Examining the viability and benefits of cement-stabilized rammed earth as an affordable and durable walling material in Addis Ababa, Ethiopia

Abstract

Cement-stabilized rammed earth, recognized as a sustainable and cost-effective alternative in the construction industry, is gaining traction. This research investigates its potential as an affordable and durable walling material in Addis Ababa, Ethiopia, and compares it with other prevalent walling products. The study evaluates red clay soil’s physical and mechanical characteristics, both pre and post-cement stabilization. Laboratory tests conducted using soil samples from Addis Ababa demonstrate the viability of cement-stabilized rammed earth. Increasing cement content enhances strength and erosion resistance, making it a cost-effective solution for addressing regional housing challenges. This study assessed the effect of cement content at 0%, 5%, 10%, and 15%. Results show that compressive strength increases as the cement content increases; the compressive strength of cement stabilized rammed earth is 1.766 MPa, 2.85 MPa, 4.79Mpa and 7.61 MPa for 0%, 5%, 10%, and 15% cement content respectively. The cement stabilized rammed earth durability test for 10% and 15% cement content against rainfall erosion show no penetration where 117.5 mm and 1 mm penetration depth for 0% and 5% cement content; the findings underscore increased cement content enhancing the material’s resistance to rainfall erosion.

Keywords:

• Cement stabilized earth block
• soil classification
• sustainability
• alternative walling

Reviewing Editor:

• Ian Phillip Jones, University of Birmingham, UK

SUBJECTS:

• Composites
• Materials Science
• Civil
• Environmental and Geotechnical Engineering
• Structural Engineering

1. Introduction

The global construction industry is profoundly transforming towards sustainable and affordable housing solutions. There is a remarkable surge in recognizing sustainability as a fundamental approach to addressing ecological concerns and societal demands. Concepts such as eco-conscious construction, circular economy principles, and energy-efficient designs have gained prominence, highlighting the importance of conscientious construction practices (UNEP, 2021). In Ethiopia, where a significant portion of the population grapples with inadequate housing, the resonance of sustainable transformation becomes incredibly profound due to the prevalence of low-income individuals. Conventional construction practices in this context often come with prohibitively high costs, necessitating a critical exploration of innovative and cost-effective alternatives (Matsumoto & Crook, 2021).

To find solutions that minimize environmental degradation while addressing housing deficits in developing countries, efforts have been made to develop materials and techniques that rationally use natural resources, require minimal financial investment, generate appropriate technologies, use low-energy consumption materials, and reuse waste materials from industries. This approach helps avoid improper disposal of waste materials. Using local materials can also reduce greenhouse gas emissions associated with transporting construction materials over long distances. Rammed earth is a building material used mainly for residential and small-scale construction. It is gaining popularity worldwide, especially among those seeking environmentally sustainable building alternatives (Rewatkar and Rajurkar, 2018).

Within sustainable construction strategies, a crucial approach involves the intelligent utilization of locally accessible alternative materials for constructing building components. Among these alternatives, rammed earth emerges as a beacon of sustainability, cost-effectiveness, and transformative potential in the construction industry (Matsumoto & Crook, 2021; Mohamed & Rahman, 2018). Rammed earth construction embodies compelling sustainable qualities by harnessing locally available resources, reducing energy consumption, and mitigating transportation-related carbon emissions (Preciado & Santos, 2020; Suresh & Anand, 2017). As the quest for eco-friendly building practices intensifies, recent innovations have aimed at enhancing the performance of rammed earth. Earth-based construction materials have shown the potential to significantly reduce production energy by 80-90%, scaling efficiency alongside production volume. Moreover, working with Earth requires essential tools and modest skill levels, promoting self-sufficiency and community engagement in housing development (Patowar et al., 2015; Zami & Lee, 2009).

In the context of the prevailing trend in Ethiopian construction, which predominantly relies on conventional materials, this research endeavors to bridge the gap between established practices and innovative solutions. By emphasizing property alterations through stabilizers, the study aims to enhance the viability of rammed earth as a sustainable construction alternative. This approach aligns with the global shift towards eco-conscious building practices, addressing the environmental impact and the practical challenges associated with rammed-earth construction in Ethiopian contexts.

