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CHAPTER TWO: LITERATURE REVIEW
2.1 Introduction
This chapter provides a comprehensive review of existing literature on the comparative analysis between rammed earth construction and traditional burned clay brick construction. It outlines the principles of sustainable construction methods, focusing particularly on the advantages of rammed earth techniques over conventional fired brick approaches. The chapter also explores the properties of earth as a construction material and details the processes involved in both construction methods. Overall, the review serves to contextualize the research by referencing prior studies and theoretical frameworks relevant to the subject matter.

2.2 Theoretical Review

2.2.1 Traditional Burned Clay Bricks
Numerous studies have investigated perceptions and applications of fired clay bricks, especially in low-income housing in tropical regions. The ELITH (Energy and Low-income Tropical Housing) project, funded by DFID, EPSRC, and DECC, aimed to address environmental concerns associated with fired clay brick production, such as high embodied energy and pollution.

Historically, clay brick masonry has been among the most durable and widely used construction techniques. From Mesopotamian to Roman times and into the modern era, its evolution has retained simplicity while benefiting from incremental advancements. Despite environmental exposure over centuries, buildings made from clay bricks remain intact, which demonstrates their durability.

However, the material’s durability is directly linked to factors like the clay’s quality and the firing process. Studies by López-Arce et al. (2003), Cultrone et al. (2004), and others focus on the physical, chemical, and mechanical properties of ancient bricks, though many analyses neglect structural performance such as compressive strength, which is essential for load-bearing capacity.

2.2.2 Manufacturing Process
Clay brick production involves four main stages: raw material extraction, clay preparation, molding, and firing. The preparation process—tempering, molding, and drying—requires precision, particularly in terms of moisture control, to avoid defects like warping and cracking. Traditional drying methods were time-consuming and climate-dependent, often taking weeks.

Firing is critical for developing strength and durability. Traditional kilns used biomass like firewood or straw, contributing significantly to deforestation and emissions. The firing process, although simple, lacks consistency in many developing countries, often leading to poor-quality bricks.

2.2.3 Physical Properties of Fired Bricks
The performance of fired clay bricks is largely influenced by physical attributes such as porosity, apparent density, and water absorption. High porosity reduces mechanical strength and increases vulnerability to environmental deterioration. Studies report porosity ranging from 12% to 43%, affecting water absorption and resistance to freeze-thaw cycles.

Apparent density values typically range between 1,200–1,900 kg/m³, where higher density correlates with better strength and durability. Water absorption, closely tied to porosity, influences brick longevity; excessive absorption can lead to structural weakening, especially in humid climates.

2.2.4 Moisture Expansion
Moisture-induced expansion or shrinkage in clay bricks is influenced by the clay’s mineral composition. While largely irrelevant for aged bricks, expansion values of 0.1–0.2% are common (Álvarez de Buergo & Limón, 1994; Fernandes, 2019).

2.2.5 Mechanical Properties
Mechanical performance, particularly compressive strength, is crucial for assessing structural integrity. Historical bricks often reflect variability due to inconsistent raw materials and primitive firing methods. Strength is generally compromised by environmental degradation, which necessitates site-specific testing. Reported compressive strengths range from 1.5 to over 30 MPa, depending on multiple factors.

2.2.6 Environmental and Economic Considerations
Although fired bricks are widely used, their production contributes to deforestation and air pollution due to inefficient kilns and biomass reliance. Studies highlight high energy use (up to five times more than regulated production in developed countries) and significant construction waste due to unskilled labor and inconsistent brick quality. Despite these issues, fired bricks remain popular due to their perceived modernity and durability.

2.2.7 Rammed Earth Construction
Rammed earth involves compacting moist subsoil into formwork to create dense structural walls. This technique minimizes environmental impact by using locally sourced materials and requiring little energy for processing. Proper particle size distribution is essential for structural performance, with ideal soils containing 8–15% clay and a high proportion of sand and gravel.

2.2.8 Stabilized Rammed Earth
Stabilization, typically using Portland cement (4–15%), enhances strength, water resistance, and durability. Artificial soils with controlled compositions have been used in laboratory experiments. Cement improves compressive strength and elasticity, and reduces susceptibility to erosion and frost damage.

2.3 Compressive Strength of Rammed Earth
The strength of rammed earth structures develops over time, similar to concrete. Compressive strength tests—often using cylindrical specimens—help determine material suitability. Field and lab studies indicate that stabilized rammed earth can achieve significant strength (e.g., 2.5 GPa modulus of elasticity), supporting its use in load-bearing applications.

2.4 Thermal Performance
Although rammed earth has lower thermal resistance than modern insulating materials, it exhibits high thermal mass, providing effective temperature regulation in buildings. Its ability to delay heat transfer makes it suitable for arid and temperate climates, reducing reliance on active HVAC systems.

2.5 Durability and Cost Implications
Durability tests, such as spray tests, confirm the robustness of cement-stabilized rammed earth. While initial construction may be labor-intensive, rammed earth offers long-term cost savings through reduced energy needs and material reuse. Comparative studies show that rammed earth construction has lower embodied energy than fired brick or steel-framed systems.

2.6 Literature Review Summary and Research Gaps
Existing literature highlights the sustainability, affordability, and thermal efficiency of rammed earth construction. However, most studies use materials and testing environments different from those found in local Ugandan contexts, such as the Gayaza Canaanite Estate. This creates a research gap in evaluating the applicability of rammed earth construction using locally available materials under region-specific conditions. Furthermore, limited local data exists on the compressive strength, thermal behavior, and environmental impact of alternative walling techniques in Uganda, necessitating contextual comparative studies like the present research.