Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define and classify bricks, cement, and aggregates.
• Explain the key properties of each material.
• Describe the manufacturing process for bricks and cement.
• Evaluate which material to use for specific jobs.
• Perform simple tests to check the quality of bricks.

Lesson summary

This week, we delve into the fundamental materials that form the backbone of almost every structure we see around us: bricks, cement, and aggregates. From houses and schools to roads and bridges, these materials are essential to the built environment. Understanding their properties is crucial for any civil technologist, allowing us to select the right materials for a specific job, ensuring structural integrity, and making informed decisions about construction methods and costs. In South Africa, with our diverse climate zones and pressing need for infrastructure development, a solid grasp of these materials is vital for building durable, sustainable, and affordable structures.

Lesson notes

2.1 Bricks: Bricks are solid masonry units, usually rectangular, used for building walls, pavements, and other structures. Their standard size makes them easy to handle and assemble.

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

Common Burnt Clay Bricks:* Used for general construction; lower strength.

Facing Bricks:* Used for exterior walls; higher strength and better appearance.

Engineering Bricks:* Very high strength, used for load-bearing walls and foundations.

Concrete Bricks:* Made from cement, aggregates, and water, molded and cured. Calcium Silicate Bricks (Sand-Lime Bricks):* Made from sand, lime, and water, hardened by autoclaving.

Properties of Bricks: Strength:* Ability to withstand compressive loads (measured in MPa – Megapascals). Engineering bricks have higher compressive strength than common bricks.

Water Absorption:* The amount of water a brick absorbs when immersed in water (expressed as a percentage of its dry weight). Lower water absorption indicates better durability and resistance to frost damage.

Durability:* Resistance to weathering, erosion, and chemical attack.

Size and Shape:* Consistent size and shape are crucial for easy laying and uniform appearance. Variations indicate poor quality.

Texture and Colour:* Affects the aesthetic appearance of the structure.

Soundness:* A good brick should produce a clear, metallic ringing sound when struck with another brick. A dull sound indicates cracks or internal flaws.

Manufacturing of Clay Bricks: Clay Preparation: Clay is dug, cleaned of impurities (stones, vegetation), and ground.

Molding: Clay is molded into the desired shape (by hand or machine).

Drying: Molded bricks are dried in the sun or in a drying kiln to remove moisture and prevent cracking during firing.

Firing: Dried bricks are fired in a kiln at high temperatures (900-1200°C) to harden them and give them their characteristic colour.

Cooling: Bricks are allowed to cool slowly to prevent cracking. 2.2 Cement: Cement is a binding agent that, when mixed with water, undergoes a chemical reaction called hydration, hardening and binding other materials (aggregates) together.

Types of Cement: Ordinary Portland Cement (OPC):* The most common type, used for general construction. Available in different strength grades (e.g., 32.5, 42.5, 52.5 MPa).

Rapid Hardening Cement:* Hardens quickly, allowing for faster construction.

Sulphate Resisting Cement (SRC):* Resistant to sulphate attack, used in marine environments and areas with high sulphate content in the soil.

Low Heat Cement:* Generates less heat during hydration, reducing the risk of cracking in mass concrete structures.

Properties of Cement: Fineness:* The degree to which cement is ground. Finer cement hydrates faster and gains strength more quickly.

Setting Time:* The time it takes for the cement paste to set (initial and final setting times).

Strength:* Compressive strength is the most important property (measured in MPa).

Soundness:* The ability of cement to retain its volume after setting. Unsound cement can expand and cause cracking.

Heat of Hydration:* The amount of heat generated during the hydration process.

Manufacturing of Cement (Simplified): Raw Materials: Limestone and clay are the primary raw materials.

Grinding: Raw materials are ground into a fine powder.

Mixing: Ground materials are mixed in the correct proportions.

Burning: The mixture is burned in a rotary kiln at high temperatures (around 1450°C) to form clinker.

Grinding and Gypsum Addition: Clinker is cooled and ground with gypsum to control the setting time.

Packaging and Distribution: Cement is packaged and distributed to construction sites. 2.3 Aggregates: Aggregates are granular materials, such as sand, gravel, and crushed stone, used as a filler in concrete and mortar. They provide bulk, strength, and stability.

Types of Aggregates: Fine Aggregates:* Sand (particle size less than 4.75 mm).

Coarse Aggregates:* Gravel and crushed stone (particle size greater than 4.75 mm).

Properties of Aggregates: Size and Grading:* The distribution of different particle sizes. Well-graded aggregates produce denser and stronger concrete.

Shape:* Rounded aggregates are easier to work with, while angular aggregates provide better interlocking and higher strength.

Surface Texture:* Rough-textured aggregates provide better bond with cement paste.

Strength:* Aggregates should be strong enough to withstand the loads applied to the concrete.

Durability:* Resistance to weathering, abrasion, and chemical attack.

Cleanliness:* Aggregates should be free from organic matter, silt, and clay, which can weaken the concrete.

Example 1: Calculating Water Absorption of a Brick A brick weighs 2.5 kg when dry. After being immersed in water for 24 hours, it weighs 2.8 kg. Calculate the percentage of water absorption.