1.1. Rammed earth

Rammed earth is an enduring walling material with a rich history, celebrated for its availability, economic advantages, compatibility with self-construction practices, and resilience in the face of bioclimatic stressors, including extreme weather conditions (Preciado & Santos, 2020). Traditionally employed for crafting load-bearing walls, recent advancements in rammed earth techniques are focused on enhancing its mechanical properties while addressing critical aspects such as thermal and acoustic performance. This progress has given rise to stabilized rammed Earth (SRE), involving incorporating diverse additives into the earth-water mixture, further elevating its multifaceted capabilities (Ávila et al., 2022; Raval, 2019; Tejus, 2019). A spectrum of stabilizing agents, including cement and fibers, synergistically enhances the behavior of rammed earth, making it one of the most stable and fitting choices for walling material (Kandamby, 2017; Patowar et al., 2015; Raval, 2019; Tejus, 2019).

1.2. Historical perspective

This ancient technique, involving complying soil, gravel, and stabilizers within formwork to create load-bearing walls, has experienced renewed interest in contemporary sustainable construction. Origins trace back to artifacts like China’s Great Wall and Mesopotamia’s ziggurats, exemplifying the longevity and durability of rammed Earth (Liu & Tong, 2017; Perić, Kraus, Kaluđer, & Kraus, 2021; Wu et al., 2016). While these historical achievements underscore its potential, modern applications have evolved through scientific inquiry and technological innovation. Recent research has deepened our understanding of the intricate interplay between material properties, construction methods, and structural performance in rammed-earth construction. Construction using Rammed Earth (RE) has a long history and has been used in numerous countries since the 1800s. The USA and Australia are pioneers in using this sustainable material in construction. RE structures make use of locally available soils stabilized with binders like lime, resulting in lower embodied energy and reduced waste compared to traditional methods. The rammed-earth wall construction method is a modern alternative to conventional building techniques. It is known for being less labor-intensive and time-consuming than other earth construction types. This method is commonly used in various countries, including the USA, Australia, Africa, New Mexico, New Zealand, France, Germany, Spain, Italy, and more (HORVATH, 2012).

1.3. Stabilized rammed earth

Rammed earth construction methods have remained largely unchanged since ancient times. However, it has become common to incorporate stabilizing agents into the raw soil, which offers several advantages. The primary benefit is that stabilizers significantly increase the strength of the material. Additionally, stabilized material is more resistant to surface wear, reducing or eliminating the need for regular maintenance compared to unsterilized construction. Furthermore, stabilizers allow for a wider range of soil types. For example, soils with lower or higher clay contents than those recommended for unsterilized construction can be utilized by incorporating stabilizers such as Portland cement or hydrated lime.

The introduction of diverse additives to create stabilized rammed Earth (SRE) has notably enhanced mechanical, thermal, and acoustic properties, extending its applicability beyond load-bearing walls (Ávila et al., 2022). In soil, stabilization is done to improve the strength and shrinkage properties. Such materials as cement, lime, and activated fly ash, among others, have been used for stabilization, among which cement is more effective. A study by Jayasinghe and Kamaladasa (2007) showed that the cement content for stabilization can be 6% or more, and higher compressive strength can be achieved with higher cement content. Apart from cement, lime and fiber stabilization is suggested by Maniatidis and Walker (2003).

However, significant challenges persist, demanding meticulous attention. Vital factors such as the meticulous selection and preparation of suitable soil, effective stabilization methods, and compatibility with local climatic conditions are pivotal in influencing the structural integrity and long-term durability of rammed earth walls (Mohamed & Rahman, 2018). A comprehensive understanding of rammed earth’s mechanical behavior under diverse loading conditions, encompassing seismic forces, dynamic stresses, and cyclic thermal expansions, remains imperative (Wu et al., 2016). Further complicating matters is the absence of standardized design codes and construction guidelines, posing considerable obstacles to its widespread adoption (Preciado & Santos, 2020). The soil used for Rammed Earth (RE) construction is a widely available resource with minimal or no negative environmental effects. Typically, subsoil is used, leaving the topsoil intact for agricultural purposes. This reduces the need for transportation and associated costs and energy. Earth materials, such as soil or aggregates, are generally inexpensive and easily accessible, leading to significant cost savings in construction.

Further cost savings can be achieved by carefully controlling the amount of cement used in RE. More than 30 percent of the world’s population currently utilizes earth as a building material. Additionally, RE provides good thermal mass, contributing to efficient building heat retention and cost savings (Anderson, 2000).

