The correct procedure for manually mixing concrete involves careful proportioning of materials and thorough mixing on a non-absorbent surface.
Here's a step-by-step guide:
1. Preparation & Safety:
- Choose a Suitable Mixing Area: Select a clean, hard, and non-absorbent surface. Avoid mixing directly on soil, as it can contaminate the concrete and absorb water. A concrete slab, plywood sheet, or mixing tray works well.
- Gather Materials and Tools: Assemble the necessary materials: cement, sand (fine aggregate), gravel or crushed stone (coarse aggregate), and clean water. Have the following tools ready: a shovel or spade, a measuring container (bucket or scoop), and safety equipment (gloves, dust mask, and eye protection).
- Safety First: Always wear gloves, a dust mask, and eye protection to prevent skin irritation, inhalation of cement dust, and eye injury.
2. Proportioning the Materials:
- Determine the Mix Ratio: A common concrete mix ratio is 1:2:3 (cement:sand:aggregate) by volume. This may vary depending on the intended use of the concrete. For example, a stronger mix (e.g., 1:1.5:2) may be required for load-bearing structures. Consult a professional for specific project requirements.
- Measure Accurately: Use a consistent measuring container to ensure accurate proportions. Avoid estimating.
3. Dry Mixing:
- Combine Cement and Sand: Pour the measured amounts of cement and sand onto the mixing surface.
- Thoroughly Mix: Using a shovel or spade, thoroughly mix the cement and sand until the mixture is a uniform color. This ensures proper distribution of the cement. The goal is to eliminate streaks of pure cement or pure sand.
4. Adding Aggregate:
- Incorporate Aggregate: Add the measured amount of gravel or crushed stone (aggregate) to the cement-sand mixture.
- Mix Again: Mix the entire dry mixture (cement, sand, and aggregate) until it is uniform in color and the aggregate is evenly distributed.
5. Adding Water:
- Create a Well: Form a crater or well in the center of the dry mixture.
- Gradually Add Water: Slowly pour clean water into the well. Avoid adding too much water at once, as this can make the concrete too soupy and weaken it.
- Mix Carefully: Use the shovel or spade to gradually incorporate the water into the dry mixture, starting from the edges of the well. Continue mixing until the concrete reaches a workable consistency. It should be moist but not runny. A good consistency is often described as "plastic" - it holds its shape when molded but is still pliable.
6. Checking Consistency:
- Slump Test (Optional): For larger projects, a slump test can be performed to ensure consistency between batches. This requires a slump cone, which is filled with concrete in layers, tamped, and then removed. The amount the concrete slumps down is measured.
- Visual Inspection: The concrete should be workable, cohesive, and free of lumps. It should hold its shape but still be easy to manipulate. If the mixture is too dry, add a little more water. If it is too wet, add a small amount of the dry cement-sand-aggregate mix.
7. Using the Concrete:
- Use Promptly: Once mixed, concrete begins to set. Use the concrete as soon as possible, ideally within 30-60 minutes.
Summary:
Manually mixing concrete involves accurately measuring cement, sand, and aggregate, thoroughly dry-mixing them, gradually adding water, and achieving the correct consistency. Proper proportioning and thorough mixing are crucial for strong and durable concrete.
