APPENDIX C-Continued 



mit easy adjustment on the basis of trial batches 

 as will be shown in the examples. 



If a theoretically exact calculation of fresh 

 concrete weight per cubic yard is desired, the fol- 

 lowing formula can be used: 



(5-1) 



V = 16.85 G, (100 - A) 

 + C(l - GJGc) - W(G, - 1) 

 where 



U = weight of fresh concrete per cubic yard, lb 



G. = weighted average specific gravity of combined 



fine and coarse aggregate, bulk SSD* 

 G, =z specific gravity of cement (generally 3.15) 

 A = air content, percent 

 W = mixing water requirement, lb per cu yd 

 C = cement requirement, lb per cu yd 



5.3.7.2 A more exact procedure for calcu- 

 lating the required amount of fine aggregate in- 

 volves the nse of volumes displaced by the in- 

 gredients. In this case, the total volume displaced 

 by the known ingredients — water, air, cement, 

 and coarse aggregate — is subtracted from the unit 

 volume of concrete to obtain the required volume 

 of fine aggregate. The volume occupied in concrete 

 by any ingredient is equal to its weight divided 

 by the density of that material (the latter being 

 the product of the unit weight of water and the 

 specific gravity of the material) . 



5.3.8 Step 8. Adjustments jot aggregate mois- 

 ture. The aggregate quantities actually to be 

 weighed out for the concrete must allow for 

 moisture in the aggregates. Generally, the ag- 

 gregates 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 con- 

 tributed by the aggregate — i.e., total moisture 

 minus absorption. 



5.3.9 Step 9. Trial batch adjustments. The 

 calculated mixture proportions should be checked 

 by means of trial batches prepared and tested in 

 accordance with ASTM C 192, "Making and Cur- 

 ing Concrete Compression and Flexure Test Speci- 

 mens in the Laboratory," or full-sized field 

 batches. Only sufficient water should be used to 

 produce the required slump regardless of the 

 amount assumed in selecting the trial propor- 

 tions. The concrete should be checked for unit 

 weight and yield (ASTM C 138) and for air con- 

 tent (ASTM C138, C173, or C231). It should 

 also be carefully observed for proper workability, 

 freedom from segregation, and finishing proper- 

 ties. Appropriate adjustments should be made in 

 th^ proportions for subsequent batches in ac- 

 cordance with the following procedure. 



5.3.9.1 Re-estimate the required mixing 

 water per cubic yard of concrete by multiplying 

 the net mixing water content of the trial batch by 



27 and dividing the product by the yield of the 

 trial batch in cubic feet. If the slump of the 

 trial batch was not correct, increase or decrease 

 the re-estimated amount of water by 10 lb for 

 each required increase or decrease of 1 in. in 

 slump. 



5.3.9.2 If the desired air content (for air- 

 entrained concrete) was not achieved, re-esti- 

 mate the admixture content required for proper 

 air content and reduce or increase the mixing 

 water content of paragraph 5.3.9.1 by 5 lb for each 

 1 percent by which the air content is to be in- 

 creased or decreased from that of the previous 

 trial batch. 



5.3.9.3 If estimated weight per cubic yard 

 of fresh concrete is the basis for proportioning, 

 re-estimate that weight by multiplying the unit 

 weight in pounds per cubic foot of the trial batch 

 by 27 and reducing or increasing the result by 

 the anticipated percentage increase or decrease in 

 air content of the adjusted batch from the first 

 trial batch. 



5.3.9.4 Calculate new batch weights starting 

 with Step 4 (Paragraph 5.3.4) , modifying the 

 volume of coarse aggregate from Table 5.3.6 if 

 necessary to provide proper workability. 



6. SAMPLE COMPUTATIONS 



6.1 — Two example problems will be used to il- 

 lustrate application of the proportioning proce- 

 dures. The following conditions are assumed: 



6.1.1 Type I non-air-entraining cement will 

 be used and its specific gravity is assumed to be 

 3.1.5.+ 



6.1.2 Coarse and fine aggregates in each case 

 are of satisfactory quality and are graded within 

 limits of generally accepted specifications.^ 



6.1.3 The coarse aggregate has a bulk specific 

 gravity of 2.68+ and an absorption of 0.5 percent. 



6.1.4 The fine aggregate has a bulk specific 

 gravity of 2.64,+ an absorption of 0.7 percent, and 

 fineness modulus of 2.8. 



6.2 — Example I. Concrete is required for a por- 

 tion of a structure which will be below ground 

 level in a location where it will not be exposed 

 to severe weathering or sulfate attack. Structural 

 considerations require it to have an average 28- 

 day compressive strength of 3500 psi.§ On the 

 basis of information in Table 5.3.1, as well as 



dry basis, and bulk SSD 



Tied to occupy 



JSuch as the "Specifications for Concrete Aggregates." (ASTM 



For the method of determining the amount by which _ 



Btrencth should exceed desiffn strength, see "Pecommended 

 Practice for Evaluation of Compression Test Results of Field 

 Concrete (ACl J14-«).- 



ACI STANDARD 



325 