A blend of soil, Portland cement, and water gives rise to cement-stabilized rammed Earth (CSRE), a successful construction medium applied across various global regions such as Australia, the United States, Europe, and Asia (Reddy & Kumar, 2011). An insightful overview reveals that substituting soil with cement enhances crucial soil attributes, including strength, durability, thermal efficiency, energy conservation, and cost-effectiveness (Patel, 2014). Comparative studies underscore the superior performance of rammed earth blocks and walls compared to cob or adobe constructions (Bui et al., 2009). The compressive strength of untreated rammed earth walls typically falls within 1.5 to 2 N/mm², but soil stabilization techniques augment this strength (Suresh & Anand, 2017). Cement, a frequently employed stabilizer, fosters chemical cohesion, bolstering the material’s resilience and water resistance (Medvey & Dobszay, 2020). Resultant compressive strength displays variance depending on the stabilizer category, soil constitution, compaction methodologies, and curing processes.

For soil to be suitable for rammed-earth construction, specific criteria must be met. The particle content, including clay, silt, and sand, should fall within 5–20%, 10–30%, and 45–75%, respectively. Additionally, preferable values for consistency limits, such as a liquid limit (LL) below 35–40% and a plasticity index (PI) below 10–30%, are recommended. Soil suitability is determined through a standard Proctor test, which identifies the optimum moisture content (OMC) for rammed-earth construction (Jayasinghe & Kamaladasa, 2007; Reddy & Kumar, 2011; Silva et al., 2013).

Compressive strength is the most essential strength metric for a load-bearing structural member. Small-scale studies using cylindrical specimens for determining compressive strength and elastic characteristics by detecting axial and lateral deformations have been described (Venkatarama Reddy and Latha, 2014; Reddy and Kumar, 2011). Cement-stabilized rammed earth must be moist for 28 days at each location to generate strength (Beckett & Ciancio 2015; Gupta, 2014). Specimens were stored in damp burlap for 28 days to cure in laboratory experiments (Reddy and Kumar, 2011; Silva et al., 2013). Because one of their fundamental flaws is their lack of permanence, it has become common practice to stabilize earthy materials with chemical binders. Cement is the most often used stabilizer, which strengthens the earth by forming chemical linkages that increase its strength and resistance to water. (Medvey & Dobszay, 2020)

Kenneth Mak et al. found that replacing cement with metal oxide and permazyme had little effect on compressive strength; however, combining cement with resin systems increased capacity by 52 to 220 percent. They conducted the following experiments for effect analysis: Compressive tests of specimens were accomplished on a universal testing machine, and RILEM did water absorption (Raval, 2019).

The researchers of this research paper, Sachin N. Bhavsar et al., 2014, determined that brick dust has a good impact on black cotton soil. Adding half of the dry weight of brick dust to the soil greatly improves the engineering properties of black cotton soil. As a result, using brick dust for stabilization is preferable since it produces beneficial outcomes as a stabilizer while also reducing waste (Raval, 2019). Ankit J. Patel and Sachin N. Bhavsar, 2014, In this research paper, the author looked at the effect of waste material on the swelling and shrinkage properties of clayey soil. By replacing the soil with a stabilizer, the maximum dry density increased by 13.45%, while the optimum moisture content decreased by 5.94% for marble powder compared to soil. They performed the following tests to investigate swelling and shrinkage qualities: Grain Size Analysis, Atterberg Limits, Modified Proctor Test, Linear Shrinkage, and Free Swell. Sachin N. Bhavsar and Ankit J. Patel determined that stabilization with marble powder can be achieved by replacing 30% of black cotton soil with marble powder and brick dust.

Amongst the variety of soil stabilizers used, cement has been the most popular in rammed-earth manufacturing. Various researchers have made attempts in the past to document the role of cement as a stabilizer in CSEBs (Spence, 1975; Reddy and Jagadish, 1989; V. Reddy, 1991; Houben and Guillaud, 1994; Walker and Stace, 1997; Kerali, 2001).

2. Materials and methods

2.1. Study area: Soil distribution of Addis Ababa

In the heart of Addis Ababa, our study area boasts a vibrant natural soil palette, ranging from reds, yellows, and browns to greys, greens, blues, white, and black, as shown in Figure 1. The northern region, distinguished by its red soil, is significant for rammed earth production. The selection of this area was guided by the Addis Ababa soil profile map, leading our study participants to identify sections rich in clay suitable for rammed earth. Notably, around Kechene, Figure 2 aligns with the map’s indication of abundant red clay soil in the northern part of the city. This strategic choice facilitated the discovery of a sufficient amount of clay for sale, making Kechene an ideal location for our sample collection.

