ali (2015) investigated the structural and thermal properties of PET bottles filled with dry sand, saturated sand, or air, bound by cement mortar to form masonry walls [3]. Their findings indicated that while the compressive strength of these plastic bottle masonry blocks was lower than that of traditional concrete blocks, their enhanced thermal insulation properties made them suitable for non-load-bearing walls. Similarly, Muyen et al. (2016) assessed the strength properties of plastic bottle bricks in Bangladesh, reporting that bricks constructed with nine PET bottles filled with sand achieved a compressive strength of 35 MPa, while those containing twelve bottles reached 33.7 MPa [4]. These values significantly surpassed those of conventional concrete cylinders, suggesting the feasibility of plastic bottle bricks for construction in regions grappling with waste management challenges in addition to being more cost effective.

Wahid et al. (2015) examined the incorporation of crushed plastic bottle waste into sand bricks, with plastic content ranging from 0% to 15% [5]. They found that higher plastic content reduced compressive strength, with the 0% plastic bricks exhibiting the highest strength. However, the addition of plastic improved water absorption characteristics, indicating potential applications where water resistance is essential. Meanwhile, Safinia and Alkalbani (2016) explored the use of recycled plastic water bottles to create voids in concrete blocks [6]. Their results demonstrated that while the inclusion of plastic bottles reduced the overall weight of the blocks, compressive strength decreased compared to conventional hollow blocks, necessitating further optimization before widespread structural use.

Dadzie and Kaliluthin (2020) reviewed the potential of recycling waste plastic bottles for construction as a solution to housing deficits in developing countries [7]. Their findings emphasized that filling plastic bottles with soil or sand to create bricks could provide an environmentally sustainable and cost-effective building alternative. More recently, Haque and Islam (2021) developed plastic bricks using sand-filled PET bottles, achieving strengths of 2.88–3.29 N/mm², offering a sustainable, low-cost solution for durable refugee shelters and putting their usage into context [8]. This study highlighted the potential for integrating plastic waste with earth materials to reduce reliance on high-carbon-footprint materials like cement while addressing plastic waste concerns.

In addition to sand, other filler materials have been tested, such as fly ash, quarry dust, and shredded plastic, offering varying results in terms of compressive strength and thermal performance. Gurumoorthy and Manoharan (2020) investigated the use of PET bottles filled with quarry dust and bound in cement mortar, noting that these composites provided favorable thermal insulation and were suitable for internal partition walls [9].

Siddique et al. (2008) explored the behavior of waste plastic fibers and granules in concrete, indicating significant improvements in crack resistance and post-peak toughness, though compressive strength may decline at high plastic contents [10].

More recent advancements focus on 3D printing with recycled plastics. Zhao et al. (2021) developed a method of 3D printing construction elements using recycled PET-based filaments, highlighting its potential for affordable housing solutions in resource-scarce areas [11].

The literature demonstrates an evolution from cement-based construction utilizing plastic bottles to incorporating them into earth-based materials, aiming to enhance sustainability. Both filled and empty plastic bottles have been assessed in different configurations, showing variations in strength, insulation, and water resistance. While research supports the potential of plastic bottle construction, further investigation is needed to optimize its structural performance, durability, and large-scale implementation. Additionally, despite significant progress in integrating plastic waste into building components, little research has examined the structural potential of PET bottles integrated into cob-based construction. This study seeks to address that gap by demonstrating an affordable, sustainable alternative capable of producing a full-scale prototype.

3. Materials and Methods

The primary focus of this research was to explore sustainable solutions to the growing issues of plastic bottle waste and the heavy reliance on cement in construction. The approach taken in this study (Figure 1) began by investigating the potential of utilizing plastic bottles as construction components, followed by exploring how they can be integrated into concrete modules to reduce cement usage. Finally, the research examined the introduction of earth-based mixtures, aiming to eliminate the overall need for cement, thereby creating lower-embodied-carbon building models. The study emphasizes compressive strength and constructability as key factors, making it essential to also evaluate the performance in various stages of the research. A systematic methodology was employed, progressing from the investigation of plastic bottle utilization to integration with concrete modules, and ultimately to cob-based structures. At each stage, semi-quantitative data from laboratory tests (including compressive strength and handling feasibility) guided material selection and design iterations. The following sections will outline the methodology and findings of the study in detail.

