Methodology
Methodology for Civil Engineering
CHAPTER THREE: MATERIALS AND METHODS
3.1. Introduction
This chapter outlines the methodology and materials utilized in the project. It highlights the procedures followed throughout the project, including sampling techniques, data collection, processing, and analysis. The focus is on stabilized rammed earth, a construction material made from subsoils mixed with stabilizing agents to enhance its physical properties. Soils for cement-stabilized rammed earth typically have higher sand and gravel content, with reduced fine particles. It is essential to avoid organic matter in rammed earth construction to minimize shrinkage, bio-deterioration, and susceptibility to insect attacks, as well as to ensure the effectiveness of stabilizers like cement. Improving mechanical strength and weather resistance involves reducing the void ratio to increase contact between soil particles.
3.2. Cement Manufacturing Process
The primary ingredients in cement production include:
- Limestone
- Calcium
- Clay or shale
- Silica/Alumina
- Quarrying
- Local resources (without a commercial market)
Limestone (CaCO₃) and clay are the main raw materials used.
3.2.1. Portland Cement Clinker
Portland cement clinker is produced by grinding raw materials, mixing them in precise proportions, and heating them in a kiln at 1400-1500°C until partially fused into balls called clinker. The cooled clinker is then ground with a small amount of gypsum rock. The mixture is burned in a rotary kiln.
3.2.2. Cement Types and Physical Properties
The cement used, Hima cement 32.5N, had its physical and chemical properties verified through a manufacturer’s testing certificate.
- Setting Time:
Setting time depends on cement fineness, water-cement ratio, chemical composition (especially gypsum content), and admixtures. Setting time tests help assess how cement paste sets. Two common setting times are defined:- Initial Set: When the paste starts to stiffen significantly.
- Final Set: When the cement hardens enough to bear a load.
Additional types of setting include: - False Set: No heat is produced, and the concrete can be remixed without adding water.
- Flash Set: Caused by the absence of gypsum and typically used for underwater repairs.
Setting times are tested using the Vicat apparatus with a 1mm needle.
- Soundness:
Soundness refers to the cement paste’s ability to retain its volume after setting without delayed expansion caused by excess free lime (CaO) or magnesia (MgO). The Le Chatelier apparatus is used for soundness testing. - Fineness:
Fineness, or particle size, affects hydration rate and strength gain. Higher fineness allows more surface area for water-cement interaction, promoting strength development, especially during the first seven days. The Blaine air-permeability method is the most common method for measuring fineness. - Strength:
Cement paste strength is measured in compressive, tensile, and flexural terms. It depends on factors like the water-cement ratio, fine aggregate quality, curing conditions, and specimen dimensions.
3.2.3. Testing Duration
Typical testing durations are:
- 1 day (for high early strength cement)
- 3 days, 7 days, 28 days, and 90 days (to monitor strength progress)
- The 28-day strength is often used as a control benchmark.
Soil used for rammed earth construction was free from organic matter and other contaminants. The particle size distribution showed 50%-70% fine gravel and sand, 15%-30% silt, and 5%-15% clay. This was verified according to BS standards. Field tests confirmed soil quality, and soil was excavated from a depth of 1m to avoid surface organic material.
3.3. The Jar Test (Particle Size Test)
To assess soil particle size distribution, a jar test was conducted. Soil was placed in a bottle with water, shaken, and allowed to settle. As the water cleared, distinct layers of sand, silt, and clay formed, revealing the soil composition. Soil was excavated to a depth of 7m, then piled to avoid mixing with topsoil and impurities. The material was aerated by sieving to remove lumps and pebbles.
The soil, sand, and stabilizers were accurately measured and mixed by hand. The mixing followed this order:
- Dry mixing of soil and stabilizer (cement) 2-3 times
- Addition of water and wet mixing 2-3 times
The mixture was kept at its Optimum Moisture Content (OMC) to achieve Maximum Dry Density (MDD), ensuring maximum strength. The mixed material was then transported to the construction site using wheelbarrows and poured into formwork.
The formwork was set with a plumb line to ensure vertical alignment. Each 120mm layer of soil was checked for thickness, leveled, and rammed until a sharp sound was heard. The formwork was dismantled after 2-3 hours, and the process was repeated.
The rammed earth structure was constructed with a 450mm wall thickness on the ground floor and 350mm on the upper floor for thermal mass in tropical regions. False walls were included to accommodate entertainment systems. The mix ratio used was 1:7 (1 bag of cement to 7 wheelbarrows of soil).
For comparison, traditional burned clay brick construction involves laying individual masonry units with bricks and cement mortar. This method provides aesthetically pleasing and cost-effective structures.
Soil stabilization was tested using a 2MPa compressive strength criterion. Soil with a Linear Shrinkage (LS) of <6.0% and a Plasticity Index (PI) of <15% showed stabilization success.