Figure 1. Modified geological map of the Addis Ababa city (BCEOM, 1996).

Figure 2. Site (Kechene) Topography Satellite Image (Google Earth, 2023).

2.2. Materials

Soil samples were collected from the Kechene quarry site in Northern Addis Ababa to investigate the suitability of natural soil for cement-stabilized rammed earth (CSRE). The soil was extracted from 1 to 2 meters below the ground surface to minimize organic content. This specific soil was chosen because it resembles the typical red soil used in conjunction with CSRE. Laboratory tests were then conducted to assess its physical properties. The material’s physical properties are tested to determine its ability to satisfy requirements for rammed earth construction (Ávila et al., 2022; Bruno et al., 2015; Perić et al., 2021). As demonstrated below in Figure 3, the mechanical properties of the soil were measured in the laboratory as per ASTM standards and AASHTO standards. The tests conducted are particle size analysis (D6913/D6913M-17 2017 and D7928-16), shrinkage limit test (ASTM S4380), Atterberg limits (ASTM D423/424, 2017), and soil compaction test (AASHTO T99).

Figure 3. Mechanical property test for the soil.

The results of these tests, summarized in Table 1, revealed that the soil had a high fine content, with clay and silt accounting for over 92% of the soil composition. This composition placed it within the suboptimal region on the USDA soil texture triangle (USDA, 2021). Typically, such soil compositions are considered unsuitable for CSRE construction. However, it is essential to note that with some mechanical adjustments, this soil could still be used for CSRE despite not meeting the standard criteria. Furthermore, the Atterberg limits measured for this soil did not fall within the usual ranges deemed appropriate for earth wall fabrication.

Table 1. Test values of the specimen.

Laboratory test	Standard	Property	Value
Particle size analysis	ASTM Standard D6913/D6913M-17 2017 and ASTM D7928-16e1 2016	Gravel	0.03
		Sand	8.12
		Silt	37.03
		clay	54.82
Atterberg limit	ASTM D423/424	Liquid limit LL	62
		Plastic limit PL	26.6
		Plasticity index PI	35.4
Soil compaction test	ASTM D698-12 2012	Optimum moisture content (%)	25
		Maximum dry density (Kg/m3)	1.375
		Specific gravity	2.52

This study used CSRE blocks, which had undergone mechanical stabilization to improve their suitability for construction purposes. The soil was layered in thicknesses ranging from 150 mm to 200 mm, and each layer was compacted using a manually operated hammer. Ordinary Portland cement (OPC) was employed per ASTM C150/C150M-17 standards to achieve stabilization (ASTM C150/C150M-17 2017).

3. Rammed earth block production

The earth block pecimens in this study were carefully conducted using a manual standard Proctor test rammer, which employed multiple strokes capable of exerting a pressure of 600 KJ/m³ within molds measuring 15x15x15 cm, as in Figure 4 below. For Cement Stabilized Rammed Earth (CSRE) material preparation, pulverized dry soil, potable water, sand, and cement were amalgamated using a shovel. The process involved gradual water addition to the soil-cement blend, with continuous mixing until uniform moisture distribution was achieved. A standard Proctor test established the optimum moisture content for each soil-cement blend, accounting for cement’s water demand. This determined moisture level was employed during the production of the earth block specimens.

Figure 4. Earth block specimen preparation according to standards.

Stringent control was maintained over the production timeframe, ensuring the interval between material mixing and block fabrication remained under 45 minutes. This precaution prevented excessive cement hardening. Each batch comprised four blocks. Post-manufacturing, the blocks underwent a 3-day curing phase involving wet-and-dry conditions at room temperature, followed by subsequent testing.

3.1. Experimental test matrix

The experimental test matrix encompassed key variable parameters: (1) the weight percentage of cement concerning the soil, comprising three distinct levels: 5%, 10%, and 15%, demonstrated below in Figure 5. The water content for soil-sand-cement mixtures containing 5%, 10%, and 15% cement stood at 4 L, 4.23 L, and 4.45 L, respectively. Each composition underwent the fabrication of three earth blocks with nominal dimensions of 150 mm × 150 mm × 150 mm. An extra block was also fashioned for durability testing, culminating in 9 earth blocks.

Figure 5. Percentage of cement content.

