proportions of the several ingredients are thrown somewhat at random, and from which the concrete is supposed to continuously fall upon the work in a properly mixed condition. This method is probably fully equal to hand mixing, but is not as satisfactory as the use of the cubical box above described.

Concrete should always be laid in courses of from six to nine inches in depth, and thoroughly rammed in place in order to compact it effectually. If several courses are to be laid in order to obtain the required depth of concrete, one course should follow another as rapidly as possible, in order that they may become effectually joined, and form finally one monolithic mass. The courses already laid, however, should not be disturbed, after the concrete has begun to take a set. When the work is interrupted at the end of a day, and other courses of concrete are to be laid the following day, and especially when Sunday intervenes, the top of the concrete should be covered and kept wet, and when the next course is laid the top surface of the former should be thoroughly water soaked, and all earthy matter removed from it.

Masonry or other heavy weights should not be laid upon concrete until it has been allowed to harden, usually as much as twenty-four hours. In the case of quick setting natural cements, however, twelve hours may be sufficient.

Since successive freezing and thawing will prevent the ultimate hardening of cement mortar, it is customary to prescribe that no masonry or concrete in which cement mortar is employed shall be laid in freezing weather. It is a well established fact, however, that Portland cement mortar is not injured by freezing if it remains in a frozen condition for a considerable length of time. Again, when the temperature is not too low, but below freezing, freezing of the mortar may be prevented by adding salt to the water in making the mortar, or the ingredients of concrete may be heated so that the concrete will have set before freezing can take place.

The following specification for cement concrete includes as a constituent part of it the specification for cement mortar in the previous article. That which is given below is supposed to follow directly upon the previous quotation, the whole constituting a specification for cement concrete.

The broken stone shall be wetted down and then thoroughly mixed with the mortar by turning it over with shovels; no more stone shall be used than can be covered on all surfaces with mortar, and the proportion of broken stone in the concrete must not exceed five measures of stone to one measure of cement. All material must be actually measured in bulk.

Concrete must be mixed in small and convenient quantities and immediately deposited in the work. It must be carefully placed, and not dropped from any height. It shall be laid in sections, and in horizontal layers not exceeding nine (9) inches in thickness, and it must be thoroughly rammed until the stone is covered with mortar and a film of water appears on the surface. In no case shall concrete be permitted to remain in the work if it has begun to set before the ramming is completed. When concrete is properly made the whole mass becomes one stone when it has set, and it is very important that it shall be deposited continuously in the work. All surfaces upon which concrete is to be laid must be wetted before the concrete is deposited. Plank or timber forms must be provided when necessary to confine the concrete to the shape and dimensions shown on the plans.

Before any weight is placed on concrete it shall have as much time to set as can conveniently be allowed, and in no case less than twelve (12) hours.

In cold weather material for concrete shall be heated as directed by the engineer.

The engineer will issue special instructions for concrete which is to be deposited under water.

O. B.

The following method of making concrete by using sea washed gravel of standard sizes as obtained from graduated screens has given most excellent results. In this mixture there were three grades of sand and gravel employed, namely, fine sand, coarse sand, and small gravel stones up to one fourth of an inch in diameter, and large gravel from one half to two inches in diameter. The proportions were one part cement, two parts fine sand, four parts coarse sand and small gravel, *See article by C. H. Platte, C. E. in Engineering News of February 21, 1895.

and eight parts of the larger gravel, making in all one part of cement to fourteen parts of sand and gravel, by measure. The cement and fine sand were mixed dry. The two grades of gravel were then thoroughly mixed and saturated with water, the surplus water being allowed to drain off. The dry mixture of cement and sand was then uniformly spread over the wet gravel and thoroughly mixed with it. The water which remained adhering to the gravel was found sufficient to moisten the cement, and also to insure a uniform distribution of such water through the mass. The mixture was then deposited in place and thoroughly rammed, and it was found to give a very solid and strong concrete. It was found that three and one half barrels of cement were used for each four and onehalf cubic yards of concrete in place. It is said that the concrete was equal in every particular to that made of one part of cement, three parts sand, and five parts broken lime stone. This species of concrete was used in the foundations of the New York and Brooklyn bridge, and also on some of the New York City cable railways. This mixture comes very near being the ideal concrete for both solidity and economy.

The following complete specification for cement mortar and concrete indicates the present practice of one of the leading civil engineers of this country, and has many excellent qualities. These specifications were used in the construction of the 155th street bridge over the Harlem river, New York City.

