Following the IS (Indian Standard) code for the preparation, casting, and curing of concrete cubes is essential for ensuring quality and compliance with Indian construction standards. Here is a detailed step-by-step guide based on IS 516: 1959 "Method of Tests for Strength of Concrete" and IS 1199: 1959 "Methods of Sampling and Analysis of Concrete":
Step 1: Preparation
1. **Materials**: Gather concrete mix, cube molds (typically 150mm x 150mm x 150mm or 100mm x 100mm x 100mm), tamping rod, scoop, trowel, curing equipment, and a clean cloth.
2. **Mold Preparation**: Ensure the molds are clean, free from any previous residues, and lightly oiled on the inside surfaces to facilitate easy removal.
Step 2: Sampling
1. **Collecting Concrete Sample**:
- Collect the concrete sample from the middle portion of the concrete batch.
- Follow the procedures in IS 1199: 1959 for sampling, ensuring that the sample is representative of the entire batch.
2. **Mixing**: Thoroughly remix the sample before filling the molds to ensure uniformity.
Step 3: Filling the Molds
1. **Layering**:
- Fill the mold in layers, typically three layers of approximately equal depth (each about one-third of the mold’s height).
2. **Compaction**:
- **Hand Compaction**: For each layer, use a standard tamping rod (16mm in diameter, 600mm in length) to tamp the concrete. Tamp each layer 35 times uniformly.
- **Vibration**: Alternatively, use a vibration table if available, particularly for high-strength concrete mixes. Ensure that each layer is vibrated adequately to remove air bubbles.
3. **Striking Off**: After the top layer is compacted, strike off the excess concrete with a trowel or straight edge to achieve a smooth, level surface.
Step 4: Curing
1. **Initial Curing**:
- Leave the filled molds undisturbed at a place free from vibration and at a temperature of 27°C ± 2°C for 24 ± 0.5 hours.
2. **Demolding**:
- After 24 hours, carefully remove the concrete cubes from the molds.
3. **Further Curing**:
- Submerge the demolded cubes in a curing tank filled with clean water or in a moist curing room. Maintain the curing temperature at 27°C ± 2°C.
- Ensure that the cubes are fully submerged and not exposed to air during the curing period.
Step 5: Testing
1. **Testing Age**:
- Standard testing ages are 7 days and 28 days.
2. **Compressive Strength Testing**:
- Clean the surface of the cubes and remove any loose sand or grit.
- Place the cube in the compression testing machine with the cast faces in contact with the platens of the testing machine.
- Apply the load uniformly without shock and continuously at a rate of approximately 140 kg/cm² per minute until failure occurs.
- Record the maximum load applied to the cube and calculate the compressive strength.
Notes:
- Ensure the molds and other equipment comply with IS 10086: 1982 “Specification for Moulds for Use in Tests of Cement and Concrete.”
- Maintain a log of all samples, including date of casting, batch number, and specific conditions during sampling and curing.
- The curing tank should have adequate water circulation to maintain uniform temperature.
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By following these detailed steps as per the IS code, you can ensure that the concrete cubes are prepared, cured, and tested accurately, providing reliable results for the assessment of concrete strength.