To ensure the blocks were prepared to exact standards, each batch was subjected to a throw-ball test, where the moisture levels were carefully controlled to attain Optimum Moisture Content (OMC). The dimensional tolerance along the two primary directions was held within an impressive ± 0.1%, showcasing the precision of the fabrication process. Furthermore, the thickness deviation was limited to ± 4.0%, emphasizing the commitment to quality and consistency.

Curing procedures were pivotal in developing these earth blocks, with particular attention to submerged curing for twenty-four specimens. Submerged curing, a method involving complete immersion in water, was chosen for most specimens to facilitate an even and controlled curing process, fostering structural integrity and enhanced strength development.

3.2. Strength test

For the experimental assessment of compressive strength in the earth block specimens, as demonstrated below in Figure 6, the Test Resources compressive strength testing machine was employed. This machine boasts a load cell capacity specifically designed for compression testing, ensuring accurate and consistent results. To enhance the testing process, the digital compression testing machine was outfitted with an internal displacement transducer, a sophisticated addition that facilitated gradual compression while also enabling precise measurement of compressive strength.

Figure 6. Compressive strength test.

Throughout the testing procedure, particular attention was given to the assessment of block displacement. After each load application, a comprehensive visual measurement of block displacement was conducted, ensuring that even the minutest changes in structural integrity were captured and analyzed.

3.3. Durability test

The earth block specimens were comprehensively assessed following the stringent ASTM C67-14 guidelines during the initial curing cycle. This assessment involved the determination of two crucial parameters: dry density (ρ) and water absorption (ω). These measurements were carried out to ensure compliance with established standards, providing valuable data on the material’s characteristics.

Subsequently, as demonstrated in Figure 7, a specialized Geelong drip test was undertaken on three specimen blocks, representing the 5%, 10%, and 15% cement-soil mixture. This durability assessment was chosen due to its specific relevance in evaluating the susceptibility of Cement Stabilized Rammed Earth (CSRE) specimen blocks to erodibility and water erosion. These factors are particularly pertinent under humid weather conditions, commonly experienced during Ethiopia’s summer (Kiremt or Meher) season. The Geelong test involved the precise positioning of the specimen on an inclined wedge, characterized by a specific ratio of 1 vertical to 2 horizontals (1 V:2H). A controlled 100 ml water droplet was released from a height of 400 mm onto the inclined surface of the specimen. This controlled release ensured consistent testing conditions. Notably, the interval for the complete dripping of the 100 ml water was regulated to fall within a range of 20 to 60 minutes, enabling the assessment of water erosion susceptibility over time.

Figure 7. Geelong test.

After the dripping process, the depth of the resultant pit was measured using a cylindrical probe featuring a terminal diameter of 3.15 mm. This comprehensive testing procedure provided valuable insights into the material’s response to water erosion and its durability under specific environmental conditions, contributing to a thorough assessment of CSRE block performance.

4. Results and discussion

4.1. Morphology and mechanical property

Table 2 provides a detailed overview of the grain size distribution of the soil after undergoing mechanical stabilization. For soil suitable for cement stabilized in the studies done by Silva et al. (2013), it is recommended that limits of the particle content, such as clay, silt, and sand and gravel content, are 5–20%, 10–30%, and 45–75%, respectively.

Table 2. Mechanically stabilized grain size distribution.

Grain size	Blended soil
Clay (%)	24.77
Silt (%)	19.74
Sand (%)	43.73
Gravel (%)	11.76

Upon completion of the mechanical stabilization process, the soil properties shown in Table 2 were satisfactory; the soil falls well within the range of the suggested grain size distribution criteria. This adherence to the recommended grain size distribution highlights the suitability of the soil for cement stabilization, aligning with the desired characteristics for effective construction material.

Table 3 provides insights into the results of the Atterberg Limit tests for sand-stabilized soil. These tests reveal that the mechanically blended soil, consisting of 60% sand and 40% soil content, exhibits medium plasticity. The introduction of a significant proportion of sand has had a notable effect on the soil sample, reducing the presence of clay, as indicated by the numerical value of the plasticity index. The Silva et al. (2013) study also recommends preferable values of consistency limits for the soil, such as a liquid limit (LL) below 35–40% and a plasticity index (PI) below 10–30%.

Table 3. Mechanically stabilized Atterberg result.

Atterberg limit	Kechene
Liquid limit (%)	46
Plastic limit (%)	32.1
Plasticity index (%)	13.9

As the values of Liquid Limit (LL) and Plasticity Index (PI) decrease, the soil transitions into a state deemed suitable for cement-stabilized rammed-earth construction. The results show that the mechanically blended soil is ideal for cement-stabilized rammed-earth construction.

