776. The use of concrete in the construction of breakwaters in the United States was suggested as early as 1845. In recent years it has been employed quite extensively, especially for harbor improvements on the Great Lakes, where it has withstood the rigorous winters, the severe storms, the attrition of ice, and the impact of boats, in a highly satisfactory manner. Its use has been confined largely to the construction of a superstructure on timber cribs, the concrete work being in the form of blocks set with derricks, or of monolithic blocks molded in place, or more frequently composed of a combination of these two forms.
Since in breakwater construction weight is of prime importance, it is not necessary, in general, to use an exceptionally strong concrete, as the increased expense had better be incurred in increasing the cross-section.
In the construction of the extensive breakwaters at Buffalo,1 concrete has been used in large quantities and according to various plans. In 1887 the superstructure of some 750 feet of timber-crib breakwater was renewed, mainly with natural cement concrete. 250 feet of this superstructure was built with a facing of Portland cement concrete, while 500 feet of it was faced with stone masonry. The concrete started two feet below mean lake level. The cross-section of the superstructure was about 350 square feet, and the cost of concrete, exclusive of materials, was about $2.36 per cubic yard.
1 Described by Mr. Emile Low, U. S. Asst. Engr. Trans. Am. Soc. C. E., December, 1903.
Breaking stone for crusher........ .38
Crushing stone............. .82
During the following year concrete footing blocks were used on both the lake and harbor faces, since it was found that the cement was washed out of the concrete laid in place below water. The blocks contained about 3 1/4 cubic yards and Cost on the average a little more than $30.00 each, or $37.35 each including the setting, or at the rate of $11.29 per cubic yard. The molds or forms, which were used repeatedly, Cost about $40.00 each.
778. Another style of concrete superstructure developed at Buffalo is that recommended by Major F. W. Symons. It consists of three longitudinal walls, connected at intervals by cross-walls, filled between with rubble stone and provided with heavy parapet and banquette decks. The longitudinal wall on the lake side is founded on heavy concrete blocks 5 feet high, 8 feet thick at the base and 7.2 feet long ; the two minor walls are formed by smaller blocks, 4 feet by 4.5 feet by 12 feet. The total width at base is 36 feet. The space between lake face blocks and center row is 14 feet, and between center row and harbor face blocks is about 5 feet. The cross-wall blocks are 7 by 6 by 4 feet under the parapet, and 4 by 3 by 4 feet under the banquette, all spaced 36 feet centers. All concrete blocks have their bases set two feet below mean lake level and have panels in their upper surfaces to provide a bond with the concrete laid in place.
The lake wall above the concrete block is 8 to 4 feet thick, with batter on face, and the decks are 3 to 4 feet thick, built of concrete in place. The forms for the harbor face wall and cross-walls were of 7/8 inch matched pine, with vertical posts two to three feet centers tied through the wall with one-half inch tie rods.
The concrete was composed of the following volumes: one part Portland cement, one part screened gravel (about 3/8 inch), two parts sand grit (nearly half of which was 1/8 inch to 1/4 inch gravel), and four parts unscreened broken limestone (about 11 per cent. dust). The Cost of the concrete in blocks was $10.00 per cubic yard, and that in place Cost $9.40 per cubic yard.
Several forms of concrete superstructure have been employed in the work at the Cleveland breakwater. One section on a thirty-two foot crib has three rows of concrete blocks, one each on lake and harbor sides and one in center of the crib, extending three feet below mean lake level. The concrete in place is started at mean lake level and is composed of a base five feet thick, with vertical faces over the entire crib, and surmounted on the lake side by a parapet five feet high and about twelve feet wide. The stone filling of the cribs was covered with a cheap decking of wood before laying the concrete in place.
In the construction of the superstructure of the breakwater at Marquette, Mich., the conditions were peculiar in that it was desirable to provide a passageway within the superstructure through which the lighthouse on the outer end might be reached in stormy weather. This was accomplished by leaving near the harbor face a conduit, 6 feet 3 inches high and 2 feet 10 inches wide, the entire length of the structure.
The old timber structure having been removed to about one foot below mean lake level, a foundation course two feet thick of Portland cement concrete was laid on a burlap carpet placed over the stone filling of the crib. Upon this the monolithic blocks were built in place, substantial molds being set up for alternate blocks ten feet apart. After these had set, the molds were removed and other molds set up to form the two faces of the intervening blocks, the ends of the blocks already completed taking the place of end molds. The monolithic blocks were of natural cement concrete in proportions of 489 pounds of cement to one-half cubic yard of sand and one cubic yard of broken stone. About twenty per cent, of these monoliths was composed of rubble stone ranging in size from one-half to three cubic feet, care being taken that no rubble should be placed nearer than one foot to any outside surface. The standard block was twenty-three feet wide on the base, which was one foot above mean lake level. The lower five feet of the face had a 45° slope. There was then a nearly level berm, 7.5 feet wide, forming the banquette deck; from the back of this deck the face sloped at an angle of 45° to the parapet deck, which was 6 ft. 4 inches wide. The harbor side of the block was vertical, 9.4 feet high. Since the structure proved very stable and free from vibrations in heavy seas, the horizontal dimensions of the block were reduced as the shore was approached.
781. The method of placing the Portland cement concrete foundation was modified as described under the head of the block and bag systems of concrete constructions (Art. 64).
The Cost of the monolithic blocks of natural cement concrete was as follows: —
490 lbs. cement, $1.04 per bbl......... $1.815
.5 cu. yd. sand, $0.50 per cu. yd......... .25
1.0 cu. yd. stone, $1.58 " " "........ 1.58
Materials in one cubic yard concrete...... $3.645
80 per cent, concrete in the finished block, .80 of $3.645 ..................... $2.91
Loading materials.................. .33
mixing concrete................... .52
Depositing concrete.................. .41
Handling rubble................... .09
Finishing blocks................... .09
Moving and setting forms............... .25
Timber waling, anchor bolts, etc............. .13
Total Cost in place per cu. yd........... $4.73
Very interesting and detailed accounts of the construction of this breakwater, which was carried out with special care as to all details, were made by Mr. Clarence Coleman, Asst. Engr., and may be found in the reports of Major Clinton B. Sears, Reports Chief of Engineers, U. S. A., 1896 and 1897.