The short answer is that the typical dosage of a concrete accelerator ranges from 2% to 6% of the cement's weight.

However the exact amount is not a fixed number and depends heavily on several factors. Here is a detailed breakdown to help you understand.

Typical Dosage Ranges

Accelerators are generally categorized by their alkali content, which affects performance and safety.

Important: These are general guidelines. You must always follow the manufacturer's specific recommendations for the product you are using.

Key Factors That Determine the Exact Dosage

The "correct" dosage is determined by the desired setting time and the specific project conditions.

Desired Setting Time (速凝时间):

Sprayed Concrete (Shotcrete - 喷射混凝土): The goal is usually a final set in 1 to 3 minutes. This requires a higher dosage to ensure the concrete sticks to the surface without sagging or falling off.

General Construction (e.g., sealing leaks, repairs): The goal might be a final set in 3 to 10 minutes. A lower dosage might be sufficient.

Cement Type and Composition:

Different types of cement (e.g., CEM I, CEM II, CEM III) react differently with accelerators. The chemical composition of the cement (especially its C3A and alkali content) has a major impact.

Ambient Temperature:

Cold Weather: A higher dosage is often needed to achieve the same setting time as in normal temperatures.

Hot Weather: A lower dosage might be sufficient.

Concrete Mix Design:

The water-cement ratio, the presence of other admixtures (like superplasticizers), and the type of aggregates can all influence the effectiveness of the accelerator.

Step-by-Step Guide to Finding the Right Dosage

To determine the precise dosage for your specific project, you should conduct a simple test.

Example Calculation:
Let's say your concrete mix uses 400 kg of cement per cubic meter, and the manufacturer's recommended starting dosage is 4%.

Calculate the Accelerator Quantity:

Accelerator Weight = Cement Weight × Dosage Rate

Accelerator Weight = 400 kg × 0.04 (which is 4%)

Accelerator Weight = 16 kg per m³ of concrete

Perform a Field Test (The "Pat Test" for Shotcrete):

Mix a small batch of concrete with the calculated dosage (e.g., 16 kg/m³).

Spray it onto a test panel or a simple vertical surface.

Observe the setting time and the build-up thickness. If the concrete sags or falls off, the dosage is too low. If it sets too quickly and creates excessive dust (rebound), the dosage might be too high.

Adjust the dosage up or down in small increments (e.g., 0.5%) and repeat the test until you achieve the perfect set time and adhesion.

Critical Warnings and Best Practices

Do Not Overdose: Adding too much accelerator will cause the concrete to flash-set, making it unworkable. It can also lead to a significant loss of long-term strength (can be more than 30%) and increased shrinkage cracking.

Consistent Mixing: The accelerator must be mixed thoroughly and uniformly into the concrete. For wet-mix shotcrete, it is usually added at the nozzle.

Follow the Data Sheet: The manufacturer's technical data sheet is your most reliable source for initial dosage, compatibility, and safety information.

Safety First: Alkaline accelerators are corrosive. Always wear appropriate Personal Protective Equipment (PPE) like gloves and goggles.

Summary

Final Advice: Start with the manufacturer's recommended mid-range dosage (e.g., 4%), perform a small-scale test, and fine-tune the amount based on the observed setting time and the requirements of your specific application.