A noteworthy observation was made in the case of cubes cast for the compressive strength test, which were mechanically stabilized and comprised 40% soil and 60% sand. These samples exhibited shrinkage after curing in ambient laboratory conditions for 28 days. The original size of the sample cubes was 15x15x15 cm, but they showed a decrease in size in all dimensions.

In contrast, the cement-stabilized soil cubical samples displayed a different behavior. These samples, incorporating 5%, 10%, and 15% of cement contents, demonstrated no dimensional shrinkage. This observation underscores the impact of cement stabilization in mitigating shrinkage, contributing to the durability and structural integrity of the rammed-earth material.

4.2. Mechanical properties

4.2.1. Compressive strength

The mechanically stabilized soil sample obtained from Kechene exhibited a notable compressive strength of 1.766 MPa, and the maximum dry density (MDD) and optimum moisture content (OMC) are 1720 kg/m³ and 16.5%, respectively. This measurement underscores the material’s inherent strength, which is crucial in assessing its suitability for construction applications. For the cement-stabilized rammed earth specimens, several fundamental material properties were determined. In the case of specimens composed of 5% cement, the Maximum Dry Density (MDD), as shown below in Figure 8, was found to be 1880.84 kg/m³, with an Optimum Moisture Content (OMC) of 16.6%. Additionally, the average mass of these specimens was recorded at 6.35 kg, with a deviation of 0.09 from the average.

Figure 8. MDD- compressive strength relation.

For specimens of 10% cement, the MDD value increased to 1948.1 kg/m³, and the OMC reached 17.7%. These specimens had an average mass of 6.57 kg, with a deviation of 0.06 from the average. Finally, for specimens comprised of 15% cement, the MDD value further rose to 1980.92 kg/m³, with an OMC of 18.75%. These specimens exhibited an average mass of 6.69 kg, with a minimal deviation of 0.04 from the average.

These values collectively provide essential insights into the material properties of cement-stabilized rammed earth, including density, moisture content, and mass, which are vital considerations in evaluating its suitability for a range of construction applications.

The results demonstrate a consistent pattern where increasing cement content corresponds to higher compressive strength in the cement-stabilized rammed earth specimens, as illustrated in Figure 9. Comparing the values stated in Table 4 with other standards, it shows that the value of compressive strength for a rammed earth sample made of only natural soil should be a minimum of 1.72 N/mm² (ACI) < 2.122 N/mm² (obtained in the Technical University of Cluj-Napoca Laboratory) and for stabilized rammed earth containing 5% Portland Cement the value obtained in the Laboratory is 2.85 > 1 N/mm² (Standards Australia) or 2.85 > 2.068 N/mm² (ASTM International).

Figure 9. Cement-compressive strength relation.

Table 4. Compressive strength of cement stabilized rammed1 earth.

Cement content (%)	Compressive strength (MPa)
0	1.766
5	2.85
10	4.79
15	7.61

It’s observed that the average compressive strength of cement-stabilized rammed earth with 10% and 15% is about 1.68 and 2.6 times higher than that of stabilized with 5% cement. The increase in strength with the increased cement content is attributed to hydration cement on curing and filling of its product in the pores of the matrix, thereby enhancing the rigidity of its structure by forming many bonds connecting sand particles (Tripura and singh 2014). During the Experimental program, it was observed that there was an increase in the strength of Rammed Earth blocks with an increase in the percentage of cement, as demonstrated below in Figure 10.

Figure 10. Curing time-compressive strength relation.

Cement stabilized rammed earth compressive strength at the 3^rd day is 1.15 MPa, 1.94 MPa, and 3.07 MPa for 5%, 10%, and 15% cement content, respectively, at the 7^th day 1.87 MPa, 3.145 MPa, and 4.99 MPa for 5%, 10% and 15% cement content respectively and at the 28^th day is 2.85 MPa, 4.79 MPa and 7.61 MPa for 5%, 10%, and 15% cement content respectively. The Rammed Earth blocks were observed for compressive strength results for curing periods of 3 days, 7 days, and 28 days. It was observed that as the curing period increased, the compressive strength of Rammed Earth blocks increased. On 28 days of curing, the compressive strength increases up to 30-40% higher than those cured for 7 days, the same as the research conducted by Rewatkar and Rajurkar (2018).

