Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Apply principles of concrete technology.
• Calculate quantities and costs of materials.
• Analyze and interpret structural drawings.
• Evaluate the impact of construction on environmental sustainability.
• Apply knowledge of the OHS Act on construction sites.

Lesson summary

This week marks a crucial stage in your Grade 12 Civil Technology journey – the final revision and dedicated examination preparation. We'll be consolidating all the knowledge and skills you've acquired throughout the year to ensure you're well-prepared to tackle the final examination. This is important because your performance in Civil Technology directly impacts your tertiary education opportunities and potential future career paths within the construction and infrastructure development sectors, which are vital for South Africa's economic growth and social development.

Lesson notes

This week's revision focuses on several key areas essential for exam success. These areas were chosen based on past papers and the overall weighting of the CAPS curriculum. 2.1 Concrete Technology: Concrete is the backbone of modern infrastructure. A thorough understanding of its properties, production, and application is paramount.

Mix Design: The process of determining the proportions of cement, aggregates (fine and coarse), water, and admixtures to achieve desired concrete properties such as strength, workability, and durability. The water-cement ratio (w/c) is crucial; lower w/c generally leads to higher strength but can reduce workability.

Types of Cement: Understanding the different types of cement (e.g., Ordinary Portland Cement (OPC), Rapid Hardening Cement, Sulphate Resisting Cement) and their specific applications is critical. OPC is the most common, while rapid hardening cement is used when quick strength gain is required, and sulphate-resisting cement is used in environments with high sulphate content (e.g., coastal areas).

Aggregates: Aggregates constitute a large portion of the concrete mix. Their size, shape, and grading significantly influence the concrete's properties. Proper grading ensures minimal voids and optimal packing, leading to a denser and stronger concrete.

Admixtures: Chemical admixtures are added to concrete to modify its properties.

Examples include: Water reducers: Increase workability without increasing the w/c ratio.

Air-entraining agents: Improve freeze-thaw resistance.

Retarders: Delay setting time, useful in hot weather.

Accelerators: Speed up setting time, useful in cold weather.

Curing: Maintaining adequate moisture content and temperature in concrete after placement to allow proper hydration of the cement and development of strength. Different curing methods exist (e.g., water curing, membrane curing, steam curing), each suitable for different conditions and applications. Incorrect curing can lead to cracking and reduced strength.

Reinforcement: Steel reinforcing bars (rebar) are embedded in concrete to enhance its tensile strength. Concrete is strong in compression but weak in tension; steel provides the necessary tensile strength to resist bending and cracking.

Example 1 (Concrete Mix Design): A concrete mix is required with a target strength of 30 MPa at 28 days.

Assume the following: Cement type: OPC (Ordinary Portland Cement)

Slump required: 50-75mm Maximum aggregate size: 20mm Exposure condition: Moderate Using relevant charts (which would be provided in an exam, or from SANS 10100-2), you would determine the appropriate water-cement ratio, cement content, and aggregate proportions based on the target strength, slump, and exposure condition.

Note: We cannot provide specific charts here due to space constraints. The point is to understand the process of using these charts.

Step 1: Determine the w/c ratio from strength requirements (e.g., From a chart, a 30 MPa strength with OPC might require a w/c ratio of 0.5).

Step 2: Determine the water content for desired slump and aggregate size (e.g., From a chart, a 50-75mm slump with 20mm aggregate might require 180 kg/m³ of water).

Step 3: Calculate the cement content: Cement content = Water content / w/c ratio = 180 kg/m³ / 0.5 = 360 kg/m³ Step 4: Determine the aggregate proportions. This often involves using tables or specific gravity information and adjusting based on the mix design method. 2.2 Quantity Surveying and Costing: Accurate quantity surveying and costing are vital for project budgeting and profitability.

Measurement Principles: Understanding standard units of measurement for different materials and work items (e.g., cubic meters for concrete, square meters for plastering, linear meters for pipes).

Taking Off Quantities: The process of extracting quantities of materials and labor from drawings and specifications. It requires meticulous attention to detail and a clear understanding of construction methods.

Bill of Quantities (BOQ): A document that lists all the items of work required for a project, along with their quantities and descriptions. The BOQ is used for tendering and contract administration.

Pricing: Assigning costs to the quantities listed in the BOQ, considering material prices, labor rates, plant hire costs, and overheads. It's important to factor in a reasonable profit margin.

Cost Estimation: Developing a comprehensive estimate of the total cost of a project, including direct costs (materials, labor, plant) and indirect costs (overheads, contingencies).

Example 2 (Quantity Surveying): Calculate the quantity of concrete required for a rectangular concrete slab that is 6m long, 4m wide, and 0.15m thick.

Solution: Volume = Length x Width x Thickness = 6m x 4m x 0.15m = 3.6 m³ Therefore, the quantity of concrete required is 3.6 cubic meters.

Example 3 (Cost Estimation): Estimate the cost of plastering a wall that is 5m long and 3m high.