Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Identify different types of bricks and cement.
• Describe the key properties of these materials.
• Explain the importance of proper mixing ratios.
• Discuss the environmental impact of their production.

Lesson summary

This week, we delve into the foundational building blocks of civil technology: bricks, cement, and aggregates. These materials are essential for constructing homes, schools, roads, and other vital infrastructure that directly impacts our lives in South Africa. Understanding their properties is crucial for ensuring the safety, durability, and sustainability of our built environment. Consider the RDP houses, the Gautrain, or even the local clinic – all rely on the careful selection and application of these basic materials. Knowing about these materials allows you to evaluate construction quality and understand the principles behind building design.

Lesson notes

2.1 Bricks Bricks are rectangular building units made from clay or other materials, hardened by firing or air-drying. They are a fundamental component in masonry construction.

Types of Bricks: Clay Bricks: The most common type. Made from clay that is molded, dried, and fired in a kiln. They come in various qualities, dependent on the manufacturing process and clay used.

Stock Bricks: Common, general-purpose bricks.

Facing Bricks: High-quality bricks with a uniform color and texture, used for visible surfaces.

Engineering Bricks: High-strength bricks used in load-bearing applications where high compressive strength and low water absorption are required, for instance, in foundations.

Concrete Bricks/Blocks: Made from cement, aggregates, and water. Often larger than clay bricks, reducing construction time. They are typically cheaper but may lack the aesthetic appeal of clay bricks. These can be solid or hollow core. Hollow core blocks reduce weight and improve insulation.

Sand-Lime Bricks: Made from sand, lime, and water, hardened by steam curing under pressure. They offer good thermal and acoustic insulation. Less common in South Africa compared to clay and concrete bricks.

Properties of Bricks: Compressive Strength: The ability of the brick to withstand a crushing force. Engineering bricks need a high compressive strength. This is measured in MPa (Megapascals). In South Africa, SANS 227 specifies the requirements for clay bricks.

Water Absorption: The amount of water a brick absorbs when immersed in water for a specific time. High water absorption can lead to damage from freeze-thaw cycles (not as prevalent in many parts of SA, but still relevant) and can affect the bonding strength with mortar.

Durability: The ability of the brick to withstand weathering and other environmental factors.

Size and Shape: Bricks must be uniform in size and shape to ensure proper bonding and stability of the masonry structure. Standard brick sizes are defined by SANS standards to allow consistent construction.

Soundness: The ability of a brick to resist disintegration or crumbling due to internal stresses.

Texture and Color: Important for aesthetic purposes, especially for facing bricks.

Worked example

A contractor is building a retaining wall and needs to choose between stock bricks and engineering bricks. Considering the retaining wall needs to withstand significant pressure from the soil, which brick should be used and why?

Solution:

Engineering bricks should be used. Retaining walls are load-bearing structures. Engineering bricks have a significantly higher compressive strength compared to stock bricks, making them suitable for withstanding the pressure exerted by the soil behind the wall.

2.2 Cement

Cement is a binder that hardens and binds other materials together. It's the key ingredient in mortar and concrete.

Types of Cement:

Ordinary Portland Cement (OPC): The most common type. Used for general construction purposes. There are different strength grades, such as 32.5, 42.5, and 52.5, indicating the compressive strength after 28 days (in MPa).

Rapid Hardening Cement: Develops strength quickly. Used when quick setting is required, e.g., for road repairs.

Sulphate Resisting Cement (SRC): Used in structures exposed to sulphate attack, such as coastal structures and structures in contact with sulphate-rich soils common in some areas of South Africa.

Pozzolanic Cement: Contains pozzolanic materials (e.g., fly ash, volcanic ash) which react with calcium hydroxide released during cement hydration to produce additional cementitious compounds. Improves durability and reduces the amount of OPC needed, leading to a lower environmental impact.

Masonry Cement: Specifically formulated for use in mortar. Contains additives to improve workability and water retention.

Properties of Cement:

Fineness: Finer cement particles hydrate faster, leading to quicker strength development.

Soundness: The ability of the cement to not expand excessively after setting, which can cause cracking.

Setting Time: The time it takes for the cement paste to harden. Initial setting time should not be too short to allow for placement and finishing. Final setting time should not be too long to avoid delays.

Compressive Strength: The ability of the hardened cement paste to withstand a crushing force.

Tensile Strength: The ability of the hardened cement paste to withstand a pulling force. Concrete has low tensile strength and needs reinforcement, typically steel.

Hydration: The chemical reaction between cement and water, which leads to hardening. This process is exothermic, meaning it generates heat.

Mixing Ratios: Cement is mixed with aggregates (sand and gravel) and water to produce concrete. The mixing ratio is crucial for achieving the desired strength and workability.

A common ratio is 1:2:4 (cement:sand:gravel) by volume. The water-cement ratio (w/c) is also critical. A lower w/c ratio generally leads to higher strength, but too little water makes the mix unworkable.

Worked

Example:

A builder needs to make 1 cubic meter of concrete with a 1:2:4 mix ratio. Calculate the volumes of cement, sand, and gravel required.

Solution:

Total parts: 1 + 2 + 4 = 7 parts

Volume of one part: 1 m³ / 7 parts = 0.143 m³ per part (approximately)

Volume of cement: 1 part * 0.143 m³/part = 0.143 m³

Volume of sand: 2 parts * 0.143 m³/part = 0.286 m³

Volume of gravel: 4 parts * 0.143 m³/part = 0.572 m³

Reference guide

• to achieve a specific compressive strength.