It is worth noting that these results align with the findings from different publications, which have established a range of compressive strength values for cement-stabilized soil on the 28th day. Importantly, the compressive strength values obtained for this study’s three cement content levels fall well within this established range. For instance, specify a compressive strength range of 1-15 N/mm², while the study by Houben & Guillaud reports a compressive strength range of 2-5 N/mm². This alignment with recognized standards and prior research not only reaffirms the reliability of the results but also underscores the material’s compliance with established norms in the construction and soil stabilization field.

A comparative analysis of compressive strength reveals the noteworthy performance of cement-stabilized rammed earth (10% and 15% cement content) when contrasted with other load-bearing wall materials, as illustrated in Table 5. It becomes evident that the compressive strength of these rammed earth variants surpasses that of conventional building bricks and Type A and Type B Hollow Concrete Blocks (HCB). Notably, the 15% cement-stabilized rammed earth exhibits higher compressive strength than second-class bricks, although it falls slightly below the compressive strength of some commonly used load-bearing materials. This assessment underscores the competitive strength characteristics of cement-stabilized rammed earth and its potential as a reliable alternative in load-bearing applications within the construction industry.

Table 5. Compressive strength of common loadbearing materials.

Wall material	Compressive strength (MPa)
First class brick	9.5
Second class brick	6
Common building brick	2.9
Concrete (residential)	15-30
Stone Masonry	22-179
TypeA HCB(min requirement	4.2
TypeB HCB(min requirement)	3.5

4.3 Durability analysis

4.3.1. Rainfall erosion

The study on the durability of rammed earth construction aimed to assess the material’s resilience to weathering effects. The results from the Geelong test revealed significant findings, as presented below in Figure 11. For the 10% and 15% cement content levels, there was no discernible pit depth after 28 minutes, 34 seconds, 32 minutes, and 35 seconds, respectively, representing the time it took for 100 ml of water to drip fully. Regarding the 5% cement content, the drip time was 26 minutes and 37 seconds, with a pit depth of only 1 mm. These findings align with established standards such as the New Zealand Standard, Standards Association of Zimbabwe, and Standards Australia, which consider specimens to have failed when the pitting depth exceeds 15 mm or the moisture penetration depth surpasses 120 mm. This demonstrates the impressive resistance of the 5% cement content rammed earth against rainfall erosion.

Figure 11. Cement-rainfall erosion relation.

Additionally, all fully immersed samples exhibited no adverse effects from water exposure, indicating the excellent water resistance of cement-stabilized rammed earth. In contrast, the control group, comprising mechanically stabilized cubes without cement, exhibited a longer drip time of 29 minutes and 35 seconds, coupled with a pitting depth of 117.5 mm, highlighting the substantial improvement in rainfall erosion resistance achieved through cement stabilization. Furthermore, the findings underscore increased cement content enhancing the material’s resistance to rainfall erosion.

4.4 Cost analysis

An analysis was conducted to assess the material cost to construct a wall having 1 m*1m*1m size for cost comparison between the different load-bearing wall materials since they all have different dimensions. The cost required to achieve the desired compressive strength by calculating the price per cubic meter of rammed earth. The results indicate that compared to other commonly used load-bearing wall materials in Ethiopia, the material cost of cement-stabilized rammed earth is notably lower. This cost-effectiveness positions cement-stabilized rammed earth as a compelling alternative in terms of affordability, further highlighting its potential to address the housing challenges faced in the region while meeting the desired structural performance standards (Table 6).

Table 6. Cost comparison for common load-bearing wall material in Ethiopia.

Load-bearing wall material	Birr/m^3
Brick Masonry	10,920
Stone Masonry	5200
Concrete	13,300
Rammed Earth with 5 % cement content without soil	1980
Rammed Earth with 10 % cement content without soil	2400
Rammed Earth with 15 % cement content without soil	2820
Rammed Earth with 5 % cement content, including soil	2143
Rammed Earth with 10 % cement content, including soil	2563
Rammed Earth has 15 % cement content, including soil	2983
Type A HCB	4400
Type B HCB	4600

The cost of cement stabilized rammed earth without including the price of soil for 10% and 15% cement is 1.21 and 1.42 times higher than that of 5% cement content, where the price of 15% cement content is 1.175 times higher than 10% cement content. And for the price, including the cost of the soil with 10% and 15%, is 1.2 and 1.39 times higher, and 15% cement content is 1.16 times higher than 10% cement content.

