PROPORTIONS OF rIATERIALS FOR CONCRETE.
To manufacture one cubic yard of concrete the following quan tities of materials are required : .
To make 100 cubic feet of concrete of the proportions 1 to 6 will require 5 bbl. cement (original package) and 4.4 yards of stone and sand.
Mixing Concrete. The concrete may be mixed by hand or machinery. In hand-mixing the cement and sand are mixed dry. About half the sand to be used in a batch of concrete is spread evenly over the mortar-board, then the dry cement is spread evenly over the sand, and then the remainder of the sand is spread on top of the cement. The sand and cement are then mixed with a hoe or by turning and re-turning with a shovel. It is very important that the sand and cement be thoroughly mixed. A baSin is then formed by drawing the mixed sand and cement to the outer edges of the board, and the whole amount of water required is poured into it. The sand and cement are then thrown back upon the water and thoroughly mixed with the hoe or shovel into a stiff mortar and then levelled off. The broken stone or gravel should be sprinkled with sufficient water to remove all dust and thoroughly wet the entire surface. The amount of water required for this purpose will vary considerably with the absorbent power of the stone and the temperature of tho air. The wet stone is then spread evenly over the top of the mortar and the whole mass thoroughly mixed by turning over with the shovel. Two, three, or more turnings may be necessary. It should be turned until every stone is coated with mortar, and the entire mass presents the uniform color of the cement, and the mortar and stones are uni formly distributed. When the aggregate consists of broken brick or other porous material it should be thoroughly wetted and time allowed for absorption previous to use; otherwise it will take away part of the water necessary to effect the setting of the cement.
When the concrete is ready for use it should be quite coherent and capable of standing at a steep slope without the water running from it.
The rules and the practice governing the mixing of concrete vary as widely as the proportion of the ingredients. It may be stated in general that if too much time is not consumed in mixing the wet materials a good result can be obtained by any of the many ways practised, if only the mixing is thorough. With four men the time required for mixing one cubic yard is about ten minutes.
the method adopted for mixing the concrete, it is advisable for the inspector to be constantly present during the opera tion, as the temptation to economize on the cement and to acid an excess of water to lighten the labor of mixing is very great.
Laying Concrete. Concrete is usually deposited in layers, the thickness of which is generally stated in the specifications for the particular work (the thickness varies between 6 and 12 in.). The concrete must be carefully deposited in place. A very common practice is to tip it from a height of several feet into the trench. This process is objected to by the best authorities on the ground that the heavy and light portions separate while falling, and that the concrete is, therefore, not uniform throughout its mass.
The best method is to wheel the concrete in barrows, imme diately after mixing, to the place where it is to be laid, gently tipping or sliding it into position and at once ramming it.
The ramming should be done before the cement begins to set, and should be continued until the water begins to ooze out upon the upper surface. When this occurs it indicates a sufficient degree of compactness. A gelatinous or quicksand condition of the Mass indicates that too much water was used in mixing. Too severe or long-continued pounding injures the strength by forcing the stones to the bottom of the layers and by distributing the incipient "set" of the cement.
The rammers need not be very heavy, 10 to 15 lb. will be suffi cient. Square ones should measure from 6 to S in. on a side and round ones from S to 12 in. in diameter.