CONCRETE 



9 



were deduced, comprised the screening of crusher-run stone and 

 bank gravel into twenty-one sizes ranging from 3 in. down to 

 material passing a No. 100 sieve, and then re-combining these 

 sized materials in a predetermined mechanical-analysis curve 

 by weighing out the necessary quantities of each size. Over 

 400 different mechanical-analysis curves were made. The 

 different mixtures were thoroughly mixed with a given weight 

 of cement to a given consistency; then they were tamped into a 

 strong cylinder, and their volume determined. These tests led 

 to the determination of valuable rules for the plotting of ideal 

 curves for density. Many of the mixtures were also made up into 

 prisms and beams, and the strength of each was determined by 

 breaking tests. These tests substantiated the two laws of the 

 theory of proportions given in the preceding article. 



The maximum density curve was found to be of substantially 

 the same form for different materials, whatever the maximum 

 size of stone. The curve in all cases may be taken as a combina- 

 tion of an ellipse and a straight line. First a straight line should 

 be drawn from the point where the largest diameter stone reaches 

 the 100 per cent line, to that point on the vertical ordinate at zero 

 diameter which is given in column (1) in the following table: 



DATA FOR PLOTTING CURVES OF MAXIMUM DENSITY 



In this table, D = the maximum diameter of the stone, in inches. 

 Next mark the tangent point on this line namely, where this 

 line is intersected by the vertical ordinate for one-tenth the 

 maximum stone. This mark should check with the values given 

 in column (2) of the above table. Then plot the location of the 

 axes of the ellipse from the values of a and (b + 7) given in 

 columns (3) and (4) respectively in the above table. The major 

 axis of the ellipse should be placed on the 7 per cent line of 

 percentages. (Fig. 3.) Now to plot the ellipse: take a strip of 



