144. Examples of Portland Cement Concrete

Foundations of Mutual Life Insurance Company's Building, New York: 1 part cement, 3 parts sand, 5 parts broken stone.

Foundation of U. S. Naval Observatory: 1 part cement, 2 sand, 3 gravel, 5 broken stone. [1 barrel of cement, 380 pounds, made 1.18 yards of concrete.]

Foundations of Cathedral of St. John the Divine, New York: 13,000 cubic yards of concrete have been used in the foundation of the tower and choir, the average depth being 15 feet. Proportions : 1 part Portland cement, 2 parts sand, 3 parts quartz gravel, 1 to 2 inches in diameter.

Filling of caissons, Johnston Building (fifteen stories) New York : 1 part Portland cement, 3 parts sand, 7 parts stone, finished on top for brickwork with 1 part cement and 3 parts gravel.

Manhattan Life Insurance Building, New York, filling of caissons: 1 part Alsen Portland cement, 2 parts sand, 4 parts broken stone.

The proportion of cement is sometimes specified as "one barrel of cement to a yard of concrete," but as it is very inconvenient to measure the concrete by the yard such a specification is not to be recommended.

145. Depositing

As soon as a batch of concrete is mixed it should be wheeled to the trenches and deposited in layers from 6 to 10 inches in thickness. Where the total thickness of concrete does not exceed 18 inches the layers should not be more than 6 inches thick. The concrete should not be dumped from a greater height than 4 feet above the bottom of the trench. If dumped from a greater height the heavy particles are apt to separate from the lighter ones.

As soon as a square yard of concrete has been deposited it should be tamped with a wooden rammer weighing about 20 pounds. The tamping should be sufficient to just flush the water to the surface. The concrete should not be permitted to dry too quickly, and if twenty-four hours elapse between depositing the successive layers the top of each layer should be sprinkled before the next is deposited.

146. Strength of Concrete

The writer is not acquainted with any reliable tests on the compressive strength of concrete, but it is generally assumed that the strength of thoroughly mixed concrete is equal to that of mortar made of the same proportions of sand and cement. The crushing strength of 6-inch cubes of 1 to 2 Portland cement mortar was found by tests made at the Watertown Arsenal to average about 500 pounds per square inch, or 36 tons per square foot. For the working strength of concrete the author recommends the following values, the larger values being for work done under strict inspection with the best of cement:

Portland cement concrete, 1 to 8, 8 to 15 tons per square foot; natural cement concrete, 1 to 6, 5 to 10 tons per square foot.

The estimated weight to be imposed on the concrete footings of the Cathedral of St. John the Divine is 10 tons per square foot.

147. Data for Estimating

There seem to be few records of careful measurements of the amount of materials required to make a cubic yard of concrete, but the following data is believed to be reasonably accurate:

Used in the proportion of 1 part cement, 3 of sand and 5 of broken stone, in sizes not exceeding 2x1x3 inches, one barrel of cement will make from 22 to 26 cubic feet of concrete, the average being about 23 cubic feet.

In putting in the foundations of the Cathedral of St. John the Divine, New York, it required 17,000 barrels of Portland cement to make 11,000 yards, or about one and one-half barrels to the yard. The proportions were 1, 2 and 3.

Concrete made of 1 part cement, 2 of sand, 3 of gravel and 5 of broken stone gave 1.18 yards of concrete to a barrel of cement.

The ordinary cement barrel contains about 3 cubic feet.

At $2 a day for labor, the cost of mixing and depositing concrete should not exceed $1 a cubic yard. The cost per yard of Portland cement concrete will generally vary from $6 to $8, according to the cost of the cement, labor and aggregates.