Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Describe types and purposes of formwork.
• Calculate required reinforcement for a simple structure.
• Explain the importance of concrete cover and compaction.
• Distinguish between different types of reinforcement steel.
• Design a simple formwork system for a column.

Lesson summary

Concrete, formwork, and reinforcement are fundamental elements in the construction of most modern structures, from houses and roads to bridges and dams. In South Africa, these materials are crucial for building infrastructure that supports economic growth, provides housing, and connects communities. Understanding their properties and proper application is essential for ensuring the safety, durability, and sustainability of our built environment. Inadequate knowledge or poor workmanship in these areas can lead to structural failures, costly repairs, and even loss of life.

Lesson notes

2.1 Formwork Formwork is the temporary mould into which concrete is poured and cast. It is essential to hold the concrete in place until it gains sufficient strength to support itself. Formwork must be strong, rigid, and watertight to prevent leakage of cement slurry and maintain the desired shape and dimensions of the concrete structure. It must also be easy to erect, dismantle, and reuse.

Types of Formwork: Timber Formwork: This is the most common type of formwork, especially for smaller projects. Timber is readily available, relatively inexpensive, and easy to work with.

However, it is susceptible to water damage and can warp or twist if not properly seasoned or treated.

Steel Formwork: Steel formwork is more durable and can be reused many times. It is typically used for large-scale projects like bridges and high-rise buildings where its initial high cost can be offset by its long lifespan and efficiency. Steel formwork provides a smooth, accurate finish.

Plywood Formwork: Plywood sheets are often used as a facing material for both timber and steel formwork. They provide a smooth surface and are relatively lightweight and easy to cut to size.

Plastic Formwork: Plastic formwork is becoming increasingly popular due to its durability, lightweight, and resistance to water damage. It is often used for specialized applications like curved surfaces or complex shapes.

Design Considerations for Formwork: Load Capacity: Formwork must be designed to withstand the weight of the wet concrete, construction loads (e.g., workers, equipment), and environmental loads (e.g., wind, rain).

Stability: The formwork must be stable and prevent any movement or deformation during concrete placement and curing.

Watertightness: The formwork must be watertight to prevent leakage of cement slurry, which can weaken the concrete and create unsightly surface defects.

Ease of Stripping: The formwork must be designed for easy removal without damaging the concrete structure. Proper form release agents should be used.

Example: Consider designing timber formwork for a rectangular concrete column that is 300mm x 400mm and 3m high.

Material Selection: We'll use 50mm thick timber planks for the sides of the formwork and timber studs for support.

Panel Construction: Each side panel will be constructed from multiple timber planks nailed together.

Stud Placement: Place vertical timber studs every 500mm along the height of the panel to provide support and prevent bulging.

Yokes/Clamps: Use steel yokes or timber clamps to hold the panels together and resist the pressure of the wet concrete. Place yokes every 500mm along the height of the column. 2.2 Reinforcement Reinforcement is steel bars or mesh embedded in concrete to improve its tensile strength. Concrete is strong in compression but weak in tension. Reinforcement provides the necessary tensile strength to resist bending, shear, and other tensile stresses.

Types of Reinforcement Steel: Mild Steel Reinforcement: This type of steel is relatively inexpensive and easy to bend. It is typically used for lightly loaded structures or for stirrups in beams.

High-Yield Steel Reinforcement: This type of steel has a higher yield strength than mild steel, allowing it to carry higher loads. It is commonly used for beams, columns, and slabs.

Deformed Bars: Deformed bars have ribs or indentations on their surface to improve bond with the concrete. This enhances the transfer of stresses between the steel and the concrete.

Welded Wire Mesh (WWM): This is a grid of steel wires welded together at intersections. It is typically used for reinforcing slabs and pavements.

Concrete Cover: Concrete cover is the distance between the surface of the reinforcement and the nearest surface of the concrete. It is essential to protect the reinforcement from corrosion and fire. Adequate concrete cover also ensures proper bond between the steel and the concrete. The required concrete cover depends on the exposure conditions and the size of the reinforcement. In South Africa, SANS 10100-1 provides guidelines for minimum concrete cover requirements.

Reinforcement Placement: Spacing: Proper spacing of reinforcement is crucial to ensure that the concrete can effectively grip the steel and prevent cracking.

Lapping: When reinforcement bars need to be joined, they are lapped, which means overlapping them by a certain length. The lap length depends on the diameter of the bars and the strength of the concrete.

Fixing: Reinforcement bars are typically tied together using wire or clips to hold them in place during concrete placement. Chairs and spacers are used to maintain the correct concrete cover.

Example: Calculating the area of steel reinforcement required for a concrete beam. A simply supported reinforced concrete beam has a width (b) of 250mm, an effective depth (d) of 400mm, and is subjected to a bending moment (M) of 80 kNm. Assume the allowable stress in the steel (fs) is 140 MPa and the lever arm factor (j) is 0.85.