A complete calibration shall be performed for each mixer used at the work site prior to performing the work.
The mixer shall be set at the appropriate operating revolutions per minute as determined by the mixer manufacturer or mixer operator. Discharge the cement until an even flow of cement is achieved. It shall then be stopped and the cement meter reset to zero. A suitable container shall be set below the discharge of cement. The time required to discharge 94 pounds of cement shall be measured with a stop watch, the number of counts on the cement meter shall be recorded, and the weight of the discharged cement determined. The process shall be repeated at least three more times or until the trials have proved consistent with one another.
The following formulas shall be used to calculate the number of counts per 94 pounds of cement and the time required to discharge 94 pounds of cement:
(Total Counts / Total Weight) x 94 pounds = Counts per bag of cement *
*Counts per bag of cement x 7 = Counts per cubic yard
(Total Time / Total Weight) x 94 pounds = Seconds per bag of cement
The accuracy of the water flow meter shall be verified by adjusting the flow to 2 gallons per minute. With the equipment operating at the required RPM’s, the water discharged during a one minute interval shall be collected and weighed. The weight in pounds of the discharged water shall be divided by 8.33 to determine the number of gallons. This procedure shall be repeated with the flow meter adjusted to 3 gallons per minute. A theoretical water setting on the flow meter can be determined by calculating the total water required according to the mix design and then deducting for aggregate moisture and water in the latex emulsion. One quart sample shall be taken from each latex concrete mobile mixer on the bridge deck before it pours to install.
Aggregate bin gates shall be calibrated based on the submitted mix design. Coarse aggregate and fine aggregate shall be weighed separately. The seconds per bag of cement for each mixer may be reduced by one half to reduce the aggregate weights to one half in order to provide a more manageable amount of material.
With the mixer set at the required operating RPM’s, calibrate the delivery of the latex modifier into a suitable container. Mobile mixers must be equipped with a latex strainer before the latex is delivered to the auger for mixing. The latex strainer should be cleaned before each calibration and load of the of the mobil mixer. Making sure the strainer is unobstructed, the latex throttling valve shall be adjusted to deliver the required amount of latex emulsion admixture for each 94 pounds of cement. The weight of the latex shall be determined, and if necessary, the valve shall be adjusted such that the amount of latex discharged is within ½ pound of the amount required for each 94 pounds of cement.
Equipment shall also be required in accordance with the manufacturer’s instructions for each specific admixture that may be required.