104. Cost of Sand. The cost of sand, including handling and freight, ranges from $O .75 to $1 .50 per cubic yard. A common price for sand delivered in the cities is $1.00 per cubic yard.
105. Cost of Broken Stone or Gravel. The cost of broken stone delivered in the cities varies from 81.25 to $1 .75 per cubic yard. The cost of gravel is usually a little less than stone.
106. Cost of Mixing. tinder ordinary conditions and where the concrete will have to be wheeled only a very short distance, the cost of hand-mixing and placing will generally range from $0.90 to $1.30 per cubic yard, if done by men skilled in this work. If a mixer is used, the cost will range from $0.50 to $0.90 per cubic yard.
107. Cost of Forms. The cost of forms for heavy walls and foundations, varies from $0.70 to $1.20 per cubic yard of concrete laid. The cost of forms and mixing concrete will be further dis cussed in Part IV.
108. Practical Methods of Proportioning. A rich mixture, proportions 1:2:4—that is, 1 barrel (4 bags) packed Portland cement (as it comes from the manufacturer), 2 barrels (7.6 cubic feet) loose sand, and 4 barrels (15.2 cubic feet) loose stone—is used in arches, reinforced-concrete floors, beams, and columns for heavy loads; engine and machine foundations subject to vibration; tanks; and for water-tight work.
A medium mixture, proportions I :21 : 5—that is, 1 barrel (4 bags) packed Portland cement, 21 barrels (9.5 cubic feet) loose sand, and 5 barrels (19 cubic feet) loose gravel or stone—may be used in arches, thin walls, floors, beams, sewers, sidewalks, founda tions, and machine foundations.
An ordinary mixture, proportions 1 :3 : 6—that is, 1 barrel (4 bags) packed Portland cement, 3 barrels (11.4 cubic feet) loose sand, and 6 barrels (22.8 cubic feet) loose gravel or broken stone— may be used for retaining walls, abutments, piers, floor slabs, and beams.
A lean mixture, proportions 1 : 4 : 8—that is, 1 barrel (4 bags) packed Portland cement, 4 barrels (15.2 cubic feet) loose sand, and S barrels (30.4 cubic feet) loose gravel or broken stone—may be used in large foundations supporting stationary loads, backing for stone masonry, or where it is subject to a plain compressive load.
These proportions must not be taken as always being the most economical to use, but they represent average practice. Cement is the most expensive ingredient; therefore a reduction of the quan tity of cement, by adjusting the proportions of the aggregate so as to produce a concrete with the same density, strength, and imper meability, is of great importance. By careful proportioning and workmanship, water-tight concrete has been made of a 1 : 3: 6 mixture. In floor construction where the span is very short and it is specified that the slab must be at least 4 inches thick, while with a high-grade concrete a 3-inch slab would carry the load, it is cer tainly more economical to use a leaner concrete.
An accurate and simple method to determine the proportions of concrete is by trial batches. The apparatus consists of a scale and a cylinder which may be a piece of wrought iron pipe 10 inches to 12 inches in diameter capped at one end. Measure and weigh the cement, sand, stone, and water and mix on a piece of sheet steel, the mixture having a consistency the same as to be used in the work. The mixture is placed in the cylinder, carefully tamped, and the height to which the pipe is filled is noted. The pipe should be weighed before and after being filled so as to check the weight of the material. The cylinder is then emptied and cleaned. Mix up another batch using the same amount of cement and water, slightly varying the ratio of the sand and stone hut having the same total weight as before. Note the height in the cylinder, which will be a guide to other batches to be tried. Several trials are made until a mixture is found that gives the least height in the cylinder, and at the same time works well while mixing,. all the stones being covered with mortar, and which makes a good appearance. This method gives very good results, but it does not indicate the changes in the physical sizes of the sand and stone so as to secure the most economical composition as would be shown in a thorough mechan ical analysis.