3.1. Plastic Bottles Utilization

The first step in this research was to understand how plastic bottles could be used in the context of construction modules. This included examining the characteristics of the bottles, potential arrangement methods, and binding techniques to guide the experimental process. As the literature has previously demonstrated the feasibility of using the bottles in their original form, without the need for shredding or crushing, this approach was selected. From a commercial standpoint, the available bottle sizes were 1000, 600, and 330 mL (Figure 2). For most of the samples, the 600 mL size was chosen due to its practicality and stability. The PET bottles used follow general manufacturing specifications, which may vary globally and were outside the control of the research team.

With regard to assembling the bottles, trials were made to determine which arrangement strategy provided the most stability to achieve the research objectives (Figure 3). Samples with different bottle quantities and mold sizes were tested, and it was found that placing the bottles in opposite directions primarily resulted in greater resilience. The binding trials included using tape, foam, rope, and glue. These were tested to ensure that if grouping the bottles was needed in any sample, the sturdiest binding method was selected, with tape proving to be the most effective.

3.2. Cement Reduction

Upon establishing familiarity with the newly introduced plastic bottle element, the next step was to integrate it into concrete samples to reduce the amount of concrete used. This approach also capitalized on the thermal properties of plastic bottles and presented an alternative similar to concrete hollow blocks but at a more affordable cost. In this direction, samples were created that integrated plastic bottles within precast concrete elements, including the following:

3.3. Different Sizes of Concrete Blocks

These blocks were intended to replace the conventional red and cementitious bricks commonly used in construction in Egypt. The designed blocks incorporated a variation in the dimensions of empty plastic bottles, all of which were relatively small; each block could typically be held in one hand (Figure 4 and

Table 1. Dimensions of the blocks and bottles.

Block ID	Dimensions (mm)	Bottles Size (mL)
I	350 × 250 × 170	1000
II	250 × 170 × 170	600
III	185 × 160 × 160	330
IV	180 × 160 × 165	330

). This configuration was designed to create voids similar to those found in regular building bricks, ensuring a comparable structural design while utilizing plastic bottles as a sustainable alternative. For each block size, three samples were created, and all samples were cured for one week before testing.

3.4. Precast Unit Using a Wooden Frame

The adoption of larger samples followed in the next phase, with precast units designed to reflect direct usage on-site and to simplify the building process. These larger units were intended to function as independent precast walls wherever possible. To verify the constructability of this concept, a sample with a wooden frame was created (Figure 5). The grouped plastic bottle units were arranged in rows, with mortar and mesh layers placed in between. The complete unit was then mortared, consisting of six rows, and had final dimensions of 1.5 m in height, 1.16 m in length, and 0.3 m in width (Figure 5).

3.5. Precast Unit Within a Concrete Frames

Another unit with larger dimensions was also created, but with some variations. This unit featured plastic bottles encased in a reinforced concrete frame and did not include the mortar and mesh layers between the rows. Instead, a mesh was wrapped around the frame, resulting in less concrete usage. However, it included steel reinforcement, which was not ideal for the objective of the study, as it would increase the overall carbon footprint. The sample dimensions were approximately 1.9 m in height, 1.9 m in length, and 0.25 m in width (Figure 6). The purpose of this unit was to test the feasibility of utilizing reinforced concrete frames in combination with precast plastic bottle units, as well as to assess the structural integrity of the resulting wall.

3.6. Cement Replacement

In an attempt to completely replace cement, cob–earth material was introduced (Figure 7). This mixture, composed of clay, straw, sand, and water, consists of locally sourced earth materials suitable for construction. The addition of lime in some of the samples was also explored, resulting in various versions of the mixtures being tested. The samples included cob mixtures with empty bottles, sand-filled bottles, and minimal percentages of cement. Additionally, different bottle arrangements were tested, including configurations without any cement, but with varying bottle orientations. For the typical clay mixture used as the base for all cob samples, the clay was sieved before being added to the dry materials, and water was gradually incorporated into the mixture, as demonstrated below.

