the batching cycle when the scale has not returned to ± 0.3 percent of zero 

 balance or when preset weighing tolerances are exceeded. An individual 

 automatic batching system provides separate scales and batchers for each 

 aggregate size and for each of the other materials batched. The weighing 

 cycle is started by a single starter switch, and individual batchers are 

 charged simultaneously. 



b . Mixing . 



(1) Total Mixing Water. Uniformity in the measurement of total 

 mixing water involves, in addition to the accurate weighing of added 

 water, control of such additional water sources as mixer wash water, ice, 

 and free moisture in aggregates. One specified tolerance (ASTM Standard 

 C94) for accuracy in measurement of total mixing water, from all sources, 

 is ± 3 percent. 



(2) Measurement of Admixtures . Use of admixtures in concrete, 

 particularly air-entraining agents, is a universally accepted practice. 

 Batching tolerance and charging-discharge interlocks should also be provided 

 for admixtures. 



(3) Measurement of Materials for Small Jobs . Occasionally the 

 concrete volume on a job is so small, e.g., 76 cubic meters (100 cubic 

 yards) or less, that it is not practical to establish and maintain a batch 

 plant and mixer at the construction site. In this case it is preferable to 

 use ready-mixed concrete or centrally dry-batched materials with truck 

 mixing at the job. If centrally dry-batched concrete is not available, 

 proper precautions should still be taken to properly measure and mix 

 concrete materials. Thorough mixing is essential for the production of 

 uniform concrete. Therefore, equipment and methods used should be capable 

 of effectively mixing concrete materials containing the largest specified 

 aggregate to produce uniform mixes of the lowest slump practical for the 

 work. 



c. Transporting . 



(1) General Considerations . Concrete can be transported by a 

 variety of methods and equipment, such as truck mixers, stationary truck 

 bodies with and without agitators, buckets hauled by truck or railroad 

 car, pipeline or hose, or conveyor belts. Each type of transportation has 

 specific advantages and disadvantages depending on the conditions of use, 

 mix ingredients, accessibility and location of placing site, required 

 capacity and time for delivery, and weather conditions. 



(2) Mixing and Transporting in Revolving-Drum Truck Bodies . Some 

 specifications limit the total drum revolutions that can be used for 

 charging, mixing, agitation, and discharge of concrete in revolving-drum 

 trucks (Fig. 21). Others place limits on the number of revolutions at 

 mixing speed only. A maximum elapsed time of 1.5 hours after the cement 

 has entered the drum until completion of discharge is also frequently 

 specified. Also, provision is made for reduction of the maximum elapsed 

 time in warm weather (ASTM C94-69) . Another specification method used is 

 to place no limits on revolutions or elapsed time as long as the specified 

 mixing water is not exceeded, no retempering water is added, the concrete 



139 



