- If oversized material is present in the stockpile, screen the aggregate prior to loading and calibrating the application equipment.
- Choose three gate settings (3”, 4”, and 5” are typical examples) at which to calibrate the aggregate and record them on the Aggregate Calibration worksheet. Also, note the unit for which the calibration is being completed as well as the date on the worksheet.
- Determine the moisture content of the aggregate and calculate the moisture factor which is used to convert the wet aggregate weight into dry aggregate weight. The moisture factor equals the moisture in the aggregate (in decimal form) plus 1.00. For each gate setting on the worksheet, record the moisture factor in the spaces provided.
Example: If the moisture is 5%, the moisture factor is 1.05.
Calculation: 0.05 + 1.00 = 1.05
Designer’s Note: A proven moisture content test method such as AASHTO T 255, Standard Method of Test for Total Evaporable Moisture Content of Aggregate by Drying, or a mutually agreed upon test method for aggregate moisture content determination, should be utilized.
- To ensure consistent calibration, ensure the aggregate belt is charged with rock/aggregate.
- Set the gate to the smallest opening chosen for calibration. (In the case of the typical examples given in Step 2 above, that would be the 3” setting).
- There are 2 options for determining the sample weights of the aggregate. Pick the option that will be used to determine how to complete the worksheet.
Option 1: The application equipment is filled with aggregate and weighed on a certified truck scale before and after each sample run in order to calculate the sample size. For this option, note the start weight in the “Weight A” column and the end weight in the “Weight B” column on the worksheet. This is the option displayed in the Aggregate Calibration worksheet example.
Option 2: A container, into which the aggregate is delivered, is weighed on a certified platform scale before and after each sample run in order to calculate the sample size. For this option, note the start weight in the “Weight B” column and the end weight in the “Weight A” column on the worksheet.
- Zero the aggregate counter before each sample run.
- Run a minimum of 50 counts of the aggregate belt. The resulting sample size should be large enough to ensure greater than 2% accuracy. Minimum sample size is calculated by dividing scale accuracy by 2%.
Examples of minimum sample size include: 50 lbs. of aggregate for a platform scale with ± 1 lb. accuracy or 1000 lbs. for a truck scale with ± 20 lb. accuracy.
- After delivery of the aggregate is complete, note the end weight in the appropriate space on the worksheet.
- Determine the net weight (weight A minus weight B) for the first sample. Divide the net weight of the aggregate by the number of counts of the aggregate belt and record the lbs./count of the wet aggregate for the first sample.
- Repeat Steps 7-10 for two additional sample runs at the same gate opening and record the results on the worksheet. It is important to run three calibration samples at each gate opening and then to average the lbs./count of wet aggregate for the three samples as illustrated in the completed worksheet example.
- Calculate the average dry aggregate lbs./count for the gate opening by dividing the average wet aggregate lbs./count by the moisture factor determined in Step 3. Record the average dry aggregate lbs./count in the appropriate space on the worksheet for the gate setting.
- Set the gate to the second gate opening and repeat Steps 7-12, then set the gate to the third gate opening and repeat Steps 7-12 again. To ensure consistent calibration ensure the aggregate belt is recharged after resetting the gate each time.
The aggregate belt should deliver consistent weights so that the deviation for any individual sample run should not exceed 2% of the mathematical average of the three runs.
Example: The average wet aggregate lbs./count at the 3” gate setting is 26.2. 2% of 26.2 is 0.5, so each individual run at the 3” gate setting should be between 25.7 and 26.7 lbs./count.
