Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Describe types of cement and aggregates.
• Explain the process of mixing concrete and the water-cement ratio.
• Identify types of formwork and their purpose.
• Explain the function and types of reinforcement.
• Calculate the volume of concrete for simple structures.

Lesson summary

Concrete, formwork, and reinforcement are fundamental elements in civil construction. Understanding these concepts is crucial for building durable, safe, and sustainable infrastructure in South Africa. From houses and schools to bridges and dams, concrete is the backbone of our built environment. This week, we'll delve into the practical aspects of mixing concrete, understanding the purpose and application of formwork, and the importance of reinforcement in strengthening concrete structures. In a country striving for infrastructure development, a solid grasp of these principles equips you with skills that are valuable and relevant to the South African context.

Lesson notes

2.1 Concrete Constituents and Mixing Concrete is a composite material made primarily of cement, aggregates (sand and gravel or crushed stone), and water. Sometimes, admixtures are added to modify the concrete's properties.

Cement: Cement acts as a binder, holding the aggregates together. Different types of cement are available, each with specific properties: Ordinary Portland Cement (OPC): The most common type, used for general construction.

Rapid Hardening Cement: Sets and gains strength faster than OPC, useful for quick repairs and projects where early strength is crucial (e.g., road patching).

Sulphate Resisting Cement (SRC): Used in environments with high sulphate content, such as coastal areas or areas with sulphate-rich soil. Sulphates can attack concrete and cause it to deteriorate. South Africa's coastal regions make SRC important.

Pozzolanic Cement: Contains pozzolans (e.g., fly ash, silica fume) which react with calcium hydroxide (produced during cement hydration) to create additional cementitious compounds, improving durability and reducing permeability. Often used in large structures like dams to reduce heat of hydration.

Aggregates: Aggregates make up the bulk of the concrete mix. They provide strength, stability, and resistance to wear.

Fine Aggregates (Sand): Fill the voids between the coarse aggregates. Should be clean and free from organic matter. Coarse Aggregates (Gravel or Crushed Stone): Provide the main structural strength. Graded aggregates (a mix of different sizes) are preferred as they pack more densely, leading to stronger concrete.

Water: Water is essential for the chemical reaction (hydration) that allows the cement to bind the aggregates together. Potable (drinkable) water is generally suitable. Seawater should be avoided, as the salts can corrode reinforcement steel.

Water-Cement Ratio (W/C Ratio): The ratio of water to cement by weight is critical. A lower W/C ratio generally results in stronger, more durable concrete, but it also makes the mix less workable. A higher W/C ratio makes the concrete easier to work with but reduces strength and increases permeability. Typical W/C ratios range from 0.4 to 0.

6. Example: If you use 50 kg of cement, and the desired W/C ratio is 0.5, you need 50 kg 0.5 = 25 kg of water (which is approximately 25 liters).

Mixing: Proper mixing ensures that the cement, aggregates, and water are evenly distributed. Over-mixing can lead to segregation (separation of aggregates from the cement paste), while under-mixing can result in weak spots in the concrete. Concrete can be mixed manually (for small projects) or with a concrete mixer (for larger projects). 2.2 Formwork Formwork is a temporary mold that supports the concrete while it cures. It provides the desired shape and size to the concrete structure.

Types of Formwork Materials: Timber: Traditional and still widely used, especially for smaller projects. Timber formwork is relatively inexpensive and easy to work with, but it can warp and absorb water.

Steel: Strong and durable, suitable for large projects where formwork needs to be reused multiple times. Steel formwork is more expensive than timber but provides a smoother finish and can withstand higher pressures.

Plywood: A common choice, offering a good balance of strength, cost, and ease of use. Plywood can be treated with a release agent to prevent the concrete from sticking to it.

Plastic: Lightweight and reusable, suitable for simple shapes and designs.

Formwork Design Considerations: Strength: Formwork must be strong enough to support the weight of the wet concrete without collapsing or deforming.

Stability: Formwork must be stable and prevent lateral movement during pouring and curing.

Tightness: Formwork must be tight enough to prevent leakage of cement slurry, which can weaken the concrete and leave a rough surface finish.

Ease of Stripping: Formwork should be designed for easy removal without damaging the concrete. 2.3 Reinforcement Concrete is strong in compression (resisting squeezing forces) but weak in tension (resisting pulling forces). Reinforcement, typically steel bars (rebar), is embedded in the concrete to provide tensile strength.

Types of Reinforcement: Reinforcing Steel Bars (Rebar): The most common type of reinforcement. Available in different sizes and grades, with varying tensile strengths. The grade of steel is typically indicated by the yield strength (e.g., Grade 400 rebar has a yield strength of 400 MPa).

Welded Wire Mesh (WWM): A grid of steel wires welded together, used for reinforcing slabs, walls, and pavements.

Fibers: Short, discontinuous fibers (steel, glass, or synthetic) added to the concrete mix to improve its tensile strength and reduce cracking.

Function of Reinforcement: Resisting Tensile Forces: Reinforcement steel carries the tensile forces in concrete structures, preventing cracking and failure.