The compressive strength of concrete is very important, as it is used more often in compression than in any other way. It is rather difficult to give average values of the compressive strength of concrete, as it is dependent on so many factors. The available aggregates are so varied, and the methods of mixing and manipulation so different, that tests must be studied before any conclusions can be drawn. For extensive work, tests should be made with the materials available to determine the strength of concrete, under conditions as nearly as possible like those in the actual structure.

A series of experiments made at the Watertown Arsenal for Mr. George A. Kimball, Chief Engineer of the Boston Elevated Railway Company, in 1899, was one of the best sets of tests that have been published, and the results are given in Table III. Portland cement, coarse, sharp sand, and stone up to 2 1/2 inches were used; and when thoroughly rammed, the water barely flushed to the surface.

Table III. Compressive Strength Of Concrete

Tests Made at Watertown Arsenal, 1899

Mixture

Brand of Cement

Strength (Pounds per Square Inch)

7 Days

1 Month

3 Months

6 Months

1:2:4

Saylor

1,724

2,238

2,702

3,510

Atlas

1,387

2,428

2,966

3,953

Alpha

904

2,420

3,123

4,411

Germania

2,219

2,642

3,082

3,643

Alsen

1,592

2,269

2,608

3,612

Average

1,565

2,399

2,896

3,826

1:3:6

Saylor

1,625

2,568

2,882

3,567

Atlas

1,050

1,816

1,538

3,170

Alpha

892

2,150

2,355

2,750

Germania

1,550

2,174

2,486

2,930

Alsen

1,438

2,114

2,349

3,026

Average

1,311

2,164

2,522

3,088

*From "Tests of Metals," 1899.

The values obtained in these tests are exceedingly high, and cannot be safely counted on in practice.

Tests made by Prof. A. N. Talbot (University of Illinois, Bulletin No. 14) on 6-inch cubes of concrete, show the average values given in Table IV. The cubes were about 60 days old when tested.

Table IV. Compressive Tests Of Concrete University'Of Illinois

No. of Tests

Mixture

Strength (Pounds per Square Inch)

3

1:2:4

2 350

6

1:3:5 1/2

1,920

7

1:3:6

1,300 .

With fair conditions as to the character of the materials and workmanship, a mixture of 1:2:4 concrete should show a compressive strength of 2,000 to 2,300 pounds per square inch in 40 to 60 days; a mixture of 1: 2 1/2: 5 concrete, a strength of 1,800 to 2,000 pounds per square inch; and a mixture of 1:3:6 concrete, a strength of 1,500 to 1,800 pounds per square inch. The rate of hardening depends upon the consistency and the temperature.

96. Tensile Strength

The tensile strength of concrete is usually considered about one-tenth of the compressive strength; that is, concrete which has a compressive value of 2,000 pounds per square inch should have a tensile strength of about 200 pounds per square inch. Although there is no fixed relation between the two values, the general law of increase in strength due to increasing the percentage of cement and the density, seems to hold in both cases.

97. Shearing Strength

The shearing strength of concrete is important on account of its intimate relation to the compressive strength and the shearing stresses to which it is subjected in structures reinforced with steel. But few tests have been made, as they are rather difficult to make; but the tests made show that the shearing strength of concrete is nearly one-half the crushing strength. By shearing is meant the strength of the material against a sliding failure when tested as a rivet would be tested for shear.

98. Modulus Of Elasticity

The principal use of the modulus of elasticity in designing reinforced concrete is in determining the relative stresses carried by the concrete and the steel. The minimum value used in designing reinforced concrete is usually taken as 2,000,-

000, and the maximum value as 3,000,000, depending on the richness of the mixture used. A value of 2,500,000 is generally taken for ordinary concrete.

99. Weight Of Concrete

The weight of stone or gravel concrete will vary from 145 pounds per cubic foot to 155 pounds per cubic foot, depending upon the specific gravity of the materials and the degree of compactness. The weight of a cubic foot of concrete is usually considered as 150 pounds.

100. Cinder Concrete

Cinder concrete has been used to some extent on account of its light weight. The strength of cinder concrete is from one-half to two-thirds the strength of stone concrete. It will weigh about 110 pounds per cubic foot.

101. Rubble Concrete

Rubble concrete is a concrete in which large stones are placed, and will be discussed in Part II.

102. Cost Of Concrete

The cost of concrete depends upon the character of the work to be done, and the conditions under which it is necessary to do this work. The cost of the material, of course, will always have to be considered, but this is not so important as the character of the work. The cost of concrete in place will range from $4.50 per cubic yard to $20, or even $25, per cubic yard. When it is laid in large masses, so that the cost of forms is relatively small, the cost will range from $4.50 per cubic yard to $6 or $7 per cubic yard, depending on the local conditions and cost of materials. Foundations and heavy walls are good examples of this class of work. For sewers and arches, the cost will vary from $7 to $13. In building construction - floors, roofs, and thin walls - the cost will range from $14 to $20 per cubic yard.

103. Cost Of Cement

The cost of Portland Cement varies with the demand. Being heavy, the freight is often a big item. The price varies from $1 to $2 per barrel. To this must be added the cost of handling.

104. Cost Of Sand

The cost of sand, including handling and freight, ranges from $0.75 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 $1.25 to $1.75 per cubic yard. The cost of gravel is usually a little less than stone.

106. Cost Of Mixing

Under 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 discussed in Part IV.