Concrete should be made by spreading the aggregate evenly over a layer of cement mortar (made as described under Mortar) in a box or on a platform, and mixing the materials thoroughly. The aggregate is usually broken stone, not over a specified size; but gravel, broken brick, etc., may be substituted. Whichever is used should be free from dirt, and be well sprinkled before mixing. The pieces should be of different sizes, so that the smaller pieces will fit in the spaces between the larger.

The voids in broken stone are one-half the bulk, which space, in good work, is practically just filled by the mortar; hence, in estimating on concrete, it is necessary to figure about as much broken stone as there are to be cubic yards of concrete. The quantity of sand will be about one-half cubic yard per yard of stone - when gravel is not also used; while the cement will vary according to proportions required. See table on page 170 giving proportions of cement and sand per cubic yard of mortar.

Probably the best proportion for a strong concrete is 1 part of cement, 2 parts of sand, and 4 or 5 parts of broken stone, these quantities being sufficient to fill all the voids.

A very good concrete may be made by using the following quantities of materials, which when mixed will make 1 cu. yd.: 2 bbl. of Rosendale cement, .5 cu. yd. of sand, and

.9 cu. yd. of broken stone. The mortar alone amounts to .55 cu. yd.

A concrete nearly as strong and considerably cheaper is made of 1 bbl. of Rosendale cement, 2 bbl. of sand, .5 cu. yd. of gravel, and .9 cu. yd. of stone. These when mixed will make 1 cu. yd. The mortar amounts to .28 cu. yd.

In laying, concrete should not be dumped from a considerable height, as the thoroughness of the mixture would be destroyed. It should be spread in layers of, say, 8 in. in thickness, and tamped enough to compact the mass well, the surface of each layer being left rough, to form a better bond with the succeeding one.

The strength of concrete increases considerably with age. For example, a Portland cement concrete 1 month old will crush under a load of about 15 tons per sq. ft., while if it is a year old, it will sustain about 100 tons per sq. ft. These figures, under favorable conditions, may be nearly doubled. Natural cement concretes, with various proportions of materials and of ages from 6 months to 4 years, showed crushing strengths of from 70 to 100 tons per sq. ft.

quantities of materials.

Proportion of Mortar in Masonry

Kind of Masonry.

Per Cent, of Mortar.

Minimum.

Maximum.

Brickwork, coarse, 1/2" to 5/8" joints......

35

40

Brickwork, ordinary, 1/4" to 3/8" joints

25

30

Brickwork, pressed, 1/8" joints..........

10

15

Ashlar, courses 12" to 20" high, joints 3/8"to 1/2"......................................

7

8

Ashlar, courses 20" to 32" high, joints 1/4"to 3/8"......................................

5

6

Rubble, coarse, not dressed ............

33

40

Rubble, roughly dressed...................

25

30

Rubble, well dressed, coursed.........

15

20

Concrete, clean stone, without gravel or screenings......................

50

55