(6) Estimation of Coarse Aggregate Content . Varying the volume 



of coarse aggregate used per unit volume of concrete changes the workability 

 of the mix. Given coarse and fine aggregates of available maximum size 

 and gradation, respectively, the correct volume of coarse aggregate per 

 unit volume of concrete must be chosen to produce satisfactorily workable 

 concrete. The fine aggregate is characterized by its fineness modulus, a 

 measure of the part of the coarse and medium sand, described in ASTM 

 Standard C125. The fineness modulus increases with coarseness and is 

 usually restricted to values between 2.3 and 3.1. The weight and volume 

 characteristics of coarse aggregate are determined by tests on dry aggregate 

 placed in thin layers and compacted by rodding. 



The volume of coarse aggregate, in cubic feet, on a dry-rodded basis, 

 for a cubic yard of concrete is equal to the value from Table 14 multiplied 

 by 27. This volume is converted to dry weight of coarse aggregate required 

 in a cubic yard of concrete by multiplying it by the dry-rodded weight per 

 cubic foot of the coarse aggregate. 



(7) Estimation of Fine Aggregate Content . At the completion of 

 (6), all ingredients of the concrete have been estimated except the fine 

 aggregate. Its quantity is determined by difference. Either of two 

 procedures may be employed- - i. e. , the weight method or the absolute volume 

 method. 



If the weight of the concrete per unit volume is assumed or can be 

 estimated from experience, the required weight of fine aggregate is simply 

 the difference between the weight of fresh concrete and the total weight of 

 the other ingredients. Often the unit weight of concrete is known with 

 reasonable accuracy from previous experience with the materials. In the 

 absence of such information, Table 15 can be used to make a first estimate. 

 Even if the estimate of concrete weight per cubic yard is rough, mixture 

 proportions will be sufficiently accurate to permit easy adjustment on the 

 basis of trial batches. 



(8) Adjustments For Aggregate Moisture . The aggregate quantities 

 actually to be weighed out for the concrete must allow for moisture in the 

 aggregates. Generally, the aggregates will be moist and their dry weights 

 should be increased by the percentage of water they contain, both absorbed 

 and surface. The mixing water added to the batch must be reduced by an 

 amount equal to the free moisture contributed by the aggregate, i.e., 

 total moisture minus absorption. 



(9) Trial Batch Adjustments . The calculated mixture proportions 

 should be checked by means of trial batches prepared and tested in accor- 

 dance with ASTM Standard C192, "Making and Curing Concrete Compression and 

 Flexure Test Specimens in the Laboratory," or full-sized field batches. 

 Only sufficient water should be used to produce the required slump regard- 

 less of the amount assumed in selecting the trial proportions. The concrete 

 should be checked for unit weight and yield (ASTM Standard C138) and for air 

 content (ASTM Standard C138, C173, or C231) . It should also be carefully 

 observed for proper workability, freedom from segregation, and finishing 

 properties. Appropriate adjustments should be made in the proportions for 

 subsequent batches. 



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