3.7. Cob Samples

The cob samples created were relatively small, with the typical unit measuring 0.5 m in height, 0.5 m in length, and 0.25 m in width (Figure 8). Initially, three cob samples were created using empty bottles. Another three samples were then made with sand-filled bottles, as literature highlighted the structural enhancement provided by sand-filled bottles in cob material. Later, another set of three samples containing small percentages of cement was also tested (

Table 2. Cob and cement mixtures.

Mix I	Mix II
20% Cement	30% Cement
80% Dry Earth Materials	70% Dry Earth Materials

). Each sample measured approximately 0.5 m in height, 0.5 m in length, and 0.25 m in width, and all samples were cured for approximately two weeks before testing.

3.8. Cob Samples Testing Setup

Since the compressive strength of the cob materials was a key factor, a dedicated testing setup was created, as shown in Figure 9. The unit was placed within a compression machine designed for such sizes, with the applied load manually increased until the sample reached failure.

4. Results

4.1. Sample Handling and Compressive Strength

Testing for compressive strength and sample handling was crucial to evaluating the feasibility of the proposed construction methods. Working with samples of various sizes revealed that larger samples were more difficult to handle and move, while smaller samples were more practical.

4.2. Concrete Blocks

The concrete blocks with empty bottles were tested to determine whether they met the IS-3495 minimum compressive strength requirement of 7.5 MPa for building applications. However, the test results (

Table 3. Compressive strength results of the blocks and bottles.

Block ID	Compressive Strength (MPa)
I	2.40
II	3.90
III	4.80
IV	6.80

) ranged from 2.40 MPa to 6.80 MPa, indicating that the blocks did not achieve the necessary strength.

The primary goal of this evaluation was to assess whether incorporating empty plastic bottles within cement mortar could serve as a viable alternative to conventional bricks. Based on the results, these blocks did not meet the required standards, suggesting the need for modifications to improve their compressive strength. Potential improvements include

• Reducing block dimensions to enhance load-bearing capacity
• Using sand-filled plastic bottles instead of empty ones
• Incorporating additives to strengthen the cement mix
• Altering bottle arrangements to optimize structural performance

4.3. Sand-Filled Cob Samples

Given the limitations of the concrete blocks, compressive strength testing shifted to sand-filled cob samples to determine their viability as load-bearing elements.

The results (Figure 10) showed that confined units, where sand-filled plastic bottles were supported laterally, resisted over 1300 kg of load, whereas unconfined units, lacking lateral support, withstood only 650 kg. This indicates that confinement significantly enhances the strength and load-bearing capacity of sand-filled bottle units, making it an essential factor in their structural design.

Compressive strength tests for cob samples with cement additions further demonstrated the impact of material composition on performance. The confined sample with a 30% cement addition exhibited the highest resistance, withstanding 4200 kg, while the confined sample with a 20% cement addition resisted 2800 kg as shown in Figure 11. These findings confirm that increasing cement content improves compressive strength, reinforcing the potential of cob–cement mixtures for construction applications.

As for the cob sample featuring a modified sand-filled bottle arrangement, this sample incorporated a dynamic combination of horizontal and vertical bottle placements to enhance structural integrity. Accordingly, this sample exhibited the highest compressive strength among all tested configurations, resisting loads exceeding 4800 kg (Figure 12 and Figure 13). The strategic bottle arrangement optimized load distribution, making this approach the most promising for future research and practical application in sustainable construction, and eliminating cement usage.

Observations of failure modes included surface cracking of cob material, minor bottle deformation, and in some unconfined cases, delamination at the cob–bottle interface. No PET bottle rupture occurred. These behaviors were more pronounced in unconfined samples, reaffirming the structural benefit of lateral confinement.

