APPENDIX C-Continued 



placed without honeycomb or void. When high 

 strength concrete is desired, best results may be 

 obtained with reduced maximum sizes of aggre- 

 gate since these produce higher strengths at a 

 given water-cement ratio. 



5.3.3 Step 3. Estimation of mixing water and 

 air content. The quantity of water per unit volume 

 of concrete required to produce a given slump is 

 dependent on the maximum size, particle shape 

 and grading of the aggregates, and on the amount 

 of entrained air. It is not greatly affected by 

 the quantity of cement. Table 5.3.3 provides esti- 

 mates of required mixing water for concretes 

 made with various maximum sizes of aggregate, 

 with and without air entrainment. Depending on 

 aggregate texture and shape, mixing water re- 

 quirements may be somewhat above or below the 

 tabulated values, but they are sufficiently ac- 

 curate for the first estimate. Such differences in 

 water demand are not necessarily reflected in 



TABLE 5.3.3— APPROXIMATE MIXING WATER AND 



AIR CONTENT REQUIREMENTS FOR DIFFERENT 

 SLUMPS AND MAXIMUM SIZES OF AGGREGATES* 



Water, lb per cu yd of 



sizes of aggregate 



iln.| ',iln.|',iln.|lln. | IW in. |2 in.t (3 In.f ] 6 ii 



Non-alr-entrained concrete 



1 to 2 

 3 to 4 

 6 to 7 



Approximate 

 amount of 

 entrapped 

 air In non- 

 air-en- 

 tralned 

 concrete. 



350 



335 



315 



300 



275 



260 



240 



385 



365 



340 



325 



300 



2fa 



265 



410 



385 



360 



340 



315 



300 



285 



3 



2.5 



2 



IS 



1 



Oi 



03 



Air -entrained 



1 to 2 



305 



295 



280 



270 



250 



240 



225 



200 



3 to 4 



340 



325 



305 



295 



275 



265 



250 



220 



6 to 7 



365 



345 



325 



310 



290 



m> 



270 







Recom- 

 mended 

 average 

 total air 

 content, 

 percent 



8 



7 



e 



5 



4.5 



4 



3.5 



3 



•The 



quantities of mixing 



computing 



cement factors for trial batches. They are maxima for reasonably 

 well'Shaped angular coarse aggregates graded within limits of 

 accepted specifications. 



ITne slump values for concrete containing aggregate larger 

 than 1*,^ in are based on slump tests made after removal of 

 particles larger than I'a In. by wet-screening. 



strength since other compensating factors may 

 be involved. For example, a rounded and an 

 angular coarse aggregate, both well and simi- 

 larly graded and of good quality, can be ex- 

 pected to produce concrete of about the same 

 compressive strength for the same cement factor 

 in spite of differences in water-cement ratio re- 

 sulting from the different mixing water require- 

 ments. Particle shape per se is not an indicator 

 that an aggregate will be either above or below 

 average in its strength-producing capacity. 



Table 5.3.3 indicates the approximate amount 

 of entrapped air to be expected in non-air-en- 



trained concrete, and shows the recommended 

 levels of average air content for concrete in 

 which air is to be purposely entrained for dura- 

 bility. Air-entrained concrete should always be 

 used for structures which will be exposed to 

 freezing and thawing, and generally for struc- 

 tures exposed to sea water or sulfates. When 

 severe exposure is not anticipated, beneficial ef- 

 fects of air entrainment on concrete workability 

 and cohesiveness can be achieved at air content 

 levels approximately half those shown for air- 

 entrained concrete. 



When trial batches are used to establish 

 strength relationships or verify strength-produc- 

 ing capability of a mixture, the least favorable 

 combination of mixing water and air content 

 should be used. This is, the air content should be 

 the maximum permitted or likely to occur, and 

 the concrete should be gaged to the highest per- 

 missable slump. This will avoid developing an 

 over-optimistic estimate of strength on the as- 

 sumption that average rather than extreme con- 

 ditions will prevail in the field. For information 

 on air content recommendations, see ACI 201, 

 301, and 302. 



5.3.4 Step 4. Selection of water-cement ratio. 

 The required water-cement ratio is determined 

 not only by strength requirements but also by 

 factors such as durability and finishing proper 

 ties.' Since different aggregates and cements gen- 

 erally produce different strengths at the same 

 water-cement ratio, it is highly desirable to have 

 or develop the relationship between strength 

 and water-cement ratio for the materials actyally 

 to be used. In the absence of such data, approxi- 

 mate and relatively conservative values for con- 

 crete containing Type I portland cement can be 

 taken from Table 5.3.4(a). With typical mate- 

 rials, the tabulated water-cement ratios should 

 produce the strengths shown, based on 28-day 

 tests of specimens cured under standard labora- 

 tory conditions. The average strength selected 

 must, of course, exceed the specified strength 

 by a sufficient margin to keep the number of low 

 tests within specified limits.* 



For severe conditions of exposure, the water- 

 cement ratio should be kept low even though 

 strength requirements ipay be met with a higher 

 value. Table 5.3.4(b) gives limiting values. 



5.3.5 Step 5. Calculation of cement content. 

 The amount of cement per unit volume of con- 

 crete is fixed by the determinations made in 

 Steps 3 and 4 above. The required cement is 

 equal to the estimated mixing water content 

 (Step 3) divided by the water-cement ratio 

 (Step 4) . If, however, the specification includes a 

 separate minimum limit on cement in addition to 



See "Recommended Practice for Evaluation of Comprcstion 



Test Result! of Field Concrete (ACI 214-65) 



ACI STANDARD 



323 



