GENERAL

The specifications require that all material proportioning equipment be calibrated and checked for accuracy. Refer to Section 2001 of the Standard Specifications for calibration tolerances. The job mix formula provides the basis for the calibrations.

The District Materials Engineer shall be afforded the opportunity to witness the calibration. The plant calibration may be monitored by and is subject to the approval of the District Materials Engineer or authorized representative. A copy of the calibration report shall be available at the plant site.

CALIBRATION

The contractor is responsible for plant calibration. The specifications require the contractor to provide personnel, scales, test weights, and equipment for calibrating each delivery component.

Scales may be calibrated by a certified scale company. Check weighing may be used to validate the calibration.

Equipment that needs to be calibrated may include these items.

1. Cold Aggregate Bins

2. RAM Bins

3. Conveyor Scales

4. Asphalt Pump/ inline flow meter

5. Hot Aggregate Bins on Batch Plants

6. Batch Plant Scales

7. Initial Plant Settings

8. Mixing Rate

Where possible it is good practice to calibrate at high, medium, and low production rates. This will help insure that the plant is in calibration when the production rate changes during the project.

The calibration tolerances for proportioning and weighing equipment are as follows:

+/– 0.2% for scales and binder meters used to determine pay quantity (platform scale, silo discharge weigh hopper or batch plant proportioning scales when counting batches).

+/- 0.5% for proportioning scales (aggregate weigh hopper or binder weigh bucket on batch plants when not counting batches for pay quantity).

+/- 1.5% for all weigh belts and binder meters not used for pay quantities (drum mix plants).

Calibration can be considered to have been achieved after two consecutive runs within tolerance.

REPORTING

See the following website for a plant calibration worksheet:

http://www.iowadot.gov/Construction_Materials/hma.html

See Appendix B for plant calibration worksheet example.