4.5 Optimal cement content

Cement is typically used in proportions between 4% and 15%, with between 6% and 10% the most commonly specified. Increased cement content improves strength and erosion resistance. A study by Jayasinghe and Kamaladasa (2007) showed that the cement content for stabilization can be 6% or more, and higher compressive strength can be achieved with higher cement content, and the compressive strength test result observed in this study aligns with this. Regarding durability and cost analysis, 10% cement content is optimal.

5. Conclusion

In this research, we delved deep into the engineering properties of soil and sand-stabilized soil through meticulous laboratory testing and data analysis. The carefully organized results became the foundation for a comprehensive geotechnical database in Microsoft Excel. Our tests covered a range of factors, including particle size distribution, Atterberg limits, linear shrinkage, and Standard Proctor Compaction, all evaluated according to both AASHTO and unified soil classification systems.

We established moisture-density correlations by employing the standard Proctor test, revealing a moderate dry density at the optimum moisture content of 25 percent. For 5%, 10% and 15% cement, the Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) was 1880.84 kg/m³ and 16.6%, 1948.1 kg/m³ and 17.7% and 1980.92 kg/m³ with 18.75% respectively. Adding sand to the soil altered its particle size distribution,24.77% Clay, 19.74% Silt, 43.73% Sand, and 11.76% Gravel, giving it a sandier texture. The medium plasticity indicated by the plasticity index LL 46%, PL 32.1%, and PI 13.9% suggested that the blended soil is well-suited for cement-stabilized rammed earth. Furthermore, post-stabilization, the soil’s shrinkage limit implied minimal expansion potential.

Cement-stabilized rammed earth was obtained, which are 2.85 MPa, 4.79 MPa, and 7.61 MPa for 5%, 10%, and 15% cement content, respectively—emerged as a superior choice among load-bearing wall materials regarding compressive strength. Not only does it excel in performance, but it also proves to be a cost-effective alternative, boasting lower material costs compared to other load-bearing materials. Cement stabilized rammed earth compressive strength on the 3rd day, the 7th day, and the 28th day shows the compressive strength increases as the curing time increases. The average characteristic strength of cured samples is about two times higher than that of uncured samples. On 28 days of curing, the Rammed earth blocks attain compressive strength 30-40% higher than those cured for seven days.

The Geelong test results, 1 mm for 5% cement content and zero for 10% and 15% cement content, affirmed the rainwater erosion resistance of cement-stabilized rammed earth. This construction material remained unaffected even under submerged curing conditions, highlighting its durability and longevity.

In conclusion, cement-stabilized rammed earth is an exceptional choice for sustainable, cost-effective load-bearing wall material. Our laboratory tests showcased its versatility in achieving the desired compressive strength through soil, sand, and cement ratio adjustments. When weighed against alternative wall materials, cement-stabilized rammed earth presents a compelling proposition for sustainable construction projects. As the construction industry evolves, the promising future of cement-stabilized rammed earth continues to gain momentum, solidifying its position as a sustainable construction solution.

Author contributions

Ermias A. Amede provided supervision conceptualization and contributed to writing and editing. Gebrella G. Aklilu and Helen W. Kidane focused on data curation, original draft preparation, and laboratory investigation. Alemayehu D. Dalibsou supervised and validated the project.

Acknowledgment

The Ethiopian Institute of Architecture, Building Construction, and City Development (EiABC) and Addis Ababa University (AAU) were acknowledged for their infrastructure support during the data-gathering stage. Thanks to Mr. Matheows from EiABC's material testing lab for his valuable contributions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data used to support the findings of this study are available from the corresponding author upon request.

Additional information

Funding

No organization or institute funded the study.

Notes on contributors

Ermias A. Amede

Ermias A. Amede is a lecturer at the Ethiopian Defense university, Adjunct lecturer, and senior researcher at the Chair of Appropriate Building Technology in Addis Ababa University.

Gebrella G. Aklilu

Gebrella G. Aklilu is a master’s student and an assistant researcher at the Chair of Appropriate Building Technology in Addis Ababa University.

Helen W. Kidane

Helen W. Kidane is a master’s student and an assistant researcher at the Chair of Appropriate Building Technology in Addis Ababa University.

Alemayehu D. Dalbiso

Alemayehu D. Dalbiso is a lecturer and senior researcher at the Chair of Appropriate Building Technology in Addis Ababa University.