ratio and, of course, produce concrete of a greater strength, than that 

 containing the 19 -millimeter aggregate. Table 9 shows maximum aggregate 

 sizes for various uses. 



Table 9. Maximum size of aggregate recommended for 

 various types of construction. 



Section 



Use or Features 



Aggregate Size 







(mm) 



Cin) 



5190.5 mm wide 



heavily reinforced floor 

 and roof slabs; parapets, 

 cobels and where space 

 is limited 



19- 



Q.75 



>190.5 mm wide 





38.1 



1.5 



with clear distance 









between reinforcement 









bars > 57 mm 









>305 mm wide unre- 



Piers, walls, baffles and 



76.2 



3 



inforced sections and 



stilling basin floor slabs 

 in which satisfactory 







>457 mm wide rein- 



placement of 152.4 mm or 







forced sections with 



cobble concrete cannot be 







clear distance between 



accomplished even though 







reinforcement bars 



reinforcement spacing 







>114.3 mm but < 229 



would permit the use of 







mm 



larger aggregates. 







Massive sections with 



Retaining walls, piers 



152.4 



6 



clear distance 



and baffles; in which 







between reinforcing 



suitable provision is 







bars > 229 mm 



made for placing concrete 

 containing the larger 

 size aggregate without 

 producing rock pockets 

 or other undesirable 

 results. 







Gap-graded aggregates can often be used effectively in areas where ASTM 

 C33 standards cannot be met. A typical gap-graded aggregate may contain 

 only one size of coarse aggregate together with sand. In this respect, the 

 resultant concrete would resemble that made with prepacked concrete. 



When natural aggregates are found to be unacceptable through service 

 records or tests, they may sometimes be improved by removing lightweight, 

 soft, or otherwise inferior particles by processing. 



(2) Lightweight Aggregate . There are a number of materials used 

 to produce lightweight concrete. Among the natural aggregates are: tuff, 



81 



