Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define the purpose of formwork and its materials.
• Describe different types of reinforcing steel.
• Explain the basic principles of a concrete mix.
• Describe how to place and compact concrete.
• Identify common defects in concrete.

Lesson summary

This week, we delve into the heart of construction: concrete, formwork, and reinforcement. These three elements work together to create the strong, durable structures that shape our world, from the foundations of our homes to the massive bridges we drive over. Understanding concrete, how to shape it (formwork), and how to strengthen it (reinforcement) is crucial for any civil technologist. In South Africa, with our growing infrastructure needs and the challenges of building sustainably, a solid understanding of these concepts is paramount. We need skilled individuals who can build safely and efficiently, using resources wisely.

Lesson notes

2.1 Concrete: The Building Block Concrete is a composite material composed primarily of: Cement: The binder that holds the aggregates together. Ordinary Portland Cement (OPC) is the most common type. It reacts with water (hydration) to form a hardened paste. The type of cement used affects the concrete's properties like setting time and strength.

Aggregates: Inert granular material, usually sand (fine aggregate) and gravel or crushed stone (coarse aggregate). Aggregates make up the bulk of the concrete and influence its workability, strength, and cost. The size and grading of aggregates are important.

Water: Essential for the hydration of cement. The water-cement ratio (w/c) is crucial; a lower w/c generally results in stronger concrete, but it can also make the mix less workable.

Admixtures: Chemical additives that modify the properties of concrete, such as workability, setting time, durability, or strength. Examples include water reducers, air-entraining agents, and accelerators/retarders.

Concrete Mix Design: The process of selecting and proportioning the ingredients to produce concrete with the desired properties.

Factors to consider include: Strength: The compressive strength of concrete is usually specified (e.g., 25 MPa, 30 MPa).

Workability: The ease with which concrete can be placed and compacted.

Durability: The ability of concrete to resist weathering, chemical attack, and abrasion.

Cost: The cost of the ingredients and the mixing process.

Example 1: Calculating Cement Required Let's say we need to create 1 cubic meter (m³) of concrete with a target strength of 25 MPa. A typical mix design for this strength might be: Cement: 300 kg/m³ Fine Aggregate (Sand): 600 kg/m³ Coarse Aggregate (Gravel): 1200 kg/m³ Water: 180 litres/m³ To calculate how much cement you need for 0.5 m³ of concrete: Cement needed = (300 kg/m³) * (0.5 m³) = 150 kg 2.2 Formwork: Shaping the Concrete Formwork is the temporary mold into which concrete is poured and allowed to harden. It supports the wet concrete and shapes it to the desired form.

Materials: Common formwork materials include: Timber: Traditional and readily available, but can be labor-intensive to construct.

Steel: Strong and durable, suitable for repetitive use, but more expensive.

Plywood: Widely used due to its versatility, strength, and ease of handling.

Aluminum: Lightweight and durable, often used for specialized applications.

Design Considerations: Formwork must be strong enough to withstand the pressure of wet concrete, stable to maintain the desired shape, and watertight to prevent leakage.

Stripping: The process of removing the formwork after the concrete has hardened sufficiently. Timing is crucial – too early, and the concrete may be damaged; too late, and it becomes difficult.

Example 2: Formwork Pressure The pressure exerted by wet concrete on formwork depends on the density of the concrete, the height of the pour, and the rate of pour. A simplified formula for calculating lateral pressure (P) is: P = γ * h Where: P = Lateral pressure (kN/m²) γ = Density of concrete (typically 24 kN/m³) h = Height of the pour (m) If we are pouring concrete to a height of 2 meters, the lateral pressure on the formwork would be: P = (24 kN/m³) * (2 m) = 48 kN/m² This pressure needs to be considered when designing the formwork to ensure it can withstand the load. 2.3 Reinforcement: Adding Strength Concrete is strong in compression (resisting being crushed) but weak in tension (resisting being pulled apart). Reinforcement, typically steel bars (rebar), is added to concrete to increase its tensile strength.

Types of Reinforcement: Deformed Bars: Most common type, with ribs or indentations to improve bond with concrete.

Welded Wire Fabric (WWF): A grid of wires welded together, used for slabs and walls.

High-Tensile Steel: Used for prestressed concrete.

Functions of Reinforcement: Resists tensile stresses. Controls cracking. Increases ductility (ability to deform without breaking).

Placement: Reinforcement must be properly placed and secured within the formwork to ensure it is effective. Minimum concrete cover (the distance between the rebar and the surface of the concrete) is required to protect the steel from corrosion.

Example 3: Calculating Reinforcement Area Let's consider a simple concrete beam subjected to bending. The amount of tensile reinforcement required depends on the bending moment (M) and the properties of the concrete and steel. A simplified formula (for illustration) is: As = M / (fy * jd)

Where: As = Area of steel reinforcement required (mm²) M = Bending moment (Nmm) – Assume M = 50 x 10^6 Nmm fy = Yield strength of steel (MPa) – Assume fy = 400 MPa jd = Lever arm (approx. 0.9d, where d is the effective depth of the beam) – Assume d = 400mm, therefore jd = 360mm As = (50 x 10^6 Nmm) / (400 MPa * 360 mm) = 347.22 mm² This tells us the total area of steel reinforcement needed. We would then select appropriate rebar sizes and spacing to achieve this area.