4.4. Embodied Carbon

Assessing the embodied carbon of different material compositions was a critical step in evaluating their environmental performance within the broader context of sustainable construction. The analysis looked at a cob mix that included a special pattern of plastic bottles, which showed better strength, compared to a regular cob mix that used cement for stability.

The embodied carbon of each composition was qualitatively examined based on the environmental impact of its constituent materials. The optimized cob mixture relied primarily on natural and repurposed inputs, including clay, sand, straw, water, and reclaimed plastic bottles used as structural components. This combination not only minimized the reliance on high-emission industrial materials but also introduced elements such as straw and reused plastic that contribute to carbon reduction through sequestration and waste diversion.

In contrast, the cement-stabilized cob mix, while structurally competent, was fundamentally linked to higher environmental costs due to the carbon-intensive nature of cement production. Cement manufacturing is associated with significant greenhouse gas emissions, primarily from calcination and fossil fuel consumption, rendering materials containing it substantially less sustainable from a lifecycle perspective.

The analysis revealed that the innovative cob mix with the integrated bottle pattern achieved a dual benefit: structural enhancement and environmental mitigation. The strategic placement of sand-filled plastic bottles not only increased the material’s load-bearing capacity but also supported a more circular and regenerative material use approach. This contrasts with the cement-based cob, which, despite acceptable mechanical properties, compromises environmental performance due to its elevated embodied carbon.

These findings highlight the potential of alternative earthen construction methods that incorporate locally available, low-carbon, and recycled materials. Such approaches align with contemporary sustainability objectives by reducing dependency on carbon-intensive resources and fostering resilience through environmentally responsible material innovation.

4.5. Continuation of Work and Prototype

Based on the results explained above of both the compressive strength and embodied carbon analysis, the larger prototype was designed using cob and sand-filled bottles arranged in the alternating pattern previously described. This decision was made to optimize both structural performance and sustainability. To guide construction, AutoCAD 2024 drawings were initially developed, illustrating the bottle placement pattern along with all necessary dimensions. These drawings (Figure 14) served as the foundation for the prototype’s construction, ensuring precision and alignment with the tested configurations.

With the designs in place, the focus shifted to the actual construction of the model. A manual approach was adopted as the construction was constructed through skilled labor. As construction progressed, critical structural elements, including the framework for windows and doors, were methodically integrated to enhance both functionality and stability. Once the primary construction was completed, the prototype underwent partial finishing, refining its form. The outcome shown in Figure 15 demonstrated the feasibility of utilizing cob–earth and sand-filled plastic bottles in sustainable construction, reinforcing the material’s potential for future applications.

5. Discussion and National Alignment

The findings of this research confirm the possibility of using cob–earth materials with plastic bottles waste to construct a room. This outcome, particularly targeting circularity and embodied carbon, strongly aligns with Egypt’s sustainability strategies and national commitments, particularly the country’s Nationally Determined Contributions (NDCs) under the Paris Agreement. The integration of plastic waste into construction aligns with Egypt’s Waste Management Law No. 202 of 2020 (issued on 13 October 2020) [12], which promotes sustainable waste utilization and circular economy practices. Moreover, the reduction in cement reliance contributes to Egypt’s Climate Change Strategy 2050, which aims to decarbonize key sectors, including the construction industry. Globally, the findings also respond to global sustainability frameworks such as the UN SDGs, particularly SDG 11 (Sustainable Cities and Communities) and SDG 12 (Responsible Consumption and Production).

5.1. Interpretation of Results

The results of this study demonstrate the potential of combining cob material with plastic bottle waste to achieve structural integrity while minimizing environmental impact. The compressive strength tests revealed that the modified bottle pattern with the cob mixture achieved the highest load resistance, exceeding 4800 kg. Compared to previous studies, these results indicate a viable alternative construction material that meets basic structural requirements while reducing reliance on traditional cement-based materials. The modified sand-filled bottle arrangements and confinement strategies were developed to mitigate the effects of the manufacturing properties of the used plastic bottles and minimize construction cracking or settlement. These findings suggest a promising pathway for integrating plastic waste into sustainable construction applications.

