Calculating the amount of admixture needed for concrete involves determining the required volume or mass based on the concrete mix design and the admixture's recommended dosage. Here's a breakdown of the process:
1. Determine the Admixture Dosage Rate:
- Admixtures are typically dosed as a percentage of the cementitious material (cement plus any supplementary cementitious materials like fly ash or slag) by weight. This dosage rate is usually provided by the admixture manufacturer in their product data sheet. For example, it might be given as 0.5% to 1.5% of cementitious materials by weight.
2. Calculate the Total Cementitious Material:
- In your concrete mix design, determine the total weight of cement and any supplementary cementitious materials (SCMs). For example:
- Cement: 300 kg/m³
- Fly Ash: 75 kg/m³
- Total Cementitious Material = 300 kg + 75 kg = 375 kg/m³
3. Calculate the Admixture Quantity:
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Multiply the total cementitious material by the desired dosage rate. Choose a dosage rate within the manufacturer's recommended range based on the specific requirements of your concrete mix (e.g., desired workability, setting time).
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Example: If the recommended dosage rate is 1.0% and the total cementitious material is 375 kg/m³, then the required admixture is:
- Admixture = (1.0/100) * 375 kg = 3.75 kg/m³
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4. Convert Weight to Volume (Optional):
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If you need to dispense the admixture by volume, you need to convert the weight to volume using the admixture's specific gravity. The specific gravity is typically provided on the admixture's data sheet.
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Formula: Volume = Mass / (Specific Gravity * Density of Water)
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Assuming the density of water is 1000 kg/m³, and the specific gravity of the admixture is 1.1:
- Volume = 3.75 kg / (1.1 * 1000 kg/m³) = 0.00341 m³
- Convert to liters: 0.00341 m³ * 1000 liters/m³ = 3.41 liters/m³
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Summary of the Calculation:
- Get the Admixture Dosage: Obtain the manufacturer's recommended dosage rate as a percentage of cementitious materials by weight.
- Determine Cementitious Material: Calculate the total weight of cement and any SCMs in your concrete mix design per cubic meter (or cubic yard) of concrete.
- Calculate Admixture Weight: Multiply the total cementitious material by the chosen dosage rate (as a decimal) to determine the required weight of the admixture.
- Convert to Volume (if needed): Divide the weight of the admixture by (its specific gravity multiplied by the density of water) to convert the required weight to a volume.
Important Considerations:
- Accuracy: Accurate measurement of both cementitious materials and admixtures is critical for consistent concrete performance.
- Dispensing Equipment: Use calibrated dispensing equipment to ensure accurate admixture delivery.
- Manufacturer's Recommendations: Always follow the admixture manufacturer's recommendations for dosage rates, mixing procedures, and storage.
- Trial Batches: It is always recommended to perform trial batches of concrete with the chosen admixture dosage to verify the desired performance characteristics (e.g., slump, setting time, strength).
- Water Reduction: Many admixtures (water reducers, superplasticizers) will impact the amount of water required in the mix. Ensure water content is adjusted to achieve the target slump while accounting for the water-reducing effects of the admixture.