105 



It Is best to mix the cement and color dry. Strength IB 

 gained and cost reduced by using the color cement for the 

 surface coating only. 



PROPORTIONING CONCRETE. 



In this country the cement and aggregates are generally 

 mixed by volume, although quite a number of engineers 

 recommend a mixture by weight. This latter method Is 

 used In testing laboratories. We have mixtures specified 

 by 1:2:3, 1:2:4, 1:2:5, 1:3:6, meaning 1 part by volume of 

 cement, 2 parts by volume of sand and 3, 4, 5 or 6 parts 

 by volume of stone or gravel. Frequently the sand and 

 stone or gravel are graded together and then the mixture 

 Is expressed by 1:5, 1:6, 1:7, etc., the sand and stone being 

 mixed to give a maximum density and then adding the 

 cement. In the following we give Mr. Fuller's rule from 

 Tuylor & Thompson. 



We find In the article of Mr. Lee Heidenrelch, In the 

 Engineering World, some sensible suggestions regarding 

 the proportioning of concrete, as follows: 



Many of the usual methods of proportioning concrete are 

 unsatisfactory, owing to the fact that the laws governing 

 the mixing and setting of concrete are not definitely 

 understood. A number of formulas and rules have been 

 devised to regulate the quantity of cement and aggregates 

 to use per cu. yd. of concrete. The usual field practice 

 In America Is to take cement and aggregates by volume; 

 In France, the cement is measured by weight, the aggre- 

 gates by volume; in Germany both are measured by 

 weight. In American testing laboratories, the practice is 

 to measure ceiaent and aggregates by weight. 



Hitherto, the practice has been to mix concrete by tak- 

 ing 1 part cement with certain parts of sand and crushed 

 stone or gravel. This practice is gradually changing, 

 however, in view of the fact that the best concrete is that 

 In which the aggregate is uniformly graded from coarse 

 to fine. Another point in favor of abandoning the former 

 method is that the 1:2:3 mixture of one contractor may 

 be identical with the 1:3:5 mixture of another, owing to 

 differences in the sizes of sand and stone. In view of 

 this, it Is preferable to abandon specifying mixtures as 

 1:2:3, 1:2:4, 1:3:5, but as 1:5, 1:6, 1:8, respectively In 

 which case the 5, the 6 or the 8 parts of aggregate are 

 mixed up, reducing the voids to whatever figure is neces- 

 sary for maximum density, and then the cement added. 

 Both methods of proportioning will be considered, since 



