Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Identify different types of bricks and cement used in South Africa.
• Explain the key properties of these materials, like strength and durability.
• Describe the basic manufacturing process for clay bricks and cement.
• Evaluate the suitability of materials for specific construction jobs.

Lesson summary

This week, we delve into the foundational materials that shape our built environment: bricks, cement, and aggregates. Understanding these materials is crucial because they are the building blocks of almost all construction projects around us, from houses and schools to roads and bridges. In South Africa, where affordable housing and infrastructure development are pressing needs, a thorough knowledge of these materials is vital for future civil technologists. Knowing their properties, strengths, and weaknesses allows us to make informed decisions about material selection, ensuring safe, durable, and cost-effective construction.

Lesson notes

2.1 Bricks Bricks are fundamental building units, typically made from clay or concrete, and used to construct walls, pavements, and other structures.

Types of Bricks: Clay Bricks: The most common type, made from clay that is moulded, dried, and then fired in a kiln.

Common Clay Bricks (Stock Bricks):* Used for general building purposes, often rendered or plastered. Lower cost, lower aesthetic appeal.

Face Bricks (Engineering Bricks):* Higher quality, more durable, and aesthetically pleasing, used for exposed brickwork. These have higher strength and lower water absorption.

Clinker Bricks:** Over-fired bricks that are very hard and durable, often used for paving or in situations requiring high resistance to weathering.

Concrete Bricks: Made from a mixture of cement, aggregates, and water, moulded and then cured. Often cheaper than clay bricks and can be manufactured in various shapes and sizes.

Hollow Concrete Blocks:* Lightweight and provide good insulation.

Manufacturing Process (Clay Bricks): Clay Extraction: Clay is mined from the earth.

Preparation: Clay is cleaned, crushed, and mixed with water.

Moulding: The clay mixture is moulded into the desired brick shape (either by hand or machine).

Drying: The moulded bricks are dried to remove moisture (air-dried or kiln-dried).

Firing: The dried bricks are fired in a kiln at high temperatures to harden them and give them their final properties.

Properties of Bricks: Strength: The ability to withstand compressive forces (measured in MPa). Engineering bricks have significantly higher strength than common clay bricks. A typical common clay brick might have a compressive strength of 7MPa while an engineering brick might have a compressive strength of 30MPa.

Durability: The ability to resist weathering, erosion, and chemical attack.

Water Absorption: The amount of water a brick absorbs (expressed as a percentage of its dry weight). Lower water absorption indicates higher durability. Face bricks should have lower water absorption than stock bricks. High water absorption can lead to cracking in freezing conditions.

Size and Shape: Consistent size and shape are important for ease of construction and a neat appearance.

Sound Insulation: Bricks provide some degree of sound insulation, which can be important in residential and commercial buildings.

Example: Consider building a single-story house in a coastal area near Durban, South Africa. Due to the high humidity and potential for exposure to salt spray, face bricks with low water absorption and high durability would be the preferred choice for the exterior walls. 2.2 Cement Cement is a binding agent that, when mixed with water, sets and hardens, adhering to other materials to bind them together. It is a key ingredient in mortar and concrete.

Types of Cement: Ordinary Portland Cement (OPC): The most common type, used for general construction purposes.

Rapid Hardening Cement: Sets and hardens more quickly than OPC, suitable for projects requiring quick turnaround times.

Sulphate Resisting Cement (SRC): Resistant to sulphate attack, used in environments with high sulphate content (e.g., coastal areas, sewage treatment plants).

Pozzolanic Cement: Contains pozzolanic materials (e.g., volcanic ash), which improve durability and resistance to chemical attack.

Manufacturing Process (OPC): Raw Materials: Limestone (calcium source) and clay or shale (silica, alumina, and iron source) are the primary raw materials.

Crushing and Grinding: The raw materials are crushed and ground into a fine powder.

Mixing: The powdered raw materials are mixed in precise proportions.

Burning (Clinkering): The mixture is heated in a rotary kiln at high temperatures (around 1450°C) to form clinker (small, hard nodules).

Grinding with Gypsum: The clinker is cooled and then ground into a fine powder with a small amount of gypsum (to control the setting time).

Properties of Cement: Setting Time: The time it takes for the cement paste to harden (initial and final setting times). Different cements have different setting times.

Fineness: The particle size of the cement. Finer cement hydrates more quickly, leading to faster strength development.

Strength: The compressive strength of the hardened cement paste (measured in MPa after specific curing periods).

Soundness: The ability of the cement to retain its volume after setting. Unsound cement can expand and crack.

Workability: The ease with which the cement paste can be mixed and placed.

Example: Imagine building a foundation for a bridge in a river near Cape Town. Due to the constant exposure to water and potential for sulphate attack from the marine environment, Sulphate Resisting Cement (SRC) would be the most suitable choice to ensure the long-term durability of the structure. 2.3 Aggregates Aggregates are granular materials, such as sand, gravel, and crushed stone, that are mixed with cement and water to form concrete or mortar. They provide bulk, strength, and stability to the mix.