All mortar used, except for pointing, shall be composed of one part Portland cement mixed in a dry state, with two parts clean, sharp sand, free from loam; the entire quantity of water to be added, and the whole mass thoroughly incorporated. Mortar shall only be made in batches for immediate consumption, and none used that has commenced to set. Pointing mortar shall be composed of cement and sand in equal parts, made in very small quantities, and mixed with just enough water to bind the grains.

The cement used will be Rosendale or its equivalent, and Portland. The Rosendale shall be of the best quality of

established brands approved by the engineer, and showing a minimum breaking strength of seventy pounds per square inch on the twenty-four hour test, viz.: Set one hour in air, remainder of time in water. Portland cement must be of a well-established brand, English or German manufacture, and approved by the engineer. It should be a moderately slowsetting cement, weighing one hundred and six to one hundred and twelve pounds per struck bushel, not less than ninetyeight per cent. fine, using a two thousand five hundred mesh sieve, and to test neat in seven days after two hours in air,* remainder of time in water, three hundred and eighty, to four hundred and twenty-five pounds per square inch of section. When tested in cakes, all cement must be free from cracking on the edges.

In general, all cements must be subject to the usual standard of inspection and testing, as recommended by the American Society of Civil Engineers. The contractor must keep on hand, under proper shelter, and convenient of access, sufficient stock of cement ahead of his wants to afford a reasonable time for its proper examination and testing.

The sand used must be clean, sharp sand, not too fine, free from pebbles and must not soil white paper.

The water used must be clean, fresh water.

The concrete will be mixed either by hand or machine, according to quantity needed and rapidity of requirements. Any machine used must be one of proved efficiency and reliability for uniform product. All component parts of a concrete batch must be accurately gauged as to relative volume. Cement and sand must be thoroughly mixed dry, and the thoroughly wetted broken stone incorporated therewith, and only enough water added by sprinkling to uniformly dampen the mass without wetting it, as may be directed. The amount of water required will depend upon atmospheric conditions of heat and moisture, and due allowance must be made therefor. In hand mixing, the broken stone will be spread out upon a tight platform, within a movable frame of gauged dimensions, holding an exact manageable batch of stone, nine to twelve inches deep. The stone to be flushed off uniform with edge of frame, and thoroughly wetted, allowance being made when stone has become highly heated from the sun, when the water quickly evaporates. The sand and cement mixed dry on an adjacent platform to be then uniformly spread over the stone prepared as above. The frame will then be lifted off, and the mass shoveled over in rows, the men working from opposite sides towards each other, care being exercised not to heap the mass, but simply turn it over, keeping the original thickness. During this process the sprinkling to be kept up to obtain the

*This is commonly given as 24 hours in air, but kept moist.

required dampness. The operation to be repeated rapidly and only so often as may be necessary to obtain a uniform homogeneous mass of concrete, which must be used as rapidly as possible. All concrete must be thoroughly rammed in layers, as may be directed, with metal rammers of an approved pattern, weighing from thirty to thirty-five pounds each. Corners inaccessible to direct long-handled ramming, as in air chambers of caissons, to have special devices provided to insure solid contact work. All concrete stone to be trap-rock, or a dense fine grained blue limestone, or may be a clean screened coarse gravel, when used with Rosendale cement. All concrete stone must be clean and free from dust. There will be three grades of concrete required designated by numbers.

No. I. Concrete mass on roof of caissons and filling of air chambers-one part Portland cement, two parts sand and four parts of one-half-inch clean, broken stone.*

No. 2. Concrete for footings of Pier III, and hearting of all piers-one part Portland cement, two parts sand and four and one half parts broken stone, miscellaneous sizes, but none over two inches in any direction.

No. 3. Common concrete in Pier II-one part Rosendale cement, two parts sand and five parts gravel or broken A. P. B.

stone.

When concrete is used for the purpose of making a wall impervious to water, it must be made of small gravel or small broken stone, and it must be unusually rich. That is to say a large excess of mortar must be employed. With these precautions, with proper care in laying, it is possible to make a practically water-tight wall of cement concrete. Such a wall or partition may be constructed between two rubble stone walls, the concrete core being relied on to make the wall practically water-tight.

129. Specifications for Stone. The following specifications for stone to be used for various purposes are those in use by the Chicago, Milwaukee and St. Paul Railway. While certain qualities of stone are here specified, no method is prescribed for determining these qualities. The qualities of building stone are often examined by means of laboratory tests for strength, specific gravity, and for effect of freezing, and also

*That is to say, crushed stone which will pass through a sieve of 1⁄4 inch mesh and be retained on a sieve of 1⁄4 inch mesh.