5.2. Implications for the Construction Industry

The implications of this research for the construction industry are significant. Given the high carbon footprint of cement production, adopting alternative materials such as cob–plastic bottle combinations can contribute to substantial embodied carbon reductions. This aligns with Egypt’s construction sector reform efforts, particularly those aimed at green building and resource efficiency. Furthermore, the feasibility of using locally available materials supports Egypt’s vision for sustainable urban development, as outlined in the Egypt Vision 2030 framework.

5.3. Challenges and Potential Solutions

Despite the promising findings, challenges remain in scaling up this approach. One major challenge is the regulatory framework, as existing building codes in Egypt may not yet recognize cob–plastic bottle structures as viable construction materials. Addressing this requires further research, pilot projects, and collaboration with policymakers to establish standards for alternative building materials.

Another challenge is public perception and market acceptance, which can be mitigated through awareness campaigns, partnerships and demonstration projects showcasing the durability and benefits of these materials. It could potentially be utilized as part of community development projects as a start to gain audience familiarity.

5.4. Long-Term Sustainability Considerations

From a long-term perspective, integrating cob and plastic waste into construction contributes to circular economy principles by diverting plastic waste from landfills and reducing reliance on virgin materials. Additionally, this approach supports climate resilience by promoting locally sourced, low-carbon materials. Technology can also play a crucial role in enhancing construction procedures, particularly in the context of Egypt’s evolving infrastructure needs. For example, 3D printing can be utilized to provide ready-made cob–bottle blocks, which can be directly and efficiently used in construction. This recommendation not only streamlines the construction process but also increases the sustainability of building practices. Future research should focus on optimizing material compositions, assessing durability under different environmental conditions, and developing cost-effective construction techniques for large-scale implementation. This will further solidify the role of sustainable materials in achieving Egypt’s green transition goals.

6. Conclusions

This study provides an in-depth exploration of sustainable construction practices, emphasizing innovative material integration, economic feasibility, and long-term environmental benefits. The findings highlight the potential of alternative materials, such as cob–earth and plastic waste, to address key challenges in the construction industry while aligning with global sustainability goals. By leveraging these materials, industry can reduce its carbon footprint, mitigate waste generation, and enhance the durability and affordability of structures, particularly in regions with high resource constraints.

The results reflect the importance of adopting a holistic approach to sustainability that incorporates not only material innovation but also economic and regulatory considerations. The study’s insights reinforce the need for a shift in the construction sector, one that prioritizes resource efficiency, lifecycle assessments, and the implementation of circular economy principles. The research also reveals significant implications for industry stakeholders, including policymakers, developers, and engineers, as they seek to balance cost-effectiveness with environmental responsibility.

Despite the promising potential, several challenges remain. The large-scale adoption of sustainable materials faces obstacles such as regulatory barriers, lack of industry awareness, and technical limitations related to durability and structural performance. Addressing these issues requires interdisciplinary collaboration, continued research, and supportive policy frameworks to drive market acceptance and standardization. Moreover, future studies should focus on optimizing material compositions, enhancing performance metrics, and conducting long-term field evaluations to further validate these solutions.

From a broader perspective, this research supports sustainable construction as a viable alternative to conventional practices. The integration of waste-based and natural materials into mainstream construction has far-reaching benefits, including reducing reliance on virgin resources, minimizing landfill waste, and promoting energy-efficient building designs. As sustainability becomes an increasingly critical priority worldwide, continued innovation and strategic investment in green construction methodologies will be essential in ensuring a resilient and environmentally conscious built environment.

In conclusion, the transition toward sustainable construction requires a multifaceted approach that combines technological advancements, policy interventions, and industry-wide commitment. By embracing sustainable alternatives and addressing key challenges, the construction industry can play a pivotal role in advancing global sustainability targets while fostering economic growth and social well-being. Further research should explore scalability, cost analysis, and the long-term impacts of sustainable construction materials, ensuring that they become integral components of future urban development strategies.