Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Identify different types of formwork and their applications.
• Explain the purpose and types of reinforcement in concrete.
• Calculate required steel reinforcement for a simple beam.
• Describe the process of placing reinforcement within formwork.
• Understand the importance of proper formwork design.

Lesson summary

Concrete is a fundamental building material used extensively in South Africa, from the foundations of our homes to the construction of roads, bridges, and even the iconic Moses Mabhida Stadium. Its strength, durability, and versatility make it indispensable for infrastructure development. Formwork provides the temporary mold that shapes the concrete, while reinforcement (typically steel) enhances its tensile strength, preventing cracking and ensuring structural integrity. Understanding these three components – concrete itself, the formwork that shapes it, and the reinforcement that strengthens it – is crucial for anyone pursuing a career in civil technology or related fields.

Lesson notes

A. Formwork Materials and Applications Formwork is a temporary structure used to contain freshly placed concrete, supporting it until it gains sufficient strength to support itself. The choice of formwork material depends on factors such as the size and shape of the concrete element, the required surface finish, cost, and reusability. Common formwork materials in South Africa include: Timber: Timber is a traditional and relatively inexpensive formwork material. It is easy to work with and can be cut to size on-site. Softwoods like pine are often used, but they need to be treated to prevent water absorption and rot. Timber formwork is suitable for smaller projects and simpler shapes.

Advantages: Readily available, cost-effective for small projects, easy to cut and shape.

Disadvantages: Limited reusability, susceptible to water damage, may require skilled labor for complex shapes.

Plywood: Plywood is a manufactured wood product made by gluing together thin layers of wood veneer. It is stronger and more durable than solid timber and provides a smoother surface finish. Plywood is widely used for formwork in South Africa, particularly for walls, slabs, and beams.

Advantages: Relatively strong and durable, provides a smooth surface, can be reused multiple times.

Disadvantages: More expensive than timber, susceptible to water damage if not treated, can be difficult to bend to complex shapes.

Steel: Steel formwork is the most durable and reusable option. It is typically used for large-scale projects, such as bridges and high-rise buildings. Steel formwork can be prefabricated into standard sizes and shapes, which speeds up the construction process.

Advantages: High strength and durability, reusable many times, provides a consistent surface finish.

Disadvantages: High initial cost, requires specialized equipment for handling and assembly, can be heavy and difficult to transport.

Aluminum: Aluminum formwork is lighter than steel formwork, making it easier to handle and assemble. It is also resistant to corrosion. Aluminum formwork is often used for high-rise buildings and other projects where weight is a concern.

Advantages: Lightweight, corrosion-resistant, reusable.

Disadvantages: Expensive, less strong than steel.

Plastic: Plastic formwork is becoming increasingly popular due to its durability, lightweight, and resistance to water damage. It is often used for columns, beams, and slabs. Plastic formwork can be molded into complex shapes, making it suitable for architectural concrete.

Advantages: Lightweight, durable, water-resistant, can be molded into complex shapes.

Disadvantages: Can be expensive, may not be suitable for all applications. B. Reinforcement in Concrete Structures Concrete is strong in compression (i.e., it can withstand crushing forces) but weak in tension (i.e., it cannot withstand pulling forces). Reinforcement is used to provide tensile strength to concrete structures, preventing cracking and ensuring that they can withstand bending and shear forces. Common types of reinforcement used in South Africa include: Steel Reinforcing Bars (Rebar): Rebar is the most common type of reinforcement. It is available in various diameters and grades of steel. The bars have deformations (ribs) on their surface to improve bond with the concrete. Common sizes are 8mm, 10mm, 12mm, 16mm, 20mm, 25mm, and 32mm.

Welded Wire Mesh (WWF): WWF is a grid of steel wires welded together at their intersections. It is often used for reinforcing concrete slabs, pavements, and walls. WWF is available in various sizes and mesh spacings.

Fibers: Fibers are short, discontinuous strands of material that are added to concrete to improve its tensile strength and crack resistance. Fibers can be made of steel, glass, plastic, or natural materials. C. Calculating Reinforcement Requirements (Example) Let's consider a simple example of calculating the required area of steel reinforcement for a reinforced concrete beam. We'll use simplified assumptions for demonstration purposes.

Example: A rectangular concrete beam has a width (b) of 250mm and an effective depth (d) of 400mm. The beam is subjected to a bending moment (M) of 80 kNm. Assuming a yield strength (f y ) of 460 MPa for the steel reinforcement and an appropriate concrete strength class, estimate the required area of steel reinforcement (A s ). We will use a simplified coefficient-based method.

Solution: Determine a suitable k value: Assuming a relatively low reinforcement ratio, we can assume a lever arm coefficient 'k' of approximately 0.

9. This needs to be checked and iterated on more complex calculations, but it's a good starting point for this simple example. This represents the ratio between the lever arm (distance between resultant compressive force in the concrete and tensile force in the steel) and the effective depth (d).