A common mistake to be guarded against is to assume that the volume of concrete produced is equal to the quantity of sand plus the gravel as indicated in the proportion. For instance a 1:2:4 mixture will not produce 6 cubic yards of concrete, if 2 yards of sand and 4 yards of gravel are used, because the sand will lodge in the voids between the pebbles. If 6 cubic yards of concrete are desired it will be necessary to use 2.7 cubic yards of sand and 5.34 cubic yards of gravel. Table 1 shows the quantity of cement, sand, and gravel required under average conditions for the indicated proportions. Table 1.--_Materials for 1 cubic yard of rammed concrete._ Proportions. | | | | | | Cement.| Sand. | Gravel.| Cement. | Sand. | Gravel. -------+-------+--------+----------+----------+----------- | | | _Sacks._ |_Cu. yds._| _Cu. yds._ 1 | 1 | --- | 19.20 | 0.74 | --- 1 | 2 | --- | 13.48 | 1.00 | --- 1 | 2-1/2| --- | 11.00 | 1.01 | --- 1 | 3 | --- | 10.40 | 1.16 | --- 1 | 1 | 2 | 10.52 | .39 | 0.78 1 | 1-1/2| 3 | 7.64 | .42 | .85 1 | 2 | 4 | 6.04 | .45 | .89 1 | 2-1/2| 5 | 4.96 | .46 | .92 1 | 3 | 5 | 4.64 | .52 | .86 1 | 3 | 6 | 4.24 | .47 | .94 -------+-------+--------+----------+----------+---------- CONSISTENCY. The quantity of water used in mixing has a very great influence on the strength of the concrete. An excess of water weakens the concrete, while an insufficient amount prevents thorough mixing. [Illustration: Fig. 2.--The result of using too dry a mixture, lack of spading and careless placing; note irregularity of layers and poor bonding.] Therefore, only sufficient water should be used to produce a workable or plastic mixture. Recent tests have proved that to secure the greatest strength the concrete should be mixed considerably drier than has heretofore been customary. Of course, for thin walls containing closely placed reinforcement, or for water-tightness, a fairly wet mix is necessary. A little experience will show the proper amount of water to use. A very rough estimate of the quantity of water required in mixing for general work is 4 to 5 gallons to each sack of cement. Three degrees of consistency (corresponding to different proportions of water) are used in general practice, namely, wet, medium, and dry. In the light of recent investigations it is thought the wet mixture of present-day practice contains too much water. The following definitions are therefore recommended: =Wet mixture.= One that does not flow readily and yet can not be piled up. It is recommended for thin sections when reinforcement is closely placed.