Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define concrete, formwork, and reinforcement.
• Identify the basic ingredients of concrete.
• Describe different types of formwork and their uses.
• Explain the purpose and types of reinforcement.
• Apply basic safety procedures.

Lesson summary

Welcome to the exciting world of concrete! In this topic, we'll be exploring concrete, formwork, and reinforcement. Concrete is one of the most widely used construction materials globally, and understanding its properties and how it's used is crucial for any budding Civil Technologist. Think about the Gautrain, the N1 highway, your own home's foundation – concrete is everywhere! Understanding these concepts will allow you to appreciate the built environment around you and even contribute to its creation in the future. In South Africa, with our growing infrastructure needs, a solid understanding of concrete technology is a valuable skill.

Lesson notes

2.1 Concrete: The Basics Concrete is a composite material made up of three main ingredients: Cement: Cement is the binding agent that, when mixed with water, hardens and binds the other ingredients together. Ordinary Portland Cement (OPC) is the most common type used.

Aggregates: These are granular materials like sand (fine aggregate) and gravel or crushed stone (coarse aggregate). They make up the bulk of the concrete mix and provide strength and stability. Different sizes of aggregates are important to achieve a well-graded mix, which reduces voids and increases density.

Water: Water is essential for the hydration of cement, the chemical reaction that causes it to harden. The water-cement ratio is a critical factor in determining the strength and durability of the concrete. Too much water weakens the concrete, while too little prevents proper hydration. A typical water-cement ratio ranges from 0.4 to 0.

6. Admixtures (Optional): These are chemicals added to the concrete mix to modify its properties.

Examples include: Water reducers:* Increase workability without adding more water.

Accelerators:* Speed up the setting time. Useful in cold weather.

Retarders:* Slow down the setting time. Useful in hot weather.

Air-entraining agents:* Create tiny air bubbles that improve freeze-thaw resistance. 2.1.1 The Hydration Process: When water is added to cement, a chemical reaction called hydration occurs. This reaction produces heat and forms a gel-like substance that gradually hardens and binds the aggregate particles together. The concrete gains strength over time as the hydration process continues. The rate of strength gain depends on factors like cement type, water-cement ratio, and temperature. 2.2 Formwork: Shaping the Concrete Formwork is a temporary structure used to contain and shape concrete while it is setting and gaining strength. It must be strong enough to withstand the pressure of the wet concrete and maintain the desired shape and dimensions. After the concrete has hardened sufficiently, the formwork is removed (stripped). 2.2.1 Formwork Materials: Common formwork materials include: Timber: Timber is a traditional formwork material, especially for small projects. It is relatively inexpensive and easy to work with, but it can be susceptible to warping and damage from moisture.

Steel: Steel formwork is strong, durable, and reusable. It is commonly used for large projects and complex shapes.

Plywood: Plywood is a versatile formwork material that is strong, lightweight, and relatively inexpensive. It is often used as a facing material for timber or steel frames.

Aluminum: Similar to steel, but lighter and easier to handle. More expensive than steel.

Engineered Plastics: These are increasing in popularity due to their reusability, durability, and ability to create complex shapes. 2.2.2 Factors influencing Formwork Selection: Cost: The cost of the formwork material is a major consideration.

Durability: The formwork must be able to withstand the pressure of the wet concrete and repeated use.

Ease of Use: The formwork should be easy to assemble, dismantle, and transport.

Shape and Size: The formwork must be able to create the desired shape and size of the concrete element.

Surface Finish: The formwork material can affect the surface finish of the concrete.

Reusability: Reusable formwork can be more cost-effective for large projects. 2.3 Reinforcement: Adding Strength Concrete is strong in compression (resisting crushing forces) but weak in tension (resisting pulling forces). Reinforcement, usually in the form of steel bars (rebar), is embedded in the concrete to improve its tensile strength. The steel and concrete work together to resist both compressive and tensile forces. 2.3.1 Types of Reinforcement: Steel Reinforcing Bars (Rebar): These are the most common type of reinforcement. They are available in various sizes and grades, each with different strength properties. In South Africa, rebar is typically manufactured to SANS 920 standard.

Welded Wire Fabric (WWF): This is a grid of welded steel wires used for reinforcing concrete slabs and other flat elements.

Fibers: Fibers made of steel, glass, or synthetic materials can be added to the concrete mix to improve its resistance to cracking. 2.3.2 Placement of Reinforcement: The placement of reinforcement is critical for the structural integrity of the concrete element. The rebar must be properly spaced and secured to prevent movement during concrete placement. Concrete cover (the distance between the rebar and the surface of the concrete) is also important to protect the steel from corrosion. The amount of cover required depends on the environmental conditions.

Example 1: Calculating Concrete Volume for a Slab A concrete slab is 5 meters long, 3 meters wide, and 0.15 meters thick. Calculate the volume of concrete required.