1 Compressive Strength and Aggregate Properties . For a 

 required concrete strength level, the strength selected should be appropriate 

 for both the service exposure and the life of the structure. In no case 

 should the compressive strength be less than 28 megapascals (4 000 pounds 

 per square inch). Suitable strength levels may be attained by: 



(a) A low water-cement ratio, 



(b) proper grading of fine and coarse aggregate (meeting ASTM 

 Standard C33) , limiting the maximum size to nominal 25 millimeters 

 (1 inch); 



(c) lowest consistency practicable for proper placing and 

 consolidation with maximum slump of 75 millimeters (3 inches), and 

 25 millimeters for toppings; 



(d) minimum air content consistent with the exposure conditions. 

 For indoor floors not subjected to freezing and thawing, air contents 

 of 3 percent or less are preferable; in addition to a detrimental 

 effect on strength, high air contents can cause blistering, 

 particularly when using dry shakes; and 



(e) when wear conditions are severe, a high strength (not less 

 than 34 megapascals (5 000 pounds per square inch)) topping layer, 

 called a two-course floor, limiting the maximum size of aggregate 

 to 12 millimeter (1/2 inch) in the topping. 



2 Proper Finishing Procedures . Delay floating and 

 troweling until the concrete has lost its surface water sheen or all free 

 water on the surface has disappeared or been carefully removed. The delay 

 period is usually for 2 or more hours after placing the concrete (depending 

 on temperatures, mix proportions, and air content). Follow the recommenda- 

 tions of ACI Standards 302-69 and 304-73 with respect to finishing unformed 

 surfaces . 



3 Vacuum Dewatering . Vacuum dewatering is a method of 

 removing excess water from concrete immediately after placement. The 

 process results in increased strength, hardness, and wear resistance of 

 concrete surfaces; it is primarily applicable to slab. 



4 Special Dry Shakes and Toppings . Where severe wear is 

 anticipated, the use of special toppings or dry shakes (such as coats of 

 cement and hard fine aggregate, or of cement and iron aggregate) should be 

 considered and, if selected, the recommendations of ACI Committee 302 (1969), 

 "Recommended Practice for Concrete Floor and Slab Construction," should be 

 followed. 



5 Proper Curing Procedures . Curing should start immedi- 

 ately after the concrete has been finished and be continued for at least 7 

 days with type I cement (5 days with type III). Curing with water by 

 spray, damp burlap, or cotton mats is preferred, provided the concrete is 

 kept continuously moist. Waterproof paper or plastic sheets are satisfactory, 

 provided the concrete is first sprayed with water and then immediatley 

 covered with the paper or plastic with the edges overlapped and sealed with 



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