Position: The batching system shall be equipped with an accurate recorder or recorders that meet the requirements of NRMCA CPMB 100. Separate bins or compartments shall be provided for each size group of aggregate and cement, pozzolan, and GGBFS. Aggregates shall be weighed either in separate weigh batchers with individual scales or cumulatively in one weigh batcher on one scale. Aggregate shall not be weighed in the same batcher with cement, pozzolan, or GGBFS. If both cement and pozzolan or GGBFS are used, they may be batched cumulatively provided that the portland cement is batched first.
If measured by mass, the mass of the water shall not be batched cumulatively with another ingredient. Water batcher filling and discharging valves shall be so interlocked that the discharge valve cannot be opened before the filling valve is fully closed. An accurate mechanical device for measuring and dispensing each admixture shall be provided. Each dispenser shall be interlocked with the batching and discharging operation of the water so that each admixture is separately batched and discharged automatically in a manner to obtain uniform distribution throughout the batch in the specified mixing period. Admixtures shall not be combined prior to introduction in water. The plant shall be arranged so as to facilitate the inspection of all operations at all times. Suitable facilities shall be provided for obtaining representative samples of aggregates from each bin or compartment. All filling ports for cementitious materials bins or silos shall be clearly marked with a permanent sign stating the contents.
2 Scales
The batching equipment by mass shall conform to the applicable requirements of NIST HB 44, except that the accuracy shall be plus or minus 0.2 percent of scale capacity. The Contractor shall provide standard reference masses and any other auxiliary equipment required for checking the operating performance of each scale or other measuring devices. The tests shall be made at the frequency required in paragraph TESTS AND INSPECTIONS and in the presence of a Government inspector.
3 Batching Tolerances
Tolerances on determination of mass: PERCENT OF REQUIRED
MATERIAL
MASS Cementitious materials -0 to +2
Aggregate ±2
Water ±1
Chemical admixture -0 to +6
For volumetric batching equipment, the following tolerances shall apply to the required volume of material being batched: Water: Plus or minus 1 percent. Chemical admixtures: Zero to plus 6 percent.
4 Water Control
The batching plant shall be capable of ready adjustment to compensate for the varying moisture content of the aggregates and to change the masses of the materials being batched.
